The contribution of soil extract composition and cyclic moisture dynamics to the physicochemical aging of superabsorbent polyacrylic acid and polyacrylamide hydrogels

The contribution of soil extract composition and cyclic moisture dynamics to the physicochemical aging of superabsorbent polyacrylic acid and polyacrylamide hydrogels | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article The contribution of soil extract composition and cyclic moisture dynamics to the physicochemical aging of superabsorbent polyacrylic acid and polyacrylamide hydrogels Janina Neff, Christian Buchmann This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7155517/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Abstract Polyacrylic acid (PAA) and polyacrylamide (PAM), two synthetic superabsorbent polymers (SAPs) commonly used in agriculture, can form three‑dimensional hydrogels that enhance soil water retention and soil structural stability. Yet, their potential aging and transformation under natural drying–rewetting dynamics and in contact with soil solutes remains unclear. In this study, we examined the effect of soil extracts from sand, loam, and clay soil in both a 72 h free swelling experiment (FSE) and in an incubation experiment (IE), where PAA and PAM hydrogels underwent ten successive drying-rewetting cycles. Samples were taken after cycles 0, 3, 5, and 10 and investigated for their swelling index (SI), water entrapment ( 1 H proton nuclear magnetic resonance relaxometry), structural stability (rheometry), morphology (scanning electron microscopy), and surface chemistry (fourier transform infrared spectroscopy). In the FSE, PAA swelling in all soil extracts reduced SI, shortened T₂ relaxation, and increased mechanical rigidity, whereas PAM properties remained stable except when swollen in sand extract. During the IE, PAA exhibited progressive hydrogel network densification, further SI loss, T 2WL shortening, band intensity shifts, and microstructural compaction, whereas PAM remained largely inert. Multivariate analysis confirmed that polymer identity and its interaction with the conducted drying-rewetting cycles and soil extract composition drove SAP aging. Overall, drying–rewetting cycles seem to induce irreversible chemo-structural alterations in anionic PAA, thereby diminishing its rehydration potential and triggering the potential formation of persistent, solid-like residues, whereas neutral PAM seems more resilient under dynamic environmental conditions. Physical sciences/Chemistry Physical sciences/Engineering Earth and environmental sciences/Environmental sciences Physical sciences/Materials science Polyacrylic acid polyacrylamide drying-rewetting cycles soil extracts hydrogel aging Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1 Introduction Increasing water scarcity in arid regions as well as periodic heavy rainfall events and high precipitation pose major challenges for agriculture. Thus, respective soils and crops that are not native to arid regions are more severely impacted compared to plants already adapted to dry periods [ 1 , 2 ] . To increase irrigation efficiency and also improve soil stability in the context of surface runoff or soil erosion, technical developments and the application of (synthetic) superabsorbent polymers (SAPs) are frequently investigated [ 3 – 6 ] . Synthetic SAPs, mainly polyacrylic acid (PAA) and polyacrylamide (PAM) are produced according to the biological model of natural hydrogels and thus can absorb large quantities of water and build a three-dimensional polymer network in the soil interparticle space, known as “junction zones” [ 7 – 8 ] . This interparticulate hydrogel swelling substantially modulates various soil physicochemical properties, including the maximum soil water holding capacity (WHC max ), soil structural stability, soil permeability, and the availability of nutrients, fertilizers, and pesticides [ 8 – 11 ] . In this regard, the term “gel effect” was coined to summarize all the previously mentioned SAP-related functions and modulations in soil [ 8 – 10 ] . Depending on the area of application and intended use, SAPs are applied to soil in different ways, including a) mix application in terms or evenly mixing SAP with soil about 0–20 cm through tillage, b) spraying/sprinkle application by evenly spraying or spreading the SAP on the leave or soil surface, c) coating or soaking seeds prior to planting, and d) point/hole application by punctual, spatially limited incorporation of the SAP to the seed burial or root zone [ 12 – 14 ] . Although various fundamental mechanisms and processes of the gel effect in soil have been explored [ 9 , 10 ] some of them are still unknown, including how the combined effects of environmental dynamics and respectively induced SAP transformation further modulate the physicochemical properties of both soil and the interparticulate hydrogel itself. In this regard, current scientific research is investigating the extent to which the aging and transformation of SAPs in soils could lead to plastic-like non-degradable SAP residues [ 15 ] . Here, one of the research focuses is on natural drying–rewetting processes, in which the composition of the soil solution - its ionic profile, colloidal clay particles, and organic constituents - significantly influences the swelling behavior, functional properties, and fate of interparticulate SAP hydrogels in soil. As drying–rewetting events concentrate or dilute soil solution constituents, osmotic gradients shift and additional crosslinking of SAP polymer chains might occur, altering SAP network architecture and functioning [ 2 , 16 – 21 ] . Because concentration changes and solute availability in soil are intrinsically linked to the water content, they must be considered and part of systematic studies in the context of moisture dynamics [ 22 , 23 ] . Although previous work has demonstrated that repeated drying–rewetting dynamics lead to progressive loss of SAP effectiveness in terms of lower maximum swelling, reduced water retention for plants, and even apparent “cementing” of soil pores [ 2 , 19 , 21 ] , there remains a significant lack of understanding regarding how environmental dynamics, particularly repeated drying–rewetting cycles and combined SAP-soil solution interactions, affect the long-term performance and transformation of SAPs. This study specifically addresses these knowledge gaps by integrating physicochemical, morphological, and relaxation-based analyses. For this, we investigated PAA and PAM, the two most common synthetic SAPs, either swollen in demineralized water (dH 2 O) or in three different soil extracts (sand, loam, clay) and subjected to ten drying-rewetting cycles. At different drying-rewetting cycles, we quantified the swelling index (SI), water dynamics ( 1 H proton nuclear magnetic resonance (NMR) relaxometry), microstructural stability (rheometry), and tracked chemical changes in terms of pH, electrical conductivity (EC), surface chemistry (attenuated total reflectance fourier transform infrared - ATR-FTIR), and morphological features (environmental scanning electron microscope - ESEM). ATR-FTIR will be used to detect general shifts in characteristic polymer bands and the appearance of new or stronger peaks, providing molecular-level evidence of ionic crosslinking and condensation dynamics. Concerning the effect of the soil extract composition, we hypothesized that the effect of cation charges on hydrogel properties is more important than the overall ion composition of the soil extract. Here, trivalent cations (especially Al³⁺) should promote stronger ionic crosslinks in the SAP hydrogels than divalent cations (e.g., Ca²⁺, Mg²⁺), promoting the formation of denser, less expandable network structures, with smaller pores (ESEM), alongside shorter transverse relaxation times ( 1 H-NMR relaxometry), reduced SI, and higher structural stability (rheometry) compared to dH 2 O controls. In the ATR-FTIR spectra, increased crosslinking degree will come along with systematic band shifts and intensity changes for characteristic absorption peaks of the two SAPs. In the course of the successive drying-rewetting cycles, SAP hydrogels swollen in dH 2 O are hypothesized to retain their original properties up to a critical number of drying-rewetting cycles, beyond which the polymer chains undergo irreversible condensation, leading to “network aging” that can be measured as permanently shorter transverse relaxation times, reduced SI, and increased stability, independent of subsequent drying-rewetting cycles. However, these effects should be amplified in the three investigated soil extracts as function of pH and soil extract composition, promoting osmotic stresses and additional ionic crosslinking. Accordingly, ATR-FTIR of the hydrogels swollen in soil extracts should show these simplified band shift trends more prominently and at earlier cycles than in dH₂O. Moreover, we hypothesized that crosslinks in the SAP hydrogels are more rapidly formed than SAP network expansion occurs during a drying-rewetting event, steadily increasing network densification, which further reduces the SI, and transverse relaxation times and increases viscosity, respectively. 2 Materials and methods Three well-characterized reference soils from the Agricultural Investigation and Research Institute (Speyer, Germany) were used. The soils differed in their texture (sand, clay and loam soil) and physicochemical properties (Table 1 ). From each soil, soil extracts were prepared by mixing soil and dH 2 O at 1:5 ratio, agitating for 24 h, and filtering through a 0.45 µm membrane. Soil extract will be referred to as sand, loam and clay in the following. For the SAP swelling experiments, hydrogel-forming PAA powder (Viscosity average molar mass M v = 4,000,000 g/mol) (Sigma-Aldrich, Germany; CAS 9003–01–4), and PAM granules (M v = 15,000,000 g/mol) (Carl Roth, Germany, CAS 9003-05-8) of high molecular weight were investigated. To assess the effect of the soil extracts on the various physicochemical SAP properties, a free swelling experiment (FSE) was conducted according to Brax et al. [ 8 ] and Buchmann et al. [ 10 ] in a modified way: for this, freely swollen PAA and PAM hydrogels were prepared by allowing dry SAP powder/granules to completely swell for 72 h on a pre-wetted dialysis membrane (flat width 44 mm, MWCO 14000, Carl Roth GmbH & Co. KG), which was positioned in a liquid reservoir with the respective soil extract (or dH 2 O as control). After swelling, the respective SAP hydrogels were gently removed from the membrane and investigated for various physicochemical properties as listed below. For the incubation experiment (IE), after an initial free swelling of the respective SAPs in dH 2 O (C0), the respective soil extracts of the three soils were used for the subsequent drying-rewetting cycles. Thus, the SAP hydrogels were subjected to a total of 10 drying-rewetting cycles of seven days each, including four days of drying at 30°C and three days of rewetting in an excess of respective liquid (dH 2 O or soil extract) at 20°C. After the 3rd, 5th, and 10th drying-rewetting cycle (C3, C5, C10), the respective PAA and PAM hydrogels were examined for the same physicochemical properties as in the previous swelling experiment. In total, 190 samples were prepared for both experiments with five replicates for each treatment. Table 1 Selected physicochemical properties of the investigated soils and the respective 1:5 soil extracts used for the SAP swelling experiment Soil name 2.1 2.4 6S Soil type sand loam clay Bulk soil Organic carbon [% C] 0.55 ± 0.10 1.83 ± 0.17 1.50 ± 0.13 Nitrogen [% N] 0.06 ± 0.01 0.23 ± 0.02 0.17 ± 0.01 pH 4.60 ± 0.10 7.50 ± 0.10 7.30 ± 0.04 CEC [meq/100g] 2.90 ± 0.20 17.40 ± 0.80 18.70 ± 1.20 Density [g/cm³] 1.47 ± 0.06 1.18 ± 0.04 1.27 ± 0.03 WHC max [g/100g] 32.50 ± 1.50 44.60 ± 2.20 41.60 ± 1.00 PSD (mm) [%] < 0.002 3.10 ± 0.90 26.60 ± 0.70 40.80 ± 1.40 0.002–0.05 10.70 ± 1.20 41.20 ± 1.30 35.10 ± 0.71 0.05-2.0 86.20 ± 0.70 32.30 ± 1.40 24.10 ± 1.80 Soil extracts EC [µS/cm] 138 ± 1 424 ± 1 86 ± 1 pH 7.0 ± 0.1 7.4 ± 0.1 8.2 ± 0.1 Al 3+ [mg/L] 0.40 ± 0.01 0.04 ± 0.00 0.11 ± 0.00 Fe 3+ [mg/L] 0.29 ± 0.04 0.23 ± 0.02 0.09 ± 0.02 Mn 2+ [mg/L] 1.17 ± 0.15 0.018 ± 0.001 0.12 ± 0.003 Zn 2+ [mg/L] 0.06 ± 0.01 0.007 ± 0.00 0.02 ± 0.00 Ca 2+ [mg/L] 31.15 ± 4.25 90.62 ± 5.10 47.51 ± 3.77 Mg 2+ [mg/L] 5.34 ± 0.13 5.87 ± 0.10 5.27 ± 0.14 K + [mg/L] 10.29 ± 0.90 2.92 ± 0.17 8.83 ± 0.96 Na + [mg/L] 2.62 ± 0.06 3.67 ± 0.05 2.63 ± 0.03 Basic parameter Before and after each drying-rewetting cycle, pH and EC of the soil extracts in the liquid reservoir were determined according to DIN EN ISO 11265 [ 24 ] and DIN 38404-5 [ 25 ] respectively, using a multi-parameter analyzer C863 (Consort, Belgium). Furthermore, the SI of both SAP hydrogels were calculated according to [ 26 , 27 ] , respectively. 1 H-NMR relaxometry Water entrapment was measured by 1 H-NMR relaxometry using a Bruker Minispec MQ (Bruker, Karlsruhe, Germany) with a magnetic field strength of 0.176T, corresponding to a proton Larmor frequency of 7.5 MHz [ 10 ] . Transverse relaxation (T 2 ) decay curves were acquired with an echo time (T E ) set at 0.3 ms. The raw data were processed using MATLAB with an inverse Laplace transformation (ILT) [ 28 ] , based on the Butler, Reeds, and Dawson (BRD) algorithm [ 29 ] to obtain the respective relaxation time distributions (RTDs). Following the method outlined by Buchmann et al. [ 30 ] , the 95th percentile of the sum of all amplitudes was employed (T 2WL ), consequently the relaxation time of 95% of the water protons within the sample was shorter than the T 2WL [ 31 ] . Further, the peak positions T 2peak within the RTDs were determined as the predominant water fraction in the sample [ 10 ] . To exclude possible (para)magnetic relaxation effects originating from the soil extracts, reference measurements were carried out as described by [ 32 ] . Rheometry Rheological measurements were conducted using an MCR 102 rheometer (Anton Paar, Germany) equipped with a cone-plate measuring geometry. For the measurements, a small amount of swollen SAP hydrogel was placed on the rheometer plate, followed by a resting period of 60 s to ensure undisturbed measurements. To assess the viscoelasticity of the hydrogels, additional amplitude sweep tests (AST) were performed at a constant frequency of 10 s⁻¹ for a total of 37 measurement points. The temperature was set to 20°C, regulated by a Peltier unit. From the AST, shear stress and at the yield point (τ YP ) as well as the maximum shear stress (τ max ) for each sample were determined. ATR-FTIR ATR- FTIR measurements of all samples were performed on the freeze-dried hydrogels using a Cary 630 ATR-FTIR (Agilent Technologies) spectrometer. The resulting data were analyzed using an open Specy R package [ 33 ] . As the spectra exhibited no significant background noise, no filtering was required during data processing. Striking absorbance bands at 4,000–3,200 cm⁻¹ (O–H), 3,000–2,800 cm⁻¹ (C–H), 1,870-1,550 cm⁻¹ (C = O), 1490 − 1150 (H–C–H), 1,040 cm⁻¹ (C–O) for PAA and 4,000–3,080 cm⁻¹ (NH), 3000 − 2800 cm⁻¹ (C–H), 1,870-1,550 cm⁻¹ (C = O), and 1,490-1,150 (H–C–H) for PAM were qualitatively evaluated for all samples [ 34 – 37 ] . ESEM ESEM images were exemplarily captured for the two SAP hydrogels at C0 and C10 using a Quanta 250 ESEM (FEI Company, Hillsboro, United States) equipped with an Everhart Thornley secondary electron detector (ETD). Prior to the measurements, the hydrogels were freeze-dried and coated with a 30 nm thick gold layer using a Q150R S sputter coater (Quorom Technologies Ltd, United Kingdom). Measurements were performed under high vacuum (< 10 − 4 Pa) with an acceleration voltage of 30kV and an average spot size of 3.5. Different resolutions were employed a) to visualize overall structural features of PAA and PAM swollen in dH 2 O and the different soil extracts and b) to examine detailed network structures, including network density, crosslinking areas, and polymeric arrangements. Statistical analysis Variations within the 5 replicates were presented as standard errors (SE) of the arithmetic means. As variance homogeneity (Levene-test) and normal distribution (Shapiro-Wilks-test) were not fulfilled, a permanova was performed based on Euclidean distance measurements using the adonis2() command from the vegan package [ 38 ] . Moreover, a scaled principal component analysis (PCA) was performed using the PCA() command from the FactoMineR package [ 39 ] . R version 4.3.1 (RStudio 2024.04.2) and Excel Office 16 were used to carry out all calculations and figures. All detailed statistical parameters, including degree of freedom, Sum of squares, R2, F-value and p-values are presented in Tables S1 and S2 of the supplementary information. 3 Results Basic parameter For the FSE, PAA substantially decreased the pH for all soil extract directly after the first swelling, from pH of 7.0 ± 0.12, 7.4 ± 0.1 and 8.2 ± 0.1 to 3.6 ± 0.1, 3.9 ± 0.0 and 4.5 ± 0.1 for sand, loam and clay, respectively (Fig. 1 a-c). In contrast, PAM slightly increased the pH of the sand and loam extracts by 0.2 and 0.1 units to 7.2 ± 0.10 and 7.5 ± 0.10, respectively, while decreasing the pH for the clay extract by 0.3 units to 7.9 ± 0.1 (Fig. 1 a-c). Furthermore, PAA substantially increased the EC after the first swelling in all soil extracts, from initially 138 ± 1 µS/cm, 424 ± 1 µS/cm and 86 ± 1 µS/cm for sand, loam and clay to 311 ± 1 µS/cm, 719 ± 1 µS/cm and 150 ± 1 µS/cm, respectively. Also, PAM substantially increased the EC in sand and clay extracts by 71 µS/cm and 177 µS/cm to 209 ± 1 µS/cm and 263 ± 1 µS/cm. In contrast, PAM decreased EC in the loam extract by 286 µS/cm to 138 ± 1 µS/cm (Fig. 1 d-f). For the IE, PAA substantially decreased the pH in the respective soil extracts from C0 to C3 and remained constant for all subsequent drying-rewetting cycles. After C3, the pH of PAA-related liquid reservoirs decreased from 7.08 ± 0.06, 7.34 ± 0.03 and 8.03 ± 0.03 to 3.5 ± 0.03, 3.86 ± 0.04 and 4.42 ± 0.03 in sand, loam and clay, respectively. In contrast, the liquid reservoirs related to PAM exhibited only minimal pH shifts (within ± 0.4 units) to 6.72 ± 0.03, 6.98 ± 0.03 and 7.84 ± 0.04 in sand, loam and clay until C10, respectively. EC substantially increased with drying-rewetting cycles for both SAPs, although the magnitude and pattern varied with the soil extracts (Fig. 1 d-f). For PAA, EC of the liquid reservoirs steadily increased from initially 138 ± 1 µS/cm, 425 ± 1 µS/cm and 81 ± 1 µS/cm (7 ± 1 µS/cm for dH 2 O) to 478 ± 0 µS/cm (sand), 1,558 ± 0 µS/cm (loam), and 233 ± 0 µS/cm (clay) after C10. In contrast, EC of the PAM-related liquid reservoir increased only modestly, peaking at 328 ± 1 µS/cm, 1,047 ± 1 µS/cm, and 388 ± 1 µS/cm for sand, loam, and clay, respectively. Concerning the FSE, the SI of PAA (SI PAA ) decreased in all three soil extracts compared to the first swelling in dH 2 O. Here, SI was the lowest for PAA swollen in clay extract (61.36 ± 0.59 ml/g compared to 76.80 ± 0.17 ml/g in dH 2 O). SI PAM decreased in loam and clay extract, while increasing in sand extract to 51.23 ± 0.58 ml/g compared to 43.80 ± 0.38 ml/g in dH 2 O (Fig. 2 ). The differences in polymer type, soil extracts (including dH 2 O) and their interactions were highly significant (p = 0.001). In the IE, SI PAA overall decreased by 20% in the course of drying-rewetting, from initially 76.80 ± 0.17 ml/g (C0) to finally 52.5 ± 0.56 ml/g after C10. In contrast, SI PAM constantly fluctuated between 43.80 ± 0.38 ml/g (C0) and 39.29 ± 0.60 ml/g (after C10) throughout all drying-rewetting cycles. Concerning the effect of the soil extracts, SI PAA decreased in all three soil extracts, whereby the loam showed the highest reduction of 76.80 ± 0.17 ml/g to 33.79 ± 0.61 ml/g after C10. When swollen in sand extract, SI PAM decreased from initially 43.80 ± 0.38 ml/g at C0 to 36.71 ± 0.57 ml/g after C3, before (re)increasing again to 44.08 ± 0.47 ml/g after C10. A similar pattern was observed in loam and clay: SI PAM decreased to 26.44 ± 0.91 ml g⁻¹ and 33.71 ± 0.59 ml g⁻¹, respectively, after C5, then (re)increased again to 39.22 ± 0.66 ml g⁻¹ (loam) and 45.98 ± 0.98 ml g⁻¹ (clay) after C10. Permanova revealed that polymer type (PAA vs. PAM), soil extract (dH 2 O, sand, loam, clay), drying–rewetting cycle (0, 3, 5, 10), and all their two- and three‐way interactions were highly significant (p < 0.001) (Fig. 2 ). Polymer-water interactions The relaxation time distributions (RTDs) together with T 2WL and T 2peak derived from the 1 H-NMR measurements were used to further characterize the SAP hydrogels in terms of polymer-water interactions (Figs. 3 and 4 ). Concerning the FSE, both PAA and PAM exhibited the same symmetric and sharp RTDs for all three soil extracts with single T 2peak at for 2,166 ± 20.16 ms for dH 2 O, 1,739.27 ± 23.65 ms for loam and 1,557.91 ± 40.10 ms for clay. PAM exhibited symmetric and sharp RTDs with T 2peak positions at 2,471.00 ± 7.81 ms for dH 2 O, 2,369.65 ± 107.28 ms for loam, and 2,233.55 ± 38.23 ms for clay extracts. In contrast, for the first swelling in sand extract, both PAA and PAM exhibited as well symmetric and sharp RTDs together with decreased T 2peak positions to 491.71 ± 8.30 ms for PAA and 650.34 ± 22.79 ms for PAM (Fig. 3 a, b and Fig. 4 d, f, h). Regarding T 2WL , PAA swollen in the different soil extracts showed lower values compared to dH 2 O (4,288.70 ± 1.44 ms) with 1,115.55 ± 19.10 ms for sand, 3435.01 ± 57.99 ms for silt, and 3,075.08 ± 67.52 ms for clay. In contrast, PAM showed approximately the same T 2WL of 4,472.018 ± 15.01 ms as dH 2 O for loam and clay, together with a decreased T 2WL of 1,322.59 ± 171.82 ms in sand (Fig. 4 c, e, g). The differences of the polymer type, the soil extracts including dH 2 O were highly significant (p = 0.001), as well as the interaction of the polymer type with the soil extracts (p = 0.002). For the IE, both PAA and PAM exhibited symmetric and sharp RTDs with a single T 2peak centered at approximately 2,166 ± 20.16 ms and 2,471 ± 7.81 ms for all drying-rewetting cycles, respectively. However, T 2WL of PAA slightly decreased with each drying-rewetting cycle, from initially 4,288.70 ± 11.44 ms at C0 to 3704.40 ± 0.00 ms after C10 (Fig. 3 a). Also, T 2peak of PAA increased from C5 on to finally 2,171.12 ± 0.00 ms after C10. In contrast, both T 2WL and T 2peak of PAM did not significantly shift (only by 88 ms on average), from initially 4,472.02 ± 15.01 ms and 2,394.13 ± 8.04 ms at C0 to 4,535.52 ± 15.01 ms and 2,427.59 ± 8.04 ms after C10, respectively. Regarding the combined effects of soil extracts and drying-rewetting cycles, RTDs of both SAPs substantially changed, with PAM being less affected than PAA, except for the sand extract. T 2WL for both PAA and PAM showed the same trend, with a decrease to 1,000.03 ± 3.49 ms after C3, which remained constant for all subsequent drying-rewetting cycles. Additionally, the RTD of PAA transitioned from a sharp single peak to a broader, slightly asymmetric distribution pattern. This was also the case in the loam extract, whereT 2WL of both PAA and PAM decreased with each drying-rewetting cycle to finally 2,329.45 ± 10.03 ms and 3,580.46 ± 9.95 ms after C10, respectively. T 2peak of PAA and PAM decreased accordingly to 1,669.91 ± 8.43 ms and 1,943.99 ± 6.53 ms after C5 and to 1,133.31 ± 7.79 ms and 1,840.78 ± 10.26 ms after C10. For the clay extract, T 2WL of PAA decreased significantly stronger (1,603.61 ± 11.02 ms after C10) with each drying-rewetting cycle than for the other extracts. In contrast, T 2WL of PAM decreased only to C3 (3,841.51 ± 19.39 ms), with only slight variations in the following drying-rewetting cycles. T 2peak showed the same course of PAA and PAM with a significantly stronger decrease of PAA from 2,295.99 ± 6.12 ms at C0 to 729.92 ± 8.03 ms after C10. In contrast, PAM decreased from 2,394.13 ± 8.04 ms at C0 to 1972.98 ± 10.30 ms after C3 (Fig. 3 c-j and Fig. 4 ) followed by slight variations in the following drying-rewetting cycles. The differences in polymer type, soil extract and the drying-rewetting cycles were highly significant (p = 0.001). Chemo-structural properties and morphology Concerning the FSE, both structural stability in terms of shear stress at the yield point (τ YP ) and maximum shear stress (τ max ) of PAA increased in all three soil extracts compared to the first swelling in dH 2 O (Fig. 5 a-b). PAA swollen in the loam extract revealed the highest τ YP and τ max of 546.78 ± 132.23 Pa and 956.29 ± 154.57 Pa, followed by 410.26 ± 112.87 Pa and 577.94 ± 19.80 Pa in sand, and 248.92 ± 12.89 Pa and 810.19 ± 131.16 Pa in clay, respectively. In contrast, τ YP of PAA swollen in dH 2 O was 141.37 ± 9.08 Pa. PAM showed the same course as PAA but with only slightly increased τ YP and τ max compared to the first swelling in dH 2 O and PAA. Both τ YP and τ max were highest in loam (48.72 ± 2.52 Pa and 274.62 ± 3.97 Pa) and lowest in clay (97.75 ± 5.93 Pa and 200.59 ± 10.49 Pa) For both stability indices, permanova revealed significances for polymer types (p = 0.001), whereas only τ max showed slight significance for the soil extracts including dH 2 O (p = 0.04). For the IE, τ YP and τ max increased for both SAPs with increasing drying-rewetting cycles and independent of the soil extract. In dH 2 O, both τ YP and τ max of PAA increased with each drying-rewetting cycle, from initially 141.37 ± 9.08 Pa and 356.08 ± 15.71 Pa to 441.67 ± 162.16 Pa and 391.88 ± 54.16 Pa after C10, respectively. However, τ YP of PAM remained constant at approximately 42.97 ± 5.26 Pa over all drying-rewetting cycles, whereas τ max slightly decreased from 95.52 ± 9.95 Pa at C0 to 82.33 ± 8.28 Pa after C3. For sand, τ YP of PAA showed the same course as in dH 2 O by increasing from 141.37 ± 9.08 Pa at C0 to 441.67 ± 162.16 Pa after C10 (Fig. 5 c-d). In contrast, τ max increased from 356.08 ± 15.71 Pa at C0 to 636.95 ± 146.65 Pa after C10 with the highest increase from C0 to 745.20 ± 105.91 Pa after C3 followed by a drop to 573.09 ± 124.09 Pa to after C5. Again, PAM showed a constant τ YP of approximately 42.97 ± 5.26 Pa during the drying-rewetting cycles with a slight increase to 49.41 ± 8.73 Pa after C5. Also, τ max remained approximately constant at 95.52 ± 9.95 Pa at C0 and 103.70 ± 13.83 Pa after C3 and increased slightly to 127.46 ± 13.63 Pa after C5 and 168.07 ± 3.44 Pa after C10. When swollen in loam, τ YP of PAA increased from 141.37 ± 9.08 Pa at C0 to finally 481.17 ± 50.87 Pa at C10, although C5 showed an intermediate decrease to 369.59 ± 93.01 Pa (Fig. 5 e-f). τ max of PAA showed a similar course with a relatively higher increase, from 356.08 ± 15.71 Pa at C0 to 621.40 ± 39.77 Pa after C10 showing a peak of 1051.80 ± 95.75 after C3. For PAM, τ YP increased from 42.97 ± 5.26 Pa at C0 to 266.07 ± 30.90 Pa after C5 and subsequently dropped to 147.57 ± 29.25 Pa after C10. τ max showed a similar trend with an increase from 95.52 ± 9.95 Pa at C0 to 536.17 ± 26.06 Pa after C5 and dropped to 440.06 ± 48.03 Pa after C10. For clay, τ YP of PAA increased from 141.37 ± 9.08 Pa at C0 to 431.80 ± 134.03 Pa after C5 (Fig. 5 g-h). With further drying-rewetting cycles, τ YP dropped to finally 209.68 \(\:\pm\:\) 67.14 Pa after C10. While τ max of PAA increased to 859.71 ± 46.65 Pa after C3, it dropped afterwards to 629.49 ± 169.95 Pa and 527.32 ± 118.88 Pa after C5 and C10. In contrast, drying-rewetting cycles increased τ YP of PAM from initially 42.97 ± 5.26 Pa at C0 to 134.84 ± 29.41 Pa at C10. Also, τ max showed this trend with an increase from 95.52 ± 9.95 Pa at C0 to 327.73 ± 49.98 Pa after C10. Both τ YP and τ max showed high significances regarding the comparison of the different polymer types and treatments (p = 0.001). Further, τ max showed a highly significant interaction for the polymer types and cycles (p = 0.001) as well. ESEM Concerning the FSE, ESEM images revealed morphological differences between the two SAPs, especially when swollen in sand and loam extract (Fig. 6 ). Compared to dH₂O, PAA exhibited more condensed structures with fragmented edges, whereas PAM formed an overall denser hydrogel network with smaller pores. When swollen in clay extract, PAA again formed a highly condensed network with large but covered pores within layered structures. Also, PAM revealed a condensed hydrogel network with larger pores compared to the one swollen in dH 2 O. For the IE, ESEM images revealed substantial morphological differences between the two SAPs as function of soil extracts and drying-rewetting cycles: C0 and swollen in dH 2 O caused a homogeneous network for both PAA and PAM, whereas clear differences in terms of fracture edges and condensed polymeric parts of the hydrogel network were observed after C10 for PAA. In contrast, PAM swollen in dH 2 O still showed a relatively intact and homogenous network structure after C10, although more condensed parts and larger spaces within the network were visible. PAA swollen in sand showed highly condensed network structures after C10, together with clear breaking edges and smaller pores. It also developed thicker pore walls and a broader pore-size distribution. Also, PAM swollen in sand exhibited elongated, plate-like lamella with highly condensed areas and fan-shaped structures. Regarding the swelling in loam and clay extract, the PAA hydrogel network was completely different compared to PAA at C0 and swollen in dH 2 O. However, both PAA and PAM hydrogels consisted of condensed structures and layer-like features. For the loam extract, PAA showed irregular pore shapes with heterogeneous wall thickness and partially merged junction zones, while PAM occasionally showed granular network structures with localized microcavities and rough walls. Especially when swollen in clay extract, PAA formed a hydrogel network with dense, radially oriented lamellar sheets and fan-shaped domains, whereas PAM transformed into stacked, sheet-like layers with clear stratification and reduced overall pore volume. ATR-FTIR PAA initially swollen in dH 2 O water showed striking absorbance bands at 4,000–3,200 cm⁻¹ (O–H), 3,000–2,800 cm⁻¹ (C–H), 1870 − 1550 cm⁻¹ (C = O), 1490 − 1150 (H–C–H), and 1,040 cm⁻¹ (C–O), whereas PAM showed further absorbance bands at 3,000–3,500 cm⁻¹ (O–H), 1,700-1,740 cm⁻¹ (C = O), and 1,040 cm⁻¹ (C–O). Interestingly, both PAA and PAM also showed an obvious peak at < 1,000 cm⁻¹ (Fig. 7 ). Concerning the FSE, the O–H stretching and C = O of PAA increased when swollen in loam extract but remained the same for dH 2 O, sand, and clay (Fig. 7 a). Further, the C–O band intensity in PAA increased markedly when swollen in loam extract, only slightly in sand, and remained unchanged in both dH₂O and clay extracts. Interestingly, the band at < 1000 cm⁻¹ showed the same course as the C–O band. Interestingly, PAM showed vice versa behavior: N–H stretching, C = O-stretching and H–C–H stretching decreased for all three soil extracts approximately the same compared to the first swelling in dH 2 O (Fig. 7 b). For the IE, drying-rewetting cycles induced cycle-dependent band intensities for PAA and all solutions: when swollen in dH 2 O, both the O–H-stretching (~ 3,400 cm⁻¹) and (3,000–2,800 cm⁻¹) C–H stretching increased from C0 to reach its maximum after C3, but then decreased again after C5 and C10 (Fig. 7 c). The C = O band (1,700 cm⁻¹), the C–O band (~ 1,040 cm⁻¹) and the < 1,000 cm⁻¹ band all followed the same pattern. In sand, the O–H band again peaked after C3 before decreasing after C5 and C10 (Fig. 7 d). The C = O band increased from C0 to C3, decreased after C5, but (re)increased after C10. The C–O band intensities showed their maxima after C3 and then decreased steadily. Interestingly, the < 1,000 cm⁻¹ band intensities showed the same course as the C–O band. For the loam extract, all four bands increased continuously with an increasing number of drying-rewetting cycles, reaching their highest intensities after C10 (Fig. 7 e). When swollen in clay extract, each band for PAA increased sharply to peak after C3 and then decreased after C5 and C10 (Fig. 7 f). PAM showed a parallel but distinct behavior: when swollen in dH 2 O, the N–H stretching band (~ 3,300 cm⁻¹), the C–H stretching band and the H–C–H band (~ 1,450 cm⁻¹) increased from C0 to their maxima after C3 before decreasing again at C5 and C10 (Fig. 7 g). In contrast, the amide C = O band (1,650 cm⁻¹) increased from C0 to C5 and decreased again to C10. For the sand, the intensities of the N–H, C–H and H–C–H bands again peaked after C3 and subsequently decreased, while the C = O band reached its highest intensity after C5 before decreasing after C10 (Fig. 7 h). PAM swollen in the loam and clay extract resulted in the same N–H, C–H and H–C–H bands maxima after C3 and their subsequent decreased with further drying-rewetting cycles (Fig. 7 i-j). Relationships between investigated parameters For the IE, the first two principal components accounted for 58.2% of the total variance (Dim 1 = 35.5%, Dim 2 = 22.7%) (Fig. 8 ). The PCA showed that SI, τ max , and Ƭ YP highly contributed to the explaining dimensions (Fig. 8 a). Loadings indicate that SI and τ max projected strongly in the positive direction of Dim1, while T 2WL and T 2peak load primarily on Dim2. τ YP contributed modestly to Dim1 and slightly negatively to Dim2. In general, T 2WL , T 2peak and τ YP exhibited modest negative correlations, respectively. Moreover, SI and τ max were also only modestly correlated. PAA clustered predominantly on the positive side of Dim1, whereas PAM occupied the negative Dim1 region (Fig. 8 b). Polymer-specific PCA revealed no relationship between T 2WL and T 2peak but did show a modest positive association between PAA and both the SI and τ max . These results concur with the permanova, which detected highly significant effects of polymer type and its interaction with drying–rewetting cycles (p = 0.001) and a marginally significant polymer–soil extract interaction (p = 0.016). The three-way interaction (polymer type, drying-rewetting cycles, and soil extract) for τ max was also significant (p = 0.007). The SI showed a high significance (p = 0.001) in terms of the polymer type itself and the interactions with soil extracts and drying-rewetting cycles. With increasing number of drying-rewetting cycles (Fig. 8 c), τ YP progressively shifted in the positive Dim1 direction, which was further confirmed by the permanova in terms of a highly significant polymer-drying-rewetting cycle interaction (p = 0.001). Moreover, SI and τ max both correlated negatively with C5. SI was highly significant (p = 0.001) for both the drying–rewetting cycles and their interaction with polymer type and soil extract, whereas τ max was only significant for the drying–rewetting drying-rewetting cycles (p = 0.002). Concerning the relevance of demineralized water and the different soil extracts, T 2WL and T 2peak projected in the positive Dim1/Dim2 quadrant alongside the hydrogels swollen in demineralized water, whereas τ YP was negatively correlated. T 2peak showed modest significance for the interaction between soil extracts and drying-rewetting cycles (p = 0.011), and for the three-way interaction among soil extracts, polymer type, and drying-rewetting cycles (p = 0.018). T 2WL exhibited highly significant effects of the soil extract (p = 0.001) and its interaction with polymer type (p = 0.001), while the interactions between soil extract and drying–rewetting cycle was only modestly significant (p = 0.026). Besides the relatively small effects of dH 2 O on τ YP , the interactions between polymer type and soil extracts were significant (p = 0.001), except for the three-way interaction between soil extracts, polymer type and drying-rewetting cycles. Concerning the soil extracts, samples swollen in sand correlated negatively with T 2WL and T 2peak , whereas only modestly negative correlations with SI and τ max were observed when for the loam soil extracts. SI was highly significant for soil extract (p = 0.001) and its interactions with polymer type and drying-rewetting cycles, and τ max was significant for the soil extract and the polymer type alone (p = 0.001). 4 Discussion When PAA and PAM were freely swollen in soil extracts, both SAP hydrogels exhibited a reduced swelling index (SI) without significant changes in their relaxation time distributions (RTDs). Compared to their initial free swelling in dH 2 O, they also showed polymerspecific increases in structural stability, further visible as more condensed hydrogel network, and uniformly decreased ATR-FTIR band intensities. This effect was more pronounced for both SAPs swollen in loam and clay extracts. On the one hand, the observed changes in the physicochemical hydrogel properties compared to dH 2 O are related to the already well-known crosslinking of polymer chains with dissolved cations that are available in the soil extracts, typically resulting in increased hydrogel network stability as also observed in our study [ 40 , 41 ] . The universal attenuation of ATR-FTIR absorbances likely reflects a loss of polymer chain mobility within increasingly compacted junction zones: when multivalent cations coordinate with carboxyl and amide moieties, vibrational freedom across the O–H, N–H and C = O groups is restricted, diminishing band intensities. This spectral signature aligns with the smaller pores observed by ESEM and the shorter T₂ relaxation times measured by NMR. Regarding the soil extract composition, although the sand extract showed a much lower Al³⁺ concentration than Ca²⁺ and Mg²⁺, it nonetheless induced the highest network compaction, confirming Hypothesis 1 that Al³⁺ mediates stronger ionic crosslinking than divalent cations. By coordinating with three carboxylate groups on adjacent polymer chains, Al³⁺ forms exceptionally stable bridges [ 42 ] , driving ionotropic hydrogelation [ 40 ] that dominates junction zone architecture. Regarding the divalent cations, Ca²⁺ still restricts (re)swelling more effectively than Mg²⁺. Consequently, the physical arrangement and morphology of the junction zones depend on cation concentration [ 8 ] , whereby calcium ions are known to stronger reduce hydrogel (re)swelling than magnesium ions in terms of possible water uptake [ 2 ] , resulting in concentration-dependent denser hydrogel networks as shown in the ESEM images of the FSE. On the other hand, PAA gets deprotonated during swelling, which results in an anionic polymer hydrogel [ 20 ] . As the stability of anionic SAPs typically increases with the amount and charge of available crosslinkers in the surrounding medium [ 2 , 19 , 40 ] , high Al 3+ and Mn 2+ /Mn 4+ concentrations in the sand likely promoted PAA crosslinking as hypothesized. Furthermore, Amjad [ 43 ] indicated that PAA stabilizes dissolved Mn 2+ / Mn 4+ more efficiently than, e.g., Fe 2+ /Fe 3+ , further underlining the possible crosslinking with Mn as well. In contrast, PAM cannot be ionized due to its neutral amide group [ 44 ] , resulting in a more stable hydrogel network during swelling together with lower water absorption efficiency than PAA [ 18 ] . Consistent with this, the ATRFTIR spectra of PAM exhibited only minor shifts in both amide I and II bands and no new peak formation, indicating that network changes arise from mechanical rearrangements rather than new covalent or ionic crosslinks. The minimal ATR-FTIR shifts in PAM aligned with its unaltered RTDs and only slight rheological changes under cyclic stress, indicating an inherently stable and compact network [ 45 ] , as further underlined by the obtained ESEM images. Besides the already mentioned crosslinking in the SAP hydrogels, the observed changes in the investigated physicochemical properties could be due to the intrinsic structure of the dry polymers themselves, as the PAM granules have a relatively low surface area compared to the investigated PAA powder, facilitating the formation of a more stable network. Here, particle-polymer interactions should be considered as e.g. clay particles could have passed the filter membranes during swelling. Especially, PAA powder promotes strong and fast interactions with soil particles like adsorption processes due to a relatively high surface area resulting in a reduced swelling potential [ 9 , 46 , 47 ] . Regarding the IE and the impact of successive drying–rewetting cycles, PAA initially and freely swollen in dH 2 O underwent pronounced physicochemical transformations and developed increasingly solidlike (elastic) behavior, demonstrated progressively lower waterabsorption capacity, exhibited localized network densification in the ESEM images, and showed amplified ATR-FTIR band intensities with each cycle. This was in line with Yunkai et al. [ 1 8] , who reported an increased hydrogel network density due to drying-rewetting events, especially from C5 on. Also, Bai et al. [ 1 7] reported direct relationships between the functional loss of SAPs in ultrapure water with severity of the drying events. Similar ageing effects in the form of a loss of the original hydrogel properties were also observed by Gu et al. [ 1 6] for sodium polyacrylate after 70 d of incubation in deionized solution. Consequently, the increased stability together with decreased T 2 and shifted RTDs, reduced SI and the morphological network changes observed in the ESEM images in our study support the assumption of a significant condensation of the PAA hydrogel network in the course of drying-rewetting, together with the breaking of polymer chains during network condensation and expansion. When polymer chains break and rearrange in the course of expansion and condensation dynamics of the three-dimensional hydrogel network [ 16 , 4 8] , a physical rearrangement of the junction zones occur [8] . This structural rearrangement, where polymer chains are packed more closely yet with reduced crosslinking degree, points to an increased vibrational freedom of specific functional groups (O–H, N–H, C = O), enhancing dipole moment changes and, thus, absorbance as shown in the ATR-FTIR spectra [ 4 9–51] . Contrary to PAA, PAM revealed no significantly changed hydrogel properties when swollen in dH 2 O and subjected to drying-rewetting cycles, as indicated by relatively stable RTDs, SIs and structural stability indices throughout all drying-rewetting cycles. Although the ESEM images showed also condensed parts as for PAA, the PAM hydrogel network was still homogeneous and intact, assuming an overall lower moisture dynamics-induced effect on the physical arrangement of the junction zones [ 1 6] . These differences were further supported by the PCA results as the two SAPs clearly occupied distinct regions in the score plot, with PAA clustering in the positive direction of Dim1 - associated with higher structural rigidity and swelling loss - while PAM aligned negatively, reflecting its more stable physicochemical profile throughout the drying-rewetting cycles. These differences underscored the fundamental chemical differences between the two SAPs: when PAA hydrogel deprotonates during swelling, it results in a concentration gradient between the hydrogel network and dH 2 O. However, the neutral amid group of PAM cannot be ionized [ 4 4] and thus remains stable during swelling, causing a relatively lower water absorption efficiency compared to PAA [ 1 8] . This, in turn likely promoted the formation of an already intrinsically more stable hydrogel network with smaller pores directly after its initial swelling [45] . The accompanied lower structural stability, swelling pressure, and volumetric expansion potential of PAM compared to PAA further restricted water uptake and resulted in a lower SI, same RTDs and similar hydrogel network as in the initial swelling, likely reflecting inherent hydrocolloid architectural differences that govern junction zone formation within the three-dimensional hydrogel network [ 8 ] . Therefore, the drying-related condensation effects were relatively lower as less water was initially absorbed by the PAM hydrogel network to cause strong swelling-shrinkage dynamics and subsequent fracture formation upon drying and rewetting. Furthermore, the overall expansion and swelling pressure of the polymer chains during reswelling was further decreased by the relatively smaller pores of the PAM hydrogel and an already more stable network after the initial swelling compared to PAA [ 18 ] , which could be due to rearranged junction zones holding less water in the interstices of the hydrogel network. Concerning the effect of the different soil extracts and their interplay with the drying-rewetting cycles, both the PCA results and the permanova findings confirmed their significant interactions. In detail, PAA swollen in sand extract showed one shift towards lower RTDs for all drying-rewetting cycles together with a decrease of SI, while structural stability increased together with hydrogel network density and polymer chains braking. This was in line with Buchmann et al. [ 20 ] , who showed that hydrogen ion release through the dissociated of the carboxylic group can gradually limit PAA crosslinking during its swelling together with an acidification of the surrounding solution, corresponding to the decrease in pH after PAA reswelling. Nonetheless, the authors found this effect only in sand while it disappeared in loam after one week due to its higher buffer capacity. In contrast to Buchmann et al. [ 20 ] , who observed variable pH shifts during drying–rewetting in PAA-treated soils, reflecting the heterogeneous buffering capacity of soil matrices, we recorded an uniform pH decline across all drying-rewetting cycles. This discrepancy likely results from the use of soil extracts or soil solutions instead of bulk soil, which typically lack the complex, insitu buffering mechanisms of soil as a whole (e.g., cation exchange, OM interactions) [ 20 , 52 , 53 ] . Interestingly and in contrast to PAA, PAM only showed shifts in the RTDs and T 2 , whereas its structural stability remained stable over the drying-rewetting cycles and hydrogel network was only more condensed with broken polymer chains when swollen and incubated in the sand extract. This may reflect either the intrinsically higher stability of the hydrogel network following initial swelling, as previously noted [ 18 , 45 ] , or a predominance of polymer chaincondensation over crosslinking in the sand extract. As the neutral amide groups of PAM cannot be protonated, fewer ionic crosslinks form in sand than in loam or clay extracts, favoring network densification over new crosslink formation. Moreover PAM granules with their relatively lower surface area can effectively counteract both the suction tension and the confining pressure of the soil matrix [ 46 , 47 ] , which resulted relatively unchanged properties of PAM compared to the initial free swelling in dH 2 O. For PAA swollen in loam and clay extract, the RTDs, T 2 , and SI decreased together with an increased structural stability as function of the drying-rewetting cycles, as also reflected in the PCA in terms of samples clustered towards the respective indices indicating increased hydrogel network densification and aging. This is consistent with the hypothesized role of multivalent cations [ 42 ] and in line with Buchmann et al. [ 20 ] , who reported reduced PAA swelling in loam with a complete functional loss over time. Several studies have already shown that composition of the soil solution significantly determines both the swelling potential and the resulting physicochemical properties of PAA [ 9 , 10 ] . Here, ionotropic hydrogelation via cation-mediated crosslinking represents one mechanism influencing the physical arrangement of junction zones [ 40 ] . In this regard, the large and condensed areas for PAA suggested a physical rearrangement of the junction zones based on the soil extract composition [ 8 ] , and ionotropic hydrogelation dynamics [ 40 ] . In line with these morphological changes, ATR-FTIR spectra of PAA revealed a distinct progression in functional group intensities, particularly in O–H and C = O bands. The initial increase and subsequent decline of O–H absorption suggest water losses and reduced hydrogen bonding, while increased C = O signals may point toward enhanced carboxylate interactions or ion-mediated crosslinking [ 49 , 54 ] . These observed spectral shifts support our hypothesis of a progressive hydrogel network densification with the formation of additional intramolecular and intermolecular crosslinks and thus stronger network structures [ 16 – 18 , 20 , 42 ] . Interestingly, PAA showed a band at < 1,000 cm − 1 , which increased especially in the soil extracts. This might underline the explained crosslinking processes, corresponding to the findings of Weng et al. [ 55 ] , who showed a similar increase with higher crosslinking in PAA. Thus, the intensified signal at < 1,000 cm⁻¹ could serve as a marker of network compaction with ionic and covalent crosslinks drawing polymer segments into tighter conformations and amplifying skeletal deformation modes in the ATR-FTIR spectrum comparable to the C–H 2n > 3 band at 730 − 720 cm − 1 [ 37 ] . Furthermore, Dong et al. [ 56 ] also described this band for PAA as a result from widely distributed backbone internal rotation angles, including both helical structure and planar zig-zag conformation over short segments in the solid state of PAA [ 57 , 58 ] . Although some studies used FT-Raman, the wavenumbers were comparable without any substantial differences to ATR-FTIR [ 55 ] . Moreover, polymer chains might have become more attached during hydrogel network condensation, reinforcing each other and therefore increasing the intensity at < 1,000 cm − 1 . Alternatively, crosslinking predominantly taking place in soil extracts may have forced polymer chain narrowing to such an extent that C–H vibrational modes in the 900–670 cm⁻¹ region were intensified, much like the outofplane bending of aromatic C–H bonds [ 3 7] . This suggests that the compacted network induced vibrational coupling comparable to aromatic structures. In contrast, PAM revealed less obvious variations in N–H and C–H band intensities, supporting a more chemically stable network structure. With successive drying–rewetting, the amide I band broadened by up to 0 %, indicating mechanical chain scission and increased conformational disorder, while the amide II band remained unchanged, confirming backbone integrity despite network stress. However, the absence of pronounced spectral changes across the drying-rewetting cycles indicates lower susceptibility to hydrolysis, chain scission, or condensation reactions. This reinforces the relative resilience of PAM under dynamic moisture conditions with a preserved backbone integrity 18,45] . Besides chemical aging or changing crosslinking degree within the two hydrogels, particle-polymer interactions need to be considered, since, e.g., clay particles could have passed the filter membranes during the drying-rewetting events and interacted with the polymer chains. On the one hand, especially the acidic nature of PAA could have promoted interactions with clay particle surfaces and thereby mobilize exchangeable cations [ 21 , 44 ] . On the other hand, clay particles are generally known to limit SAP hydrogel swelling in soil by mutual swelling restriction caused during water competition [ 9 ] . However, both investigated SAPs were able to (re)swell without restrictions in our experiment, resulting in a homogeneous hydrogel network with the maximum water absorbed into the junction zones compared to their (re)swelling directly in the soil interparticle space [ 59 – 61 ] . Consequently, clay-induced PAA swelling restriction were most likely lower in the soil extract than in the soil interparticle space due to 1) dilution effects since only 1:5 soil extracts were used and 2) the missing confining pressure and mutual swelling competition by the soil matrix [ 9 ] . Although this study revealed fist important insights into the physicochemical aging of two commonly used SAPs as function of drying-rewetting events and soil extract composition, several limitations must be considered: first, we only investigated one specific SAP application way in terms of point application, including a first swelling in dH 2 O to ensure full swelling and homogeneous hydrogel network formation without soil extract-mediated crosslinking for the first swelling. This resulted in hydrogel networks with different morphological and intrinsic properties than for those directly added as dry polymers and thus swollen in the soil interparticle space [ 20 ] . Thus, soil matrix-related crosslinking, counterpressure and mutual water competition taking place directly during SAP swelling will lead to different intrinsic physicochemical hydrogel properties and should therefore be investigated further [ 9 , 10 , 31 , 46 , 47 ] . Second, we used 1:5 soil extracts to track changes of the SAP hydrogels. Although soil extracts offer a controlled medium for assessing SAP swelling, the subsequent dilution can alter cation concentration, speciation, and pH, while excluding solid phases that typically act as adsorption sites, mechanical constraints, pore structure, and microbial or redox dynamics. Consequently, extract-based assays may overestimate SAP swelling and distort the strength and type of aging, which requires more targeted in-situ experiments [ 2 , 19 , 40 , 45 ] . Third, high molecular weight polymers as used in this study are known to maximize soil-polymer interactions [ 62 ] and increase hydrogel crosslinking degree [ 42 ] , which could have influenced the results as well. These limitations also underline the relevance of in-situ studies using native soil matrices and further commercially applied SAP products to verify the environmental relevance of the observed aging effects. Moreover, the SAP application way seems to play an important role on the density of the organ-mineral complexes [ 9 , 10 , 31 ] , as the point application used in this study has a larger surface and therefore a larger soil interface than the application way used in Buchmann et al. [ 20 ] . Future experiments should also explore varied application techniques and simulate more realistic soil moisture regimes. Regarding the question of whether SAPs in soil can form plastic-like residues [ 15 ] , our study indicated an increased structural stability of the hydrogels when subjected to drying-rewetting events as function of the number of cycles and soil extract composition. From current research, SAPs are already known to form large stable aggregates and dense organo-mineral complexes during drying, making cemented membranous polymer structures occupying large parts of the interparticle space [ 9 , 10 , 21 , 31 ] . On the one hand, the changes observed in the ATR-FTIR spectra, such as the decreasing O–H signals and the shifts in carbonyl band intensities suggest that repeated drying-rewetting events induced not only physical network densification but also chemical alterations. Such transformations could contribute to the formation of more persistent hydrogel residues with plastic-like characteristics. On the other hand, ESEM images indicated that repeated drying–rewetting cycles can irreversibly densify and restructure the SAP hydrogel networks, most notably in PAA, leading to diminished rehydration capacity, compromised longterm performance in soils, and the potential for the formation of persistent residues. Yet, while this structural consolidation and reduced swelling suggest lower degradability, definitive chemical evidence of plasticization, such as novel carbon–carbon bonding motifs or enhanced resistance to microbial breakdown, has not been demonstrated. To fully characterize these aging processes, future investigations should employ solidstate NMR and highresolution mass spectrometry to resolve specific bondlevel alterations [ 63 –68] . Moreover thermodynamic analysis in terms of the thermal stability and decomposition behaviors of the SAP hydrogels themselves and in soil seem straight-forward [ 69 –71] . 5 Conclusion This study provides important insights into the physicochemical transformation of two widely used synthetic superabsorbent polymers (SAPs), polyacrylic acid (PAA) and polyacrylamide (PAM), under repeated drying–rewetting cycles and varying soil extract compositions. SAP aging was driven by an interplay of physical densification, ion-mediated crosslinking, and chemical modifications of the hydrogel network. For PAA, these drying-rewetting cycles reduced swelling capacity and water mobility, increased mechanical rigidity, and triggered the formation of condensed structures. ATR-FTIR spectral changes, particularly reduced OH and shifting carbonyl bands, suggested progressive crosslinking and structural fixation, especially for cation-rich soil extracts. In contrast, PAM remained comparatively stable, showing minor structural and chemical changes. Its neutral, dense hydrogel network appeared less responsive to environmental stressors. PCA results supported these differences, showing clear separation between polymer types along dimensions associated with aging and network rigidity. All in all, the findings indicate that drying-rewetting cycles can cause irreversible structural and chemical changes in SAPs, particularly for anionic PAA, reducing rehydration potential and promoting the potential formation of persistent residues. While ESEM and ATR-FTIR support this interpretation, direct evidence of plasticization, such as covalent bond changes or biodegradation resistance, remains lacking. Thus, future studies should employ advanced methods such as solid-state NMR or high-resolution mass spectrometry to further elucidate aging pathways. Further, several key limitations, including the use of pre-swollen SAPs, diluted soil extracts lacking solid-phase and microbial interactions, and high molecular weight polymers not necessarily representing commercial products, need to be considered. To assess environmental relevance, future research must prioritize in-situ experiments under realistic field conditions and explore diverse SAP types and deployment strategies. Declarations Competing interests The authors declare no competing interests. Financial support: This research was financially supported by the Deutsche Forschungsgemeinschaft (Grant No. BU 3763/1–1). Acknowledgements We kindly thank Zacharias Steinmetz for his input on ATR-FTIR and statistical analysis, Anna Baskal for helping with the ICP measurements, and Gabriele E. Schaumann and Mathilde Knott for their valuable feedback on the results. Data Availability: The data that supports the findings of this study are available from the corresponding author upon reasonable request. Author contribution Conceptualization: J.N. and C.B.; Methodology and experimental setup: J.N. and C.B.; Material preparation and data collection: J.N.; Data evaluation and interpretation: J.N. and C.B., Writing-review and editing: J.N. and C.B.; Funding acquisition: C.B.; Project management: C.B.; Supervision: C.B. Funding This research was financially supported by the Deutsche Forschungsgemeinschaft (Grant No. BU 3763/1-1) References Saha, A., Sekharan, S. & Manna, U. Superabsorbent hydrogel (SAH) as a soil amendment for drought management: A review. Soil Tillage Res. 204 , 104736 (2020). Banedjschafie, S. & Durner, W. Water retention properties of a sandy soil with superabsorbent polymers as affected by aging and water quality. J. Plant Nutr. Soil Sci. 178 , 798–806 (2015). 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The Interaction of Polysaccharides with Silver Hill Illite. Clays Clay Miner. 40 , 151–156 (1992). Richardson, J. L., Gunnerson, W. T. & Giles, J. F. INFLUENCE OF IN SITU TWO-PHASE POLYMERS ON AGGREGATE STABILIZATION IN VARIOUS TEXTURED NORTH DAKOTA SOILS. Can. J. Soil Sci. 67 , 209–213 (1987). Steinmetz, Z. & Schröder, H. Plastic debris in plastic-mulched soil—a screening study from western Germany. PeerJ 10 , e13781 (2022). Huppertsberg, S., Zahn, D., Pauelsen, F., Reemtsma, T. & Knepper, T. P. Making waves: Water-soluble polymers in the aquatic environment: An overlooked class of synthetic polymers? Water Res. 181 , 115931 (2020). Vidovic, N., Krauskopf, L.-M., Jovancicevic, I., Antic, V. & Schwarzbauer, J. Determination of the water-soluble polymer poly(N-vinylcaprolactam) in wastewater effluents by continuous-flow off-line pyrolysis-GC/MS. Discov. Water 2 , (2022). Kronimus, A. & Schwarzbauer, J. Analysis of structurally modified polyacrylamides by on-line thermochemolysis-GC–MS. J. Anal. Appl. Pyrolysis 80 , 471–476 (2007). El Hariri El Nokab, M., Lasorsa, A., Sebakhy, K. O., Picchioni, F. & Van Der Wel, P. C. A. Solid-state NMR spectroscopy insights for resolving different water pools in alginate hydrogels. Food Hydrocoll. 127 , 107500 (2022). Vandersypen, L. M. K. & Chuang, I. L. NMR techniques for quantum control and computation. Rev. Mod. Phys. 76 , 1037–1069 (2005). Zaharescu, M., Predoana, L. & Pandele-Cusu, J. Thermal Analysis on Gels, Glasses, and Powders. doi:https://doi.org/10.1007/978-3-319-32101-1_99. David, J., Steinmetz, Z., Kučerík, J. & Schaumann, G. E. Quantitative Analysis of Poly(ethylene terephthalate) Microplastics in Soil via Thermogravimetry–Mass Spectrometry. Anal. Chem. 90 , 8793–8799 (2018). Kučerík, J. et al. Determination of soil properties using thermogravimetry under laboratory conditions. Eur. J. Soil Sci. 71 , 415–419 (2020). Additional Declarations No competing interests reported. Supplementary Files TableS1.pdf TableS2.pdf Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 09 Sep, 2025 Reviews received at journal 26 Aug, 2025 Reviews received at journal 26 Aug, 2025 Reviewers agreed at journal 23 Aug, 2025 Reviewers agreed at journal 21 Aug, 2025 Reviewers agreed at journal 28 Jul, 2025 Reviewers invited by journal 22 Jul, 2025 Editor invited by journal 22 Jul, 2025 Editor assigned by journal 21 Jul, 2025 Submission checks completed at journal 20 Jul, 2025 First submitted to journal 18 Jul, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7155517","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":489500431,"identity":"607fe60f-3aba-4e94-8631-7b7084fc6569","order_by":0,"name":"Janina Neff","email":"","orcid":"","institution":"IES Landau, RPTU University Kaiserslautern-Landau","correspondingAuthor":false,"prefix":"","firstName":"Janina","middleName":"","lastName":"Neff","suffix":""},{"id":489500432,"identity":"ff05ed61-49cb-4d84-9ecd-534d9daba853","order_by":1,"name":"Christian Buchmann","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAx0lEQVRIiWNgGAWjYFACHgYGxgYQg/kAROAA8VrYEhgYEkjTwmNAnBb+Bt5jEj93bJM3l+759rnwx2EGvuMN+LVIHOBLk+w9c9tw55yzm2fPSDjMIHmGgDUGDDxm0oxttxk33MjdzMwD1GJwI4E4LfYbbuQ8hmi5/4A4LYlALcxQW/DrYJA4zJds2dt2O3nDjTRj5hlp6TySZwg4jL+99+CNn223bTfcSH7MXGBjLcd3/AABa5jR2DwE1OPRPgpGwSgYBaMADgAOmURLrXPKTQAAAABJRU5ErkJggg==","orcid":"","institution":"IES Landau, RPTU University Kaiserslautern-Landau","correspondingAuthor":true,"prefix":"","firstName":"Christian","middleName":"","lastName":"Buchmann","suffix":""}],"badges":[],"createdAt":"2025-07-18 08:23:27","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7155517/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7155517/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":87575830,"identity":"ca51995e-e3a7-413f-ac1f-5fca8d01866c","added_by":"auto","created_at":"2025-07-25 11:38:37","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":108884,"visible":true,"origin":"","legend":"\u003cp\u003epH (a-c) and electrical conductivity (EC, d-f) in μS/cm in sand (a,d), loam (b,e) and clay (c,f) extract for both polyacrylic acid (PAA) and polyacrylamide (PAM)-related liquid reservoirs. Open symbols represent data from the first free swelling in the soil extracts and dH\u003csub\u003e2\u003c/sub\u003eO (free swelling experiment FSE), filled symbols represent data from the incubation experiment (IE), respectively. Error bars represent standard error of the arithmetic means and are not shown when smaller than the symbol size.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/c716e8e073601b098b6a39d7.png"},{"id":87574545,"identity":"eed4171d-b8aa-4b87-970b-c70220eb6ff6","added_by":"auto","created_at":"2025-07-25 11:30:37","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":119913,"visible":true,"origin":"","legend":"\u003cp\u003eSwelling index (SI) for both polyacrylic acid (PAA, grey) and polyacrylamide (PAM, black) as function of the drying-rewetting cycles and swollen in either (a) dH\u003csub\u003e2\u003c/sub\u003eO and soil extracts of (b) sand, (c) loam, and (d) clay. Open symbols represent data from the first free swelling in the soil extracts and dH\u003csub\u003e2\u003c/sub\u003eO (free swelling experiment FSE), filled symbols represent data from the incubation experiment (IE), respectively. Error bars represent standard error of the arithmetic means and are not shown when smaller than the symbol size.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/d7d085314f7388b86ec08c59.png"},{"id":87575833,"identity":"b1c4cd07-6478-47f2-9142-3d7737da4c64","added_by":"auto","created_at":"2025-07-25 11:38:37","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":193378,"visible":true,"origin":"","legend":"\u003cp\u003eTransverse relaxation time distribution (RTD) of (a) polyacrylic acid (PAA) and (b) polyacrylamide (PAM) for the first free swelling (free swelling experiment FSE) in dH\u003csub\u003e2\u003c/sub\u003eO (grey line), and the three soil extracts - sand (black dashed line), loam (dashed line light grey) and clay (dotted-dashed line dark grey).\u003cstrong\u003e \u003c/strong\u003eRTD of (c-f) PAA and (g-j) PAM obtained from the incubation experiment (IE) for the drying-rewetting cycle zero (C0, grey line), three (C3, black dashed line), five (C5, dashed line light grey) and ten (C10, dotted-dashed line dark grey) and swollen in dH\u003csub\u003e2\u003c/sub\u003eO and soil extracts of sand, loam and clay.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/56dafb78de436e5f224a5d50.png"},{"id":87575836,"identity":"81657c50-f9d5-4501-b7b4-29ca2f5ae66d","added_by":"auto","created_at":"2025-07-25 11:38:37","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":189998,"visible":true,"origin":"","legend":"\u003cp\u003eTransverse relaxation times\u003cstrong\u003e \u003c/strong\u003eT\u003csub\u003e2WL\u003c/sub\u003e (a ,c, e, g) and (b) T\u003csub\u003e2peak\u003c/sub\u003e (b, d, f, h) for polyacrylic acid (PAA, grey) and polyacrylamide (PAM, black) as function of the drying-rewetting cycles for dH\u003csub\u003e2\u003c/sub\u003eO and soil extracts of sand, loam, and clay. Open symbols represent data from the first free swelling in the soil extracts and dH\u003csub\u003e2\u003c/sub\u003eO (free swelling experiment FSE), filled symbols represent data from the incubation experiment (IE), respectively. Error bars represent standard error of the arithmetic means and are not shown when smaller than the symbol size.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/40a4aa72dff0596f8693512e.png"},{"id":87574563,"identity":"1e73e9a1-e591-4491-87ee-e0d189324605","added_by":"auto","created_at":"2025-07-25 11:30:37","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":194991,"visible":true,"origin":"","legend":"\u003cp\u003eShear stress at the yield point\u003cstrong\u003e \u003c/strong\u003e(τ\u003csub\u003eYP\u003c/sub\u003e) (a, c, e, g) and maximum shear stress (τ\u003csub\u003emax\u003c/sub\u003e)\u003csub\u003e \u003c/sub\u003e(b, d, f, h) for both polyacrylic acid (PAA, grey) and polyacrylamide (PAM, black) as function of the drying-rewetting cycles for (a, b) dH\u003csub\u003e2\u003c/sub\u003eO and soil extracts of (c, d) sand, (e, f) loam, and (g, h) clay. Open symbols represent data from the first free swelling in the soil extracts and dH\u003csub\u003e2\u003c/sub\u003eO (free swelling experiment FSE), filled symbols represent data from the incubation experiment (IE), respectively. Error bars represent standard error of the arithmetic means and are not shown when smaller than the symbol size.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/93c800e82f6e3b5231c39a29.png"},{"id":87574577,"identity":"578e19ad-a098-410d-8bb3-54d8d4191d32","added_by":"auto","created_at":"2025-07-25 11:30:38","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":500005,"visible":true,"origin":"","legend":"\u003cp\u003eESEM pictures of swollen and freeze-dried hydrogel networks of polyacrylic acid (PAA) and polyacrylamide (PAM) after the first swelling (C0) in the soil extracts of sand (a, i), loam (b, j), clay (c, k), and dH\u003csub\u003e2\u003c/sub\u003eO (d, l) and after ten drying-rewetting cycles (C10) in dH\u003csub\u003e2\u003c/sub\u003eO (e, m), sand (f, n), loam (g, o) and clay (h, p) extract.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/aea918478f147e147c83671d.png"},{"id":87574576,"identity":"eaae4316-8a80-49f8-b4e4-6aaf65ebc364","added_by":"auto","created_at":"2025-07-25 11:30:38","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":350685,"visible":true,"origin":"","legend":"\u003cp\u003eATR-FTIR absorbance spectra of the absorbance intensitiy (A [-]) over the wavenumbers ( \u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;[1/cm]) of PAA (subfigures a, c–f) and PAM (subfigures b, g–j) plotted against wavenumber. Subfigures a–b show the free swelling experiment (FSE) with the first swelling in dH₂O (solid grey), sand extract (black dashed), loam extract (light grey dashed) and clay extract (dark grey dash–dot). Subfigures c–j show the incubation experiment (IE) following drying–rewetting cycles zero (C0, solid grey), three (C3, black dashed), five (C5, light grey dashed) and ten (C10, dark grey dash–dot). ATR-FTIR band assignments appear in the top‑right inset.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/ded7e09987c4b228469e91bd.png"},{"id":87574572,"identity":"67647768-555a-4e33-99b5-0bfa9fed6e0e","added_by":"auto","created_at":"2025-07-25 11:30:38","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":262317,"visible":true,"origin":"","legend":"\u003cp\u003ePrincipal component analysis (PCA) of the investigated parameters showing (a) the contribution of the variables to PC1 and PC2 and (b-d) the sample clustering by the different treatments, including (b) the soil extracts and dH\u003csub\u003e2\u003c/sub\u003eO, (c) the different polymer types, and (d) the drying-rewetting cycles.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/ee8e780d9a512f2872115371.png"},{"id":87577992,"identity":"5d957c78-a807-4663-8130-a9c043bd0521","added_by":"auto","created_at":"2025-07-25 11:54:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2855813,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/13602a99-3c73-42fb-a2bd-83451c171b5e.pdf"},{"id":87574548,"identity":"29f8cb6f-c0b7-4374-8cde-37351e1e4de3","added_by":"auto","created_at":"2025-07-25 11:30:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":289583,"visible":true,"origin":"","legend":"","description":"","filename":"TableS1.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/f3b47b7f6a87510fca9898be.pdf"},{"id":87574549,"identity":"67549224-f3e7-4a66-bd3b-15ec9f76885b","added_by":"auto","created_at":"2025-07-25 11:30:37","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":228997,"visible":true,"origin":"","legend":"","description":"","filename":"TableS2.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7155517/v1/a29aa19c67a058ac7c0d2cf9.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"The contribution of soil extract composition and cyclic moisture dynamics to the physicochemical aging of superabsorbent polyacrylic acid and polyacrylamide hydrogels","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eIncreasing water scarcity in arid regions as well as periodic heavy rainfall events and high precipitation pose major challenges for agriculture. Thus, respective soils and crops that are not native to arid regions are more severely impacted compared to plants already adapted to dry periods \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. To increase irrigation efficiency and also improve soil stability in the context of surface runoff or soil erosion, technical developments and the application of (synthetic) superabsorbent polymers (SAPs) are frequently investigated \u003csup\u003e[\u003cspan additionalcitationids=\"CR4 CR5\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. Synthetic SAPs, mainly polyacrylic acid (PAA) and polyacrylamide (PAM) are produced according to the biological model of natural hydrogels and thus can absorb large quantities of water and build a three-dimensional polymer network in the soil interparticle space, known as \u0026ldquo;junction zones\u0026rdquo; \u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. This interparticulate hydrogel swelling substantially modulates various soil physicochemical properties, including the maximum soil water holding capacity (WHC\u003csub\u003emax\u003c/sub\u003e), soil structural stability, soil permeability, and the availability of nutrients, fertilizers, and pesticides \u003csup\u003e[\u003cspan additionalcitationids=\"CR9 CR10\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. In this regard, the term \u0026ldquo;gel effect\u0026rdquo; was coined to summarize all the previously mentioned SAP-related functions and modulations in soil \u003csup\u003e[\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eDepending on the area of application and intended use, SAPs are applied to soil in different ways, including a) mix application in terms or evenly mixing SAP with soil about 0\u0026ndash;20 cm through tillage, b) spraying/sprinkle application by evenly spraying or spreading the SAP on the leave or soil surface, c) coating or soaking seeds prior to planting, and d) point/hole application by punctual, spatially limited incorporation of the SAP to the seed burial or root zone \u003csup\u003e[\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. Although various fundamental mechanisms and processes of the gel effect in soil have been explored \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e some of them are still unknown, including how the combined effects of environmental dynamics and respectively induced SAP transformation further modulate the physicochemical properties of both soil and the interparticulate hydrogel itself. In this regard, current scientific research is investigating the extent to which the aging and transformation of SAPs in soils could lead to plastic-like non-degradable SAP residues \u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. Here, one of the research focuses is on natural drying\u0026ndash;rewetting processes, in which the composition of the soil solution - its ionic profile, colloidal clay particles, and organic constituents - significantly influences the swelling behavior, functional properties, and fate of interparticulate SAP hydrogels in soil. As drying\u0026ndash;rewetting events concentrate or dilute soil solution constituents, osmotic gradients shift and additional crosslinking of SAP polymer chains might occur, altering SAP network architecture and functioning \u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan additionalcitationids=\"CR17 CR18 CR19 CR20\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. Because concentration changes and solute availability in soil are intrinsically linked to the water content, they must be considered and part of systematic studies in the context of moisture dynamics \u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eAlthough previous work has demonstrated that repeated drying\u0026ndash;rewetting dynamics lead to progressive loss of SAP effectiveness in terms of lower maximum swelling, reduced water retention for plants, and even apparent \u0026ldquo;cementing\u0026rdquo; of soil pores \u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e, there remains a significant lack of understanding regarding how environmental dynamics, particularly repeated drying\u0026ndash;rewetting cycles and combined SAP-soil solution interactions, affect the long-term performance and transformation of SAPs.\u003c/p\u003e\u003cp\u003eThis study specifically addresses these knowledge gaps by integrating physicochemical, morphological, and relaxation-based analyses. For this, we investigated PAA and PAM, the two most common synthetic SAPs, either swollen in demineralized water (dH\u003csub\u003e2\u003c/sub\u003eO) or in three different soil extracts (sand, loam, clay) and subjected to ten drying-rewetting cycles. At different drying-rewetting cycles, we quantified the swelling index (SI), water dynamics (\u003csup\u003e1\u003c/sup\u003eH proton nuclear magnetic resonance (NMR) relaxometry), microstructural stability (rheometry), and tracked chemical changes in terms of pH, electrical conductivity (EC), surface chemistry (attenuated total reflectance fourier transform infrared - ATR-FTIR), and morphological features (environmental scanning electron microscope - ESEM). ATR-FTIR will be used to detect general shifts in characteristic polymer bands and the appearance of new or stronger peaks, providing molecular-level evidence of ionic crosslinking and condensation dynamics.\u003c/p\u003e\u003cp\u003eConcerning the effect of the soil extract composition, we hypothesized that the effect of cation charges on hydrogel properties is more important than the overall ion composition of the soil extract. Here, trivalent cations (especially Al\u0026sup3;⁺) should promote stronger ionic crosslinks in the SAP hydrogels than divalent cations (e.g., Ca\u0026sup2;⁺, Mg\u0026sup2;⁺), promoting the formation of denser, less expandable network structures, with smaller pores (ESEM), alongside shorter transverse relaxation times (\u003csup\u003e1\u003c/sup\u003eH-NMR relaxometry), reduced SI, and higher structural stability (rheometry) compared to dH\u003csub\u003e2\u003c/sub\u003eO controls. In the ATR-FTIR spectra, increased crosslinking degree will come along with systematic band shifts and intensity changes for characteristic absorption peaks of the two SAPs.\u003c/p\u003e\u003cp\u003eIn the course of the successive drying-rewetting cycles, SAP hydrogels swollen in dH\u003csub\u003e2\u003c/sub\u003eO are hypothesized to retain their original properties up to a critical number of drying-rewetting cycles, beyond which the polymer chains undergo irreversible condensation, leading to \u0026ldquo;network aging\u0026rdquo; that can be measured as permanently shorter transverse relaxation times, reduced SI, and increased stability, independent of subsequent drying-rewetting cycles. However, these effects should be amplified in the three investigated soil extracts as function of pH and soil extract composition, promoting osmotic stresses and additional ionic crosslinking. Accordingly, ATR-FTIR of the hydrogels swollen in soil extracts should show these simplified band shift trends more prominently and at earlier cycles than in dH₂O. Moreover, we hypothesized that crosslinks in the SAP hydrogels are more rapidly formed than SAP network expansion occurs during a drying-rewetting event, steadily increasing network densification, which further reduces the SI, and transverse relaxation times and increases viscosity, respectively.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cp\u003eThree well-characterized reference soils from the Agricultural Investigation and Research Institute (Speyer, Germany) were used. The soils differed in their texture (sand, clay and loam soil) and physicochemical properties (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). From each soil, soil extracts were prepared by mixing soil and dH\u003csub\u003e2\u003c/sub\u003eO at 1:5 ratio, agitating for 24 h, and filtering through a 0.45 \u0026micro;m membrane. Soil extract will be referred to as sand, loam and clay in the following. For the SAP swelling experiments, hydrogel-forming PAA powder (Viscosity average molar mass M\u003csub\u003ev\u003c/sub\u003e = 4,000,000 g/mol) (Sigma-Aldrich, Germany; CAS 9003\u0026ndash;01\u0026ndash;4), and PAM granules (M\u003csub\u003ev\u003c/sub\u003e= 15,000,000 g/mol) (Carl Roth, Germany, CAS 9003-05-8) of high molecular weight were investigated.\u003c/p\u003e\u003cp\u003eTo assess the effect of the soil extracts on the various physicochemical SAP properties, a free swelling experiment (FSE) was conducted according to Brax et al. \u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e and Buchmann et al. \u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e in a modified way: for this, freely swollen PAA and PAM hydrogels were prepared by allowing dry SAP powder/granules to completely swell for 72 h on a pre-wetted dialysis membrane (flat width 44 mm, MWCO 14000, Carl Roth GmbH \u0026amp; Co. KG), which was positioned in a liquid reservoir with the respective soil extract (or dH\u003csub\u003e2\u003c/sub\u003eO as control). After swelling, the respective SAP hydrogels were gently removed from the membrane and investigated for various physicochemical properties as listed below.\u003c/p\u003e\u003cp\u003eFor the incubation experiment (IE), after an initial free swelling of the respective SAPs in dH\u003csub\u003e2\u003c/sub\u003eO (C0), the respective soil extracts of the three soils were used for the subsequent drying-rewetting cycles. Thus, the SAP hydrogels were subjected to a total of 10 drying-rewetting cycles of seven days each, including four days of drying at 30\u0026deg;C and three days of rewetting in an excess of respective liquid (dH\u003csub\u003e2\u003c/sub\u003eO or soil extract) at 20\u0026deg;C. After the 3rd, 5th, and 10th drying-rewetting cycle (C3, C5, C10), the respective PAA and PAM hydrogels were examined for the same physicochemical properties as in the previous swelling experiment. In total, 190 samples were prepared for both experiments with five replicates for each treatment.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSelected physicochemical properties of the investigated soils and the respective 1:5 soil extracts used for the SAP swelling experiment\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSoil name\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e2.1\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.4\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e6S\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSoil type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003esand\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eloam\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eclay\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"5\" rowspan=\"6\"\u003e\u003cp\u003eBulk soil\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eOrganic carbon [% C]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e0.55\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNitrogen [% N]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e0.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003epH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e4.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e7.50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e7.30\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCEC [meq/100g]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e2.90\u0026thinsp;\u0026plusmn;\u0026thinsp;0.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e17.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.80\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e18.70\u0026thinsp;\u0026plusmn;\u0026thinsp;1.20\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDensity [g/cm\u0026sup3;]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e1.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1.18\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eWHC\u003csub\u003emax\u003c/sub\u003e [g/100g]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e32.50\u0026thinsp;\u0026plusmn;\u0026thinsp;1.50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e44.60\u0026thinsp;\u0026plusmn;\u0026thinsp;2.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e41.60\u0026thinsp;\u0026plusmn;\u0026thinsp;1.00\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003ePSD (mm) [%]\u003c/p\u003e\u003cp\u003e\u003cem\u003e\u0026lt;\u0026thinsp;0.002\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e3.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003e26.60\u0026thinsp;\u0026plusmn;\u0026thinsp;0.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e40.80\u0026thinsp;\u0026plusmn;\u0026thinsp;1.40\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003e0.002\u0026ndash;0.05\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e10.70\u0026thinsp;\u0026plusmn;\u0026thinsp;1.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003e41.20\u0026thinsp;\u0026plusmn;\u0026thinsp;1.30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e35.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.71\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003e0.05-2.0\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e86.20\u0026thinsp;\u0026plusmn;\u0026thinsp;0.70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003e32.30\u0026thinsp;\u0026plusmn;\u0026thinsp;1.40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e24.10\u0026thinsp;\u0026plusmn;\u0026thinsp;1.80\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"9\" rowspan=\"10\"\u003e\u003cp\u003eSoil extracts\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eEC [\u0026micro;S/cm]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e138\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e424\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e86\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003epH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e7.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e7.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e8.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAl\u003csup\u003e3+\u003c/sup\u003e [mg/L]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e0.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFe\u003csup\u003e3+\u003c/sup\u003e [mg/L]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e0.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMn\u003csup\u003e2+\u003c/sup\u003e [mg/L]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e1.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.018\u0026thinsp;\u0026plusmn;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eZn\u003csup\u003e2+\u003c/sup\u003e [mg/L]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e0.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.007\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.02\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eCa\u003csup\u003e2+\u003c/sup\u003e [mg/L]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e31.15\u0026thinsp;\u0026plusmn;\u0026thinsp;4.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e90.62\u0026thinsp;\u0026plusmn;\u0026thinsp;5.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e47.51\u0026thinsp;\u0026plusmn;\u0026thinsp;3.77\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMg\u003csup\u003e2+\u003c/sup\u003e [mg/L]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e5.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e5.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e5.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e [mg/L]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e10.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e8.83\u0026thinsp;\u0026plusmn;\u0026thinsp;0.96\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e [mg/L]\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e2.62\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e3.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e2.63\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eBasic parameter\u003c/b\u003e\u003c/p\u003e\u003cp\u003eBefore and after each drying-rewetting cycle, pH and EC of the soil extracts in the liquid reservoir were determined according to DIN EN ISO 11265 \u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e and DIN 38404-5 \u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e respectively, using a multi-parameter analyzer C863 (Consort, Belgium). Furthermore, the SI of both SAP hydrogels were calculated according to \u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e, respectively.\u003c/p\u003e\u003cp\u003e\u003csup\u003e\u003cb\u003e1\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eH-NMR relaxometry\u003c/b\u003e\u003c/p\u003e\u003cp\u003eWater entrapment was measured by \u003csup\u003e1\u003c/sup\u003eH-NMR relaxometry using a Bruker Minispec MQ (Bruker, Karlsruhe, Germany) with a magnetic field strength of 0.176T, corresponding to a proton Larmor frequency of 7.5 MHz \u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. Transverse relaxation (T\u003csub\u003e2\u003c/sub\u003e) decay curves were acquired with an echo time (T\u003csub\u003eE\u003c/sub\u003e) set at 0.3 ms. The raw data were processed using MATLAB with an inverse Laplace transformation (ILT) \u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e, based on the Butler, Reeds, and Dawson (BRD) algorithm \u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e to obtain the respective relaxation time distributions (RTDs). Following the method outlined by Buchmann et al. \u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e, the 95th percentile of the sum of all amplitudes was employed (T\u003csub\u003e2WL\u003c/sub\u003e), consequently the relaxation time of 95% of the water protons within the sample was shorter than the T\u003csub\u003e2WL\u003c/sub\u003e \u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. Further, the peak positions T\u003csub\u003e2peak\u003c/sub\u003e within the RTDs were determined as the predominant water fraction in the sample \u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. To exclude possible (para)magnetic relaxation effects originating from the soil extracts, reference measurements were carried out as described by \u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003cb\u003eRheometry\u003c/b\u003e\u003c/p\u003e\u003cp\u003eRheological measurements were conducted using an MCR 102 rheometer (Anton Paar, Germany) equipped with a cone-plate measuring geometry. For the measurements, a small amount of swollen SAP hydrogel was placed on the rheometer plate, followed by a resting period of 60 s to ensure undisturbed measurements. To assess the viscoelasticity of the hydrogels, additional amplitude sweep tests (AST) were performed at a constant frequency of 10 s⁻\u0026sup1; for a total of 37 measurement points. The temperature was set to 20\u0026deg;C, regulated by a Peltier unit. From the AST, shear stress and at the yield point (τ\u003csub\u003eYP\u003c/sub\u003e) as well as the maximum shear stress (τ\u003csub\u003emax\u003c/sub\u003e) for each sample were determined.\u003c/p\u003e\u003cp\u003e\u003cb\u003eATR-FTIR\u003c/b\u003e\u003c/p\u003e\u003cp\u003eATR- FTIR measurements of all samples were performed on the freeze-dried hydrogels using a Cary 630 ATR-FTIR (Agilent Technologies) spectrometer. The resulting data were analyzed using an open Specy R package \u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e. As the spectra exhibited no significant background noise, no filtering was required during data processing. Striking absorbance bands at 4,000\u0026ndash;3,200 cm⁻\u0026sup1; (O\u0026ndash;H), 3,000\u0026ndash;2,800 cm⁻\u0026sup1; (C\u0026ndash;H), 1,870-1,550 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;O), 1490\u0026thinsp;\u0026minus;\u0026thinsp;1150 (H\u0026ndash;C\u0026ndash;H), 1,040 cm⁻\u0026sup1; (C\u0026ndash;O) for PAA and 4,000\u0026ndash;3,080 cm⁻\u0026sup1; (NH), 3000\u0026thinsp;\u0026minus;\u0026thinsp;2800 cm⁻\u0026sup1; (C\u0026ndash;H), 1,870-1,550 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;O), and 1,490-1,150 (H\u0026ndash;C\u0026ndash;H) for PAM were qualitatively evaluated for all samples \u003csup\u003e[\u003cspan additionalcitationids=\"CR35 CR36\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003cb\u003eESEM\u003c/b\u003e\u003c/p\u003e\u003cp\u003eESEM images were exemplarily captured for the two SAP hydrogels at C0 and C10 using a Quanta 250 ESEM (FEI Company, Hillsboro, United States) equipped with an Everhart Thornley secondary electron detector (ETD). Prior to the measurements, the hydrogels were freeze-dried and coated with a 30 nm thick gold layer using a Q150R S sputter coater (Quorom Technologies Ltd, United Kingdom). Measurements were performed under high vacuum (\u0026lt;\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e Pa) with an acceleration voltage of 30kV and an average spot size of 3.5. Different resolutions were employed a) to visualize overall structural features of PAA and PAM swollen in dH\u003csub\u003e2\u003c/sub\u003eO and the different soil extracts and b) to examine detailed network structures, including network density, crosslinking areas, and polymeric arrangements.\u003c/p\u003e\u003cp\u003e\u003cb\u003eStatistical analysis\u003c/b\u003e\u003c/p\u003e\u003cp\u003eVariations within the 5 replicates were presented as standard errors (SE) of the arithmetic means. As variance homogeneity (Levene-test) and normal distribution (Shapiro-Wilks-test) were not fulfilled, a permanova was performed based on Euclidean distance measurements using the adonis2() command from the vegan package \u003csup\u003e[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e. Moreover, a scaled principal component analysis (PCA) was performed using the PCA() command from the FactoMineR package \u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e. R version 4.3.1 (RStudio 2024.04.2) and Excel Office 16 were used to carry out all calculations and figures. All detailed statistical parameters, including degree of freedom, Sum of squares, R2, F-value and p-values are presented in Tables S1 and S2 of the supplementary information.\u003c/p\u003e"},{"header":"3 Results","content":"\u003cp\u003e\u003cb\u003eBasic parameter\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFor the FSE, PAA substantially decreased the pH for all soil extract directly after the first swelling, from pH of 7.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12, 7.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 and 8.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 to 3.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1, 3.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0 and 4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 for sand, loam and clay, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-c). In contrast, PAM slightly increased the pH of the sand and loam extracts by 0.2 and 0.1 units to 7.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10 and 7.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10, respectively, while decreasing the pH for the clay extract by 0.3 units to 7.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-c). Furthermore, PAA substantially increased the EC after the first swelling in all soil extracts, from initially 138\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm, 424\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm and 86\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm for sand, loam and clay to 311\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm, 719\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm and 150\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm, respectively. Also, PAM substantially increased the EC in sand and clay extracts by 71 \u0026micro;S/cm and 177 \u0026micro;S/cm to 209\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm and 263\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm. In contrast, PAM decreased EC in the loam extract by 286 \u0026micro;S/cm to 138\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed-f).\u003c/p\u003e\u003cp\u003eFor the IE, PAA substantially decreased the pH in the respective soil extracts from C0 to C3 and remained constant for all subsequent drying-rewetting cycles. After C3, the pH of PAA-related liquid reservoirs decreased from 7.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06, 7.34\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 and 8.03\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 to 3.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03, 3.86\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 and 4.42\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 in sand, loam and clay, respectively. In contrast, the liquid reservoirs related to PAM exhibited only minimal pH shifts (within \u0026plusmn;\u0026thinsp;0.4 units) to 6.72\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03, 6.98\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 and 7.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 in sand, loam and clay until C10, respectively. EC substantially increased with drying-rewetting cycles for both SAPs, although the magnitude and pattern varied with the soil extracts (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed-f). For PAA, EC of the liquid reservoirs steadily increased from initially 138\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm, 425\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm and 81\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm (7\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm for dH\u003csub\u003e2\u003c/sub\u003eO) to 478\u0026thinsp;\u0026plusmn;\u0026thinsp;0 \u0026micro;S/cm (sand), 1,558\u0026thinsp;\u0026plusmn;\u0026thinsp;0 \u0026micro;S/cm (loam), and 233\u0026thinsp;\u0026plusmn;\u0026thinsp;0 \u0026micro;S/cm (clay) after C10. In contrast, EC of the PAM-related liquid reservoir increased only modestly, peaking at 328\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm, 1,047\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm, and 388\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S/cm for sand, loam, and clay, respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eConcerning the FSE, the SI of PAA (SI\u003csub\u003ePAA\u003c/sub\u003e) decreased in all three soil extracts compared to the first swelling in dH\u003csub\u003e2\u003c/sub\u003eO. Here, SI was the lowest for PAA swollen in clay extract (61.36\u0026thinsp;\u0026plusmn;\u0026thinsp;0.59 ml/g compared to 76.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17 ml/g in dH\u003csub\u003e2\u003c/sub\u003eO). SI\u003csub\u003ePAM\u003c/sub\u003e decreased in loam and clay extract, while increasing in sand extract to 51.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.58 ml/g compared to 43.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38 ml/g in dH\u003csub\u003e2\u003c/sub\u003eO (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The differences in polymer type, soil extracts (including dH\u003csub\u003e2\u003c/sub\u003eO) and their interactions were highly significant (p\u0026thinsp;=\u0026thinsp;0.001).\u003c/p\u003e\u003cp\u003eIn the IE, SI\u003csub\u003ePAA\u003c/sub\u003e overall decreased by 20% in the course of drying-rewetting, from initially 76.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17 ml/g (C0) to finally 52.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.56 ml/g after C10. In contrast, SI\u003csub\u003ePAM\u003c/sub\u003e constantly fluctuated between 43.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38 ml/g (C0) and 39.29\u0026thinsp;\u0026plusmn;\u0026thinsp;0.60 ml/g (after C10) throughout all drying-rewetting cycles. Concerning the effect of the soil extracts, SI\u003csub\u003ePAA\u003c/sub\u003e decreased in all three soil extracts, whereby the loam showed the highest reduction of 76.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17 ml/g to 33.79\u0026thinsp;\u0026plusmn;\u0026thinsp;0.61 ml/g after C10. When swollen in sand extract, SI\u003csub\u003ePAM\u003c/sub\u003e decreased from initially 43.80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.38 ml/g at C0 to 36.71\u0026thinsp;\u0026plusmn;\u0026thinsp;0.57 ml/g after C3, before (re)increasing again to 44.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.47 ml/g after C10. A similar pattern was observed in loam and clay: SI\u003csub\u003ePAM\u003c/sub\u003e decreased to 26.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.91 ml g⁻\u0026sup1; and 33.71\u0026thinsp;\u0026plusmn;\u0026thinsp;0.59 ml g⁻\u0026sup1;, respectively, after C5, then (re)increased again to 39.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.66 ml g⁻\u0026sup1; (loam) and 45.98\u0026thinsp;\u0026plusmn;\u0026thinsp;0.98 ml g⁻\u0026sup1; (clay) after C10. Permanova revealed that polymer type (PAA vs. PAM), soil extract (dH\u003csub\u003e2\u003c/sub\u003eO, sand, loam, clay), drying\u0026ndash;rewetting cycle (0, 3, 5, 10), and all their two- and three‐way interactions were highly significant (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003ePolymer-water interactions\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe relaxation time distributions (RTDs) together with T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e derived from the \u003csup\u003e1\u003c/sup\u003eH-NMR measurements were used to further characterize the SAP hydrogels in terms of polymer-water interactions (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eConcerning the FSE, both PAA and PAM exhibited the same symmetric and sharp RTDs for all three soil extracts with single T\u003csub\u003e2peak\u003c/sub\u003e at for 2,166\u0026thinsp;\u0026plusmn;\u0026thinsp;20.16 ms for dH\u003csub\u003e2\u003c/sub\u003eO, 1,739.27\u0026thinsp;\u0026plusmn;\u0026thinsp;23.65 ms for loam and 1,557.91\u0026thinsp;\u0026plusmn;\u0026thinsp;40.10 ms for clay. PAM exhibited symmetric and sharp RTDs with T\u003csub\u003e2peak\u003c/sub\u003e positions at 2,471.00\u0026thinsp;\u0026plusmn;\u0026thinsp;7.81 ms for dH\u003csub\u003e2\u003c/sub\u003eO, 2,369.65\u0026thinsp;\u0026plusmn;\u0026thinsp;107.28 ms for loam, and 2,233.55\u0026thinsp;\u0026plusmn;\u0026thinsp;38.23 ms for clay extracts.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn contrast, for the first swelling in sand extract, both PAA and PAM exhibited as well symmetric and sharp RTDs together with decreased T\u003csub\u003e2peak\u003c/sub\u003e positions to 491.71\u0026thinsp;\u0026plusmn;\u0026thinsp;8.30 ms for PAA and 650.34\u0026thinsp;\u0026plusmn;\u0026thinsp;22.79 ms for PAM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, b and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, f, h). Regarding T\u003csub\u003e2WL\u003c/sub\u003e, PAA swollen in the different soil extracts showed lower values compared to dH\u003csub\u003e2\u003c/sub\u003eO (4,288.70\u0026thinsp;\u0026plusmn;\u0026thinsp;1.44 ms) with 1,115.55\u0026thinsp;\u0026plusmn;\u0026thinsp;19.10 ms for sand, 3435.01\u0026thinsp;\u0026plusmn;\u0026thinsp;57.99 ms for silt, and 3,075.08\u0026thinsp;\u0026plusmn;\u0026thinsp;67.52 ms for clay. In contrast, PAM showed approximately the same T\u003csub\u003e2WL\u003c/sub\u003e of 4,472.018\u0026thinsp;\u0026plusmn;\u0026thinsp;15.01 ms as dH\u003csub\u003e2\u003c/sub\u003eO for loam and clay, together with a decreased T\u003csub\u003e2WL\u003c/sub\u003e of 1,322.59\u0026thinsp;\u0026plusmn;\u0026thinsp;171.82 ms in sand (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, e, g). The differences of the polymer type, the soil extracts including dH\u003csub\u003e2\u003c/sub\u003eO were highly significant (p\u0026thinsp;=\u0026thinsp;0.001), as well as the interaction of the polymer type with the soil extracts (p\u0026thinsp;=\u0026thinsp;0.002).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFor the IE, both PAA and PAM exhibited symmetric and sharp RTDs with a single T\u003csub\u003e2peak\u003c/sub\u003e centered at approximately 2,166\u0026thinsp;\u0026plusmn;\u0026thinsp;20.16 ms and 2,471\u0026thinsp;\u0026plusmn;\u0026thinsp;7.81 ms for all drying-rewetting cycles, respectively. However, T\u003csub\u003e2WL\u003c/sub\u003e of PAA slightly decreased with each drying-rewetting cycle, from initially 4,288.70\u0026thinsp;\u0026plusmn;\u0026thinsp;11.44 ms at C0 to 3704.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms after C10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Also, T\u003csub\u003e2peak\u003c/sub\u003e of PAA increased from C5 on to finally 2,171.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 ms after C10. In contrast, both T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e of PAM did not significantly shift (only by 88 ms on average), from initially 4,472.02\u0026thinsp;\u0026plusmn;\u0026thinsp;15.01 ms and 2,394.13\u0026thinsp;\u0026plusmn;\u0026thinsp;8.04 ms at C0 to 4,535.52\u0026thinsp;\u0026plusmn;\u0026thinsp;15.01 ms and 2,427.59\u0026thinsp;\u0026plusmn;\u0026thinsp;8.04 ms after C10, respectively.\u003c/p\u003e\u003cp\u003eRegarding the combined effects of soil extracts and drying-rewetting cycles, RTDs of both SAPs substantially changed, with PAM being less affected than PAA, except for the sand extract. T\u003csub\u003e2WL\u003c/sub\u003e for both PAA and PAM showed the same trend, with a decrease to 1,000.03\u0026thinsp;\u0026plusmn;\u0026thinsp;3.49 ms after C3, which remained constant for all subsequent drying-rewetting cycles. Additionally, the RTD of PAA transitioned from a sharp single peak to a broader, slightly asymmetric distribution pattern. This was also the case in the loam extract, whereT\u003csub\u003e2WL\u003c/sub\u003e of both PAA and PAM decreased with each drying-rewetting cycle to finally 2,329.45\u0026thinsp;\u0026plusmn;\u0026thinsp;10.03 ms and 3,580.46\u0026thinsp;\u0026plusmn;\u0026thinsp;9.95 ms after C10, respectively. T\u003csub\u003e2peak\u003c/sub\u003e of PAA and PAM decreased accordingly to 1,669.91\u0026thinsp;\u0026plusmn;\u0026thinsp;8.43 ms and 1,943.99\u0026thinsp;\u0026plusmn;\u0026thinsp;6.53 ms after C5 and to 1,133.31\u0026thinsp;\u0026plusmn;\u0026thinsp;7.79 ms and 1,840.78\u0026thinsp;\u0026plusmn;\u0026thinsp;10.26 ms after C10. For the clay extract, T\u003csub\u003e2WL\u003c/sub\u003e of PAA decreased significantly stronger (1,603.61\u0026thinsp;\u0026plusmn;\u0026thinsp;11.02 ms after C10) with each drying-rewetting cycle than for the other extracts. In contrast, T\u003csub\u003e2WL\u003c/sub\u003e of PAM decreased only to C3 (3,841.51\u0026thinsp;\u0026plusmn;\u0026thinsp;19.39 ms), with only slight variations in the following drying-rewetting cycles. T\u003csub\u003e2peak\u003c/sub\u003e showed the same course of PAA and PAM with a significantly stronger decrease of PAA from 2,295.99\u0026thinsp;\u0026plusmn;\u0026thinsp;6.12 ms at C0 to 729.92\u0026thinsp;\u0026plusmn;\u0026thinsp;8.03 ms after C10. In contrast, PAM decreased from 2,394.13\u0026thinsp;\u0026plusmn;\u0026thinsp;8.04 ms at C0 to 1972.98\u0026thinsp;\u0026plusmn;\u0026thinsp;10.30 ms after C3 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-j and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) followed by slight variations in the following drying-rewetting cycles. The differences in polymer type, soil extract and the drying-rewetting cycles were highly significant (p\u0026thinsp;=\u0026thinsp;0.001).\u003c/p\u003e\u003cp\u003e\u003cb\u003eChemo-structural properties and morphology\u003c/b\u003e\u003c/p\u003e\u003cp\u003eConcerning the FSE, both structural stability in terms of shear stress at the yield point (τ\u003csub\u003eYP\u003c/sub\u003e) and maximum shear stress (τ\u003csub\u003emax\u003c/sub\u003e) of PAA increased in all three soil extracts compared to the first swelling in dH\u003csub\u003e2\u003c/sub\u003eO (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea-b). PAA swollen in the loam extract revealed the highest τ\u003csub\u003eYP\u003c/sub\u003e and τ\u003csub\u003emax\u003c/sub\u003e of 546.78\u0026thinsp;\u0026plusmn;\u0026thinsp;132.23 Pa and 956.29\u0026thinsp;\u0026plusmn;\u0026thinsp;154.57 Pa, followed by 410.26\u0026thinsp;\u0026plusmn;\u0026thinsp;112.87 Pa and 577.94\u0026thinsp;\u0026plusmn;\u0026thinsp;19.80 Pa in sand, and 248.92\u0026thinsp;\u0026plusmn;\u0026thinsp;12.89 Pa and 810.19\u0026thinsp;\u0026plusmn;\u0026thinsp;131.16 Pa in clay, respectively. In contrast, τ\u003csub\u003eYP\u003c/sub\u003e of PAA swollen in dH\u003csub\u003e2\u003c/sub\u003eO was 141.37\u0026thinsp;\u0026plusmn;\u0026thinsp;9.08 Pa. PAM showed the same course as PAA but with only slightly increased τ\u003csub\u003eYP\u003c/sub\u003e and τ\u003csub\u003emax\u003c/sub\u003e compared to the first swelling in dH\u003csub\u003e2\u003c/sub\u003eO and PAA. Both τ\u003csub\u003eYP\u003c/sub\u003e and τ\u003csub\u003emax\u003c/sub\u003e were highest in loam (48.72\u0026thinsp;\u0026plusmn;\u0026thinsp;2.52 Pa and 274.62\u0026thinsp;\u0026plusmn;\u0026thinsp;3.97 Pa) and lowest in clay (97.75\u0026thinsp;\u0026plusmn;\u0026thinsp;5.93 Pa and 200.59\u0026thinsp;\u0026plusmn;\u0026thinsp;10.49 Pa) For both stability indices, permanova revealed significances for polymer types (p\u0026thinsp;=\u0026thinsp;0.001), whereas only τ\u003csub\u003emax\u003c/sub\u003e showed slight significance for the soil extracts including dH\u003csub\u003e2\u003c/sub\u003eO (p\u0026thinsp;=\u0026thinsp;0.04).\u003c/p\u003e\u003cp\u003eFor the IE, τ\u003csub\u003eYP\u003c/sub\u003e and τ\u003csub\u003emax\u003c/sub\u003e increased for both SAPs with increasing drying-rewetting cycles and independent of the soil extract. In dH\u003csub\u003e2\u003c/sub\u003eO, both τ\u003csub\u003eYP\u003c/sub\u003e and τ\u003csub\u003emax\u003c/sub\u003e of PAA increased with each drying-rewetting cycle, from initially 141.37\u0026thinsp;\u0026plusmn;\u0026thinsp;9.08 Pa and 356.08\u0026thinsp;\u0026plusmn;\u0026thinsp;15.71 Pa to 441.67\u0026thinsp;\u0026plusmn;\u0026thinsp;162.16 Pa and 391.88\u0026thinsp;\u0026plusmn;\u0026thinsp;54.16 Pa after C10, respectively. However, τ\u003csub\u003eYP\u003c/sub\u003e of PAM remained constant at approximately 42.97\u0026thinsp;\u0026plusmn;\u0026thinsp;5.26 Pa over all drying-rewetting cycles, whereas τ\u003csub\u003emax\u003c/sub\u003e slightly decreased from 95.52\u0026thinsp;\u0026plusmn;\u0026thinsp;9.95 Pa at C0 to 82.33\u0026thinsp;\u0026plusmn;\u0026thinsp;8.28 Pa after C3.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFor sand, τ\u003csub\u003eYP\u003c/sub\u003e of PAA showed the same course as in dH\u003csub\u003e2\u003c/sub\u003eO by increasing from 141.37\u0026thinsp;\u0026plusmn;\u0026thinsp;9.08 Pa at C0 to 441.67\u0026thinsp;\u0026plusmn;\u0026thinsp;162.16 Pa after C10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec-d). In contrast, τ\u003csub\u003emax\u003c/sub\u003e increased from 356.08\u0026thinsp;\u0026plusmn;\u0026thinsp;15.71 Pa at C0 to 636.95\u0026thinsp;\u0026plusmn;\u0026thinsp;146.65 Pa after C10 with the highest increase from C0 to 745.20\u0026thinsp;\u0026plusmn;\u0026thinsp;105.91 Pa after C3 followed by a drop to 573.09\u0026thinsp;\u0026plusmn;\u0026thinsp;124.09 Pa to after C5. Again, PAM showed a constant τ\u003csub\u003eYP\u003c/sub\u003e of approximately 42.97\u0026thinsp;\u0026plusmn;\u0026thinsp;5.26 Pa during the drying-rewetting cycles with a slight increase to 49.41\u0026thinsp;\u0026plusmn;\u0026thinsp;8.73 Pa after C5. Also, τ\u003csub\u003emax\u003c/sub\u003e remained approximately constant at 95.52\u0026thinsp;\u0026plusmn;\u0026thinsp;9.95 Pa at C0 and 103.70\u0026thinsp;\u0026plusmn;\u0026thinsp;13.83 Pa after C3 and increased slightly to 127.46\u0026thinsp;\u0026plusmn;\u0026thinsp;13.63 Pa after C5 and 168.07\u0026thinsp;\u0026plusmn;\u0026thinsp;3.44 Pa after C10.\u003c/p\u003e\u003cp\u003eWhen swollen in loam, τ\u003csub\u003eYP\u003c/sub\u003e of PAA increased from 141.37\u0026thinsp;\u0026plusmn;\u0026thinsp;9.08 Pa at C0 to finally 481.17\u0026thinsp;\u0026plusmn;\u0026thinsp;50.87 Pa at C10, although C5 showed an intermediate decrease to 369.59\u0026thinsp;\u0026plusmn;\u0026thinsp;93.01 Pa (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee-f). τ\u003csub\u003emax\u003c/sub\u003e of PAA showed a similar course with a relatively higher increase, from 356.08\u0026thinsp;\u0026plusmn;\u0026thinsp;15.71 Pa at C0 to 621.40\u0026thinsp;\u0026plusmn;\u0026thinsp;39.77 Pa after C10 showing a peak of 1051.80\u0026thinsp;\u0026plusmn;\u0026thinsp;95.75 after C3. For PAM, τ\u003csub\u003eYP\u003c/sub\u003e increased from 42.97\u0026thinsp;\u0026plusmn;\u0026thinsp;5.26 Pa at C0 to 266.07\u0026thinsp;\u0026plusmn;\u0026thinsp;30.90 Pa after C5 and subsequently dropped to 147.57\u0026thinsp;\u0026plusmn;\u0026thinsp;29.25 Pa after C10. τ\u003csub\u003emax\u003c/sub\u003e showed a similar trend with an increase from 95.52\u0026thinsp;\u0026plusmn;\u0026thinsp;9.95 Pa at C0 to 536.17\u0026thinsp;\u0026plusmn;\u0026thinsp;26.06 Pa after C5 and dropped to 440.06\u0026thinsp;\u0026plusmn;\u0026thinsp;48.03 Pa after C10.\u003c/p\u003e\u003cp\u003eFor clay, τ\u003csub\u003eYP\u003c/sub\u003e of PAA increased from 141.37\u0026thinsp;\u0026plusmn;\u0026thinsp;9.08 Pa at C0 to 431.80\u0026thinsp;\u0026plusmn;\u0026thinsp;134.03 Pa after C5 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg-h). With further drying-rewetting cycles, τ\u003csub\u003eYP\u003c/sub\u003e dropped to finally 209.68 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\pm\\:\\)\u003c/span\u003e\u003c/span\u003e 67.14 Pa after C10. While τ\u003csub\u003emax\u003c/sub\u003e of PAA increased to 859.71 \u0026plusmn; 46.65 Pa after C3, it dropped afterwards to 629.49 \u0026plusmn; 169.95 Pa and 527.32 \u0026plusmn; 118.88 Pa after C5 and C10. In contrast, drying-rewetting cycles increased τ\u003csub\u003eYP\u003c/sub\u003e of PAM from initially 42.97 \u0026plusmn; 5.26 Pa at C0 to 134.84 \u0026plusmn; 29.41 Pa at C10. Also, τ\u003csub\u003emax\u003c/sub\u003e showed this trend with an increase from 95.52 \u0026plusmn; 9.95 Pa at C0 to 327.73 \u0026plusmn; 49.98 Pa after C10. Both τ\u003csub\u003eYP\u003c/sub\u003e and τ\u003csub\u003emax\u003c/sub\u003e showed high significances regarding the comparison of the different polymer types and treatments (p = 0.001). Further, τ\u003csub\u003emax\u003c/sub\u003e showed a highly significant interaction for the polymer types and cycles (p\u0026thinsp;=\u0026thinsp;0.001) as well.\u003c/p\u003e\u003cp\u003e\u003cb\u003eESEM\u003c/b\u003e\u003c/p\u003e\u003cp\u003eConcerning the FSE, ESEM images revealed morphological differences between the two SAPs, especially when swollen in sand and loam extract (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Compared to dH₂O, PAA exhibited more condensed structures with fragmented edges, whereas PAM formed an overall denser hydrogel network with smaller pores. When swollen in clay extract, PAA again formed a highly condensed network with large but covered pores within layered structures. Also, PAM revealed a condensed hydrogel network with larger pores compared to the one swollen in dH\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFor the IE, ESEM images revealed substantial morphological differences between the two SAPs as function of soil extracts and drying-rewetting cycles: C0 and swollen in dH\u003csub\u003e2\u003c/sub\u003eO caused a homogeneous network for both PAA and PAM, whereas clear differences in terms of fracture edges and condensed polymeric parts of the hydrogel network were observed after C10 for PAA.\u003c/p\u003e\u003cp\u003eIn contrast, PAM swollen in dH\u003csub\u003e2\u003c/sub\u003eO still showed a relatively intact and homogenous network structure after C10, although more condensed parts and larger spaces within the network were visible. PAA swollen in sand showed highly condensed network structures after C10, together with clear breaking edges and smaller pores. It also developed thicker pore walls and a broader pore-size distribution. Also, PAM swollen in sand exhibited elongated, plate-like lamella with highly condensed areas and fan-shaped structures.\u003c/p\u003e\u003cp\u003eRegarding the swelling in loam and clay extract, the PAA hydrogel network was completely different compared to PAA at C0 and swollen in dH\u003csub\u003e2\u003c/sub\u003eO. However, both PAA and PAM hydrogels consisted of condensed structures and layer-like features. For the loam extract, PAA showed irregular pore shapes with heterogeneous wall thickness and partially merged junction zones, while PAM occasionally showed granular network structures with localized microcavities and rough walls. Especially when swollen in clay extract, PAA formed a hydrogel network with dense, radially oriented lamellar sheets and fan-shaped domains, whereas PAM transformed into stacked, sheet-like layers with clear stratification and reduced overall pore volume.\u003c/p\u003e\u003cp\u003e\u003cb\u003eATR-FTIR\u003c/b\u003e\u003c/p\u003e\u003cp\u003ePAA initially swollen in dH\u003csub\u003e2\u003c/sub\u003eO water showed striking absorbance bands at 4,000\u0026ndash;3,200 cm⁻\u0026sup1; (O\u0026ndash;H), 3,000\u0026ndash;2,800 cm⁻\u0026sup1; (C\u0026ndash;H), 1870\u0026thinsp;\u0026minus;\u0026thinsp;1550 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;O), 1490\u0026thinsp;\u0026minus;\u0026thinsp;1150 (H\u0026ndash;C\u0026ndash;H), and 1,040 cm⁻\u0026sup1; (C\u0026ndash;O), whereas PAM showed further absorbance bands at 3,000\u0026ndash;3,500 cm⁻\u0026sup1; (O\u0026ndash;H), 1,700-1,740 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;O), and 1,040 cm⁻\u0026sup1; (C\u0026ndash;O). Interestingly, both PAA and PAM also showed an obvious peak at \u0026lt;\u0026thinsp;1,000 cm⁻\u0026sup1; (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eConcerning the FSE, the O\u0026ndash;H stretching and C\u0026thinsp;=\u0026thinsp;O of PAA increased when swollen in loam extract but remained the same for dH\u003csub\u003e2\u003c/sub\u003eO, sand, and clay (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea). Further, the C\u0026ndash;O band intensity in PAA increased markedly when swollen in loam extract, only slightly in sand, and remained unchanged in both dH₂O and clay extracts. Interestingly, the band at \u0026lt;\u0026thinsp;1000 cm⁻\u0026sup1; showed the same course as the C\u0026ndash;O band. Interestingly, PAM showed vice versa behavior: N\u0026ndash;H stretching, C\u0026thinsp;=\u0026thinsp;O-stretching and H\u0026ndash;C\u0026ndash;H stretching decreased for all three soil extracts approximately the same compared to the first swelling in dH\u003csub\u003e2\u003c/sub\u003eO (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb).\u003c/p\u003e\u003cp\u003eFor the IE, drying-rewetting cycles induced cycle-dependent band intensities for PAA and all solutions: when swollen in dH\u003csub\u003e2\u003c/sub\u003eO, both the O\u0026ndash;H-stretching (~\u0026thinsp;3,400 cm⁻\u0026sup1;) and (3,000\u0026ndash;2,800 cm⁻\u0026sup1;) C\u0026ndash;H stretching increased from C0 to reach its maximum after C3, but then decreased again after C5 and C10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). The C\u0026thinsp;=\u0026thinsp;O band (1,700 cm⁻\u0026sup1;), the C\u0026ndash;O band (~\u0026thinsp;1,040 cm⁻\u0026sup1;) and the \u0026lt;\u0026thinsp;1,000 cm⁻\u0026sup1; band all followed the same pattern. In sand, the O\u0026ndash;H band again peaked after C3 before decreasing after C5 and C10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed). The C\u0026thinsp;=\u0026thinsp;O band increased from C0 to C3, decreased after C5, but (re)increased after C10. The C\u0026ndash;O band intensities showed their maxima after C3 and then decreased steadily. Interestingly, the \u0026lt;\u0026thinsp;1,000 cm⁻\u0026sup1; band intensities showed the same course as the C\u0026ndash;O band. For the loam extract, all four bands increased continuously with an increasing number of drying-rewetting cycles, reaching their highest intensities after C10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ee). When swollen in clay extract, each band for PAA increased sharply to peak after C3 and then decreased after C5 and C10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ef).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003ePAM showed a parallel but distinct behavior: when swollen in dH\u003csub\u003e2\u003c/sub\u003eO, the N\u0026ndash;H stretching band (~\u0026thinsp;3,300 cm⁻\u0026sup1;), the C\u0026ndash;H stretching band and the H\u0026ndash;C\u0026ndash;H band (~\u0026thinsp;1,450 cm⁻\u0026sup1;) increased from C0 to their maxima after C3 before decreasing again at C5 and C10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eg). In contrast, the amide C\u0026thinsp;=\u0026thinsp;O band (1,650 cm⁻\u0026sup1;) increased from C0 to C5 and decreased again to C10. For the sand, the intensities of the N\u0026ndash;H, C\u0026ndash;H and H\u0026ndash;C\u0026ndash;H bands again peaked after C3 and subsequently decreased, while the C\u0026thinsp;=\u0026thinsp;O band reached its highest intensity after C5 before decreasing after C10 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eh). PAM swollen in the loam and clay extract resulted in the same N\u0026ndash;H, C\u0026ndash;H and H\u0026ndash;C\u0026ndash;H bands maxima after C3 and their subsequent decreased with further drying-rewetting cycles (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ei-j).\u003c/p\u003e\u003cp\u003e\u003cb\u003eRelationships between investigated parameters\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFor the IE, the first two principal components accounted for 58.2% of the total variance (Dim 1\u0026thinsp;=\u0026thinsp;35.5%, Dim 2\u0026thinsp;=\u0026thinsp;22.7%) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). The PCA showed that SI, τ\u003csub\u003emax\u003c/sub\u003e, and Ƭ\u003csub\u003eYP\u003c/sub\u003e highly contributed to the explaining dimensions (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea). Loadings indicate that SI and τ\u003csub\u003emax\u003c/sub\u003e projected strongly in the positive direction of Dim1, while T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e load primarily on Dim2. τ\u003csub\u003eYP\u003c/sub\u003e contributed modestly to Dim1 and slightly negatively to Dim2. In general, T\u003csub\u003e2WL\u003c/sub\u003e, T\u003csub\u003e2peak\u003c/sub\u003e and τ\u003csub\u003eYP\u003c/sub\u003e exhibited modest negative correlations, respectively. Moreover, SI and τ\u003csub\u003emax\u003c/sub\u003e were also only modestly correlated.\u003c/p\u003e\u003cp\u003ePAA clustered predominantly on the positive side of Dim1, whereas PAM occupied the negative Dim1 region (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). Polymer-specific PCA revealed no relationship between T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e but did show a modest positive association between PAA and both the SI and τ\u003csub\u003emax\u003c/sub\u003e. These results concur with the permanova, which detected highly significant effects of polymer type and its interaction with drying\u0026ndash;rewetting cycles (p\u0026thinsp;=\u0026thinsp;0.001) and a marginally significant polymer\u0026ndash;soil extract interaction (p\u0026thinsp;=\u0026thinsp;0.016). The three-way interaction (polymer type, drying-rewetting cycles, and soil extract) for τ\u003csub\u003emax\u003c/sub\u003e was also significant (p\u0026thinsp;=\u0026thinsp;0.007). The SI showed a high significance (p\u0026thinsp;=\u0026thinsp;0.001) in terms of the polymer type itself and the interactions with soil extracts and drying-rewetting cycles.\u003c/p\u003e\u003cp\u003eWith increasing number of drying-rewetting cycles (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec), τ\u003csub\u003eYP\u003c/sub\u003e progressively shifted in the positive Dim1 direction, which was further confirmed by the permanova in terms of a highly significant polymer-drying-rewetting cycle interaction (p\u0026thinsp;=\u0026thinsp;0.001). Moreover, SI and τ\u003csub\u003emax\u003c/sub\u003e both correlated negatively with C5. SI was highly significant (p\u0026thinsp;=\u0026thinsp;0.001) for both the drying\u0026ndash;rewetting cycles and their interaction with polymer type and soil extract, whereas τ\u003csub\u003emax\u003c/sub\u003e was only significant for the drying\u0026ndash;rewetting drying-rewetting cycles (p\u0026thinsp;=\u0026thinsp;0.002).\u003c/p\u003e\u003cp\u003eConcerning the relevance of demineralized water and the different soil extracts, T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e projected in the positive Dim1/Dim2 quadrant alongside the hydrogels swollen in demineralized water, whereas τ\u003csub\u003eYP\u003c/sub\u003e was negatively correlated. T\u003csub\u003e2peak\u003c/sub\u003e showed modest significance for the interaction between soil extracts and drying-rewetting cycles (p\u0026thinsp;=\u0026thinsp;0.011), and for the three-way interaction among soil extracts, polymer type, and drying-rewetting cycles (p\u0026thinsp;=\u0026thinsp;0.018). T\u003csub\u003e2WL\u003c/sub\u003e exhibited highly significant effects of the soil extract (p\u0026thinsp;=\u0026thinsp;0.001) and its interaction with polymer type (p\u0026thinsp;=\u0026thinsp;0.001), while the interactions between soil extract and drying\u0026ndash;rewetting cycle was only modestly significant (p\u0026thinsp;=\u0026thinsp;0.026). Besides the relatively small effects of dH\u003csub\u003e2\u003c/sub\u003eO on τ\u003csub\u003eYP\u003c/sub\u003e, the interactions between polymer type and soil extracts were significant (p\u0026thinsp;=\u0026thinsp;0.001), except for the three-way interaction between soil extracts, polymer type and drying-rewetting cycles. Concerning the soil extracts, samples swollen in sand correlated negatively with T\u003csub\u003e2WL\u003c/sub\u003e and T\u003csub\u003e2peak\u003c/sub\u003e, whereas only modestly negative correlations with SI and τ\u003csub\u003emax\u003c/sub\u003e were observed when for the loam soil extracts. SI was highly significant for soil extract (p\u0026thinsp;=\u0026thinsp;0.001) and its interactions with polymer type and drying-rewetting cycles, and τ\u003csub\u003emax\u003c/sub\u003e was significant for the soil extract and the polymer type alone (p\u0026thinsp;=\u0026thinsp;0.001).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"4 Discussion","content":"\u003cp\u003eWhen PAA and PAM were freely swollen in soil extracts, both SAP hydrogels exhibited a reduced swelling index (SI) without significant changes in their relaxation time distributions (RTDs). Compared to their initial free swelling in dH\u003csub\u003e2\u003c/sub\u003eO, they also showed polymerspecific increases in structural stability, further visible as more condensed hydrogel network, and uniformly decreased ATR-FTIR band intensities. This effect was more pronounced for both SAPs swollen in loam and clay extracts. On the one hand, the observed changes in the physicochemical hydrogel properties compared to dH\u003csub\u003e2\u003c/sub\u003eO are related to the already well-known crosslinking of polymer chains with dissolved cations that are available in the soil extracts, typically resulting in increased hydrogel network stability as also observed in our study \u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e. The universal attenuation of ATR-FTIR absorbances likely reflects a loss of polymer chain mobility within increasingly compacted junction zones: when multivalent cations coordinate with carboxyl and amide moieties, vibrational freedom across the O\u0026ndash;H, N\u0026ndash;H and C\u0026thinsp;=\u0026thinsp;O groups is restricted, diminishing band intensities. This spectral signature aligns with the smaller pores observed by ESEM and the shorter T₂ relaxation times measured by NMR.\u003c/p\u003e\u003cp\u003eRegarding the soil extract composition, although the sand extract showed a much lower Al\u0026sup3;⁺ concentration than Ca\u0026sup2;⁺ and Mg\u0026sup2;⁺, it nonetheless induced the highest network compaction, confirming Hypothesis 1 that Al\u0026sup3;⁺ mediates stronger ionic crosslinking than divalent cations. By coordinating with three carboxylate groups on adjacent polymer chains, Al\u0026sup3;⁺ forms exceptionally stable bridges \u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e, driving ionotropic hydrogelation \u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e that dominates junction zone architecture. Regarding the divalent cations, Ca\u0026sup2;⁺ still restricts (re)swelling more effectively than Mg\u0026sup2;⁺. Consequently, the physical arrangement and morphology of the junction zones depend on cation concentration \u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e, whereby calcium ions are known to stronger reduce hydrogel (re)swelling than magnesium ions in terms of possible water uptake \u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e, resulting in concentration-dependent denser hydrogel networks as shown in the ESEM images of the FSE.\u003c/p\u003e\u003cp\u003eOn the other hand, PAA gets deprotonated during swelling, which results in an anionic polymer hydrogel \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. As the stability of anionic SAPs typically increases with the amount and charge of available crosslinkers in the surrounding medium \u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e, high Al\u003csup\u003e3+\u003c/sup\u003e and Mn\u003csup\u003e2+\u003c/sup\u003e/Mn\u003csup\u003e4+\u003c/sup\u003e concentrations in the sand likely promoted PAA crosslinking as hypothesized. Furthermore, Amjad \u003csup\u003e[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]\u003c/sup\u003e indicated that PAA stabilizes dissolved Mn\u003csup\u003e2+\u003c/sup\u003e/ Mn\u003csup\u003e4+\u003c/sup\u003e more efficiently than, e.g., Fe\u003csup\u003e2+\u003c/sup\u003e/Fe\u003csup\u003e3+\u003c/sup\u003e, further underlining the possible crosslinking with Mn as well. In contrast, PAM cannot be ionized due to its neutral amide group \u003csup\u003e[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e, resulting in a more stable hydrogel network during swelling together with lower water absorption efficiency than PAA \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. Consistent with this, the ATRFTIR spectra of PAM exhibited only minor shifts in both amide I and II bands and no new peak formation, indicating that network changes arise from mechanical rearrangements rather than new covalent or ionic crosslinks. The minimal ATR-FTIR shifts in PAM aligned with its unaltered RTDs and only slight rheological changes under cyclic stress, indicating an inherently stable and compact network \u003csup\u003e[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e, as further underlined by the obtained ESEM images.\u003c/p\u003e\u003cp\u003eBesides the already mentioned crosslinking in the SAP hydrogels, the observed changes in the investigated physicochemical properties could be due to the intrinsic structure of the dry polymers themselves, as the PAM granules have a relatively low surface area compared to the investigated PAA powder, facilitating the formation of a more stable network. Here, particle-polymer interactions should be considered as e.g. clay particles could have passed the filter membranes during swelling. Especially, PAA powder promotes strong and fast interactions with soil particles like adsorption processes due to a relatively high surface area resulting in a reduced swelling potential \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eRegarding the IE and the impact of successive drying\u0026ndash;rewetting cycles, PAA initially and freely swollen in dH\u003csub\u003e2\u003c/sub\u003eO underwent pronounced physicochemical transformations and developed increasingly solidlike (elastic) behavior, demonstrated progressively lower waterabsorption capacity, exhibited localized network densification in the ESEM images, and showed amplified ATR-FTIR band intensities with each cycle. This was in line with Yunkai et al. \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e8]\u003c/sup\u003e, who reported an increased hydrogel network density due to drying-rewetting events, especially from C5 on. Also, Bai et al. \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e7]\u003c/sup\u003e reported direct relationships between the functional loss of SAPs in ultrapure water with severity of the drying events. Similar ageing effects in the form of a loss of the original hydrogel properties were also observed by Gu et al. \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e6]\u003c/sup\u003e for sodium polyacrylate after 70 d of incubation in deionized solution. Consequently, the increased stability together with decreased T\u003csub\u003e2\u003c/sub\u003e and shifted RTDs, reduced SI and the morphological network changes observed in the ESEM images in our study support the assumption of a significant condensation of the PAA hydrogel network in the course of drying-rewetting, together with the breaking of polymer chains during network condensation and expansion. When polymer chains break and rearrange in the course of expansion and condensation dynamics of the three-dimensional hydrogel network \u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e8]\u003c/sup\u003e, a physical rearrangement of the junction zones occur \u003csup\u003e[8]\u003c/sup\u003e. This structural rearrangement, where polymer chains are packed more closely yet with reduced crosslinking degree, points to an increased vibrational freedom of specific functional groups (O\u0026ndash;H, N\u0026ndash;H, C\u0026thinsp;=\u0026thinsp;O), enhancing dipole moment changes and, thus, absorbance as shown in the ATR-FTIR spectra \u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e9\u0026ndash;51]\u003c/sup\u003e. Contrary to PAA, PAM revealed no significantly changed hydrogel properties when swollen in dH\u003csub\u003e2\u003c/sub\u003eO and subjected to drying-rewetting cycles, as indicated by relatively stable RTDs, SIs and structural stability indices throughout all drying-rewetting cycles. Although the ESEM images showed also condensed parts as for PAA, the PAM hydrogel network was still homogeneous and intact, assuming an overall lower moisture dynamics-induced effect on the physical arrangement of the junction zones \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e6]\u003c/sup\u003e. These differences were further supported by the PCA results as the two SAPs clearly occupied distinct regions in the score plot, with PAA clustering in the positive direction of Dim1 - associated with higher structural rigidity and swelling loss - while PAM aligned negatively, reflecting its more stable physicochemical profile throughout the drying-rewetting cycles. These differences underscored the fundamental chemical differences between the two SAPs: when PAA hydrogel deprotonates during swelling, it results in a concentration gradient between the hydrogel network and dH\u003csub\u003e2\u003c/sub\u003eO. However, the neutral amid group of PAM cannot be ionized \u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e4]\u003c/sup\u003e and thus remains stable during swelling, causing a relatively lower water absorption efficiency compared to PAA \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e8]\u003c/sup\u003e. This, in turn likely promoted the formation of an already intrinsically more stable hydrogel network with smaller pores directly after its initial swelling \u003csup\u003e[45]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe accompanied lower structural stability, swelling pressure, and volumetric expansion potential of PAM compared to PAA further restricted water uptake and resulted in a lower SI, same RTDs and similar hydrogel network as in the initial swelling, likely reflecting inherent hydrocolloid architectural differences that govern junction zone formation within the three-dimensional hydrogel network \u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. Therefore, the drying-related condensation effects were relatively lower as less water was initially absorbed by the PAM hydrogel network to cause strong swelling-shrinkage dynamics and subsequent fracture formation upon drying and rewetting. Furthermore, the overall expansion and swelling pressure of the polymer chains during reswelling was further decreased by the relatively smaller pores of the PAM hydrogel and an already more stable network after the initial swelling compared to PAA \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e, which could be due to rearranged junction zones holding less water in the interstices of the hydrogel network.\u003c/p\u003e\u003cp\u003eConcerning the effect of the different soil extracts and their interplay with the drying-rewetting cycles, both the PCA results and the permanova findings confirmed their significant interactions. In detail, PAA swollen in sand extract showed one shift towards lower RTDs for all drying-rewetting cycles together with a decrease of SI, while structural stability increased together with hydrogel network density and polymer chains braking. This was in line with Buchmann et al. \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, who showed that hydrogen ion release through the dissociated of the carboxylic group can gradually limit PAA crosslinking during its swelling together with an acidification of the surrounding solution, corresponding to the decrease in pH after PAA reswelling. Nonetheless, the authors found this effect only in sand while it disappeared in loam after one week due to its higher buffer capacity. In contrast to Buchmann et al. \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, who observed variable pH shifts during drying\u0026ndash;rewetting in PAA-treated soils, reflecting the heterogeneous buffering capacity of soil matrices, we recorded an uniform pH decline across all drying-rewetting cycles. This discrepancy likely results from the use of soil extracts or soil solutions instead of bulk soil, which typically lack the complex, insitu buffering mechanisms of soil as a whole (e.g., cation exchange, OM interactions) \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eInterestingly and in contrast to PAA, PAM only showed shifts in the RTDs and T\u003csub\u003e2\u003c/sub\u003e, whereas its structural stability remained stable over the drying-rewetting cycles and hydrogel network was only more condensed with broken polymer chains when swollen and incubated in the sand extract. This may reflect either the intrinsically higher stability of the hydrogel network following initial swelling, as previously noted \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e, or a predominance of polymer chaincondensation over crosslinking in the sand extract. As the neutral amide groups of PAM cannot be protonated, fewer ionic crosslinks form in sand than in loam or clay extracts, favoring network densification over new crosslink formation. Moreover PAM granules with their relatively lower surface area can effectively counteract both the suction tension and the confining pressure of the soil matrix \u003csup\u003e[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]\u003c/sup\u003e, which resulted relatively unchanged properties of PAM compared to the initial free swelling in dH\u003csub\u003e2\u003c/sub\u003eO.\u003c/p\u003e\u003cp\u003eFor PAA swollen in loam and clay extract, the RTDs, T\u003csub\u003e2\u003c/sub\u003e, and SI decreased together with an increased structural stability as function of the drying-rewetting cycles, as also reflected in the PCA in terms of samples clustered towards the respective indices indicating increased hydrogel network densification and aging. This is consistent with the hypothesized role of multivalent cations \u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e and in line with Buchmann et al. \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, who reported reduced PAA swelling in loam with a complete functional loss over time. Several studies have already shown that composition of the soil solution significantly determines both the swelling potential and the resulting physicochemical properties of PAA \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. Here, ionotropic hydrogelation via cation-mediated crosslinking represents one mechanism influencing the physical arrangement of junction zones \u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e. In this regard, the large and condensed areas for PAA suggested a physical rearrangement of the junction zones based on the soil extract composition \u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e, and ionotropic hydrogelation dynamics \u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn line with these morphological changes, ATR-FTIR spectra of PAA revealed a distinct progression in functional group intensities, particularly in O\u0026ndash;H and C\u0026thinsp;=\u0026thinsp;O bands. The initial increase and subsequent decline of O\u0026ndash;H absorption suggest water losses and reduced hydrogen bonding, while increased C\u0026thinsp;=\u0026thinsp;O signals may point toward enhanced carboxylate interactions or ion-mediated crosslinking \u003csup\u003e[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]\u003c/sup\u003e. These observed spectral shifts support our hypothesis of a progressive hydrogel network densification with the formation of additional intramolecular and intermolecular crosslinks and thus stronger network structures \u003csup\u003e[\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e. Interestingly, PAA showed a band at \u0026lt;\u0026thinsp;1,000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which increased especially in the soil extracts. This might underline the explained crosslinking processes, corresponding to the findings of Weng et al. \u003csup\u003e[\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]\u003c/sup\u003e, who showed a similar increase with higher crosslinking in PAA. Thus, the intensified signal at \u0026lt;\u0026thinsp;1,000 cm⁻\u0026sup1; could serve as a marker of network compaction with ionic and covalent crosslinks drawing polymer segments into tighter conformations and amplifying skeletal deformation modes in the ATR-FTIR spectrum comparable to the C\u0026ndash;H\u003csub\u003e2n\u0026thinsp;\u0026gt;\u0026thinsp;3\u003c/sub\u003e band at 730\u0026thinsp;\u0026minus;\u0026thinsp;720 cm\u003csup\u003e\u0026minus;\u0026thinsp;1 [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e. Furthermore, Dong et al. \u003csup\u003e[\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]\u003c/sup\u003e also described this band for PAA as a result from widely distributed backbone internal rotation angles, including both helical structure and planar zig-zag conformation over short segments in the solid state of PAA \u003csup\u003e[\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]\u003c/sup\u003e. Although some studies used FT-Raman, the wavenumbers were comparable without any substantial differences to ATR-FTIR \u003csup\u003e[\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]\u003c/sup\u003e. Moreover, polymer chains might have become more attached during hydrogel network condensation, reinforcing each other and therefore increasing the intensity at \u0026lt;\u0026thinsp;1,000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eAlternatively, crosslinking predominantly taking place in soil extracts may have forced polymer chain narrowing to such an extent that C\u0026ndash;H vibrational modes in the 900\u0026ndash;670 cm⁻\u0026sup1; region were intensified, much like the outofplane bending of aromatic C\u0026ndash;H bonds \u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e7]\u003c/sup\u003e. This suggests that the compacted network induced vibrational coupling comparable to aromatic structures. In contrast, PAM revealed less obvious variations in N\u0026ndash;H and C\u0026ndash;H band intensities, supporting a more chemically stable network structure. With successive drying\u0026ndash;rewetting, the amide I band broadened by up to 0 %, indicating mechanical chain scission and increased conformational disorder, while the amide II band remained unchanged, confirming backbone integrity despite network stress. However, the absence of pronounced spectral changes across the drying-rewetting cycles indicates lower susceptibility to hydrolysis, chain scission, or condensation reactions. This reinforces the relative resilience of PAM under dynamic moisture conditions with a preserved backbone integrity \u003csup\u003e18,45]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eBesides chemical aging or changing crosslinking degree within the two hydrogels, particle-polymer interactions need to be considered, since, e.g., clay particles could have passed the filter membranes during the drying-rewetting events and interacted with the polymer chains. On the one hand, especially the acidic nature of PAA could have promoted interactions with clay particle surfaces and thereby mobilize exchangeable cations \u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e. On the other hand, clay particles are generally known to limit SAP hydrogel swelling in soil by mutual swelling restriction caused during water competition \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. However, both investigated SAPs were able to (re)swell without restrictions in our experiment, resulting in a homogeneous hydrogel network with the maximum water absorbed into the junction zones compared to their (re)swelling directly in the soil interparticle space \u003csup\u003e[\u003cspan additionalcitationids=\"CR60\" citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]\u003c/sup\u003e. Consequently, clay-induced PAA swelling restriction were most likely lower in the soil extract than in the soil interparticle space due to 1) dilution effects since only 1:5 soil extracts were used and 2) the missing confining pressure and mutual swelling competition by the soil matrix \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eAlthough this study revealed fist important insights into the physicochemical aging of two commonly used SAPs as function of drying-rewetting events and soil extract composition, several limitations must be considered: first, we only investigated one specific SAP application way in terms of point application, including a first swelling in dH\u003csub\u003e2\u003c/sub\u003eO to ensure full swelling and homogeneous hydrogel network formation without soil extract-mediated crosslinking for the first swelling. This resulted in hydrogel networks with different morphological and intrinsic properties than for those directly added as dry polymers and thus swollen in the soil interparticle space \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. Thus, soil matrix-related crosslinking, counterpressure and mutual water competition taking place directly during SAP swelling will lead to different intrinsic physicochemical hydrogel properties and should therefore be investigated further \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eSecond, we used 1:5 soil extracts to track changes of the SAP hydrogels. Although soil extracts offer a controlled medium for assessing SAP swelling, the subsequent dilution can alter cation concentration, speciation, and pH, while excluding solid phases that typically act as adsorption sites, mechanical constraints, pore structure, and microbial or redox dynamics. Consequently, extract-based assays may overestimate SAP swelling and distort the strength and type of aging, which requires more targeted in-situ experiments \u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThird, high molecular weight polymers as used in this study are known to maximize soil-polymer interactions \u003csup\u003e[\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e]\u003c/sup\u003e and increase hydrogel crosslinking degree \u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e, which could have influenced the results as well. These limitations also underline the relevance of in-situ studies using native soil matrices and further commercially applied SAP products to verify the environmental relevance of the observed aging effects. Moreover, the SAP application way seems to play an important role on the density of the organ-mineral complexes \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e, as the point application used in this study has a larger surface and therefore a larger soil interface than the application way used in Buchmann et al. \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. Future experiments should also explore varied application techniques and simulate more realistic soil moisture regimes.\u003c/p\u003e\u003cp\u003eRegarding the question of whether SAPs in soil can form plastic-like residues \u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e, our study indicated an increased structural stability of the hydrogels when subjected to drying-rewetting events as function of the number of cycles and soil extract composition. From current research, SAPs are already known to form large stable aggregates and dense organo-mineral complexes during drying, making cemented membranous polymer structures occupying large parts of the interparticle space \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. On the one hand, the changes observed in the ATR-FTIR spectra, such as the decreasing O\u0026ndash;H signals and the shifts in carbonyl band intensities suggest that repeated drying-rewetting events induced not only physical network densification but also chemical alterations. Such transformations could contribute to the formation of more persistent hydrogel residues with plastic-like characteristics. On the other hand, ESEM images indicated that repeated drying\u0026ndash;rewetting cycles can irreversibly densify and restructure the SAP hydrogel networks, most notably in PAA, leading to diminished rehydration capacity, compromised longterm performance in soils, and the potential for the formation of persistent residues. Yet, while this structural consolidation and reduced swelling suggest lower degradability, definitive chemical evidence of plasticization, such as novel carbon\u0026ndash;carbon bonding motifs or enhanced resistance to microbial breakdown, has not been demonstrated. To fully characterize these aging processes, future investigations should employ solidstate NMR and highresolution mass spectrometry to resolve specific bondlevel alterations \u003csup\u003e[\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u0026ndash;68]\u003c/sup\u003e. Moreover thermodynamic analysis in terms of the thermal stability and decomposition behaviors of the SAP hydrogels themselves and in soil seem straight-forward \u003csup\u003e[\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u0026ndash;71]\u003c/sup\u003e.\u003c/p\u003e"},{"header":"5 Conclusion","content":"\u003cp\u003eThis study provides important insights into the physicochemical transformation of two widely used synthetic superabsorbent polymers (SAPs), polyacrylic acid (PAA) and polyacrylamide (PAM), under repeated drying\u0026ndash;rewetting cycles and varying soil extract compositions. SAP aging was driven by an interplay of physical densification, ion-mediated crosslinking, and chemical modifications of the hydrogel network. For PAA, these drying-rewetting cycles reduced swelling capacity and water mobility, increased mechanical rigidity, and triggered the formation of condensed structures. ATR-FTIR spectral changes, particularly reduced OH and shifting carbonyl bands, suggested progressive crosslinking and structural fixation, especially for cation-rich soil extracts. In contrast, PAM remained comparatively stable, showing minor structural and chemical changes. Its neutral, dense hydrogel network appeared less responsive to environmental stressors. PCA results supported these differences, showing clear separation between polymer types along dimensions associated with aging and network rigidity.\u003c/p\u003e\u003cp\u003eAll in all, the findings indicate that drying-rewetting cycles can cause irreversible structural and chemical changes in SAPs, particularly for anionic PAA, reducing rehydration potential and promoting the potential formation of persistent residues. While ESEM and ATR-FTIR support this interpretation, direct evidence of plasticization, such as covalent bond changes or biodegradation resistance, remains lacking. Thus, future studies should employ advanced methods such as solid-state NMR or high-resolution mass spectrometry to further elucidate aging pathways. Further, several key limitations, including the use of pre-swollen SAPs, diluted soil extracts lacking solid-phase and microbial interactions, and high molecular weight polymers not necessarily representing commercial products, need to be considered. To assess environmental relevance, future research must prioritize in-situ experiments under realistic field conditions and explore diverse SAP types and deployment strategies.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFinancial support:\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis research was financially supported by the Deutsche Forschungsgemeinschaft (Grant No. BU 3763/1\u0026ndash;1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe kindly thank Zacharias Steinmetz for his input on ATR-FTIR and statistical analysis, Anna Baskal for helping with the ICP measurements, and Gabriele E. Schaumann and Mathilde Knott for their valuable feedback on the results.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability:\u003c/strong\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe data that supports the findings of this study are available from the corresponding author upon reasonable request.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contribution\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization: J.N. and C.B.; Methodology and experimental setup: J.N. and C.B.; Material preparation and data collection: J.N.; Data evaluation and interpretation: J.N. and C.B., Writing-review and editing: J.N. and C.B.; Funding acquisition: C.B.; Project management: C.B.; Supervision: C.B. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was financially supported by the Deutsche Forschungsgemeinschaft (Grant No. BU 3763/1-1)\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eSaha, A., Sekharan, S. \u0026amp; Manna, U. 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Soil Sci.\u003c/em\u003e \u003cstrong\u003e71\u003c/strong\u003e, 415\u0026ndash;419 (2020).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Polyacrylic acid, polyacrylamide, drying-rewetting cycles, soil extracts, hydrogel aging","lastPublishedDoi":"10.21203/rs.3.rs-7155517/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7155517/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePolyacrylic acid (PAA) and polyacrylamide (PAM), two synthetic superabsorbent polymers (SAPs) commonly used in agriculture, can form three‑dimensional hydrogels that enhance soil water retention and soil structural stability. Yet, their potential aging and transformation under natural drying\u0026ndash;rewetting dynamics and in contact with soil solutes remains unclear.\u003c/p\u003e\u003cp\u003eIn this study, we examined the effect of soil extracts from sand, loam, and clay soil in both a 72 h free swelling experiment (FSE) and in an incubation experiment (IE), where PAA and PAM hydrogels underwent ten successive drying-rewetting cycles. Samples were taken after cycles 0, 3, 5, and 10 and investigated for their swelling index (SI), water entrapment (\u003csup\u003e1\u003c/sup\u003eH proton nuclear magnetic resonance relaxometry), structural stability (rheometry), morphology (scanning electron microscopy), and surface chemistry (fourier transform infrared spectroscopy).\u003c/p\u003e\u003cp\u003eIn the FSE, PAA swelling in all soil extracts reduced SI, shortened T₂ relaxation, and increased mechanical rigidity, whereas PAM properties remained stable except when swollen in sand extract. During the IE, PAA exhibited progressive hydrogel network densification, further SI loss, T\u003csub\u003e2WL\u003c/sub\u003e shortening, band intensity shifts, and microstructural compaction, whereas PAM remained largely inert. Multivariate analysis confirmed that polymer identity and its interaction with the conducted drying-rewetting cycles and soil extract composition drove SAP aging. Overall, drying\u0026ndash;rewetting cycles seem to induce irreversible chemo-structural alterations in anionic PAA, thereby diminishing its rehydration potential and triggering the potential formation of persistent, solid-like residues, whereas neutral PAM seems more resilient under dynamic environmental conditions.\u003c/p\u003e","manuscriptTitle":"The contribution of soil extract composition and cyclic moisture dynamics to the physicochemical aging of superabsorbent polyacrylic acid and polyacrylamide hydrogels","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-25 11:30:32","doi":"10.21203/rs.3.rs-7155517/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-09-09T12:07:14+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-26T14:16:06+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-26T13:41:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"112312133945491045473644660477532059926","date":"2025-08-23T14:03:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"124428783348447726719870609128737456025","date":"2025-08-21T23:28:58+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"335229328681420617267576235149882100409","date":"2025-07-28T12:34:10+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-07-23T03:56:28+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-07-23T03:08:12+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-21T10:17:40+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-07-21T03:54:54+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-07-18T08:18:53+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"1d034a34-3993-4f1b-b4e3-657f970d8410","owner":[],"postedDate":"July 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":51966373,"name":"Physical sciences/Chemistry"},{"id":51966374,"name":"Physical sciences/Engineering"},{"id":51966375,"name":"Earth and environmental sciences/Environmental sciences"},{"id":51966376,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-05-12T06:09:47+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-25 11:30:32","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7155517","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7155517","identity":"rs-7155517","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}