Salt-Resistant Swelling Behavior and Methylene Blue Adsorption Performance of Chitosan-Modified Poly(acrylic acid-acrylamide-sodium styrene sulfonate) Superabsorbent Resin

Salt-Resistant Swelling Behavior and Methylene Blue Adsorption Performance of Chitosan-Modified Poly(acrylic acid-acrylamide-sodium styrene sulfonate) Superabsorbent Resin | 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 Research Article Salt-Resistant Swelling Behavior and Methylene Blue Adsorption Performance of Chitosan-Modified Poly(acrylic acid-acrylamide-sodium styrene sulfonate) Superabsorbent Resin Xiaoxuan Jv, Xiangchi Liu, Ailing Liu, Baijun Liu, Mingyao Zhang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8930030/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 9 You are reading this latest preprint version Abstract In response to the escalating global water pollution problem, a three-dimensional crosslinked poly(acrylic acid-acrylamide-sodium styrene sulfonate)/chitosan (P(AA-AM-SSS)/CTS) composite superabsorbent resin was successfully synthesized via inverse suspension polymerization. A series of systematic experiments were performed to elucidate the effects of initiator dosage, crosslinker content, and monomer composition on the network structure, swelling behavior, and adsorption performance. Through controlled optimization of the polymerization conditions, a structurally stable three-dimensional polymer network was established. Under optimized conditions, the composite resin exhibited a high swelling capacity of 1233.7 g/g in deionized water and 137.3 g/g in saline solution, demonstrating effective enhancement of both water absorbency and salt tolerance. Meanwhile, the material maintained favorable water retention at 60°C, achieving a water retention rate of approximately 85%, and displays outstanding reswelling capability. Thermogravimetric analysis revealed a residual mass of 42.9% at 600°C, indicating enhanced thermal stability. In addition, adsorption experiments toward the cationic dye methylene blue were systematically conducted under varying adsorbent dosage, contact time, temperature, and initial dye concentration. The optimized composite achieved a maximum adsorption capacity of 874.6 mg/g under the studied conditions. Therefore, the incorporation of sulfonic acid groups and chitosan-based structural regulation into the superabsorbent resin network enables the simultaneous improvement of water absorbency, salt tolerance, and dye adsorption performance, indicating its potential applicability in wastewater treatment. Superabsorbent polymer Sodium styrene sulfonate Chitosan Salt-resistant swelling Methylene blue adsorption Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. INTRODUCTION With the rapid advancement of industrialization, the global annual production of dyes reaches several hundred thousand tons, of which a considerable fraction is discharged as wastewater [ 1 , 2 ]. Dye molecules are typically characterized by pronounced biological toxicity, posing severe threats to aquatic ecosystems and impacting human health [ 3 – 5 ]. Consequently, the development of efficient and economically viable materials for dye wastewater treatment has become a sustained focus of research efforts. To date, a variety of technologies have been explored for dye removal, including adsorption [ 6 , 7 ], photocatalytic degradation [ 8 , 9 ], chemical oxidation [ 10 ], and electrochemical treatment [ 11 ]. Among these approaches, adsorption is widely regarded as one of the most promising strategies owing to its relatively low operational cost and process simplicity [ 12 ]. However, many existing adsorbents suffer from high production costs or limited scalability, which hampers their practical implementation [ 13 ]. Therefore, the design of low-cost, facilely fabricated adsorbents with high removal efficiency remains a critical objective in the field. Superabsorbent polymers (SAPs) are a class of advanced functional macromaterials featuring three-dimensional crosslinked network structures [ 14 , 15 ]. Benefiting from their pronounced swelling capabilities, superabsorbent polymers have found broad applications in wastewater treatment and soil moisture conservation [ 16 ]. Despite this exceptional swelling capacity, the water absorbency of conventional SAPs deteriorates sharply in saline environments. This behavior mainly arises from the electrostatic screening of fixed charges on the polymer chains by dissolved ions, which weakens the electrostatic repulsion between polymer segments, reduces the osmotic pressure difference between the interior and exterior of the polymer network, and induces network contraction. Consequently, achieving a balance between ultrahigh water absorbency and enhanced salt tolerance, while simultaneously endowing SAPs with efficient pollutant adsorption capability, remains a critical engineering and materials-design challenge for their practical application in water treatment [ 17 , 18 ]. Chitosan is a naturally derived polymer that has attracted extensive attention owing to its nontoxicity, renewability, abundant amino functionalities, and excellent biocompatibility. In particular, chitosan has shown considerable promise in water treatment applications, as the plentiful amino and hydroxyl groups along its molecular backbone can serve as effective adsorption sites for dye molecules [ 19 , 20 ]. In recent years, chitosan-based adsorbent materials have been increasingly explored for dye wastewater remediation. For example, Ren et al. exploited the environmental friendliness and adsorption capability of chitosan to fabricate a porous gelatin/chitosan composite, which exhibited maximum adsorption capacities of 294.1 mg/g for methyl orange, 221.2 mg/g for Congo red, and 209.2 mg/g for methylene blue [ 21 ]. In addition, Nakhjiri et al. prepared an N-maleoylated chitosan-based adsorbent, achieving an adsorption capacity of 66.89 mg/g for methylene blue [ 22 ]. Collectively, these studies indicate that the abundant amino and hydroxyl groups of chitosan play a key role in governing dye-adsorbent interfacial interactions, thereby enhancing adsorption performance. However, the adsorption capacity and structural stability of conventional chitosan-based materials remain insufficient for practical deployment in complex wastewater systems, underscoring the need for further molecular-level design and functional integration. Nevertheless, conventional chitosan-based superabsorbent resins generally suffer from insufficient salt tolerance, which severely limits their performance in ionic environments. To address this limitation, sodium styrene sulfonate was introduced in this study as a copolymerizable monomer. Owing to the presence of strongly polar sulfonic acid groups, the incorporation of this monomer markedly enhances the affinity of the polymer network toward water molecules, thereby improving the overall swelling capacity of the material [ 23 , 24 ]. More importantly, upon ionization in aqueous media, the sulfonic groups generate a high density of negatively charged sites, which not only strengthens electrostatic interactions with cationic dye molecules but also mitigates the charge-screening effect of inorganic salts on hydrophilic functional groups [ 25 , 26 ]. As a result, the superabsorbent resin exhibits improved swelling performance under saline conditions. Previous studies have demonstrated that the introduction of sulfonic acid-containing functional monomers is an effective strategy for enhancing both adsorption performance and swelling behavior in high-ionic-strength solutions [ 27 , 28 ]. For instance, Li et al. reported a lignosulfonate-tannin composite adsorbent with a saturated adsorption capacity of 629.5 mg/g for methylene blue [ 29 ]. Guo et al. incorporated 2-acrylamido-2-methylpropane sulfonic acid into a superabsorbent composite, achieving a maximum swelling ratio of 119% in saline water [ 30 ]. Collectively, these findings underscore the pivotal role of sulfonic functionalities in improving the swelling and adsorption performance of superabsorbent resins under high-salinity conditions [ 31 ]. Although chitosan-modified SAPs and sulfonated polymer systems have been widely investigated, most reported studies focus primarily on either swelling enhancement or adsorption improvement. The simultaneous integration of salt resistance, water retention stability, and efficient cationic dye adsorption within a single crosslinked SAP network remains insufficiently explored. In this study, a series of three-dimensional P(AA-AM-SSS)/CTS composite superabsorbent resins were constructed via inverse suspension polymerization technology using acrylic acid (AA), acrylamide (AM), chitosan (CTS), and sodium styrene sulfonate (SSS) as monomers. The effects of monomer composition, crosslinker dosage, and initiator content on the swelling and water retention behaviors of the composite resin were systematically investigated. On this basis, adsorption experiments toward the cationic dye methylene blue were conducted, and the adsorption behavior and underlying mechanisms were elucidated through kinetic and isotherm analyses, suggesting its potential applicability in dye wastewater treatment. 2. EXPERIMENTS 2.1. Materials All chemicals used in this study were of commercial grade and were used as received without further purification. Acrylic acid (AA, AR), acrylamide (AM, AR), sodium hydroxide (NaOH, AR), potassium persulfate (KPS, AR), acetic acid (AR), and sodium chloride (NaCl, AR) were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Span-80 (CP) was supplied by Kelong Chemical Co., Ltd. (Chengdu, China). Absolute ethanol (EtOH) was obtained from Tianjin Fuyu Fine Chemical Co., Ltd. Cyclohexane (AR), chitosan (CTS, Product code: C804728), N, N′-methylenebisacrylamide (MBA, AR), and sodium styrene sulfonate (SSS, AR) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Methylene blue (MB) was supplied by Tianjin Dengfeng Chemical Reagent Factory. Deionized water (DDI) was prepared in the laboratory and used throughout the experiments. 2.2. Characterization The chemical structure and functional groups of the resin were characterized by Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50). The surface morphology and microstructure were examined using scanning electron microscopy (SEM, Phenom Pro). The crystalline properties were analyzed by X-ray diffraction (XRD, Bruker D8 Advance). Thermal stability was evaluated by thermogravimetric analysis (TGA, PerkinElmer). The concentrations of methylene blue were determined using an ultraviolet-visible spectrophotometer (UV-vis, Cary5000), and the adsorption capacity was calculated accordingly. 2.3. Swelling Capacity Test A dried sample of the superabsorbent resin (M₀ = 0.3 g) was placed in a 500 mL beaker containing deionized water and left to swell for several hours until equilibrium was reached. Afterward, excess water was removed by filtration, and the swollen resin was weighed (M₁, g). The water absorption capacity (Q₁, g/g) was calculated using Eq. ( 1 ), where Q₁ represents the grams of water absorbed per gram of the sample [ 32 ]. $$\:{\text{Q}}_{\text{1}}\left(\text{g}\text{/}\text{g}\right)\text{=}\frac{{\text{M}}_{\text{1}}\text{-}{\text{M}}_{\text{0}}}{{\text{M}}_{\text{0}}}$$ 1 2.4. Salt Absorption Test A 0.9wt% sodium chloride solution was prepared. A dried sample of the superabsorbent resin (M₂ = 0.1 g) was placed in a beaker containing the sodium chloride solution and allowed to swell for several hours until equilibrium was reached. Excess saline solution was removed by filtration, and the swollen resin was weighed (M₃, g). The salt absorption capacity (Q₂, g/g) was calculated using Eq. ( 2 ), where Q₂ represents the grams of saline absorbed per gram of the sample. $$\:{\text{Q}}_{\text{2}}\left(\text{g/g}\right)\text{=}\frac{{\text{M}}_{\text{3}}\text{-}{\text{M}}_{\text{2}}}{{\text{M}}_{\text{2}}}$$ 2 2.5. Re-swelling Performance Test A dried resin sample (0.3 g) was placed in a 500 mL beaker containing deionized water and allowed to swell at room temperature until equilibrium was reached. After removing excess water by filtration, the swollen sample was weighed and then dried in an oven to constant weight. The dried resin was subsequently re-immersed in the same volume of deionized water to achieve re-swelling. This swelling-drying process was repeated for multiple cycles. The water absorption capacity of each cycle was calculated using Eq. ( 1 ) to evaluate the re-swelling performance of the resin. 2.6. Water Retention Test The fully swollen resin (W 0 ) was placed in a constant-temperature oven at 20, 40, and 60°C, respectively. At predetermined time intervals, the samples were removed, excess surface water was eliminated, and the remaining mass (W 1 ) was recorded. The water retention ratio (W t , %) was calculated using Eq. ( 3 ), where W t represents the percentage of retained water relative to the initial swollen mass at time t [ 32 ]. 3 2.7. Adsorption Experiments Methylene blue was selected as a representative cationic dye model molecule to evaluate the adsorption behavior. Adsorption was conducted under limited swelling conditions. Adsorption experiments were conducted to evaluate the dye adsorption capacity of the resin. Different amounts of the superabsorbent resin (0.01–0.05 g) were added to methylene blue solutions with a fixed initial concentration at a controlled temperature and agitated using a magnetic stirrer until adsorption equilibrium was reached. The supernatant was collected, and the residual dye concentration was determined by UV-vis spectroscopy. The adsorption capacity was calculated using Eq. ( 4 ) [ 33 ]. $$\:\text{q}\left(\text{mg}\text{/}\text{g}\right)\text{=}\frac{\left({\text{C}}_{\text{0}}\text{-}{\text{C}}_{\text{t}}\right)\text{V}}{\text{m}}$$ 4 where C 0 (mg/L) is the initial dye concentration, C t (mg/L) is the dye concentration at time t, V (L) is the volume of the dye solution, and m (g) is the mass of the resin. 3. RESULTS AND DISCUSSION 3.1. Synthesis of P(AA-AM-SSS)/CTS Superabsorbent Resin The preparation procedure of the superabsorbent resin is illustrated in Fig. 1 . Chitosan was first dissolved in a 2wt% aqueous acetic acid solution under stirring at 50°C until a transparent solution was obtained. Subsequently, a mixed monomer solution containing acrylic acid (AA), acrylamide (AM), and sodium styrene sulfonate (SSS), together with potassium persulfate (KPS, 0.65wt%) as the initiator and N, N′-methylenebisacrylamide (MBA, 0.1wt%) as the crosslinker, was added to form the aqueous phase. The resulting aqueous phase was then introduced into cyclohexane containing Span-80 as a suspension stabilizer (oil-to-water ratio = 3:1) to obtain a uniform suspension system. The polymerization was carried out under a nitrogen atmosphere at 68°C with increased stirring speed for 2.5 h. After completion of the reaction, the product was washed with ethanol, dried, and ground to obtain the final P(AA-AM-SSS)/CTS composite superabsorbent resin. Figure 2 illustrates the swelling mechanism of the P(AA-AM-SSS)/CTS superabsorbent resin. Upon immersion in water, water molecules diffuse into the resin through hydrogen bonding interactions with hydrophilic functional groups. The ionization of carboxyl and sulfonate groups generates negatively charged sites along the polymer chains, inducing electrostatic repulsion that promotes network expansion. Meanwhile, the accumulation of counterions within the polymer network establishes an osmotic pressure gradient between the interior and exterior of the resin, driving further water uptake. The presence of crosslinking points restricts excessive chain extension, and the swelling process reaches equilibrium when the osmotic pressure is balanced by the elastic retractive force of the crosslinked network [ 34 ] 3.2. Characterization of P(AA-AM-SSS)/CTS Superabsorbent Resin Figure 3 systematically illustrates the effects of synthesis parameters, including initiator dosage, crosslinker content, and monomer composition, on the water absorption and salt resistance of the P(AA-AM-SSS)/CTS superabsorbent resin. In this study, the equilibrium swelling capacity is evaluated to compare the effectiveness of structural optimization. As shown in Fig. 3 A, the water absorption capacity of the resin initially increased and then decreased with increasing potassium persulfate (KPS) content. The maximum water absorption capacity (1233.33 g/g) and salt absorption capacity (135.67 g/g) were achieved at a KPS dosage of 0.65wt%. KPS thermally decomposes to generate sulfate radical species, which initiate the free-radical polymerization of the monomers. At low initiator concentrations, the number of active radical sites is insufficient, resulting in a reduced polymerization rate and an incompletely developed network structure, thereby limiting resin swelling. In contrast, excessive initiator dosage leads to an increased density of active centers and accelerated polymerization, which promotes the formation of an overly compact crosslinked network. This dense structure restricts polymer chain mobility and water diffusion, ultimately suppressing the swelling capacity [ 35 ]. Figure 3 B illustrates the effect of crosslinker (MBA) content on the swelling behavior of the resin. A similar trend was observed, with the swelling capacity first increasing and then decreasing as the MBA dosage increased. Insufficient crosslinker content resulted in the formation of partially linear or soluble polymer chains, whereas an optimal MBA content of 0.1wt% produced a well-developed three-dimensional network with maximal swelling performance. Further increasing the crosslinker content led to excessive crosslinking density, which significantly constrained network expansion and hindered the diffusion of water and saline into the polymer matrix, thereby reducing both water and salt absorption capacities [ 36 ]. The influence of chitosan (CTS) content on the swelling and salt resistance of the composite resin is presented in Fig. 3 C. Both water and salt absorption capacities increased initially with increasing CTS content and reached a maximum at 4.5wt%, followed by a decline at higher CTS loadings. Moderate incorporation of CTS provided effective grafting backbones, enabling enhanced monomer grafting and expansion of the polymer network. In addition, the abundant amino and hydroxyl groups in CTS contributed additional hydrophilic sites, further improving water uptake [ 37 ]. However, excessive CTS content (6wt%) markedly increased the viscosity of the reaction system, which impeded mass transfer, reduced polymerization efficiency, and restricted segmental chain motion, ultimately leading to decreased swelling and salt absorption performance. As shown in Fig. 3 D, the incorporation of sodium styrene sulfonate (SSS) significantly enhanced the swelling behavior of the resin. The optimal SSS content was 10wt%, at which the highest water absorption capacity was achieved. This enhancement can be attributed to the introduction of highly hydrophilic sulfonic acid groups (-SO₃⁻), which enhance the hydration capacity of the polymer chains and help mitigate the electrostatic charge-screening effect induced by salt ions in solution, thereby contributing to improved salt resistance. Nevertheless, further increasing the SSS content to 14wt% resulted in a decline in swelling performance. Excessive sulfonic group density intensified intermolecular interactions and electrostatic repulsion, which restricted polymer chain mobility and limited further network expansion, thereby reducing both water and salt absorption capacities [ 38 ]. Compared with some previously reported superabsorbent resins [ 39 – 41 ], the P(AA-AM-SSS)/CTS composite resin exhibits relatively high swelling capacities in both deionized water and saline solutions. Notably, the resin maintains favorable water absorbency even under saline conditions, indicating that the incorporation of sodium styrene sulfonate and chitosan effectively enhances the salt tolerance of the polymer system. Such improved swelling behavior provides a favorable structural basis for adsorption of dye molecules. The re-swelling performance of the composite resin was evaluated to assess its cyclic stability and reusability. As shown in Fig. 4 A, the water absorption capacity gradually decreased with increasing swelling-drying cycles. This decline is primarily attributed to partial disruption of physical crosslinking points within the polymer network during repeated swelling and dehydration processes, leading to structural fatigue and the leaching of low-molecular-weight compounds. Nevertheless, after five cycles, the resin retained approximately 76% of its initial swelling capacity, indicating favorable re-swelling behavior and good recycling potential. The enhanced cyclic stability of the composite resin can be attributed to the synergistic effects of chitosan and sodium styrene sulfonate. The rigid backbone of chitosan reinforces the mechanical strength and toughness of the crosslinked network, thereby mitigating structural degradation during repeated cycles. Meanwhile, the chemically stable sulfonate groups (-SO₃⁻) contribute to maintaining network integrity under varying environmental conditions, effectively reducing adverse structural changes induced by medium fluctuations. As a result, the P(AA-AM-SSS)/CTS resin exhibits improved structural robustness and sustained swelling performance during repeated use [ 42 ]. In practical applications, superabsorbent resins are required not only to exhibit high equilibrium swelling capacity but also to maintain effective water retention under varying thermal conditions. The water retention behavior of the fully swollen composite resin was therefore evaluated at 20, 40, and 60°C, and the results are presented in Fig. 4 B. As expected, the water retention ratio gradually decreased with increasing temperature and exposure time. At 20°C, the resin showed a relatively slow water loss rate, indicating favorable water retention performance under ambient conditions. Notably, even at elevated temperature, the composite resin maintained relatively high water retention, retaining approximately 85% of its absorbed water after 6 h at 60°C. The enhanced water retention performance can be attributed to the combined effects of chitosan incorporation and sodium styrene sulfonate introduction. The rigid backbone and multichain structure of chitosan reinforce the crosslinked network, resulting in a reduced diffusion rate of water molecules within the polymer matrix and suppressing rapid water release after equilibrium swelling [ 43 ]. In addition, the strong hydrophilicity of sodium styrene sulfonate and the electrostatic repulsion between ionized sulfonate and carboxylate groups help maintain an expanded network conformation, thereby strengthening water-polymer interactions and further improving the overall water retention capacity of the composite resin [ 44 ]. Figure 5 A presents the Fourier transform infrared (FT-IR) spectra of CTS and the P(AA-AM-SSS)/CTS superabsorbent resin. In the FT-IR spectrum of chitosan, the broad absorption band in the range of 3100–3500 cm − 1 is attributed to the stretching vibrations of -OH and -NH, indicating the presence of extensive hydrogen bonding along the chitosan molecular chains. The characteristic peak at 1153 cm − 1 corresponds to the asymmetric stretching vibration of -C-O-C, while the peaks at 1066 cm − 1 and 1019 cm − 1 are assigned to the stretching vibrations of -C-O bonds in the chitosan backbone. These absorption features are consistent with the typical molecular structure of chitosan. Compared with pristine CTS [ 45 ], the FT-IR spectrum of the P(AA-AM-SSS)/CTS composite resin exhibits noticeable changes. The absorption band in the 3100–3500 cm − 1 region shifts and becomes broader, suggesting the formation of new hydrogen-bonding interactions between chitosan and the introduced acrylic acid, acrylamide, and sodium styrene sulfonate monomers, thereby enhancing intermolecular interactions within the polymer network. In addition, the characteristic peak at 1657 cm − 1 is attributed to the -C = O stretching vibration of the amide group from acrylamide, confirming the successful participation of AM in the copolymerization process. [ 46 ]. Furthermore, the peaks at 1549 cm − 1 and 1444 cm − 1 correspond to the -C = O and -COO⁻ stretching vibrations of acrylic acid, respectively, further indicating the grafting of AA and its partial neutralization [ 47 ]. Notably, the appearance of absorption bands at 1180 and 1040 cm − 1 , corresponding to the symmetric stretching vibration of sulfonate groups and the -S-O bond stretching, respectively, provides clear evidence for the successful introduction of sodium styrene sulfonate into the polymer matrix [ 48 ]. In addition, the -C-O stretching vibration peaks of the CTS backbone are significantly weakened, providing further evidence that CTS actively participated in the graft copolymerization with AA, AM, and SSS. Collectively, these spectral features demonstrate that effective graft copolymerization occurred among CTS, AA, AM, and SSS, leading to the successful formation of the P(AA-AM-SSS)/CTS composite superabsorbent resin. Figure 5 B shows the X-ray diffraction (XRD) patterns of the pristine P(AA-AM-SSS) copolymer and the P(AA-AM-SSS)/CTS composite resin to elucidate their crystalline characteristics. The diffraction pattern of P(AA-AM-SSS) exhibits a weak peak at 2θ = 8.1°, which can be attributed to short-range ordering associated with hydroxyl and carboxyl groups. In addition, a broad and low-intensity diffraction halo centered at 2θ = 21.4° is observed, which is characteristic of an amorphous polymeric structure. This amorphous nature arises from the random arrangement of polymer chains formed during free-radical polymerization, which inhibits the development of long-range crystalline order. After incorporation of chitosan, no distinct new diffraction peaks are detected in the XRD pattern of the composite resin, and only slight peak shifts are observed. This result indicates that the grafted P(AA-AM-SSS)/CTS resin maintains an overall amorphous structure. The absence of crystallinity may be related to two main factors. First, strong intermolecular interactions between chitosan and the copolymer chains, including extensive hydrogen bonding between amino, hydroxyl, and carboxyl groups, disrupt the intrinsic crystalline domains of chitosan and prevent ordered chain packing. Second, the relatively low content of chitosan leads to its effective dispersion within the amorphous copolymer matrix, hindering the formation of a continuous crystalline phase [ 49 ]. Consequently, the composite resin exhibits a predominantly amorphous structure, which is favorable for network expansion and mass transport during swelling and adsorption processes. Figure 6 presents the scanning electron microscopy (SEM) images of the P(AA-AM-SSS)/CTS composite superabsorbent resin. The resin particles exhibit a relatively uniform spherical morphology with rough and indented surfaces. Such surface features may facilitate water diffusion into the polymer matrix. At higher magnification, the resin displays a three-dimensional porous network structure with rough and wrinkled pore walls. This interconnected porous morphology may increase the effective contact area between the resin and the solution and provide continuous diffusion pathways for water molecules, enabling efficient transport within the polymer network [ 50 ]. The presence of such a hierarchical porous structure is highly favorable for rapid swelling and mass transfer processes, which is consistent with the observed high water absorption capacity of the composite resin. Thermogravimetric analysis (TGA) was performed to compare the thermal stability of the P(AA-AM) resin and the P(AA-AM-SSS)/CTS composite resin, and the corresponding TG curves are shown in Fig. 7 . The thermogravimetric analysis was performed from 30°C to 600°C under nitrogen atmosphere. The first stage occurs between 30 and 200°C and is associated with a slight mass loss, which can be attributed to the evaporation of physically adsorbed and bound water within the polymer network. Notably, the P(AA-AM-SSS)/CTS composite resin displays a lower weight-loss rate in this region, indicating a reduced water release rate and enhanced water retention capability. The second stage, observed from 200 to 400°C, corresponds to the major thermal degradation process. In this temperature range, the P(AA-AM) resin undergoes rapid decomposition, with a significantly higher mass-loss rate compared to the composite resin. This degradation is primarily associated with the decomposition of carboxyl groups from acrylic acid, amide groups from acrylamide, and the scission of the polymer backbone. In contrast, the P(AA-AM-SSS)/CTS resin exhibits improved thermal resistance, suggesting that the incorporation of chitosan and sodium styrene sulfonate stabilizes the polymer network. The third stage, occurring between 400 and 600°C, is attributed to the progressive destruction of the crosslinked network structure [ 51 ]. After thermal decomposition, the composite resin shows a substantially higher residual mass than the P(AA-AM) resin, with a residual mass of 42.9% at 600°C. This behavior may be associated with the incorporation of chitosan and sulfonate groups. 3.3. Adsorption Performance of P(AA-AM-SSS)/CTS for Methylene Blue The absorbance of methylene blue (MB) solutions with different concentrations was measured at 664 nm using a UV-vis spectrophotometer. According to the Beer-Lambert law, a linear calibration curve was established by plotting absorbance versus MB concentration, as shown in Fig. 8 A, which was subsequently used for the quantitative determination of MB concentrations in adsorption experiments. The effect of adsorbent dosage on the adsorption capacity of methylene blue (MB) is shown in Fig. 8 B. Adsorption experiments were conducted at 30°C with a fixed initial MB concentration of 200 mg/L, using different amounts of P(AA-AM-SSS)/CTS resin (0.01–0.05 g). As the adsorbent dosage increased, the adsorption capacity per unit mass gradually decreased. The maximum adsorption capacity of 874.6 mg/g was achieved at a dosage of 0.01 g. This decline at higher dosages can be attributed to particle aggregation and the partial overlap of active sites, which reduce the effective accessibility of MB molecules to adsorption sites. The strong electrostatic interaction between the negatively charged functional groups and cationic dye molecules suggests potential selectivity. The effect of temperature on MB adsorption was investigated at an initial MB concentration of 200 mg/L with an adsorbent dosage of 0.01 g over the temperature range of 20–40°C (Fig. 8 C). At all temperatures, the adsorption capacity increased rapidly within the first 30 min and then gradually approached equilibrium. An increase in temperature from 20 to 30°C resulted in a higher adsorption capacity, indicating that elevated temperature enhanced molecular diffusion and promoted interactions between MB molecules and active sites. However, a further increase to 40°C led to a decrease in adsorption capacity, which may be related to weakened adsorbate–adsorbent interactions at elevated temperature [ 52 ]. Therefore, 30°C was identified as the optimal adsorption temperature under the studied conditions. These results indicate that the adsorption performance is sensitive to environmental conditions, suggesting potential tunability under different wastewater scenarios. To elucidate the adsorption mechanism of methylene blue (MB) onto the P(AA-AM-SSS)/CTS composite resin, adsorption kinetics were analyzed at a constant temperature using time-dependent adsorption data. The experimental results were fitted with the pseudo-first-order and pseudo-second-order kinetic models to evaluate the rate-controlling steps and adsorption behavior. The mathematical expressions of the pseudo-first-order and pseudo-second-order kinetic models are given in Eqs. ( 5 ) and ( 6 ), respectively [ 53 ]. $$\:\text{ln}\left(\text{q-}{\text{q}}_{\text{t}}\right)\text{=}\text{ln}\text{q}\text{-}{\text{K}}_{\text{1}}\text{t}$$ 5 $$\:\frac{\text{t}}{{\text{q}}_{\text{t}}}\text{=}\frac{\text{1}}{{\text{K}}_{\text{2}}{\text{q}}^{\text{2}}}\text{+}\frac{\text{t}}{\text{q}}$$ 6 In the kinetic models, t (min) represents the adsorption time, q (mg/g) is the equilibrium adsorption capacity, qₜ (mg/g) is the adsorption capacity at time t, and K₁ and K₂ are the rate constants of the pseudo-first-order and pseudo-second-order models, respectively. As shown in Fig. 9 A, MB adsorption on the P(AA-AM-SSS)/CTS composite increased rapidly at the initial stage and gradually approached equilibrium after approximately 180 min, with an equilibrium adsorption capacity of 874.6 mg/g. A further extension of contact time to 240 min resulted in only negligible changes, indicating that adsorption equilibrium was essentially reached at 180 min. The experimental data were fitted using the pseudo-first-order and pseudo-second-order kinetic models (Fig. 9 B and Fig. 9 C), and the corresponding kinetic parameters are summarized in Table 1 . The pseudo-second-order kinetic model exhibited a significantly higher correlation coefficient (R² = 0.99903) than the pseudo-first-order model (R² = 0.98039), and the calculated q values from the pseudo-second-order kinetic model were in good agreement with the experimental results [ 54 ]. These results demonstrate that the pseudo-second-order kinetic model provides a more accurate description of the adsorption process. The adsorption behavior of the composite resin toward MB is likely governed by multiple interactions. The negatively charged functional groups (-COO⁻ and -SO₃⁻) may provide electrostatic attraction toward the cationic dye molecules. Meanwhile, the conjugated aromatic rings of the SSS units may engage in π-π stacking interactions with the phenothiazine aromatic structure of MB, further enhancing the stability of the adsorption system. In addition, the hydroxyl groups (-OH) of chitosan and the amide groups (-CONH₂) of acrylamide units are capable of forming intermolecular hydrogen bonds with nitrogen-containing groups in MB molecules. The combined contribution of these interactions synergistically improves the adsorption capacity of the composite resin toward the dye [ 55 ]. Therefore, the adsorption process can be better described by the pseudo-second-order kinetic model, suggesting that chemical interactions may contribute significantly to the overall adsorption behavior. Table 1 Kinetic parameters of the pseudo-first-order and pseudo-second-order q exp Pseudo-first-order Pseudo-second-order q 1 K 1 R 2 q 2 K 2 R 2 874.6 979.3 0.03329 0.98039 914.6 5.499E-5 0.99903 Adsorption isotherm experiments were conducted at 30°C with a resin dosage of 0.01 g and a contact time of 180 min using MB solutions with different initial concentrations. As shown in Fig. 10 A, the adsorption capacity of the P(AA-AM-SSS)/CTS composite increased with increasing initial MB concentration and gradually approached saturation at approximately 200 mg/L, indicating that adsorption equilibrium had been reached. At low MB concentrations, the increase in adsorption capacity can be attributed to the higher availability of active adsorption sites relative to dye molecules. In contrast, at higher concentrations, most active sites became occupied, resulting in a plateau in adsorption capacity. To further elucidate the interaction between MB molecules and the composite resin surface, the equilibrium adsorption data were fitted using the Langmuir and Freundlich isotherm models. The mathematical expressions of the Langmuir and Freundlich models are given in Eqs. ( 7 ) and ( 8 ), respectively [ 56 ]. $$\:\frac{{\text{C}}_{\text{e}}}{\text{q}}\text{=}\frac{{\text{C}}_{\text{e}}}{{\text{q}}_{\text{m}}}\text{+}\frac{\text{1}}{{\text{K}}_{\text{L}}{\text{q}}_{\text{m}}}$$ 7 $$\:\text{lg}\text{q}\text{=}\text{lg}{\text{K}}_{\text{F}}\text{+}\frac{\text{1}}{\text{n}}\text{lg}{\text{C}}_{\text{e}}$$ 8 where C e (mg/L) is the equilibrium concentration of MB in solution, q (mg/g) is the equilibrium adsorption capacity, q m (mg/g) represents the maximum monolayer adsorption capacity, K L and K F are the Langmuir and Freundlich isotherm constants, respectively, and n is the Freundlich heterogeneity parameter. The Langmuir and Freundlich fitting curves are shown in Fig. 10 B and Fig. 10 C, and the corresponding isotherm parameters are summarized in Table 2 . The Langmuir model exhibited a significantly higher correlation coefficient (R² = 0.99912) than that of the Freundlich model (R² = 0.98749), indicating that the Langmuir model provides a more accurate description of MB adsorption on the P(AA-AM-SSS)/CTS composite resin. This result suggests that MB adsorption predominantly occurs on energetically homogeneous active sites via a monolayer adsorption mechanism. To further evaluate the favorability of the adsorption process, the dimensionless separation factor (R L ) derived from the Langmuir model was calculated according to Eq. ( 9 ). $$\:{\text{R}}_{\text{L}}\text{=}\frac{\text{1}}{\text{1+}{\text{K}}_{\text{L}}{\text{C}}_{\text{0}}}$$ 9 where K L is the Langmuir constant and C 0 (mg/L) denotes the initial concentration of MB. Generally, R L > 1 indicates unfavorable adsorption, whereas 0 < R L < 1 corresponds to favorable adsorption behavior. In this study, the calculated R L value was 0.02782, confirming that MB adsorption on the P(AA-AM-SSS)/CTS composite resin is highly favorable. A comparison of MB adsorption capacities of various adsorbent materials is summarized in Table 3 . Compared with many previously reported polymer-based composite materials, the P(AA–AM–SSS)/CTS composite resin exhibits a higher adsorption capacity, indicating its promising potential for application in dye wastewater treatment. Table 2 Parameters of the Langmuir and Freundlich adsorption isotherm models q exp Langmuir model fitting Freundlich model fitting q m K L R 2 n K F R 2 874.6 890.8 0.699 0.99912 3.905 381.83 0.98749 Table 3 Adsorption performance of different materials for methylene blue solutions adsorbent q exp (mg/g) ref NR-g-PAM hydrogels 538.3 [ 57 ] HC-g-Am-BIS-BT 140.66 [ 58 ] Poly(acrylic acrylamide) slag composite 463 [ 59 ] PSBMA-NaSS 760 [ 60 ] SPC-SAP 62.52 [ 61 ] Poly(AA-AM-SSS)/CTS 874.6 this work 4. CONCLUSION A P(AA-AM-SSS)/CTS composite superabsorbent polymer was successfully synthesized via inverse suspension polymerization. The incorporation of sodium styrene sulfonate and chitosan markedly enhanced the water absorbency, salt tolerance, thermal stability, and dye adsorption performance of the material. Under optimal conditions, the composite resin exhibited a water absorption capacity of 1233.7 g/g and a saline absorption capacity of 137.3 g/g. Notably, approximately 75% of the initial swelling capacity was retained after five swelling-drying cycles, and a high water retention of 84.2% was maintained at 60°C. FT-IR results verified the successful copolymerization of the P(AA-AM-SSS) resin and the structural optimization induced by chitosan incorporation. SEM images demonstrated the formation of an interconnected three-dimensional network within the composite resin. Thermogravimetric analysis further revealed a residual mass of 42.9% at 600°C, indicating excellent thermal stability. In terms of adsorption performance, the composite demonstrated a high maximum adsorption capacity toward methylene blue of 874.6 mg/g. The adsorption behavior was well described by the pseudo-second-order kinetic model and the Langmuir isotherm model, suggesting that monolayer adsorption and chemical interactions may contribute to the overall adsorption process. Overall, the P(AA-AM-SSS)/CTS composite superabsorbent polymer exhibits a favorable combination of high water absorbency, salt resistance, and efficient dye removal capability, highlighting its potential for dye wastewater treatment applications. Declarations CREDIT AUTHORSHIP CONTRIBUTION STATEMENT Xiaoxuan Jv: Writing-original draft, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Xiangchi Liu: Investigation, Formal analysis, Conceptualization. Ailing Liu: Formal analysis, Data curation. Baijun Liu: Writing-review & editing, Writing-original draft. Mingyao Zhang: Writing-review & editing, Writing-original draft. DECLARATION OF COMPETING INTEREST The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. DATA AVAILABILITY Data will be made available on request. ACKNOWLEDGEMENTS The authors appreciate the financial support from the Jilin Province Science and Technology Development Plan Project (20240602019RC). References Wan X, Rong Z, Zhu K, Wu Y (2022) Chitosan-based dual network composite hydrogel for efficient adsorption of methylene blue dye. Int J Biol Macromol 222:725–735. https://doi.org/10.1016/j.ijbiomac.2022.09.213 Tang X, Ran G, Li J, Zhang Z, Xiang C (2021) Extremely efficient and rapidly adsorb methylene blue using porous adsorbent prepared from waste paper: kinetics and equilibrium studies. 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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-8930030","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":602837827,"identity":"3ffa4703-0653-4087-befc-9033b14d6ef6","order_by":0,"name":"Xiaoxuan Jv","email":"","orcid":"","institution":"Changchun University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiaoxuan","middleName":"","lastName":"Jv","suffix":""},{"id":602837828,"identity":"1be7dde0-94bd-4259-9731-27a0cd87311c","order_by":1,"name":"Xiangchi Liu","email":"","orcid":"","institution":"Changchun University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiangchi","middleName":"","lastName":"Liu","suffix":""},{"id":602837829,"identity":"74524c64-df31-4d1f-996f-6b0d52d2d00b","order_by":2,"name":"Ailing Liu","email":"","orcid":"","institution":"Changchun University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Ailing","middleName":"","lastName":"Liu","suffix":""},{"id":602837830,"identity":"966e6349-d06c-4d0a-91cc-d4512991999e","order_by":3,"name":"Baijun Liu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA80lEQVRIiWNgGAWjYBACfmbmAwcSeCR42NibD0DFEvBrkWxvS3zwQMZGjp/nGEipAWEtBj1njA0f2KQZS87wMSBSi0SCmURCzuHEDTd4Pn742faHgZ89x4Dh5w7cWswlEtIkEs4Atdzu3SzZ22bAINnzxoCx9wxuLZYzEo5JJPYAtdw5u42ZEajF4EaOAZCBx2E3EtskEv+BHJbzDKzFnqCWM4eZDRJ4QN7PYYPYIkFACzCQGR8k8IAD2Viy55wxj8SZZwUHe/Fo4Wfm/3DwByQqH374USYnx9+evPHBTzxaMAAPiDhAgoZRMApGwSgYBVgAACxmVOlaaSYUAAAAAElFTkSuQmCC","orcid":"","institution":"Changchun University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Baijun","middleName":"","lastName":"Liu","suffix":""},{"id":602837831,"identity":"cd3fad92-94a3-4f25-847c-ae1c706cad90","order_by":4,"name":"Mingyao Zhang","email":"","orcid":"","institution":"Changchun University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Mingyao","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2026-02-21 02:38:15","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8930030/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8930030/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":104294067,"identity":"4f387bdb-b5a1-4868-a483-cd2d296c3441","added_by":"auto","created_at":"2026-03-10 07:27:56","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":200098,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the preparation process of the P(AA-AM-SSS)/CTS superabsorbent resin\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/3be0668c0af16970e63e47bd.png"},{"id":104294065,"identity":"b437b5ef-b03a-443b-b485-252e508398fa","added_by":"auto","created_at":"2026-03-10 07:27:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3990595,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the polymerization mechanism of P(AA-AM-SSS)/CTS superabsorbent resin\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/e678dd8e249460f099ae5337.png"},{"id":104294071,"identity":"dfdb1648-65ae-4ed7-9562-40d09d03e5cb","added_by":"auto","created_at":"2026-03-10 07:27:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":367437,"visible":true,"origin":"","legend":"\u003cp\u003eEquilibrium swelling ratios of the superabsorbent resin in deionized water and 0.9wt% NaCl solution as a function of the contents of (\u003cstrong\u003eA\u003c/strong\u003e) KPS, (\u003cstrong\u003eB\u003c/strong\u003e) MBA, (\u003cstrong\u003eC\u003c/strong\u003e) CTS, and (\u003cstrong\u003eD\u003c/strong\u003e) SSS\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/8ced620413e26cfe6e3436bd.png"},{"id":104294077,"identity":"859cc261-1aa1-4e0f-8586-a3df0e5c1445","added_by":"auto","created_at":"2026-03-10 07:27:59","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":151784,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cstrong\u003eA\u003c/strong\u003e) Reswelling performance of the P(AA-AM-SSS)/CTS superabsorbent resin during repeated swelling–drying cycles; (\u003cstrong\u003eB\u003c/strong\u003e) Water retention behavior of the P(AA-AM-SSS)/CTS superabsorbent resin at different temperatures\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/05be0bfbc3201ef52ab68bfc.png"},{"id":104294075,"identity":"4536ba88-69dd-4a74-97fa-c95c10e696e6","added_by":"auto","created_at":"2026-03-10 07:27:59","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":184574,"visible":true,"origin":"","legend":"\u003cp\u003e(\u003cstrong\u003eA\u003c/strong\u003e) FT-IR spectra of CTS and P(AA-AM-SSS)/CTS; (\u003cstrong\u003eB\u003c/strong\u003e) XRD patterns of P(AA-AM-SSS) and P(AA-AM-SSS)/CTS\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/3675c8b4e25a00cc2fab9161.png"},{"id":104294064,"identity":"25ed2887-a072-4638-9eb2-2d3241034a99","added_by":"auto","created_at":"2026-03-10 07:27:55","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":430508,"visible":true,"origin":"","legend":"\u003cp\u003eScanning electron microscopy (SEM) images of the P(AA-AM-SSS)/CTS superabsorbent resin at 5k× magnification\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/163cf942db10c1d2656fb3d7.png"},{"id":104294076,"identity":"be272aa3-93be-4624-87d4-88755741fec6","added_by":"auto","created_at":"2026-03-10 07:27:59","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":428172,"visible":true,"origin":"","legend":"\u003cp\u003eThermogravimetric analysis (TGA) curves of the P(AA-AM-SSS)/CTS and P(AA-AM) resins\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/ea38cdc10019ee9e33fa79de.png"},{"id":104294062,"identity":"1568beab-5191-4d7e-8201-fcc8df5d1dd6","added_by":"auto","created_at":"2026-03-10 07:27:55","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":58837,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA\u003c/strong\u003e) Calibration curve of methylene blue; (\u003cstrong\u003eB\u003c/strong\u003e) Effect of adsorbent dosage on the adsorption performance of methylene blue; (\u003cstrong\u003eC\u003c/strong\u003e) Effect of temperature on the adsorption performance of methylene blue\u003c/p\u003e","description":"","filename":"floatimage81.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/77d4fbdb1ccc6d72050c2a6b.png"},{"id":104294079,"identity":"d2538f86-c4c9-4534-9533-f14dbc347110","added_by":"auto","created_at":"2026-03-10 07:27:59","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":118674,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA\u003c/strong\u003e) Variation of methylene blue (MB) adsorption capacity of P(AA-AM-SSS)/CTS resin with time; (\u003cstrong\u003eB\u003c/strong\u003e) fitting with the pseudo-first-order kinetic model; (\u003cstrong\u003eC\u003c/strong\u003e) fitting with the pseudo-second-order kinetic model\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/98a3a98edbae097ee2e0f8cd.png"},{"id":104294063,"identity":"dca0a329-2c71-4445-9403-e437e847e485","added_by":"auto","created_at":"2026-03-10 07:27:55","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":120748,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eA\u003c/strong\u003e) Effect of initial dye concentration on the adsorption capacity of the superabsorbent resin; (\u003cstrong\u003eB\u003c/strong\u003e) fitting with the Langmuir isotherm model; (\u003cstrong\u003eC\u003c/strong\u003e) fitting with the Freundlich isotherm model.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/4f6459bcbe4465a3def78f88.png"},{"id":104294083,"identity":"a1864443-fb9f-4839-8c8f-e4588ec1125d","added_by":"auto","created_at":"2026-03-10 07:28:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6860856,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/d84fd00d-9722-490e-96e7-6ffb22316cde.pdf"},{"id":104294073,"identity":"7124eff4-87e9-46a4-afaf-0e06452b9014","added_by":"auto","created_at":"2026-03-10 07:27:58","extension":"jpeg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":788975,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical Abstract\u003c/p\u003e","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8930030/v1/c2f8a47ab955229f936aec57.jpeg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Salt-Resistant Swelling Behavior and Methylene Blue Adsorption Performance of Chitosan-Modified Poly(acrylic acid-acrylamide-sodium styrene sulfonate) Superabsorbent Resin","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eWith the rapid advancement of industrialization, the global annual production of dyes reaches several hundred thousand tons, of which a considerable fraction is discharged as wastewater [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Dye molecules are typically characterized by pronounced biological toxicity, posing severe threats to aquatic ecosystems and impacting human health [\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Consequently, the development of efficient and economically viable materials for dye wastewater treatment has become a sustained focus of research efforts. To date, a variety of technologies have been explored for dye removal, including adsorption [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], photocatalytic degradation [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], chemical oxidation [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], and electrochemical treatment [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Among these approaches, adsorption is widely regarded as one of the most promising strategies owing to its relatively low operational cost and process simplicity [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. However, many existing adsorbents suffer from high production costs or limited scalability, which hampers their practical implementation [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Therefore, the design of low-cost, facilely fabricated adsorbents with high removal efficiency remains a critical objective in the field.\u003c/p\u003e \u003cp\u003eSuperabsorbent polymers (SAPs) are a class of advanced functional macromaterials featuring three-dimensional crosslinked network structures [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Benefiting from their pronounced swelling capabilities, superabsorbent polymers have found broad applications in wastewater treatment and soil moisture conservation [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Despite this exceptional swelling capacity, the water absorbency of conventional SAPs deteriorates sharply in saline environments. This behavior mainly arises from the electrostatic screening of fixed charges on the polymer chains by dissolved ions, which weakens the electrostatic repulsion between polymer segments, reduces the osmotic pressure difference between the interior and exterior of the polymer network, and induces network contraction. Consequently, achieving a balance between ultrahigh water absorbency and enhanced salt tolerance, while simultaneously endowing SAPs with efficient pollutant adsorption capability, remains a critical engineering and materials-design challenge for their practical application in water treatment [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eChitosan is a naturally derived polymer that has attracted extensive attention owing to its nontoxicity, renewability, abundant amino functionalities, and excellent biocompatibility. In particular, chitosan has shown considerable promise in water treatment applications, as the plentiful amino and hydroxyl groups along its molecular backbone can serve as effective adsorption sites for dye molecules [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. In recent years, chitosan-based adsorbent materials have been increasingly explored for dye wastewater remediation. For example, Ren et al. exploited the environmental friendliness and adsorption capability of chitosan to fabricate a porous gelatin/chitosan composite, which exhibited maximum adsorption capacities of 294.1 mg/g for methyl orange, 221.2 mg/g for Congo red, and 209.2 mg/g for methylene blue [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. In addition, Nakhjiri et al. prepared an N-maleoylated chitosan-based adsorbent, achieving an adsorption capacity of 66.89 mg/g for methylene blue [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Collectively, these studies indicate that the abundant amino and hydroxyl groups of chitosan play a key role in governing dye-adsorbent interfacial interactions, thereby enhancing adsorption performance. However, the adsorption capacity and structural stability of conventional chitosan-based materials remain insufficient for practical deployment in complex wastewater systems, underscoring the need for further molecular-level design and functional integration.\u003c/p\u003e \u003cp\u003eNevertheless, conventional chitosan-based superabsorbent resins generally suffer from insufficient salt tolerance, which severely limits their performance in ionic environments. To address this limitation, sodium styrene sulfonate was introduced in this study as a copolymerizable monomer. Owing to the presence of strongly polar sulfonic acid groups, the incorporation of this monomer markedly enhances the affinity of the polymer network toward water molecules, thereby improving the overall swelling capacity of the material [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. More importantly, upon ionization in aqueous media, the sulfonic groups generate a high density of negatively charged sites, which not only strengthens electrostatic interactions with cationic dye molecules but also mitigates the charge-screening effect of inorganic salts on hydrophilic functional groups [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. As a result, the superabsorbent resin exhibits improved swelling performance under saline conditions. Previous studies have demonstrated that the introduction of sulfonic acid-containing functional monomers is an effective strategy for enhancing both adsorption performance and swelling behavior in high-ionic-strength solutions [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. For instance, Li et al. reported a lignosulfonate-tannin composite adsorbent with a saturated adsorption capacity of 629.5 mg/g for methylene blue [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Guo et al. incorporated 2-acrylamido-2-methylpropane sulfonic acid into a superabsorbent composite, achieving a maximum swelling ratio of 119% in saline water [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Collectively, these findings underscore the pivotal role of sulfonic functionalities in improving the swelling and adsorption performance of superabsorbent resins under high-salinity conditions [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Although chitosan-modified SAPs and sulfonated polymer systems have been widely investigated, most reported studies focus primarily on either swelling enhancement or adsorption improvement. The simultaneous integration of salt resistance, water retention stability, and efficient cationic dye adsorption within a single crosslinked SAP network remains insufficiently explored.\u003c/p\u003e \u003cp\u003eIn this study, a series of three-dimensional P(AA-AM-SSS)/CTS composite superabsorbent resins were constructed via inverse suspension polymerization technology using acrylic acid (AA), acrylamide (AM), chitosan (CTS), and sodium styrene sulfonate (SSS) as monomers. The effects of monomer composition, crosslinker dosage, and initiator content on the swelling and water retention behaviors of the composite resin were systematically investigated. On this basis, adsorption experiments toward the cationic dye methylene blue were conducted, and the adsorption behavior and underlying mechanisms were elucidated through kinetic and isotherm analyses, suggesting its potential applicability in dye wastewater treatment.\u003c/p\u003e"},{"header":"2. EXPERIMENTS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eAll chemicals used in this study were of commercial grade and were used as received without further purification. Acrylic acid (AA, AR), acrylamide (AM, AR), sodium hydroxide (NaOH, AR), potassium persulfate (KPS, AR), acetic acid (AR), and sodium chloride (NaCl, AR) were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Span-80 (CP) was supplied by Kelong Chemical Co., Ltd. (Chengdu, China). Absolute ethanol (EtOH) was obtained from Tianjin Fuyu Fine Chemical Co., Ltd. Cyclohexane (AR), chitosan (CTS, Product code: C804728), N, N\u0026prime;-methylenebisacrylamide (MBA, AR), and sodium styrene sulfonate (SSS, AR) were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Methylene blue (MB) was supplied by Tianjin Dengfeng Chemical Reagent Factory. Deionized water (DDI) was prepared in the laboratory and used throughout the experiments.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Characterization\u003c/h2\u003e \u003cp\u003eThe chemical structure and functional groups of the resin were characterized by Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50). The surface morphology and microstructure were examined using scanning electron microscopy (SEM, Phenom Pro). The crystalline properties were analyzed by X-ray diffraction (XRD, Bruker D8 Advance). Thermal stability was evaluated by thermogravimetric analysis (TGA, PerkinElmer). The concentrations of methylene blue were determined using an ultraviolet-visible spectrophotometer (UV-vis, Cary5000), and the adsorption capacity was calculated accordingly.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Swelling Capacity Test\u003c/h2\u003e \u003cp\u003eA dried sample of the superabsorbent resin (M₀ = 0.3 g) was placed in a 500 mL beaker containing deionized water and left to swell for several hours until equilibrium was reached. Afterward, excess water was removed by filtration, and the swollen resin was weighed (M₁, g). The water absorption capacity (Q₁, g/g) was calculated using Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), where Q₁ represents the grams of water absorbed per gram of the sample [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{\\text{Q}}_{\\text{1}}\\left(\\text{g}\\text{/}\\text{g}\\right)\\text{=}\\frac{{\\text{M}}_{\\text{1}}\\text{-}{\\text{M}}_{\\text{0}}}{{\\text{M}}_{\\text{0}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Salt Absorption Test\u003c/h2\u003e \u003cp\u003eA 0.9wt% sodium chloride solution was prepared. A dried sample of the superabsorbent resin (M₂ = 0.1 g) was placed in a beaker containing the sodium chloride solution and allowed to swell for several hours until equilibrium was reached. Excess saline solution was removed by filtration, and the swollen resin was weighed (M₃, g). The salt absorption capacity (Q₂, g/g) was calculated using Eq.\u0026nbsp;(\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), where Q₂ represents the grams of saline absorbed per gram of the sample.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{\\text{Q}}_{\\text{2}}\\left(\\text{g/g}\\right)\\text{=}\\frac{{\\text{M}}_{\\text{3}}\\text{-}{\\text{M}}_{\\text{2}}}{{\\text{M}}_{\\text{2}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Re-swelling Performance Test\u003c/h2\u003e \u003cp\u003eA dried resin sample (0.3 g) was placed in a 500 mL beaker containing deionized water and allowed to swell at room temperature until equilibrium was reached. After removing excess water by filtration, the swollen sample was weighed and then dried in an oven to constant weight. The dried resin was subsequently re-immersed in the same volume of deionized water to achieve re-swelling. This swelling-drying process was repeated for multiple cycles. The water absorption capacity of each cycle was calculated using Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) to evaluate the re-swelling performance of the resin.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6. Water Retention Test\u003c/h2\u003e \u003cp\u003eThe fully swollen resin (W\u003csub\u003e0\u003c/sub\u003e) was placed in a constant-temperature oven at 20, 40, and 60\u0026deg;C, respectively. At predetermined time intervals, the samples were removed, excess surface water was eliminated, and the remaining mass (W\u003csub\u003e1\u003c/sub\u003e) was recorded. The water retention ratio (W\u003csub\u003et\u003c/sub\u003e, %) was calculated using Eq.\u0026nbsp;(\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), where W\u003csub\u003et\u003c/sub\u003e represents the percentage of retained water relative to the initial swollen mass at time t [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. \u003cp\u003e\u003cimg src=\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAOQAAABACAYAAAAK2tt1AAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAAFiUAABYlAUlSJPAAAAyVSURBVHhe7Z3NixzFG8e/8/sDhGZvKh6mZ8HbiPQSkY0QQWcNIQiadFQQDxLtvYb1ZdCTgpkNuSYTiBcPzkSTizjLugsK9iqIQWZA8JD0noKnHteXP6B+l3nap5/p6pcxO9OdrQ807FZVT/dU1beep56q7qkppRQMBkMp+J9MMBgMi8MI0mAoEUaQBkOJMII0GEqEEaTBUCKMIA2GEmEEaTCUCCNIg6FEGEFWhL29PdRqtakDANbW1qbS19bWYufL/L29vVi+oRwYQVaE1dVVBEEA27YBAK7rgjZZbW9vw/f9qGy328X29nb0PwAopeB5HhzHQRAEWF1djeUbyoERZIWo1+v4+OOPZTIwEazneTI5hmVZuHDhAur1uswylISa2ctaLcbjMZaXl3FwcIAgCGLi2t/fh23bcBwHP//8c+w8AGg0Grh7965MNpQIYyErxtLSEs6dOwcA2N3dldkAgNu3b2N/fz+WtrW1heeffz6WZigfRpAV5K233gIAXLp0KZa+u7sLx3EAAJ9++mks77PPPovOM5QX47JWlEajgSAIMBwO0Ww2AQArKyu4fv06nnjiCdi2Hbmn4/EYx44dM+5qBTAWsqJsbGwAAK5duwYAGI1GsG0bzWYTnuchCAJsbW0BAG7evInz58/Hzudsbm6apZCSYARZUZ577jkAQL/fx3g8xhdffIEXX3wRAHDq1Clg4qYCwPXr1/Hyyy+zs/+l0Wjg3XfflcmGBVE5QY7HY1y7dg0rKyuRBThsRqMRGo0GNjc3MR6PZfZCqNfrcF0XBwcH2N3dxc7OTiTSkydPwrIs3LhxI7J6uqWOu3fvxtYwDYulUoLc2trC8vIy3n//fZw5cwaPP/64LHIoPProo9jY2MDFixexvLwcuYmL5vXXXwcAfPDBB1hZWcHS0lKUR2uSp0+fxoULF6J0Q8lRFaHdbisAqtVqqTAMZfZcCMNQua6rACjP82T2QrAsSwFQg8Eglj4cDhUABSCzvnzfVwCU7/syyzBnoFiD6I6ocEIeHZ1Oh3+utnyr1ZLFMul2u7nOHQwGynEcBUBZlpV4Tzq63a5yHCez8yqllOd5CoBqt9sya+60221l27ZMVkopZdt2roGjLIL0fV+1Wq1C7Sbb3PM8FQSBLJZIp9NRtm0rAMq2bdXpdFLbPwxD5XleNAi2Wi01HA5lsUTCMFS2bateryezYsQsJHV8OrrdLs9OLOO6buqXGAwGUWXN0uA00luWlXqdXq+nwCwX/Z9HNMPhUFmWVbhyy9CJh8Oh9jt2u90py5nEogXZ6/UiUUEzuCdBAyP10yAIlOu6mW0ZhqFyHCfWJ33fV47jaAdlfs5wOIz9n2cAcF031+A45bLyitF9qU6nU6jyAGSODDparVau69CoxSuTRJNWYVSxRe+PBK+zTlVikYL0fV/5vh8N3HnaWrE+KDt5GIbKsqxUoVCfkm1O9eA4TixdMUPEBz9pBHTQgJMkdMmUIOki0FhINfnSVMZ1XZkdYzAYKMuyZHIuqIKk0CTUmLIiab6X1sCe52VWqA7d/K1qzCrItDbh5C1HxiCtvdTEElK/SBIdxRuS+mbWQEr3IPs+pfO25vN0HcPhUNm2rTVukqkoK4XOkbA1i1haWoqieDdu3EhdCvj666+jvZdF+fzzz4HJo0Y8gij59ddfgcl9cZ588kkAwC+//BJLJ/r9PnZ2dvDRRx/JrFzQ96L1vqPGsWPH0O/3ZXKM0WiE5eXlqb21Scj200HbAh3HSVzOeeGFF4BJ35TXvXz5MjDpU0mcOXMGSOj7t2/fBgA89NBDURrtkMLke0rG4zHefPNNbGxsxMqmMSVILrYgCBIvBHEzN2/ejOUR4/EY/X4fr776qszKxc7ODgDgxIkTMqsQf/75p0zCaDTC+vo6bt26lbsjSJ555hmA3WcVWVtbw/HjxwEAx48fx/r6uiyi5datW1hfX9eKcjQa4cSJE7hy5UqicGbl6tWrABA9Gyrhz3ryDfij0SgS1mOPPRalc55++mkgo+8n8c8//8gkfPjhh7Btu9geYmkylXAVde4cuYNIcBWJXq+ndQ2y4G5Jlis1i8vqOM6UW1IUXk/cJeFz7KJHViS5bFBATM7HdOlp5IkXcDcxrVxSffKApK5P8Tbln1/UZR0MBsq27dzuOjH9SRMoIJI0/6M5JFUgNAGgoiFsDq+YpHkChyby0AR1ZOW32+3E+cUs0D3yhgqCIApWFD2S6rHsSPHJ//OSR5C+RjAS+ixuEPhA6WsEqVibcmNE5yYFdeQgGgRBZqRXh1aQ/OZlxZLl41ZMWlLKyxKTDh5cyoOMeOmicLOOXDroHtM6x1GARNjpdGYSo8opSN4v04Jp3FgkpaW1P5XhQpPLHkEQKNu2E4X3X7wvbW/nYpPWhF+QTLm0pLTQPiu84vPCF4lpoZdDI5ccHeV5RToT3aO81lFEDopFKSpI2Y6cLEGmQWWk5QvFxgDXdafE2G63M8/zUjYvpN4Z/wL0AdLycUvGO3LRji2ZRZBZOI4z1diyEyWtN6VB98g/9zBcVrrOPI8iLMJCzluQWdASH7e+0rKGk00llmadNPXOuNjIInY6nSlTTmXIktJkN80tyIJPwO8H8r5VjrlnUoVJ6B55J+Kdpugh75GQ5eZx5EXOGeX/eckjSL6BIO3zqV25l0brk0hpWx6oyTsoK9aX5CBBfSFp7pnkSaTWOu+wNDlOsnw84hoEgfI8b8rN1ZF0U6pgUCcL3/enRi6VEp3N0zEIuse0+cyDjE58uvQ08tQ7F0xaOSrD+2HRKGuReWBLE8BMis7ya0im1iE5/IVKQRCg3+/jjz/+iG0eAHsMCJN1n36/H0vT0e/3pxZuiYcffjj6+/fff4/lFWE8HuP06dP46quvptYbdRsKnn32WSBlQwHBn7B/5JFHYnlHAb7OKDd/NJtNfPfdd6nrlLPQbDZhWRYA4K+//pLZwKTNCb6G/dRTT0V///3339HfHJ7Oy6exubkJAHjnnXdkVuKGAr5OKtc6UwUJ9kIlAHjllVdw7ty5qQ5MD8QCwNtvvx2lpUEL8zrq9Xr0mT/++KPMzs1rr70Gz/MO5cXA9+7dAybvO827E+NB4qWXXkoUI8FFqRt4Z4Gup9uQ8dtvv0V/c+PRbDajzQQ//PBDlM6hQZpeh5LF3t4eLl68iCtXrsis2ZAmMwmaU0nTy+H+uc4NJWjyS+WhmTt5k938ed1fSafTmXJHOTqXNW1DAee/3l/VkVMAHXnL5XFZVYG9rEn9kOZvug0r5GLmcbVpSqfThNK4rNztlkynJEC+t+5LKHERXaRQAo0QCe5r521UguYwSQ1GZAV1sr4HnZvWIIb85BWkYsESWZbaNK3t6TpyHkn9TQ7QOlzXzQz8pAV1kgbyXIKkESnr4rZtp4pWkiVIVbCRCAot5xGKjHhRBSaNrpyskdZQDD742pNNJ1mQh8Kfh+RLDDpCzfOQlmUpJ+djUnnX2fm1hhkbClReQarJaJD0AZxut1soMpVHkGR5k6KkOvKMXBy5MSBL/CT4pFHWUAwuxKQji263G7WFNXljQJ5+EoaharfbkZeTp92JPN4XJ8yxoYDI/saHSB5BKma18pSdBzTHLCJ6gyEPlRCkYu5J64i+5EpnSZTYgUKHrFeZbyx7OamMIBWLzlqTLVrzEmYYhqrb7UbXLuKW309o/oGEgAAXrO7+PM9TjuPkdrUM8ydzHbJMnDx5Enfu3MEnn3yCL7/8Ej/99JMscijcu3cPly5dwnvvvYc7d+4Ue+D0PmJ+H/IIIBU6TzCZTPu+P3cXsKrwpRpp6Sgarov+mYhw+Vmohez1egiCAG+88cbCrE7VML8P+YAjFWooP7QUJC0erY0hIQKcFmo3lAfz+5AVxfw+5IPJQl1Ww+zcr9+H3N/fR61WQ61WQ6PRkNmGOWMEWVHux+9D7u/vw7Zt+L4PpRTOnz9vRLlopA9rqA60SaGX8Kp6isT6k9+sSMJLeGs7fZ5hMRgLWWH+6+9DXr16NXrZM+F5Hr7//vtYmmF+GEFWGHowPAiCyE0lzp49CwA4ODiYesMDJu4qJj9GK9E9+Gs4fIwgK47nebBte+oNDfR0vOd5U294MJQXI8iKc/bsWe0Px2xsbExZTkO5MeuQR5harYZerxd7J87a2hrq9fr9e0eMoRDGQh5hkgI433zzzVSgxzA/jIU8wvB1yNXVVWxubuLbb7/F9va2LGqYE0aQR5y9vb3o9yFbrZYR44IxgjQYSoSZQxoMJcII0mAoEUaQBkOJMII0GErE/wHg3vOQlETlXwAAAABJRU5ErkJggg==\"\u003e\u003c/p\u003e3\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.7. Adsorption Experiments\u003c/h2\u003e \u003cp\u003eMethylene blue was selected as a representative cationic dye model molecule to evaluate the adsorption behavior. Adsorption was conducted under limited swelling conditions. Adsorption experiments were conducted to evaluate the dye adsorption capacity of the resin. Different amounts of the superabsorbent resin (0.01\u0026ndash;0.05 g) were added to methylene blue solutions with a fixed initial concentration at a controlled temperature and agitated using a magnetic stirrer until adsorption equilibrium was reached. The supernatant was collected, and the residual dye concentration was determined by UV-vis spectroscopy. The adsorption capacity was calculated using Eq.\u0026nbsp;(\u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:\\text{q}\\left(\\text{mg}\\text{/}\\text{g}\\right)\\text{=}\\frac{\\left({\\text{C}}_{\\text{0}}\\text{-}{\\text{C}}_{\\text{t}}\\right)\\text{V}}{\\text{m}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere C\u003csub\u003e0\u003c/sub\u003e (mg/L) is the initial dye concentration, C\u003csub\u003et\u003c/sub\u003e (mg/L) is the dye concentration at time t, V (L) is the volume of the dye solution, and m (g) is the mass of the resin.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Synthesis of P(AA-AM-SSS)/CTS Superabsorbent Resin\u003c/h2\u003e \u003cp\u003eThe preparation procedure of the superabsorbent resin is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Chitosan was first dissolved in a 2wt% aqueous acetic acid solution under stirring at 50\u0026deg;C until a transparent solution was obtained. Subsequently, a mixed monomer solution containing acrylic acid (AA), acrylamide (AM), and sodium styrene sulfonate (SSS), together with potassium persulfate (KPS, 0.65wt%) as the initiator and N, N\u0026prime;-methylenebisacrylamide (MBA, 0.1wt%) as the crosslinker, was added to form the aqueous phase. The resulting aqueous phase was then introduced into cyclohexane containing Span-80 as a suspension stabilizer (oil-to-water ratio\u0026thinsp;=\u0026thinsp;3:1) to obtain a uniform suspension system. The polymerization was carried out under a nitrogen atmosphere at 68\u0026deg;C with increased stirring speed for 2.5 h. After completion of the reaction, the product was washed with ethanol, dried, and ground to obtain the final P(AA-AM-SSS)/CTS composite superabsorbent resin.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e illustrates the swelling mechanism of the P(AA-AM-SSS)/CTS superabsorbent resin. Upon immersion in water, water molecules diffuse into the resin through hydrogen bonding interactions with hydrophilic functional groups. The ionization of carboxyl and sulfonate groups generates negatively charged sites along the polymer chains, inducing electrostatic repulsion that promotes network expansion. Meanwhile, the accumulation of counterions within the polymer network establishes an osmotic pressure gradient between the interior and exterior of the resin, driving further water uptake. The presence of crosslinking points restricts excessive chain extension, and the swelling process reaches equilibrium when the osmotic pressure is balanced by the elastic retractive force of the crosslinked network [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Characterization of P(AA-AM-SSS)/CTS Superabsorbent Resin\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e systematically illustrates the effects of synthesis parameters, including initiator dosage, crosslinker content, and monomer composition, on the water absorption and salt resistance of the P(AA-AM-SSS)/CTS superabsorbent resin. In this study, the equilibrium swelling capacity is evaluated to compare the effectiveness of structural optimization.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA, the water absorption capacity of the resin initially increased and then decreased with increasing potassium persulfate (KPS) content. The maximum water absorption capacity (1233.33 g/g) and salt absorption capacity (135.67 g/g) were achieved at a KPS dosage of 0.65wt%. KPS thermally decomposes to generate sulfate radical species, which initiate the free-radical polymerization of the monomers. At low initiator concentrations, the number of active radical sites is insufficient, resulting in a reduced polymerization rate and an incompletely developed network structure, thereby limiting resin swelling. In contrast, excessive initiator dosage leads to an increased density of active centers and accelerated polymerization, which promotes the formation of an overly compact crosslinked network. This dense structure restricts polymer chain mobility and water diffusion, ultimately suppressing the swelling capacity [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB illustrates the effect of crosslinker (MBA) content on the swelling behavior of the resin. A similar trend was observed, with the swelling capacity first increasing and then decreasing as the MBA dosage increased. Insufficient crosslinker content resulted in the formation of partially linear or soluble polymer chains, whereas an optimal MBA content of 0.1wt% produced a well-developed three-dimensional network with maximal swelling performance. Further increasing the crosslinker content led to excessive crosslinking density, which significantly constrained network expansion and hindered the diffusion of water and saline into the polymer matrix, thereby reducing both water and salt absorption capacities [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. The influence of chitosan (CTS) content on the swelling and salt resistance of the composite resin is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC. Both water and salt absorption capacities increased initially with increasing CTS content and reached a maximum at 4.5wt%, followed by a decline at higher CTS loadings. Moderate incorporation of CTS provided effective grafting backbones, enabling enhanced monomer grafting and expansion of the polymer network. In addition, the abundant amino and hydroxyl groups in CTS contributed additional hydrophilic sites, further improving water uptake [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. However, excessive CTS content (6wt%) markedly increased the viscosity of the reaction system, which impeded mass transfer, reduced polymerization efficiency, and restricted segmental chain motion, ultimately leading to decreased swelling and salt absorption performance. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD, the incorporation of sodium styrene sulfonate (SSS) significantly enhanced the swelling behavior of the resin. The optimal SSS content was 10wt%, at which the highest water absorption capacity was achieved. This enhancement can be attributed to the introduction of highly hydrophilic sulfonic acid groups (-SO₃⁻), which enhance the hydration capacity of the polymer chains and help mitigate the electrostatic charge-screening effect induced by salt ions in solution, thereby contributing to improved salt resistance. Nevertheless, further increasing the SSS content to 14wt% resulted in a decline in swelling performance. Excessive sulfonic group density intensified intermolecular interactions and electrostatic repulsion, which restricted polymer chain mobility and limited further network expansion, thereby reducing both water and salt absorption capacities [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCompared with some previously reported superabsorbent resins [\u003cspan additionalcitationids=\"CR40\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e], the P(AA-AM-SSS)/CTS composite resin exhibits relatively high swelling capacities in both deionized water and saline solutions. Notably, the resin maintains favorable water absorbency even under saline conditions, indicating that the incorporation of sodium styrene sulfonate and chitosan effectively enhances the salt tolerance of the polymer system. Such improved swelling behavior provides a favorable structural basis for adsorption of dye molecules.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe re-swelling performance of the composite resin was evaluated to assess its cyclic stability and reusability. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eA, the water absorption capacity gradually decreased with increasing swelling-drying cycles. This decline is primarily attributed to partial disruption of physical crosslinking points within the polymer network during repeated swelling and dehydration processes, leading to structural fatigue and the leaching of low-molecular-weight compounds. Nevertheless, after five cycles, the resin retained approximately 76% of its initial swelling capacity, indicating favorable re-swelling behavior and good recycling potential. The enhanced cyclic stability of the composite resin can be attributed to the synergistic effects of chitosan and sodium styrene sulfonate. The rigid backbone of chitosan reinforces the mechanical strength and toughness of the crosslinked network, thereby mitigating structural degradation during repeated cycles. Meanwhile, the chemically stable sulfonate groups (-SO₃⁻) contribute to maintaining network integrity under varying environmental conditions, effectively reducing adverse structural changes induced by medium fluctuations. As a result, the P(AA-AM-SSS)/CTS resin exhibits improved structural robustness and sustained swelling performance during repeated use [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn practical applications, superabsorbent resins are required not only to exhibit high equilibrium swelling capacity but also to maintain effective water retention under varying thermal conditions. The water retention behavior of the fully swollen composite resin was therefore evaluated at 20, 40, and 60\u0026deg;C, and the results are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eB. As expected, the water retention ratio gradually decreased with increasing temperature and exposure time. At 20\u0026deg;C, the resin showed a relatively slow water loss rate, indicating favorable water retention performance under ambient conditions. Notably, even at elevated temperature, the composite resin maintained relatively high water retention, retaining approximately 85% of its absorbed water after 6 h at 60\u0026deg;C. The enhanced water retention performance can be attributed to the combined effects of chitosan incorporation and sodium styrene sulfonate introduction. The rigid backbone and multichain structure of chitosan reinforce the crosslinked network, resulting in a reduced diffusion rate of water molecules within the polymer matrix and suppressing rapid water release after equilibrium swelling [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. In addition, the strong hydrophilicity of sodium styrene sulfonate and the electrostatic repulsion between ionized sulfonate and carboxylate groups help maintain an expanded network conformation, thereby strengthening water-polymer interactions and further improving the overall water retention capacity of the composite resin [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eA presents the Fourier transform infrared (FT-IR) spectra of CTS and the P(AA-AM-SSS)/CTS superabsorbent resin. In the FT-IR spectrum of chitosan, the broad absorption band in the range of 3100\u0026ndash;3500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to the stretching vibrations of -OH and -NH, indicating the presence of extensive hydrogen bonding along the chitosan molecular chains. The characteristic peak at 1153 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the asymmetric stretching vibration of -C-O-C, while the peaks at 1066 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1019 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are assigned to the stretching vibrations of -C-O bonds in the chitosan backbone. These absorption features are consistent with the typical molecular structure of chitosan. Compared with pristine CTS [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e], the FT-IR spectrum of the P(AA-AM-SSS)/CTS composite resin exhibits noticeable changes. The absorption band in the 3100\u0026ndash;3500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e region shifts and becomes broader, suggesting the formation of new hydrogen-bonding interactions between chitosan and the introduced acrylic acid, acrylamide, and sodium styrene sulfonate monomers, thereby enhancing intermolecular interactions within the polymer network. In addition, the characteristic peak at 1657 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to the -C\u0026thinsp;=\u0026thinsp;O stretching vibration of the amide group from acrylamide, confirming the successful participation of AM in the copolymerization process. [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. Furthermore, the peaks at 1549 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1444 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to the -C\u0026thinsp;=\u0026thinsp;O and -COO⁻ stretching vibrations of acrylic acid, respectively, further indicating the grafting of AA and its partial neutralization [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. Notably, the appearance of absorption bands at 1180 and 1040 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to the symmetric stretching vibration of sulfonate groups and the -S-O bond stretching, respectively, provides clear evidence for the successful introduction of sodium styrene sulfonate into the polymer matrix [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. In addition, the -C-O stretching vibration peaks of the CTS backbone are significantly weakened, providing further evidence that CTS actively participated in the graft copolymerization with AA, AM, and SSS. Collectively, these spectral features demonstrate that effective graft copolymerization occurred among CTS, AA, AM, and SSS, leading to the successful formation of the P(AA-AM-SSS)/CTS composite superabsorbent resin.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eB shows the X-ray diffraction (XRD) patterns of the pristine P(AA-AM-SSS) copolymer and the P(AA-AM-SSS)/CTS composite resin to elucidate their crystalline characteristics. The diffraction pattern of P(AA-AM-SSS) exhibits a weak peak at 2θ\u0026thinsp;=\u0026thinsp;8.1\u0026deg;, which can be attributed to short-range ordering associated with hydroxyl and carboxyl groups. In addition, a broad and low-intensity diffraction halo centered at 2θ\u0026thinsp;=\u0026thinsp;21.4\u0026deg; is observed, which is characteristic of an amorphous polymeric structure. This amorphous nature arises from the random arrangement of polymer chains formed during free-radical polymerization, which inhibits the development of long-range crystalline order. After incorporation of chitosan, no distinct new diffraction peaks are detected in the XRD pattern of the composite resin, and only slight peak shifts are observed. This result indicates that the grafted P(AA-AM-SSS)/CTS resin maintains an overall amorphous structure. The absence of crystallinity may be related to two main factors. First, strong intermolecular interactions between chitosan and the copolymer chains, including extensive hydrogen bonding between amino, hydroxyl, and carboxyl groups, disrupt the intrinsic crystalline domains of chitosan and prevent ordered chain packing. Second, the relatively low content of chitosan leads to its effective dispersion within the amorphous copolymer matrix, hindering the formation of a continuous crystalline phase [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. Consequently, the composite resin exhibits a predominantly amorphous structure, which is favorable for network expansion and mass transport during swelling and adsorption processes.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents the scanning electron microscopy (SEM) images of the P(AA-AM-SSS)/CTS composite superabsorbent resin. The resin particles exhibit a relatively uniform spherical morphology with rough and indented surfaces. Such surface features may facilitate water diffusion into the polymer matrix. At higher magnification, the resin displays a three-dimensional porous network structure with rough and wrinkled pore walls. This interconnected porous morphology may increase the effective contact area between the resin and the solution and provide continuous diffusion pathways for water molecules, enabling efficient transport within the polymer network [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. The presence of such a hierarchical porous structure is highly favorable for rapid swelling and mass transfer processes, which is consistent with the observed high water absorption capacity of the composite resin.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThermogravimetric analysis (TGA) was performed to compare the thermal stability of the P(AA-AM) resin and the P(AA-AM-SSS)/CTS composite resin, and the corresponding TG curves are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The thermogravimetric analysis was performed from 30\u0026deg;C to 600\u0026deg;C under nitrogen atmosphere. The first stage occurs between 30 and 200\u0026deg;C and is associated with a slight mass loss, which can be attributed to the evaporation of physically adsorbed and bound water within the polymer network. Notably, the P(AA-AM-SSS)/CTS composite resin displays a lower weight-loss rate in this region, indicating a reduced water release rate and enhanced water retention capability. The second stage, observed from 200 to 400\u0026deg;C, corresponds to the major thermal degradation process. In this temperature range, the P(AA-AM) resin undergoes rapid decomposition, with a significantly higher mass-loss rate compared to the composite resin. This degradation is primarily associated with the decomposition of carboxyl groups from acrylic acid, amide groups from acrylamide, and the scission of the polymer backbone. In contrast, the P(AA-AM-SSS)/CTS resin exhibits improved thermal resistance, suggesting that the incorporation of chitosan and sodium styrene sulfonate stabilizes the polymer network. The third stage, occurring between 400 and 600\u0026deg;C, is attributed to the progressive destruction of the crosslinked network structure [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. After thermal decomposition, the composite resin shows a substantially higher residual mass than the P(AA-AM) resin, with a residual mass of 42.9% at 600\u0026deg;C. This behavior may be associated with the incorporation of chitosan and sulfonate groups.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Adsorption Performance of P(AA-AM-SSS)/CTS for Methylene Blue\u003c/h2\u003e \u003cp\u003eThe absorbance of methylene blue (MB) solutions with different concentrations was measured at 664 nm using a UV-vis spectrophotometer. According to the Beer-Lambert law, a linear calibration curve was established by plotting absorbance versus MB concentration, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eA, which was subsequently used for the quantitative determination of MB concentrations in adsorption experiments.\u003c/p\u003e \u003cp\u003eThe effect of adsorbent dosage on the adsorption capacity of methylene blue (MB) is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eB. Adsorption experiments were conducted at 30\u0026deg;C with a fixed initial MB concentration of 200 mg/L, using different amounts of P(AA-AM-SSS)/CTS resin (0.01\u0026ndash;0.05 g). As the adsorbent dosage increased, the adsorption capacity per unit mass gradually decreased. The maximum adsorption capacity of 874.6 mg/g was achieved at a dosage of 0.01 g. This decline at higher dosages can be attributed to particle aggregation and the partial overlap of active sites, which reduce the effective accessibility of MB molecules to adsorption sites. The strong electrostatic interaction between the negatively charged functional groups and cationic dye molecules suggests potential selectivity.\u003c/p\u003e \u003cp\u003eThe effect of temperature on MB adsorption was investigated at an initial MB concentration of 200 mg/L with an adsorbent dosage of 0.01 g over the temperature range of 20\u0026ndash;40\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eC). At all temperatures, the adsorption capacity increased rapidly within the first 30 min and then gradually approached equilibrium. An increase in temperature from 20 to 30\u0026deg;C resulted in a higher adsorption capacity, indicating that elevated temperature enhanced molecular diffusion and promoted interactions between MB molecules and active sites. However, a further increase to 40\u0026deg;C led to a decrease in adsorption capacity, which may be related to weakened adsorbate\u0026ndash;adsorbent interactions at elevated temperature [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. Therefore, 30\u0026deg;C was identified as the optimal adsorption temperature under the studied conditions. These results indicate that the adsorption performance is sensitive to environmental conditions, suggesting potential tunability under different wastewater scenarios.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the adsorption mechanism of methylene blue (MB) onto the P(AA-AM-SSS)/CTS composite resin, adsorption kinetics were analyzed at a constant temperature using time-dependent adsorption data. The experimental results were fitted with the pseudo-first-order and pseudo-second-order kinetic models to evaluate the rate-controlling steps and adsorption behavior. The mathematical expressions of the pseudo-first-order and pseudo-second-order kinetic models are given in Eqs.\u0026nbsp;(\u003cspan refid=\"Equ5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) and (\u003cspan refid=\"Equ6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), respectively [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e].\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\:\\text{ln}\\left(\\text{q-}{\\text{q}}_{\\text{t}}\\right)\\text{=}\\text{ln}\\text{q}\\text{-}{\\text{K}}_{\\text{1}}\\text{t}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$$\\:\\frac{\\text{t}}{{\\text{q}}_{\\text{t}}}\\text{=}\\frac{\\text{1}}{{\\text{K}}_{\\text{2}}{\\text{q}}^{\\text{2}}}\\text{+}\\frac{\\text{t}}{\\text{q}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eIn the kinetic models, t (min) represents the adsorption time, q (mg/g) is the equilibrium adsorption capacity, qₜ (mg/g) is the adsorption capacity at time t, and K₁ and K₂ are the rate constants of the pseudo-first-order and pseudo-second-order models, respectively.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eA, MB adsorption on the P(AA-AM-SSS)/CTS composite increased rapidly at the initial stage and gradually approached equilibrium after approximately 180 min, with an equilibrium adsorption capacity of 874.6 mg/g. A further extension of contact time to 240 min resulted in only negligible changes, indicating that adsorption equilibrium was essentially reached at 180 min. The experimental data were fitted using the pseudo-first-order and pseudo-second-order kinetic models (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eB and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eC), and the corresponding kinetic parameters are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The pseudo-second-order kinetic model exhibited a significantly higher correlation coefficient (R\u0026sup2; = 0.99903) than the pseudo-first-order model (R\u0026sup2; = 0.98039), and the calculated q values from the pseudo-second-order kinetic model were in good agreement with the experimental results [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. These results demonstrate that the pseudo-second-order kinetic model provides a more accurate description of the adsorption process. The adsorption behavior of the composite resin toward MB is likely governed by multiple interactions. The negatively charged functional groups (-COO⁻ and -SO₃⁻) may provide electrostatic attraction toward the cationic dye molecules. Meanwhile, the conjugated aromatic rings of the SSS units may engage in π-π stacking interactions with the phenothiazine aromatic structure of MB, further enhancing the stability of the adsorption system. In addition, the hydroxyl groups (-OH) of chitosan and the amide groups (-CONH₂) of acrylamide units are capable of forming intermolecular hydrogen bonds with nitrogen-containing groups in MB molecules. The combined contribution of these interactions synergistically improves the adsorption capacity of the composite resin toward the dye [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. Therefore, the adsorption process can be better described by the pseudo-second-order kinetic model, suggesting that chemical interactions may contribute significantly to the overall adsorption behavior.\u003c/p\u003e \u003cp\u003e \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\u003eKinetic parameters of the pseudo-first-order and pseudo-second-order\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\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 \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eq\u003csub\u003eexp\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003ePseudo-first-order\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003ePseudo-second-order\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eq\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eK\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eq\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eK\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e874.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e979.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.03329\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.98039\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e914.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e5.499E-5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.99903\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\u003eAdsorption isotherm experiments were conducted at 30\u0026deg;C with a resin dosage of 0.01 g and a contact time of 180 min using MB solutions with different initial concentrations. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eA, the adsorption capacity of the P(AA-AM-SSS)/CTS composite increased with increasing initial MB concentration and gradually approached saturation at approximately 200 mg/L, indicating that adsorption equilibrium had been reached. At low MB concentrations, the increase in adsorption capacity can be attributed to the higher availability of active adsorption sites relative to dye molecules. In contrast, at higher concentrations, most active sites became occupied, resulting in a plateau in adsorption capacity. To further elucidate the interaction between MB molecules and the composite resin surface, the equilibrium adsorption data were fitted using the Langmuir and Freundlich isotherm models. The mathematical expressions of the Langmuir and Freundlich models are given in Eqs.\u0026nbsp;(\u003cspan refid=\"Equ7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) and (\u003cspan refid=\"Equ8\" class=\"InternalRef\"\u003e8\u003c/span\u003e), respectively [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e].\u003cdiv id=\"Equ7\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e\n$$\\:\\frac{{\\text{C}}_{\\text{e}}}{\\text{q}}\\text{=}\\frac{{\\text{C}}_{\\text{e}}}{{\\text{q}}_{\\text{m}}}\\text{+}\\frac{\\text{1}}{{\\text{K}}_{\\text{L}}{\\text{q}}_{\\text{m}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ8\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ8\" name=\"EquationSource\"\u003e\n$$\\:\\text{lg}\\text{q}\\text{=}\\text{lg}{\\text{K}}_{\\text{F}}\\text{+}\\frac{\\text{1}}{\\text{n}}\\text{lg}{\\text{C}}_{\\text{e}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e8\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere C\u003csub\u003ee\u003c/sub\u003e (mg/L) is the equilibrium concentration of MB in solution, q (mg/g) is the equilibrium adsorption capacity, q\u003csub\u003em\u003c/sub\u003e (mg/g) represents the maximum monolayer adsorption capacity, K\u003csub\u003eL\u003c/sub\u003e and K\u003csub\u003eF\u003c/sub\u003e are the Langmuir and Freundlich isotherm constants, respectively, and n is the Freundlich heterogeneity parameter.\u003c/p\u003e \u003cp\u003eThe Langmuir and Freundlich fitting curves are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eB and Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003eC, and the corresponding isotherm parameters are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The Langmuir model exhibited a significantly higher correlation coefficient (R\u0026sup2; = 0.99912) than that of the Freundlich model (R\u0026sup2; = 0.98749), indicating that the Langmuir model provides a more accurate description of MB adsorption on the P(AA-AM-SSS)/CTS composite resin. This result suggests that MB adsorption predominantly occurs on energetically homogeneous active sites via a monolayer adsorption mechanism. To further evaluate the favorability of the adsorption process, the dimensionless separation factor (R\u003csub\u003eL\u003c/sub\u003e) derived from the Langmuir model was calculated according to Eq.\u0026nbsp;(\u003cspan refid=\"Equ9\" class=\"InternalRef\"\u003e9\u003c/span\u003e).\u003cdiv id=\"Equ9\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ9\" name=\"EquationSource\"\u003e\n$$\\:{\\text{R}}_{\\text{L}}\\text{=}\\frac{\\text{1}}{\\text{1+}{\\text{K}}_{\\text{L}}{\\text{C}}_{\\text{0}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e9\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere K\u003csub\u003eL\u003c/sub\u003e is the Langmuir constant and C\u003csub\u003e0\u003c/sub\u003e (mg/L) denotes the initial concentration of MB. Generally, R\u003csub\u003eL\u003c/sub\u003e \u0026gt; 1 indicates unfavorable adsorption, whereas 0\u0026thinsp;\u0026lt;\u0026thinsp;R\u003csub\u003eL\u003c/sub\u003e \u0026lt; 1 corresponds to favorable adsorption behavior. In this study, the calculated R\u003csub\u003eL\u003c/sub\u003e value was 0.02782, confirming that MB adsorption on the P(AA-AM-SSS)/CTS composite resin is highly favorable. A comparison of MB adsorption capacities of various adsorbent materials is summarized in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Compared with many previously reported polymer-based composite materials, the P(AA\u0026ndash;AM\u0026ndash;SSS)/CTS composite resin exhibits a higher adsorption capacity, indicating its promising potential for application in dye wastewater treatment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eParameters of the Langmuir and Freundlich adsorption isotherm models\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\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 \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eq\u003csub\u003eexp\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eLangmuir model fitting\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003eFreundlich model fitting\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eq\u003csub\u003em\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eK\u003csub\u003eL\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003en\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eK\u003csub\u003eF\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e874.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e890.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.699\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.99912\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e3.905\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e381.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.98749\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 \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAdsorption performance of different materials for methylene blue solutions\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eadsorbent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eq\u003csub\u003eexp\u003c/sub\u003e(mg/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eref\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNR-g-PAM hydrogels\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e538.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHC-g-Am-BIS-BT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e140.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePoly(acrylic acrylamide) slag composite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e463\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePSBMA-NaSS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e760\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSPC-SAP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e62.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e[\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e]\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePoly(AA-AM-SSS)/CTS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e874.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003ethis work\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. CONCLUSION","content":"\u003cp\u003eA P(AA-AM-SSS)/CTS composite superabsorbent polymer was successfully synthesized via inverse suspension polymerization. The incorporation of sodium styrene sulfonate and chitosan markedly enhanced the water absorbency, salt tolerance, thermal stability, and dye adsorption performance of the material. Under optimal conditions, the composite resin exhibited a water absorption capacity of 1233.7 g/g and a saline absorption capacity of 137.3 g/g. Notably, approximately 75% of the initial swelling capacity was retained after five swelling-drying cycles, and a high water retention of 84.2% was maintained at 60\u0026deg;C. FT-IR results verified the successful copolymerization of the P(AA-AM-SSS) resin and the structural optimization induced by chitosan incorporation. SEM images demonstrated the formation of an interconnected three-dimensional network within the composite resin. Thermogravimetric analysis further revealed a residual mass of 42.9% at 600\u0026deg;C, indicating excellent thermal stability. In terms of adsorption performance, the composite demonstrated a high maximum adsorption capacity toward methylene blue of 874.6 mg/g. The adsorption behavior was well described by the pseudo-second-order kinetic model and the Langmuir isotherm model, suggesting that monolayer adsorption and chemical interactions may contribute to the overall adsorption process. Overall, the P(AA-AM-SSS)/CTS composite superabsorbent polymer exhibits a favorable combination of high water absorbency, salt resistance, and efficient dye removal capability, highlighting its potential for dye wastewater treatment applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCREDIT AUTHORSHIP CONTRIBUTION STATEMENT\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eXiaoxuan Jv:\u0026nbsp;\u003c/strong\u003eWriting-original draft, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. \u003cstrong\u003eXiangchi Liu:\u0026nbsp;\u003c/strong\u003eInvestigation, Formal analysis, Conceptualization. \u003cstrong\u003eAiling Liu:\u0026nbsp;\u003c/strong\u003eFormal analysis, Data curation. \u003cstrong\u003eBaijun Liu:\u003c/strong\u003e Writing-review \u0026amp; editing, Writing-original draft. \u003cstrong\u003eMingyao Zhang:\u003c/strong\u003e Writing-review \u0026amp; editing, Writing-original draft.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDECLARATION OF COMPETING INTEREST\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDATA AVAILABILITY\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eACKNOWLEDGEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors appreciate the financial support from the Jilin Province Science and Technology Development Plan Project (20240602019RC).\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWan X, Rong Z, Zhu K, Wu Y (2022) Chitosan-based dual network composite hydrogel for efficient adsorption of methylene blue dye. 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J Environ Chem Eng 10(5):108425. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jece.2022.108425\u003c/span\u003e\u003cspan address=\"10.1016/j.jece.2022.108425\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"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":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Superabsorbent polymer, Sodium styrene sulfonate, Chitosan, Salt-resistant swelling, Methylene blue adsorption","lastPublishedDoi":"10.21203/rs.3.rs-8930030/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8930030/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn response to the escalating global water pollution problem, a three-dimensional crosslinked poly(acrylic acid-acrylamide-sodium styrene sulfonate)/chitosan (P(AA-AM-SSS)/CTS) composite superabsorbent resin was successfully synthesized via inverse suspension polymerization. A series of systematic experiments were performed to elucidate the effects of initiator dosage, crosslinker content, and monomer composition on the network structure, swelling behavior, and adsorption performance. Through controlled optimization of the polymerization conditions, a structurally stable three-dimensional polymer network was established. Under optimized conditions, the composite resin exhibited a high swelling capacity of 1233.7 g/g in deionized water and 137.3 g/g in saline solution, demonstrating effective enhancement of both water absorbency and salt tolerance. Meanwhile, the material maintained favorable water retention at 60\u0026deg;C, achieving a water retention rate of approximately 85%, and displays outstanding reswelling capability. Thermogravimetric analysis revealed a residual mass of 42.9% at 600\u0026deg;C, indicating enhanced thermal stability. In addition, adsorption experiments toward the cationic dye methylene blue were systematically conducted under varying adsorbent dosage, contact time, temperature, and initial dye concentration. The optimized composite achieved a maximum adsorption capacity of 874.6 mg/g under the studied conditions. Therefore, the incorporation of sulfonic acid groups and chitosan-based structural regulation into the superabsorbent resin network enables the simultaneous improvement of water absorbency, salt tolerance, and dye adsorption performance, indicating its potential applicability in wastewater treatment.\u003c/p\u003e","manuscriptTitle":"Salt-Resistant Swelling Behavior and Methylene Blue Adsorption Performance of Chitosan-Modified Poly(acrylic acid-acrylamide-sodium styrene sulfonate) Superabsorbent Resin","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-10 07:25:56","doi":"10.21203/rs.3.rs-8930030/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-23T13:15:12+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-23T11:45:18+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-17T02:34:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"302852566397299205824301979829596519358","date":"2026-03-15T13:07:40+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"114971225880894882332034821461197516030","date":"2026-03-13T21:24:13+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-03-13T11:56:48+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-26T08:40:46+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-26T08:39:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Colloid and Polymer Science","date":"2026-02-21T02:30:10+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"colloid-and-polymer-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Colloid and Polymer Science](https://www.springer.com/journal/396) ","snPcode":"396","submissionUrl":"https://mc.manuscriptcentral.com/cps","title":"Colloid and Polymer Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"6811f639-93e4-4d8e-9b36-5cfc8adcea60","owner":[],"postedDate":"March 10th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-13T21:09:23+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-10 07:25:56","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8930030","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8930030","identity":"rs-8930030","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}