Sustainable synthesis and optimization of geothermal silica reinforced sodium carboxymethylcellulose (CMCNa)-based hydrogels with enhanced swelling performance

Sustainable synthesis and optimization of geothermal silica reinforced sodium carboxymethylcellulose (CMCNa)-based hydrogels with enhanced swelling performance | 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 Sustainable synthesis and optimization of geothermal silica reinforced sodium carboxymethylcellulose (CMCNa)-based hydrogels with enhanced swelling performance Anisa Galuh Arisanti, Vincent Sutresno Hadi Sujoto, Fatimah Tresna Pratiwi, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8905255/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 11 You are reading this latest preprint version Graphical Abstract Abstract Natural polymer-based hydrogels are environtmentally friendly and biodegradable polymers capable of absorbing an enormous volume of water, yet they often suffer from poor structural stability upon extensive swelling. Reinforcement using inorganic fillers such as silica can overcome this limitation. Geothermal silica (GS), a silica-rich byproduct (> 95% SiO₂) from geothermal power plants, offers a sustainable reinforcing agent owing to its porous and hydrophilic characteristics. In this study, sodium carboxymethylcellulose (CMCNa)-based hydrogels were synthesized using citric acid (CA) as a crosslinker agent and GS as an inorganic filler to enhance swelling performance. Fourier Transform Infrared (FTIR) spectroscopy and SEM analysis confirmed the successful of the hydrogel synthesis and silica incorporation into the hydrogel network. Swelling behavior evaluated over 24 hours showed that moderate GS and CA ratios produced the best performance, achieving a maximum swelling ratio of 216.95 g g⁻¹. Optimization using Response Surface Methodology (RSM) identified citric acid concentration as the most significant parameter influencing swelling, with a strong correlation (R² = 0.9049); the optimized swelling ratio variabel is 226.44 g water g⁻¹ hydrogel, achieved at an CA/CMCNa mass ratio of 1.29 wt% and an GS/CMCNa mass ratio of 2.17 wt%. The overall process is technically and economically feasible, utilizing low-cost materials and mild reaction conditions. These results demonstrate that geothermal silica–CMCNa hydrogels represent a sustainable, scalable, and eco-friendly material system for environmental and agricultural applications. geothermal silica carboxymethylcellulose (CMCNa) citric acid hydrogel optimization sustainable materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Hydrogels are three-dimensional (3D) crosslinked polymeric networks that exhibit remarkable hydrophilicity, enabling them to absorb and retain large amounts of water or aqueous solutions without compromising their structural integrity [ 1 ]. Depending on the type of crosslinking, whether physical or chemical, they display a broad range of physicochemical and mechanical properties that make them useful in diverse technological and scientific applications. Because of their tunable swelling behavior, biocompatibility, and permeability, hydrogels have found widespread applications in environmental engineering, biomedicine, and agriculture. Typical applications include controlled drug delivery systems [ 2 , 3 ], wastewater treatment and pollutant adsorption [ 4 ], biofuel dehydration and separation processes [ 5 – 8 ], as well as soil conditioning and moisture retention in agricultural practices [ 9 – 13 ]. Natural polymer–based hydrogels have recently attracted increasing scientific attention as sustainable and environmentally friendly alternatives to synthetic polymer–based hydrogels. This interest is driven by their biodegradability, non-toxicity, and compatibility with biological systems, which make them suitable for environmentally friendly and biomedical applications. Among natural polymers, sodium carboxymethylcellulose (CMCNa) is a water-soluble cellulose derivative with valuable properties such as high viscosity, excellent film-forming capability, and the ability to form gels through hydrogen bonding and electrostatic interactions [ 14 ]. Its abundance, renewability, and low cost make it an attractive base material for the design of green hydrogel systems. However, CMCNa-based hydrogels often suffer from weak mechanical strength and poor structural stability under high-swelling conditions, which restricts their durability and practical performance [ 15 ]. Several modification strategies have been investigation to address these limitations, including chemical crosslinking and the incorporation of inorganic reinforcing agents. Chemical crosslinking enhances the mechanical stability and elasticity of polymer networks by introducing covalent bonds between polymer chains, thereby strengthening the overall structure. Among various crosslinkers, citric acid (CA), a naturally occurring tricarboxylic acid, has been widely studied as a non-toxic and multifunctional crosslinking agent that forms ester bonds with hydroxyl groups of polysaccharide chains during thermal treatment [ 14 ]. This esterification reaction increases the rigidity and chemical stability of the polymer network while preserving its biocompatibility. At the same time, the addition of inorganic fillers such as silica particles further improves mechanical strength, porosity, and water absorption. The silanol groups (Si–OH) on the silica surface form secondary hydrogen bonds with polymer hydroxyl groups, which increase hydrophilicity, enlarge pore volume, and create microchannels that facilitate the penetration and diffusion of water molecules [ 16 – 22 ]. One up-and-coming inorganic source is geothermal silica (GS), a high-purity amorphous silica byproduct from geothermal power plants that contains more than 96 percent silica [ 23 – 26 ]. GS offers a sustainable and low-cost material that supports waste valorization and the circular economy. The reuse of GS not only reduces the environmental burden associated with geothermal plant waste but also provides sustainable raw material for the synthesis of advanced materials. Previous studies have demonstrated the suitability of geothermal silica for producing silica gels, nanoparticles, and composites with excellent surface area and reactivity. However, the integration of GS into biopolymer hydrogel matrices has not been systematically investigated. The potential of GS as a reinforcing agent that could improve the swelling properties of hydrogels remains largely unexplored. This study aims to develop a novel sodium carboxymethylcellulose (CMCNa)-based hydrogel chemically crosslinked with citric acid and reinforced with geothermal silica as an inorganic filler to enhance swelling performance. The research focuses on understanding the synergistic effects of CA and GS on the structural and swelling behavior of the hydrogel. The incorporation of GS is expected to enhance water absorption through increased porosity and hydrophilicity, while CA controls the network rigidity by adjusting the crosslinking density. The interaction between the organic polymer matrix and the inorganic filler is anticipated to result in the formation of hydrogen bonding, which are key factors influencing the swelling behavior and structural morphology of the resulting hydrogel. To identify the optimal synthesis conditions for maximizing swelling capacity, Response Surface Methodology (RSM) was employed as a statistical optimization method. RSM combines mathematical modeling and regression analysis to evaluate the effects of multiple process variables, such as the ratios of CA/CMCNa and GS/CMCNa, on a defined response [ 27 , 28 ]. This approach enables the establishment of a predictive model that describes the interaction among synthesis parameters and the corresponding swelling performance. It also allows for accurate optimization of formulation conditions to achieve the desired hydrogel properties. In addition to swelling evaluation, Fourier Transform Infrared (FTIR) spectroscopy was employed to verify the chemical structure and confirm the formation of ester and siloxane bonds within the hydrogel matrix while Scanning Electron Microscopy (SEM) was utilized to characterize the morphological change of hydrogel. The swelling behavior was assessed through 24-hour immersion tests in water to determine equilibrium swelling ratios and to understand how the variations in GS and CA concentrations affect hydrogel stability and porosity. Overall, this study provides a sustainable strategy for hydrogel synthesis by integrating natural polymers with industrial by-products, thereby supporting the development of environmentally friendly and high-performance materials. This approach promotes both material innovation and environmental responsibility by demonstrating that geothermal silica can be transformed into a high-value reinforcing component for hydrogel synthesis. The findings are expected to guide the design of biodegradable superabsorbent materials with improved swelling performance, offering potential applications in agriculture, biomedical engineering, and environmental remediation where controlled water retention and release are critical. 2. Method and material 2.1. Equipment and materials The materials and instruments used in this research were selected to ensure consistency and reproducibility of the synthesis process. Sodium carboxymethylcellulose (CMCNa, average molecular weight 700,000; degree of substitution 0.81; viscosity 5520 cps) obtained from Sigma-Aldrich served as the polymer backbone because of its hydrophilic and film-forming properties. Anhydrous citric acid (CA) from the same supplier was used as an environmentally friendly crosslinking agent to establish ester bonds with CMCNa chains during thermal treatment. The inorganic component, geothermal silica (GS), was supplied by PT Geo Dipa Energi, Dieng, Central Java, Indonesia. The raw silica contained roughly 95% SiO 2 , accompanied by minor impurities such as Fe 2 O 3 , Al 2 O 3 , and CaO derived from geothermal scaling deposits. To obtain material suitable for hydrogel reinforcement, the silica underwent a series of purification steps, including washing, acid leaching, and calcination, which increased its purity to over 96% SiO 2 and produced a fine, amorphous powder. Other reagents comprised sodium hydroxide (NaOH) pellets for sodium silicate preparation, sulfuric acid (H 2 SO 4 , 98%), and acetone (analytical grade), all purchased from Supelco. Distilled water was used for all dissolution, purification, and washing processes. Experimental work utilized a magnetic stirrer with temperature control, an overhead mechanical stirrer, a vacuum filtration unit, a laboratory oven, and a high-temperature furnace for calcination. Characterization employed a Shimadzu IR Spirit QATR-S FTIR spectrometer with ATR accessories, and all weighing was performed on an analytical balance with ± 0.0001 g precision to ensure accuracy in mass measurements. The morphology of samples was analyzed by scanning electron microscopy (SEM, Hitachi SU3500) conducted with an acceleration voltage of 3kV. All samples were sputter coated with ultrathin coating of Au before analysis. 2.2. Methodology The preparation of the geothermal silica–reinforced hydrogel followed a systematic workflow beginning with raw material purification and ending with statistical optimization of swelling behavior. The geothermal silica was first purified to remove metallic and mineral impurities through a sequence of grinding, sieving, water washing, acid leaching, and calcination following the method of Kusumastuti et al. [ 29 ]. The raw powder was ground to pass a 60-mesh sieve, mixed with distilled water at a 1:10 mass ratio, and stirred at 90°C for 2 hours. After filtration and drying at 100°C, the sample was treated with 20% H₂SO₄ solution at a 4:1 solid-to-liquid ratio and again stirred at 90°C for 2 hours. The acid-treated silica was rinsed with distilled water to neutral pH, dried at 100°C for 1 hour, and finally calcined at 700°C for 4 hours with a heating rate of 350°C h⁻¹ to produce high-purity geothermal silica. Figure 1 illustrates the physical appearance of raw geothermal silica in comparison with purified geothermal silica. The purified silica was then converted into a sodium silicate (SS) precursor by dissolving 10 g of silica in 100 mL of 0.0077 M NaOH and stirring at 90°C for 1 hour. After cooling and filtration, the clear SS solution was used immediately for hydrogel synthesis. This solution acted as both a silica source and a structural reinforcing agent within the polymer network. For hydrogel synthesis, 2 g of CMCNa was dissolved in 80 mL of distilled water and stirred for 1 hour until homogeneous. A predetermined mass of CA dissolved in 20 mL of water and a measured volume of SS solution were subsequently added and stirred for 2 hours at 300 rpm. The resulting mixture was poured into 15 cm Petri dishes and left overnight to eliminate trapped air bubbles before thermal crosslinking at 80°C for 8 hours. The crosslinked gels were washed with acetone to remove unreacted components and dried at 80°C for 3 hours to yield solid hydrogel sheets. Finally, the swelling ratio test quantified the hydrogel’s ability to absorb water. Dried hydrogel samples (m 0 ​) were immersed in distilled water for 24 hours at room temperature, then surface-blotted and re-weighed (m 1 ​). The swelling ratio was calculated using Eq. 1 . $$\:\text{x}=\:\frac{{\text{m}}_{1}-{\text{m}}_{0}}{{\text{m}}_{0}}\times\:100\:\text{\%}$$ 1 where m 0 ​ and m 1 ​ represent the dry and swollen masses, respectively. Each experiment was conducted in triplicate to ensure reproducibility. The collected data were analyzed by analysis of variance (ANOVA) to determine significant effects and to identify the optimal CA/CMCNa and GS/CMCNa ratios that maximize swelling performance. 2.1. Response surface methodology Response Surface Methodology (RSM) combined with a Central Composite Design (CCD) was employed to optimize the formulation of geothermal silica–reinforced CMCNa hydrogels. This statistical approach was selected because it reduces the number of required experimental runs while enabling the simultaneous evaluation of multiple variables and their interaction effects, thereby improving the efficiency and reliability of the optimization process. Two independent variables were selected for optimization: the mass ratio of citric acid to CMCNa (CA/CMCNa) and the mass ratio of geothermal silica to CMCNa (GS/CMCNa). These factors were chosen based on their critical influence on hydrogel crosslinking density and water absorption capacity. The swelling ratio (SR) was used as the response variable. The experimental design consisted of five coded levels (− 1.414, − 1, 0, + 1, and + 1.414) for each variable to capture both linear and quadratic effects. A total of 13 experimental runs were designed by the CCD method, along with three control samples (A0-1, A02, and A0-3) prepared without the addition of geothermal silica. These two samples were used for comparison purposes only and were not included in statistical modeling. The details of the experimental combinations are presented in Table 1 . Table 1 Variants of hydrogels based on the Central Composite Design (CCD) method No Level (Coded) Actual Value Sample Code A (CA/CMCNa) B (GS/CMCNa) CA/CMCNa Ratio GS/CMCNa Ratio 1 −1 −1 1.5% 3.0% A1 2 + 1 −1 2.5% 3.0% A2 3 −1 + 1 1.5% 7.0% A3 4 + 1 + 1 2.5% 7.0% A4 5 −1.414 0 1.293% 5.0% A5 6 + 1.414 0 2.707% 5.0% A6 7 0 −1.414 2.0% 2.172% A7 8 0 + 1.414 2.0% 7.828% A8 9 0 0 2.0% 5.0% A9 10 0 0 2.0% 5.0% A10 11 0 0 2.0% 5.0% A11 12 0 0 2.0% 5.0% A12 13 0 0 2.0% 5.0% A13 14 – – 1.5% 0.0% A0-1* 15 – – 2.0% 0.0% A0-2* 16 – – 2.5% 0.0% A0-3* * This formulation was not included in the RSM optimization and was instead employed as a control to evaluate the baseline properties of the GS-free hydrogel. 3. Results and discussion 3.1. Structural Confirmation by FTIR Analysis The analysis of functional groups using Fourier Transform Infrared (FTIR) spectroscopy is crucial for verifying the chemical interactions and structural evolution occurring during hydrogel synthesis. It provides direct evidence of bond formation, crosslinking reactions, and inorganic–organic interactions within polymeric materials. In the context of this study, FTIR analysis serves to confirm both the esterification reaction between citric acid (CA) and sodium carboxymethylcellulose (CMCNa) and the successful incorporation of geothermal silica (GS) into the hybrid hydrogel matrix, which are essential for understanding the structure–property relationship governing swelling behavior. ATR–FTIR spectroscopy was utilized to identify specific functional groups and to verify the crosslinking process and successful incorporation of GS into the hydrogel matrix. Figure 2 presents the FTIR spectra of hydrogels synthesized with and without GS. Compared with the characteristic bands of CMCNa, both hydrogel spectra, with and without GS exhibit a distinct absorption band around 1735 cm⁻¹, corresponding to the C = O stretching vibration of ester bonds formed between the carboxyl groups of CA and the hydroxyl groups of CMCNa. This result confirms that the crosslinking reaction proceeded successfully through esterification. In addition, the broad absorption band centered around 3314 cm⁻¹, assigned to O–H stretching vibrations, shows a noticeable decrease in intensity after hydrogel synthesis. This reduction indicates the consumption of hydroxyl groups during crosslinking process, further supporting the formation of ester bonds within the hydrogel network [ 30 ]. Upon the incorporation of GS, the O–H stretching band becomes broader and of lower intensity compared to the hydrogel without GS. This change reflects the additional hydrogen bonding and potential interaction between the silanol (Si–OH) groups of GS and the hydroxyl groups within the polymer matrix. These interactions suggest that GS participates not only through physical reinforcement but also via interfacial bonding with polymer network. Consequently, the participation of GS enhances network integrity and promotes stability of the resulting hydrogel structure. Moreover, in the spectral range of 1021–1054 cm⁻¹, the hydrogel incorporating GS exhibits a broader band compared to the hydrogel without GS. This broadening is attributed to the overlapping contributions of asymmetric vibrations of asymmetric Si–O–Si stretching from GS and C–O–C stretching from the CMCNa backbone. In contrast, the hydrogel without GS displays a lower peak in this region, associated solely with C–O–C vibrations of the polysaccharide chains. The emergence of Si–O–Si vibration confirms the successful integration of GS into the hydrogel network, leading to improved interfacial bonding between the inorganic filler and the polymer matrix. The formation of the CMCNa – based hydrogel reinforced with geothermal silica (GS) proceeds through a series of chemical and physical interactions as schematically illustrated in Figures (3a) – (3g). In aqueous medium, sodium silicate undergoes hydrolysis to generate silanol (Si-OH) groups, which subsequently condense to form siloxane (Si-O-Si) structures. Simultaneously, CA undergoes dehydration to form a cyclic anhydride intermediate that subsequently reacts with the hydroxyl groups of CMCNa to form ester linkages, confirming the formation of chemical crosslinks within the hydrogel matrix. In addition to covalent bonding, the silanol groups on GS interact with hydroxyl and carboxylate groups of CMCNa via hydrogen bonding. These physical interactions enhance interfacial adhesion between the organic polymer and inorganic silica phases. The coexistence of ester crosslinks and silica–polymer interactions produce synergistic reinforcing effect, improving network integrity, mechanical stability, and swelling behaviour of the hydrogel [ 30 ]. This dual crosslinking mechanism is consistent with the FTIR results, which confirm ester bond formation and silica incorporation within the hydrogel network. Figure 3. Proposed crosslinking mechanism of CMCNa with CA and GS as a reinforcing agent. 3.2. Morfology Images by SEM analysis Scanning Electron Microscopy (SEM) was utliized to investigate the morphological surface of the synthesized CMCNa – CA hydrogels at a magnification of 1000x, as shown in Fig. 4 . The hydrogel synthesized without geothermal silica (GS) (Fig. 4 a) exhibits relatively smooth and homogenous surface, indicating the formation of a dense and polymer matrix dominated by the crosslinked CMCNa network with limited microstructural heterogeneity. This morphology suggests a compact structure with minimal contribution from inorganic phases. In contrast, the hydrogel incorporating GS (Fig. 4 b) shows a noticeably rougher and more textured surface compared to the GS – free sample. The increased surface roughness and compact appearance are attributed to the successful incorporation of geothermal silica within the polymer matrix, introducing additional physical interactions and inorganic domains. These silica - rich regions promote interfacial bonding between the silanol (Si–OH) groups and the hydroxyl groups of CMCNa, resulting in a more consolidated and reinforced network structure. The observed morphological differences corroborate the FTIR results, confirming that GS effectively participates in the hydrogel network and contributes to enhanced structural integrity. 3.3. Swelling Characteristics and Network Behavior A critical parameter in evaluating hydrogel performance is the swelling ratio, which represents the material’s capacity to absorb and retain water within its three-dimensional polymeric network. This property is primarily governed by the degree of crosslinking and the abundance of hydrophilic functional groups, both of which determine the mobility of polymer chains and their interaction with water molecules. Understanding these characteristics provides insight into the hydrogel’s network structure and guides its potential application in wastewater treatment, biomedical engineering, and agricultural systems. In this study, the swelling behavior of CMCNa-based hydrogels was assessed over a 24-hour immersion period under static conditions. Figure 5 illustrates the morphological transformation of hydrogel films during the swelling process. Before immersion (Fig. 5 a), the samples appeared dry, rigid, and maintained their original dimensions, indicating the formation of a stable crosslinked matrix. After 24 hours of immersion (Fig. 5 b), the hydrogels expanded significantly. They developed a soft, gel-like texture as water diffused into the polymeric network, causing relaxation and volumetric expansion until equilibrium was reached [ 31 , 32 ]. This transformation demonstrates the superabsorbent nature of the synthesized hydrogels and confirms their suitability for applications requiring high fluid uptake. A visual comparison between the A5 and A7 samples after immersion reveals that A5 appeared more fragile and loosely structured than A7. The lower citric-acid (CA) concentration in A5 resulted in a less densely crosslinked polymer network, allowing greater chain mobility and water penetration but reducing mechanical rigidity. Although A5 contained a higher geothermal-silica (GS) content than A7, the hydrogen and ionic interactions provided by GS were weaker than the covalent ester bonds produced by CA, leading to higher flexibility but lower mechanical stability [ 33 ]. These observations indicate that while GS enhances hydrophilicity and porosity, CA plays the dominant role in defining network stability and stiffness through strong covalent crosslinks with CMCNa [ 34 ]. The quantitative results of the 24-hour swelling ratio (SR₂₄) are summarized in Fig. 6 a, which presents the swelling behavior of all hydrogel formulations. The baseline sample A0-2, synthesized without GS, exhibited a swelling ratio of 55.94 g water per g hydrogel, representing a moderately crosslinked structure with limited hydrophilic sites. Upon the incorporation of GS, a notable improvement in swelling performance was observed. As shown in Fig. 6 b, which illustrates the effect of varying the GS/CMCNa mass ratio at a fixed CA/CMCNa ratio of 2%, the swelling capacity increased initially and then decreased at higher filler loadings. Sample A7, with a low GS/CMCNa ratio of 2.17%, achieved the highest swelling ratio of 69.11 g water per g hydrogel. This improvement can be attributed to the hydrophilic silanol (Si–OH) groups of silica that enhance water affinity and enlarge network porosity [ 19 , 20 ]. At this level, GS likely functions as a structural modifier, generating additional free volume and water-accessible domains without compromising matrix flexibility [ 3 ]. However, excessive GS loading led to performance deterioration. Sample A8, with a GS/CMCNa ratio of 7.83%, exhibited a much lower swelling ratio of 40.72 g water per g hydrogel, mainly due to particle agglomeration and pore blockage, which restricted water diffusion [ 19 ]. Overloading of GS also increased matrix rigidity, limiting chain expansion. In contrast, A9 (GS/CMCNa = 5%) displayed a moderate swelling ratio of 64.12 g water per g hydrogel, representing an optimum balance between porosity and mechanical stability. Similarly, A5, which combined moderate GS and low CA content, also surpassed the swelling performance of the baseline. These results confirm that moderate GS addition enhances swelling by improving porosity and hydrophilicity, whereas excessive filler causes pore collapse and reduced performance [ 20 ]. The influence of crosslinker concentration is shown in Fig. 6 c, which demonstrates the effect of varying CA/CMCNa mass ratios at a constant GS/CMCNa ratio of 5%. The data reveal an inverse correlation between CA content and swelling capacity. Among all samples, A5, containing the lowest CA/CMCNa ratio of 1.29%, achieved the highest swelling ratio of 216.95 g water per g hydrogel, significantly exceeding the baseline A0-2 (55.94 g water per g hydrogel). The relatively low crosslinking density in A5 allowed higher chain mobility and greater pore volume for water diffusion. As CA concentration increased, the swelling ratio progressively declined: A9 (CA/CMCNa = 2%) reached 64.12 g water per g hydrogel, while A6 (CA/CMCNa = 2.71%) dropped sharply to 8.75 g water per g hydrogel. These results indicate that excessive CA produces a rigid, tightly crosslinked matrix that suppresses polymer relaxation and water uptake [ 7 , 22 , 35 – 38 ]. 3.4. Optimization of swelling response by response surface methodology Response Surface Methodology (RSM) was employed to statistically evaluate and model the relationships between synthesis variables and the swelling performance of the hydrogels, thereby enabling optimization of the hydrogel preparation parameters. In this study, the swelling ratio after 24 hours (SR₂₄) was used as the response variable, while the CA/CMCNa mass ratio (Factor A) and GS/CMCNa mass ratio (Factor B) were selected as the independent variables. A total of 13 hydrogel formulations were analyzed according to the Central Composite Design (CCD), enabling the assessment of both linear and interaction effects of these parameters on the hydrogel’s swelling behavior. Based on regression analysis, the relationship between the swelling ratio (SR₂₄) and the synthesis parameters was described by the empirical polynomial equation shown in Eq. (2), achieving a coefficient of determination (R²) of 90.49%, which indicates excellent correlation between the model and the experimental results. SR24 = 755–49981*A – 1877*B + 697034*AA − 28855*BB + 198470*AB (2) The Pareto chart of standardized effects, presented in Fig. 7 a, illustrates the relative significance of each factor on the swelling response. The CA/CMCNa mass ratio (Factor A) exerts the most substantial influence, surpassing the reference t -value of 2.365 at a 95% confidence level. This finding confirms that the hydrogel’s swelling ability is primarily controlled by the degree of crosslinking, which is directly influenced by the citric acid concentration [ 35 , 39 ]. Increasing CA concentration promotes a denser network structure that constraints polymer chain mobility and water diffusion, consequently decreasing the swelling capacity. In contrast, the effect of the GS/CMCNa mass ratio (Factor B) was comparatively less significant but still contributed to modifying the porosity and hydrophilicity of the hydrogel matrix. To visualize the interaction effects between both variables, a contour plot of the 24-hour swelling ratio (SR₂₄) was generated and is presented in Fig. 7 b. The plot demonstrates that the highest swelling performance occurs in regions characterized by a low CA/CMCNa mass ratio (< 1.5%) and a moderate GS/CMCNa mass ratio (2.5–4.0%). This condition corresponds to a hydrogel network with lower crosslinking density, allowing higher flexibility and sufficient pore volume for water absorption. At the same time, moderate GS incorporation enhances hydrophilicity and micro-porosity, providing additional diffusion pathways and active sites that facilitate water uptake and retention within the hydrogel network. Based on the RSM optimization results shown in Fig. 7 c, the maximum predicted swelling ratio at 24 h (SR 24 ) was 226.44 g water g⁻¹ hydrogel, achieved at an CA/CMCNa mass ratio of 1.29 wt% and an GS/CMCNa mass ratio of 2.17 wt%. To validate this prediction, an experimental verification was conducted under the same conditions, yielding an actual SR 24 value of 195.72 g water g⁻¹ hydrogel. The discrepancy between the predicted and experimental results corresponds to a deviation of 15.69%. This deviation is likely attributed to insufficient network stability at the predicted optimal composition. At low CA and GS contents, the ester crosslinks formed by citric acid and the physical interactions contributed by geothermal silica are not strong enough to generate a robust and durable crosslinked network. Consequently, the hydrogel structure becomes excessively loose and prone to structural failure during the swelling process. These results emphasize the critical importance of achieving a balanced combination between the crosslinker (CA) and inorganic filler (GS) concentrations. Excessive CA concentration yields a rigid and compact structure that limits swelling, while insufficient CA may result in a weak and unstable network. Conversely, moderate GS levels reinforce the polymer matrix through physical and hydrogen bonding interactions without restricting water uptake. Therefore, an optimal synthesis condition, characterized by a low CA/CMCNa mass ratio and a moderate GS/CMCNa mass ratio, offers the best compromise between flexibility and mechanical stability, ultimately maximizing the hydrogel’s water absorption capacity [ 35 , 39 ]. 3.5. Feasibility and cost–benefit analysis of scaling-up geothermal silica-based hydrogel production The techno-economic evaluation was performed to assess the feasibility and scalability of the proposed geothermal silica–based hydrogel production process. This analysis provides a quantitative understanding of the cost structure, investment requirement, and operational efficiency, offering a comparison against a conventional PVA/CMC–Silica Hydrogel process. The discussion also demonstrates how the valorization of geothermal silica can create economic and environmental value by transforming an industrial byproduct into a functional hydrogel material. The results bridge the laboratory-scale process optimization with industrial relevance, outlining the potential for large-scale application under realistic production conditions. The assessment followed the cost estimation methodology described by Capanema et al. (2023), with all cost parameters updated to reflect local market prices in Indonesia (2025) [ 40 ]. The evaluation assumes an independent small-scale pilot facility, without shared infrastructure, designed to produce 1,000 kg of dried hydrogel per year, equivalent to about 4 kg per day with 250 operating days per year and a plant utilization rate of 80%. The process operates at atmospheric pressure and a maximum temperature of 90°C, followed by drying at 80°C for 8 hours. The expected equipment lifetime is 10 years, with a straight-line depreciation of 10% per year. Electricity cost is assumed to be 0.15 $ /kWh, while the plant is operated by two operators and one supervisor, each with an average wage of 500 $ per month. The feedstock includes sodium CMC (5 $ /kg) and citric acid (2 $ /kg) as a green crosslinker, while geothermal silica is considered a zero-cost raw material (0 $ /t), sourced from geothermal brine residue. The washing stage uses deionized water and acetone with 90% solvent recovery, resulting in an effective cost of 0.2 $ /kg of product. Annual maintenance and overhead are estimated at 5% of CAPEX, and waste management involves only neutralization and filtration, as the process generates non-hazardous wastewater. The selling price of agricultural-grade hydrogel is assumed at 15 $ /kg. Based on these parameters, the total capital expenditure (CAPEX) for the proposed geothermal silica–CMCNa process is estimated at approximately 26,800 $ , while the reference PVA/CMC–Silica Hydrogel process requires 35,200 $ , as shown in Fig. 8 . The proposed method achieves a 24% reduction in total capital investment due to its simplified reactor configuration, low-temperature operation, and the absence of sterilization or solvent-recovery systems. The reactor operates at atmospheric pressure and does not require an inert gas environment or stainless-steel construction. Therefore, epoxy-coated mild steel vessels are sufficient, reducing material and fabrication costs. The drying and curing oven consumes less power because it operates at 80°C rather than 120°C, minimizing insulation and electrical requirements. The filtration and washing setup is also simplified, employing a single-stage gravity or vacuum system instead of a multi-stage controlled system. Moreover, the water purification and analytical instruments are limited to those essential for environmental-grade product quality, further lowering investment costs. Overall, Fig. 8 shows that the simplified design and elimination of specialized biomedical equipment lead to significant capital savings. The operating expenditure (OPEX) comparison presented in Fig. 9 highlights an even greater difference. The total OPEX for the geothermal silica–CMCNa process is approximately 4.45 $ /kg, which is nearly 50% lower than the 9.15 $ /kg of the reference process. The most significant savings come from the raw materials, as geothermal silica is a freely available byproduct and citric acid is considerably cheaper and safer than glutaraldehyde. The process consumes less energy because of its low-temperature synthesis and short drying duration, reducing electricity demand by about 70%. Labor costs are also lower since only two operators are required, while the reference process includes additional personnel for sterile quality control. Maintenance and overhead are reduced because there are no pressurized or stainless-steel systems, and waste handling costs are minimal. After all, only neutralized rinse water is generated. The OPEX structure in Fig. 9 clearly indicates that the geothermal silica–CMCNa system is substantially more economical to operate, primarily due to low-cost raw materials, reduced energy input, and simplified maintenance requirements. Under these base-case conditions, the total annual production cost for 1,000 kg of dried hydrogel is approximately 4,450 $ per year, resulting in a gross margin of about 70% at the assumed selling price of 15 $ /kg. The estimated payback period is less than two years, with an internal rate of return (IRR) of about 38%, and a positive net present value (NPV) achieved within the first three years of operation. These results confirm that the geothermal silica–CMCNa process offers a strong techno-economic advantage by combining low capital investment, reduced energy consumption, and waste minimization, while simultaneously contributing to sustainable resource management. Overall, the analysis demonstrates that scaling up this production system under local market conditions is economically feasible and consistent with the objectives of green manufacturing and circular economy development. 4. Conclusion This study successfully established a sustainable and technically feasible approach for synthesizing geothermal silica–reinforced carboxymethylcellulose (CMCNa) hydrogels using citric acid as an environmentally benign crosslinker under mild reaction conditions. Optimization through response surface methodology (RSM) verified that the citric acid-to-CMCNa mass ratio was the most influential factor controlling the hydrogel’s network structure and swelling performance, yielding a strong correlation with an R² value of 0.9049. The optimized hydrogel achieved a maximum 24-hour swelling ratio of 216.5 g g⁻¹, confirming that moderate crosslinking effectively enhanced hydrophilicity while maintaining structural stability. Incorporation of geothermal silica improved the mechanical robustness and reusability of the hydrogel while enabling the valorization of geothermal waste into a high-value functional material. The techno-economic evaluation further demonstrated that the proposed process is economically competitive, requiring an estimated capital expenditure (CAPEX) of 26,800 $ and operating expenditure (OPEX) of 4.45 $ kg⁻¹, both significantly lower than conventional PVA/CMC–silica hydrogel systems. The combination of low-temperature processing, non-toxic reagents, and zero-cost geothermal silica feedstock contributes to substantial cost savings, improved energy efficiency, and environmental compatibility. Overall, this research highlights the integration of material innovation, cost efficiency, and sustainability principles, illustrating that geothermal silica–CMCNa hydrogels can serve as a scalable and eco-friendly alternative for industrial applications, while advancing the circular utilization of geothermal resources toward sustainable material production. Declarations Conflict of Interest The authors declare that there are no conflicts of interest associated with this work. Funding The authors gratefully acknowledge financial support from the Lembaga Pengelola Dana Pendidikian (LPDP) under the Republic of Indonesia, as well as research facilities provided by the Mineral Processing Research Group, Department of Chemical Engineering, Gadjah Mada University, The authors also sincerely thank PT Geo Dipa Energi (Persero) for their support in delivering geothermal solid waste materials in this study. Author Contribution All authors contributed to the conception and design of the study. Material preparation, data collection, analysis, data curation, and conceptualization were primaryly performed by Anisa Galuh Arisanti. Reviewing and editing were conducted by Vincent Sutresno Hadi Sujoto. Characterization, analysis and data curation were carried out by Fatimah Tresna Pratiwi, Rina Dewi Mayasari, Adhi Priyo Pamungkas. Supervision, conceptualization, investigation, review, and funding acquisition were performed by Rochmadi Rochmadi, Eka Tarwaca Susila Putra, Himawan Tri Bayu Murti Petrus. The first draft of the manuscript was written by Anisa Galuh Arisanti and all authors commented on previous versions of the manuscript. All authors read and approved the final version of the manuscript. Data Availability Data supporting the findings of this study will be made available upon reasonable request. References Anjali J, Jose VK, Lee JM (2019) Carbon-based hydrogels: Synthesis and their recent energy applications. J Mater Chem A 7:15491–15518. https://doi.org/10.1039/c9ta02525a Mali KK, Dhawale SC, Dias RJ et al (2018) Citric acid crosslinked carboxymethyl cellulose-based composite hydrogel films for drug delivery. 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Surf Interfaces 24. https://doi.org/10.1016/j.surfin.2021.101124 Pitaloka AB, Rukmana AS, Nur’afiani TY (2021) Synthesis and Characterization of Carboxy Methyl Cellulose-Based Hydrogel Cross-linked with Citric Acid. World Chem Eng J 5:7. https://doi.org/10.48181/wcej.v5i1.12082 Gorshkova MY, Volkova IF, Grigoriyan ES, Molchanov SP (2024) Structure and properties of hydrogels based on sodium alginate and synthetic polyacids. Mendeleev Commun 34:372–375. https://doi.org/10.1016/j.mencom.2024.04.019 Kusumastuti Y, Petrus HTBM, Yohana F et al (2017) Synthesis and characterization of biocomposites based on chitosan and geothermal silica. AIP Conf Proc 1823. https://doi.org/10.1063/1.4978200 Ninciuleanu CM, Ianchis R, Alexandrescu E et al (2021) The effects of monomer, crosslinking agent, and filler concentrations on the viscoelastic and swelling properties of poly(methacrylic acid) hydrogels: A cOMPARISON. Mater (Basel) 14. https://doi.org/10.3390/ma14092305 Reddy JP, Varada Rajulu A, Rhim JW, Seo J (2018) Mechanical, thermal, and water vapor barrier properties of regenerated cellulose/nano-SiO2 composite films. Cellulose 25:7153–7165. https://doi.org/10.1007/s10570-018-2059-x Capanema NSV, Mansur AAP, Carvalho IC et al (2023) Bioengineered Water-Responsive Carboxymethyl Cellulose/Poly(vinyl alcohol) Hydrogel Hybrids for Wound Dressing and Skin Tissue Engineering Applications. Gels 9. https://doi.org/10.3390/gels9020166 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 05 May, 2026 Reviews received at journal 28 Apr, 2026 Reviews received at journal 23 Apr, 2026 Reviewers agreed at journal 23 Apr, 2026 Reviewers agreed at journal 22 Apr, 2026 Reviewers agreed at journal 22 Apr, 2026 Reviewers agreed at journal 22 Apr, 2026 Reviewers invited by journal 22 Apr, 2026 Editor assigned by journal 23 Feb, 2026 Submission checks completed at journal 23 Feb, 2026 First submitted to journal 17 Feb, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-8905255","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":631409651,"identity":"c5fd338d-605b-401b-9a7a-9f992ced0da1","order_by":0,"name":"Anisa Galuh Arisanti","email":"","orcid":"","institution":"Universitas Gadjah Mada","correspondingAuthor":false,"prefix":"","firstName":"Anisa","middleName":"Galuh","lastName":"Arisanti","suffix":""},{"id":631409656,"identity":"32da1ded-ec5d-4560-b85b-7463c14da82d","order_by":1,"name":"Vincent Sutresno Hadi Sujoto","email":"","orcid":"","institution":"Universitas Gadjah 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02:54:21","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8905255/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8905255/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":108240638,"identity":"0de41790-90d9-40e1-a06b-791dcfdfb772","added_by":"auto","created_at":"2026-04-30 20:31:51","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":161121,"visible":true,"origin":"","legend":"\u003cp\u003eGeothermal silica (a) before purification, (b) after purification\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/ed9ff7eaee2611c53cb590eb.png"},{"id":108491379,"identity":"293a0d21-f678-408d-abb0-f6f3c5f7ef23","added_by":"auto","created_at":"2026-05-05 09:53:37","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":51776,"visible":true,"origin":"","legend":"\u003cp\u003eATR–FTIR spectra of CMCNa–CA hydrogels synthesized with and without geothermal silica (GS), showing the formation of ester and Si–O–Si bonds, confirming crosslinking and silica incorporation.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/e2b742ca85cb0ae781602818.png"},{"id":108240640,"identity":"d11b67d4-b6f1-4c51-87ba-7dd0eeac5766","added_by":"auto","created_at":"2026-04-30 20:31:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":243352,"visible":true,"origin":"","legend":"\u003cp\u003eProposed crosslinking mechanism of CMCNa with CA and GS as a reinforcing agent.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/f72b30d76323257b5399ec9d.png"},{"id":108240641,"identity":"1fd831b6-96e8-4fe7-b023-14c71b9d8912","added_by":"auto","created_at":"2026-04-30 20:31:51","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":350354,"visible":true,"origin":"","legend":"\u003cp\u003eSEM Images of CMCNa–CA hydrogels synthesized (a) without geothermal silica (GS) and (b) with geothermal silica (GS)\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/99dd5a9d169430338bcd273d.png"},{"id":108491694,"identity":"be67acf9-b3b0-4b8b-8818-b0fc0ff04cdc","added_by":"auto","created_at":"2026-05-05 09:55:14","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":250570,"visible":true,"origin":"","legend":"\u003cp\u003eCMCNa–GS hydrogel appearance in A5 and A7, respectively: (a) before swelling; (b) after 24 hours of immersion (216× swelling; 69× swelling).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/1009eea99ca56761d800bc61.png"},{"id":108240643,"identity":"a17e0392-a19f-4c2d-aa7a-1f9c0e551c2e","added_by":"auto","created_at":"2026-04-30 20:31:51","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":394442,"visible":true,"origin":"","legend":"\u003cp\u003eSwelling performance during 24 hours of immersion: (a) all samples; (b) variation of GS/CMCNa mass ratios; (c) variation of CA/CMCNa mass ratios.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/f5afb1dbdac4a9724310f994.png"},{"id":108491862,"identity":"89345811-1d41-44e0-9efb-df8daf548292","added_by":"auto","created_at":"2026-05-05 09:56:00","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":107567,"visible":true,"origin":"","legend":"\u003cp\u003eRSM analysis results: (a) Pareto chart of standardized effects; (b) Contour plot of swelling ratio distribution; (c) Optimization result analysis\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/8842c9f0dfaedc4c82dd3d81.png"},{"id":108240645,"identity":"fcd42d43-ead0-4749-bd0e-3b8224ee381b","added_by":"auto","created_at":"2026-04-30 20:31:51","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":79973,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of total capital expenditure (CAPEX) between the proposed Geothermal Silica–CMCNa Hydrogel and the reference PVA/CMC–Silica Hydrogel; data marked with (*) are calculated based on the methodology obtained from Capanema et al (2023) [40].\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/a5047b1146b7e64b7983fba8.png"},{"id":108491879,"identity":"38af2d02-ebcb-4bd9-b32d-f3cf354ece29","added_by":"auto","created_at":"2026-05-05 09:56:01","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":68622,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of total operating expenditure (OPEX) between the proposed Geothermal Silica–CMCNa Hydrogel and the reference PVA/CMC–Silica Hydrogel; data marked with (*) are calculated based on the methodology obtained from Capanema et al (2023) [40].\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/1f5bb3f57ae208f770966e1b.png"},{"id":108240646,"identity":"e5650281-ff93-438b-b115-4a66aaf69805","added_by":"auto","created_at":"2026-04-30 20:31:51","extension":"png","order_by":20,"title":"","display":"","copyAsset":false,"role":"graphical-abstract","size":39053,"visible":true,"origin":"","legend":"Natural polymer-based hydrogels are environtmentally friendly and biodegradable polymers capable of absorbing an enormous volume of water, yet they often suffer from poor structural stability upon extensive swelling. Reinforcement using inorganic fillers such as silica can overcome this limitation. Geothermal silica (GS), a silica-rich byproduct (\u0026gt;\u0026thinsp;95% SiO₂) from geothermal power plants, offers a sustainable reinforcing agent owing to its porous and hydrophilic characteristics. In this study, sodium carboxymethylcellulose (CMCNa)-based hydrogels were synthesized using citric acid (CA) as a crosslinker agent and GS as an inorganic filler to enhance swelling performance. Fourier Transform Infrared (FTIR) spectroscopy and SEM analysis confirmed the successful of the hydrogel synthesis and silica incorporation into the hydrogel network. Swelling behavior evaluated over 24 hours showed that moderate GS and CA ratios produced the best performance, achieving a maximum swelling ratio of 216.95 g g⁻\u0026sup1;. Optimization using Response Surface Methodology (RSM) identified citric acid concentration as the most significant parameter influencing swelling, with a strong correlation (R\u0026sup2; = 0.9049); the optimized swelling ratio variabel is 226.44 g water g⁻\u0026sup1; hydrogel, achieved at an CA/CMCNa mass ratio of 1.29 wt% and an GS/CMCNa mass ratio of 2.17 wt%. The overall process is technically and economically feasible, utilizing low-cost materials and mild reaction conditions. These results demonstrate that geothermal silica\u0026ndash;CMCNa hydrogels represent a sustainable, scalable, and eco-friendly material system for environmental and agricultural applications.","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/2e902d2c0b75795430d72670.png"},{"id":108494412,"identity":"c247cc24-10de-4731-952e-e549bc3e0f0e","added_by":"auto","created_at":"2026-05-05 10:05:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1912296,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8905255/v1/4914708f-0331-4668-ad35-2e4a8135355b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Sustainable synthesis and optimization of geothermal silica reinforced sodium carboxymethylcellulose (CMCNa)-based hydrogels with enhanced swelling performance","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eHydrogels are three-dimensional (3D) crosslinked polymeric networks that exhibit remarkable hydrophilicity, enabling them to absorb and retain large amounts of water or aqueous solutions without compromising their structural integrity [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Depending on the type of crosslinking, whether physical or chemical, they display a broad range of physicochemical and mechanical properties that make them useful in diverse technological and scientific applications. Because of their tunable swelling behavior, biocompatibility, and permeability, hydrogels have found widespread applications in environmental engineering, biomedicine, and agriculture. Typical applications include controlled drug delivery systems [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], wastewater treatment and pollutant adsorption [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], biofuel dehydration and separation processes [\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], as well as soil conditioning and moisture retention in agricultural practices [\u003cspan additionalcitationids=\"CR10 CR11 CR12\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eNatural polymer\u0026ndash;based hydrogels have recently attracted increasing scientific attention as sustainable and environmentally friendly alternatives to synthetic polymer\u0026ndash;based hydrogels. This interest is driven by their biodegradability, non-toxicity, and compatibility with biological systems, which make them suitable for environmentally friendly and biomedical applications. Among natural polymers, sodium carboxymethylcellulose (CMCNa) is a water-soluble cellulose derivative with valuable properties such as high viscosity, excellent film-forming capability, and the ability to form gels through hydrogen bonding and electrostatic interactions [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Its abundance, renewability, and low cost make it an attractive base material for the design of green hydrogel systems. However, CMCNa-based hydrogels often suffer from weak mechanical strength and poor structural stability under high-swelling conditions, which restricts their durability and practical performance [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSeveral modification strategies have been investigation to address these limitations, including chemical crosslinking and the incorporation of inorganic reinforcing agents. Chemical crosslinking enhances the mechanical stability and elasticity of polymer networks by introducing covalent bonds between polymer chains, thereby strengthening the overall structure. Among various crosslinkers, citric acid (CA), a naturally occurring tricarboxylic acid, has been widely studied as a non-toxic and multifunctional crosslinking agent that forms ester bonds with hydroxyl groups of polysaccharide chains during thermal treatment [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. This esterification reaction increases the rigidity and chemical stability of the polymer network while preserving its biocompatibility. At the same time, the addition of inorganic fillers such as silica particles further improves mechanical strength, porosity, and water absorption. The silanol groups (Si\u0026ndash;OH) on the silica surface form secondary hydrogen bonds with polymer hydroxyl groups, which increase hydrophilicity, enlarge pore volume, and create microchannels that facilitate the penetration and diffusion of water molecules [\u003cspan additionalcitationids=\"CR17 CR18 CR19 CR20 CR21\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOne up-and-coming inorganic source is geothermal silica (GS), a high-purity amorphous silica byproduct from geothermal power plants that contains more than 96 percent silica [\u003cspan additionalcitationids=\"CR24 CR25\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. GS offers a sustainable and low-cost material that supports waste valorization and the circular economy. The reuse of GS not only reduces the environmental burden associated with geothermal plant waste but also provides sustainable raw material for the synthesis of advanced materials. Previous studies have demonstrated the suitability of geothermal silica for producing silica gels, nanoparticles, and composites with excellent surface area and reactivity. However, the integration of GS into biopolymer hydrogel matrices has not been systematically investigated. The potential of GS as a reinforcing agent that could improve the swelling properties of hydrogels remains largely unexplored.\u003c/p\u003e \u003cp\u003eThis study aims to develop a novel sodium carboxymethylcellulose (CMCNa)-based hydrogel chemically crosslinked with citric acid and reinforced with geothermal silica as an inorganic filler to enhance swelling performance. The research focuses on understanding the synergistic effects of CA and GS on the structural and swelling behavior of the hydrogel. The incorporation of GS is expected to enhance water absorption through increased porosity and hydrophilicity, while CA controls the network rigidity by adjusting the crosslinking density. The interaction between the organic polymer matrix and the inorganic filler is anticipated to result in the formation of hydrogen bonding, which are key factors influencing the swelling behavior and structural morphology of the resulting hydrogel.\u003c/p\u003e \u003cp\u003eTo identify the optimal synthesis conditions for maximizing swelling capacity, Response Surface Methodology (RSM) was employed as a statistical optimization method. RSM combines mathematical modeling and regression analysis to evaluate the effects of multiple process variables, such as the ratios of CA/CMCNa and GS/CMCNa, on a defined response [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. This approach enables the establishment of a predictive model that describes the interaction among synthesis parameters and the corresponding swelling performance. It also allows for accurate optimization of formulation conditions to achieve the desired hydrogel properties.\u003c/p\u003e \u003cp\u003eIn addition to swelling evaluation, Fourier Transform Infrared (FTIR) spectroscopy was employed to verify the chemical structure and confirm the formation of ester and siloxane bonds within the hydrogel matrix while Scanning Electron Microscopy (SEM) was utilized to characterize the morphological change of hydrogel. The swelling behavior was assessed through 24-hour immersion tests in water to determine equilibrium swelling ratios and to understand how the variations in GS and CA concentrations affect hydrogel stability and porosity.\u003c/p\u003e \u003cp\u003eOverall, this study provides a sustainable strategy for hydrogel synthesis by integrating natural polymers with industrial by-products, thereby supporting the development of environmentally friendly and high-performance materials. This approach promotes both material innovation and environmental responsibility by demonstrating that geothermal silica can be transformed into a high-value reinforcing component for hydrogel synthesis. The findings are expected to guide the design of biodegradable superabsorbent materials with improved swelling performance, offering potential applications in agriculture, biomedical engineering, and environmental remediation where controlled water retention and release are critical.\u003c/p\u003e"},{"header":"2. Method and material","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Equipment and materials\u003c/h2\u003e \u003cp\u003eThe materials and instruments used in this research were selected to ensure consistency and reproducibility of the synthesis process. Sodium carboxymethylcellulose (CMCNa, average molecular weight 700,000; degree of substitution 0.81; viscosity 5520 cps) obtained from Sigma-Aldrich served as the polymer backbone because of its hydrophilic and film-forming properties. Anhydrous citric acid (CA) from the same supplier was used as an environmentally friendly crosslinking agent to establish ester bonds with CMCNa chains during thermal treatment. The inorganic component, geothermal silica (GS), was supplied by PT Geo Dipa Energi, Dieng, Central Java, Indonesia. The raw silica contained roughly 95% SiO\u003csub\u003e2\u003c/sub\u003e, accompanied by minor impurities such as Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and CaO derived from geothermal scaling deposits. To obtain material suitable for hydrogel reinforcement, the silica underwent a series of purification steps, including washing, acid leaching, and calcination, which increased its purity to over 96% SiO\u003csub\u003e2\u003c/sub\u003e and produced a fine, amorphous powder. Other reagents comprised sodium hydroxide (NaOH) pellets for sodium silicate preparation, sulfuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 98%), and acetone (analytical grade), all purchased from Supelco. Distilled water was used for all dissolution, purification, and washing processes. Experimental work utilized a magnetic stirrer with temperature control, an overhead mechanical stirrer, a vacuum filtration unit, a laboratory oven, and a high-temperature furnace for calcination. Characterization employed a Shimadzu IR Spirit QATR-S FTIR spectrometer with ATR accessories, and all weighing was performed on an analytical balance with \u0026plusmn;\u0026thinsp;0.0001 g precision to ensure accuracy in mass measurements. The morphology of samples was analyzed by scanning electron microscopy (SEM, Hitachi SU3500) conducted with an acceleration voltage of 3kV. All samples were sputter coated with ultrathin coating of Au before analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Methodology\u003c/h2\u003e \u003cp\u003eThe preparation of the geothermal silica\u0026ndash;reinforced hydrogel followed a systematic workflow beginning with raw material purification and ending with statistical optimization of swelling behavior. The geothermal silica was first purified to remove metallic and mineral impurities through a sequence of grinding, sieving, water washing, acid leaching, and calcination following the method of Kusumastuti et al. [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The raw powder was ground to pass a 60-mesh sieve, mixed with distilled water at a 1:10 mass ratio, and stirred at 90\u0026deg;C for 2 hours. After filtration and drying at 100\u0026deg;C, the sample was treated with 20% H₂SO₄ solution at a 4:1 solid-to-liquid ratio and again stirred at 90\u0026deg;C for 2 hours. The acid-treated silica was rinsed with distilled water to neutral pH, dried at 100\u0026deg;C for 1 hour, and finally calcined at 700\u0026deg;C for 4 hours with a heating rate of 350\u0026deg;C h⁻\u0026sup1; to produce high-purity geothermal silica. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e illustrates the physical appearance of raw geothermal silica in comparison with purified geothermal silica.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe purified silica was then converted into a sodium silicate (SS) precursor by dissolving 10 g of silica in 100 mL of 0.0077 M NaOH and stirring at 90\u0026deg;C for 1 hour. After cooling and filtration, the clear SS solution was used immediately for hydrogel synthesis. This solution acted as both a silica source and a structural reinforcing agent within the polymer network. For hydrogel synthesis, 2 g of CMCNa was dissolved in 80 mL of distilled water and stirred for 1 hour until homogeneous. A predetermined mass of CA dissolved in 20 mL of water and a measured volume of SS solution were subsequently added and stirred for 2 hours at 300 rpm. The resulting mixture was poured into 15 cm Petri dishes and left overnight to eliminate trapped air bubbles before thermal crosslinking at 80\u0026deg;C for 8 hours. The crosslinked gels were washed with acetone to remove unreacted components and dried at 80\u0026deg;C for 3 hours to yield solid hydrogel sheets.\u003c/p\u003e \u003cp\u003eFinally, the swelling ratio test quantified the hydrogel\u0026rsquo;s ability to absorb water. Dried hydrogel samples (m\u003csub\u003e0\u003c/sub\u003e​) were immersed in distilled water for 24 hours at room temperature, then surface-blotted and re-weighed (m\u003csub\u003e1\u003c/sub\u003e​). The swelling ratio was calculated using Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:\\text{x}=\\:\\frac{{\\text{m}}_{1}-{\\text{m}}_{0}}{{\\text{m}}_{0}}\\times\\:100\\:\\text{\\%}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere m\u003csub\u003e0\u003c/sub\u003e​ and m\u003csub\u003e1\u003c/sub\u003e​ represent the dry and swollen masses, respectively. Each experiment was conducted in triplicate to ensure reproducibility. The collected data were analyzed by analysis of variance (ANOVA) to determine significant effects and to identify the optimal CA/CMCNa and GS/CMCNa ratios that maximize swelling performance.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Response surface methodology\u003c/h2\u003e \u003cp\u003eResponse Surface Methodology (RSM) combined with a Central Composite Design (CCD) was employed to optimize the formulation of geothermal silica\u0026ndash;reinforced CMCNa hydrogels. This statistical approach was selected because it reduces the number of required experimental runs while enabling the simultaneous evaluation of multiple variables and their interaction effects, thereby improving the efficiency and reliability of the optimization process. Two independent variables were selected for optimization: the mass ratio of citric acid to CMCNa (CA/CMCNa) and the mass ratio of geothermal silica to CMCNa (GS/CMCNa). These factors were chosen based on their critical influence on hydrogel crosslinking density and water absorption capacity. The swelling ratio (SR) was used as the response variable. The experimental design consisted of five coded levels (\u0026minus;\u0026thinsp;1.414, \u0026minus;\u0026thinsp;1, 0, +\u0026thinsp;1, and +\u0026thinsp;1.414) for each variable to capture both linear and quadratic effects.\u003c/p\u003e \u003cp\u003eA total of 13 experimental runs were designed by the CCD method, along with three control samples (A0-1, A02, and A0-3) prepared without the addition of geothermal silica. These two samples were used for comparison purposes only and were not included in statistical modeling. The details of the experimental combinations are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\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\u003eVariants of hydrogels based on the Central Composite Design (CCD) method\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNo\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLevel (Coded)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eActual Value\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample Code\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eA (CA/CMCNa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eB (GS/CMCNa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCA/CMCNa Ratio\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eGS/CMCNa Ratio\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026minus;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026minus;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.5%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e3.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e+\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026minus;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.5%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e3.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026minus;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e+\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.5%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e7.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e+\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e+\u0026thinsp;1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.5%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e7.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026minus;1.414\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.293%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e+\u0026thinsp;1.414\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.707%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026minus;1.414\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2.172%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e+\u0026thinsp;1.414\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e7.828%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA11\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.5%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA0-1*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA0-2*\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u0026ndash;\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.5%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.0%\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003eA0-3*\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=\"BlockQuote\"\u003e \u003cp\u003e \u003cb\u003e* This formulation was not included in the RSM optimization and was instead employed as a control to evaluate the baseline properties of the GS-free hydrogel.\u003c/b\u003e \u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Structural Confirmation by FTIR Analysis\u003c/h2\u003e \u003cp\u003eThe analysis of functional groups using Fourier Transform Infrared (FTIR) spectroscopy is crucial for verifying the chemical interactions and structural evolution occurring during hydrogel synthesis. It provides direct evidence of bond formation, crosslinking reactions, and inorganic\u0026ndash;organic interactions within polymeric materials. In the context of this study, FTIR analysis serves to confirm both the esterification reaction between citric acid (CA) and sodium carboxymethylcellulose (CMCNa) and the successful incorporation of geothermal silica (GS) into the hybrid hydrogel matrix, which are essential for understanding the structure\u0026ndash;property relationship governing swelling behavior.\u003c/p\u003e \u003cp\u003eATR\u0026ndash;FTIR spectroscopy was utilized to identify specific functional groups and to verify the crosslinking process and successful incorporation of GS into the hydrogel matrix. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents the FTIR spectra of hydrogels synthesized with and without GS. Compared with the characteristic bands of CMCNa, both hydrogel spectra, with and without GS exhibit a distinct absorption band around 1735 cm⁻\u0026sup1;, corresponding to the C\u0026thinsp;=\u0026thinsp;O stretching vibration of ester bonds formed between the carboxyl groups of CA and the hydroxyl groups of CMCNa. This result confirms that the crosslinking reaction proceeded successfully through esterification. In addition, the broad absorption band centered around 3314 cm⁻\u0026sup1;, assigned to O\u0026ndash;H stretching vibrations, shows a noticeable decrease in intensity after hydrogel synthesis. This reduction indicates the consumption of hydroxyl groups during crosslinking process, further supporting the formation of ester bonds within the hydrogel network [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eUpon the incorporation of GS, the O\u0026ndash;H stretching band becomes broader and of lower intensity compared to the hydrogel without GS. This change reflects the additional hydrogen bonding and potential interaction between the silanol (Si\u0026ndash;OH) groups of GS and the hydroxyl groups within the polymer matrix. These interactions suggest that GS participates not only through physical reinforcement but also via interfacial bonding with polymer network. Consequently, the participation of GS enhances network integrity and promotes stability of the resulting hydrogel structure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMoreover, in the spectral range of 1021\u0026ndash;1054 cm⁻\u0026sup1;, the hydrogel incorporating GS exhibits a broader band compared to the hydrogel without GS. This broadening is attributed to the overlapping contributions of asymmetric vibrations of asymmetric Si\u0026ndash;O\u0026ndash;Si stretching from GS and C\u0026ndash;O\u0026ndash;C stretching from the CMCNa backbone. In contrast, the hydrogel without GS displays a lower peak in this region, associated solely with C\u0026ndash;O\u0026ndash;C vibrations of the polysaccharide chains. The emergence of Si\u0026ndash;O\u0026ndash;Si vibration confirms the successful integration of GS into the hydrogel network, leading to improved interfacial bonding between the inorganic filler and the polymer matrix.\u003c/p\u003e \u003cp\u003eThe formation of the CMCNa \u0026ndash; based hydrogel reinforced with geothermal silica (GS) proceeds through a series of chemical and physical interactions as schematically illustrated in Figures (3a) \u0026ndash; (3g). In aqueous medium, sodium silicate undergoes hydrolysis to generate silanol (Si-OH) groups, which subsequently condense to form siloxane (Si-O-Si) structures. Simultaneously, CA undergoes dehydration to form a cyclic anhydride intermediate that subsequently reacts with the hydroxyl groups of CMCNa to form ester linkages, confirming the formation of chemical crosslinks within the hydrogel matrix. In addition to covalent bonding, the silanol groups on GS interact with hydroxyl and carboxylate groups of CMCNa via hydrogen bonding. These physical interactions enhance interfacial adhesion between the organic polymer and inorganic silica phases. The coexistence of ester crosslinks and silica\u0026ndash;polymer interactions produce synergistic reinforcing effect, improving network integrity, mechanical stability, and swelling behaviour of the hydrogel [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. This dual crosslinking mechanism is consistent with the FTIR results, which confirm ester bond formation and silica incorporation within the hydrogel network.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFigure\u0026nbsp;3.\u003c/b\u003e Proposed crosslinking mechanism of CMCNa with CA and GS as a reinforcing agent.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Morfology Images by SEM analysis\u003c/h2\u003e \u003cp\u003eScanning Electron Microscopy (SEM) was utliized to investigate the morphological surface of the synthesized CMCNa \u0026ndash; CA hydrogels at a magnification of 1000x, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The hydrogel synthesized without geothermal silica (GS) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) exhibits relatively smooth and homogenous surface, indicating the formation of a dense and polymer matrix dominated by the crosslinked CMCNa network with limited microstructural heterogeneity. This morphology suggests a compact structure with minimal contribution from inorganic phases.\u003c/p\u003e \u003cp\u003eIn contrast, the hydrogel incorporating GS (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) shows a noticeably rougher and more textured surface compared to the GS \u0026ndash; free sample. The increased surface roughness and compact appearance are attributed to the successful incorporation of geothermal silica within the polymer matrix, introducing additional physical interactions and inorganic domains. These silica - rich regions promote interfacial bonding between the silanol (Si\u0026ndash;OH) groups and the hydroxyl groups of CMCNa, resulting in a more consolidated and reinforced network structure. The observed morphological differences corroborate the FTIR results, confirming that GS effectively participates in the hydrogel network and contributes to enhanced structural integrity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3. Swelling Characteristics and Network Behavior\u003c/h2\u003e \u003cp\u003eA critical parameter in evaluating hydrogel performance is the swelling ratio, which represents the material\u0026rsquo;s capacity to absorb and retain water within its three-dimensional polymeric network. This property is primarily governed by the degree of crosslinking and the abundance of hydrophilic functional groups, both of which determine the mobility of polymer chains and their interaction with water molecules. Understanding these characteristics provides insight into the hydrogel\u0026rsquo;s network structure and guides its potential application in wastewater treatment, biomedical engineering, and agricultural systems. In this study, the swelling behavior of CMCNa-based hydrogels was assessed over a 24-hour immersion period under static conditions.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e illustrates the morphological transformation of hydrogel films during the swelling process. Before immersion (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003ea), the samples appeared dry, rigid, and maintained their original dimensions, indicating the formation of a stable crosslinked matrix. After 24 hours of immersion (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), the hydrogels expanded significantly. They developed a soft, gel-like texture as water diffused into the polymeric network, causing relaxation and volumetric expansion until equilibrium was reached [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. This transformation demonstrates the superabsorbent nature of the synthesized hydrogels and confirms their suitability for applications requiring high fluid uptake. A visual comparison between the A5 and A7 samples after immersion reveals that A5 appeared more fragile and loosely structured than A7. The lower citric-acid (CA) concentration in A5 resulted in a less densely crosslinked polymer network, allowing greater chain mobility and water penetration but reducing mechanical rigidity. Although A5 contained a higher geothermal-silica (GS) content than A7, the hydrogen and ionic interactions provided by GS were weaker than the covalent ester bonds produced by CA, leading to higher flexibility but lower mechanical stability [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. These observations indicate that while GS enhances hydrophilicity and porosity, CA plays the dominant role in defining network stability and stiffness through strong covalent crosslinks with CMCNa [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe quantitative results of the 24-hour swelling ratio (SR₂₄) are summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, which presents the swelling behavior of all hydrogel formulations. The baseline sample A0-2, synthesized without GS, exhibited a swelling ratio of 55.94 g water per g hydrogel, representing a moderately crosslinked structure with limited hydrophilic sites. Upon the incorporation of GS, a notable improvement in swelling performance was observed. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003eb, which illustrates the effect of varying the GS/CMCNa mass ratio at a fixed CA/CMCNa ratio of 2%, the swelling capacity increased initially and then decreased at higher filler loadings. Sample A7, with a low GS/CMCNa ratio of 2.17%, achieved the highest swelling ratio of 69.11 g water per g hydrogel. This improvement can be attributed to the hydrophilic silanol (Si\u0026ndash;OH) groups of silica that enhance water affinity and enlarge network porosity [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. At this level, GS likely functions as a structural modifier, generating additional free volume and water-accessible domains without compromising matrix flexibility [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHowever, excessive GS loading led to performance deterioration. Sample A8, with a GS/CMCNa ratio of 7.83%, exhibited a much lower swelling ratio of 40.72 g water per g hydrogel, mainly due to particle agglomeration and pore blockage, which restricted water diffusion [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Overloading of GS also increased matrix rigidity, limiting chain expansion. In contrast, A9 (GS/CMCNa\u0026thinsp;=\u0026thinsp;5%) displayed a moderate swelling ratio of 64.12 g water per g hydrogel, representing an optimum balance between porosity and mechanical stability. Similarly, A5, which combined moderate GS and low CA content, also surpassed the swelling performance of the baseline. These results confirm that moderate GS addition enhances swelling by improving porosity and hydrophilicity, whereas excessive filler causes pore collapse and reduced performance [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe influence of crosslinker concentration is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003ec, which demonstrates the effect of varying CA/CMCNa mass ratios at a constant GS/CMCNa ratio of 5%. The data reveal an inverse correlation between CA content and swelling capacity. Among all samples, A5, containing the lowest CA/CMCNa ratio of 1.29%, achieved the highest swelling ratio of 216.95 g water per g hydrogel, significantly exceeding the baseline A0-2 (55.94 g water per g hydrogel). The relatively low crosslinking density in A5 allowed higher chain mobility and greater pore volume for water diffusion. As CA concentration increased, the swelling ratio progressively declined: A9 (CA/CMCNa\u0026thinsp;=\u0026thinsp;2%) reached 64.12 g water per g hydrogel, while A6 (CA/CMCNa\u0026thinsp;=\u0026thinsp;2.71%) dropped sharply to 8.75 g water per g hydrogel. These results indicate that excessive CA produces a rigid, tightly crosslinked matrix that suppresses polymer relaxation and water uptake [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan additionalcitationids=\"CR36 CR37\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Optimization of swelling response by response surface methodology\u003c/h2\u003e \u003cp\u003eResponse Surface Methodology (RSM) was employed to statistically evaluate and model the relationships between synthesis variables and the swelling performance of the hydrogels, thereby enabling optimization of the hydrogel preparation parameters. In this study, the swelling ratio after 24 hours (SR₂₄) was used as the response variable, while the CA/CMCNa mass ratio (Factor A) and GS/CMCNa mass ratio (Factor B) were selected as the independent variables. A total of 13 hydrogel formulations were analyzed according to the Central Composite Design (CCD), enabling the assessment of both linear and interaction effects of these parameters on the hydrogel\u0026rsquo;s swelling behavior.\u003c/p\u003e \u003cp\u003eBased on regression analysis, the relationship between the swelling ratio (SR₂₄) and the synthesis parameters was described by the empirical polynomial equation shown in Eq.\u0026nbsp;(2), achieving a coefficient of determination (R\u0026sup2;) of 90.49%, which indicates excellent correlation between the model and the experimental results.\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eSR24\u0026thinsp;=\u0026thinsp;755\u0026ndash;49981*A \u0026ndash; 1877*B\u0026thinsp;+\u0026thinsp;697034*AA\u0026thinsp;\u0026minus;\u0026thinsp;28855*BB\u0026thinsp;+\u0026thinsp;198470*AB (2)\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe Pareto chart of standardized effects, presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ea, illustrates the relative significance of each factor on the swelling response. The CA/CMCNa mass ratio (Factor A) exerts the most substantial influence, surpassing the reference \u003cem\u003et\u003c/em\u003e-value of 2.365 at a 95% confidence level. This finding confirms that the hydrogel\u0026rsquo;s swelling ability is primarily controlled by the degree of crosslinking, which is directly influenced by the citric acid concentration [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Increasing CA concentration promotes a denser network structure that constraints polymer chain mobility and water diffusion, consequently decreasing the swelling capacity. In contrast, the effect of the GS/CMCNa mass ratio (Factor B) was comparatively less significant but still contributed to modifying the porosity and hydrophilicity of the hydrogel matrix.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo visualize the interaction effects between both variables, a contour plot of the 24-hour swelling ratio (SR₂₄) was generated and is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003eb. The plot demonstrates that the highest swelling performance occurs in regions characterized by a low CA/CMCNa mass ratio (\u0026lt;\u0026thinsp;1.5%) and a moderate GS/CMCNa mass ratio (2.5\u0026ndash;4.0%). This condition corresponds to a hydrogel network with lower crosslinking density, allowing higher flexibility and sufficient pore volume for water absorption. At the same time, moderate GS incorporation enhances hydrophilicity and micro-porosity, providing additional diffusion pathways and active sites that facilitate water uptake and retention within the hydrogel network.\u003c/p\u003e \u003cp\u003eBased on the RSM optimization results shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003ec, the maximum predicted swelling ratio at 24 h (SR\u003csub\u003e24\u003c/sub\u003e) was 226.44 g water g⁻\u0026sup1; hydrogel, achieved at an CA/CMCNa mass ratio of 1.29 wt% and an GS/CMCNa mass ratio of 2.17 wt%. To validate this prediction, an experimental verification was conducted under the same conditions, yielding an actual SR\u003csub\u003e24\u003c/sub\u003e value of 195.72 g water g⁻\u0026sup1; hydrogel. The discrepancy between the predicted and experimental results corresponds to a deviation of 15.69%. This deviation is likely attributed to insufficient network stability at the predicted optimal composition. At low CA and GS contents, the ester crosslinks formed by citric acid and the physical interactions contributed by geothermal silica are not strong enough to generate a robust and durable crosslinked network. Consequently, the hydrogel structure becomes excessively loose and prone to structural failure during the swelling process.\u003c/p\u003e \u003cp\u003eThese results emphasize the critical importance of achieving a balanced combination between the crosslinker (CA) and inorganic filler (GS) concentrations. Excessive CA concentration yields a rigid and compact structure that limits swelling, while insufficient CA may result in a weak and unstable network. Conversely, moderate GS levels reinforce the polymer matrix through physical and hydrogen bonding interactions without restricting water uptake. Therefore, an optimal synthesis condition, characterized by a low CA/CMCNa mass ratio and a moderate GS/CMCNa mass ratio, offers the best compromise between flexibility and mechanical stability, ultimately maximizing the hydrogel\u0026rsquo;s water absorption capacity [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.5. Feasibility and cost\u0026ndash;benefit analysis of scaling-up geothermal silica-based hydrogel production\u003c/h2\u003e \u003cp\u003eThe techno-economic evaluation was performed to assess the feasibility and scalability of the proposed geothermal silica\u0026ndash;based hydrogel production process. This analysis provides a quantitative understanding of the cost structure, investment requirement, and operational efficiency, offering a comparison against a conventional PVA/CMC\u0026ndash;Silica Hydrogel process. The discussion also demonstrates how the valorization of geothermal silica can create economic and environmental value by transforming an industrial byproduct into a functional hydrogel material. The results bridge the laboratory-scale process optimization with industrial relevance, outlining the potential for large-scale application under realistic production conditions.\u003c/p\u003e \u003cp\u003eThe assessment followed the cost estimation methodology described by Capanema et al. (2023), with all cost parameters updated to reflect local market prices in Indonesia (2025) [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. The evaluation assumes an independent small-scale pilot facility, without shared infrastructure, designed to produce 1,000 kg of dried hydrogel per year, equivalent to about 4 kg per day with 250 operating days per year and a plant utilization rate of 80%. The process operates at atmospheric pressure and a maximum temperature of 90\u0026deg;C, followed by drying at 80\u0026deg;C for 8 hours. The expected equipment lifetime is 10 years, with a straight-line depreciation of 10% per year. Electricity cost is assumed to be 0.15 \u003cspan\u003e$\u003c/span\u003e/kWh, while the plant is operated by two operators and one supervisor, each with an average wage of 500 \u003cspan\u003e$\u003c/span\u003e per month. The feedstock includes sodium CMC (5 \u003cspan\u003e$\u003c/span\u003e/kg) and citric acid (2 \u003cspan\u003e$\u003c/span\u003e/kg) as a green crosslinker, while geothermal silica is considered a zero-cost raw material (0 \u003cspan\u003e$\u003c/span\u003e/t), sourced from geothermal brine residue. The washing stage uses deionized water and acetone with 90% solvent recovery, resulting in an effective cost of 0.2 \u003cspan\u003e$\u003c/span\u003e/kg of product. Annual maintenance and overhead are estimated at 5% of CAPEX, and waste management involves only neutralization and filtration, as the process generates non-hazardous wastewater. The selling price of agricultural-grade hydrogel is assumed at 15 \u003cspan\u003e$\u003c/span\u003e/kg.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBased on these parameters, the total capital expenditure (CAPEX) for the proposed geothermal silica\u0026ndash;CMCNa process is estimated at approximately 26,800 \u003cspan\u003e$\u003c/span\u003e, while the reference PVA/CMC\u0026ndash;Silica Hydrogel process requires 35,200 \u003cspan\u003e$\u003c/span\u003e, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e. The proposed method achieves a 24% reduction in total capital investment due to its simplified reactor configuration, low-temperature operation, and the absence of sterilization or solvent-recovery systems. The reactor operates at atmospheric pressure and does not require an inert gas environment or stainless-steel construction. Therefore, epoxy-coated mild steel vessels are sufficient, reducing material and fabrication costs. The drying and curing oven consumes less power because it operates at 80\u0026deg;C rather than 120\u0026deg;C, minimizing insulation and electrical requirements. The filtration and washing setup is also simplified, employing a single-stage gravity or vacuum system instead of a multi-stage controlled system. Moreover, the water purification and analytical instruments are limited to those essential for environmental-grade product quality, further lowering investment costs. Overall, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows that the simplified design and elimination of specialized biomedical equipment lead to significant capital savings.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe operating expenditure (OPEX) comparison presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e highlights an even greater difference. The total OPEX for the geothermal silica\u0026ndash;CMCNa process is approximately 4.45 \u003cspan\u003e$\u003c/span\u003e/kg, which is nearly 50% lower than the 9.15 \u003cspan\u003e$\u003c/span\u003e/kg of the reference process. The most significant savings come from the raw materials, as geothermal silica is a freely available byproduct and citric acid is considerably cheaper and safer than glutaraldehyde. The process consumes less energy because of its low-temperature synthesis and short drying duration, reducing electricity demand by about 70%. Labor costs are also lower since only two operators are required, while the reference process includes additional personnel for sterile quality control. Maintenance and overhead are reduced because there are no pressurized or stainless-steel systems, and waste handling costs are minimal. After all, only neutralized rinse water is generated. The OPEX structure in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e clearly indicates that the geothermal silica\u0026ndash;CMCNa system is substantially more economical to operate, primarily due to low-cost raw materials, reduced energy input, and simplified maintenance requirements.\u003c/p\u003e \u003cp\u003eUnder these base-case conditions, the total annual production cost for 1,000 kg of dried hydrogel is approximately 4,450 \u003cspan\u003e$\u003c/span\u003e per year, resulting in a gross margin of about 70% at the assumed selling price of 15 \u003cspan\u003e$\u003c/span\u003e/kg. The estimated payback period is less than two years, with an internal rate of return (IRR) of about 38%, and a positive net present value (NPV) achieved within the first three years of operation. These results confirm that the geothermal silica\u0026ndash;CMCNa process offers a strong techno-economic advantage by combining low capital investment, reduced energy consumption, and waste minimization, while simultaneously contributing to sustainable resource management. Overall, the analysis demonstrates that scaling up this production system under local market conditions is economically feasible and consistent with the objectives of green manufacturing and circular economy development.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study successfully established a sustainable and technically feasible approach for synthesizing geothermal silica\u0026ndash;reinforced carboxymethylcellulose (CMCNa) hydrogels using citric acid as an environmentally benign crosslinker under mild reaction conditions. Optimization through response surface methodology (RSM) verified that the citric acid-to-CMCNa mass ratio was the most influential factor controlling the hydrogel\u0026rsquo;s network structure and swelling performance, yielding a strong correlation with an R\u0026sup2; value of 0.9049. The optimized hydrogel achieved a maximum 24-hour swelling ratio of 216.5 g g⁻\u0026sup1;, confirming that moderate crosslinking effectively enhanced hydrophilicity while maintaining structural stability. Incorporation of geothermal silica improved the mechanical robustness and reusability of the hydrogel while enabling the valorization of geothermal waste into a high-value functional material. The techno-economic evaluation further demonstrated that the proposed process is economically competitive, requiring an estimated capital expenditure (CAPEX) of 26,800 \u003cspan\u003e$\u003c/span\u003e and operating expenditure (OPEX) of 4.45 \u003cspan\u003e$\u003c/span\u003e kg⁻\u0026sup1;, both significantly lower than conventional PVA/CMC\u0026ndash;silica hydrogel systems. The combination of low-temperature processing, non-toxic reagents, and zero-cost geothermal silica feedstock contributes to substantial cost savings, improved energy efficiency, and environmental compatibility. Overall, this research highlights the integration of material innovation, cost efficiency, and sustainability principles, illustrating that geothermal silica\u0026ndash;CMCNa hydrogels can serve as a scalable and eco-friendly alternative for industrial applications, while advancing the circular utilization of geothermal resources toward sustainable material production.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of Interest\u003c/h2\u003e \u003cp\u003eThe authors declare that there are no conflicts of interest associated with this work.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThe authors gratefully acknowledge financial support from the Lembaga Pengelola Dana Pendidikian (LPDP) under the Republic of Indonesia, as well as research facilities provided by the Mineral Processing Research Group, Department of Chemical Engineering, Gadjah Mada University, The authors also sincerely thank PT Geo Dipa Energi (Persero) for their support in delivering geothermal solid waste materials in this study.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAll authors contributed to the conception and design of the study. Material preparation, data collection, analysis, data curation, and conceptualization were primaryly performed by Anisa Galuh Arisanti. Reviewing and editing were conducted by Vincent Sutresno Hadi Sujoto. Characterization, analysis and data curation were carried out by Fatimah Tresna Pratiwi, Rina Dewi Mayasari, Adhi Priyo Pamungkas. Supervision, conceptualization, investigation, review, and funding acquisition were performed by Rochmadi Rochmadi, Eka Tarwaca Susila Putra, Himawan Tri Bayu Murti Petrus. The first draft of the manuscript was written by Anisa Galuh Arisanti and all authors commented on previous versions of the manuscript. All authors read and approved the final version of the manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData supporting the findings of this study will be made available upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAnjali J, Jose VK, Lee JM (2019) Carbon-based hydrogels: Synthesis and their recent energy applications. J Mater Chem A 7:15491\u0026ndash;15518. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1039/c9ta02525a\u003c/span\u003e\u003cspan address=\"10.1039/c9ta02525a\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMali KK, Dhawale SC, Dias RJ et al (2018) Citric acid crosslinked carboxymethyl cellulose-based composite hydrogel films for drug delivery. 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Gels 9. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/gels9020166\u003c/span\u003e\u003cspan address=\"10.3390/gels9020166\" 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":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"silicon","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scon","sideBox":"Learn more about [Silicon](https://www.springer.com/journal/12633)","snPcode":"12633","submissionUrl":"https://submission.nature.com/new-submission/12633/3","title":"Silicon","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"geothermal silica, carboxymethylcellulose (CMCNa), citric acid, hydrogel optimization, sustainable materials","lastPublishedDoi":"10.21203/rs.3.rs-8905255/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8905255/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Natural polymer-based hydrogels are environtmentally friendly and biodegradable polymers capable of absorbing an enormous volume of water, yet they often suffer from poor structural stability upon extensive swelling. Reinforcement using inorganic fillers such as silica can overcome this limitation. Geothermal silica (GS), a silica-rich byproduct (\u0026gt;\u0026thinsp;95% SiO₂) from geothermal power plants, offers a sustainable reinforcing agent owing to its porous and hydrophilic characteristics. In this study, sodium carboxymethylcellulose (CMCNa)-based hydrogels were synthesized using citric acid (CA) as a crosslinker agent and GS as an inorganic filler to enhance swelling performance. Fourier Transform Infrared (FTIR) spectroscopy and SEM analysis confirmed the successful of the hydrogel synthesis and silica incorporation into the hydrogel network. Swelling behavior evaluated over 24 hours showed that moderate GS and CA ratios produced the best performance, achieving a maximum swelling ratio of 216.95 g g⁻\u0026sup1;. Optimization using Response Surface Methodology (RSM) identified citric acid concentration as the most significant parameter influencing swelling, with a strong correlation (R\u0026sup2; = 0.9049); the optimized swelling ratio variabel is 226.44 g water g⁻\u0026sup1; hydrogel, achieved at an CA/CMCNa mass ratio of 1.29 wt% and an GS/CMCNa mass ratio of 2.17 wt%. The overall process is technically and economically feasible, utilizing low-cost materials and mild reaction conditions. These results demonstrate that geothermal silica\u0026ndash;CMCNa hydrogels represent a sustainable, scalable, and eco-friendly material system for environmental and agricultural applications.","manuscriptTitle":"Sustainable synthesis and optimization of geothermal silica reinforced sodium carboxymethylcellulose (CMCNa)-based hydrogels with enhanced swelling performance","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-30 20:31:47","doi":"10.21203/rs.3.rs-8905255/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2026-05-05T06:35:45+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-29T02:25:40+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-23T18:01:14+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"105934066197702322637464445641089668120","date":"2026-04-23T12:18:21+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"143066538768025660920610957455817340259","date":"2026-04-23T03:05:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"208761936569579632651267688488695908979","date":"2026-04-23T00:37:27+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"42144799121452056600485600776954038201","date":"2026-04-22T20:04:35+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-22T16:24:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-23T23:26:13+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-23T23:25:48+00:00","index":"","fulltext":""},{"type":"submitted","content":"Silicon","date":"2026-02-18T02:46:33+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"silicon","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scon","sideBox":"Learn more about [Silicon](https://www.springer.com/journal/12633)","snPcode":"12633","submissionUrl":"https://submission.nature.com/new-submission/12633/3","title":"Silicon","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8e48536f-0dd2-4f3b-9d24-3beb855a9c68","owner":[],"postedDate":"April 30th, 2026","published":true,"recentEditorialEvents":[{"type":"editorInvitedReview","content":"","date":"2026-05-05T06:35:45+00:00","index":22,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-29T02:25:40+00:00","index":21,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-30T20:31:47+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-30 20:31:47","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8905255","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8905255","identity":"rs-8905255","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}