Chitosan-g-poly (styrene sulfonate) hydrogel with crosslinked polymeric for the efficient remediation of cationic dyeing wastewater

2.1 Chemicals

CS and sodium styrene sulfonate (NaSS, ˃93.0%) were sourced from TCI Co., Ltd. N, N’-methylene bisacrylamide (MBA) and Ammonium persulfate (APS) were obtained from Sigma-Aldrich Chemical Co. MB, crystal violet (CV), rose Bengal (RB) and acid yellow 23 (AY23) were acquired from Thermo Fisher Scientific (Alfa Aesar, USA).

2.2 Hydrogel synthesis

The hydrogel was synthesized via free radical polymerization of NaSS monomers with different feed ratios and CS in the presence of MBA as a crosslinking agent. First, a 50 mL three-necked flask (equipped with a condenser and argon inlet) was charged with 25 mL of purified water. The distilled water was purged with inert gas for 10 minutes to remove dissolved oxygen. Subsequently, the NaSS (0.5, 1, 1.5, and 2 g) and CS (1 g) were added to the flask and dissolved in distilled water containing 2% acetic acid as a solvent. To ensure a homogeneous mixture, continuous stirring was maintained for roughly 10 min before introducing MBA as a crosslinking agent. APS was then introduced as the initiator at a concentration of 1.0 wt% relative to the total NaSS mass. Polymerization was performed over 24 h at 65°C under constant agitation using an oil heating system. Once the reaction finished, the flask was left to cool down to room temperature. The resulting hydrogel was filtered and washed multiple times with ethanol to remove any unreacted monomers or impurities. Finally, the purified hydrogel was dried in a vacuum oven at 50°C for 72 h to obtain the final product, which was then ready for further characterization and use. The prepared samples were coded according to the mass ratio of monomer to CS as C1S0.5, C1S1, C1S1.5, and C1S2, where the numerical values correspond to the mass of the used NaSS monomer in the synthesis.

2.3 Hydrogel characterization

Fourier-transform infrared (FTIR) spectroscopy was conducted using a Bruker Tensor 27 FTIR spectrophotometer (Germany). The samples were analyzed via the potassium bromide (KBr) disk method, with spectral data collected over a wavenumber range of 4000 to 400 cm⁻¹. Thermogravimetric analysis (TGA) was performed using a TA-Q500 instrument (USA) at a heating rate of 10°C/min. Additionally, the surface morphology of the hydrogel was characterized by scanning electron microscopy (SEM) (model: JSM-6380 LA).

2.4 Adsorption experiments

Prior to adsorption experiments, a stock solution of MB dye (1000 mg/L concentration) was prepared through dissolution of 1 g of dye in 1 L of distilled water. A calibration curve was established using diluted dye solutions (2.5–10 mg/L), and their absorbance was recorded at 660 nm via UV-Vis spectrophotometry. A preliminary screening was conducted to evaluate the adsorption efficiency of the four synthesized hydrogel samples (C1S0.5, C1S1, C1S1.5, and C1S2). For this purpose, batch adsorption experiments were conducted by introducing 10.0 mg hydrogel aliquots into 10 mL of 500 mg/L dye solution, followed by continuous agitation (150 rpm) in a temperature-controlled water bath shaker (25°C) for 24 h. Further adsorption experiments with sample C1S2 investigated key parameters: pH (2-10, adjusted using 0.1 M HCl/NaOH), ionic strength (0.05-0.3 M KCl), initial dye concentration (50-700 mg/L at 25-40°C), and adsorption kinetics (5-90 min), using 300 mg/L dye solutions. To evaluate selectivity, a binary mixture was prepared by combining 5 mL of MB dye (50 mg/L) with 5 mL of AY23 dye (50 mg/L). Subsequently, 10 mg of the gel was introduced into the mixture, which was then agitated in a shaker for 4 h. Following the equilibration period, the absorption was measured to assess dye uptake.

The adsorbed dye (qₑ) was calculated as:

where Cₒ and Cₑ are initial/equilibrium concentrations (mg/L), V is solution volume (L), and m is hydrogel mass (g).

For the reusability study, the used hydrogel was treated with 10 mL of a 0.1 M HCl/acetone mixture (7.5:2.5 v/v) under 2 h of shaking. The hydrogel was then recovered, sequentially washed with distilled water and a 0.01 M NaOH solution, rinsed again with distilled water, and finally prepared for reuse.

The mathematical equations for the isothermal and kinetic models employed in this study, along with the equations used to determine the thermodynamic parameters, are provided in the Supporting Information.

3.1 Characterization

Fig. 1 presents the SEM images of the four prepared samples. The morphology of C1S0.5 exhibits a smooth surface devoid of particles or pores. In contrast, C1S1 displays a rougher surface with the presence of agglomerates. C1S1.5 demonstrates further agglomeration, resulting in a more pronounced surface roughness. Notably, C1S2 reveals a distinct morphology characterized by a porous structure, suggesting the formation of a porous gel-like framework. This structural variation highlights the differences in the physical characteristics among the samples. The observed morphological diversity suggests that the ratio of NaSS feed monomer plays a pivotal role in the formation and structural characteristics of the hydrogels. Specifically, variations in the ratio of monomer appear to influence surface roughness, agglomeration behavior, and porosity, as evidenced by the distinct morphologies of the prepared samples.

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Fig. 1.

SEM images of the prepared hydrogel with varying ratios of NaSS feed monomer: (a) C1S0.5, (b) C1S1, (c) C1S1.5, and (d) C1S2.

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FTIR analysis has been extensively used to characterize prepared hydrogel samples with varying ratios of NaSS, as displayed in Fig. 2. The FTIR spectra typically reveal distinct peaks corresponding to the functional groups of both CS and polystyrene sulfonate for the C1S2 sample. For instance, the presence of CS is confirmed by characteristic peaks such as the O-H stretching vibration around 3428 cm⁻¹ and the N-H deformation vibration near 1648 cm⁻¹. When polystyrene sulfonate is grafted onto CS, new peaks emerge, such as the symmetric and asymmetric stretching vibrations of the sulfonate group (SO₃⁻) around 1035 cm⁻¹ and 1177 cm⁻¹, respectively, which are indicative of successful grafting. (Castro et al., 2011; Men et al., 2021).

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Fig. 2.

FTIR of the prepared hydrogel with varying ratios of NaSS feed monomer; C1S0.5 (black); C1S1(red); C1S1.5 (blue) and C1S2 (green).

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Additionally, the intensity of these sulfonate peaks increases with higher ratios of NaSS, demonstrating the proportional incorporation of the monomer into the grafted structure. These findings collectively confirm the successful synthesis of CS-g-PSS and highlight the influence of NaSS ratio on the chemical structure and functional group interactions within the grafted material.

TGA/DTA analysis of prepared hydrogel samples reveal significant insights into their thermal stability and decomposition behavior (Fig. 3). These analyses show that varying the ratio of NaSS affects the thermal properties of the hydrogels. Higher NaSS content generally reduces thermal stability, as indicated by increased decomposition temperatures and reduced weight loss at elevated temperatures. The TGA curves demonstrate a multi-step degradation process, with initial weight loss attributed to the loss of water and subsequent stages corresponding to the decomposition of the organic components. For example, C1S2 exhibits a distinct third degradation step beyond 320°C, at a lower temperature range compared to other samples. This suggests a higher percentage of NaSS in C1S2, impacting its thermal behavior significantly. In contrast, C1S0.5 lacks a clear third step, further validating that this thermal step is specific to the polystyrene sulfonate linked to the CS structure.

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Fig. 3.

TGA/DTA analysis of the prepared hydrogel with varying ratios of NaSS feed monomer; C1S0.5 (black); C1S1 (red); C1S1.5 (blue), and C1S2 (green).

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TGA analysis also reveals that the residual char content at 800 °C decreases significantly following the grafting of PSS onto CS. This reduction is primarily attributed to the synergistic effects of thermal and structural modifications introduced by PSS. The sulfonate groups (-SO₃⁻) in PSS decompose through desulfonation at moderate temperatures (200–300°C), while the polystyrene backbone undergoes complete thermal degradation between 400-600°C without contributing to char formation (Knauth et al., 2011; Seo et al., 2009). In addition, PSS disrupts the hydrogen-bonding network of native CS, weakening its thermal stability and promoting earlier decomposition. The acidic nature of the sulfonate groups may further accelerate the degradation process by catalyzing chain scission and enhancing volatilization of breakdown products.

3.2 Adsorption study

This study initially investigated the effect of NaSS monomer content (0.5–2 g) on the properties of CS-based hydrogels (C1S0.5-C1S2) synthesized via radical polymerization. FTIR spectroscopic analysis revealed that formulations with lower monomer content (C1S0.5-C1S1.5) exhibited weak S=O stretching vibrations at 1040 cm⁻¹, indicating insufficient sulfonate group incorporation that directly correlated with their poor adsorption performance. In contrast, the C1S2 hydrogel demonstrated markedly enhanced properties, including intense S=O stretching vibrations confirming higher sulfonate density, an interconnected porous morphology observed by SEM, and superior MB adsorption capacity. These improvements are attributed to the significantly higher grafting efficiency achieved at 2 g monomer loading, which simultaneously enhanced both the structural integrity and adsorption efficacy of the hydrogel. The established correlation between monomer content, grafting efficiency, and functional performance identifies the 2 g formulation as optimal for this system, justifying its selection as the representative sample for this study.

The C1S2 sample’s dye adsorption capacity was first evaluated using two cationic dyes (MB and CV) and two anionic dyes (RB and MO), with results presented in Fig. 4(a). The results revealed that the C1S2 hydrogel exhibited a higher adsorption capacity for cationic dyes, with MB showing the highest uptake (291.18 mg/g), while for CV dye (272.13 mg/g). In contrast, the adsorption capacity for the anionic dyes RB and MO was significantly lower 64.97 and 19.45 mg/g, respectively. The reduced dye uptake arises from Coulombic repulsion between the hydrogel’s sulfonate (–SO₃⁻) moieties and the anionic sites on the dye, as governed by their shared negative charge density. These findings suggest that the hydrogel has a strong affinity for cationic dyes, making it a promising candidate for the selective removal of such pollutants from wastewater.

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Fig. 4.

(a) Adsorption of various dyes (b) selectivity study (c) pH effect on adsorption capacity (d) point zero charge measurement, (e) ionic strength effect on adsorption capacity and (f) swelling ratio of C1S2 hydrogel at different pH.

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The adsorption selectivity of the gel was evaluated using a binary dye system containing MB as cationic dye and AY23 as an anionic dye. As evidenced by UV-Vis spectroscopic analysis (Fig. 4b), the C1S2 hydrogel demonstrated pronounced selectivity toward the cationic dye, as indicated by a substantial decrease in the characteristic absorption peak of MB at 660 nm. In contrast, the absorption band of AY23 at 430 nm remained virtually unchanged, confirming negligible interaction between the C1S2 hydrogel and the anionic dye. This distinct differential adsorption behavior clearly establishes the C1S2 hydrogel’s preferential affinity for cationic dyes, likely attributable to electrostatic interactions between the hydrogel’s–SO₃⁻ moieties and cationic sites on the dye molecules.

The sulfonate groups (-SO₃⁻) in the prepared sulfonated hydrogel play a critical role in the pH-dependent adsorption behavior of cationic MB dyes as displayed in Fig. 4(c). The -SO₃⁻ groups are strongly acidic and remain negatively charged across a wide pH range, including the studied range of 3–10 (Thamer et al., 2023b). At lower pH (pH 3–6), the sulfonate groups contribute significantly to the electrostatic attraction of cationic MB dye molecules, as their negative charge is not affected by protonation. This explains the increase in adsorption capacity from 149 to 273 mg/g as the pH rises from 3 to 6, as the hydrogel’s overall negative charge density increases due to the deprotonation of CS’s amino and hydroxyl groups, in addition to the persistent negative charge of the sulfonate groups. At higher pH (9–10), the slight decrease in adsorption capacity may be attributed to factors such as increased competition from hydroxide ions (OH⁻) for adsorption sites or changes in the hydrogel’s swelling behavior, which could reduce accessibility to the sulfonate groups. However, the sulfonate groups themselves remain negatively charged and active for adsorption, ensuring that the hydrogel retains a relatively high adsorption capacity even at alkaline pH. The combined effect of sulfonate groups and CS’s pH-responsive functional groups makes the hydrogel highly effective for cationic MB dye removal, particularly in slightly acidic to neutral conditions. This result aligns with the point of zero charge (pH_PZC) measurement, where the pH_PZC of C1S2 was determined to be pH 6, as illustrated in Fig. 4(d). At this pH_PZC, the hydrogel’s surface exhibits no net charge, marking the transition between positively charged protonated amino groups (−NH₃⁺) at lower pH and negatively charged sulfonate groups (−SO₃⁻) at higher pH (Akl and Serage, 2024). This pH PZC value is consistent with the observed pH-dependent swelling and adsorption behavior, confirming the critical role of electrostatic interactions in the hydrogel’s properties.

As shown in Fig. 4(e), the ionic strength of the MB dye solution, influenced by the concentration of KCl, significantly affects the adsorption capacity of prepared sulfonated hydrogel for MB dye removal. The result has shown that as the KCl concentration increases from 0.1 to 0.25 mol/L, the adsorption capacity of the sulfonated hydrogel for MB dye rises from 222 to 255 mg/g. This increase can be attributed to the screening effect of the electrolyte (KCl), which reduces the electrostatic repulsion between the negatively charged sulfonate groups (-SO₃⁻) on the hydrogel and the cationic MB dye molecules. This allows for closer interaction and enhanced adsorption. However, at higher KCl concentrations (e.g., 0.3 mol/L), the adsorption capacity slightly decreases. This is likely due to excessive screening, which may compress the double layer around the hydrogel and reduce the availability of active sites for dye binding. Additionally, high ionic strength can alter the swelling behavior of the hydrogel, potentially reducing its porosity and accessibility to dye molecules (Zhu et al., 2015). These findings are consistent with the pH-dependent adsorption behavior, where electrostatic interactions dominate.

The swelling characteristic of C1S2 hydrogel sample was investigated at various pH as displayed in Fig. 4(f). At acidic media (pH 3), CS’s amino groups (−NH₂​) become protonated (−NH₃⁺), creating electrostatic repulsion between chains and causing high swelling ratio (223%), despite ionic crosslinks with PSS’s sulfonate groups (−SO₃⁻​) (Rizwan et al., 2017). At neutral pH, CS’s protonation decreases, reducing electrostatic repulsion and leading to a more compact structure with lower swelling ratio (132%). At alkaline media (pH 10), CS is fully deprotonated, weakening ionic crosslinks and allowing osmotic pressure from PSS’s sulfonate groups to dominate, increasing swelling ratio (328%).

3.3 Initial concentration effect and isotherm analysis

The initial concentration of MB dye and system temperature significantly influence the adsorption capacity of the prepared sulfonated hydrogel. Fig. 5 reveals a significant enhancement in adsorption capacity across the 50-400 mg/L dye concentration range, driven by both intensified mass transfer gradients and increased probability of dye-active site interactions on the hydrogel surface. This is attributed to the enhanced concentration gradient, which promotes diffusion and adsorption. However, beyond 400 mg/L, the adsorption capacity increases only slightly to reach the equilibrium state, suggesting that the active sites on the hydrogel become saturated, limiting further adsorption despite the higher MB dye concentration (Hu et al., 2018). Additionally, the adsorption capacity increases with temperature, as higher temperatures (25-40°C) enhance the mobility of dye molecules and the swelling of the hydrogel, improving access to internal adsorption sites. This temperature-dependent behavior also indicates that the adsorption process is endothermic, driven by stronger interactions between the dye and the hydrogel at elevated temperatures.

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Fig. 5.

Non-linear regression fits Langmuir, Freundlich, and Langmuir-Freundlich isotherm models for MB adsorption on the synthesized hydrogel at different temperatures: (a) 25°C, (b) 30°C, (c) 35°C, and (d) 40°C. Conditions: 24 h contact time, 10 mg hydrogel dose, 10 mL solution volume, pH 9.0.

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The adsorption behavior of MB dye onto the prepared sulfonate hydrogel was investigated using three non-linear isotherm models: Langmuir (Langmuir, 1918), Freundlich (Freundlich, 1906), and Langmuir-Freundlich (Sips, 1948). Although the Langmuir-Freundlich model, which combines features of Langmuir and Freundlich models, showed slightly higher R² values, it was less reliable due to the high standard error in qₘₐₓ values (Table 1). These discrepancies rendered the Langmuir model more effective at representing the adsorption behavior. The Freundlich model, which assumes heterogeneous adsorption, was less applicable according R² values. Accordingly, the experimental findings were best described by the Langmuir model, given its excellent R² values and small standard deviation. Based on the Langmuir model’s assumptions, MB adsorption occurs as a monolayer on uniform surface sites with limited capacity, indicating MB molecules are arranged in a single molecular layer across the hydrogel’s surface. According to Langmuir modeling, the qₘₐₓ rose by 17% (from 337 to 394 mg/g) when the temperature increased from 25 to 35°C, reflecting enhanced dye mobility and hydrogel swelling at higher temperatures, which improved access to adsorption sites. However, beyond 35°C, qₘₐₓ remained constant, indicating saturation of active sites.

3.4 Contact time effect and kinetic study

The effect of contact time on the adsorption of MB dye onto the prepared sulfonated hydrogel was studied at different dye concentrations (50, 100, and 200 mg/L) as displayed in Fig. 6(a-c). The results showed that the adsorption process was rapid, reaching equilibrium within 5, 10, and 30 min for dye concentrations of 50, 100, and 200 mg/L, respectively. This trend can be explained by the hydrogel’s structure and the availability of active sites. At lower dye concentrations (50 ppm), the number of dye molecules is relatively small compared to the abundant active sites on the hydrogel surface, such as sulfonate (-SO₃⁻) and amino (-NH₂) groups, allowing for faster adsorption and quicker equilibrium. As the dye concentration increases (100 and 200 ppm), more dye molecules compete for the available active sites, leading to a longer time required to reach equilibrium.

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Fig. 6.

Adsorption kinetics of MB dye at concentration 50, 100 and 200 mg/L on prepared hydrogel and fitting by non-linear (a) PFO, (b) PSO, (c) Elovich and (d) linear intraparticle diffusion kinetic model.

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The adsorption kinetics of MB dye onto sulfonated hydrogel was investigated using three non-linear kinetic models: pseudo-first-order (PFO) (Lagergren, 1898), pseudo-second-order (PSO) (Ho and McKay, 1998), and Elovich (Chien and Clayton, 1980). Among these, the PFO model provided the best fit to the experimental data, as evidenced by its high R² values, low r-χ² values, and minimal standard error (SE) across different dye concentrations (50-200 mg/L). The PFO model assumes that the rate-limiting step is physical adsorption, driven by diffusion and weak interactions such as van der Waals forces or hydrogen bonding between the dye molecules and the hydrogel’s surface. This indicates that the adsorption process is primarily governed by physical interactions rather than chemical bonding. The PSO model, which assumes chemisorption as the dominant mechanism, and the Elovich model, which describes heterogeneous adsorption, were less suitable due to their lower R² values and higher r-χ² and SE values (Table 2). The superior fit of the PFO model suggests that the adsorption process is controlled by physical interactions, such as electrostatic attraction between the cationic dye and the negatively charged sulfonate (-SO₃⁻) groups on the hydrogel surface.

The linear intraparticle diffusion model was applied to study the adsorption mechanism of MB dye onto the prepared sulfonated hydrogel, as shown in Fig. 6(d). The results revealed that the adsorption process occurred in two distinct steps. The first step involved rapid surface adsorption, where MB dye molecules quickly bound to the active sites on the hydrogel’s surface, such as sulfonate (-SO₃⁻) and amino (-NH₂) groups. This step was attributed to the high accessibility of these functional groups and the strong electrostatic interactions between the adsorbate-adsorbent interface. The second step was characterized by slower intraparticle diffusion, where dye molecules gradually migrated into the porous structure of the hydrogel to reach internal adsorption sites (Weber, 1963). This step was influenced by the hydrogel’s three-dimensional network and the diffusion resistance within its pores. The two-step mechanism suggests that while surface adsorption dominates the initial phase, internal pore diffusion substantially contributes to the adsorption kinetics, particularly at higher dye concentrations or longer contact times. These findings align with the kinetic study results, further supporting the importance of both surface and internal adsorption sites in the hydrogel’s efficient dye removal capabilities.

3.5 Thermodynamic study

The thermodynamic analysis of the adsorbate-adsorbent interaction system elucidates key characteristics of the underlying sorption behavior. In this study, the Gibbs free energy change (ΔG) and enthalpy change (ΔH) were both found to be negative, indicating that the adsorption process is spontaneous and exothermic, as shown in Table 3. The negative ΔG values suggest that the adsorption of MB dye molecules onto the hydrogel is thermodynamically favorable, driven by a decrease in free energy as the dye molecules interact with an adsorption site on the hydrogel surface, such as sulfonate (-SO₃⁻) and amino (-NH₂) groups. The negative ΔH values further confirm that the process releases heat, highlighting the exothermic nature of the adsorption, which is often associated with electrostatic interactions and physical adsorption mechanisms. Additionally, the positive entropy change (ΔS > 0) suggests increased molecular disorder at the solid-liquid interface, consistent with the displacement of solvated species (e.g., water molecules or counterions) from the hydrogel surface upon dye adsorption.

3.6 Reusability study

Reusability is a critical parameter for evaluating the practical applicability of an adsorbent material. It not only reflects the economic viability of the adsorbent but also its sustainability in long-term applications. In this study, the reusability of the prepared hydrogel for the adsorption of MB dye was systematically investigated over eight consecutive cycles. As illustrated in Fig. 7(a), the adsorption capacity of the prepared hydrogel for MB dye demonstrated no significant decline with increasing cycle numbers, highlighting its robust reusability. Moreover, a slight enhance in the q_max was observed in the second and third cycles, which could be attributed to the activation of adsorption sites and structural changes in the hydrogel. The minor reduction in performance observed after four cycles can be primarily attributed to the physical loss of the hydrogel adsorbent during the regeneration process.

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Fig. 7.

(a) Reusability of C1S2 sample, (b) FTIR spectra of C1S2 before and after MB dye adsorption and (c) proposed adsorption mechanism.

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3.7 Proposed adsorption mechanism

Understanding the adsorption mechanism is crucial for interpreting adsorption behavior. The adsorption of MB on the prepared hydrogel surface has been examined using FTIR (see Fig. 7b). The FTIR spectrum before and after MB adsorption on C1S2 reveals significant changes, indicating dye adsorption and interactions between the hydrogel and the dye. The appearance of distinct absorption bands at 1387 cm⁻¹ (assigned to dimethylamino C-N stretching) and 1329 cm⁻¹ (attributed to C=S⁺ ring vibrations) demonstrates successful MB adsorption (Ovchinnikov et al., 2016). Additionally, some hydrogel bands shift, reflecting interactions such as electrostatic forces, π-π stacking, and hydrogen bonding. For example, the peak at 1648 cm⁻¹ shifts to 1596 cm⁻¹, indicating π-π interactions between the aromatic rings of MB and the hydrogel. Similarly, the peaks at 1035 cm⁻¹ and 1177 cm⁻¹, corresponding to the sulfonate (SO₃⁻) groups, shift to 1031cm⁻¹ and 1168 cm⁻¹, suggesting electrostatic interactions between the anionic sulfonate groups and the cationic MB dye. Furthermore, the intensity of some hydrogel bands decreases after adsorption, confirming the involvement of functional groups in the dye adsorption process. These changes collectively demonstrate the adsorption mechanism, driven by electrostatic, π-π, and hydrogen bonding interactions. The potential interaction mechanisms have been illustrated in Fig 7(c).

3.8 Comparison with reported adsorbents

A comparative evaluation of the prepared hydrogel with other sulfonated hydrogels for the maximum adsorption of MB dye has been presented in Table 4. The results demonstrate that the prepared hydrogel exhibit a significantly higher adsorption capacity compared to other sulfonated hydrogel adsorbents. This enhanced performance underscores the superior efficiency of prepared hydrogel in adsorbing cationic dyes like MB. The findings suggest that the unique structural and functional properties of prepared hydrogel contribute to their exceptional adsorption capabilities, making them a highly promising candidate for the removal of cationic dyes from aqueous solutions.