Adsorptive Removal of Crystal Violet Dye Using Sodium Alginate-g-Poly (Acrylic Acid-Co-Itaconic Acid)/Titanium Dioxide [SA-g-p(AA-IA)/ TiO2] Hydrogel Nanocomposite

INTRODUCTION
The introduction of coloured wastewater from industrial processes into the environment poses a significant and growing threat to the health of aquatic ecosystems. Industries such as textiles, leather finishing, printing, and food production utilize a vast array of synthetic dyes, which are discharged into water bodies without adequate treatment [1]. These dyes, often characterized by complex aromatic structures, exhibit considerable stability and are resistant to biodegradation, leading to their accumulation in aquatic environments [2, 3]. The presence of these organic pollutants not only detracts from the aesthetic quality of water but also impedes the penetration of sunlight, which is essential for photosynthesis by aquatic plants, thereby disrupting the ecological balance and potentially reducing dissolved oxygen levels [4]. Furthermore, a significant number of dyes and their metabolic byproducts are known to be toxic, carcinogenic, or mutagenic, posing direct risks to aquatic organisms and potentially entering the food chain, with implications for human health [5-7]. Crystal Violet (CV), a dye belonging to the triphenylmethane family, finds widespread application. It serves as a biological stain in both human and animal medicine, a colorant in the textile industry during processing, and contributes a deep violet hue to paints and printing inks and also used in medical field. Despite its diverse applications, CV is recognized as a persistent and environmentally challenging dye molecule that can exert toxic effects. Notably, it functions as a mitotic inhibitor, a potential carcinogen that can promote tumor development in certain fish species [5, 8-11]. Among the various physical and chemical methods available for the removal of dyes from wastewater, adsorption has emerged as a particularly attractive and extensively studied technique [12-14]. This method offers several compelling advantages, including its simplicity of operation, high efficiency in removing dyes even at relatively low concentrations [15-17], cost-effectiveness [18-20], and the potential for the adsorbent material to be regenerated and reused, thereby promoting sustainability [21]. The fundamental principle underlying adsorption involves the transfer of dye molecules from the liquid phase to the surface of a solid adsorbent material, where they are retained through a variety of physical and chemical interactions [22]. These interactions can include electrostatic forces between charged dye molecules and the adsorbent surface, van der Waals forces arising from intermolecular attractions, hydrogen bonding between functional groups on the dye and the adsorbent, and even formation of chemical bonds in the case of chemisorption. The effectiveness of an adsorbent in capturing and retaining dye molecules is determined by its intrinsic properties, as specific surface area, distribution of pore sizes, net surface charge, and presence of functional groups that can interact with the dye molecules [23]. Consequently, a significant focus of research in this field is directed towards development of novel adsorbent materials that possess tailored properties to maximize their dye removal performance. This includes exploring various materials, optimizing their synthesis, and modifying their surface characteristics to enhance affinity for specific types of dyes [23]. The recurring emphasis on the cost-effectiveness and the potential for regeneration of adsorbents underscores the practical viability of adsorption as a sustainable solution for large-scale wastewater treatment applications [23]. Hydrogels, which are three-dimensional networks of hydrophilic polymers capable of absorbing and retaining remarkably large volumes of water or biological fluids relative to their mass, have emerged as a significant class of materials with immense potential for wastewater treatment, particularly in the removal of organic dyes [24]. The inherent physicochemical characteristics, as high porosity, large surface area available for adsorption, modifiable surface chemistry, and significant water retention capacity, make hydrogels excellent candidates for effectively capturing and removing a wide range of pollutants from aqueous solutions [25-27]. Hydrogels can be synthesized from a diverse array of natural polysaccharides, such as sodium alginate (SA), chitosan, cellulose, starch, gellan gum, and guar gum, as well as synthetic polymers, including poly(acrylic acid) (AA), poly(Itaconic acid) (IA) and poly(N-vinyl pyrrolidone) (PNVP) [25, 28, 29]. Polysaccharide-based hydrogels are particularly attractive due to their inherent biocompatibility, biodegradability, low cost, and natural abundance, aligning well with the principles of green chemistry and sustainable development for environmental remediation [30]. Hydrogels creates a vast network structure with a large contact area, facilitating the diffusion of dye molecules into the polymer matrix and enhancing the overall adsorption capacity. Furthermore, the chemical composition and physical structure of hydrogels can be precisely tailored during synthesis to introduce specific functional groups that exhibit a high affinity towards particular classes of dyes, enabling the design of highly selective and efficient adsorbents for targeted pollutant removal [31]. The ongoing research into novel hydrogel synthesis methods, including graft copolymerization, the formation of interpenetrating polymer networks, and the incorporation of various nanomaterials, aims to overcome inherent limitations such as mechanical strength and adsorption capacity, ultimately paving the way for the development of more efficient and robust hydrogel-based adsorbents for practical applications in wastewater treatment [32]. This investigation focuses on the synthesis and application of a specific hydrogel nanocomposite i.e., sodium alginate-g-poly(acrylic acid-co-Itaconic acid)/titanium dioxide, SA-g-p(AA-IA)/TiO2. This material is designed to leverage the combined benefits of a natural polysaccharide (sodium alginate), synthetic monomers (acrylic acid and Itaconic acid), and inorganic nanoparticles (titanium dioxide) for the efficient removal of CV dye from aqueous solutions [4]. Sodium alginate, an anionic polysaccharide derived from seaweed, is a readily available, cost-effective, and biodegradable biopolymer that offers a biocompatible matrix for hydrogel formation [33]. The grafting of acrylic acid onto the sodium alginate backbone introduces pH-responsive carboxylic acid functional groups, which can significantly enhance the adsorption of cationic dyes, particularly under alkaline conditions. These groups are deprotonated and exhibit increased electrostatic attraction towards the positively charged dye molecules [34]. The incorporation of itaconic acid, a non-ionic monomer, into the polymer network contributes to the hydrogel’s overall structural integrity, mechanical strength, and water swelling capacity, facilitating the accessibility of adsorption sites within the hydrogel matrix [35]. The inclusion of TiO2 nanoparticles within the hydrogel matrix introduces a component known for its high surface area [36, 37]. The specific objectives of this research include: 
Synthesizing and thoroughly characterizing the SA-g-p(AA-IA)/TiO2 hydrogel nanocomposite using appropriate analytical techniques to confirm its successful formation and to understand its structural and morphological properties.
Systematically evaluating the influence of key operational parameters, such as time, pH of solution, temperature, the initial dye concentration, and the dosage of nanocomposite, on efficiency of adsorption process.
Determining the adsorption kinetics by applying established kinetic models, including the pseudo-first-order and pseudo-second-order models, to elucidate the rate-controlling steps and the mechanism of adsorption.
Calculating the thermodynamic parameters, namely the Gibbs free energy change (ΔG), the enthalpy change (ΔH), and the entropy change (ΔS), to assess the spontaneity of adsorption process and to determine whether it is endothermic or exothermic in nature. 
Analyzing the adsorption equilibrium data using various isotherm models, as Langmuir, Freundlich, and Temkin models, to determine the maximum adsorption capacity of nanocomposite for CV dye and to gain insights into the nature of adsorption mechanism at solid-liquid interface. 
A comprehensive investigation of isothermal, kinetics and thermodynamics of CV adsorption on SA-g-p(AA-IA)/TiO2 nanocomposite will provide a thorough understanding of the adsorption mechanism, enabling the optimization of its performance for efficient and sustainable removal of this dye from wastewater. This knowledge can also contribute for developing more effective hydrogel-based nanocomposite for the removal of a wider range of organic pollutants from contaminated water resources. 

MATERIALS AND METHODS 
Chemicals 
Sodium alginate, with a purity of 99.0 % (Himedia), TiO2 with 99.7 % purity (Merck). itaconic acid with 99 % purity (Himedia), potassium persulfate with purity of 99 % (Riedel-dehaenag seclze hannover), N, N’-Methylene-bis-Itaconic acid at 99 % purity (Himedia laboratories), ethanol with a purity of 95 % (Merck Energy), acrylic acid at 99 % purity (Himedia), nitrogen gas (Xinrui), sodium hydroxide with 99 % purity (Alpha Chemika), hydrochloric acid at a concentration of 38 % (J.T. Baker), calcium carbonate with 99 % purity (Fluka), potassium chloride at 99.5 % purity (Fluka), CV with 99.0 % (Merck) and tetramethylethylenediamine (TEMED) at 99.0 % purity (Fluka) were used throughout the study. 

Preparation SA-g-p(AA-IA)/ TiO2 hydrogel nanocomposite 
The SA-g-p(AA-IA)/TiO2 hydrogel nanocomposite was synthesized via free-radical copolymerization in an aqueous solution. This involved dissolving 0.12 g of TiO2 particles in 20 mL of deionized water with continuous stirring for 4 h, followed by transfer to an ultrasonication device for another 4 h. The reaction mixture was then transferred to a three-neck round-bottom flask equipped with a condenser, a dropping funnel, and a nitrogen gas inlet, placed in a water bath with stirring until complete dissolution was achieved. Subsequently, 0.5 g of sodium alginate (SA) was added portion-wise and slowly to the reaction mixture with stirring for 1 h until a homogeneous solution was obtained. Next, 4 g of acrylic acid (AA) was added portion-wise to the preceding solution with continuous stirring for 15 min. Afterwards, a solution prepared by dissolving 2 g of itaconic acid (IA) in 2 mL of H2O was added portion-wise to the reaction mixture with continuous stirring for 15 min. Similarly, 0.05 g of N, N’-methylenebisItaconic acid (MBA) (cross-linking agent) was prepared in 2 mL of H2O and added gradually to solution with stirring for 15 min. Afterwards, four drops of TEMED solution were added with continuous stirring for 5 min. Additionally, a solution of 0.03 g of potassium persulfate (KPS) in 2 mL of H2O was prepared and added gradually with continuous stirring for 15 min at room temperature under a nitrogen atmosphere via the dropping funnel. The temperature was then raised to 70 °C and maintained for 3 h to complete the polymerization reaction. The synthesized nanocomposite hydrogel SA-g-p(AA-IA)/TiO2 was immersed in deionized water to remove unreacted monomers. Finally, it was dried in an electric oven at 70 °C to achieve a constant weight. 

Adsorption studies and calibration curve 
For investigating the potential of synthesized SA-g-p(AA-IA)/ TiO2 hydrogel nanocomposite as an adsorbent, experiments were conducted in a batch mode for removing CV dye from water. Parameters studied for dye adsorption include time (0-180 min), temperature (10, 15, 20, 30 °C), dye concentration (10-500 mg/L), dose of nanocomposite used (0.001-0.04 g) and pH (2-10). For each experiment, the shaking speed was adjusted to 130 rpm using 10 mL dye solution in a conical flask. For the adjustment of pH, NaOH and HCl, 0.1 M, were used and pH meter was used for pH measurement. After the equilibrium time, the absorbance of solution was measured by using UV-vis spectrophotometer with a λmax of 574 nm. Afterwards, the amount of adsorbed dye on adsorbent surface (mg/g) and % removal of dye were calculated by Eq. 1 and 2 correspondingly [38].  

here, Co and Ce refers to initial and equilibrium concentrations of CV dye in mg/L, correspondingly while “m” denotes dose of nanocomposite used in “g”. The volume of dye solution is represented by V in “L”. 
The calibration curve for CV dye (Fig. 1), established by measuring the absorbance of dye at its maximum absorbance wavelength (around 590-600 nm), demonstrates a strong linear relationship between absorbance and concentration, consistent with Beer-Lambert’s law. The resulting linear equation (y = mx + b) with a high coefficient of determination (R2 = 0.998) provides a reliable tool for quantifying CV dye concentrations in adsorption studies, enabling calculation of residual dye concentration, adsorption capacity, and dye removal efficiency. 

RESULTS AND DISCUSSION
Characterization
FTIR analysis of the SA-g-p(AA-IA)/TiO₂ nanocomposite, before and after CV dye adsorption, reveals significant changes in its functional groups. Before dye exposure (Fig. 2), the nanocomposite’s spectrum showed characteristic peaks: a broad band at 3300–3400 cm⁻¹ for –OH and –NH stretches (from alginate hydroxyls and IA amides), a strong peak at 1650 cm⁻¹ for C=O stretching (amide I), and bands near 1550–1600 cm⁻¹ (N–H bending/asymmetric –COO⁻ stretch from AA) and 1400–1450 cm⁻¹ (symmetric –COO⁻ stretch). The C–O–C stretch of polysaccharides appeared around 1020–1080 cm⁻¹, while TiO2 was confirmed by a band at 500–700 cm⁻¹ (Ti–O–Ti vibrations). After CV dye adsorption, noticeable shifts and intensity changes occurred. The –OH/–NH band broadened and shifted, suggesting hydrogen bonding or electrostatic interactions with the dye. The C=O peak at 1650 cm⁻¹ showed minor shifts and decreased intensity, indicating interaction with amide or carboxylic acid groups. Shifts or intensity changes near 1550 cm⁻¹ point to carboxylate or amine groups involved in electrostatic binding or π−π stacking with the dye’s aromatic rings. Changes in the 1400–1450 cm⁻¹ regions suggest ionic exchange or electrostatic attraction between the dye’s positive charges and the nanocomposite’s carboxyl groups. Additionally, the C–O–C peak near 1020–1080 cm⁻¹ have changed intensity due to slight polymer backbone rearrangement upon dye binding [39-42]. These findings align with our prior research on Pb (II) adsorption [43]. 
The X-ray diffraction (XRD) patterns for both pure TiO2 and SA-g-p(AA-IA)/TiO2 nanocomposite (Fig. 3) offer key information about the material’s crystal structure. For, TiO2 (Fig. 3a), the XRD pattern shows distinct and sharp peaks, indicating its crystalline structure. These peaks match the anatase form of TiO2, known for its high photoactivity. These peaks were observed at 2θ angles around 25.3°, 37.8°, 48.0°, 53.9°, and 62.7°, which correspond to (101), (004), (200), (105), and (204) crystal planes. The sharpness and strength of these peaks confirm that TiO2 nanoparticles are highly crystalline and pure. When TiO2 is added to SA-g-p(AA-IA) hydrogel matrix, the XRD pattern changes in several ways (Fig. 3b). The TiO2 peaks become weaker, since the TiO2 nanoparticles are dispersed within the non-crystalline polymer matrix. This decrease in peak strength often happens when nanoparticles are embedded and partially covered by a hydrogel network. Some of the characteristic anatase TiO2 peaks (like at 25.3° for (101) plane) are still visible, showing that the TiO2 keeps its crystal structure even in nanocomposite. However, these peaks are broader and less intense, suggesting that size of TiO2 crystals might be slightly smaller, and crystal structure might be distorted due to interactions with polymer’s functional groups. The broad peak feature in XRD pattern of nanocomposite (between 15° and 30°) indicates that it is non-crystalline. This confirms that the polymer matrix is mostly amorphous, and TiO2 is dispersed in polymeric basic [44]. These findings are from our previously reported paper where we studied Pb (II) adsorption from water [43]. 
FESEM images in Fig. 4 show the nanocomposite’s surface after CV dye adsorption. Before adsorption (Fig. 4a), the material has a bumpy and porous surface with visible openings, suggesting a large surface area for dye binding. TiO2 nanoparticles appear well-dispersed, likely enhancing strength and dye attraction. After dye adsorption (Fig. 4b), the surface becomes noticeably smoother and more densely packed, with most openings filled, confirming that dye molecules have attached. This change from a porous to a smooth surface visually demonstrates the material’s strong adsorption capability [42]. 

Kinetic study 
The influence of time (Fig. 5(a, b)) on adsorption of dye by SA-g-p(AA-IA)/TiO2 hydrogel nanocomposite was investigated by evaluating the adsorption capacity (qe) and percentage removal (% removal). The results indicated a swift and efficient adsorption process characterized by distinct phases. Initially, within the first minute, a qe of 114.44 mg/g and % removal of 57.22% were observed, signifying rapid dye uptake attributed to abundance of accessible active sites on nanocomposite surface. This was followed by a significant enhancement within the first 10 min, reaching a qe of 180.33 mg/g and % removal of 90.16%, highlighting a strong initial mass transfer driven by concentration gradient and surface availability. Subsequently, adsorption rate showed a reduction in intermediate phase (10–45 min) due to progressive saturation of active sites. Eventually, the system approached equilibrium in later stages (60–180 min) confirming that equilibrium was attained at 90 min. The minimal change beyond this point suggests adsorption saturation and equilibrium obtained at 90 min [45]. 
To understand how CV dye is adsorbed onto hydrogel nanocomposite over time, two common kinetic models were applied: pseudo-first-order and pseudo-second-order models. These models help determine the mechanism and slowest step in adsorption process. Pseudo first-order model proposes that rate is related to available number of sites while pseudo second model suggests that the rate-limiting step involves chemical adsorption. Eq. 3 and 4 represents pseudo-first-order and pseudo-second-order model expressions, correspondingly: 

here, qt and qe refers to amounts of dye adsorbed at time t and at equilibrium, respectively (mg/g), while k1 and k2 refers to rate constant for pseudo-first (min⁻¹) and second order model (g/mg·min), correspondingly. Results of kinetic study (Fig. 6(a, b) and Table 1 revealed that pseudo-first-order model does not accurately describe how CV dye was adsorbed by nanocomposite due to its lower regression coefficient value in comparison to pseudo second model. This suggests that process is not primarily controlled by physical diffusion alone. The pseudo-second model provides the best description of kinetics, indicating that chemical adsorption is main mechanism. This likely involves interactions as electrostatic forces, π–π stacking, or H-bonds between CV molecules and functional groups (as –COOH, –NH₂) of nanocomposite [46].  

Isothermal and temperature study
The adsorption capacity (mg/g) of nanocomposite was increased significantly at higher initial concentrations of CV dye (Fig. 7). Specifically, at a temperature of 10 °C, the adsorption capacities were recorded as 17.92 mg/g at a dye concentration of 10 mg/L, 197.41 mg/g at 100 mg/L, and 928.14 mg/g at 500 mg/L. At 15 °C, qe values were 17.55 mg/g, 197.15 mg/g, and 919.42 mg/g for the same respective concentrations. When the temperature was raised to 25 °C, the adsorption capacities were 16.45 mg/g, 197.04 mg/g, and 915.76 mg/g. Finally, at 30°C, qe values were 16.09 mg/g, 192.72 mg/g, and 891.59 mg/g, respectively. These values demonstrate that a high concentration of dye molecules in solution provides a stronger driving force for the movement of dye molecules towards the adsorbent surface and their subsequent attachment. Furthermore, moderate enhancement in temperature, specifically within the range of 10–15 °C, appears to enhance the uptake of the dye by nanocomposite. This enhancement is likely due to the increased kinetic energy of dye molecules and the adsorbent surface, which can facilitate more effective interaction and binding. However, at the highest temperature tested (30 °C), a slight reduction in the adsorption capacity was observed, particularly at higher dye concentrations. This suggests that at elevated temperatures, the equilibrium might shift slightly towards desorption, or that saturation phenomena become more pronounced, limiting further dye uptake [44, 47, 48].  
Thermodynamic parameters, including Gibbs free energy (ΔG), enthalpy change (ΔH), and entropy change (ΔS), provide crucial insights into feasibility, spontaneity of adsorption process, as well as the degree of order or disorder at solid–solution interface. Based on the data obtained at 20 °C (from Van’t Hoff plot, Fig. 8), the thermodynamic parameters were determined (Table 2) that include ΔG = –5.812 kJ/mol, indicating that the adsorption of CV dye onto nanocomposite is a spontaneous process under these conditions. Further, ΔH is –15.737 kJ/mol, revealing that adsorption process releases heat, and is therefore exothermic. Additionally, ΔS is –34.255 J/mol·K, suggesting a reduction in entropy at the solid–solution interface. This reduction in randomness is likely due to ordering of dye molecules as they become immobilized on specific sites of adsorbent surface, thereby reducing their degrees of freedom as compared to their state in the bulk solution [5]. 
To gain a deeper understanding of the interaction between CV dye molecules and surface of the nanocomposite, three adsorption isotherm models were employed i.e., Langmuir, Freundlich, and Temkin isotherm. Langmuir model (Eq. 5) is based on assumptions of monolayer adsorption, where single layer of adsorbate molecules forms on homogeneous adsorbent surface containing a finite number of identical adsorption sites. Freundlich isotherm (Eq. 6) describes multilayer adsorption on heterogeneous surfaces, where the heat of adsorption varies with the degree of surface coverage. While Temkin model (Eq. 7) takes into account the interactions between adsorbed molecules and assumes that heat of adsorption decreases linearly with surface coverage. 

where, qe is equilibrium adsorption capacity, qmax is maximum adsorption capacity, KL is Langmuir constant, Ce is equilibrium dye concentration, Kf is Freundlich constant related to adsorption capacity, 1/n is an empirical parameter related to surface heterogeneity, R is gas constant, T is absolute temperature, b is Temkin constant related to heat of adsorption, and A is Temkin isotherm constant. Isotherm analysis (Fig. 9(a-c)) revealed that the adsorption behaviour can be described by both the Langmuir and Freundlich models (due to their high R² values), indicating that the composite surface presents a combination of both energetically uniform sites capable of monolayer adsorption and heterogeneous sites allowing for multilayer adsorption. The Temkin isotherm also provided insights into the energy of adsorption and adsorbate-adsorbate interactions [49].  

Effect of pH and nanocomposite dose 
The swelling percentage of SA-g-p(AA-IA)/TiO2 hydrogel nanocomposite is significantly affected by pH, as the ionic environment influences the charged state of functional groups within its polymer structure and is calculated by Eq. 8: 

The hydrogel’s swelling capacity is significantly influenced by pH, as illustrated in Fig. 10a. In acidic conditions, swelling is limited: 130% at pH 2, gradually increasing to 240% at pH 3, and 330% at pH 4. This restricted swelling is due to the protonation of carboxylic (–COOH) and amide groups, which reduces repulsive forces between polymer chains and limits water absorption. As pH increases to 5 and beyond, a substantial enhancement in swelling is observed. Swelling reaches 470% at pH 5, climbing sharply to 670% at pH 6, 850% at pH 7, and peaking at 1060% at pH 8. This significant rise is attributed to the deprotonation of –COOH groups to –COO⁻, leading to increased electrostatic repulsion within the polymer network and greater water absorption. Beyond pH 8, swelling continues to increase but at a slower rate, reached 1100% at pH 9, 1120% at pH 10, and stabilized at 1155% at pH 12. This plateau indicates that the ionizable groups are fully deprotonated, and the hydrogel has reached its maximum expansion in alkaline conditions [50]. These findings are consistent with our previous research on Pb (II) adsorption from water [43]. The adsorption capacity of SA-g-p(AA-IA)/TiO2 hydrogel nanocomposite for CV dye was clearly influenced by pH of solution (Fig. 10b). At a highly acidic pH of 2, the qe was 195.54 mg/g. This comparatively lower adsorption is likely due to the abundance of H⁺ ions, which cause protonation of functional groups like –COO⁻ and –OH on adsorbent surface. This protonation results in a positively charged hydrogel surface, leading to electrostatic repulsion with positively charged dye and consequently reducing its adsorption. As the pH was raised to 4 and then to 6, the adsorption capacity showed improvement, reaching 196.31 mg/g and 197.08 mg/g, respectively. This enhancement can be attributed to gradual loss of protons from the hydrogel’s functional groups, imparting more negatively charged active sites that show interactions with cationic dyes. An enhancement in adsorption was observed at pH 7 and 8, where qe values were 197.33 mg/g and 197.44 mg/g, respectively. Further increasing in pH to 9 and 10 resulted in a slight additional growth in adsorption capacity to 197.55 mg/g and a peak of 197.66 mg/g, indicating that optimal adsorption efficiency is achieved under weakly basic conditions. Beyond this pH, no significant increase in adsorption was observed [50].  
Fig. 11(a, b) revealed how different amounts of nanocomposite adsorbent affects both the adsorption capacity and removal efficiency of CV dye. As the quantity of the SA-g-p(AA-IA)/ TiO2 nanocomposite was increased from 0.001 g to 0.01 g, a significant improvement in the percentage of dye removed was observed, rising from 93.84% to 99.70%. This enhancement is attributed to the greater availability of active adsorption sites as more adsorbent material is introduced into the system. However, beyond an adsorbent dose of 0.01 g, the removal efficiency tended to level off, reaching 99.45% at a dose of 0.04 g. This plateau indicates that the majority of the dye present in the solution had been adsorbed. Consequently, further increasing in the adsorbent amount yielded only a minimal improvement in removal. Conversely, the amount of dye adsorbed per unit mass of adsorbent (mg/g) showed a sharp reduction as adsorbent dose was increased. At the lowest dose of 0.001 g, the qe value was exceptionally high at 1876.78 mg/g, reflecting a large ratio of dye molecules to the amount of adsorbent available. As the dose was increased to 0.005 g and then to 0.01 g, the qe values decreased to 385.67 mg/g and 199.41 mg/g, respectively. At the highest adsorbent dose of 0.04 g, the adsorption capacity per unit mass fell to only 49.73 mg/g. This trend occurs when a high quantity of adsorbent is added to the system. Furthermore, at higher adsorbent concentrations, the particles may tend to clump together, which can reduce the effective surface area available for adsorption and limit the accessibility of dye molecules to the active sites [50, 51].  

CONCLUSION 
In this study, a versatile SA-g-p(AA-IA)/TiO2 hydrogel nanocomposite was successfully created and its performance in removing CV dye from water solutions was assessed. The material displayed desirable structural and surface properties, as confirmed by FTIR, XRD, and FESEM analyses. The adsorption process was significantly affected by operational factors, like notably pH, where basic conditions increased the electrostatic attraction between negatively charged areas and the positively charged dye. Kinetic modeling indicated that the adsorption process adhered to pseudo-second-order kinetics, suggesting a chemical adsorption mechanism. While equilibrium data aligned with both Langmuir and Freundlich isotherm models, indicating the presence of both uniform and varied binding sites. The thermodynamic parameters confirmed that the adsorption was a spontaneous and heat-releasing process. Moreover, the composite showed excellent swelling capabilities and high removal efficiency (exceeding 99.7 %) even with a small amount of adsorbent. These results strongly suggest that the SA-g-p(AA-IA)/TiO2 hydrogel nanocomposite presents a significant potential as an affordable, effective, and environmentally friendly adsorbent for treating industrial wastewater contaminated with cationic dyes. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.