Abstract Crystal violet (CV) is a persistent synthetic dye that must be effectively removed from contaminated water. This study investigated how biopolymer composition affects the swelling behavior and equilibrium CV adsorption of xanthan gum–alginate hydrogel beads containing either chitosan (XAC) or guar gum (XAG). The beads were prepared by Ca²⁺-mediated ionic gelation and characterized by Fourier-transform infrared spectroscopy, scanning electron microscopy, and swelling analysis at different pH values. XAG demonstrated a higher degree of swelling at a pH of 2. Conversely, XAC exhibited a greater degree of swelling at pH 6.8 and 13. Equilibrium adsorption was evaluated using 30 milligrams of dried beads in 5 mL of CV solution at initial concentrations ranging from 10-1,000 mg/mL. This was followed by 2 h of shaking and 22 h of static equilibration at room temperature. XAC demonstrated more consistent adsorption behavior, with Langmuir Qmax values ranging from 0.87-1.0437 mg/g and R² > 0.99 across three replicates. In contrast, XAG exhibited highly variable Langmuir Qmax values (0.2436–99.97 mg/g), indicating unstable parameter estimation despite high R² values, while the Freundlich model provided more consistent fitting. These findings demonstrate that differences in ternary biopolymer composition substantially affect the swelling and adsorption behavior of xanthan gum–alginate hydrogel beads. The findings provide a comparative framework for understanding how polysaccharide composition influences the performance and consistency of simple hydrogel bead adsorbents for CV-contaminated water, while further studies are needed to establish their cost-effectiveness, scalability, and practical applicability.

Keywords: Hydrogel bead, Crystal violet, Xanthan gum, Alginate, Chitosan, Guar gum

Funding: The authors acknowledge funding from Riset Kolaborasi Indonesia with contract no. 1549/UN1/DITLIT/Dit-Lit/PT.01.03/2022.

Citation:  Zai, K., Hastuti, A.A.M.B., and Purwitasari, E. 2027. Influence of biopolymer composition on swelling behavior and crystal violet adsorption of xanthan gum–alginate hydrogel beads. Natural and Life Sciences Communications. 26(1): e2027004.

Graphical Abstract:

INTRODUCTION

Crystal violet (CV; C₂₅H₃₀ClN₃) has been utilized extensively in the textile, paper, and printing industries (Krishnaswamy, 2018; Piaskowski et al., 2018; Kerkez et al., 2020). CV's complex aromatic structure and high stability contribute to its resistance to photodegradation and biodegradation, resulting in its persistent presence in aquatic environments (Krishnaswamy, 2018; Piaskowski et al., 2018; Kerkez et al., 2020). Once released into water bodies, CV has the potential to induce ecological and health risks, including cytotoxic, mutagenic, and potentially carcinogenic effects (Krishnaswamy, 2018; Piaskowski et al., 2018; Kerkez et al., 2020). Consequently, effectively removing CV from contaminated water is crucial for environmental protection and wastewater management.

Despite the development of various methods for dye removal, including advanced oxidation processes, membrane filtration, and photocatalysis, these approaches may involve relatively high operational costs, chemical consumption, energy requirements, and the generation of secondary waste (Samsami et al., 2020; Ayedi et al., 2023; Najim et al., 2025; Parida et al., 2025). Conversely, adsorption has emerged as an effective and comparatively simple approach for removing dyes from aqueous media because of its operational simplicity, versatility, and potential cost-effectiveness (Bulgariu et al., 2019; Din et al., 2024). This approach is consistent with the findings of previous studies, which have demonstrated the efficacy of adsorption in removing dyes from wastewater (Sawasdee and Watcharabundit, 2016). However, the adsorption performance of a material is contingent upon its surface chemistry, porosity, swelling behavior, and accessibility of functional groups.

In recent years, biopolymer-based adsorbents have received significant attention due to their renewable sourcing, biodegradability, accessibility, and functional versatility. Polysaccharide-based materials, including xanthan gum, sodium alginate, chitosan, and guar gum, possess a high density of hydroxyl, carboxyl, and/or amino functional groups. These functional groups can engage in hydrogen bonding, electrostatic interactions, and other intermolecular interactions with dissolved pollutants (Kahya and Erim, 2022; Hevira et al., 2024; Kim et al., 2024; Mei et al., 2024; Ponlawat et al., 2025). Recent studies have further demonstrated the potential of natural-polymer-based hydrogel systems for water remediation, including guar gum-based hydrogel composites and modified guar gum hydrogels for the removal of dyes and other aqueous pollutants (Keshawy et al., 2025; Yousry et al., 2025). Chitosan-containing hybrid materials have also been investigated as adsorbents for organic and inorganic pollutants because of their abundant amino and hydroxyl functionalities and their ability to form structured composite matrices (Buanpech et al., 2024; Abdelaziz et al., 2025; Zai et al., 2026). Earlier research has also demonstrated the application of natural-polymer-based hydrogel nanoparticles for removing metal ions from an aqueous solution (Farag et al., 2016). The extant studies indicate that the chemical composition and structural organization of biopolymer-based hydrogels can substantially influence their interaction with aqueous pollutants. Furthermore, composite hydrogels formed by combining different polysaccharides may provide tunable swelling, water retention, and structural properties that affect pollutant diffusion and accessibility of adsorption sites (Cofelice et al., 2023; Cömez et al., 2023; Le et al., 2023; Naji-Tabasi et al., 2023; Nafisah et al., 2024; Pal et al., 2025).

In this study, two composite hydrogel bead formulations were developed: xanthan gum–alginate–chitosan (XAC) and xanthan gum–alginate–guar gum (XAG). The two formulations under consideration both contain xanthan gum and sodium alginate as the common polysaccharide matrix. However, they differ in that one contains chitosan, and the other contains guar gum, which were selected to introduce different chemical and structural characteristics. Chitosan provides amino and hydroxyl functionalities and can contribute to network interactions and structural reinforcement, whereas guar gum is a highly hydrophilic galactomannan that can contribute to viscosity, water retention, and hydration of the polymer network. The beads were prepared using an ionic gelation technique, with calcium ions serving as the cross-linking agent. A comparison of XAC and XAG thus presents an opportunity to examine how differences in ternary biopolymer composition influence the physicochemical and adsorption-related properties of the xanthan gum–alginate hydrogel system.

The physicochemical properties of the hydrogel beads were evaluated using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and swelling-ratio analysis at different pH conditions. Fourier transform infrared spectroscopy (FTIR) was utilized to examine the characteristic functional groups of the constituent polymers. Scanning electron microscopy (SEM) was employed to assess the morphology of dried beads. Finally, swelling analysis was used to compare their pH-dependent water uptake behavior. These characteristics are pertinent to adsorption because hydration and polymer-network expansion can affect the accessibility and diffusion of dissolved molecules within hydrogel matrices.

Despite the growing interest in natural polymer-based adsorbents, the comparative influence of different polysaccharide compositions on swelling behavior and equilibrium CV adsorption in simple ternary hydrogel bead systems remains insufficiently described. Previous studies have frequently focused on the following: chemically modified polymers; grafted hydrogels; graphene-containing composites; or inorganic nanocomposites. These materials can provide enhanced adsorption capacity, but they introduce additional structural and compositional complexity. In contrast, the present study focuses on two simple ternary polysaccharide hydrogel bead formulations, xanthan gum–alginate–chitosan (XAC) and xanthan gum–alginate–guar gum (XAG), prepared by Ca²⁺-mediated ionic gelation. The objective of this study was to investigate how differences in ternary biopolymer composition affect pH-dependent swelling and equilibrium CV adsorption and to compare their adsorption behavior using Langmuir and Freundlich models. By examining these two natural-polysaccharide formulations under the same adsorption protocol, the study provides comparative insight into the relationship between polymer composition, swelling behavior, and the consistency of CV adsorption parameters. The findings are intended to support the rational selection of polysaccharide combinations for developing simple and potentially sustainable hydrogel-based adsorbents for dye-contaminated water.

MATERIALS AND METHODS

Materials

Sodium hydroxide (NaOH), Calcium chloride (CaCl₂), crystal violet (C₂₅H₃₀ClN₃), and acetic acid were purchased from Merck. Xanthan gum was purchased from Qingdao ICD Biochemistry (China). Chitosan (blue crab shell, mol. wt. ~ 168 kDa, degree of deacetylation: 96.19%) was purchased from Chimultiguna (Indonesia). Sodium alginate (mol. wt. ~ 75-100 kDa) was purchased from Kimica International. Guar gum was purchased from Lucid (India).

Preparation of polymer solution

Xanthan gum (XG) solution was prepared by dissolving 10 g of XG in 1,000 mL of hot distilled water (90 °C) under continuous stirring at 500 rpm. Sodium alginate (SA) solution was prepared by dissolving 10 g of SA in 1,000 mL of 0.1 M NaOH solution under stirring at 500 rpm and 45 °C. Chitosan (CS) solution was prepared by dissolving 10 g of CS powder in 1,000 mL of 1% (v/v) acetic acid under continuous stirring at 500 rpm at room temperature until a homogeneous, pale-yellow viscous solution was obtained. Guar gum (GG) solution was prepared by dissolving 10 g of GG in 1,000 mL of distilled water under stirring at 500 rpm at room temperature. All polymer solutions had a nominal concentration of 1% (w/v).

Preparation of hydrogel beads

Two ternary hydrogel bead formulations, xanthan gum–sodium alginate–chitosan (XAC) and xanthan gum–sodium alginate–guar gum (XAG), were prepared by ionic gelation using Ca²⁺ as crosslinker agent. The detailed preparation procedure is described in the following text and illustrated in Figure 1. The polymer compositions used for the two formulations are presented in Table 1.

Table 1. The ratio of polymer combination.

XAC hydrogel bead

For XAC hydrogel beads, 50 g of CS solution was mixed with 50 g of XG solution at 300 rpm and room temperature for 10 min. The resulting XG–CS mixture was allowed to rest for 30 min. Subsequently, 100 g of SA solution was added and mixed at 300 rpm and room temperature for 10 min. The resulting XG–SA–CS polymer mixture was then allowed to rest for 30 min.

The complete polymer mixture was subsequently added dropwise into 500 mL of 2% (w/v) CaCl₂ solution using a 20-mL medical syringe fitted with a specialized needle. The droplets underwent ionic gelation in the CaCl₂ bath and were maintained overnight at ambient temperature. The resulting hydrogel beads were collected and thoroughly rinsed with distilled water. The quantities of the 1% (w/v) stock solutions corresponded to a polymer composition of 25:50:25 (% w/w) for XG:SA, respectively, based on the total polymer mass.

XAG hydrogel bead

For XAG hydrogel beads, 100 g of GG solution was mixed with 50 g of XG solution at 300 rpm and room temperature for 10 min. The resulting XG–GG mixture was allowed to rest for 30 min. Subsequently, 50 g of SA solution was added and mixed at 300 rpm and room temperature for 10 min. The resulting XG–SA–GG polymer mixture was then allowed to rest for 30 min.

The complete polymer mixture was subsequently added dropwise into 500 mL of 2% (w/v) CaCl₂ solution using a 20-mL medical syringe fitted with a specialized needle. The droplets underwent ionic gelation in the CaCl₂ bath and were maintained overnight at ambient temperature. The resulting hydrogel beads were collected and thoroughly rinsed with distilled water. The quantities of the 1% (w/v) stock solutions corresponded to a polymer composition of 25:25:50 (% w/w) for XG:SA, respectively, based on the total polymer mass.

Figure 1. Schematic illustration of preparing xanthan gum–sodium alginate–chitosan (XAC) and xanthan gum–sodium alginate–guar gum (XAG) hydrogel beads by Ca²⁺-mediated ionic gelation.

Characterization of hydrogel beads

Scanning electron microscopy (SEM)

The sample was initially mounted onto a specimen holder using carbon adhesive tape and allowed to dry. A thin gold (Au) coating was applied using a JEOL JEC-3000FC auto fine coater (JEOL, Tokyo, Japan) for approximately 120 seconds under a vacuum pressure of about 3.2 Pa. Subsequently, the hydrogel surface morphology was examined using SEM. The sample was introduced into the microscope chamber and subjected to vacuum conditions for roughly 60 seconds before imaging. Electron beams were directed at the sample at a defined probe current, and the resulting surface topography was captured.

Fourier-transform infrared analysis

The experimental procedure involved acquiring Fourier-transform infrared (FTIR) spectra. This was achieved by means of a Thermo Nicolet iS10 spectrometer (Thermo Fisher Scientific, Madison, USA) that was outfitted with a Smart iTR diamond ATR accessory. Samples were meticulously positioned onto the ATR crystal and secured using a pressure clamp. Samples were placed directly onto the ATR crystal and secured using a pressure clamp. Spectral acquisition was carried out over the wavenumber range of 4,000 to 600 cm⁻¹ with a resolution interval of 0.964 cm⁻¹. Between measurements, the crystal surface was thoroughly cleaned with water, followed by acetone, and a new background spectrum was collected before each sample analysis.

Hydrogel swelling ratio

The dried hydrogels were weighed and immersed in buffer solutions with different pH levels (2-13) at 25°C. The calculation of the swelling ratio was derived from three replicate measurements using the following equation:

wherein, W_t and W₀ refer to the weight of swollen beads for t h and 0 h, respectively.

Equilibrium adsorption study

Equilibrium adsorption experiments were performed by adding 5 mL of CV solution at varying initial concentrations (10–1,000 mg/L) to 30 mg of dried hydrogel beads. The mixtures were shaken at 100 rpm and room temperature for 2 h, followed by static incubation for 22 h, giving a total contact period of 24 h. The adsorbent dosage, solution volume, agitation speed, contact-time protocol, and temperature were kept constant for XAC and XAG to enable direct comparison of their equilibrium adsorption behavior. These parameters were not individually optimized in the present study, as the experiment was designed to evaluate the adsorption isotherms over a range of initial CV concentrations. After incubation, the samples were filtered, and the equilibrium CV concentration (Cₑ) in the filtrate was determined using a UV–Vis spectrophotometer (Hitachi U-2900) at 615.8 nm. The equilibrium adsorption capacity, qₑ (mg/g), was calculated as:

Wherein, C₀ and C_e are the initial and equilibrium concentrations of CV (mg/mL), V is the volume of the solution (L), and m is the mass of the dried adsorbent (g).

The adsorption capacity was calculated from the decrease in CV concentration between the initial solution and the equilibrium filtrate; however, an independent experimental mass-balance verification of CV in both the solution and hydrogel phases was not performed.

RESULTS

Characterization of hydrogel beads

Hydrogel beads formed in CaCl₂ solution, serving as the ionic crosslinker, initially exhibited a spherical, ball-like morphology. However, after freeze-drying, the beads collapsed, resulting in irregular and shrunken shapes, as shown in Figure 2. SEM analysis revealed differences in the surface morphology of the dried hydrogel beads. The XAC hydrogel exhibited a relatively smooth matrix at lower magnification, while the higher-magnification image showed numerous rounded and granular surface deposits (Figure 3a). In contrast, XAG exhibited a more irregular and rough surface with wrinkled and clustered structures at higher magnification (Figure 3b). These observations indicate differences in the surface morphology of the two hydrogel formulations after drying. The elemental or chemical identity of the observed deposits was not determined in the present study.

Figure 2. Physical appearance of calcium-ionically crosslinked composite hydrogel beads: freshly prepared hydrated beads (a) and freeze-dried beads (b).

Figure 3. SEM image of XAC (a) and XAG (b) hydrogel beads with scale bars: 500, 100, and 5 µm.

Fourier-transform infrared analysis

Figure 4 presents the FTIR spectra of the individual polymers and the XAC and XAG hydrogel bead formulations. Both hydrogel formulations exhibited broad O–H/N–H stretching bands in the high-wavenumber region and characteristic polysaccharide vibrations in the fingerprint region. Several bands in the 1,000–1,200 cm⁻¹ region were associated with overlapping C–O and C–O–C vibrations, reflecting the carbohydrate-rich structure of the hydrogel matrices. Bands around 1,425 cm⁻¹ and in the region around 1,600–1,650 cm⁻¹ were also observed and may be associated with carboxylate and other overlapping functional-group vibrations.

Differences were observed between the XAC and XAG spectra in several regions. XAC exhibited bands around 1,650 and 1,598 cm⁻¹ that are consistent with amide I and amide II vibrations, respectively, as well as a band around 1,328 cm⁻¹ that may be associated with amino-containing functionalities of chitosan. In contrast, these chitosan-associated bands were not apparent in XAG, while additional bands were observed around 1,149, 1,237, and 3,340 cm⁻¹ in the XAG spectrum. These differences are consistent with the different ternary polymer compositions of the two hydrogel formulations and the contribution of guar gum in XAG.

Overall, the FTIR spectra showed that both hydrogel formulations retained characteristic functional groups of polysaccharide-based materials, while differences in the fingerprint region reflected differences in their constituent polymer composition. The observed spectra provide compositional and functional-group evidence for the two hydrogel formulations but do not by themselves establish a specific chemical reaction or directly confirm the Ca²⁺ crosslinking mechanism.

Figure 4. FTIR spectra of the individual polymers (xanthan gum, alginate, chitosan, and guar gum), and hydrogel beads (XAC and XAG).

Table 2. Unique peaks for each hydrogel bead sample.

Swelling ratio of hydrogel beads

The swelling ratios of XAC and XAG hydrogel beads at pH 2, 6.8, and 13 are shown in Figure 5. XAG beads exhibited higher swelling at pH 2, whereas XAC beads demonstrated higher swelling ratios at pH 6.8 and pH 13.

Figure 5. Swelling ratio (%) of XAC and XAG hydrogel beads at pH 2, 6.8, and 13. Error bars represent standard deviations (n = 3).

Equilibrium adsorption

We evaluated the equilibrium adsorption behavior of crystal violet (CV) onto XAC and XAG hydrogel beads using the Langmuir and Freundlich isotherm models. Figure 6-7 and Table 3 present the fitting results.

For XAC hydrogel beads, the Langmuir model showed Qmax values ranging from 0.87 to 1.0437 mg/g and KL values ranging from 0.14241 to 0.21752, with R² values above 0.99. The Freundlich model also demonstrated high correlation coefficients (R² > 0.993), with KF values ranging from 0.4160 to 0.9316 and n values ranging from 0.4311 to 0.9503.

For XAG hydrogel beads, the Langmuir model produced highly variable Qmax values, ranging from 0.2436 to approximately 99 mg/g, with KL values between 0.00019 and 0.09338. Despite high R² values (>0.98), substantial variability was observed among replicates. The Freundlich model generated KF values between 0.0190 and 0.0232, while n values ranged from 0.9218 to 1.0760, with R² values above 0.986.

Table 3. Isotherm model parameters for the adsorption of CV onto hydrogel beads.

Figure 6. Linear and non-linear plots of the Langmuir (a) and the Freundlich (b) adsorption models for the XAC hydrogel bead.

Figure 7. Linear and non-linear plots of the Langmuir (a) and the Freundlich (b) adsorption models for the XAG hydrogel bead.

DISCUSSION

Morphological characteristics of hydrogel beads

The formation of spherical beads following dropwise addition of the polymer mixtures into CaCl₂ solution is consistent with Ca²⁺-mediated ionic gelation of the alginate-containing polymer network. The subsequent collapse and shrinkage observed after freeze-drying can be attributed to the removal of water from the hydrated polymer network, which may result in substantial structural contraction and deformation. Similar morphological changes have been observed in dried hydrogel systems because removal of the liquid phase reduces the internal support provided by the hydrated network.

SEM analysis revealed differences in the surface morphology of the dried XAC and XAG hydrogel beads. XAC exhibited a relatively smooth matrix at lower magnification, whereas numerous rounded and granular deposits were observed at higher magnification. XAG showed a more irregular, rough, and wrinkled surface with clustered structures. These differences may reflect variations in polymer–polymer interactions and network organization resulting from the different ternary biopolymer compositions. The granular deposits observed on XAC may be associated with residual material from the CaCl₂ gelation process; however, their chemical identity cannot be confirmed from SEM morphology alone. Complementary elemental or crystallographic characterization, such as EDS or XRD, would be required to determine their composition and crystalline nature. Therefore, the SEM observations are interpreted primarily as morphological differences between the two formulations rather than as direct evidence of differences in crosslinking density. Such differences in surface morphology and network organization may contribute to the contrasting hydration and adsorption behavior observed for XAC and XAG.

FTIR analysis and polymer interactions

The FTIR spectra provide evidence that the characteristic functional groups associated with the polysaccharide components were retained in both hydrogel bead formulations. The broad O–H/N–H stretching region and the bands in the fingerprint region associated with C–O and C–O–C vibrations are consistent with the carbohydrate-rich structures of xanthan gum, sodium alginate, chitosan, and guar gum. The common spectral features observed for XAC and XAG are therefore consistent with their shared polysaccharide components.

The differences between XAC and XAG are particularly evident in the fingerprint and 1,600–1,700 cm⁻¹ regions. The bands observed around 1,650 and 1,598 cm⁻¹ in XAC are consistent with amide I and amide II vibrations associated with chitosan, while the band around 1,328 cm⁻¹ may also reflect a contribution from chitosan amino-containing functionalities. These features were not apparent in the XAG spectrum, consistent with the absence of chitosan in that formulation. Conversely, the additional bands observed in XAG around 1,149 and 1,237 cm⁻¹, together with differences in the O–H stretching region, are consistent with contributions from the guar gum-containing polymer network. These spectral differences support the compositional distinction between the two formulations.

The FTIR spectra should, however, be interpreted primarily as evidence of the functional groups present in the resulting polymer matrices rather than as definitive proof of new chemical bonds or a specific crosslinking mechanism. The formation of the beads was achieved by Ca²⁺-mediated ionic gelation, primarily involving interactions between Ca²⁺ ions and carboxylate groups of alginate. Changes in peak position or intensity could potentially reflect differences in the molecular environment of these functional groups after blending and gel formation; however, FTIR alone is insufficient to establish the nature or extent of such interactions. Overall, the differences in FTIR profiles are consistent with the different ternary biopolymer compositions of XAC and XAG and may contribute to their different swelling and adsorption behaviors.

Swelling behavior of hydrogel beads

The swelling behavior of the hydrogel beads varied with both polymer composition and environmental pH, indicating that the structure of the polysaccharide network influenced its interaction with water. The hydrogel beads were formed primarily through Ca²⁺-mediated ionic gelation of the alginate component, in which Ca²⁺ ions interact with carboxylate groups of alginate to form an ionically crosslinked network. Xanthan gum, chitosan, and guar gum can additionally contribute through hydrogen bonding, chain entanglement, and other intermolecular interactions, resulting in differences in network organization between XAC and XAG. Consequently, the swelling response reflects the combined effects of crosslinking and the composition of the polymer network rather than the contribution of a single polymer alone.

XAG exhibited higher swelling than XAC at pH 2. Under strongly acidic conditions, protonation of carboxyl groups reduces electrostatic repulsion within the alginate-containing network, which would generally limit expansion of the ionically crosslinked matrix. The comparatively greater swelling of XAG at this pH may therefore be related to the highly hydrophilic nature of guar gum and its contribution to water retention within the polymer network. In contrast, the presence of protonatable amino groups in chitosan may alter the intermolecular interactions and network organization of XAC under acidic conditions. However, the present data do not allow the individual contribution of guar gum or chitosan to be quantitatively separated.

In contrast, XAC beads exhibited higher swelling at pH 6.8 and 13. At these pH conditions, carboxyl-containing groups, particularly those associated with alginate, are increasingly ionized, increasing electrostatic repulsion between negatively charged sites and promoting expansion of the hydrogel network (Cömez et al., 2023; Nafisah et al., 2024). The greater swelling observed for XAC may therefore reflect differences in the organization and hydration of the XG–SA–CS network compared with the XG–SA–GG network. Importantly, this observation should not be interpreted as evidence that chitosan itself directly causes electrostatic repulsion, because the charge state of chitosan is strongly pH-dependent and its contribution under neutral and alkaline conditions differs from that under acidic conditions.

Overall, the results demonstrate that the swelling response of the hydrogel beads is governed by the combined effects of Ca²⁺-mediated ionic gelation, pH-dependent ionization of functional groups, and the composition and intermolecular organization of the ternary polysaccharide networks. The observed differences in swelling may influence water accessibility and polymer-chain mobility and may therefore contribute to differences in CV adsorption. However, swelling was not independently demonstrated to be the sole determinant of adsorption performance.

Adsorption behavior of hydrogel beads

The adsorption behavior of CV onto the hydrogel beads was analyzed using both Langmuir and Freundlich isotherm models to evaluate the adsorption characteristics of the hydrogel systems.

For XAC hydrogel beads, both Langmuir and Freundlich models provided high coefficients of determination (R² > 0.99), with very similar values across the three replicates and Qmax values ranging from 0.87 to 1.0437 mg/g across the three replicates. Although the high R² indicates that the Langmuir equation described the experimental data well, a high R² value alone does not establish that adsorption occurred exclusively through monolayer adsorption on homogeneous sites (Gao et al., 2021; Compton et al., 2025). The Freundlich model also provided a high correlation coefficient, indicating that a heterogeneous adsorption model could also describe the adsorption data (Kekes et al., 2021). Therefore, the adsorption behavior of XAC is more appropriately interpreted as being consistent with both Langmuir- and Freundlich-type descriptions rather than being assigned to a single adsorption mechanism.

In contrast, the Langmuir model produced highly variable Qmax estimates for XAG hydrogel beads, ranging from 0.2436 to 99.97 mg/g, despite high R² values. This large variation indicates poor stability of the fitted Langmuir parameters and limits the reliability of assigning a single Langmuir maximum adsorption capacity to XAG. The very low KL values obtained for XAG also indicate weak apparent affinity within the Langmuir model. In comparison, the Freundlich model produced more consistent parameter estimates, suggesting that the Freundlich equation provided a more stable empirical description of the XAG adsorption data under the conditions investigated.

The adsorption of CV onto the XAC and XAG hydrogel beads may involve several simultaneous interactions between the cationic dye and the polysaccharide-based hydrogel network. One possible contribution is electrostatic attraction between positively charged CV molecules and negatively charged carboxylate groups (–COO⁻), primarily originating from alginate and xanthan gum. The extent of this interaction is expected to depend on the ionization state of the carboxyl groups and the solution pH. Hydrogen bonding may also contribute to CV retention through interactions between hydroxyl- and carboxyl-containing groups of the polysaccharide matrix and heteroatom-containing groups of the dye. In addition, water uptake and swelling of the hydrogel network can facilitate CV diffusion from the surrounding solution into the hydrated polymer matrix, increasing access to internal regions of the beads. Other non-covalent interactions, including hydrophobic or aromatic interactions, may also contribute to dye retention; however, these interactions were not directly demonstrated in the present study.

For XAC, the presence of chitosan should be considered in relation to its pH-dependent protonation behavior. Under acidic conditions, the amino groups of chitosan are predominantly protonated; therefore, they would not be expected to provide direct electrostatic attraction toward the cationic CV molecules. Instead, chitosan may influence CV adsorption indirectly by modifying the hydrogen-bonding environment, hydrophilicity, and structural organization of the polymer network. Consequently, the observed adsorption behavior is likely to reflect the combined contribution of the functional groups and structural characteristics of the entire polysaccharide matrix rather than adsorption onto a single specific polymer component.

The adsorption results can also be considered in relation to the swelling behavior of hydrogel beads at near-neutral conditions. Because the adsorption experiments were conducted in distilled water without adjustment of the solution pH, the actual adsorption environment was not equivalent to the controlled pH conditions used in the swelling experiment. Nevertheless, the greater swelling observed for XAC at pH 6.8 may indicate a more hydrated and expanded polymer network under near-neutral conditions, which could facilitate penetration of CV into the bead matrix and increase accessibility of adsorption regions. Similar relationships between hydrogel swelling and dye uptake have been reported for polymeric hydrogel systems (Torun and Şolpan, 2015; Saraydın et al., 2018).

The greater adsorption performance and parameter consistency observed for XAC compared with XAG may therefore be associated with differences in the overall polymer composition and network structure. However, the present study does not establish a direct causal relationship between swelling and adsorption, because swelling and adsorption were evaluated under different experimental conditions and no direct diffusion or pore-accessibility measurements were performed. Thus, swelling should be considered a possible factor facilitating dye transport rather than the sole determinant of CV adsorption.

The adsorption capacity of the prepared hydrogel beads was further compared with previously reported natural-polymer-based hydrogel adsorbents for CV removal. Pashaei-Fakhri et al. (2021) reported CV adsorption capacities of 62.07 mg/g for an acrylamide–sodium alginate hydrogel and 100.30 mg/g for an acrylamide/graphene oxide–sodium alginate nanocomposite hydrogel. Xanthan gum-based graphene oxide crosslinked hydrogel nanocomposites have also been reported for CV adsorption, demonstrating the potential of xanthan gum-containing hydrogel networks for cationic dye removal (Mittal et al., 2021). Sodium alginate beads have likewise been investigated for CV removal, providing a more directly comparable natural-polymer bead system (Arshad et al., 2024). More recently, a sodium alginate-g-polyacrylamide/hydroxyapatite/Cu–Fe layered double hydroxide nanocomposite demonstrated high CV removal efficiency (Massrour et al., 2025). Compared with these reported systems, the Qmax obtained for XAC in the present study (0.87–1.0437 mg/g) was lower. This difference may be related to the simpler composition of the present hydrogel beads, which contain only natural polysaccharides and Ca²⁺-mediated ionic gelation without grafting, graphene oxide, or inorganic nanocomponents. In addition, direct comparison of Qmax values should be interpreted cautiously because adsorption capacity is influenced by adsorbent composition, initial dye concentration, adsorbent dosage, pH, contact time, and isotherm fitting methodology. Therefore, the present results are best considered as a benchmark for simple ternary polysaccharide hydrogel beads and provide insight into the relationship between biopolymer composition, swelling behavior, and CV adsorption rather than demonstrating superior adsorption capacity.

Study limitations

The present study has several limitations that should be considered when interpreting the results. Reusability of the hydrogel beads was not evaluated through repeated adsorption–desorption cycles; therefore, the stability of their adsorption performance over multiple cycles cannot be established from the present data. In addition, the effects of competing ions and the adsorption behavior in real wastewater matrices were not investigated, as the adsorption experiments were designed using distilled water as a controlled system to evaluate the interaction between CV and the hydrogel beads. Therefore, the selectivity and performance of the hydrogels in complex wastewater matrices remain to be established. The reversibility of swelling and structural integrity of the hydrogel beads after exposure to extreme pH conditions, particularly pH 13, were also not evaluated; therefore, their stability under repeated or prolonged alkaline exposure cannot be established from the present data. Furthermore, swelling kinetics were not investigated; therefore, the rate of water uptake and the time required to reach swelling equilibrium could not be determined, limiting direct assessment of the suitability of the beads for flow-through wastewater treatment systems. Surface-area analysis, elemental mapping, and temperature-dependent adsorption studies were also beyond the scope of the present experimental design. These aspects should be addressed in future studies to further evaluate the regeneration capability, swelling kinetics, structural characteristics, and practical performance of the hydrogel beads under more realistic wastewater conditions.

CONCLUSION

The study demonstrated that XAC hydrogel beads (xanthan–alginate–chitosan) exhibited superior adsorption performance for crystal violet (CV) compared to XAG (xanthan–alginate–guar gum), as supported by both the Langmuir and the Freundlich isotherm models. The Langmuir model provided the best fit for XAC, indicating monolayer adsorption with high capacity and strong affinity. At the same time, XAG showed more heterogeneous adsorption with lower capacity, better described by the Freundlich model. These results align with swelling ratio data, where XAC exhibited greater swelling at near-neutral pH (pH 6.8), which matches the typical pH range of general wastewater (pH 6–8). The enhanced swelling under these conditions facilitated greater dye diffusion and increased exposure of active binding sites, resulting in more efficient adsorption. Overall, the findings underscore the significance of hydrogel composition and pH-responsive swelling behavior in determining adsorption performance, highlighting their potential for practical applications in wastewater treatment.

ACKNOWLEDGEMENTS

The authors acknowledge funding from Riset Kolaborasi Indonesia with contract no. 1549/UN1/DITLIT/Dit-Lit/PT.01.03/2022.

AUTHOR CONTRIBUTIONS

Khadijah Zai: Conceptualization (Lead), Methodology (Lead), Data Curation (Equal), Formal Analysis (Lead), Validation (Lead), Resource (Lead), Writing – Original Draft (Lead), Writing – Review & Editing (Lead), Investigation (Lead), Supervision (Lead), Project Administration (Lead); Agustina Ari Murti Budi Hastuti: Data Curation (Equal), Formal Analysis (Supporting), Writing – Original Draft (Supporting), Writing – Review & Editing (Supporting), Investigation (Supporting); Eriyani Purwitasari: Data Curation (Supporting), Formal Analysis (Supporting), Writing – Original Draft (Supportingl), Writing – Review & Editing (Supporting), Investigation (Supporting).

CONFLICT OF INTEREST

The authors declare that they have no conflicts of interest.

REFERENCES

Abdelaziz, A.I.E., Farag, R.K., Hasan, A.M.A., Farag, A.A., Keshawy, M., El-Azabawy, R.E., and Abdel-Raouf, M.E.-S. 2025. Chitosan-based Zeolitic Imidazolate Framework-8 for water remediation: Kinetic and isotherm insights into the removal of organic and inorganic pollutants. Environmental Science and Pollution Research. 32(51): 29222-29247. https://doi.org/10.1007/s11356-025-37233-2

Arshad, R., Javed, T., and Thumma, A. 2024. Exploring the efficiency of sodium alginate beads and Cedrus deodara sawdust for adsorptive removal of crystal violet dye. Journal of Dispersion Science and Technology. 45(12): 2330-2343. https://doi.org/10.1080/01932691.2023.2265464

Ayedi, K., Innocenzi, V., Di Celso, G.M., and Prisciandaro, M. 2023. Water reuse in the textile industry with integrated treatments: Membranes and advanced oxidation processes. Desalination and Water Treatment. 309: 158-164. https://doi.org/10.5004/dwt.2023.29875

Buanpech, P., Chaijareenont, P., Wanachantararak, P., and Silthampitag, P. 2024. Antifungal effect and durability of chitosan oligosaccharide coating on heat-cured polymethylmethacrylate surface. Natural and Life Sciences Communications. 23(1): e2024004. https://doi.org/10.12982/NLSC.2024.004

Bulgariu, L., Escudero, L.B., Bello, O.S., Iqbal, M., Nisar, J., Adegoke, K.A., Alakhras, F., Kornaros, M., and Anastopoulos, I. 2019. The utilization of leaf-based adsorbents for dyes removal: A review. Journal of Molecular Liquids. 276: 728-747. https://doi.org/10.1016/j.molliq.2018.12.001

Cofelice, M., Messia, M.C., Marconi, E., and Lopez, F. 2023. Effect of the xanthan gum on the rheological properties of alginate hydrogels. Food Hydrocolloids. 139: 108528. https://doi.org/10.1016/j.foodhyd.2023.108768

Cömez, B., Şahbaz, S., and Özbaş, S. 2023. In vitro characterization of alginate-chitosan hydrogels prepared with pH modification. Journal of Research in Pharmacy. 27(3): 532-538. https://doi.org/10.29228/jrp.528

Compton, D., Chiu, N.-C., Stylianou, K.C., and Stadie, N.P. 2025. The temperature dependence of the Langmuir adsorption model for a single-site metal-organic framework. Langmuir. 41(30): 11234-11243. https://doi.org/10.1021/acs.langmuir.5c00757

Din, H., Kiran, M., Haq, F., and Jilani, S. 2024. Synergizing date palm seeds-derived oxidized activated carbon: Sustainable innovation for enhanced water retention, efficient wastewater treatment, and synthetic dye removal. Chemical Engineering Research and Design. 187: 248-259. https://doi.org/10.1016/j.cherd.2024.02.040

Farag, R.K., El-Saeed, S.M., and Abdel-Raouf, M.E. 2016. Synthesis and investigation of hydrogel nanoparticles based on natural polymer for removal of lead and copper(II) ions. Desalination and Water Treatment. 57(34): 16150-16160. https://doi.org/10.1080/19443994.2015.1077744

Gao, J., Zhang, X., Liu, C., Zhao, L., and Wang, Y. 2021. Construction and experimental study of Langmuir model for multi-molecular layer adsorption of porous materials. IOP Conference Series: Earth and Environmental Science. 792: 012012.

Hevira, L., Ighalo, J.O., and Sondari, D. 2024. Chitosan-based polysaccharides for effective synthetic dye adsorption. Journal of Molecular Liquids. 390: 123604. https://doi.org/10.1016/j.molliq.2023.123604

Kekes, T., Tsakanika, L.A., Kolliopoulos, G., and Tzia, C. 2021. Adsorption of indigo carmine onto chitosan-cerium oxide nanobiosorbent: Adsorption isotherms, kinetics, and thermodynamics. IOP Conference Series: Earth and Environmental Science. 882: 012005. https://doi.org/10.1088/1755-1315/690/1/012048

Kahya, N. and Erim, F.B. 2022. Graphene oxide/chitosan-based composite materials as adsorbents in dye removal. Chemical Engineering Communications. 209(12): 1741-1754. https://doi.org/10.1080/00986445.2021.1986700

Kerkez, Đ., Bečelić-Tomin, M., Pucar Milidrag, G., Gvoić, V., Kulić Mandić, A., Leovac Maćerak, A., and Tomašević Pilipović, D. 2020. Treatment of wastewater containing printing dyes: Summary and perspectives. In: Proceedings of the 10^th International Symposium on Graphic Engineering and Design (GRID 2020). Novi Sad: University of Novi Sad, Faculty of Technical Sciences, Department of Graphic Engineering and Design. p. 289–295. https://doi.org/10.24867/GRID-2020-p31.

Keshawy, M., Kamal, R.S., Abdelhamid, A.E., Labena, A., Amin, A., Hasan, A.M., and Abdel-Raouf, M.E. 2025. Novel green sustainable hydrogel composites based on guar gum and algal species for wastewater remediation. International Journal of Environmental Science and Technology. 22: 8895-8918. https://doi.org/10.1007/s13762-024-06159-6

Kim, J.W., Heo, J.W., Xia, Q., Kim, M.S., Oh, D.H., and Kim, Y.S. 2024. Synthesis of chitosan-derived Schiff bases for enhanced antioxidant activity and dye adsorption capabilities. Journal of Korea Technical Association of the Pulp and Paper Industry. 56(4): 14-27. https://doi.org/10.7584/JKTAPPI.2024.8.56.4.14

Krishnaswamy, V.G. 2019. Bioremediation of azo dyes: Applied treatment technologies. In: Bhakta, J.N., Lahiri, S., Jana, B.B., editors. Green Technology for Bioremediation of Environmental Pollution. New York: Nova Science Publishers. p. 261–298.

Le, V.T., Joo, S.W., Berkani, M., Mashifana, T., Kamyab, H., Wang, C., and Vasseghian, Y. 2023. Sustainable cellulose-based hydrogels for water treatment and purification. Industrial Crops and Products. 205: 117525. https://doi.org/10.1016/j.indcrop.2023.117525

Massrour, A., Peighambardoust, S.H., Foroughi, M., Foroutan, R., and Ramavandi, B. 2025. Crystal violet removal by sodium alginate-g-polyacrylamide/hydroxyapatite/Cu-Fe LDH nanocomposite. Environmental Technology and Innovation. 38: 104149. https://doi.org/10.1016/j.eti.2025.104149

Mei, Z., Szczepanski, C.R., Montreuil, O., Kuzhir, P., and Godeau, G. 2024. Investigation on novel chitin and chitosan from dung beetle Heteronitis castelnaui (Harold, 1865) and its potential application for organic dyes removal from aqueous solution. International Journal of Biological Macromolecules. 280(Pt 1): 135605. https://doi.org/10.1016/j.ijbiomac.2024.135605

Mittal, H., Al Alili, A., Morajkar, P.P., and Alhassan, S.M. 2021. Graphene oxide crosslinked hydrogel nanocomposites of xanthan gum for the adsorption of crystal violet dye. Journal of Molecular Liquids. 323: 115034. https://doi.org/10.1016/j.molliq.2020.115034

Nafisah, A.N.,   Tarmidzi, F.M., Setiawan, D., and Ngabito, D.R. 2024. Synthesis and characterization of chitosan/alginate hydrogel using CaCl₂ as a crosslinking agent. Materials Science Forum. 1140: 85-91. https://doi.org/10.4028/p-bj4cHv

Najim, A.A., Radeef, A.Y., and Jabbar, Z.H. 2025. Recent trends in physio-chemo technologies and their role in dyes removal: Effectiveness, benefits, and limitations. Chemical Engineering Research and Design. 219: 198-221. https://doi.org/10.1016/j.cherd.2025.06.005

Naji-Tabasi, S., Shahidi-Noghabi, M., and Dovom, A.M. 2023. Investigating the fabrication and functional properties of new composite hydrogels containing gellan/alginate/xanthan gum. Journal of Sol-Gel Science and Technology. 107(2): 469-478. https://doi.org/10.1007/s10971-022-06014-5

Pal, I., Chopra, L., Ganesan, S., Sharma, G.C., Bhowmik, A., and Santhosh, A.J. 2025. Guar gum: Superabsorbent hydrogels for dye remediation. Polymers for Advanced Technologies. 36(1): e70080. https://doi.org/10.1002/pat.70080

Parida, V.K., Singh, N., Priyadarshini, M., and Tambi, A. 2025. Insights into the synthetic dye contamination in textile wastewater: Impacts on aquatic ecosystems and human health, and eco-friendly remediation strategies for environmental sustainability. Journal of Industrial and Engineering Chemistry. 128: 24-41. https://doi.org/10.1016/j.jiec.2025.04.019

Pashaei-Fakhri, S., Peighambardoust, S.J., Foroutan, R., Arsalani, N., and Ramavandi, B. 2021. Crystal violet dye sorption over acrylamide/graphene oxide bonded sodium alginate nanocomposite hydrogel. Chemosphere. 270: 129419. https://doi.org/10.1016/j.chemosphere.2020.129419

Piaskowski, K., Świderska-Dąbrowska, R., and Zarzycki, P.K. 2018. Dye removal from water and wastewater using various physical, chemical, and biological processes. Journal of AOAC International. 101(5): 1371-1387. https://doi.org/10.5740/jaoacint.18-0051

Ponlawat P., Prathum N., Pittayavinai P., Tuanudom R., Srisawang N., Kumchai T., Sricharoen P., Somsook E., and Meechai T. 2025. Biopolymer beads composed of gamma-irradiated chitosan and dental-waste-derived alginate for dye removal applications. Carbohydrate Polymer Technologies and Applications. 11: 100922. https://doi.org/10.1016/j.carpta.2025.100922

Samsami, S., Mohamadi, M., Sarrafzadeh, M.-H., and Firoozbahr, M. 2020. Recent advances in the treatment of dye-containing wastewater from textile industries: Overview and perspectives. Process Safety and Environmental Protection. 141: 83-103. https://doi.org/10.1016/j.psep.2020.05.034

Saraydın, D., Işıkver, Y., and Karadağ, E. 2018. A study on the correlation between adsorption and swelling for poly(hydroxamic acid) hydrogels-triarylmethane dyes systems. Journal of Polymers and the Environment. 26(10): 4402-4410. https://doi.org/10.1007/s10924-018-1257-9

Sawasdee, S. and Watcharabundit, P. 2016. Effect of temperature on brilliant green adsorption by shrimp shell: Equilibrium and kinetics. Chiang Mai University Journal of Natural Sciences. 15(3): 261-274. https://doi.org/10.12982/cmujns.2016.0017

Torun, M. and Şolpan, D. 2015. The removal of cationic dyes from aqueous solutions by using poly(N-vinylpyrrolidone-co-methacrylic acid) hydrogel adsorbents: Effects of pH and composition on adsorption. Fibers and Polymers. 16(11): 2432-2441. https://doi.org/10.1007/s12221-015-0735-0

Yousry, R., Sayed, A., Behalo, M.S., Abdel-Raouf, M.E., and Feteha, A. 2025. Tailoring of carboxymethyl guar gum hydrogels via gamma irradiation for remarkable removal of cationic and anionic dyes from simulated solutions. International Journal of Biological Macromolecules. 284(1): 137867. https://doi.org/10.1016/j.ijbiomac.2024.137867

Zai, K., Ratri, L.K., Rosydiana, C.S., and Dewanto, K.A.P. 2026. Potential and characteristic combinations of collagen with natural polymer-based hydrogel for burn wound dressing applications. Natural and Life Sciences Communications. 25(1): e2026013. https://doi.org/10.12982/NLSC.2026.013

OPEN access freely available online

Natural and Life Sciences Communications

Chiang Mai University, Thailand. https://cmuj.cmu.ac.th