Immobilization of a highly efficient adsorbent in biopolymer hydrogel beads with an established interconnected network explored for selective dye adsorption†

Kshama D. Lokhande, Ankita Kadam, Kartiki Bhosale, Madhuri A. Bhakare and Surajit Some*
Department of Speciality Chemicals Technology, Institute of Chemical Technology, Matunga, Mumbai-400019, India. E-mail: sr.some@ictmumbai.edu.in

First published on 12th July 2025

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Introduction

Synthetic and natural dyes are widely used as industrial-coloured raw materials in industries such as printing, dying, paper, leather, plastics, and other industries. After use, a significant portion of these dyes is often discharged with industrial waste water, without further treatment, into the environment.¹ This dye-containing wastewater harms ecosystems and human health directly through exposure to carcinogenic/mutagenic dyes and their byproducts, and by indirectly affecting the aesthetic quality of water bodies.^2,3 Dyes in water bodies decrease transparency, thereby blocking sunlight penetration to sufficient depths. The lack of sunlight adversely affects aquatic biodiversity by reducing the rate of photosynthesis.⁴ Meanwhile, the prolonged presence of dyes in water can lead to bioaccumulation in humans through consumption of contaminated aquatic animals.⁵ This discussion indicates that the harmful effects can impact organisms and spread throughout the entire ecosystem. Some dyes have complex organic structures and high water solubility, which makes them extremely difficult to remove using traditional dye removal methods.^6,7

Effective dye removal methods must balance environmental impact, cost, and efficiency.⁸ Although various techniques like biological treatment, membrane filtration, flocculation, catalytic oxidation, adsorption, and degradation^9,10 have been explored, many either generate secondary pollution or are expensive.¹¹ As a result, there is a continued need for more efficient and sustainable solutions.¹² Among the above-mentioned methods, adsorption is the most promising method due because it is simple, cost-effective, easy to operate, highly efficient, mechanically stable, and reusable.^13,14 The adsorption process involves two mechanisms: chemisorption and physisorption. Chemisorption or chemical adsorption results from chemical bonding between adsorbate and adsorbent and is irreversible.¹⁵ Meanwhile, physisorption or physical adsorption is a reversible process caused by physical interactions such as dipole–dipole interactions, hydrogen bonds, and van der Waals forces.¹⁵ The careful optimization of factors such as pH, temperature, adsorbent doses, pollutant initial concentration, and contact time should be considered to scale up the adsorption process to an industrial level as the adsorption efficiency is greatly influenced by the mentioned factors.¹⁶

Polymers are increasingly used for dye removal due to their biodegradability, biocompatibility, and environmental safety.¹⁷ They offer advantages like versatility, low toxicity, and abundant functional groups.¹⁸ Among them, alginate stands out for its carboxyl and hydroxyl groups, making it easily modifiable.¹⁹ Sodium alginate (SA) can form hydrogels, membranes, and beads through crosslinking with agents like bivalent cations and acids, enhancing its dye removal efficiency.²⁰ Especially, the anionic carboxylate groups of alginate have a strong affinity towards cationic dyes which improves the adsorption of the latter. In addition, it has a higher ability to immobilize dye adsorbents in the form of hydrogel beads.²¹ These properties make alginate hydrogels ideal for encapsulating dye adsorbents. Hydrogels, with their 3D network and hydrophilic nature, offer high sorption capacity, durability,²² reusability,²³ and the ability to retain large amounts of water.²⁴

Graphene oxide (GO) is one of the important graphene derivatives widely utilized in environmental fields. It has become a leading adsorption material with many oxygen-containing functionalities, large specific surface area, and high mechanical strength.^25,26 These oxygen functionalities, such as carboxyl, hydroxyl, and epoxy, are readily functionalized to form efficient composites.²⁷ Hence, hydrogen bonds can interact with the hydroxyl groups of SA, which considerably enhances the mechanical strength and sorption efficiency of SA hydrogel beads to remove different pollutants.²⁸ Yang and coworkers developed 3D reduced GO/SA double network beads to address the problem of local water pollution caused by antimony.²⁹ Ma et al. immobilized Ag₂O in SA-reduced graphene oxide (RGO) aerogel beads and explored it for degradation of cationic Rhodamine B and anionic Orange II dyes.³⁰ Clay minerals have attracted extensive attention due to their remarkable properties, such as low cost, large abundance, high surface area, and excellent sorption capacity.³¹ The anionic nature of clay minerals enables removal of cationic dyes from wastewater. Bentonite is a cheap and naturally occurring clay mineral composed of interlinked layered structure of tetrahedral Si and octahedral Al and contains large number of reactive hydroxyl groups.³² Oussalah et al. encapsulated natural bentonite clay in calcium alginate (Alg) to form alginate/bentonite adsorbent beads (Alg/Ben) with different ratios of Alg/Ben during the preparation of beads. The as-prepared Alg/Ben has been applied for the adsorption of methylene blue (MB) dye.³³ Belhouchat and coworkers synthesized activated organo-bentonite (AOBent)/SA composite beads and utilized them for the adsorption of MB and methyl orange.³⁴

Synthetic dyes like MB pose serious environmental and health risks due to their toxicity and persistence. Traditional water treatments often fail at low concentrations, making adsorption a preferred, efficient alternative. The presence of synthetic dyes, such as MB, in wastewater poses a significant environmental threat due to their toxicity, persistence, and resistance to conventional treatment methods. The present study addresses this challenge by introducing a green and efficient adsorption-based solution using an eco-friendly composite material. By employing choline hydroxide as a green reducing agent and catalyst for the synthesis of RGO, the work contributes to the advancement of sustainable materials for environmental remediation. The research focuses on optimizing key parameters that influence adsorption, such as pH, adsorbent dosage, contact time, and dye concentration, while also characterizing the adsorption kinetics and isotherms to gain a deeper understanding of the mechanism and scalability of the process. The findings demonstrate a green synthesis approach, composite bead design, selective adsorption, and strong potential for scale up, making it a significant step toward practical and environmentally friendly wastewater treatment technologies. The innovative encapsulation of RGO and bentonite in SA beads enhances the mechanical stability, ease of handling, and reusability of the adsorbent, making it more suitable for real-world applications.

Materials and methods

Graphite powder, potassium permanganate, hydrochloric acid, sulphuric acid, sodium nitrate, hydrogen peroxide (30%), choline chloride and potassium hydroxide were purchased from Sigma Aldrich. Bentonite clay was purchased from Oxford Chemicals.

Experimental section

The complete multistep synthesis of SA/RGO/BENT hydrogel is systematically given in Fig. S1 (ESI†).

Preparation of GO

GO was prepared using a modified Hummers’ method.³⁵ The Experimental section is divided into three parts: (i) preparation of the ionic liquid (IL); (ii) reduction of GO using the IL; and (iii) immobilization of the as-prepared RGO and bentonite clay with SA to form hydrogel beads.

Preparation of Ch–OH

To prepare Ch–OH, 2 g of choline chloride and 0.8 g of KOH were weighed and placed in a 50 mL two-neck round-bottom flask. 20 mL methanol was added to this solution, and the mixture was heated at 60 °C overnight. After the reaction was completed, the reaction mixture was filtered to separate the solid KCl salt. The filtrate was vacuum-evaporated to remove excess methanol until the product reached a constant weight. The Ch–OH thus obtained was used to further reduce GO without purification.

Reduction of GO by Ch–OH

The second part includes the reduction of GO by Ch–OH; for this purpose, 100 mg of GO synthesized by the modified Hummers’ method was dispersed in 50 mL of deionized water by ultrasonication for 30 min. To this dispersion, Ch–OH was added, and the solution was refluxed at 90 °C overnight. The residue was separated by vacuum filtration through 0.47 μm. The product was washed several times with deionized water and dried at 60 °C for 12 h.

Preparation of SA/RGO/BENT hydrogel beads

The third part involves the preparation of hydrogel beads from SA. To prepare hydrogel beads, 50 mg of RGO was dispersed in 10 mL of deionized water by ultrasonication. In a separate beaker, bentonite clay was washed several times thoroughly using deionized water by ultrasonication to remove any impurities. 50 mg of purified bentonite clay was mixed with deionized water. Separately, 2 g of SA was weighed and dissolved in 30 mL of deionized water by continuous stirring at room temperature to obtain a homogeneous gel. To this gel, dispersed RGO and bentonite dispersion were immobilized by constant stirring. The hydrogel beads were prepared by dropping the gel solution through a syringe into 10% CaCl₂ solution. The beads were kept in the CaCl₂ solution for 2 h for crosslinking, after which they were washed with deionized water several times to remove excess CaCl₂. The obtained SA/RGO/BENT hydrogel beads were stored in water for further sorption of MB experiment.

Characterization techniques

The as-prepared composite was characterized using a Brucker ALPHA II FTIR instrument in the range of 500–4000 cm⁻¹ for functional group detection. The thermal stability was analysed using a Hitachi STA7300. The as-prepared adsorbent was characterized by X-ray photoelectronic spectroscopy. The thermogram was recorded from room temperature to 800 °C at 20 °C min⁻¹. The morphological analysis was done using a scanning electron microscope. The adsorption experiments were carried out in an orbital shaker (REMI RSB-12). The adsorption efficiency was checked and calculated using a UV spectrophotometer (JASCO V-750).

Results and discussion

The functional group detection of as-prepared samples was carried out using FTIR analysis (Fig. 1a and Fig. S4, ESI†). In the FTIR spectrum of bentonite (Fig. S4, ESI†), the peaks observed in the range 3000–3700 cm⁻¹ indicate the –OH and water vibrations, specifically the bands at 3695 and 3623 cm⁻¹ being associated with –OH bond coordinated to Si–OH and Al–OH in tetrahedral and octahedral sheets. A band with high intensity at 1005 cm⁻¹, with a small one at 1111 cm⁻¹, has been observed indicating Si–O stretching vibrations confirming a silicate structure,³⁶ while a peak appeared at 786 cm⁻¹ indicating the Si–O–Si stretching vibrations. The FTIR spectrum of Ch–OH (Fig. S4, ESI†) reveals the presence of band at 3380 cm⁻¹ attributed to –OH stretching vibration. The C–H stretching vibration and methyl deformation account for the appearance of bands at 2995 and 1475 cm⁻¹, respectively. The stretching vibrations for C–N and C–O (in primary alcohol) are evident by the presence of bands at 1362 and 1081 cm⁻¹, respectively. While bands appearing at around 954 and 863 cm⁻¹ correspond to –CH₂ deformation.³⁷ The conversion of natural graphite into GO has been examined using its FTIR spectra. The –OH stretching vibrations have been proven by a broad band at 3390 cm⁻¹. The presence of carbonyl and aromatic CC was confirmed by the appearance of sharp peaks at 1725 and 1620 cm⁻¹, respectively. The bands at 1412, 1225, and 1050 cm⁻¹ were assigned to C–H bending, epoxy group, and C–O stretching vibration, respectively (Fig. S4, ESI†).³⁸ The FTIR spectrum of RGO shows a peak that appeared at 3360 cm⁻¹ which may be attributed to –OH functionality in choline hydroxide, while the peaks at 2919 and 1463 cm⁻¹ are attributed to C–H stretching vibrations and deformation of a methyl group, respectively, from Ch–OH. The band at 1342 cm⁻¹ is evidence for the presence of a C–N bond, which confirms the grafting of Ch–OH on the surface of GO, resulting in the formation of RGO. New peaks at 1554 and 1140 cm⁻¹ correspond to CC stretching of the aromatic ring and C–O stretching, respectively, in RGO.³⁸ The peaks appearing at 947 and 867 cm⁻¹ correspond to –CH₂ deformation from Ch–OH. The peak at around 1725 cm⁻¹ has completely disappeared in the FTIR spectra of RGO and RGO/BENT due to substantial GO reduction into RGO by Ch–OH (Fig. 1a). In the case of RGO/BENT, peaks appeared at 3692 and 3621 cm⁻¹, attributed to Si–OH and Al–OH in tetrahedral and octahedral sheets.³⁹ The peaks at 1340 and 1472 cm⁻¹ are from Ch–OH attributed to C–N and deformation of methyl group. The peak at 1630 cm⁻¹ gives information about CC bonds in aromatic rings of RGO. The bands appearing at 1006, 916, 685, and 521 cm⁻¹ are attributed to Si–O for silicate structure, Al–Al–OH, bending of Si–OH and Al–O–Si, respectively (Fig. 1a), from bentonite.⁴⁰ This discussion reveals the reduction of GO into RGO by Ch–OH and grafting of Ch–OH and bentonite on the surface of RGO. The functional group changes before and after the dye adsorption process have been analysed with the help of FTIR analysis (Fig. S3, ESI†).

The thermal stability of GO, RGO, and the RGO/BENT composite was investigated using TGA. The thermogram of GO revealed a two-step weight-loss pattern. The initial step occurred within the temperature range of 100–190 °C, attributable to the evaporation of moisture and CO₂ content within GO. Subsequently, a second weight-loss phase commenced at 200 °C, continuing until reaching 700 °C, indicative of the decomposition of the carbon skeleton. No additional weight loss was noted beyond 700 °C. The cumulative weight loss for GO amounted to 70–75%. In the case of the thermogram of RGO, a two-step weight loss was observed. The first step was between room temperature and 145 °C, which is commonly attributed to the removal of trapped water molecules from the RGO surface. However, the second step is the major weight-loss step, which starts at 145 °C and ends at 400 °C, which is mainly attributable to the decomposition of the carbon skeleton³⁶ with almost 58.7% weight loss. In contrast, no further weight loss was seen up to 700 °C. The total weight loss for RGO was recorded up to 63.78%. RGO/BENT shows a three-step weight loss in the TGA; the first weight loss was observed between room temperature and 145 °C, which was attributed to moisture loss. Whereas for the second step, weight loss is attributed to volatile gases, which appeared in the range 145–225 °C with approximately 21% weight loss. The third step appeared between 225 and 400 °C, attributable to the degradation of the polymer matrix and dehydroxylation of silicate layers,⁴¹ and ended at 700 °C.

XPS analysis provides information about qualitative chemistry and bonding nature in the RGO/BENT composite. Fig. 2a reveals the full scan of the RGO/BENT composite, which shows the presence of C, O, N, Si and Al, whereas that of GO shows the presence of C and O elements. The high-resolution spectrum of C 1s demonstrated the appearance of a peak at around 284.8 eV for the C–C/CC bond. Peaks appeared at a binding energy of 285.7 and 288.5 eV, which were attributed to carbonyl functionality: CO/C–O and O–CO, respectively. In the high-resolution scan of O 1s, the appearance of peaks at 531.5, 532.9 and 533.7 eV reveals the presence of CO, C–O/Si–O, and Si–OH bonds, respectively.⁴¹ The deconvoluted peak of N 1s shows the presence of peaks at binding energies of 398.6 and 399.2 eV, which indicates the presence of the C–N and N–H bonds from choline hydroxide. The high-resolution spectrum of Si 2p consists of three distinct peaks centered at 102.3, 103.4 and 104.3 eV attributed to Si–O–C, Si–O–Si and Si–OH, respectively (Fig. S2a, ESI†). At the same time, the deconvoluted peak of Al 2p splits into three main peaks at binding energies of 74.5, 75.4, and 76.7 eV, representing the presence of Al–O–C, Al–O–Al, and Al–OH, respectively (Fig. S2b, ESI†).⁴² These findings reveal the covalent bonding of RGO with bentonite clay supported by the appearance of peaks corresponding to Si–O–C and Al–O–C.

The pore size distribution shows that the mean diameter of pores is 3.48 nm (Fig. S6, ESI†), i.e., between 2 and 50 nm,⁴³ which suggests the presence of mesopores in SA/RGO/BENT beads. The surface morphology and texture of SA/RGO/BENT hydrogel beads were examined using SEM analysis, and the corresponding images are shown in Fig. 3. The low-magnification SEM image of hydrogel beads shows a rough surface, while the high-magnification image displays a coarse and uneven surface (Fig. 3a and b). The cross-section of a hydrogel bead shows numerous pores (Fig. 3c). The presence of spherical walls of hydrogel beads proves the cross-linked three-dimensional network structure. The interconnected filamentous network inside the hydrogel beads could be linked with graphene sheets. This network structure provides abundant active sites for the adsorption of dye molecules, which results in enhanced adsorption. The EDX image (Fig. 3d) and the respective elemental mapping show the presence of C, O, N, Al, Si and Ca elements (Fig. 3e–j). The atomic percentages are shown in Table S1 (ESI†).

Dye adsorption study

To perform a dye adsorption experiment, a 1000 ppm stock solution was prepared by dissolving 1 g of MB dye in 1000 mL of distilled water. The desired concentration of dye (20, 40, 60, 80, 100, 120, 140 ppm) was made by diluting the appropriate volume from the stock solution. The adsorption process was carried out in an orbital bath shaker at a constant 150 rpm speed to ensure continuous stirring. For this purpose, 100 mL Erlenmeyer flasks were charged with 50 mL of different concentrations of dye solution and an optimized mass of adsorbent beads was added. The samples extracted every 2 min and subsequently analysed by UV spectroscopy to monitor the adsorption progress. The percentage adsorption and the quantity of MB adsorbed per unit mass of adsorbent, i.e., adsorption capacity (q_e) of the adsorbent, were calculated using eqn (1) and (2), respectively:

	(1)

	(2)

where C₀ and C_t are the initial and final concentrations of MB dye, while C_e is the concentration of dye at equilibrium, V is the volume of dye solution used during adsorption experiments, and m is the mass of adsorbent.

Effect of adsorbent dose

The effect of dosage of SA/RGO/BENT beads is shown in Fig. S7 (ESI†). The effect of adsorbent dose on dye adsorption is a crucial aspect to consider in adsorption processes. The adsorption experiment was carried out by varying the adsorbent dose at 2, 4 and 6 g (adsorbent was weighed in wet condition) in order to optimize the adsorbent dose. It was observed that the percent adsorption was increased from 74.56 to 91.96 and 97.92%, respectively, as the adsorption dose was increased from 2 to 6 g. There might be an initial rapid increase in dye adsorption at lower adsorbent doses due to the abundance of available active sites. This phase often saturates quickly as active sites become occupied. It was observed that the adsorption rate of MB for 6 g dose was increased by 5.96% compared to that for 4 g (Fig. 4a). This phenomenon can be attributed to the abundance of available sites during the adsorption process, resulting in a low ratio between the molecules of MB and the dosage of the adsorbent.⁴⁴ Hence the optimum adsorbent dose for adsorption of MB by SA/RGO/BENT was 6 g.

Effect of concentration of MB dye solution

To examine the influence of initial dye concentration on the adsorption process, the adsorption of dye was carried out using different concentrations of dye, namely 20, 40, 60, 80, 100, 120, and 140 ppm, with 6 g of adsorbent. The corresponding UV spectra are shown in Fig. S8a–g (ESI†). It was observed that at low initial dye concentrations, i.e., 20 to 80 ppm, the adsorption sites on the adsorbent surface are plentiful compared to the dye molecules, leading to a higher adsorption efficiency. The adsorption process completed within 28 min. However, as the initial dye concentration increases, the available adsorption sites become saturated, and the adsorption capacity reaches a plateau. Higher initial dye concentrations provide a greater driving force for mass transfer between the aqueous phase and the solid adsorbent. This can enhance the adsorption rate initially but may also lead to quicker saturation of the adsorption sites⁴¹ as observed in case of 100, 120 and 140 ppm, and the adsorption process took 28 min to complete. The detailed percent adsorption with respect to dye concentration is shown in Fig. 4b. The adsorption efficiency dropped slightly at 120 and 140 ppm; therefore, the entire adsorption process was carried out using a dye concentration of 100 ppm.

Effect of temperature

The adsorption experiments were carried out at 30, 40, 50 and 60 °C to examine the influence of temperature on the adsorption process (Fig. S9, ESI†). The percent adsorption estimated from obtained data for 30 °C was 97.92%, while it was up to 97.80, 97.57, and 98.71% for 40, 50 and 60 °C, respectively (Fig. 4c). These results revealed that there is no considerable change in the adsorption efficiencies with increasing temperature. Therefore, further adsorption experiments were performed at room temperature.

Effect of contact time

Contact time is a critical factor in the adsorption of dyes onto adsorbents, influencing both the adsorption capacity and the efficiency of the process. The contact time was optimized for the adsorption of MB by SA/RGO/BENT at optimized conditions (6 g adsorbent, 100 ppm dye concentration, and without adjusting the pH of the dye solution). It was observed that the adsorption percentage of MB increased rapidly up to 8 min, reached equilibrium at 18 min, and achieved a final adsorption of 95.23%. After 18 min, a slight yet considerable change in adsorption was observed. It was visualized that the process took 30 min to complete, but the UV spectroscopic data reveal that almost 97.92% of dye was adsorbed by SA/RGO/BENT in 28 min (Fig. 4d and Fig. S10, ESI†).

Effect of pH

The pH level is a crucial factor influencing dye uptake, primarily because it alters the ionization process of dye molecules and the configurations of functional groups on the adsorbent.⁴⁵ 6 g of SA/RGO/BENT was added to 50 mL of 100 ppm dye solution, the pH of solution was adjusted to 3, 5, 9 and 10 using 0.1 M HCl and 0.1 M NaOH. Samples were extracted every 2 min up to 28 min, and analysed by UV-spectrometry to monitor the adsorption progress (Fig. S11, ESI†). The adsorption of MB on SA/RGO/BENT remained relatively constant under aqueous conditions across different pH levels (Fig. 5a and b), indicating that pH has minimal impact on the adsorption of MB by SA/RGO/BENT. Therefore, all further adsorption experiments were performed at a nearly neutral pH without adjusting the pH of the dye solution.

Selective dye adsorption

The as-prepared SA/RGO/BENT adsorbent was explored for the adsorption of anionic dye. For this purpose, acid orange II (AO II) was chosen. For convenience, the AO II adsorption experiment was conducted under the same optimized conditions used for MB solution (6 g adsorbent, 100 ppm dye concentration, 50 mL dye volume, contact time of 28 min). Samples were taken out at 2 min intervals, which were then analyzed by a UV spectrophotometer. The results are shown in Fig. S9 (ESI†), which demonstrates that only 49% of AO II was adsorbed by SA/RGO/BENT beads (Fig. S17a, ESI†). The selective dye adsorption experiment was performed by using another cationic dye, Brilliant Green (BG), which shows an adsorption efficiency up to 96.28% (Fig. S17b, ESI†), suggesting that the beads enable the selective adsorption of cationic dyes only. The negatively charged surface of the adsorbent effectively adsorbed positively charged dye molecules, indicating that SA/RGO/BENT is selective towards cationic dyes only. Additionally, π–π stacking can occur between the aromatic structure of MB and the π network of RGO. In addition, electrostatic interaction, π–π stacking, and hydrogen bonding also enhance adsorption of cationic dye on the surface of SA/RGO/BENT hydrogel beads (Fig. S14 and S15, ESI†). Table S2 compares the adsorption capacity of the SA/RGO/BENT beads with existing materials. SA and other biopolymers are biodegradable substances with a large number of carboxylate and hydroxyl functional groups in their internal structure.^46,47 Especially, the negatively charged carboxylic ions improve cationic dye adsorption. SA has a greater effect in immobilizing the dye adsorbents.²¹ As a result, SA/RGO/BENT beads might be regarded as a novel option for eliminating cationic contaminants from wastewater due to their abundance of anionic groups (much like cationic dyes). Electrostatic forces between adsorbents and pollutants enable the adsorption of dyes on biopolymer-based adsorbents.⁴⁸ Bentonite has a layered structure composed of two silica sheets surrounding an alumina layer, which gives it a strong ability to adsorb substances. In bentonite clay, the isomorphous alteration of Al³⁺ for Si⁴⁺ in tetrahedral sheets and Mg²⁺ for Al³⁺ occurs, which gives the clay a stable negative charge on its surface. This charge is balanced by positively charged ions like sodium, hydrogen, or calcium within the clay layers. When the clay gets wet, its layers expand, and these ions also absorb water, making the clay surface hydrophilic. These exchangeable positive ions can be replaced by cationic pollutants (such as organic dye molecules) through cationic exchange, allowing bentonite to effectively adsorb positively charged contaminants.^49–51 As clay minerals are abundant in nature and exhibit permanent negative charges and exchangeable cations, they have been employed extensively as inexpensive, sustainable, and efficient adsorbents for the removal of MB from aqueous solution.^52,53

MB adsorption with control samples

To compare the adsorption efficiency of SA/RGO/BENT hydrogel beads, the same adsorption experiment was carried out with pure SA beads, SA/RGO beads, and SA/BENT beads. The results were obtained using a UV-visible spectrophotometer; the adsorption efficiency was calculated (Fig. S18a–d, ESI†). Pure SA beads have poor adsorption efficiency up to 18.35%, while the addition of RGO and bentonite to the SA matrix enhances the adsorption. The adsorption achieved by SA/RGO and SA/BENT hydrogel beads was 91.8% and 83.96% respectively. Hence, combining RGO and BENT together with the SA polymer matrix considerably improves the adsorption efficiency towards MB, which was 97.92%

Kinetic study

The time-dependent experimental results were examined using two different kinetic models—the pseudo-first-order kinetic model and the pseudo-second-order kinetic model—to examine the adsorbent's uptake rate and adsorption efficiency, which are expressed by eqn (3) and (4), respectively:^54,55

ln(qe − qt) = lnqe − k1t	(3)

	(4)

In these equations, q_t and q_e are the dye adsorbed at time t and at equilibrium (mg g⁻¹), t is the time (min), and k₁ and k₂ are the pseudo-first-order rate constant (min⁻¹) and pseudo-second-order rate constant (g mg⁻¹ min⁻¹) respectively. The kinetic parameters for pseudo-first- and second-order kinetics were obtained by fitting the curves plotted of t vs. ln(q_e − q_t) and t vs. t/q_t, respectively (Fig. 6a, b and Table 1). Comparing the pseudo-first-order and pseudo-second-order kinetics models revealed that the pseudo-first-order model better fits the adsorption kinetics. This conclusion is based on the correlation coefficient (R²) values obtained from the pseudo-first-order model, which are closer to 1 and correlate to the adsorption capacity (Table 1).

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Adsorption isotherm

Two significant variables influencing the adsorption performance are temperature and the initial concentration of the MB solution. The adsorption data showed that increasing the temperature does not promote MB adsorption on SA/RGO/BENT beads. Furthermore, a significant portion of the adsorption sites remain unoccupied when equilibrium is reached at low concentrations. As the initial concentration increases, the unoccupied adsorption sites are gradually filled, which results in higher adsorption capacity and utilisation rate. It will be challenging to improve the adsorption capacity after the adsorption reaches saturation. The possible interaction between MB dye molecules and SA/RGO/BENT beads is shown in Fig. S8 (ESI†).

Langmuir and Freundlich isotherm models have been widely employed to study the adsorption mechanism. When the adsorption process reaches equilibrium at a particular temperature, the relation between the adsorbent and dye concentration can be illustrated by using an adsorption isotherm. In this study, the Langmuir and Freundlich isotherm models have been extensively evaluated to correlate the adsorbent and initial concentration of dye. The Langmuir isotherm model describes a monolayer adsorption process on a uniform solid–liquid interface, where each adsorption site has an equal attraction power with zero interaction among adsorbate molecules.⁵⁶ In contrast, the Freundlich isotherm model explains multilayer adsorption occurring on a heterogeneous interface.^57,58

The Langmuir and Freundlich isotherm models can be represented by the following equations:

	(5)

	(6)

where C_e (mg L⁻¹) is the equilibrium concentration, Q_e (mg g⁻¹) is the adsorption capacity at equilibrium, Q_m (mg g⁻¹) is the maximum adsorption capacity, K_L is the Langmuir constant of the isotherm model, K_F is the Freundlich isotherm constant, and is the adsorption intensity. The Langmuir and Freundlich adsorption isotherms are shown in Fig. 7a and b. The relevant parameters are shown in Table 2. The linear fit for Langmuir and Freundlich isotherms was obtained by plotting C_e against and logC_e vs. logQ_e, respectively. The Langmuir adsorption model offers a better description than the Freundlich isotherm, although the Freundlich isotherm also shows good agreement with the MB adsorption data. The findings suggest that there may be a simultaneous involvement of multilayer adsorption on heterogeneous surfaces and monolayer adsorption on homogeneous surfaces in the adsorption process.

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Intraparticle diffusion model

To elucidate the adsorption mechanism and identify the rate-controlling steps of the adsorption process, the diffusion model was employed to analyse the adsorption of MB. This approach aimed to recognize the intraparticle diffusion mechanism. Using the kinetic model established by Weber and Morris, the study explored the potential impact of diffusion resistance within the particles on the adsorption process.⁵⁸ The intraparticle diffusion model can be expressed using the following equation:

Qt = kwt1/2 + C	(7)

In the above equation, k_w (mg g⁻¹ min^−1/2) expresses the intraparticle diffusion rate, which is called as Weber-Morris rate constant. C (mg g⁻¹) is the intercept, which is related to the thickness of the outer surface of the adsorbent. While the other parameters are similar to those of the pseudo-first- and second-order kinetics. The three stages of the adsorption process are shown in Fig. 8. The plot of Q_t vs. t^1/2 shows that the line does not pass through the origin which means that the intraparticle diffusion is not the only rate-determining step. In the initial stage, rapid adsorption was observed on the outer surface or macropores of beads with higher slope and the rate-determining step is external diffusion. Slow adsorption was observed in the second stage in which the adsorbate interacts with inner surface and pores of the adsorbent attributable to intraparticle diffusion. Equilibrium was attained in the third stage, during which the adsorption capacity increased slowly a lower slope. It has been observed that the intraparticle diffusion rate constant k_w1 is higher than k_w2, while C₂ is higher than C₁. This result indicates that greater adsorption takes place in the first stage than in the third stage; whereas, the intercept C > 0 means that both external and internal diffusion control the adsorption process.

Adsorption study by gas-sparged column

A gas-sparged column was designed as per the requirements (Fig. S12, ESI†). The top surface of the column has an inlet to fill SA/RGO/BENT beads, while there is an inlet at the bottom of column to sparge the gas to ensure the interaction of beads with the dye molecules as the gas enables the beads to continue bubbling in the column. There is an inlet and outlet for dye and residual water, respectively. For this process, 100 ppm MB dye solution was pumped into the column by a submersible pump. The treated water was collected from the outlet (Video V1, ESI†). For better understanding, samples were taken after 10, 15 and 20 min to examine the adsorption pattern, which shows that the dye was effectively adsorbed up to 97.07, 98.25 and 98.70%, respectively (Fig. S13, ESI†).

Reusability

The reusability of the adsorbent determines its feasibility for large-scale practical applications. In addition to reducing adsorption costs, reusability also minimises the risk of the adsorbent being released into the environment. SA/RGO/BENT beads can be quickly separated from dye solution by simple filtration after completion of the adsorption process. The separated beads were immersed in methanol for desorption. It was noticed that the adsorption efficiency was unaltered up to three cycles of adsorption–desorption with adsorption rates of 97.64, 97.03, and 96.47%, respectively. After three cycles, the adsorption rate was considerably reduced for the fourth and fifth cycles, which was 84.51 and 81.27%, respectively (Fig. S16, ESI†). In summary, regeneration and recycling of SA/RGO/BENT beads enable transforming laboratory-scale adsorption studies into economically viable and environmentally friendly industrial-scale wastewater treatment solutions.

Conclusion

The study concludes that sodium alginate/reduced graphene oxide/bentonite (SA/RGO/BENT) hydrogel beads can be effectively synthesized and used as a novel adsorbent for removing MB dye from wastewater. An environmentally friendly method for reduction of graphene oxide is to employ choline hydroxide (Ch–OH), and adding bentonite will increase its adsorption capability. Ch–OH enables the surface of the adsorbent to be negatively charged, which leads to the selective adsorption of cationic dyes. According to the experimental results, these composite beads have remarkable dye removal effectiveness; under ideal circumstances, they can remove up to 97.92% of MB. The adsorption mechanism is consistent with the Langmuir isotherm model and pseudo-second-order kinetics, suggesting monolayer adsorption on a homogeneous surface. The study also highlights the potential for scaling up the adsorption process in a gas-sparged column, making these beads promising candidates for real-world wastewater treatment applications. This research provides valuable insights into the development of sustainable and efficient adsorbents for environmental remediation.

Author contributions

DST-FIST 2018 (SR/FST/ETI/2018/156(C)), NTTM (1/1/2024-NTTM/8thMSG/4), SERB (CRG/2023/000527(C)) and CHT. K. D. L. and S. S. wrote the manuscript, K. D. L. prepared SA/RGO/BENT hydrogel beads, A. K., K. B. and M. A. B. helped with the adsorption study, K. D. L. analyzed all data, S. S supervised all work.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data are available from the authors upon request.

Acknowledgements

The authors are grateful to DST-FIST 2018 [grant number (SR/FST/ETI/2018/156(C))], NTTM (1/1/2024-NTTM/8thMSG/4), SERB (CRG/2023/000527(C)) and CHT as this work was supported by DST-FIST, NTTM, SERB and CHT funding.

References

1. K. T. Wong, N. C. Eu, S. Ibrahim, H. Kim, Y. Yoon and M. Jang, Recyclable magnetite loaded palm shell-waste based activated carbon for the effective removal of methylene blue from aqueous solution, J. Clean. Prod., 2016, 115, 337–342 CrossRef CAS .
2. Q. Wang and Z. Yang, Industrial water pollution, water environment treatment, and health risks in China, Environ. Pollut., 2016, 218, 358–365 CrossRef CAS PubMed .
3. S. K. Mani, R. Bhandari and A. Nehra, Self-assembled cylindrical Zr (IV), Fe (III) and Cu (II) impregnated polyvinyl alcohol-based hydrogel beads for real-time application in fluoride removal, Colloids Surf. A, 2021, 610, 125751 CrossRef CAS .
4. V. Pande, S. C. Pandey, T. Joshi, D. Sati, S. Gangola, S. Kumar and M. Samant, Biodegradation of toxic dyes: a comparative study of enzyme action in a microbial system, Smart Biorem. Technol., 2019, 255–287 CAS .
5. S. C. Pandey, A. Pandey, T. Joshi, V. Pande, D. Sati and M. Samant, Microbiological Monitoring in the Biodegradation of Food Waste, Global Initiatives for Waste Reduction and Cutting Food Loss, 2019, pp. 116–140 Search PubMed .
6. O. J. Hao, H. Kim and P.-C. Chiang, Decolorization of wastewater, Crit. Rev. Environ. Sci. Technol., 2000, 30, 449–505 CrossRef CAS .
7. V. Pande, S. Pandey, T. Joshi, D. Sati, S. Gangola, S. Kumar and M. Samant, Biodegradation of Toxic Dyes: A Comparative Study of Enzyme Action in a Microbial System, Smart Bioremediation Technologies Microbial Enzymes, 2019, pp. 255–287 Search PubMed .
8. R. L. Levin, M. A. Degrange, G. F. Bruno, C. D. Del Mazo, D. J. Taborda, J. J. Griotti and F. J. Boullon, Methylene blue reduces mortality and morbidity in vasoplegic patients after cardiac surgery, Ann. Thorac. Surg., 2004, 77, 496–499 CrossRef PubMed .
9. C. F. Mok, et al., Adsorption of dyes using poly (vinyl alcohol)(PVA) and PVA-based polymer composite adsorbents: a review, J. Polym. Environ., 2020, 28(3), 775–793 CrossRef CAS .
10. S. Venkatesh, K. Venkatesh and A. R. Quaff, Dye decomposition by combined ozonation and anaerobic treatment: cost effective technology, J. Appl. Res. Technol., 2017, 15(4), 340–345 CrossRef .
11. A. Verma, S. Thakur, G. Mamba, Prateek, R. K. Gupta, P. Thakur and V. Kumar Thakur, Graphite modified sodium alginate hydrogel composite for efficient removal of malachite green dye, Int. J. Biol. Macromol., 2020, 148, 1130–1139 CrossRef CAS PubMed .
12. Y. Anjaneyulu, N. Sreedhara Chary and S. S. Raj, Decolourization of industrial effluents—Available methods and emerging technologies—A review, Rev. Environ. Sci. Biotechnol., 2005, 4, 245 CrossRef CAS .
13. A. Oussalah and A. Boukerroui, Alginate-bentonite beads for efficient adsorption of methylene blue dye, Eur. Mediterr. J. Environ. Integrate, 2020, 5(2), 1–10 Search PubMed .
14. S. Dutta, et al., Recent advances on the removal of dyes from wastewater using various adsorbents: a critical review, Mater. Adv., 2021, 2, 4497–4531 RSC .
15. A. Allangawi, M. A. Aziz Aljar, K. Ayub and A. Abd El-Fattah, Removal of methylene blue by using sodium alginate-based hydrogel; validation of experimental findings via DFT calculations, J. Mol. Graph. Model, 2023, 122, 108468 CrossRef CAS .
16. A. Ahmad, et al., Recent advances in new generation dye removal technologies: novel search for approaches to reprocess wastewater, RSC Adv., 2015, 5(39), 30801–30818 RSC .
17. M. S. Rostami and M. M. Khodaei, Chitosan-based composite films to remove cationic and anionic dyes simultaneously from aqueous solutions: Modeling and optimization using RSM, Int. J. Biol. Macromol., 2023, 235, 123723 CrossRef CAS PubMed .
18. R. S. Dassanayake, S. Acharya and N. Abidi, Recent advances in biopolymer-based dye removal technologies, Molecules, 2021, 26, 4697 CrossRef CAS PubMed .
19. X. Hou, S. Tang, X. Guo, L. Wang, X. Liu, X. Lu and Y. Guo, Preparation and application of guanidyl-functionalized graphene oxide-grafted silica for efficient extraction of acidic herbicides by BoxBehnken design, J. Chromatogr. A, 2018, 1571, 65–75 CrossRef CAS PubMed .
20. Y. Wu, H. Qi, C. Shi, R. Ma, S. Liu and Z. Huang, Preparation and adsorption behaviors of sodium alginate-based adsorbent-immobilized β-cyclodextrin and graphene oxide, RSC Adv., 2017, 7, 31549–31557 RSC .
21. Z. Zhang, P. Tran, S. Rumi, N. Bergfeld, T. W. Reid, N. Abidi and N. Abidi, Alginate/organo-selenium composite hydrogel beads: Dye adsorption and bacterial deactivation, Int. J. Biol. Macromol., 2024, 280, 135908 CrossRef CAS PubMed .
22. S. Thakur, B. Sharma, A. Verma, J. Chaudhary, S. Tamulevicius and V. K. Thakur, Recent approaches in guar gum hydrogel synthesis for water purification, Int. J. Polym. Anal. Char., 2018, 23, 621–632 CrossRef CAS .
23. N. Pandey, S. K. Shukla and N. B. Singh, Water purification by polymer nanocomposites: an overview, Nanocomposites, 2017, 3, 47–66 CrossRef CAS .
24. M. J. Zohuriaan-Mehr and K. Kabiri, Superabsorbent polymer materials: a review, Iran. Polym. J., 2008, 17, 451 CAS .
25. Y. Zhu, M. D. Stoller, W. Cai, A. Velamakanni, R. D. Piner, D. Chen and R. S. Ruoff, Exfoliation of Graphite Oxide in Propylene Carbonate and Thermal Reduction of Resulting Graphene Oxide Platelets, ACS Nano, 2010, 4, 1227–1233 CrossRef CAS PubMed .
26. J. Fan, Z. Shi, M. Lian, H. Li and J. Yin, Mechanically strong graphene oxide/sodium alginate/polyacrylamide nanocomposite hydrogel with improved dye adsorption capacity, J. Mater. Chem. A, 2013, 1, 7433–7443 RSC .
27. L. Shen, W. Zhao, K. Wang and J. Xu, GO-Ti3C2 two-dimensional heterojunction nanomaterial for anticorrosion enhancement of epoxy zinc-rich coatings, J. Hazard. Mater., 2021, 417, 126048 CrossRef CAS PubMed .
28. M. Yadav, K. Y. Rhee and S. J. Park, Synthesis and characterization of graphene oxide/carboxymethylcellulose/alginate composite blend films, Carbohydr. Polym., 2014, 110, 18–25 CrossRef CAS PubMed .
29. X. Yang, T. Zhou, R. Deng, Z. Zhu, A. Saleem and Y. Zhang, Removal of Sb(III) by 3D-reduced graphene oxide/sodium alginate double-network composites from an aqueous batch and fixed-bed system, Sci. Rep., 2021, 11, 22374 CrossRef CAS PubMed .
30. Y. Ma, J. Wang, S. Xu, S. Feng and J. Wang, Ag₂O/sodium alginate-reduced graphene oxide aerogel beads for efficient visible light driven photocatalysis, Appl. Surf. Sci., 2018, 430, 155–164 CrossRef CAS .
31. G. Palani, A. Arputhalatha, K. Kannan, S. Lakkaboyana, M. Hanafiah, V. Kumar and R. Marella, Current Trends in the Application of Nanomaterials for the Removal of Pollutants from Industrial Wastewater Treatment—A Review, Molecules, 2021, 26, 2799 CrossRef CAS PubMed .
32. J. V. Fernandes, A. M. Rodrigues, R. R. Menezes and G. D. A. Neves, Adsorption of Anionic Dye on the Acid-Functionalized Bentonite, Materials, 2020, 13, 3600 CrossRef CAS PubMed .
33. A. Oussalah and A. Boukerroui, Alginate-bentonite beads for efficient adsorption of methylene blue dye, Eur. – Mediterr. J. Environ. Integr., 2020, 5, 31 CrossRef .
34. N. Belhouchat and H. Zaghouane-Boudiaf, César Viseras, Removal of anionic and cationic dyes from aqueous solution with activated organo-bentonite/sodium alginate encapsulated beads, Appl. Clay Sci., 2017, 135, 9–15 CrossRef CAS .
35. W. S. Hummers and R. E. Oferman, Preparation of graphite oxide, J. Am. Chem. Soc., 1958, 80, 1339 CrossRef CAS .
36. D. Gandhia, R. Bandyopadhyay and B. Soni, Naturally occurring bentonite clay: Structural augmentation, characterization and application as catalyst, Materials Today: Proceedings, 2022, 57, 194–201 Search PubMed .
37. S. N. Vajekar and G. S. Shankarling, Highly efficient green synthesis of the photochromic spironaphthoxazines using an ecofriendly choline hydroxide catalyst, Synth. Commun., 2019, 50, 338–347 CrossRef .
38. P. H. Wadekar, D. J. Ahirrao, R. V. Khose, D. A. Pethsangave, N. Jha and S. Some, Synthesis of Aqueous Dispersible Reduced Graphene Oxide by the Reduction of Graphene Oxide in Presence of Carbonic Acid, ChemistrySelect, 2018, 3, 5630–5638 CrossRef CAS .
39. D. Gandhi, R. Bandyopadhyay and B. Soni, Naturally occurring bentonite clay: Structural augmentation, characterization and application as catalyst, Mater. Today: Proc., 2022, 57, 194–201 CAS .
40. S. Wang, Y. Dong, M. He, L. Chen and X. Yu, Characterization of GMZ bentonite and its application in the adsorption of Pb(II) from aqueous solutions, Appl. Clay Sci., 2009, 43, 164–171 CrossRef CAS .
41. P. S. Dhumal, K. D. Lokhande, M. P. Bondarde, M. A. Bhakare and S. Some, Heat resistive, binder-free 3d-dough composite as a highly potent flame-retardant, J. Appl. Polym. Sci., 2022, 139, e52146 CrossRef .
42. P. D. Narayanan, A. Gopalakrishnan, Z. Y. S. Sugunan and B. N. Narayanan, facile synthesis of clay – graphene oxide nanocomposite catalysts for solvent free multicomponent Biginelli reaction Arab, J. Chem., 2020, 13, 318–334 Search PubMed .
43. A. Zsirka, E. Horvath, Z. Jarvas, A. Dallos, E. Mako and J. Kristof, Structural and energetical characterization of exfoliated kaolinite surfaces, Appl. Clay Sci., 2016, 124, 54–61 CrossRef .
44. X. Liu, B. Cui, S. Liu and Q. Ma, Methylene Blue Removal by Graphene Oxide/Alginate Gel Beads, Fibers Polym., 2019, 20, 1666–1672 CrossRef CAS .
45. E. Rápó and S. Tonk, Factors Affecting Synthetic Dye Adsorption; Desorption Studies: A Review of Results from the Last Five Years (2017–2021), Molecules, 2021, 26, 5419 CrossRef PubMed .
46. Saruchi, B. S. Kaith, R. Jindal and V. Kumar, Biodegradation of Gum tragacanth acrylic acid based hydrogel and its impact on soil fertility, Polym. Degrad. Stab., 2015, 115, 24–31 CrossRef CAS .
47. Saruchi, B. S. Kaith, R. Jindal, V. Kumar and M. S. Bhatti, Optimal response surface design of Gum tragacanth-based poly [(acrylic acid)-co-acrylamide] IPN hydrogel for the controlled release of the antihypertensive drug losartan potassium, RSC Adv., 2014, 4, 39822–39829 RSC .
48. H. Mittal, R. Jindal, B. S. Kaith, A. Maity and S. S. Ray, Flocculation and adsorption properties of biodegradable gum-ghatti-grafted poly(acrylamide-co-methacrylic acid) hydrogels, Carbohydr. Polym., 2015, 115, 617–628 CrossRef CAS PubMed .
49. Q. H. Hu, S. Z. Qiao, F. Haghseresht, M. A. Wilson and G. Q. Lu, Adsorption study for removal of basic red dye using bentonite, Ind. Eng. Chem. Res., 2006, 45, 733–738 CrossRef CAS .
50. J. Ma, J. Qi, C. Yao, B. Cui, T. Zhang and D. Li, A novel bentonite-based adsorbent for anionic pollutant removal from water, Chem. Eng. J., 2012, 200–202, 97–103 CrossRef CAS .
51. A. S. Özcan, B. Erdem and A. Özcan, Adsorption of acid blue 193 from aqueous solutions onto BTMA-Bentonite, Colloids Surf., A, 2005, 266, 73–81 CrossRef .
52. M. Pentrák, A. Czímerová, J. Madejová and P. Komadel, Changes in layer charge of clay minerals upon acid treatment as obtained from their interactions with methylene blue, Appl. Clay Sci., 2012, 55, 100–107 CrossRef .
53. Y. El Mouzdahir, A. Elmchaouri, R. Mahboub, A. Gil and S. A. Korili, Equilibrium modeling for the adsorption of methylene blue from aqueous solutions on activated clay minerals, Desalination, 2010, 250, 335–338 CrossRef CAS .
54. H. Liu, B. Wang, H. Liu, Y. Zheng, M. Li, K. Tang, B. Pan, C. Liu, J. Luo and X. Pang, Multi-crosslinked robust alginate/polyethyleneimine modified graphene aerogel for efficient organic dye removal, Colloids Surf., A, 2024, 683, 133034 CrossRef CAS .
55. C. Liu, H. Liu, Y. Zheng, J. Luo, C. Lu, Y. He, X. Pang and R. Layek, Schiff base crosslinked graphene/oxidized nanofibrillated cellulose/chitosan foam: An efficient strategy for selective removal of anionic dyes, Int. J. Biol. Macromol., 2023, 252, 126448 CrossRef CAS PubMed .
56. Y. Liu, Y. Kang, B. Mu and A. Wang, Attapulgite/bentonite interactions for methylene blue adsorption characteristics from aqueous solution, Chem. Eng. J., 2014, 237, 403–410 CrossRef CAS .
57. Y. Yao, F. Xu, M. Chen, Z. Xu and Z. Zhu, Adsorption behaviour of methylene blue on carbon nanotubes, Bioresour. Technol., 2010, 101, 3040–3046 CrossRef CAS PubMed .
58. H. Kim, S. Kang, S. Park and H. S. Park, Adsorption isotherms and kinetics of cationic and anionic dyes on three-dimensional reduced graphene oxide macrostructure, J. Ind. Eng. Chem., 2015, 21, 1191–1196 CrossRef CAS .

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nj01310k

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