Structure and Performance of Bentonite-Enhanced Superabsorbent Gels for Water Absorption and Methylene Blue Adsorption

Abstract

To address the limitations of conventional superabsorbent polymers in complex aqueous environments, a novel ternary composite gel (BT-SAP) based on xanthan gum, poly(acrylic acid-co-acrylamide), and bentonite was synthesized via a facile one-pot polymerization. Characterization confirmed the formation of a stable organic–inorganic hybrid three-dimensional network. The gel demonstrated outstanding comprehensive performance: a maximum water absorption capacity of 378.6 g/g; good adaptability to various pH levels, salt ions, and real water bodies; and rapid absorption kinetics and reusable potential over multiple cycles. Simultaneously, it exhibited a high adsorption capacity of 181.3 mg/g for methylene blue. The adsorption isotherm followed the Freundlich model, indicating adsorption on a heterogeneous surface. Kinetic studies revealed that the process was best described by the pseudo-second-order model, suggesting chemisorption as the rate-controlling step. XPS analysis further elucidated that the adsorption primarily occurred through the synergistic effect of electrostatic attraction from carboxyl groups and hydrogen bonding from amide/hydroxyl groups within the gel. This work provides a new strategy for developing smart materials integrating efficient water absorption and dye removal functionalities.

1. Introduction

Global water scarcity and industrial wastewater pollution have driven the development of multifunctional polymeric materials capable of efficient water absorption, retention, and pollutant adsorption [1]. Superabsorbent polymers can absorb hundreds to thousands of times their weight in water. They are widely used in agricultural moisture retention, hygiene products, and ecological restoration [2]. However, conventional single-component superabsorbent polymers, such as polyacrylic-based gels, exhibit significant limitations. These include poor salt tolerance (owing to the cation-induced “charge screening effect” that suppresses anionic repulsion and reduces osmotic swelling), low gel strength, rapid water loss under load or heat, and poor selectivity for target pollutants like toxic dyes [3]. These drawbacks restrict their application in complex environments, particularly in wastewater treatment.

Constructing organic–inorganic hybrid gels presents a promising strategy to overcome these challenges [4]. Among various inorganic fillers, bentonite, a natural montmorillonite-rich clay, has been extensively investigated as a reinforcing agent in superabsorbent composites due to its high aspect ratio, cation exchange capacity, and cost-effectiveness [5]. Previous studies have demonstrated that the incorporation of bentonite can significantly enhance the mechanical strength and thermal stability of the hydrogels [6,7]. Furthermore, the hydrophilic layered structure and surface functional groups of bentonite can provide additional sites for water interaction and pollutant adsorption [8]. It is also theorized that the ionic groups on clay surfaces can partially mitigate the charge screening effect in saline environments, contributing to improved salt tolerance. However, challenges remain. The homogeneous dispersion of bentonite within the polymer matrix is critical to avoid aggregation that can compromise performance [9]. Moreover, while improving gel strength, excessive clay content often leads to a trade-off by reducing the ultimate equilibrium swelling ratio [10]. Most reported bentonite-composite gels focus primarily on swelling capacity or mechanical properties. Research on integrated materials that simultaneously provide high swelling, ionic stability, and efficient dye adsorption for practical wastewater treatment remains scarce.

A key advancement in this work is the strategic selection of xanthan gum as the biopolymer component. Compared to other polysaccharides, xanthan gum offers a unique combination of a rigid cellulose backbone and charged trisaccharide side chains. This specific structure is hypothesized to form a more robust and interconnected network with bentonite platelets and the synthetic polymer, leading to superior mechanical stability and synergistic adsorption sites. Furthermore, its inherent biodegradability addresses a critical environmental limitation of persistent, purely synthetic hydrogels. Meanwhile, xanthan gum can form interpenetrating networks with synthetic polymers like poly(acrylic acid/acrylamide) [11]. This multi-component design aims to improve swelling kinetics, pore structure, water retention, and environmental responsiveness. Notably, we employed a simple, scalable “one-pot” method to integrate all components, which is crucial for potential industrial translation.

Methylene blue poses serious risks to aquatic ecosystems and human health [12]. Developing adsorbents that combine high water absorption with effective dye removal is crucial for integrated wastewater treatment and resource recovery. Although bentonite-composite superabsorbents and various dye adsorbents have been studied separately, materials that simultaneously offer high water management performance and targeted dye adsorption remain underexplored [13,14,15]. The structure–swelling–adsorption relationships in such multifunctional systems are not fully understood.

Herein, we successfully synthesized a ternary composite superabsorbent gel based on xanthan gum, poly(acrylic acid/acrylamide), and bentonite via a facile one-pot polymerization. This design utilizes the synergistic effects of the biopolymer network and the silicate layers to enhance mechanical properties, stability, and functionality. The incorporation of xanthan gum is specifically aimed at creating a more biodegradable and mechanically coherent network, while the bentonite is intended to improve salt resilience and provide additional adsorption sites. We systematically investigated the swelling behaviors of the composite, along with its adsorption performance toward methylene blue. This study provides new insights into the design of integrated water-management materials with potential environmental remediation functions.

2. Results and Discussion

2.1. Preparation Mechanism and Characterization Analysis

The formation mechanism of BT-SAP is illustrated in Figure 1a. Under heating, APS decomposes to generate sulfate anion radicals. These radicals initially abstract hydrogen atoms from the hydroxyl groups of XG, producing macroradicals on XG that serve as active sites for grafting. Subsequently, AA and AM undergo copolymerization initiated by these macroradicals and other radicals in the system [16]. Meanwhile, they are grafted onto the XG backbone via covalent bonds. During this process, NMBA reacts with the growing polymer chains, establishing chemical crosslinks and constructing a preliminary three-dimensional network. Simultaneously, the dispersed BT nanoplatelets are incorporated into the network through hydrogen bonding and electrostatic interactions between their surface silanol groups and the polymer chains. These platelets act as multifunctional physical crosslinking points and rigid reinforcing phases. Ultimately, a multi-level organic–inorganic hybrid three-dimensional network is formed.

As shown in the FT-IR spectra [Figure 1b], the BT-SAP exhibits key absorption bands similar to those of the pure polymer: 3200–3300 cm⁻¹ (O–H/N–H stretching vibrations), around 2931 cm⁻¹ (C–H stretching), around 1660 cm⁻¹ (amide I, C=O stretching), around 1552 cm⁻¹ and 1400 cm⁻¹ (asymmetric and symmetric stretching vibrations of carboxylate anions), and around 1317 cm⁻¹ (amide III or skeleton vibrations) [17]. This confirms the successful synthesis of the XG-g-p(AA-co-AM) and indicates that the introduction of BT does not alter the fundamental chemical structure. A distinct new peak at 1041 cm⁻¹ in the BT-SAP, assigned to the Si–O–Si stretching vibration of the silicate framework, provides direct evidence for the successful incorporation of BT nanoplatelets into the polymer matrix. Slight shifts observed in the positions of major peaks (e.g., C=O and O–H/N–H regions) can be attributed to hydrogen-bond interactions between surface silanol groups of BT and functional groups (carboxyl and amide) on the polymer chains. Thus, the FT-IR results support the mechanism in which BT acts as a physical crosslinking site via secondary interactions.

The XRD patterns are presented in Figure 1c. The SAP (without BT) exhibits only a broad hump, characteristic of amorphous polymers, confirming the long-range disordered structure. In contrast, the BT-SAP displays distinct diffraction peaks at approximately 7° and 21°. The peak around 7° corresponds to the (001) crystal plane of montmorillonite (the main component of BT), directly confirming the presence of its layered structure in the composite [18]. The persistence of this characteristic peak, without a significant shift in its position, indicates that the layered crystalline framework of BT remains largely intact during the one-pot polymerization. The peak near 21° may be attributed to other montmorillonite crystal planes or weak ordering induced in the polymer chains under confinement. Thus, the XRD pattern of the gel is best described as a superposition of the amorphous polymer halo and the crystalline signature of BT, confirming the coexistence of both components in a solid-state hybrid. This result corroborates the FT-IR findings of interfacial interactions and supports the proposed mechanism of physical crosslinking and surface complexation within the composite.

As shown in Figure 1d, SEM analysis reveals the multiscale microstructure of the BT-SAP. At low magnification, the material exhibits a continuous, interconnected three-dimensional porous network. The irregular pore morphology and broad pore size distribution result from the freeze-drying of the swollen hydrogel, providing both storage space and rapid transport pathways for water. At higher magnification, the pore walls show visible folds and uniformly dispersed micrometer-sized bentonite particles, indicating effective distribution and interfacial bonding during polymerization. Further high-resolution imaging confirms that bentonite exists as partially exfoliated nano-aggregates. This nanoscale dispersion not only enhances the mechanical properties of the matrix through a “nano-reinforcement effect” but also strengthens interactions with water molecules via the hydrophilic groups on its surface. Together, this hierarchical structure contributes to the material’s water absorption capacity and cycling stability.

Figure 1e presents the nitrogen adsorption–desorption isotherm of BT-SAP, which exhibits characteristics of a type IV isotherm. At p/p₀ < 0.8, the increase in nitrogen adsorption capacity is gradual, corresponding to the initial adsorption process in mesoporous structures. As p/p₀ approaches 1.0, the adsorption capacity rises sharply, a phenomenon attributed to capillary condensation of nitrogen in macropores or interparticle voids. This indicates a hierarchical pore structure in the material, comprising both mesopores and macropores (or textural pores).

The pore size distribution in Figure 1f further reveals that the pore structure of BT-SAP is predominantly mesoporous, with the main pore distribution concentrated in the mesopore range and an average pore diameter of 13.59 nm. The mesoporous structure can serve as transport channels for water and methylene blue molecules. Together with the three-dimensional swelling network of the material, it supports its water absorption and adsorption performance. The relatively low specific surface area and the meso-macroporous structure are consistent with the macroporous characteristics (rather than high-surface-area microporosity) of the composite superabsorbent polymer.

2.2. Polymerization Optimization

The polymerization conditions of the BT-SAP were optimized through single-factor experiments, as shown in Figure 2. The results indicate that each component exhibits a trend of initial enhancement followed by suppression: an appropriate amount of APS promotes effective polymerization and network formation, whereas excess APS reduces molecular weight and damages the network due to excessive radicals. Increasing NMBA improves crosslinking stability, but overly high crosslinking density severely restricts swelling space. XG enhances hydrophilicity and structural toughness at suitable levels, but excessive amounts cause chain entanglement and inhibit swelling. BT incorporation optimizes pore structure and strengthens hydration via nanodispersion and surface hydrophilic groups, yet overloading leads to aggregation, pore blockage, and network discontinuity. The optimal polymerization conditions were determined as follows: APS, 0.03 g; NMBA, 0.025 g; XG, 0.8 g; and BT, 0.8 g. The composite gel prepared under these conditions exhibited the highest water absorption capacity of 378.6 g/g. All subsequent BT-SAP gels in this study were fabricated based on this optimized formulation.

2.3. Water Absorption Capacity

This study systematically evaluated the water absorption performance of the BT-SAP under different conditions. As shown in Figure 3a, in the pH range of 2–5, the gradual dissociation of carboxyl groups into carboxylate anions leads to electrostatic repulsion that expands the three-dimensional network, resulting in a continuous increase in water absorption capacity. The absorption peaks in the pH 5–9 range, where carboxyl groups are fully dissociated without excessive cation interference. At pH > 9, excess Na⁺ causes charge screening, weakening the electrostatic repulsion between –COO⁻ groups, contracting the network, and gradually reducing water absorption [19]. These results indicate optimal absorption performance under neutral to weakly acidic/alkaline conditions.

In saline environments [Figure 3b], water absorption decreases with increasing salt concentration and is strongly influenced by cation valence: Fe³⁺ > Ca²⁺ > Na⁺. The FeCl₃ solution shows the most significant reduction due to strong charge shielding [20]. In real water systems (tap water, river water, lake water), the gel maintains high absorption capacities (306.8, 240.6, and 269.8 g/g, respectively), demonstrating good adaptability to complex aqueous environments.

The water absorption kinetics [Figure 3c] shows a rapid increase within the first 40 min, attributed to the abundant hydrophilic groups and interconnected porous structure that facilitate rapid capillary action and surface absorption. Absorption gradually plateaus as the swelling network reaches equilibrium between osmotic pressure and elastic retraction. The kinetic data were empirically fitted to common models to quantify the uptake rate. The process shows an excellent correlation with the pseudo-second-order kinetic model (Equation (1)) (R² = 0.9963). While this model is often associated with chemisorption in adsorption studies, in the context of hydrogel swelling, its high fitting quality primarily indicates that the rate of water uptake is proportional to both the available swelling capacity and the driving force for network expansion, effectively describing the observed “fast-then-slow” approach to equilibrium [21]. Therefore, it serves as a useful empirical descriptor of the overall absorption kinetics rather than as evidence for a specific chemical reaction-controlled step. The model parameters confirm the rapid initial absorption, which is suitable for applications requiring fast water uptake.

$${\displaystyle \frac{t}{Q_t}}={\displaystyle \frac{t}{Q_e}}+{\displaystyle \frac{1}{k}}{\displaystyle \frac{1}{Q_e^2}}$$

(1)

where Q_t is the swelling ratio (g/g) or adsorption capacity (mg/g) at time t; t is the duration (min) of swelling or adsorption; Q_e is the equilibrium swelling ratio or adsorption capacity; and k is the pseudo-second-order rate constant.

Cyclic absorption tests [Figure 3d] were conducted to evaluate the reusability and structural resilience of BT-SAP. The results show that the material maintains a functional absorption capacity over multiple cycles. During the first seven absorption–desorption cycles, the water absorption capacity remains above 340 g/g. After ten cycles, approximately 44% of the initial capacity (165.8 g/g) is retained. The gradual decline is attributed to partial pore fatigue or slight collapse during repeated swelling–shrinking and irreversible relaxation of some physical crosslinks, while the primary network skeleton remains largely intact, allowing for continued, albeit reduced, water uptake. This demonstrates a degree of practical reusability, although long-term durability requires further improvement for sustained cyclic applications.

In summary, the composite superabsorbent gel combines strong environmental adaptability, rapid absorption response, and excellent cycling stability, making it suitable for diverse practical water-absorption applications.

2.4. Water Retention Performance

Figure 4a shows the water retention rate under different drying conditions over time. Under oven-drying at 50 °C, the water retention rate decreased most rapidly, approaching zero after about 6.5 h. At 30 °C, the decline was slower, nearing zero after approximately 19 h. Under ambient conditions, the water retention rate decreased most gradually, only approaching zero after around 130 h. These results clearly demonstrate that temperature is the key factor controlling water retention persistence: higher temperatures significantly accelerate the evaporation and desorption of bound water from the resin network [22]. Thus, the material exhibits better long-term water retention capability under ambient or low-temperature conditions.

Figure 4b further evaluates the water retention stability of the gel under centrifugal force. At a fixed speed of 8000 r/min, the water retention rate only decreased slowly from 100% to about 97% as the centrifugation time increased from 0 to 60 min. Meanwhile, at a fixed centrifugation time of 10 min, even when the speed was increased from 2000 to 12,000 r/min, the water retention rate remained high at approximately 98%. These findings indicate that neither prolonged high-speed centrifugation nor increased centrifugal speed caused a significant drop in water retention. This performance is consistent with a stable three-dimensional crosslinked network capable of physically confining water molecules. The incorporation of bentonite, which is known to enhance network cohesion and mechanical stability as observed in other tests [23], likely contributes to this resilience against centrifugal force. Thus, the gel structure maintains good water retention stability and integrity under mechanical stress.

2.5. Moisture Resistance

The moisture absorption behavior was evaluated by monitoring the mass change of dried samples exposed to ambient laboratory conditions. As shown in Figure 5, the moisture absorption rate increased continuously with prolonged exposure time: starting at 7.14% on day 1 and rising gradually to 32.57% by day 7. This indicates a notable affinity of the material for atmospheric moisture, driven by the combined effects of environmental vapor pressure and the hydrophilic groups within the resin network.

From the perspective of moisture resistance, an ideal moisture-resistant material should exhibit both a low equilibrium absorption and slow absorption kinetics. The absence of a plateau and the clear upward trend in absorption suggest that the resin continues to adsorb moisture under ambient humidity, reflecting limited long-term moisture stability [24]. Therefore, for applications requiring strict moisture protection or long-term storage, further modification (such as enhancing hydrophobicity or applying surface/structural treatments) would be necessary to suppress moisture uptake. In contrast, for short-term exposure or less demanding humidity conditions, the relatively gradual initial absorption may still meet certain practical requirements.

2.6. MB Adsorption Performance

To directly evaluate the effect of modification, the adsorption performance of the pure SAP (without bentonite) was tested under identical conditions (30 mg/L MB). As shown in

Table 1. Comparison of the adsorption performance between SAP and BT-SAP in a 30 mg/L MB solution.

	SAP	BT-SAP
Q (mg/g)	96.3	105.9

, the equilibrium capacity increased from 96.3 mg/g for the pure SAP to 105.9 mg/g for the BT-SAP composite, representing a ~10% enhancement. This improvement confirms the beneficial role of bentonite. The added clay likely contributes additional active sites, and may enhance the accessibility of functional groups involved in the chemisorption process. This result validates the design of the ternary composite for improved adsorption functionality.

The adsorption performance of the BT-SAP toward MB was systematically investigate. Figure 6a shows the effect of initial MB concentration. As the concentration increased, the adsorption capacity (Q_MB) rose continuously, which can be attributed to the enhanced mass transfer driving force at higher concentrations, facilitating dye diffusion toward the adsorption sites. In contrast, the removal rate (R_MB) gradually decreased with increasing concentration, indicating progressive saturation of available adsorption sites on the resin surface, consistent with typical adsorption behavior.

Figure 6b illustrates the influence of solution pH. Over the pH range of 2–12, both Q_MB and R_MB first increased and then slightly decreased. Under acidic conditions, high H⁺ concentrations competed with MB cations for negatively charged sites (e.g., –COO⁻) on the gel surface, inhibiting adsorption. As pH increased, the dissociation of functional groups such as carboxyl enhanced the surface negative charge density, strengthening electrostatic attraction [25]. The maximum adsorption capacity of 116.8 mg/g was achieved at pH 10. The slight decline under strongly alkaline conditions may be related to changes in the swelling state of the resin network.

Figure 6c compares adsorption in different water matrices. The gel exhibited the highest Q_MB and R_MB in deionized water. In tap, river, and lake water, the presence of competing cations (e.g., Na⁺, Ca²⁺) led to a slight reduction in performance. Nevertheless, the gel maintained considerable adsorption capacity, demonstrating its adaptability to complex ionic environments in real wastewater.

Figure 6d evaluates the cyclic adsorption stability. After six adsorption–desorption cycles, the adsorption capacity decreased to 40.8% of its initial value, mainly due to irreversible occupation of active sites and slight structural fatigue. However, a high removal rate was maintained through the first four cycles. Therefore, from a practical application standpoint, the adsorbent can be effectively reused for four cycles. Overall, the resin retained notable adsorption ability after repeated use, indicating good structural stability and potential for repeated application in dye wastewater treatment.

Figure 7a illustrates the variation in MB adsorption capacity of BT-SAP over time. The adsorption process exhibits a typical “fast adsorption-slow equilibrium” characteristic: within the initial 60 min, the adsorption capacity increases rapidly with time; after 60 min, it gradually stabilizes and eventually reaches saturation. This behavior indicates that at the initial stage, the gel surface possesses abundant active sites capable of efficiently capturing MB molecules, accompanied by a strong mass transfer driving force. As adsorption proceeds, the surface active sites become progressively occupied, leading to a decline in adsorption rate until the system reaches equilibrium. Such kinetic performance demonstrates that BT-SAP exhibits a favorable adsorption response toward MB, enabling efficient removal within a relatively short period.

Figure 7b compares the fitting results of the pseudo-first-order (Equation (2)) and pseudo-second-order (Equation (1)) kinetic models to the experimental data. The correlation coefficient for the pseudo-second-order model is significantly higher than that for the pseudo-first-order model, indicating that the adsorption process follows the pseudo-second-order kinetics more closely. The pseudo-second-order model is generally applicable to processes controlled mainly by chemisorption, which assumes that the adsorption rate is governed by electron transfer or chemical bond formation between the adsorbent and the adsorbate.

$$\log \left(Q_e-Q_t\right)=-{\displaystyle \frac{k_1}{2.303}}t+\log Q_e$$

(2)

where Q_t is the adsorption capacity at time t, mg/g; t is the adsorption time, min; Q_e is the equilibrium adsorption capacity, mg/g; k₁ is pseudo-first adsorption rate constant.

The adsorption behavior and molecular interaction mechanism of the BT-SAP toward MB were systematically elucidated through adsorption isotherm fitting (Equation (3): Langmuir model; Equation (4): Freundlich model) and XPS surface analysis. The fitting results of the adsorption isotherms [Figure 8a] show that both the Langmuir (R² = 0.9880) and Freundlich (R² = 0.9946) models describe the process well, with the Freundlich model providing a superior fit. This indicates that the adsorption is more characteristic of multilayer adsorption on a heterogeneous surface, reflecting a non-uniform energy distribution of adsorption sites and the accumulation of MB molecules on the resin surface [26].

$${\displaystyle \frac{{\rho }_e}{Q_e}}={\displaystyle \frac{1}{Q_0{K}_b}}+{\displaystyle \frac{{\rho }_e}{Q_0}}$$

(3)

$$\ln Q_e=\ln K_f+{\displaystyle \frac{1}{n}}\ln {\rho }_e$$

(4)

where ρ_e denotes the remaining MB concentration after adsorption, mg/L; Q_e represents the equilibrium adsorption capacity, mg/g; Q₀ is the maximum theoretical adsorption capacity, mg/g; K_b is the adsorption constant, L/mg; K_f is another adsorption constant, mg¹⁻ⁿ·Lⁿ·g⁻¹; n is the adsorption index.

XPS analysis provides direct molecular-level evidence for this adsorption mode. The N1s spectra [Figure 8b] show that before adsorption, the N peak corresponds solely to the –CONH₂ group (399.53 eV). After MB adsorption, a new characteristic peak appears at 402.2 eV, attributed to the –N(CH₃)₂ group of MB, confirming its successful adsorption. Concurrently, the binding energy of the gel’s –CONH₂ peak shifts by +0.25 eV (to 399.78 eV), indicating the participation of its nitrogen atom in interactions with MB (e.g., hydrogen bonding) [27]. The C1s [Figure 8c] and O1s [Figure 8d] spectra jointly reveal the crucial role of oxygen-containing functional groups. After adsorption, the binding energies of the O–C=O and C–O, C–N components in the C1s spectrum shift by 0.1 eV and 0.05 eV, respectively. Similarly, the C=O and C–O–C components in the O1s spectrum shift by 0.1 eV and 0.5 eV, respectively. These consistent, minor chemical shifts clearly demonstrate that carboxyl (–COO⁻), ether, and amide groups are the core active sites for MB adsorption, primarily interacting with MB molecules via electrostatic attraction and hydrogen bonding [28].

The FT-IR spectral analysis before and after MB adsorption provides direct evidence for the interaction mechanisms involved. After adsorption, several characteristic peaks exhibited notable shifts [Figure 1b and Figure 8e]: the broad O–H/N–H stretching vibration shifted from 3305 to 3370 cm⁻¹, indicating enhanced hydrogen bonding involving the hydroxyl and amide groups of the adsorbent with MB molecules. More significantly, the amide I band (C=O stretching) shifted from 1661 to 1675 cm⁻¹, and the amide II band (N–H bending coupled with C–N stretching) moved from 1552 to 1568 cm⁻¹, confirming the participation of amide groups in the adsorption process, likely through hydrogen bonding interactions. The peak around 1400 cm⁻¹, associated with the symmetric stretching of COO⁻, shifted to 1406 cm⁻¹, supporting the role of electrostatic attraction between the anionic carboxylate groups and the cationic MB. The appearance of two new peaks at 1453 and 1174 cm⁻¹ after adsorption can be attributed to the aromatic C=C skeletal vibrations and C–N stretching of the MB molecule, respectively, directly confirming the successful loading of MB onto BT-SAP. The persistence of the Si–O–Si stretching band near 1041 cm⁻¹ indicates that the silicate framework of bentonite remained stable. Collectively, these spectral changes substantiate a synergistic adsorption mechanism involving hydrogen bonding (via –OH/–CONH₂), electrostatic interaction (via –COO⁻), and likely van der Waals interactions.

The EDX analysis of BT-SAP before and after MB adsorption [Figure 8f] revealed distinct elemental changes. Prior to adsorption, the main elements detected were C, O, Na, Si, and Ca. Carbon and oxygen originated from the polymer backbone and oxygen-containing functional groups of the resin matrix. Sodium derived from partially neutralized carboxyl groups, while Si and Ca were characteristic of the bentonite filler, with their contents consistent with the designed composition. Following MB adsorption, two new elements S and Cl appeared in the spectrum, corresponding to the thiazine ring sulfur and the counter chloride ion in the MB molecule. This provides direct evidence of successful MB uptake on the BT-SAP surface. Concurrently, the relative mass fractions of the original elements shifted noticeably: the proportions of C and O decreased, and those of Na, Si, and Ca were also redistributed, reflecting the overall compositional change induced by MB loading. These EDX results corroborate the chemisorption mechanism indicated by earlier XPS analysis and the pseudo-second-order kinetic behavior, offering elemental-level evidence for the adsorption of MB onto BT-SAP.

In conclusion, as shown in Figure 9, the adsorption of MB onto BT-SAP is driven by multiple synergistic mechanisms inherent to its composite structure. Electrostatic attraction occurs between the cationic dye molecules and the anionic functional groups on the polymer-clay network. Concurrently, hydrogen bonding forms between the –NH₂, –OH, and –COO⁻ groups of the adsorbent and the heteroatoms in MB. Additionally, the three-dimensional porous architecture of BT-SAP facilitates physical entrapment and pore-filling of MB molecules. Together, these interactions underscore the heterogeneous and multifunctional nature of the adsorbent, enabling efficient and robust dye removal.

A comparative analysis of the BT-SAP prepared in this work with reported composite superabsorbent/adsorbent materials is summarized in

Table 2. Comparison of the maximum water absorption capacity and MB adsorption capacity between BT-SAP and other reported composite materials.

Absorbents	Qmax (g/g)	QMB(max) (mg/g)	Adsorption Parameters	Reference
Chitosan-g-poly(acrylamide)/attapulgite SAP	319	NA	NA (Not appliable)	[29]
HPMC-g-poly(AM-co-SPA)	121	NA	NA	[30]
Bio-SAH	29.46	6.737	MB concentration: 20 ppm; Hydrogel dosage: 0.03 g; T: 55 °C; pH: 8	[31]
Bentonite/sodium alginate composite	NA	100	MB concentration: 100 mg/L; Hydrogel dosage: 0.01 g; T: 25 °C; Shaking speed: 200 rpm	[32]
Chitosan-MMT/PEI	NA	111	MB concentration: 20 mg/L; Hydrogel dosage: 0.05 g	[33]
Alginate-based vinylated SiO2	889.76	139.31	MB concentration: 10 mg/L; Hydrogel dosage: 0.05 g/mL; T: 25 °C; pH: 6.4; Shaking speed: 70 rpm	[34]
HG/MTWBC	NA	20.79	pH: 8: Hydrogel dosage: 1.5 g/L; MB concentration: 10 mg/L; T: 25 °C	[35]
BT-SAP	378.6	181.3	Hydrogel dosage: 0.01 g; MB concentration: 60 mg/L; T: 25 °C; Unadjusted pH	This work

. Focusing on the two core performance metrics (maximum water absorption capacity and maximum MB adsorption capacity), BT-SAP demonstrates a superior water absorption level among its peers, matching or exceeding most reported systems. Its MB adsorption capacity is particularly outstanding, significantly surpassing that of many existing adsorbents.

Critically, BT-SAP achieves a synergistic integration of high water absorption and efficient dye adsorption, a dual functionality that is uncommon in comparable materials. This integrated advantage stems from its bentonite-enhanced three-dimensional porous structure and the multi-active-site adsorption mechanism, positioning BT-SAP as a highly competitive candidate for applications demanding both high water retention and dye wastewater treatment.

3. Conclusions

This study successfully developed a ternary composite gel (BT-SAP) with dual functionality of high water absorbency and efficient dye adsorption. Systematic characterization and performance tests validate the effectiveness of the strategy combining biopolymer, synthetic polymer networks, and bentonite nanoplatelets. The material’s dual functionality originates from its unique structure: bentonite not only acts as a physical crosslinker enhancing the mechanical and swelling stability of the network, but also, together with the abundant oxygen- and nitrogen-containing functional groups on the polymer chains, constructs a hierarchical porous structure with numerous active sites. This enables the efficient absorption and retention of water through osmosis and hydrophilicity, while simultaneously allowing the specific capture of cationic dye molecules via a synergistic mechanism of electrostatic attraction and hydrogen bonding. Consequently, BT-SAP shows potential for integrated applications requiring simultaneous water management and pollutant removal, such as agricultural wastewater reuse and ecological restoration. Future work could focus on expanding its pollutant adsorption spectrum and further enhancing its long-term stability in extreme environments.

4. Materials and Methods

4.1. Materials

Acrylic acid (AA, 99%), acrylamide (AM, 98%), xanthan gum (XG, USP), N, N-methylenebisacrylamide (NMBA, 99%), ammonium persulfate (APS, 98%), sodium hydroxide (96%), sodium chloride (99%), calcium chloride (98%), ferric chloride (97%), hydrochloric acid (36.0%~38.0%), bentonite (BT), and methylene blue (MB, 98%) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Deionized water, tap water, lake water (Qingfeng Lake, Dongying, China), and river water (Guangli River, Dongying, China) were used for the experiments.

4.2. Preparation of BT-XG/P(AA-AM) Superabsorbent (BT-SAP)

In one representative polymerization, the synthesis was performed as follows: 0.80 g of XG was accurately weighed and dissolved in 40.0 mL of deionized water under magnetic stirring until complete dissolution. Then, 0.80 g of BT was added, and the mixture was ultrasonically dispersed for 10 min to obtain a uniformly dispersed solution (Solution 1). Separately, 6.40 g of NaOH was dissolved in 20.00 mL of deionized water, followed by the slow addition of 14.4 g of AA under stirring (corresponding to an 80% degree of neutralization). After the system cooled to room temperature, 7.10 g of AM was added and stirred until completely dissolved, yielding Solution 2. Subsequently, Solution 1 and Solution 2 were combined. NMBA (0.02 g) and APS (0.03 g) were added sequentially, and the mixture was homogenized by ultrasonication (53 kHz) for 10 min. The resulting mixture was transferred into a beaker, sealed with plastic film, and placed in an oven at 70 °C for 4 h to complete the polymerization. The obtained product was cut into small pieces, dried at 105 °C for 3 h, ground into powder, and sieved through a 30–50 mesh standard sieve to obtain the target superabsorbent resin sample.

4.3. Swelling Properties

To determine the swelling performance of the superabsorbent polymer, a precisely weighed sample was immersed in an excess of the test solution and allowed to swell fully. Following complete swelling, the gel was separated from any unabsorbed liquid by filtration using a nylon mesh bag. The mass of the swollen hydrogel was then recorded. The water absorption capacity, Q (g/g), was calculated according to Equation (5).

$$Q={\displaystyle \frac{M_2-M_1}{M_1}}$$

(5)

where M₁ and M₂ represent the mass of the superabsorbent SAP before and after water absorption (g), respectively.

The repeated absorption capacity was assessed by completely drying the superabsorbent sample after its first full swelling, followed by re-immersion in an adequate volume of deionized water to measure its secondary absorption ratio. This procedure was repeated iteratively until a significant decrease in the water absorption capacity of the superabsorbent resin was observed, at which point the experiment was terminated, thereby determining its cyclic usability.

4.4. Water Retention Capacity

To systematically evaluate the water retention performance, this study investigated its behavior under both mechanical dewatering and thermal evaporation. The centrifugal water retention ratio was determined by weighing the swollen hydrogel and subjecting it to centrifugation (2000–12,000 r/min), where centrifugal force simulated external pressure, followed by weighing the residual gel mass. The evaporative water retention ratio was assessed by exposing the saturated hydrogel to static air at different temperatures (30 °C, 40 °C, 50 °C): the room-temperature group was kept in the laboratory environment to simulate natural evaporation, while the high-temperature group was placed in a precision oven to accelerate the evaporation process. All samples were collected and weighed at designated time intervals, and the water retention rate R (%) was calculated according to Equation (6).

$$R={\displaystyle \frac{M_t}{M_0}}\times 100\%$$

(6)

where M_t (g) is the mass of the hydrogel at time t and M₀ is the mass of the initial saturated hydrogel (g).

4.5. Moisture Resistance

To evaluate the moisture resistance of the composite superabsorbent resin, a dried sample was placed in a beaker and exposed to ambient laboratory conditions. The weight of the sample after moisture absorption was measured daily. The moisture absorption rate was then calculated according to Equation (7) to assess its hygroscopic performance.

$$M={\displaystyle \frac{M_2-M_1}{M_1}}\times 100\%$$

(7)

where M₂ represents the weight after moisture absorption (g) and M₁ denotes the weight of the dried sample (g).

4.6. MB Adsorption Performance

Specifically, 0.01 g of BT-SAP was added to 50 mL of MB solution for adsorption. After a predetermined adsorption period, the supernatant was collected, and the residual MB concentration was determined using a TU-190 dual-beam UV-visible spectrophotometer (Shanghai Analytical Instruments Co., Ltd., Shanghai, China) (λ_max of MB: 664 nm). The effect of pH on adsorption was examined across a pH range of 2 to 12. Adsorption isotherms were established by testing initial MB concentrations of 10, 20, 30, 40, 50, and 60 mg/L. Following saturation with MB, the adsorbent was regenerated by desorption with 0.1 mol/L of hydrochloric acid. The adsorption capacity (Q_MB, mg/g) and removal efficiency (R_MB, %) were calculated according to Equations (8) and (9), respectively [36,37]. All the MB adsorption experiments were performed in triplicate, and the average values are reported.

$$Q_{MB}={\displaystyle \frac{\left(C_0-C_e\right)V}{m}}$$

(8)

$$R_{MB}={\displaystyle \frac{C_0-C_e}{C_0}}\times 100\%$$

(9)

where C₀ (mg/L) and C_e (mg/L) are the initial and equilibrium MB concentration; m (g) is the mass of the dried BT-SAP; and V (L) is the volume of the MB solution.

4.7. Characterization

The chemical structure of the materials was characterized by Fourier transform infrared (FTIR) spectroscopy using a NICOLET iN10 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), with spectra recorded from 4000 to 500 cm⁻¹. Surface morphological features were observed with an FEI QUANTA FEG 450 field emission scanning electron microscope (FEI, Hillsboro, OR, USA). Phase composition and crystal structure were determined by X-ray diffraction (XRD) analysis performed on a Rigaku D/Max-2500VB2/PC diffractometer (Rigaku, Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB 250Xi system Thermo Fisher Scientific, Waltham, MA, USA) to characterize the chemical states. The specific surface area and pore structure of the samples were analyzed by nitrogen adsorption–desorption isotherms using a Micromeritics ASAP 2460 surface area and porosity analyzer (Micromeritics, Norcross, GA, USA). Energy-dispersive X-ray spectroscopy (EDX) was conducted with an Oxford Instruments X-Max spectrometer (Oxford Instruments, Oxford, UK) attached to the scanning electron microscope to determine the elemental composition and distribution.