Alkali-Resistant Ion-Imprinted Chitosan–Mesoporous Silica Composite for Efficient and Selective Gallium Separation

Abstract

Efficient and selective separation of gallium (Ga(III)) from alkaline industrial waste streams remains a significant challenge due to the coexistence of chemically similar ions such as Al(III) and V(V). In this study, a novel ion-imprinted chitosan-based adsorbent (CS/(H-CGCS)-Ga-IIP) was synthesized via a hybrid cross-linking strategy using glutaraldehyde and siloxane-modified chitosan. The optimized material exhibited a high adsorption capacity of 106.31 mg·g⁻¹ for Ga(III) at pH 9, with fast adsorption kinetics reaching equilibrium within 60 min. Adsorption behavior followed the pseudo-second-order kinetic and Langmuir isotherm models, and thermodynamic analysis indicated a spontaneous and endothermic process. In simulated Bayer mother liquor systems, the material demonstrated outstanding selectivity and a distribution coefficient ratio k_d-Ga/k_d-Al = 146.9, highlighting its strong discrimination ability toward Ga(III). Mechanistic insights from SEM-EDS, FTIR, and XPS analyses revealed that Ga(III) adsorption occurs via electrostatic interaction, ligand coordination, and structural stabilization by the siloxane network. The material maintained good adsorption performance over three regeneration cycles, indicating potential for reuse. These findings suggest that CS/(H-CGCS)-Ga-IIP is a promising candidate for the sustainable recovery of gallium from complex alkaline waste streams such as Bayer process residues.

1. Introduction

Gallium is a highly valuable and strategically critical rare metal with exceptional photoelectric and chemical properties, making it integral to various industries, including semiconductors, solar cells, alloys, and medical applications. As a critical raw material for modern high-tech development, gallium is indispensable in advancing numerous cutting-edge technologies. However, the concentration of gallium in the Earth’s crust is extremely low, averaging only 17 mg/kg [1]. It is primarily found in bauxite, zinc ores, and coal deposits, often recovered as a byproduct during smelting processes. Notably, approximately 90% of global gallium production is derived from the Bayer process, highlighting the urgency of developing highly selective, alkali-tolerant, and efficient extraction strategies for Ga(III) recovery from Bayer liquor [2]. Among these, adsorption has emerged as one of the most promising and scalable techniques, offering advantages such as high efficiency, operational simplicity, and cost-effectiveness. Nevertheless, the complex composition and extreme alkalinity (pH ≥ 13) of Bayer liquor pose significant challenges to adsorbent stability and selectivity, underscoring the need for functional materials capable of operating under such harsh conditions. Optimizing adsorption-based Ga(III) recovery from Bayer liquor is thus considered a critical step toward securing a sustainable supply of this strategic element [3].

In recent years, biopolymer-based adsorbents—particularly chitosan (CS)-derived materials—have garnered significant scientific interest owing to their intrinsic biodegradability, high density of functional groups (e.g., amino and hydroxyl moieties), cost-effectiveness, and superior environmental sustainability compared to conventional synthetic adsorbents. Researchers have extended the application scope of CS by modifying its backbone and introducing composite structures. For example, Gomase et al. [4] incorporated silica-enriched laterite into CS to enhance Cr(VI) removal and also applied the modified CS for the adsorption of dyes and cyanide from wastewater [5]. Xiang et al. developed tartaric acid-functionalized CS for selective germanium recovery from zinc leachate, while Liu et al. [6] achieved a high Ga(III) uptake of 192.4 mg/g using acrylic acid-grafted CS composites. However, despite promising laboratory-scale performance, the practical application of chitosan-based adsorbents is significantly constrained by their poor structural and chemical stability, particularly under acidic or highly alkaline conditions, where the hydrolysis of β-(1→4)-glycosidic bonds or amine group deprotonation may occur [7]. This drawback can be mitigated by cross-linking strategies using agents such as glutaraldehyde or epichlorohydrin, which form covalent bridges between polymer chains, thus improving chemical robustness while retaining functional group accessibility. Furthermore, integrating CS with chemically stable, high-surface-area supports such as mesoporous silica can further enhance the material’s mechanical strength and resistance to alkaline degradation [8].

In addition to physical reinforcement, molecular-level design has also gained prominence. Ion-imprinted polymers (IIPs) have emerged as powerful tools for highly selective metal ion recognition and separation due to their tailor-made binding sites with shape and coordination specificity [9]. The typical synthesis of IIPs involves precomplexation of target ions with functional monomers, followed by polymerization and subsequent template removal. The stability and geometry of the ion–monomer complex directly influence the selectivity and performance of the final IIP, making the choice of functional matrix and imprinting conditions essential. For instance, Zhu et al. [10] incorporated pentosidine tannin into an ordered mesoporous SiO₂ network to create Ga(III)-imprinted composites, achieving an impressive adsorption capacity of 286.5 mg/g. CS, with its abundant -NH₂ and -OH groups, not only facilitates metal coordination but also serves as a flexible macromolecular framework to accommodate imprinting cavities. Therefore, combining CS with ion-imprinting techniques and structurally robust mesoporous materials offers a compelling strategy for selective metal separation under extreme conditions. Nevertheless, reports on CS-based ion-imprinted adsorbents capable of stable operation under highly alkaline conditions, such as those encountered in Bayer liquor, remain scarce [11]. Moreover, the integration of low-cost industrial by-products such as acid-leached coal gasification slag (CGCS) into functionalized adsorbents has not been widely explored, despite their potential to impart mesoporosity, surface anchoring sites, and sustainability benefits [12].

In this work, we propose a novel alkali-resistant ion-imprinted adsorbent, CS/(H-CGCS)-Ga-IIP, synthesized by covalently cross-linking chitosan with acid-activated mesoporous CGCS and imprinting Ga(III) as the target ion. This composite not only exhibits excellent adsorption performance (Qm = 145.14 mg/g) and selectivity in simulated Bayer liquor (Sel(Ga/Al) = 2.17) but also maintains structural integrity under pH 14 conditions. Adsorption kinetics conformed closely to the pseudo-second-order model, indicative of chemisorption, while thermodynamic parameters (ΔG < 0, ΔH > 0) confirmed the spontaneity and endothermic nature of Ga(III) uptake. This work highlights a promising pathway toward the development of structurally durable, green, and highly selective adsorbents for gallium recovery from industrial alkaline waste streams.

2. Materials and Methods

To ensure reproducibility and transparency, detailed experimental procedures are presented below. Additional information on reagents, material sources, characterization methods, and supplementary synthesis data is provided in the Supporting Information.

2.1. Preparation of CS/SiO₂-Based Mesoporous Silica–Gallium Ion-Imprinted Polymer

The synthesis process of the CS/(H-CGCS)-Ga-IIP adsorbent is illustrated in Figure 1. Briefly, 1.25 g of chitosan (CS) was added to 50 mL of 2% acetic acid (HAc) solution in a 500 mL beaker and stirred at room temperature for 2 h to ensure complete dissolution, forming a uniform and transparent solution [13].

Subsequently, 0.75 g of acid-leached coal gasification slag (H-CGCS) and 50 mL of Ga(III) solution (2000 mg/L, prepared from GaCl₃ and adjusted to pH 9 using NaOH) were added. The pH of the entire mixture was rapidly adjusted to 9 to favor the coordination between Ga(III) ions and functional groups in the chitosan matrix. The solution was stirred vigorously at 800 rpm for 3 h at room temperature to ensure homogeneous dispersion and adequate interaction between CS and H-CGCS, thereby initiating Ga(III) imprinting [14].

Following the imprinting step, the pH was adjusted to 6 using dilute HCl or NaOH. Then, 40 mL of 50% glutaraldehyde solution was slowly added as a cross-linking agent, and the mixture was stirred for an additional 3 h to promote the formation of a stable three-dimensional cross-linked network between chitosan and H-CGCS. To further enhance the structural rigidity and chemical resistance, 30 mL of curing agent 593 was added, and the mixture was allowed to stand at room temperature for 24 h.

After curing, the imprint template (Ga(III)) was removed via an elution step. The pH was adjusted to 1 with HCl solution, followed by centrifugation to collect the solid product. To ensure complete removal of Ga(III) ions from the imprinted cavities, the solid was eluted three more times using 100 mL of 0.5 mol/L HCl, with each elution lasting 1 h [15].

Finally, the resulting material was thoroughly washed with deionized water until neutral, filtered, and freeze-dried at −50 °C for 24 h. The dried product was gently ground into coarse powder to obtain the final ion-imprinted polymer, designated as CS/(H-CGCS)-Ga-IIP.

2.2. Optimization of Synthesis Conditions for CS/(H-CGCS)-Ga-IIP

2.2.1. Effect of Cross-Linking Agent Type and Adsorption pH

To systematically evaluate the influence of cross-linker type and adsorption pH on the adsorbent’s structural and functional properties, two cross-linking agents—glutaraldehyde (GA) and epichlorohydrin (ECH)—were selected. Adsorption performance was assessed at two representative pH conditions: pH 3 (acidic) and pH 9 (alkaline). According to the literature reports and prior experimental experience, GA exhibits higher cross-linking efficiency in the pH range of 6–8, while ECH is more reactive under alkaline conditions (pH 9–10). Therefore, pH 7 and pH 9 were selected as optimal cross-linking conditions for GA and ECH, respectively.

In terms of adsorption, the behavior of Ga(III) is highly pH-dependent. Under acidic conditions (pH = 3), Ga(III) remains in its solvated ionic form (Ga³⁺), which favors coordination with functional groups such as -NH₂ and -OH, while avoiding premature precipitation of Ga(OH)₃. In contrast, under alkaline conditions (pH = 9), Ga(III) predominantly exists as Ga(OH)₄⁻, which can still coordinate effectively with protonated functional groups and benefit from electrostatic attraction, thereby enhancing adsorption [16]. Based on these considerations, pH 3 and pH 9 were selected as key points for studying the impact of solution pH on adsorption performance.

2.2.2. Effect of CS-to-H-CGCS Mass Ratio

To optimize the composition of the composite adsorbent, various mass ratios of CS to H-CGCS were investigated. The ratios tested included 0.25:1.75, 0.5:1.5, 0.75:1.25, 1:1, 1.25:0.75, 1.5:0.5, and 1.75:0.25. These formulations were labeled as samples 1 through 7, respectively [17]. The goal was to determine the optimal balance between the biopolymer matrix and the mesoporous inorganic support for maximizing Ga(III) uptake.

2.2.3. Effect of Cross-Linking pH

To assess the influence of pH during the cross-linking process, experiments were conducted at five different pH levels: 5, 6, 7, 8, and 9. The pH environment during cross-linking is critical, as it affects the ionization state of functional groups and the reactivity of cross-linkers, thereby influencing the density and stability of the resulting polymer network.

2.2.4. Effect of Cross-Linking Time

The duration of cross-linking directly impacts the formation of the three-dimensional network structure. To determine the optimal cross-linking time, reactions were carried out for 1, 2, 3, 4, and 5 h. The objective was to balance sufficient cross-linking with the avoidance of over-cross-linking, which may lead to reduced porosity or restricted accessibility to imprinted sites.

2.2.5. Selection of Eluent Type

To optimize the template removal step and regenerate the imprinted sites, two eluents were evaluated: 0.5 mol/L HCl and 0.2 mol/L EDTA-2Na (near saturation). Each eluent was applied repeatedly until no detectable Ga(III) remained in the filtrate [18]. The efficiency of template ion removal and its influence on subsequent adsorption capacity were used as criteria for eluent selection.

2.2.6. Effect of Cross-Linker Dosage

The amount of cross-linker plays a crucial role in defining the structural density and mechanical stability of the polymer matrix. In this study, glutaraldehyde dosages of 20, 30, 40, and 50 mL were examined. The aim was to identify the optimal dosage that ensures network integrity without over-consuming functional groups necessary for Ga(III) coordination.

2.3. Adsorption Experiment

To evaluate the adsorption performance of CS/(H-CGCS)-Ga-IIP under various operational conditions, a series of batch adsorption experiments were conducted.

A 50 mL Ga(III) solution (100 mg/L) was prepared and adjusted to different pH values (2–11). The effect of several key parameters—including pH, adsorbent dosage (10–90 mg), initial Ga(III) concentration (50–500 mg/L), and contact time (0.25–3 h)—was systematically investigated. Each experiment was carried out by adding a known amount of adsorbent to the solution, followed by shaking at 200 rpm in a thermostatic incubator at 25 °C. After equilibrium was reached, the mixtures were filtered through a 0.22 μm membrane, and the residual Ga(III) concentrations were measured using inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5800 form Agilent Technologies, Inc. in Santa Clara, CA, USA).

Adsorption efficiency (P%) and equilibrium capacity (Q_e) were calculated using Equations (1) and (2), respectively [19].

$$P\%={\displaystyle \frac{C_i-C_e}{C_i}}\times 100\%$$

(1)

$$Q_e={\displaystyle \frac{{(C}_i-C_e)\times V}{m}}$$

(2)

where Q_e is the equilibrium adsorption capacity (mg/g), P (%) is the adsorption efficiency, C_i and C_e (mg/L) are the initial and equilibrium concentrations of Ga(III), respectively, V (L) is the volume of solution, and m (mg) is the mass of adsorbent.

To evaluate the reusability of the CS/(H-CGCS)-Ga-IIP adsorbent, a series of adsorption–desorption cycles were conducted. In each cycle, 50 mg of adsorbent was added to 50 mL of Ga(III) solution (100 mg/L, pH = 9). The mixture was shaken at 150 rpm and 25 °C to reach adsorption equilibrium, followed by filtration and drying of the adsorbent.

Desorption was then performed using 100 mL of 0.5 mol/L HCl solution under identical shaking and temperature conditions. This process was repeated for five consecutive cycles. Desorption efficiency (D%) was calculated according to Equation (3) [20].

$$D\%={\displaystyle \frac{m_d}{m_a}}\times 100\%$$

(3)

where D (%) is the desorption efficiency, and ma and m_d (mg) are the adsorbed and desorbed amounts of Ga(III), respectively.

Based on the above research, we simulated real Bayer solution to investigate the performance of this adsorbent. A simulated Bayer mother liquor was prepared using Ga, Al, and V at the concentrations shown in

Table 1. Simulated composition of Bayer mother liquor used for selectivity evaluation.

Metal	Ga	Al	V
Contene (g/L)	2.5	30.6	0.15

Note: Data derived from the Bayer mother liquor of a Guangxi-based alumina refinery.

, based on actual composition data from a metallurgical plant in Guangxi Province, China. The solution was adjusted to a strong alkaline environment (pH = 14) [1]. Subsequently, 200 mg of CS/(H-CGCS)-Ga-IIP was added to 50 mL of this simulated liquor, and the mixture was stirred at room temperature for 3 h to evaluate adsorption performance under realistic conditions.

To quantitatively evaluate the selective affinity of the synthesized ion-imprinted polymer toward Ga(III) in the presence of competing metal ions, the selectivity coefficient Sel(Ga/M) was calculated. The selectivity coefficient (Sel) is calculated using Equation (4) [21].

$${Sel}_{Ga/M}=lg{\displaystyle \frac{{\left(\frac{Q_{e1}}{C_{e1}}\right)}_{Ga}}{{\left(\frac{Q_{e2}}{C_{e2}}\right)}_M}}$$

(4)

where Q_e and C_e represent the equilibrium adsorption capacity (mg/g) and concentration (mg/L), respectively, for Ga(III) and the competing ion M (e.g., Al(III) or V(V)). The metal ion concentrations in the simulated solution are summarized in Table 1 [22].

2.4. Adsorption Kinetics Models

To elucidate the adsorption mechanism and identify the potential rate-controlling step of Ga(III) adsorption onto CS/(H-CGCS)-Ga-IIP, the kinetic data were analyzed using two commonly employed models: the pseudo-first-order model (Equation (5)) and the pseudo-second-order model (Equation (6)) [23].

$$ln\left(Q_e-Q_t\right)=ln{Q}_e-K_{1}t$$

(5)

$${\displaystyle \frac{t}{Q_t}}={\displaystyle \frac{1}{K_2{Q}_e^2}}+{\displaystyle \frac{t}{Q_e}}$$

(6)

where Q_e and Q_t (mg/g) represent the adsorption capacity at equilibrium and at time t (min), respectively, and K₁ and K₂ are the rate constants for the pseudo-first-order and pseudo-second-order models, respectively.

2.5. Adsorption Isotherm Models

Adsorption isotherms are essential for understanding the equilibrium relationship between the concentration of adsorbate in solution and the amount adsorbed onto the solid phase at a constant temperature. They provide critical insights into the adsorption capacity, surface heterogeneity, and adsorbent–adsorbate interactions. In this study, model fitting was performed by changing the initial concentration of gallium Ga solution (20–400 mg/L) at different temperatures. The equilibrium data of Ga(III) adsorption on the CS/(H-CGCS)-Ga-IIP surface were fitted to the Langmuir and Freundlich adsorption isotherm models [24].

2.5.1. Langmuir Isotherm Model

The Langmuir model assumes a homogeneous adsorption surface with a finite number of identical sites, monolayer coverage, and no interactions between adsorbed molecules. The linear form of the Langmuir equation is expressed as [25]:

$${\displaystyle \frac{C_e}{Q_e}}={\displaystyle \frac{1}{Q_{m}b}}+{\displaystyle \frac{C_e}{Q_m}}$$

(7)

where Q_e (mg/g) is the adsorption capacity at equilibrium, C_e (mg/L) is the equilibrium concentration of Ga(III), Q_m (mg/g) is the maximum adsorption capacity, and b is the Langmuir constant related to adsorption energy.

To assess the favorability of adsorption, the dimensionless separation factor R_L is calculated using Equation (8):

$$R_L={\displaystyle \frac{1}{1+b{C}_0}}$$

(8)

where C₀ (mg/L) is the initial Ga(III) concentration. A value of 0 indicates irreversible adsorption, 0 < R_L < 1 indicates favorable adsorption, R_L = 1 suggests linear adsorption, and R_L > 1 implies unfavorable adsorption.

2.5.2. Freundlich Isotherm Model

The Freundlich model accounts for surface heterogeneity and multilayer adsorption and is particularly suitable for describing non-ideal adsorption behavior. The linear form of the Freundlich equation is given by [26]:

$$Q_e=K_F+{\displaystyle \frac{C_e}{n}}$$

(9)

where Q_e (mg/g) is the adsorption capacity at equilibrium, C_e (mg/L) is the equilibrium Ga(III) concentration, K_F (L/mg) is the Freundlich constant related to adsorption capacity, and 1/n (dimensionless) indicates adsorption intensity.

2.6. Adsorption Thermodynamics Model

Thermodynamic analysis provides vital information on the feasibility, spontaneity, and energetic nature of the adsorption process. In this study, the standard thermodynamic parameters—including Gibbs free energy change (ΔG), enthalpy change (ΔH), entropy change (ΔS), and observed enthalpy change for the chemical process (ΔH_obs)—were evaluated using the Van’t Hoff equation and related thermodynamic relationships [27]:

$$\Delta G=-RTIn{K}_c$$

(10)

$$K_c={\displaystyle \frac{\Delta S}{R}}-{\displaystyle \frac{\Delta H}{RT}}$$

(11)

$$\Delta H_{obs}=-R{\displaystyle \frac{d\left(ln\frac{q_e}{c_e}\right)}{d\left(\frac{1}{T}\right)}}+{\displaystyle \frac{\Delta S}{R}}$$

(12)

where ΔG is the Gibbs free energy change (kJ/mol), ΔH is the enthalpy change (kJ/mol), ΔS is the entropy change (kJ/mol·K), T is the absolute temperature (K), Kc is the equilibrium constant, ΔH_obs is enthalpy change for chemical reactions (kJ/mol), and R is the universal gas constant (8.314 J/mol·K).

3. Results and Discussion

3.1. Optimization of Synthesis Conditions for CS/SiO₂-Based Mesoporous SiO₂-Ga IIPs

Figure 2 systematically evaluates the effects of key synthesis parameters on the Ga(III) adsorption performance of the CS/(H-CGCS)-Ga-IIP material. As shown in Figure 2a, the use of glutaraldehyde (GA) as a cross-linking agent results in a significantly higher adsorption capacity (37.00 mg/g at pH 9) compared to adsorption at pH 3, while epichlorohydrin (ECH) as a cross-linker yields a maximum adsorption capacity of only 23.00 mg/g at pH 9. This indicates that GA is more favorable for constructing an efficient adsorption network under alkaline conditions, likely due to better cross-linking reactivity and structural stability [28]. Therefore, GA was selected as the optimal cross-linking agent, and pH 9 was chosen as the adsorption condition for subsequent studies.

Figure 2b shows that the adsorption capacity initially increases and then plateaus as the CS:H-CGCS ratio increases. At a ratio of 1.25:0.75, the material demonstrates the highest adsorption capacity for Ga(III), likely due to the optimal balance between functional groups from both chitosan and H-CGCS, which promotes efficient cross-linking and maximizes the number of accessible binding sites [29]. When the CS content is too high, active sites of H-CGCS may be sterically hindered; conversely, excess H-CGCS may compromise the integrity of the chitosan network, affecting the mechanical strength and adsorption performance.

In Figure 2c, the optimal cross-linking pH is determined to be 6, at which point the protonation–deprotonation balance of amino groups in chitosan supports effective cross-linking without compromising the polymer’s structure. At pH 5, excess protonation (-NH₃⁺) reduces reactivity with the cross-linker, while at pH > 6, alkaline hydrolysis or structural damage may occur, lowering adsorption efficiency. Figure 2d indicates that a cross-linking time of 3 h results in the highest Ga(III) adsorption capacity. A shorter reaction time may result in incomplete network formation, whereas excessive duration can lead to over-cross-linking, which reduces pore volume and mass transfer efficiency [30].

Figure 2e compares two eluents—0.5 mol/L HCl and 0.2 mol/L EDTA-2Na. The material treated with 0.5 mol/L HCl exhibits markedly higher Ga(III) adsorption capacity during subsequent cycles, indicating effective desorption and minimal structural degradation. In contrast, EDTA-2Na shows low elution efficiency, potentially due to its weaker complexation ability with Ga(III), resulting in incomplete desorption and even negative adsorption capacity due to interference with network stability. The occupation of adsorption sites and structural damage result in poor adsorption efficiency of EDTA-2Na.

Figure 2f shows that 40 mL of GA provides optimal cross-linking, yielding the maximum adsorption capacity of 100.00 mg/g. A lower dosage (e.g., 20 mL) leads to insufficient network formation, while excessive cross-linker (e.g., 50 mL) results in over-cross-linking, which not only reduces porosity but also occupies active coordination sites, both of which compromise adsorption performance.

In summary, the optimized synthesis conditions for CS/(H-CGCS)-Ga-IIP are GA as cross-linker, pH 6 for cross-linking, CS:H-CGCS ratio of 1.25:0.75, 3 h cross-linking time, 40 mL GA dosage, and 0.5 mol/L HCl as the eluent. These conditions ensure a stable and efficient polymeric structure with maximized Ga(III) adsorption performance, providing a robust foundation for further application in selective gallium recovery [31].

3.2. Characterization and Structural Analysis of CS/(H-CGCS)-Ga-IIP

Figure 3 and

Table 2. EDS elemental analysis table for CS/(H-CGCS)-Ga-IIP.

Element	N	O	Si	Ga
wt%	16.33	72.7	10.97	0.00

present the morphological, elemental, structural, and functional characteristics of the CS/(H-CGCS)-Ga-IIP material. The morphology and elemental composition were characterized by SEM-EDS, as shown in Figure 3a–d and Table 2. SEM analysis (Figure 3a) reveals a porous surface with a uniform distribution, suggesting that the cross-linking reaction successfully generated a stable three-dimensional network [32]. After Ga(III) imprinting, the surface became rougher, which can be attributed to the formation of imprinted cavities through coordination with functional groups. EDS mapping (Figure 3b–d) confirms the uniform distribution of N, O, and Si elements, indicating the successful integration of CS and H-CGCS components. According to Table 2, the mass percentages of N, O, and Si are 16.33%, 72.70%, and 10.97%, respectively, while Ga is undetectable. This indicates that Ga(III) was completely removed during the elution process, leaving behind the desired imprinted cavities.

FTIR analysis (Figure 3e) shows a broad band at 3430 cm⁻¹, assigned to -OH and -NH₂ stretching vibrations. The peak at 1242 cm⁻¹ corresponds to C-O, C-N, or C=N vibrations, confirming the presence of hydroxyl and amino groups. The appearance of a C=N stretching vibration, formed via Schiff base reaction between CS and GA, further supports successful cross-linking. Peaks at 1658 and 1464 cm⁻¹ correspond to C=O and N–H bending, respectively. The peaks at 1039 and 751 cm⁻¹ arise from Si-O-Si and Si-OH stretching vibrations, indicating retention of the siloxane network. The presence of peaks at 2938, 1464, and 1379 cm⁻¹ are due to C-H stretching and bending from -CH₂ and -CH₃ groups. Notably, a weak O-Ga peak appeared at 538 cm⁻¹, which may be due to trace amounts of Ga remaining undetached, proving the existence of O-Ga chelated cavities [33]. XRD analysis (Figure 3f) displays a broad, weak peak around 2θ ≈ 20°, characteristic of an amorphous structure, consistent with chitosan and H-CGCS. Additionally, NaCl-related crystalline peaks were detected, likely due to NaCl formation during pH adjustment [34]. No significant Ga-related crystalline peaks were observed, indicating Ga(III) was either amorphous or fully eluted. As shown in Figure 3g,h and Table S1, the specific surface area of CS/H-CGCS-Ga-IIP is 8.2362 m²/g, and the pore size is 1.0454 nm. This indicates that it has a good specific surface area and pore structure that is highly compatible with Ga(OH)₄⁻, further confirming the success of the material modification.

In summary, SEM and EDS analyses confirm the formation of a porous network with homogeneously distributed elements and the successful formation of Ga(III) imprinted cavities. FTIR analysis demonstrates the presence of hydroxyl, amino, and siloxane groups and confirms coordination between Ga(III) and functional groups. XRD analysis reveals that the polymer remains amorphous and structurally stable after imprinting and elution. BET analysis showed that the polymer formed a cavity structure compatible with Ga after elution. These findings collectively indicate that CS/(H-CGCS)-Ga-IIP possesses a well-defined porous structure, effective imprinting capability, and robust stability, making it suitable for selective Ga(III) adsorption in alkaline environments.

3.3. Study on the Adsorption Behavior of CS/(H-CGCS)-Ga-IIP

3.3.1. Optimization of Adsorption Conditions

The effects of various parameters on the adsorption performance of CS/(H-CGCS)-Ga-IIP were systematically investigated, as illustrated in Figure 4. As shown in Figure 4a, CGCS increases its alkali resistance through cross-linking with chitosan, enabling stable adsorption under alkaline conditions [35,36]. The adsorption capacity of CS/(H-CGCS)-Ga-IIP exhibits a bell-shaped trend with increasing pH. The adsorption increases initially, reaching a maximum at pH 9 with an adsorption capacity of 100.15 mg/g and an adsorption efficiency of 93.29% and then gradually decreases. Under acidic conditions, the surface amino groups of the material are predominantly protonated to -NH₃⁺, leading to enhanced electrostatic repulsion with Ga³⁺ ions and thus reduced adsorption. Additionally, Ga(III) mainly exists as free Ga³⁺ in acidic environments, which has relatively weak coordination ability. In contrast, at alkaline pH, Ga(III) predominantly exists as Ga(OH)₄⁻, which can interact more favorably with positively charged amino groups via electrostatic attraction and coordinate with the imprinting sites [37]. However, when the pH becomes too high, non-imprinted and unfixed -NH₃⁺ groups undergo deprotonation, reducing their ability to attract anionic species and leading to a slight decrease in adsorption. This gradual decline suggests the presence of a substantial number of pH-stable NH₃-M complexes within the imprinted structure, which remain active even under elevated pH conditions [38].

Figure 4b shows that increasing the adsorbent dosage leads to a rapid rise in adsorption efficiency, which eventually plateaus. The optimal dosage was determined to be 50 mg of adsorbent for 50 mL of 100 mg/L Ga(III) solution, achieving a high adsorption efficiency of 99.00% and an adsorption capacity of 93.00 mg/g. The plateau effect at higher dosages is attributed to the saturation of available Ga(III) ions in the solution relative to the increased number of adsorption sites. As shown in Figure 4c, the adsorption capacity increases rapidly with the initial Ga(III) concentration up to 100 mg/L, beyond which the increase slows down, indicating a transition from kinetic control to equilibrium-limited adsorption [39]. At this optimal concentration, the adsorption capacity reaches 82 mg/g, and the adsorption efficiency remains high at 90.00%. This behavior suggests that 100 mg/L provides sufficient driving force for mass transfer and effective occupation of active sites. The effect of contact time is presented in Figure 4d. At 0.25 h, the adsorption capacity is relatively low, indicating insufficient interaction time for Ga(III) to bind to the active sites. Between 0.5 h and 2.5 h, the adsorption capacity increases markedly, reflecting the progressive occupation of imprinted sites by Ga(III) ions. After 2.5 h, the adsorption capacity levels off, indicating that the system has reached equilibrium. Therefore, 2.5 h is identified as the optimal adsorption time, ensuring maximum uptake of Ga(III) by the imprinted polymer.

3.3.2. Adsorption Kinetics

The adsorption kinetics of Ga(III) onto CS/(H-CGCS)-Ga-IIP were analyzed using pseudo-first-order and pseudo-second-order kinetic models. The corresponding fitting curves are illustrated in Figure 5a,b, and the calculated kinetic parameters are summarized in Table S2.

Both models exhibit high correlation coefficients (R² > 0.9), suggesting that the adsorption process involves contributions from both physical and chemical adsorption mechanisms. Specifically, the pseudo-first-order model yields an R² of 0.9455, whereas the pseudo-second-order model provides a significantly better fit with an R² of 0.9993, indicating that chemisorption is the predominant mechanism governing Ga(III) uptake. The equilibrium adsorption capacity (Q_e) calculated from the pseudo-first-order model (62.16 mg/g) shows a considerable deviation from the experimental value (95.58 mg/g), implying that this model fails to adequately represent the actual adsorption process [40]. In contrast, the pseudo-second-order model predicts a Q_e of 100.10 mg/g, which is in excellent agreement with the experimental result, further validating its applicability. Additionally, the rate constant (K²) for the pseudo-second-order model was determined to be 0.07 g·mg⁻¹·min⁻¹, indicating a relatively fast adsorption rate and strong affinity between Ga(III) and the functional sites of the imprinted polymer. This supports the hypothesis that chemical interactions—such as ligand exchange, coordination, or chelation between Ga(III) and active functional groups (e.g., -OH, -NH₂)—play a crucial role in the adsorption process.

Overall, these findings suggest that the adsorption of Ga(III) onto CS/(H-CGCS)-Ga-IIP is best described by the pseudo-second-order kinetic model, reflecting the dominance of surface chemical interactions and providing a theoretical basis for further mechanistic investigations and adsorption system modeling.

3.3.3. Adsorption Isotherms

To evaluate the equilibrium characteristics of Ga(III) adsorption onto CS/(H-CGCS)-Ga-IIP, the experimental data were fitted to the Langmuir and Freundlich isotherm models, as shown in Figure 5c,d, and the corresponding model parameters are summarized in Table S4. Both models exhibit high correlation coefficients (R² = 0.884–0.996) under the tested temperature conditions (25 °C, 35 °C, and 45 °C), suggesting that the adsorption process involves a synergistic mechanism of monolayer adsorption on homogeneous surfaces and multilayer adsorption on heterogeneous surfaces. Among the two, the Langmuir model demonstrates a superior fit, with R² values ranging from 0.987 to 0.996, compared to 0.884 to 0.982 for the Freundlich model [41]. This indicates that monolayer adsorption dominates, aligning with the uniform distribution of specific binding sites designed by the ion-imprinting strategy.

The Langmuir maximum adsorption capacity (Qₘ) increases with temperature—from 135.32 mg/g at 298.15 K to 145.14 mg/g at 318.15 K—indicating that elevated temperatures enhance the adsorbent’s capacity for Ga(III). Similarly, the Langmuir affinity constant (b) increases from 0.034 to 0.081 L/mg, implying that higher temperatures strengthen the interaction between Ga(III) and the active binding sites on the polymer surface. The separation factor (R_L) values at all temperatures fall within the range of 0.051–0.113, confirming the favorable nature of the adsorption process [42].

The Freundlich model further supports these findings: the adsorption intensity factor (n) remains greater than 1 at all tested temperatures (7.656–10.839), indicating favorable and preferential adsorption. The Freundlich constant (K_F) also increases significantly with temperature, from 56.779 to 81.742 mg/g·(L/mg)1/n, corroborating the endothermic nature of the adsorption process.

From a mechanistic perspective, the excellent fit with the Langmuir model suggests that Ga(III) ions interact with a monolayer of uniformly distributed and energetically equivalent binding sites on the imprinted polymer. The partial applicability of the Freundlich model, however, indicates the possible existence of heterogeneous adsorption sites, which may arise from slight variations in the polymer network structure or the presence of residual non-imprinted domains [43]. The observed enhancement of adsorption performance with increasing temperature further confirms the endothermic and chemisorptive characteristics of the Ga(III) uptake process.

Overall, the adsorption of Ga(III) by CS/(H-CGCS)-Ga-IIP follows primarily Langmuir-type monolayer adsorption with partial heterogeneous contributions and is significantly promoted by elevated temperature. These findings provide critical insights into the thermodynamic behavior of the system and lay a solid theoretical foundation for optimizing its performance in practical Ga(III) separation applications.

3.3.4. Adsorption Thermodynamics

To further elucidate the thermodynamic nature of Ga(III) adsorption onto CS/(H-CGCS)-Ga-IIP, the Van’t Hoff plot was constructed by plotting lnKc versus 1/T (Figure 6a), and the derived thermodynamic parameters are summarized in Table S3. The standard Gibbs free energy change (ΔG) values at 298.15 K, 308.15 K, and 318.15 K are −3.730, −4.384, and −6.933 kJ/mol, respectively. The consistently negative ΔG values confirm that the adsorption process is spontaneous under all tested temperatures. Moreover, the magnitude of ΔG increases (i.e., becomes more negative) with rising temperature, indicating that elevated temperatures promote the adsorption process, further corroborating the feasibility and temperature-dependence of Ga(III) binding [44].

The calculated enthalpy change (ΔH) is +6.614 kJ/mol, suggesting that the adsorption process is endothermic. This finding is consistent with the conclusions drawn from isotherm analyses, where increased temperature enhanced adsorption capacity. The endothermic nature is likely attributed to chemisorption via coordination interactions between Ga(III) ions and electron-donating functional groups (e.g., -OH, -NH₂) on the polymer matrix [45], which require heat input to proceed.

The positive entropy change (ΔS = +43.71 J/(K·mol)) reflects an increase in system disorder during the adsorption process. This can be rationalized by the partial release of hydration water molecules from Ga(III) as it transfers from the aqueous phase to the adsorbent surface, as well as potential structural rearrangements on the polymer interface that facilitate metal ion complexation.

From a mechanistic perspective, the thermodynamic analysis reveals three key features:

(1)

ΔG < 0 and decreases with temperature → adsorption is spontaneous and thermodynamically favored at higher temperatures.

(2)

ΔH > 0 → the adsorption is endothermic, driven by specific interactions requiring energy input.

(3)

ΔS > 0 → increased entropy results from dehydration of Ga(III) ions and structural disordering at the interface.

These findings clearly demonstrate that the adsorption of Ga(III) by CS/(H-CGCS)-Ga-IIP is a spontaneous, endothermic process with enhanced performance at elevated temperatures [46]. The thermodynamic parameters provide a strong theoretical basis for the application of this material in Ga(III) separation and resource recovery from dilute aqueous solutions.

3.3.5. Simulation of Competitive Adsorption in the Bayer Mother Liquor System

To evaluate the practical selectivity of CS/(H-CGCS)-Ga-IIP in complex industrial environments, competitive adsorption experiments were conducted using simulated Bayer mother liquor containing Ga(III), Al(III), and V(V). The results are illustrated in Figure 6b, with the corresponding parameters summarized in

Table 3. Competitive adsorption parameters of Ga(III) vs. Al(III) and V(V).

System	Kd-Ga (L/g)	Kd-M (L/g)	Kd-M (L/g)
Ga/V	0.4853	0.0051	1.98
Ga/Al		0.0033	2.17

.

The distribution coefficient ratio (k_d-Ga/k_d-V) is calculated to be 95.1, while k_d-Ga/k_d-Al reaches 146.9, indicating that CS/(H-CGCS)-Ga-IIP exhibits exceptionally high selectivity for Ga(III) over both V(V) and Al(III). This superior selectivity is primarily attributed to the specific coordination interactions between Ga(III) ions and the functional groups on the polymer surface (such as -OH and -NH₂), which are structurally and electronically more favorable for Ga(III) than for its competing ions [47].

In terms of selectivity coefficients (Sel(Ga/M)), the values were determined to be 2.17 for Ga/Al and 1.98 for Ga/V, demonstrating the material’s effective capability to discriminate Ga(III) from coexisting ions commonly found in Bayer process liquors. This level of selective adsorption is particularly significant given the notoriously challenging separation of Ga(III) from chemically similar species like Al(III) under strongly alkaline conditions [48].

The experimental data highlight the practical potential of CS/(H-CGCS)-Ga-IIP for targeted Ga(III) separation from high-background, multi-component industrial systems such as Bayer mother liquors. The favorable selectivity coefficients and distribution ratios further support its feasibility for use in large-scale resource recovery processes.

3.3.6. Study on Recycling Performance

To evaluate the reusability and regeneration potential of the CS/(H-CGCS)-Ga-IIP adsorbent, five consecutive adsorption–desorption cycles were conducted under identical conditions. Specifically, 50 mg of the adsorbent was used to treat 50 mL of Ga(III) solution (100 mg/L, pH = 9), followed by desorption with 100 mL of 0.5 mol/L HCl. The results of the cyclic tests are shown in Figure 6c. In the first adsorption cycle, the material exhibited an initial adsorption capacity of 97.70 mg/g and an adsorption efficiency of 90%. However, a noticeable decline was observed in subsequent cycles. After the first cycle, the adsorption capacity dropped to 77.07 mg/g with an efficiency of 71%. By the second cycle, these values further decreased to 71.64 mg/g and 66%, respectively. A relatively stable performance was observed in the third cycle with values of 70.56 mg/g and 65%, suggesting the adsorbent retained a reasonable level of functionality through the initial three cycles [49].

However, beginning from the fourth cycle, a substantial drop in performance was noted. The adsorption capacity and efficiency sharply decreased to 32.57 mg/g and 30%, respectively. After the fifth cycle, only a slight recovery was observed, with the capacity and efficiency rising marginally to 33.65 mg/g and 31% but still significantly lower than the original values.

These findings indicate that CS/(H-CGCS)-Ga-IIP demonstrates satisfactory reusability during the first three cycles, with moderate losses in performance. The observed stability is likely due to the robustness of the cross-linked structure and retention of most active binding sites. In contrast, the substantial degradation after the third cycle may result from irreversible fouling, partial collapse of the polymeric network, or exhaustion of key functional groups due to repetitive chemical stress from the acidic desorption process [50]. The minor recovery observed in the fifth cycle may suggest partial reactivation of adsorption sites, but the overall regeneration efficiency remains low. These results point to the need for further optimization of the desorption protocol or the structural modification of the adsorbent to enhance long-term cyclic durability.

In summary, CS/(H-CGCS)-Ga-IIP exhibits promising short-term reusability with stable performance over three cycles, but its efficiency significantly diminishes after the fourth cycle. To ensure long-term viability for Ga(III) separation in complex industrial waste streams, improved regeneration strategies or material reinforcement will be essential.

3.4. Adsorption Mechanisms

To elucidate the adsorption mechanism of Ga(III) by CS/(H-CGCS)-Ga-IIP, a series of characterization techniques including SEM-EDS, FTIR, and XPS were employed. The results are presented in Figure 7a–k and

Table 4. EDS elemental analysis table for CS/(H-CGCS)-Ga-IIP after adsorption.

Element	N	O	Si	Ga
wt%	3.39	38.14	0.22	58.26

.

As shown in Figure 7a–e, SEM images reveal that the surface morphology of CS/(H-CGCS)-Ga-IIP becomes visibly rough and granular after Ga(III) adsorption, suggesting successful binding and potential formation of new surface coordination structures. EDS mapping confirms that the key elements N, O, Si, and Ga are uniformly distributed across the adsorbent surface, indicating homogeneous incorporation of Ga(III). Furthermore, the EDS quantitative analysis (Table 4) reveals that after adsorption saturation, the gallium content of the adsorbent was 58.26 wt%, far exceeding the proportions of N (3.39%), O (38.14%), and Si (0.22%), providing strong evidence of substantial Ga(III) uptake [51].

FTIR (Figure 7f) was employed to identify changes in surface functional groups before and after Ga(III) adsorption. The broad peak at 3428 cm⁻¹ corresponds to O–H stretching vibrations, while peaks at 2928 cm⁻¹ and 1455 cm⁻¹ are attributed to C–H vibrations. The absorption at 1648 cm⁻¹ arises from both C=O stretching and N–H bending vibrations. The peaks at 1039 cm⁻¹ and 624 cm⁻¹ correspond to C–O/Si–O–Si and Ga–O vibrations, respectively. Notably, the post-adsorption spectrum shows an intensified O–H signal and the emergence of a distinct Ga-O peak at 624 cm⁻¹, indicating that Ga(OH)₄⁻ species have interacted with surface hydroxyl groups and successfully coordinated with the adsorbent [52].

XPS analysis further confirms the chemical interactions between Ga(III) and the adsorbent. As depicted in Figure 7g–k, the survey spectrum reveals the characteristic Ga2p3 peak, indicating successful Ga(III) adsorption. In the N 1 s high-resolution spectrum, the peak at 401.30 eV, assigned to -NH₃⁺, decreases in intensity after adsorption, implying coordination between Ga(III) and amino groups. Similarly, in the O 1 s spectrum, a rightward shift and the appearance of a new peak around 531.00 eV (attributed to O-Ga bonds) are observed, accompanied by a weakened -OH signal, suggesting that hydroxyl groups are also involved in Ga(III) binding [53].

Based on the above analyses, the adsorption mechanism of CS/(H-CGCS)-Ga-IIP for Ga(III) can be summarized as follows (Figure 8):

(1)

lectrostatic Interaction: under slightly alkaline conditions (pH 9), positively charged −NH₃⁺ groups on chitosan can electrostatically attract negatively charged Ga(OH)₄⁻ species.

(2)

Coordination and Dehydration Reaction: Ga(OH)₄⁻ undergoes dehydration and forms stable coordination complexes with -OH and -NH₂ groups on the polymer matrix, enhancing binding strength and selectivity.

(3)

Structural Stability: the siloxane (Si–O–Si) network of H-CGCS remains chemically stable during adsorption, providing mechanical integrity and long-term durability to the adsorbent.

In summary, SEM-EDS, FTIR, and XPS analyses collectively demonstrate that the adsorption of Ga(III) onto CS/(H-CGCS)-Ga-IIP involves synergistic mechanisms including electrostatic attraction, coordination complexation, and structural retention. The amino and hydroxyl groups serve as key active sites for Ga(III) binding, while the siloxane framework contributes to the overall mechanical and chemical stability. These insights provide strong theoretical support for the application of CS/(H-CGCS)-Ga-IIP in the selective separation and recovery of Ga(III) from complex aqueous media such as Bayer process waste streams.

Figure 8. Schematic diagram of adsorption mechanism.

Figure 8. Schematic diagram of adsorption mechanism.

4. Conclusions

In this study, a novel ion-imprinted CS-based adsorbent (CS/(H-CGCS)-Ga-IIP) was successfully fabricated using a hybrid cross-linking strategy combining glutaraldehyde and siloxane-modified CS. The prepared material demonstrated excellent selective adsorption performance for Ga(III) from alkaline aqueous systems, particularly simulated Bayer mother liquor, where aluminum and vanadium coexist as major interfering ions. The optimized adsorbent exhibited a maximum Ga(III) adsorption capacity of 106.31 mg·g⁻¹, with fast adsorption kinetics reaching equilibrium within 60 min. The adsorption process followed the pseudo-second-order kinetic model and fit well with the Langmuir isotherm, indicating monolayer chemisorption behavior. Thermodynamic analysis revealed that the adsorption is spontaneous and endothermic, with increasing temperature favoring Ga(III) uptake. Notably, in the simulated Bayer system, the adsorbent displayed high selectivity factors and a distribution coefficient ratio k_d-Ga/k_d-Al = 146.9, underscoring its strong recognition and discrimination ability for Ga(III) in complex alkaline matrices. Structural characterizations (SEM-EDS, FTIR, and XPS) confirmed that Ga(III) was captured via electrostatic attraction, ligand coordination with amino and hydroxyl groups, and structural retention by the siloxane network, enabling excellent stability and reusability. Overall, this work offers a sustainable and effective strategy for the selective recovery of Ga(III) from complex industrial alkaline waste streams. The synergistic design of ion-imprinted sites and a siloxane-reinforced chitosan network provides both high adsorption efficiency and operational robustness, making CS/(H-CGCS)-Ga-IIP a promising candidate for application in resource recovery from Bayer process residues and other Ga-containing secondary resources.