The Sustainable Remediation of Antimony(III)-Contaminated Water Using Iron and Manganese-Modified Graphene Oxide–Chitosan Composites: A Comparative Study of Kinetic and Isotherm Models

Abstract

This study introduces a series of Fe/Mn-GOCS composites using high-temperature impregnation with graphene oxide and chitosan as substrates, modified by diverse manganese salts, including MnCl₂∙4H₂O, KMnO₄, and MnSO₄. Among these, FeCl₂/MnSO₄-GOCS demonstrated the highest adsorption capacity for Sb(III), peaking at 57.69 mg/g. The adsorption performance was extensively evaluated under various conditions, such as different initial concentrations, pH levels, solid–liquid ratios, and adsorption durations. It was observed that when the Fe/Mn molar ratio exceeded 4:1, there was a notable decrease in both the adsorption capacity and removal rate. Kinetic analyses using the pseudo-second-order model revealed a better fit (R² > 0.99) compared to the pseudo-first-order model, indicating that chemisorption dominated the adsorption process. Additionally, isothermal modeling highlighted the efficiency of Fe/Mn-GOCS, particularly in high-concentration environments, with the Sips model demonstrating the best fit, integrating characteristics of both Langmuir and Freundlich models. These results not only offer a robust theoretical and practical basis for efficient Sb(III) removal but also underscore the potential of multi-metal-modified adsorbents as sustainable solutions for environmental remediation.

1. Introduction

With the development of industrialization, antimony (Sb) is widely used as a flame retardant, in battery materials, and as an additive. However, large amounts of Sb leaking into the environment due to illegal emissions can cause serious ecological damage and environmental pollution [1]. Sb is mainly present in the environment in trivalent (Sb(III)) and pentavalent (Sb(V)) forms, and Sb(III) poses a greater threat to human health and ecosystems due to its higher bioavailability and toxicity [2]. Among the many methods for removing pollutants, adsorption technology is widely used in water treatment due to its simplicity, efficiency, and cost-effectiveness [3]. Conventional adsorbent materials are usually inefficient in removing Sb(III) from water [4,5]. While materials such as activated carbon, silica gel, and certain natural minerals perform well in removing some common pollutants, they often perform poorly in treating antimony-containing wastewater due to the strong hydration stability and low reactivity of Sb(III) [6,7,8]. The unique chemical properties of Sb(III) ions make them difficult to effectively capture and immobilize with conventional adsorbents, resulting in low adsorption capacity and removal efficiency [9]. Additionally, conventional adsorbent materials may require longer contact time and higher dosages when treating large volumes of wastewater, further limiting their practical application [10]. Therefore, there is an urgent need to develop new and efficient adsorbent materials for the more effective removal of Sb(III) from water.

Graphene oxide (GO) facilitates chemical modification due to its high specific surface area, unique nanostructure, and abundant surface functional groups, making it an ideal substrate for adsorption materials [11]. As a natural biopolymer, chitosan (CS) contains amino and hydroxyl groups that can form stable chelates with heavy metal ions, significantly improving the adsorption capacity and selectivity of the composite [12]. The introduction of iron further optimizes the properties of the composite, providing additional adsorption sites and enhancing the affinity for heavy metals through surface coordination and ion exchange mechanisms [13]. Iron-modified graphene oxide chitosan (Fe-GOCS) composites attract extensive attention due to their high specific surface area and excellent electron conductivity [14,15,16]. Shan et al. [16] confirm that the adsorption capacity of GO chitosan-based materials for Cr(VI) is significantly improved by the introduction of iron. Compared with GOCS, the adsorption capacity of Fe-GOCS for Cr(VI) increases by nearly 30 mg/g [16]. Recent studies also show that the selectivity and adsorption capacity of adsorbent materials can be significantly improved by introducing a variety of metal ions [17,18,19]. Manganese (Mn) is a polyvalent transition metal capable of forming various oxidation states, making it particularly effective in improving the properties of adsorbent materials [20]. For example, Shan et al. [21] have prepared Fe/Mn-GOCS microbeads by simultaneously encapsulating FeO_x and MnO_x into GOCS using an embedding method, showing excellent performance in As(III) adsorption experiments, with a maximum adsorption capacity of 108.89 mg/g at 25 °C. Nevertheless, relatively few studies have investigated the loading of metal ions onto GOCS using high-temperature impregnation methods. Most current studies still rely on conventional room-temperature or low-temperature impregnation techniques, which do not fully exploit the chemical reactions that may occur between the metal ions and the substrate under high-temperature conditions [1,22,23].

Previous studies discussing the adsorption effects of different media on heavy metals and the improvement of adsorption performance through various modification techniques have received extensive attention [24,25]. However, the analysis of adsorption data in these studies is often limited to one or two classical adsorption models, such as the Langmuir model and the Freundlich model. Although these models play an important role in explaining adsorption behaviors, their limited scope of application often fails to fully reflect the multiple mechanisms of action in complex adsorption systems. In adsorption technology research, relying solely on experience and a single adsorption model makes it difficult to fully reveal the adsorption mechanism and properties of materials [26]. Therefore, multiple adsorption models and comprehensive analytical methods are used for systematic studies. The Temkin model considers the interaction energy between adsorbent and adsorbate, making it suitable for high-concentration adsorbent studies [27]. The Dubinin–Radushkevich (D-R) model describes the non-uniform energy distribution of adsorption in porous media, suitable for analyzing energy changes and mechanisms in the adsorption process [28]. Multi-parameter models such as the Redlich–Peterson model and the Sips model are introduced to better describe the complex behavior of real adsorption systems [29,30]. For example, Ahmad et al. [31] have evaluated the adsorption performance of different biochars on trichloroethylene (TCE) using Freundlich, Langmuir, Temkin, and Dubinin–Radushkevich adsorption models, finding that the Temkin and Dubinin–Radushkevich models best described TCE adsorption, indicating pore filling as the main adsorption mechanism.

Commonly used models in adsorption kinetic studies include the pseudo-first-order and pseudo-second-order kinetic models. The Elovich model and Avrami model are also widely used to describe different adsorption processes. The pseudo-second-order and pseudo-first-order kinetic models analyze the adsorption rate over time, and comparing their fitting effects reveals the rate-controlling steps and mechanisms in the adsorption process more accurately [32]. The Elovich model describes chemical adsorption, especially when active sites on the surface gradually decrease. The Avrami model describes multistep adsorption processes in complex systems [33]. The Weber–Morris intraparticle diffusion model is used to analyze diffusion mechanisms, assuming the adsorption process includes external diffusion, internal diffusion, and surface adsorption reactions [33]. Fitting experimental data to this model can determine rate-controlling steps. For example, Bi et al. [34] have analyzed the adsorption capacity and mechanism of amorphous nano-alumina (nano-Al₂O₃) on AsO₄³⁻ ions in an aqueous solution using the Weber–Morris intraparticle diffusion model and Boyd’s model. It is demonstrated that the adsorption process of AsO₄³⁻ ions by nano-Al₂O₃ is controlled by both membrane and internal diffusion, with the adsorption rate determined by membrane diffusion. Using various models and methods allows for a comprehensive understanding of the kinetic and thermodynamic properties of the adsorption process, providing a theoretical basis and technical support for developing efficient adsorption materials.

Therefore, in this study, graphene oxide and chitosan (GOCS) are used as carrier support, and FeO_x and MnO_x are loaded onto GOCS by high-temperature impregnation to obtain a new type of recoverable particle adsorbent, which is applied to the removal of Sb(III) in aqueous solution. A batch experiment is conducted to study the effects of Mn salt type, Fe/Mn molar ratio, and other experimental parameters on Sb(III) removal. At the same time, a variety of kinetic models and isothermal adsorption model data are fitted to determine the applicability of each model, and the adsorption process of the Fe/Mn-GOCS composite is analyzed.

2. Materials and Methods

FeCl₂∙4H₂O, MnCl₂∙4H₂O, KMnO₄, MnSO₄, HCl, and NaOH, all of the analytical grades, were purchased from Xilong Technology Co., Ltd. (Shanghai, China). C₈H₄K₂O₁₂Sb₂∙3H₂O (Sb(III)) was purchased from Shanghai McLean Biochemistry & Science Co. (Shanghai, China). Graphene oxide was purchased from Jiangsu Suzhou Carbonfund Graphene Technology Co. (Suzhou, China), and chitosan was purchased from Xilong Chemical Co. Deionized water (18.2 mΩ∙cm) prepared using a Milli-Q water system (Millipore, St. Louis, MO, USA) was used throughout the study. A 1000 mg/L stock solution of Sb(III) was prepared by dissolving C₈H₄K₂O₁₂Sb₂∙3H₂O in deionized water and was then diluted to the required concentration for batch experiments.

2.1. Sample Preparation

Based on our previous studies, GOCS loaded with Fe and Mn were synthesized by high-temperature impregnation, with some adjustments as follows [16]: 0.4 g of GO was poured into 100 mL of 1.5% v/v acetic acid solution and ultrasonically stirred for 30 min to make a homogeneous mixture. Then, 2.0 g of CS was added and ultrasonically stirred until it was completely dissolved, obtaining the GO/CS mixture. The mixture was obtained by dropping it into 500 mL of 7% NaOH solution and left to form GOCS composite microspheres for 24 h. The NaOH solution was filtered, and the spheres were washed with deionized water until the washing solution was nearly neutral. The microspheres were then placed in 200 mL of 5% v/v glutaraldehyde–methanol mixture for 6 h. After this, the particles were washed several times with ethanol and deionized water and then dried to obtain GO/CS composite particles. Subsequently, the microspheres were placed in 50 mL of 0.1 mol/L FeCl₂∙4H₂O solution, heated, and evaporated to dryness at 300 °C on a graphite hot plate. After cooling, they were washed with deionized water to remove excess iron salts and dried at 45 °C to obtain Fe-GOCS composite spheres. Next, the spheres were placed in 50 mL of 0.025 mol/L MnSO₄ solution and heated to evaporation at 300 °C on the graphite hot plate. After cooling, they were washed with deionized water to remove excess Mn salts and dried at 45 °C to obtain Fe/Mn-GOCS composite spheres.

2.2. Batch Adsorption Experiments

Fe/Mn-GOCS composite microspheres were added to 10 mg/L Sb(III) solution at a mass-to-volume ratio (m/v) of 1 g/L. After 48 h of reaction at 25 °C and pH 4.0, the supernatant was collected to determine the Sb(III) concentration. Additionally, MnO_x-modified Fe-GOCS composite samples prepared using different manganese salt species (KMnO₄, MnO₂, and MnCl₂) were used for Sb(III) removal experiments according to the above procedure. Among the MnO_x-modified Fe-GOCS composite microspheres, the ones with the best Sb(III) removal performance were selected for batch adsorption experiments to further determine the effects of initial Sb(III) concentration (5–300 mg/L), pH (3.0–11.0), mass-to-volume ratio (m/v) (0.4–2.0 g/L), contact time (5–3840 min), temperature (25 °C, 35 °C, 45 °C), and coexisting ions on adsorption. On this basis, kinetic and isothermal experiments were carried out at the optimum reaction conditions of pH and m/v. The above experiments were conducted in triplicate in a constant temperature water bath shaker at 150 rpm. The removal efficiency of Sb(III) (R_e, %), equilibrium adsorption (Q_e, mg/g), and time t (Q_t, mg/g) were calculated using the following equations, Equations (1) and (2).

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

(1)

$$Q_e={\displaystyle \frac{C_0-C_e}{m}}\times V$$

(2)

where R_e represents the removal efficiency (%) of the material for Sb(III) at adsorption equilibrium, C₀ is the initial concentration of Sb(III) (mg/L), C_e is the concentration of Sb(III) at adsorption equilibrium (mg/L), Q_e is the adsorption capacity (mg/g), V is the volume of the solution containing Sb(III) (L), and m is the mass of the adsorbent (g).

2.3. Analytical Techniques

The concentration of Sb(III) in an aqueous solution was determined with an inductively coupled plasma optical emission spectrometer (Optima 7000DV, Platinum Elmer Instruments, Inc., Waltham, MA, USA). Surface morphology and elemental analyses of Fe/Mn-GOCS were determined with JSM-7900F SEM-EDS (JEOL, Tokyo, Japan). The IS10 FTIR spectrometer (Thermo Fisher, Waltham, MA, USA) was used to determine the functional groups of Fe/Mn-GOCS. The crystal structure of Fe/Mn-GOCS was determined with X’Pert3 powdered multifunctional XRD (Panaco, Vaassen, The Netherlands, copper target, λ = 1.54056 Å). The scanning step, speed, and range were 0.02626°, 0.6565°/s, and 5–90° (2θ), respectively.

3. Results

3.1. Sb(III) Removal by Different MnOx Decorated Fe-GOCS

To investigate the effect of Fe-GOCS modified with different Mn salts on the adsorption of Sb(III), the content of FeCl₂∙4H₂O is fixed at 0.1 mol, and different Mn salts are added according to an Fe/Mn molar ratio of 1:1. Four composites, FeCl₂/MnO₂-GOCS, FeCl₂/KMnO₄-GOCS, FeCl₂/MnSO₄-GOCS, and FeCl₂/MnCl₂-GOCS, are prepared to investigate their adsorption properties of Sb(III). The results are shown in Figure 1a. It is found that the removal efficiency(R_e) of FeCl₂/MnO₂-GOCS on Sb(III) is only about 19%, while FeCl₂/KMnO₄-GOCS and FeCl₂/MnCl₂-GOCS have a removal rate of about 25%, and FeCl₂/MnSO₄-GOCS has R_e values of up to 28%. It can be seen that FeCl₂/MnSO₄-GOCS has the highest removal efficiency among the four materials, showing significant advantages. Therefore, FeCl₂/MnSO₄-GOCS is selected for subsequent experimental studies and named Fe/Mn-GOCS.

3.2. Molar Ratio of Fe/Mn

To investigate the effect of the ratio between FeCl₂∙4H₂O and MnSO₄ on the adsorption of Sb(III) by Fe/Mn-GOCS, the composite spheres with different Fe/Mn molar ratios are prepared for static adsorption experiments, and the results are shown in Figure 1b. It is found that the adsorption capacity (Q_e) of Sb(III) by Fe/Mn-GOCS is 2.00 mg/g, 2.43 mg/g, 2.90 mg/g, 2.80 mg/g, and 2.70 mg/g when the Fe/Mn molar ratios are 1:1, 2:1, 4:1, 6:1, and 10:1, respectively. The removal efficiency (R_e) is 18.51%, 22.54%, 26.93%, 25.84%, and 25.13%, respectively. This phenomenon indicates that different ratios of Fe/Mn loaded in the Fe/Mn-GOCS composite spheres have varying adsorption effects on Sb(III). The Q_e values and R_e values decrease when MnSO₄ is added beyond the Fe/Mn ratio of 4:1. The possible reason is that excessive Fe loading blocks the surface and internal channels of spheres, deteriorating its structure [35]. Therefore, Fe/Mn-GOCS composite spheres with an Fe/Mn molar ratio of 4:1 are selected for subsequent experimental studies, considering the adsorption capacity and removal efficiency of Sb(III).

3.3. Characterization

3.3.1. XRD

The XRD pattern of Fe/Mn-GOCS (Figure 2a) lacks sharp characteristic peaks, consistent with Li et al. [36], suggesting that Fe/Mn-GOCS synthesized by high-temperature impregnation is amorphous. The broad peak observed at 20–25° is attributed to the complexation of GOCS [21]. Furthermore, no new characteristic peaks appear in the XRD pattern of Fe/Mn-GOCS after Sb(III) adsorption. This might be due to the minimal crystalline material produced during the removal process, rendering Sb compounds undetectable [36].

3.3.2. FTIR

Figure 2b shows the FTIR spectra of Fe/Mn-GOCS before and after Sb(III) adsorption, highlighting the changes in its functional groups and confirming Sb(III) adsorption by Fe/Mn-GOCS. Before Sb(III) adsorption, major peaks of Fe/Mn-GOCS appear near 3398, 1699, 1651, 1515, and 1041 cm⁻¹, corresponding to –OH, C=O, N–H, C=N, and C–O stretching vibrations, respectively [36,37]. The peak at 1457 cm⁻¹ is attributed to –OH deformation vibrations, consistent with earlier reports. Additionally, peaks at 508 and 598 cm⁻¹ represent Mn–O and Fe–O stretching vibrations, respectively [38,39]. After Sb(III) adsorption, the FTIR spectrum of Fe/Mn-GOCS shows a new peak at 668 cm⁻¹ corresponding to the Sb–O stretching vibration reported in other studies, indicating that Sb species are adsorbed on the Fe/Mn-GOCS surface [40,41]. Furthermore, the characteristic peaks of –OH, Mn–O, and Fe–O at 1457, 508, and 598 cm⁻¹, respectively, become weaker, while the intensity of the C–O peak at 1041 cm⁻¹ increases, implying potential coordination between Sb(III) on the Fe/Mn-GOCS surface and the –OH, C–O, Fe–O, and Mn–O groups [25].

3.3.3. SEM

The SEM images of Fe/Mn-GOCS before and after the adsorption of Sb(III) are shown in Figure 2c. Before the adsorption of Sb(III), the surface of Fe/Mn-GOCS shows rough features and the presence of many agglomerates of white particles, which indicate that the surface structure of the adsorbent is more complex and may be related to the particles formed during the synthesis process or the residues of precursors. However, after the adsorption of Sb(III), the surface morphology of the adsorbent changes significantly and becomes smoother, and the aggregation of particles is reduced. This suggests that the Sb(III) ions interact effectively with the surface active sites, possibly through chemical or physical adsorption, leading to the coating of the surface particles and the formation of a more homogeneous and dense structure. This increase in surface smoothness indicates the successful adsorption of Sb(III) on the Fe/Mn-GOCS surface.

3.3.4. XPS

To further confirm the loading of Fe/Mn-GOCS and its adsorption efficiency for Sb(III), the XPS full spectra and Sb 3d fine spectra of the adsorbent before and after Sb(III) adsorption are evaluated (Figure 2d,e). In the full spectrum of Fe/Mn-GOCS, characteristic peaks corresponding to Fe 3d and Mn 2p are clearly identified, verifying the successful incorporation of Fe and Mn onto the surface of GOCS. After Sb(III) adsorption, the presence of Sb in the spectrum further confirms the adsorption of Sb(III) on Fe/Mn-GOCS (Figure 2e). Only the characteristic peaks of Sb(V) are detected at 539.93 and 530.63 eV, indicating that the adsorbed Sb(III) is completely oxidized to Sb(V) on Fe/Mn-GOCS, reflecting the oxidative role of Mn in the adsorption process.

3.4. Influencing Factors

3.4.1. Effect of Initial Sb(III) Concentration

Figure 3a shows the variation in equilibrium adsorption (Q_e) and removal efficiency (R_e) for Fe/Mn-GOCS with different initial Sb(III) concentrations (C₀). The figure shows that the R_e values gradually decrease from 29.56% to 14.27% as the initial concentration (C₀) increases. In contrast, the Q_e values gradually increase from 1.82 mg/g to 43.17 mg/g, showing that the adsorption has not reached equilibrium. The Q_e values continue to increase with C₀, indicating that Fe/Mn-GOCS has a strong ability to adsorb Sb(III). Overall, the adsorption of Sb(III) by Fe/Mn-GOCS increases with C₀, but the removal efficiency decreases. When the mass of Fe/Mn-GOCS is fixed, the number of active adsorption sites is also fixed. Most of the Sb(III) can bind to the active sites at low concentrations, resulting in a high removal efficiency and a low adsorption amount. As the concentration increases, the active sites become saturated, causing the efficiency rate to decrease while the adsorption amount increases [42].

3.4.2. Effect of pH

The initial pH of the solution can affect the existence form of Sb(III) in the water body, thus affecting the adsorption of Sb(III) by Fe/Mn-GOCS. Figure 3b shows the effect of pH (3–11) on the adsorption of Sb(III) by Fe/Mn-GOCS. The results show that the removal efficiency (R_e) gradually decreases from 31.78% to 12.87% and the equilibrium adsorption (Q_e) decreases from 3.43 mg/g to 1.35 mg/g as the initial pH of the solution increases, indicating that higher pH levels have a greater effect on the adsorption of Sb(III) by Fe/Mn-GOCS. It is observed that Fe/Mn-GOCS has a strong removal capacity for Sb(III) at pH = 3–4. The R_e values rapidly decrease from 32.36% to 20.76% when the initial pH of the solution increases from 4 to 5, and R_e shows a slight decreasing trend with the increase of pH from 5 to 11. This is contrary to the conclusion of Cheng et al. [43]; that is, the adsorption of Sb(III) is not affected in a wide pH range but is similar to the reported change trend of Sb(V) [44]. This may be attributed to the species transformation of Sb(III) into Sb(V) on the surface of Fe/Mn-GOCS because it is found that the adsorbed Sb on the composite is entirely pentavalent (Section 3.3). Considering the solubility of Fe and Mn in the aqueous solution, subsequent experiments are carried out at pH = 4.

3.4.3. Effect of m/v

To investigate the effect of the mass-to-volume ratio (m/v) on the adsorption of Sb(III) by Fe/Mn-GOCS, static adsorption experiments are set up with the mass of the Fe/Mn-GOCS material and the volume of the Sb(III) solution at ratios of 0.25, 0.5, 0.75, 1, 1.25, and 1.5 g/L. The results are shown in Figure 3c. The figure shows that the removal efficiency (R_e) increases from 9.34% to 43.50% with the increase in the m/v, while the equilibrium adsorption (Q_e) decreases from 5.31 mg/g to 4.07 mg/g. The volume of the fixed solution is 50 mL, and according to Equations (1) and (2), when the amount of Sb(III) in the solution is fixed, increasing the amount of adsorbent provides more adsorption sites, and R_e gradually increases. Therefore, the adsorption of Sb(III) by Fe/Mn-GOCS can be improved by appropriately increasing the amount of adsorbent. When the m/v is 1.5 g/L, the R_e of Sb(III) is 43.50%, and the Q_e is only 4.07 mg/g, indicating that the adsorption of Sb(III) by Fe/Mn-GOCS has not yet reached saturation under this m/v condition. Studies have shown that the number of available adsorption sites on the surface of the adsorbent increases as the m/v increases, while the amount of Sb(III) in the solution remains constant [25]. This results in an increase in removal efficiency and a decrease in the equilibrium adsorption capacity of Fe/Mn-GOCS for Sb(III). Considering the Q_e values and the R_e values, m/v = 1 g/L is selected as the optimum condition for Sb(III) removal.

3.4.4. Effect of Adsorption Time

As shown in Figure 3d, static adsorption experiments are carried out at intervals ranging from 5 to 3840 min to investigate the effect of adsorption time on the adsorption of Sb(III) by Fe/Mn-GOCS. The figure shows that the removal efficiency (R_e) and adsorption amount (Q_e) gradually increase with the increase in reaction time, reaching adsorption equilibrium at 2280 min. The whole adsorption process can be divided into three stages: initial fast adsorption (0–1020 min), where the adsorbent shows good performance and high efficiency for the rapid removal of Sb(III) from the solution; a slower adsorption rate (1020–2280 min); and finally, a leveling off of the adsorption amount and removal rate as equilibrium is approached. The results show that in the initial stage, many active adsorption sites exist on the surface of the adsorbent, and the solution concentration does not decrease significantly, leading to a rapid increase in the adsorption amount and removal rate. As the reaction proceeds, fewer active adsorption sites remain, and the solution concentration gradually decreases, slowing the growth of the adsorption amount and removal rate. Finally, the active adsorption sites become saturated, and the solution concentration is very low, leading to the saturation of adsorption.

3.4.5. Effect of Coexisting Ions

To study the influence of common cations and anions in groundwater on the Fe/Mn-GOCS adsorption of Sb(III), 10 mM solutions of Cl⁻ (NaCl), HCO_3‾ (NaHCO₃), NO_3‾ (NaNO₃), SO₄²⁻ (Na₂SO₄), H₂PO_4‾ (NaH₂PO₄), Ca²⁺ (CaCl₂), and Mg²⁺ (MgCl₂) are added to Sb(III) solution for adsorption experiments. The results are shown in Figure 3e. The addition of cations Mg²⁺ and Ca²⁺ and anions Cl⁻, NO₃⁻, and PO₄³⁻ to the Sb(III) adsorption system has little influence on the adsorption capacities of 6.64 mg/g, 5.23 mg/g, 8.15 mg/g, 7.91 mg/g, and 7.17 mg/g, respectively. Among them, Cl⁻ has the least effect on the Fe/Mn-GOCS adsorption of Sb(III), possibly due to the weak binding of Cl⁻ to the adsorbent surface through the outer-sphere complex, consistent with the findings of Deng et al. [24]. The presence of SO₄²⁻ and HCO₃⁻ significantly inhibits the adsorption of Sb(III), with adsorption amounts of 2.89 mg/g and 3.11 mg/g, respectively. The presence of cations Ca²⁺ and Mg²⁺ enhances the adsorption of Sb(III), significantly increasing the adsorption capacity. The increased adsorption capacity may be due to the increased ionic strength of the solution caused by these cations. Studies show that increased ionic strength can compress the double electric layer more tightly, correcting or making the surface potential of the adsorbent more negative, ultimately increasing the removal rate [45].

3.5. Adsorption Kinetic Characteristics

The total adsorption rate of the adsorbent can be controlled by one or more steps. To investigate the adsorption rate and behavior of the adsorbent during the adsorption of target pollutants, pseudo-first-order kinetics, pseudo-second-order kinetics, the Elovich model, the Weber–Morris intraparticle diffusion model, and the Boyd kinetic model are applied to simulate the adsorption kinetics of Sb(III) on Fe/Mn-GOCS composites.

3.5.1. Pseudo-First-Order and Pseudo-Second-Order Kinetic Models

The proposed first-order kinetic model (Equation (3)), grounded in membrane diffusion theory, posits that the adsorption process is predominantly governed by physical adsorption [46]. In contrast, the pseudo-second-order kinetic model (Equation (4)) suggests that adsorption involves the sharing or transfer of electron pairs between the adsorbent and sorbate, indicating a process primarily driven by chemisorption [47,48].

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

(3)

$${\displaystyle \frac{t}{Q_t}}={\displaystyle \frac{1}{{k_2{Q}_e}^2}}+{\displaystyle \frac{t}{Q_e}}$$

(4)

where Q_e (mg/g) and Q_t (mg/g) are the amounts of Sb(III) adsorbed by the material at equilibrium and at reaction time t, respectively. K₁ and K₂ are the first-order rate constant and second-order rate constant diffusion coefficients, respectively.

The fitting results are illustrated in Figure 4a, with the corresponding parameters detailed in

Table 1. Pseudo-first-order and pseudo-second-order kinetic model parameters.

Temperature °C	Pseudo-First-Order			Pseudo-Second-Order
	Qe (mg/g)	k1	R2	Qe (mg/g)	k2	R2
25	3.17	1.92 × 10−3	0.9839	4.40	1.48 × 10−3	0.9958

. Compared to the first-order kinetic model (R² > 0.98), the pseudo-second-order kinetic model exhibits a higher coefficient of determination (R² > 0.99), and the fitted Q_e value (4.41 mg/g) aligns more closely with the experimental value (4.42 mg/g). It can be inferred that the adsorption of Sb(III) by Fe/Mn-GOCS is better described by the pseudo-second-order kinetic model, and chemisorption is the dominant mechanism.

3.5.2. Elovich Model

The Elovich model explores the effect of adsorption time on the adsorption rate and the time required for reaction equilibrium [47]. It predicts an infinite amount of adsorption over a long period and is physically incomplete [47]. Therefore, it is suitable for dynamics far from equilibrium, where desorption does not occur due to low surface coverage [49]. The specific expression is shown in Equation (5).

$$Q_t={\displaystyle \frac{1}{\beta }}\ln \left(\alpha \beta t+1\right)$$

(5)

where α is the initial adsorption rate (mg/g∙min⁻¹) and β (g/mg) is the adsorption constant.

The results fitted by the Elovich model are shown in Figure 4c, and the fitting parameters are shown in

Table 2. Elovich model parameters.

Temperature °C	Elovich Model
	α	ꞵ	R2
25	0.10	1.43	0.9481

. The curves fitted by the Elovich model show two stages: the first stage is the fast response stage (0–1020 min), where the fitted curves show a clear increasing trend in a short period; the second stage is the slow response stage (after 1020 min), where the fitted curves show a slowly increasing trend over a long period. The coefficient of determination (R² > 0.94) obtained from the Elovich model shows a good fit. The initial adsorption rate (α) is 0.10 mg/g∙min⁻¹. The fitting results of the Elovich model show that Qt increases rapidly in a short period of time and slows down with time. The fast reaction rate may be related to surface coverage, the activation energy of chemical adsorption, and the number of available adsorption sites. Over time, the number of sites on the adsorbent surface becomes progressively fewer, and the reaction rate of adsorption decreases.

3.5.3. Weber–Morris Intra-Particle Diffusion Model

Pseudo-first-order, pseudo-second-order, and Elovich models explain the adsorption process, where surface adsorption is the main mechanism [47]. However, intra-particle diffusion also plays an important role in the adsorption process, which cannot be fitted by the pseudo-first-order, pseudo-second-order, and Elovich kinetic models. Therefore, diffusion-related models should be considered for such cases. The Weber–Morris intraparticle diffusion model is often used to analyze the control steps in the reaction, as shown in Equation (6).

$$Q_t=K_{ip}{t}^{0.5}+C$$

(6)

where the values of the K_ip intraparticle diffusion coefficient and C can be obtained from the slopes of Q_e and t^0.5. The value of C is a constant involving the thickness and boundary layer.

The fitting results of the diffusion coefficient in Weber–Morris particles are shown in Figure 4c, and the relevant parameters are shown in

Table 3. Weber–Morris model of intragranular diffusion.

Temperature °C	Weber-Morris Model of Intragranular Diffusion
	K1p	C1 (mg/g)	R2	K2p	C2 (mg/g)	R2	K3p	C3 (mg/g)	R2
25	0.10	0.65	0.9789	4.88 × 10−2	2.14	0.91	3.39 × 10−3	4.02	0.8691

. The adsorption process of Fe/Mn-GOCS is divided into rapid adsorption on the adsorbent surface, slow diffusion in the pores, and adsorption equilibrium. The graphs for Q_t and t^0.5 do not pass through the origin and show multiple linear relationships According to Milmile et al. [50], if Q_t and t^0.5 have a good linear relationship and pass through the origin, then intraparticle diffusion is the limiting step of the adsorption process. This indicates that many mechanisms are involved in the adsorption of Sb(III) by Fe/Mn-GOCS, and the intraparticle diffusion mechanism is not the only control pathway.

3.5.4. Boyd Model

The external diffusion model assumes that the diffusion of the adsorbate in the boundary liquid film around the adsorbent is the slowest step [47]. As a type of external diffusion model, the Boyd kinetic model mainly describes the diffusion of the adsorbate through the bonded liquid film and is used to analyze whether liquid film diffusion is a rate-limiting step in the reaction [47]

$$B_t=-0.4997-\ln \left(1-{\displaystyle \frac{Q_t}{Q_e}}\right)$$

(7)

where B_t is a function of Q_t/Q_e. If the relationship curve of B_t and t is linear and passes through the origin, then intraparticle diffusion is the controlling step of the adsorption reaction [25].

The fitting results of the Boyd dynamic model to the data are shown in Figure 3e, and the relevant parameters are shown in

Table 4. Boyd dynamic model parameters.

Temperature °C	Boyd Dynamic Model Parameters
	Slope	Intercept	R2
25	$1.92 \times$ 10−3	−0.21	0.98392

. Fe/Mn-GOCS adsorbed Sb(III) with a good linear relationship between B_t and t (R² > 0.98) but does not pass through the origin. Therefore, liquid film diffusion is the rate-limiting step in the adsorption process.

3.6. Adsorption Isotherm Model

An adsorption isotherm is a mathematical model that describes the interaction between an adsorbent and an adsorbate under specific conditions. To study the isothermal adsorption characteristics of Fe/Mn-GOCS on Sb(III), Langmuir, Freundlich, Sips, Dubinin–Radushkevich, and Temkin models are used to fit the experimental data.

3.6.1. Langmuir Model and Freundlich Model

The Langmuir model assumes that adsorption occurs on a uniform surface of the material, where all sites have the same affinity for the adsorbate, with no adsorption migration on the surface plane [26]. This model primarily describes monolayer adsorption [26]. The Freundlich adsorption isotherm model describes a reversible and non-ideal adsorption process [26]. Unlike the Langmuir model, the Freundlich model assumes that the adsorbent has a heterogeneous surface structure and considers multilayer adsorption to be the dominant process [26]. Therefore, the Langmuir and Freundlich models are given by the following equations, Equations (8) and (9):

$$Q_e={\displaystyle \frac{Q_m{K}_L{C}_e}{1+K_L{C}_e}}$$

(8)

$$Q_e={K_F{C}_e}^{1/n}$$

(9)

Among them, Q_e is the adsorption capacity (mg/g) of the material for Sb(III) at adsorption equilibrium, Q_m is the maximum adsorption capacity (mg/g) for Sb(III), C_e is the mass concentration (mg/L) of Sb(III) solution at adsorption equilibrium, and K_L is the Langmuir adsorption equilibrium constant, related to the strength of adsorption interaction. K_F and 1/n are the adsorption equilibrium constant and adsorption strength constant of Freundlich, respectively.

Figure 5a and

Table 5. Adsorption isotherm model.

Adsorption Isotherm Model		25 °C	35 °C	45 °C
Langmuir model	Qm (mg/g)	57.69	79.36	81.35
	KL	5.19 × 10−3	1.52 × 10−2	3.55 × 10−2
	R2	0.99	0.96	0.96
Freundlich model	KF	1.17	7.47	9.29
	1/n	0.66	0.36	0.39
	R2	0.9884	0.9416	0.8563
Sips model	Qm (mg/g)	80.09	58.26	72.20
	Ks	4.34 × 10−3	4.10 × 10−2	0.97
	n	0.96	1.48	14.07
	R2	0.9968	0.9682	0.9832
Dubinin–Radushkevich model	Qm (mg/g)	38.76	43.68	62.11
	K	2586.49	323.46	174.94
	E (kJ/mol)	1.39 × 10−2	3.93 × 10−2	5.34 × 10−2
	R2	0.8856	0.8899	0.9458
Temkin model	AT (L/mg)	0.23	0.21	0.38
	B1 (J/mol)	9.91	11.84	16.54
	R2	0.8841	0.9447	0.9545

present the fitting parameters and results for Sb(III) adsorption using the Langmuir and Freundlich models at different temperatures. The results indicate that the adsorption capacity (Q_e) of Fe/Mn-GOCS increases with the equilibrium concentration of Sb(III) (C_e), suggesting a strong adsorption capacity for high concentrations of Sb(III). The adsorption constant (K_L) of the Langmuir model increases with increasing temperatures, indicating that the adsorption process is endothermic [51]. The high correlation coefficient (R² > 0.97) of the Langmuir model suggests that Sb(III) adsorption on Fe/Mn-GOCS predominantly follows monolayer adsorption, with uniform binding sites playing a crucial role [7].

The Freundlich model also shows good applicability (R² > 0.92), indicating that both monolayer and multilayer adsorption may be involved in Sb(III) adsorption by Fe/Mn-GOCS. The n value of the Freundlich model is greater than 1 at different temperatures which according to Manjot et al. [52], indicates a favorable adsorption process. Although the fitting performance (R_e) of the Langmuir model is slightly better than that of the Freundlich model, both models adequately describe the adsorption behavior. At 25 °C, the maximum adsorption capacity (Q_m) of Sb(III) is 57.69 mg/g, which is in good agreement with the value predicted by the Langmuir model.

3.6.2. Sips Model

The Sips model combines the Langmuir and Freundlich models. At low adsorption concentrations, the Sips model reduces to the Freundlich isotherm. At high concentrations, the Sips model predicts the single-layer adsorption capacity characteristics of the Langmuir isotherm [25]. The Sips model formula and the fitting results at different temperatures are shown in the corresponding Equation (10) and Figure 5b.

$$Q_e={\displaystyle \frac{{Q_m\left(K_s{C}_e\right)}^n}{1{+\left(K_s{C}_e\right)}^n}}$$

(10)

where K_s and n are isotherm constants of Sips.

The results show that the regression coefficients fitted by the Sips model (R² > 0.98) are higher than those fitted by the Langmuir model (R² > 0.97) and the Freundlich model (R² > 0.92), indicating the superior fitting effect of the Sips model. At 25 °C, the maximum adsorption capacity (Q_m) of Sb(III) fitted by the Sips model is 80.09 mg/g.

3.6.3. Dubinin–Radushkevich Model

The significance of the Dubinin–Radushkevich model lies in its ability to determine the nature of adsorption, whether physical or chemical [53]. If the calculated average adsorption energy (E) is less than 8 kJ/mol, the process involves physical adsorption. An E value between 8 and 16 kJ/mol indicates that chemisorption is the rate-controlling step [53].

$$Q_e=Q_m\exp \left(-K{\epsilon }^2\right)$$

(11)

Here, ε represents the Dubinin–Radushkevich model adsorption potential ($\epsilon =RTln(1+{\displaystyle \frac{1}{C_e}})$), and E denotes the average adsorption-free energy per mole of adsorption (kJ/mol, E$={\displaystyle \frac{1}{\sqrt{2K}}}$). The fitting results of the Dubinin–Radushkevich model at different temperatures are shown in Figure 5c, and the relevant parameters are listed in Table 5. The Dubinin–Radushkevich model is poorly fitted, with R² values of less than 0.90 at 25 °C and 35 °C. In addition, the E values of 0.01 kJ/mol, 0.04 kJ/mol, and 0.05 kJ/mol are obtained at 25 °C, 35 °C, and 45 °C, respectively, indicating that physical adsorption also plays a role in the adsorption of Sb(III) on Fe/Mn-GOCS.

3.6.4. Temkin Model

In the Temkin model (Equation (12)), the adsorption process is assumed to be multilayer, and the interaction between the adsorbate and the adsorbent is considered.

$$Q_e=B_1\left(\mathrm{l}\mathrm{n}{\mathrm{K}}_T\right)+{\mathrm{B}}_1\left(ln{C}_e\right)$$

(12)

where A_T (L/mg) and B₁ (J/mol, $B_1={\displaystyle \frac{RT}{b}})$ are constants of the Temkin model.

The fitting results of the Temkin model at different temperatures are shown in Figure 5d, and the relevant parameters are listed in Table 5. It can be seen that the Temkin model fits the adsorption data well (R² > 0.92), indicating a strong interaction between Sb(III) and the reaction group. The adsorption energy values (B) obtained at 25 °C, 35 °C, and 45 °C are 9.91 J/mol, 11.85 J/mol, and 16.54 J/mol, respectively. The adsorption energy increases with temperature, indicating an endothermic reaction [54]. The fitting results show that the adsorption energy increases with temperature, confirming that the Fe/Mn-GOCS adsorption of Sb(III) is an endothermic process. Studies have shown that when the adsorption energy is below 20 J/mol (0 < B < 20 J/mol) at all temperatures, adsorption occurs through physical adsorption [32]. The fitted adsorption energy values are all below 20 J/mol, indicating that Fe/Mn-GOCS physically adsorbs Sb(III).

4. Conclusions

In this study, we successfully prepare a series of Fe/Mn-GOCS composites modified with different Mn salts by the high-temperature impregnation method, using graphene oxide and chitosan as substrates, and FeCl₂∙4H₂O, MnCl₂∙4H₂O, KMnO₄, and MnSO₄ as modifiers. Based on preliminary adsorption experiments, FeCl₂/MnSO₄-GOCS composites with the best adsorption performance for Sb(III) are selected. The effects of different initial concentrations, pH values, solid–liquid ratios, and adsorption times on the adsorption properties of Sb(III) are further discussed. The experimental results show that Fe/Mn-GOCS modified with MnSO₄ has a high removal efficiency for Sb(III), with a removal efficiency of 28.16%. When the molar ratio of Fe/Mn exceeds 4:1, both the adsorption capacity and removal rate show a decreasing trend.

For the selection and fitting of dynamic models, various models are compared in this study. The determination coefficient (R²) of the pseudo-second-order kinetic model (>0.99) is significantly higher than that of the pseudo-first-order kinetic model (>0.98) and is closer to the experimentally obtained adsorption capacity value (4.42 mg/g versus 4.41 mg/g), indicating that the adsorption process of Fe/Mn-GOCS for Sb(III) is mainly controlled by chemisorption. Additionally, the fitting results of the Elovich model and Weber–Morris model for intraparticle diffusion reveal complex mechanisms involved in the adsorption process, such as the gradual saturation of adsorption sites and multiple diffusion stages.

In the isothermal adsorption experiment, Fe/Mn-GOCS also exhibits excellent adsorption behavior for a high concentration of Sb(III). The regression coefficients of both the Langmuir model and Sips model exceed 0.97, indicating that the adsorption process mainly occurs at homogeneous adsorption sites on the surface of the material. The Sips model shows the best fitting effect due to its integration of the Langmuir and Freundlich models, and its high determination coefficient (R² > 0.98) emphasizes its applicability in describing the non-ideal adsorption process. Additionally, the results of the Dubinin–Radushkevich model and the Temkin model also indicate the involvement of physical adsorption in the overall adsorption process. Through these comprehensive analyses, the Sb(III) removal capacity of different modified Fe/Mn-GOCS materials is deeply investigated, and the mechanism and kinetic properties of the adsorption process are comprehensively evaluated using various kinetic and isothermal models. These findings provide an important theoretical basis and practical guidance for developing and designing efficient Sb(III) removal materials.