Adsorption of Sb(III) from Solution by Immobilized Microcystis aeruginosa Microspheres Loaded with Magnetic Nano-Fe₃O₄

Abstract

In this study, a renewable and reusable immobilized Microcystis aeruginosa microsphere loaded with magnetic Nano-Fe₃O₄ composite adsorbent material is designed to study the treatment of wastewater containing heavy metal Sb(III). Through static absorption experiments combined with various characterization methods, this article studies the absorption process and mechanism of Sb(III), and investigates the optimal preparation conditions and environmental influencing factors. The results show that the optimal preparation conditions for immobilized Microcystis aeruginosa microspheres loaded with magnetic Nano-Fe₃O₄ adsorbent materials are 50.0% mass fraction of Microcystis suspension, 1.5% mass fraction of Nano-Fe₃O₄, and 2.5% mass fraction of sodium alginate. When the pH of the solution is 4, the reaction temperature is 25 °C, and the adsorbent dosage is 8.5 g/L, the removal rate of Sb(III) is the highest, reaching 83.62% within 120 min. The adsorption process conforms to the pseudo-second order kinetic model and Langmuir adsorption isotherm model, mainly characterized by chemical adsorption and surface complexation. Therefore, the composite material has been proven to be an efficient Sb (III) adsorption material.

1. Introduction

Antimony is an important non-ferrous metal element, widely used in industries such as ceramics, batteries, papermaking, plastics, paint, glass, alloys, catalysts, and combustion aids [1]. However, it has chronic toxicity and carcinogenicity to the human body and environmental organisms. It can also cause various diseases to skin and respiratory and cardiovascular systems [2]. In aquatic environments, antimony mainly exists in the forms of Sb (III) and Sb (V) [3], with Sb (III) being 10 times more toxic than Sb (V) and more likely to remain in living organisms [4]. The excessive exploitation and abuse of antimony by humans has caused serious antimony pollution [5]. Every year, approximately 3.8 × 10⁴ tons of antimony from human production activities have been released into the environment [6], posing a serious threat to human health and the water ecosystem [7]. The Chinese Ministry of Ecology and Environment, the US Environmental Protection Agency, and the European Union have all prioritized the control of antimony and its compounds as pollutants, and have established strict antimony emission standards to reduce antimony pollution [8]. Therefore, studying the removal of antimony and its compounds from wastewater is of great significance and has attracted widespread attention from scholars.

At present, the treatment technologies for antimony wastewater mainly include adsorption [9], coagulation/flocculation, electrochemical method, ion exchange method, membrane treatment method, etc. Among them, coagulation/flocculation and electrochemistry are widely used in engineering, but there is a risk of secondary pollution. Although membrane and ion exchange technology have high antimony removal efficiency, the high initial investment and operating costs hinder the development of these two technologies [10]. The adsorption method can effectively treat low-concentration heavy metal ion wastewater [11], but it is hard to be regenerated and reused. In recent years, immobilized microbial adsorption technology has solved the problem of regeneration and reuse of microbial adsorbents [12]. Therefore, it has been frequently applied in the treatment of heavy metals and organic polluted wastewater [13]. The commonly adsorbed microorganisms mainly include Bacillus, Chlorella, Sulfate reducing bacteria, Beer yeast, Pseudomonas [14,15,16,17,18], etc. And the common immobilization carriers include biochar, sodium alginate, graphene oxide, polyvinyl alcohol, chitosan [19], etc. For example, Zhao et al. [20] prepared a microbial graphene oxide composite material, which had a maximum adsorption capacity of 149.3 mg/g for U (VI) in aqueous solution. Chen et al. [21] synthesized an immobilized graphene Burkholderia C09V biomaterial, and after four rounds of regeneration and reuse, the removal rate of Sb(V) could still reach 72%.

Sodium alginate (SA) is an ideal natural polysaccharide adsorbent material [22], which has advantages such as low cost, non-toxicity, and biodegradability [23]. For example, Bustos Terranes et al. [13] used sodium alginate (SA) to fix the microbial community in activated sludge and introduced it into a fluidized bed reactor. The experimental results show that its removal efficiency of organic matter in domestic wastewater is 93%. Nano-Fe₃O₄, as a new type of nanomaterial, is widely used in adsorbent materials because of its large specific surface area, ease of preparation, strong magnetic properties, and ease of separation [24]. For example, Sun et al. [22] prepared a magnetic composite microsphere using Fe₃O₄ as the raw material, which was used to adsorb Pb²⁺ in the solution. After five adsorption and desorption cycles, the adsorption capacity still reached 165.5 mg/g. Therefore, based on the existing research, we selected nano-Fe₃O₄ to be added into the adsorbent material to try to prepare the magnetic composite material. In recent years, the research and application of algae bioremediation technology have received much attention. A large number of studies have found that algae show good adsorption capacity for Cd, Cr, Cu, Ni, As, Sb, etc. [25,26,27,28,29,30]. Wu et al. [31] studied the biosorption behavior of Microcystis on Sb(III) under different environmental conditions, and Sun et al. [32] studied the biosorption mechanism of the Cyanobacteria from Taihu Lake on Sb(V). All these studies show the biosorption potential of Microcystis on antimony in wastewater, but there are few studies on the adsorption of antimony by immobilized algae.

Based on these issues, this article takes Microcystis aeruginosa [33], the main alga of blue-green algae blooms in eutrophic lakes in China, as the main adsorbent, and loads Nano-Fe₃O₄ particles. The two are embedded in sodium alginate (SA) for fixation to prepare a new type of immobilized magnetic biosorbent material. And it is used to study the biological adsorption characteristics and environmental factors of Sb(III), and the optimal preparation conditions, adsorption process, and mechanism. In this way, this article can provide scientific basis for the removal of Sb(III) in mining wastewater and the regeneration and reuse of new adsorbents.

2. Materials and Methods

2.1. Preparation of Immobilized Microcystis aeruginosa Microspheres Loaded with Magnetic Nano-Fe₃O₄ Adsorbent Materials

Main reagents: sodium alginate ((C₆H₇NaO₆)_n, chemically pure, Sinopharm); calcium chloride (CaCl₂, analytically pure, Beijing Chemical Industry); magnetic Nano-Fe₃O₄ (99.99% purity, particle size 20 nm, bulk density 0.67 g/cm³, macklin).

The algal species used in the experiment is Microcystis aeruginosa (Fachb-905), which was purchased from the freshwater algal species bank of the Chinese Academy of Sciences. The medium used is BG-11 [34], with the environment of a constant temperature and timed-light incubator. The incubation constant temperature is 25.0 °C, the light intensity is 2500 lx, and the light-to-darkness ratio is 12 h:12 h. It is shaken regularly every day, and the exponential growth is maintained by subculture.

The preparation process of the adsorbent material is shown in Figure 1. Add the sodium alginate to physiological saline, heat it to 80 °C for dissolution, and place it on a super clean bench for sterile cooling for later use. Mix the magnetic Nano-Fe₃O₄ particles with the Microcystis suspension cultured to the stable stage, add it into the sodium alginate solution cooled to room temperature (25 °C), mix it evenly with a stirring device, and then drop it into the pre-configured 4 °C, 2% CaCl₂ solution with a 30 mL sterile syringe. The dropping process is carried out in a constant temperature shaker, shaking while dropping at a speed of 100 r/min. The formed gel particles are left at room temperature for 2.0 h and washed three times with sterile physiological saline, thus obtaining immobilized Microcystis aeruginosa microspheres loaded with magnetic Nano-Fe₃O₄ adsorbent material (hereinafter referred to as immobilized Microcystis aeruginosa microspheres).

2.2. Orthogonal Experimental Design

To explore the optimal preparation ratio of immobilized Microcystis aeruginosa microspheres, an orthogonal experiment is designed. A certain amount of 10 mg/L Sb (III) solution is prepared, and the pH is adjusted to 4.0 using 1 mol/L HCl solution and 1 mol/L NaOH solution. The solution is placed in a 100 mL conical flask, 6.0 g of adsorbent is added in, and is shaken at a constant temperature of 25 °C and 125 r/min for 12 h. The removal rate of Sb (III) is used as the evaluation index. The experimental factors are as follows: A: mass fraction of sodium alginate (%), B: mass fraction of Nano-Fe₃O₄ (%), C: the mass fraction (%) of Microcystis suspension and the range of values for each factor, which are shown in

Table 1. Value range and level of each factor.

Level	Mass Fraction of Sodium Alginate A	Mass Fraction of Nano Fe3O4 B	Mass Fraction of Microcystis Suspension C
1	2.5%	1.5%	50%
2	2.0%	1%	30%
3	1.5%	0.5%	15%
4	1.0%	0	0

.

2.3. Characterisation Methods and Sb(III) Concentration Determination

The morphology and structure of the samples are analyzed using a field emission scanning electron microscope (Zeiss Gemini 300, Zeiss, Oberkochen, Germany). The specific surface area, pore size, and pore volume of the material are measured using a porous physical adsorption instrument (Quadrasorb evo, Quantachrome, FL, USA). And Fourier transform infrared (FTIR) spectra of the material before and after adsorption are obtained using KBr compression method in the wavelength range of 400–4000 cm by a Fourier transform infrared spectrometer (Nicolet 670, Thermal Fisher, MA, USA).

The concentration of Sb (III) in the sample is determined by using a flame atomic absorption spectrometer (AA7002A, EWAI, Beijing, China). The Sb (III) solution used in the experiment is prepared using the following method: 2.748 g of C₈H₄K₂O₁₂Sb₂·3H₂O (analytical pure, China National Pharmaceutical Group) is weighed and dissolved in 1000 mL of ultrapure water, which is 1000 mg/L Sb (III) reserve solution. The prepared reserve solution is sealed and stored in a refrigerator at 4 °C. Each working solution is diluted with the reserve solution and prepared before use.

2.4. Adsorption Probe Test

Prepare a Sb(III) solution with a concentration of 10.0 mg/L and a volume of 40 mL, and place it in a 250 mL conical flask. Adjust the pH of the solution between 2 and 9 using HCl and NaOH solution (1 mol/L, 0.1 mol/L), and place it in a constant temperature shaker maintained at 25 °C. Then add a specific amount of immobilized Microcystis aeruginosa microspheres, and begin shaking at a speed of 125 r/min to start the oscillation timing and timed sampling. After infiltrating the sample with a 0.45 μm membrane, the remaining Sb(III) concentration in the sample is determined by flame atomic absorption spectrophotometry. Three parallel groups are set for each group, and the average value of the measurement results is taken.

2.5. Adsorption Isotherm Model and Adsorption Kinetics Experiment

Isothermal adsorption experiments are conducted in a series of Sb(III) solutions with pH = 4 and Sb(III) concentrations ranging from 5 mg/L to 200 mg/L. Each group is treated with a certain amount of immobilized Microcystis aeruginosa microspheres and placed in a constant temperature shaker (125 r/min) at temperatures of 20 °C, 25 °C, and 30 °C. After shaking for 6 h, the adsorbent is separated and the supernatant is taken to determine the concentration of Sb (III). The isothermal adsorption model was selected to fit the experimental data.

Exploring the adsorption kinetics process of immobilized Microcystis aeruginosa microspheres is conducted under conditions of pH = 4, temperature of 25 °C, rotational speed of 125 r/min, and Sb (III) concentration of 10.0 mg/L. During the adsorption process from 0 min to 720 min, samples are taken at regular intervals to determine the concentration of Sb(III). Finally, the adsorption capacity of the adsorbent for Sb(III) during the entire adsorption process is obtained, and the obtained data is dynamically fitted.

2.6. Desorption Test

Prepare an Sb(III) solution with a concentration of 10.0 mg/L and a pH of 4. Add a certain amount of immobilized Microcystis aeruginosa microspheres and shake them at a constant temperature of 25 °C and 125 r/min for 2 h. Separate the adsorbed microspheres with a magnet and rinse them three times with sterile physiological saline. Then add them to 40 mL of physiological saline, 1 mol/L NaOH, 1 mol/L HNO₃, and 1 mol/L HCl solution separately, and shake and desorb the solution at 25 °C and 125 r/min for 1 h to measure the ion concentration. Repeat the adsorption and desorption steps 5 times, and then take samples separately to calculate the removal and desorption rates of Sb (III) by the small balls. The calculation formulas are shown in Equations (1) and (2).

R₀ = (C₀ − C_e)/C₀ × 100%

(1)

D₀ = q_d/q_e × 100%

(2)

In the equations, R₀ is the antimony removal rate, %; D₀ is the desorption rate, %; C₀ is the initial liquid-phase Sb(III) concentration, mg/L; C_e is the measured Sb(III) concentration after adsorption, mg/L; q_d is the desorption amount, mg/g; q_e is the equilibrium adsorption amount, mg/g.

3. Results and Discussion

3.1. Optimal Preparation Conditions for Immobilized Microcystis aeruginosa Microspheres Loaded with Magnetic Nano-Fe₃O₄ Adsorbent Materials

An orthogonal table L16(4³) (see

Table 2. Orthogonal experimental design scheme and results.

Numbers	Factors			Removal Rate (%)
	A	B	C
1	1	1	1	90.5%
2	1	2	2	87.4%
3	1	3	3	79.8%
4	1	4	4	60.2%
5	2	1	2	86.8%
6	2	2	1	84.2%
7	2	3	4	66.7%
8	2	4	3	68.5%
9	3	1	3	73.2%
10	3	2	4	73.0%
11	3	3	1	82.9%
12	3	4	2	70.0%
13	4	1	4	72.6%
14	4	2	3	77.5%
15	4	3	2	76.4%
16	4	4	1	70.8%
K1	317.8%	323.1%	328.4%
K2	306.2%	322.2%	320.6%
K3	299.2%	305.7%	299.0%
K4	297.3%	269.5%	272.5%
R	5.13%	13.39%	13.99%

) was designed and orthogonal tests were conducted according to the factors in Table 1 and the number of levels, and the results obtained are shown in Table 2. K₁, K₂, K₃, and K₄ are used to represent the sum of assessment results for each factor at different levels, respectively. Based on the magnitude of the results, it can be seen that factors A, B, and C are best at level 1. And based on the extreme variance R, it can be concluded that the effect size of each factor is C > B > A. Based on the experimental values, using three-factor analysis of variance (see

Table 3. Results of three-factor analysis of variance.

Source of Variance	Sum of Squares	df	Mean Square	F	p
Intercept	9.310	1	9.310	8586.664	0.000
Mass fraction of sodium alginate A	0.006	3	0.002	1.992	0.217
Mass fraction of Nano-Fe3O4 B	0.047	3	0.016	14.452	0.004
Mass fraction of Microcystis suspension C	0.047	3	0.016	14.495	0.004
Residual	0.007	6	0.001

) to study the impact of factors A, B, and C on the removal rate, it was found that factor A did not show significant differences (F = 1.992, p = 0.217 > 0.05), indicating that the sodium alginate mass fraction A did not have a differential relationship with the removal rate. However, Nano-Fe₃O₄ mass fraction B and Microcystis suspension mass fraction C showed significance (F = 14.452, p = 0.004 < 0.05), indicating the presence of main effects. Therefore, Nano-Fe₃O₄ mass fraction B and Microcystis aeruginosa mass fraction C will have a differential relationship on removal rates. Based on the above results, it can be concluded that the suitable embedding condition for immobilized Microcystis aeruginosa microspheres is C₁B₁A₁. That is, 50 percent mass fraction of Microcystis aeruginosa; 1.5 percent mass fraction of Nano-Fe₃O₄; and 2.5 percent mass fraction of sodium alginate.

3.2. Morphology and Performance Characterization of Immobilized Microcystis aeruginosa Microspheres

The surface morphology and structure of the spherical particles is shown in Figure 2. The surface is relatively flat and dense, with low porosity, and is covered with spherical substances with a diameter of about 200 nm. Fe₃O₄ particles are loaded on the surface and inside of the material, and there are a few cracks on the surface of the small ball, which may be caused by the drying process.

The nitrogen adsorption and desorption curve and pore size distribution of immobilized Microcystis aeruginosa microspheres are shown in Figure 3. The adsorption and desorption curve is approximately S-shaped. When the relative pressure is low, it is mainly single-layer adsorption, and when the pressure is close to saturation, it is mainly multi-layer adsorption. The inflection point at the front of the curve corresponds to the saturation adsorption amount of the single layer. When P/P₀ = 0.5, the curve shows an insignificant inflection point. At this point, the coverage of the monolayer and the initial amount of multi-layer adsorption may overlap. When P/P₀ = 1.0, the adsorption process has not yet reached saturation. Based on the pore size distribution map, it was determined that the immobilized Microcystis aeruginosa microspheres should be non-porous materials, which is consistent with the results of SEM images.

3.3. Effect of pH on Adsorption Performance

From the results of Figure 4, it can be seen that pH has little effect on the adsorption of Sb(III) by immobilized Microcystis aeruginosa microspheres when the solution pH is 2–9, and the adsorption removals are all more than 80%. Previous studies have found that the morphology and chemical characteristics of Sb(III) in solution are closely related to the pH of the solution [35]. Sun et al. [34] calculated the main forms and contents of Sb(III) after hydrolysis of potassium antimony tartrate solutions at pH 2–8 under room temperature conditions (25 °C). It was found that the pH of the solution affects the properties of the functional groups on the surface of the Microcystis adsorbent. When the solution pH is 2 and 3, 76.0% and 96.0% of Sb(III) in the solution exists in the form of Sb(OH)_3. At pH 4–7, Sb(OH)₃ accounts for more than 99.0%, while the rest exists in the cationic form Sb(OH)²⁺. When the pH is 8, Sb(OH)₃ accounts for more than 90.0%. At this point, the surface functional groups of the adsorbent prepared by direct freeze-drying of Microcystis aeruginosa may undergo deprotonation [36]. This reduces the number of sites on the adsorbent surface that can interact with neutral Sb(OH)₃, significantly reducing the biological adsorption efficiency of Sb(III) [37]. However, after immobilization, Microcystis aeruginosa microspheres have a stronger tolerance to the pH of the solution compared to individual Microcystis adsorbents. When pH = 8 and pH = 9, the removal rates of Sb(III) by immobilized Microcystis aeruginosa microspheres can still reach 85.04% and 87.89%, almost unaffected by the morphology of Sb(III) in the solution.

3.4. Influence of Adsorption Material Dosage and Adsorption Time on the Adsorption Process

The effect of adsorbent dosage on the adsorption process is shown in Figure 5. As the dosage of immobilized Microcystis aeruginosa microspheres increases, the removal rate of Sb(III) gradually increases. When the adsorbent dosage is 15%, the removal rate reaches 88.03%. And the removal rate does not change much, even though the dosage of adsorbent is increased. When the adsorbent dosage continues to increase to 20%, the removal rate is 87.65 percent. It may be due to the fact that as the number of Microcystis aeruginosa microspheres increases, some of them become agglomerated with each other, making the effective surface area of the adsorbent decrease, thus not reaching the saturated adsorption state [38]. Therefore, it can be assumed that the adsorption process reaches equilibrium when the adsorbent dosage is 15–20 percent. The optimum adsorption dose is selected to be 15 percent.

As shown in Figure 6, the removal rate of Sb(III) by immobilized Microcystis aeruginosa microspheres gradually increases as adsorption time increases. In addition, the adsorption rate is relatively fast in the early stage, and the removal rate reached 83.62% after 2 h of adsorption. It continued to grow slowly and reached 90.88% after 12 h. Between 12 h and 24 h, the concentration of Sb(III) in the solution remained almost unchanged, indicating that the adsorption process tends to an equilibrium. Therefore, 12 h will be used as the adsorption equilibrium time.

3.5. Characteristics of Isothermal Adsorption Model

The Langmuir adsorption isotherm model (Equation (3)) and Freundlich adsorption isotherm model (Equations (4) and (5)) are used to fit the adsorption process of Sb(III). The fitted graph lines are shown in Figure 7, and the parameters are shown in

Table 4. Fitting parameters of isothermal adsorption model.

T/K	Langmuir				Freundlich
	qm (mg/g)	KL	RL	R2	KF	1/n	R2
293	12.1622	0.0446	0.1008~0.8177	0.9782	1.4154	0.4295	0.9639
298	13.4515	0.0452	0.0996~0.8157	0.9849	1.5242	0.4358	0.9373
303	9.7630	0.0582	0.0791~0.7746	0.9866	1.4454	0.3864	0.9649

:

$$\frac{C_e}{q_e}=\frac{C_e}{q_m}+\frac{1}{q_m\times K_L}$$

(3)

$$\ln q_e=\frac{1}{n}\times \ln C_e+\ln K_F$$

(4)

$$R_L=\frac{1}{1+K_L\times C_i}$$

(5)

where C_e is the adsorption equilibrium concentration, mg/L; q_e is the equilibrium adsorption capacity, mg/g; q_m is the maximum adsorption capacity, mg/g; K_L is Langmuir’s adsorption constant; K_F is the Freundlich’s adsorption capacity constant; and 1/n is the constant related to the adsorption density. R_L is the equilibrium constant associated with the type of reaction.

The R² values fitted by the Langmuir model and the Freundlich model are both relatively large. This indicates that both models can well fit the adsorption process of Sb(III) by Microcystis aeruginosa microspheres. Between them, the 1/n value in the Freundlich model can reflect the strength of adsorption. When the reaction temperature is 25 °C, the adsorption capacity of immobilized Microcystis aeruginosa microspheres is maximum, with 1/n values ranging from 0.1 to 0.5 and R_L values ranging from 0 to 1; this indicates that the adsorption process was easy to carry out, and that the material had a good adsorption capacity for Sb(III). At this point, the Langmuir model has a better fit with an R² of 0.985, which is more suitable for describing the isothermal adsorption process of Sb(III). And the immobilized Microcystis aeruginosa microspheres adsorbed Sb(III) in solution predominantly as a monomolecular layer [39], with a maximum adsorption of 13.45 mg/g (dry weight).

3.6. Adsorption Kinetics Characteristics

The adsorption kinetics characteristics of Sb(III) are as shown in Figure 8 and

Table 5. Adsorption kinetic model fitting parameters.

Pseudo-First Order Kinetic Model			Pseudo-Second Order Kinetic Model
k1 (min−1)	qe (mg/g)	R2	k2 (min−1)	qe (mg/g)	R2
1.0741	1.9935	0.9943	0.7481	2.1751	0.9948

, described using a c model (Equation (6)) and a pseudo-second order kinetic model (Equation (7)). In the initial stage of adsorption, the adsorption rate is relatively fast, and the adsorption amount was 1.6640 mg/g when the reaction was carried out for 2 h, which was more than 80% of the equilibrium adsorption state. Afterward, it began to decrease and gradually reached equilibrium around 8 h. This may be because Sb(III) in the solution first reacts with higher active adsorption sites on the surface of immobilized Microcystis aeruginosa microspheres. With the increase in contact time, the adsorption active sites of the balls decrease [40], and the adsorption rate decreases.

From the fitting results, it can be seen that both kinetic models are fitted better. However, compared to the pseudo-first order kinetic model, the fitting effect of the quasi-secondary order kinetic model is better. The quasi-secondary kinetic model with an R² value of 0.995 can be better used to predict and describe the adsorption kinetic process of Sb(III). This indicates that the adsorption process of Sb(III) by immobilized Microcystis aeruginosa microspheres is greatly affected by the adsorption rate, and is mainly characterized by chemical adsorption [41]. In addition to being influenced by the surface active sites of the microspheres, it is also affected by the initial concentration of the solution [22].

The rate control steps of the adsorption process were analyzed using the intraparticle diffusion model (Equation (8)), and the results are shown in Figure 9 and

Table 6. Intraparticle diffusion model fitting parameters.

Surface Diffusion			Endodiffusion
kp (mg·g−1·h0.5)	Cp	R2	kp (mg·g−1·h0.5)	Cp	R2
0.7800	0.5033	0.8885	0.0782	1.7788	0.7122

. For the intraparticle diffusion model, if the straight line passes through the origin (C_p = 0), the adsorption process is controlled only by intraparticle diffusion; otherwise, the adsorption process involves multiple diffusion resistances. The fitting results show (Figure 9) that C_p ≠ 0, which implies that the adsorption process is more complex due to the presence of the boundary layer [42], and the rate-controlling step of the adsorption process is controlled by other diffusion resistances in addition to intraparticle diffusion. The adsorption rate is very fast in the surface diffusion stage, and the adsorption process is mainly caused by diffusion on the surface or in the macro pores; the adsorption rate slows down in the internal diffusion stage, and adsorption occurs in the small pores. And after reaching the equilibrium state, the adsorption amount is gradually saturated and no longer undergoes any significant change.

$$\ln \left(q_e-q_t\right)=\ln q_e-t\times k_1$$

(6)

$$\frac{t}{q_t}=\frac{t}{q_e}+\frac{1}{k_2\times {q_e}^2}$$

(7)

$$q_t=k_p\times t^{1/2}+C_p$$

(8)

where q_t and q_e are the adsorption capacity at time t and adsorption equilibrium, mg/g, respectively; t is the adsorption time, k₁ is the quasi-primary adsorption rate constant, and k₂ is the pseudo-second order adsorption rate constant; K_p is the intraparticle diffusion constant, and C_p is the intercept.

3.7. Adsorption Mechanism of Sb(III) by Adsorbent Materials

The FTIR spectra of Microcystis aeruginosa microspheres before and after adsorption of Sb(III) are shown in Figure 10:

The characteristic peaks of Microcystis aeruginosa microspheres before adsorption are present at 3380.91 cm⁻¹, 1608.71 cm⁻¹, 1426.77 cm⁻¹, 1081.85 cm⁻¹, 1028.74 cm⁻¹, and 577.79 cm⁻¹. Among them, the stretching vibration peak of O-H is at 3380.91 cm⁻¹, the asymmetric stretching vibration peak of COO- is at 1608.71 cm⁻¹, the bending vibration peak of CH₂ is at 1426.77 cm⁻¹, the stretching vibration peak of C-O-C is at 1081.85 and 1028.74 cm⁻¹ [43], and the stretching vibration peak of Fe-O is at 577.79 cm⁻¹ [44]. After adsorption, there are significant changes in the infrared spectra: the infrared characteristic peaks of O-H, COO-, and CH₂ shifted to 3355.74 cm⁻¹, 1603.25 cm⁻¹, and 1423.00 cm⁻¹, while the characteristic peaks of C-O-C and Fe-O shifted to the peak range, indicating that active groups such as O-H and COO- interacted with Sb(III). Moreover, Nano-Fe₃O₄ particles also incorporated into the adsorbent and participated in the adsorption reaction process of Sb(III).

Previous studies have shown that carboxyl and hydroxyl functional groups can chelate with Sb(III), forming a pentacyclic complex containing two Sb-O-C bonds [45]. When pH = 4, carboxyl groups undergo protonation and exist in the carboxylic acid molecular state (-COOH), which will chelate with neutral Sb(OH)₃ to produce endogenous complexes. Moreover, hydroxyl groups also undergo complexation with Sb (OH)₃ [34], further confirming the previous inference. Therefore, based on the infrared spectroscopy analysis results, combined with the adsorption kinetics characteristics and Langmuir adsorption isotherm model fitting results mentioned earlier, it can be concluded that O-H, COO-, Fe-O and other functional groups are the main adsorption sites of Sb(III), playing a major role in the adsorption process of Sb(III). The adsorption of Sb(III) by Microcystis aeruginosa microspheres is mainly chemical adsorption, mainly manifested by surface complexation.

3.8. Desorption Characteristics of Immobilized Microcystis aeruginosa Microspheres

Using physiological saline, 1.0 mol/L NaOH, 1.0 mol/L HNO₃, and 1.0 mol/L HCl to elute Sb(III) adsorbed on the surface of Microcystis aeruginosa microspheres, the desorption efficiencies are 39.76%, 36.98%, 65.58%, and 77.75%, respectively. It can be concluded that 1.0 mol/L HCl has the highest desorption efficiency, while 1.0 mol/L NaOH has the lowest desorption efficiency. The results of five cycles of adsorption–desorption regeneration using 1.0 mol/L HCl are shown in Figure 11: after five regeneration cycles, the removal rate of Sb(III) by Microcystis aeruginosa microspheres is only 11.84% lower than the first adsorption, and the desorption rate can still reach 78.78%. Sun et al. [34] directly prepared Microcystis aeruginosa into an adsorbent, which was desorbed with 1.0 mol/L HCl after adsorption, and the second adsorption efficiency was 68.4%. After four uses, the adsorption efficiency significantly decreased to 39.7%. Therefore, compared to conventional Microcystis adsorption, the recycling efficiency of immobilized Microcystis aeruginosa microspheres is significantly improved.

4. Conclusions

(1) Immobilized Microcystis aeruginosa microspheres have good removal efficiency for Sb(III) in solution, and the removal rate of Sb(III) in a solution with a concentration of 10.0 mg/L can reach over 90%. The optimal preparation conditions are 50% mass fraction of Microcystis, 1.5% mass fraction of Nano-Fe₃O₄, and 2.5% mass fraction of sodium alginate; in addition, immobilized Microcystis aeruginosa microspheres have good regeneration and reuse performance, and can still maintain over 85% of the initial adsorption capacity after five regeneration and reuse cycles.

(2) The adsorption process of immobilized Microcystis aeruginosa microspheres has a wide range of suitable pH values. The solution pH in the range of 2–9 had little effect on the adsorption of Sb(III) by immobilized Microcystis aeruginosa microspheres, and the removal rate could reach more than 80%. Adsorption is fast in the early stages and slow in the later stages. And the rate can hit more than 90 percent of equilibrium by 2 h and reach equilibrium by 12 h. So, the optimal adsorption conditions for immobilized microspheres are: a solution with a pH =4, reaction temperature of 25 °C, and adsorbent dosage of 8.5 g/L (dry weight).

(3) The adsorption of Sb(III) by immobilized Microcystis aeruginosa microspheres conforms to a pseudo-second order kinetic model, with chemisorption dominating, mainly in the form of surface complexation. The main adsorption sites include O-H, COO, Fe-O, and other functional groups, which are influenced by the surface site activity of the adsorbent and the initial concentration of the solution. The entire adsorption process can be well fitted using the Langmuir adsorption isotherm model, with single-layer adsorption as the main method and a maximum single-layer adsorption capacity of 13.45 mg/g.