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 × 10
4 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-Fe
3O
4, 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 Fe
3O
4 as the raw material, which was used to adsorb Pb
2+ 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-Fe
3O
4 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-Fe
3O
4 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-Fe
3O
4 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% CaCl
2 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-Fe
3O
4 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-Fe
3O
4 (%), C: the mass fraction (%) of Microcystis suspension and the range of values for each factor, which are shown in
Table 1.
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 HNO
3, 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).
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; Ce is the measured Sb(III) concentration after adsorption, mg/L; qd is the desorption amount, mg/g; qe is the equilibrium adsorption amount, mg/g.
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.