1 Introduction
Arsenic (As) and antimony (Sb) are group Va elements in the periodic table of elements, which are usually accompanied in nature. As two types of metallic elements, arsenic and antimony are widely used. Arsenic can be used in agricultural production such as pesticide and herbicide synthesis and industrial production such as semiconductor synthesis and metallurgy
[1]. Antimony can be used in the production of industrial materials such as semiconductors, infrared detectors and diodes, and antimony compounds can also be used as catalysts for chemical fiber synthesis, as well as drugs to treat parasitic diseases and leishmaniasis
[2]. However, the two elements and their compounds pose a potential threat to human health. Natural and occupational exposure to arsenic and antimony can easily lead to respiratory diseases such as laryngitis and tracheobronchitis, cardiovascular diseases such as Raynaud's disease, and gastrointestinal diseases
[1,3]. As a result, arsenic and antimony are listed as priority pollutants of concern by the Council of the European Communities (1976) and the Environmental Protection Agency of the United States (1979). The World Health Organization (WHO) standards for As and Sb in drinking water are 10 and 5 μg/L, respectively
[4].
Both As and Sb have four oxidation States (-Ⅲ, 0, Ⅲ, Ⅴ), among which As (Ⅲ)/Sb (Ⅲ) and As (Ⅴ)/Sb (Ⅴ) are the most common valence States in the environment, which are dominant under reducing and oxidizing conditions, respectively
[5]. In natural water, As (Ⅴ) and Sb (Ⅴ) exist in the form of negatively charged HAsO
42-/H
2AsO
4- and Sb(OH)
6-, while As (Ⅲ) and Sb (Ⅲ) exist in the form of uncharged As(OH)
3 and Sb(OH)
3[6,7]. The oxidation state and chemical form have an important influence on the environmental behavior and biological toxicity of arsenic and antimony
[8]. For example, the biological toxicity of As (Ⅲ)/Sb (Ⅲ) is higher than that of As (Ⅴ)/Sb (Ⅴ)
[4]. As (Ⅲ)/Sb (Ⅲ) is considered to be more difficult to remove than As (Ⅴ)/Sb (Ⅴ) because of the low charge dependence of the two species on the solid phase due to the electrical neutrality of the As(OH)
3/Sb(OH)
3[7].
Arsenic and antimony mainly enter the biogeochemical cycle through rock weathering, microbial activities, complexation with natural organic matter and other ways. The participation of human activities leads to a significant increase in the concentration of arsenic and antimony in the environment, resulting in serious arsenic and antimony pollution of soil and water
[9,10]. Taking Sb as an example, the concentration of dissolved Sb in typical unpolluted water is usually less than 1 μg/L, while the concentration of Sb in man-made pollution sources can be as high as 100 times the natural level
[9]. In view of the current situation of arsenic and antimony pollution, a variety of technologies have been used to remove arsenic and antimony, including adsorption, coagulation/flocculation, membrane separation, ion exchange, extraction and so on
[11⇓~13]. Iron-based materials are widely used in the removal and remediation of As and Sb in the environment because of their advantages such As efficient adsorption, environmental friendliness and abundant reserves in the earth.
In recent years, the removal of heavy metals by iron-based materials such as nano-zero-valent iron, iron skeleton materials and layered double hydroxides has become a hot research field, while the removal of heavy metals such as arsenic and antimony by iron-based materials with microorganisms has been paid less attention, especially the removal of antimony by iron oxides with microorganisms
[14⇓⇓~17]. In fact, microorganisms play an important role in the fixation and flow of Fe/As/Sb. Firstly, the process of iron mineral synthesis involving microorganisms is accompanied by the fixation of arsenic and antimony
[18,19]. Secondly, the microbial dissolution of iron minerals under long-term reducing conditions will lead to the release of arsenic and antimony. Finally, the process of microbially induced secondary iron mineral formation can refix arsenic and antimony to the surface or interior of the mineral. In addition to the redox of Fe, some microorganisms also have the ability to redox As/Sb. The most common form of As/Sb in the natural environment is the binding of As/Sb with iron (hydr) oxides by adsorption or coprecipitation, and microorganisms play an important role in the binding process
[5,8,20].
In view of the common phenomenon of arsenic and antimony fixation by microbial synthesis of iron minerals, the remediation of arsenic and antimony pollution by microbial synthesis of iron minerals will be an economical, efficient and simple remediation technology. In this paper, the research progress on the effect of microbial iron redox on As (Ⅲ)/As (Ⅴ) and Sb (Ⅲ)/Sb (Ⅴ) removal was reviewed, focusing on the As/Sb fixation by microbial iron minerals. The cycle of "synthesis-dissolution-transformation" of iron minerals mediated by microorganisms and the fixation, dissolution and transformation of arsenic and antimony caused by microorganisms participating in the cycle are introduced in detail. In addition, the biochemical mechanism and influencing factors involved in the process of arsenic and antimony fixation were also explained in this paper.Finally, the existing problems of arsenic and antimony removal by microbial iron redox were discussed, and the future development direction of this field was prospected.
2 As/Sb Fixation by Microorganisms Involved in Fe (Ⅲ) Mineral Synthesis
Fe (Ⅱ) oxidizing microorganisms have the ability to oxidize Fe (Ⅱ) to produce Fe (Ⅲ) minerals, which can be used for As/Sb removal. The microbial oxidation of Fe (Ⅱ) is also accompanied by the microbial oxidation of As (Ⅲ)/Sb (Ⅲ), which changes the As/Sb fixation behavior of Fe (Ⅲ) minerals. This section mainly introduces the processes of Fe (Ⅲ) mineral synthesis, As/Sb fixation and As/Sb valence transformation involving microorganisms. As the engineering application of Fe (Ⅲ) minerals is very common, the method of As/Sb fixation by Fe (Ⅲ) minerals has become the key process of wastewater treatment because of its advantages of high efficiency and low cost
[21]. In addition, the biological toxicity of As (Ⅴ)/Sb (Ⅴ) is lower than that of As (Ⅲ)/Sb (Ⅲ), so the process of microbial synthesis of Fe (Ⅲ) minerals and oxidation of As (Ⅱ)/Sb described in this section has good industrial application potential in the removal of As/Sb
[21].
2.1 Biosynthesis of Fe (Ⅲ) -bearing minerals
There are two ways for microorganisms to synthesize Fe (Ⅲ) (hydroxide) oxide: (1) under oxygenated and neutral pH conditions, microorganisms obtain Fe (Ⅲ) (hydroxide) oxide to meet their biochemical needs; (2) Microorganisms dissolve Fe (Ⅱ) and oxidize it to Fe (Ⅲ) to obtain energy. The oxidation modes can be divided into three types: aerobic, microaerobic and photosynthetic Fe (Ⅱ) oxidation
[23]. Microbially involved iron oxidation processes synthesize various forms of iron minerals (Fig. 1) and are an important part of the iron cycle in nature
[24⇓~26]. Rentz et al. measured the potential iron oxidation rate of freshwater microbial community in a simulated environment with unsaturated dissolved oxygen concentration. By comparing the iron oxidation rate of live samples and samples poisoned by azide (respiratory inhibitor), they found that up to 75% of the iron oxidation process was mediated by microorganisms, while the pure chemical oxidation rate of Fe (II) was relatively slow
[27]. Under laboratory conditions, Maisch et al. Found that under the conditions of O
2 concentration of 20 μM and in the absence of Fe (Ⅲ) minerals, microaerobic Fe (Ⅱ) -oxidizing complex bacteria (and aerobic Fe (Ⅱ) -oxidizing bacteria Sideroxydans spp
[28]. The similarity is 99.6%, and the contribution rate of Fe (Ⅱ) oxidation is about 40% within 26 H, which fully illustrates the important participation of microorganisms in Fe (Ⅱ) oxidation.
2.2 As/Sb fixation and valence conversion
After Fe (Ⅱ) is oxidized to Fe (Ⅲ) by denitrifying bacteria and photosynthetic Fe (Ⅱ) -oxidizing bacteria, As and Sb will be removed by adsorption and/or co-precipitation with the formed Fe (Ⅲ) minerals
[29]. Hohmann et al. Carried out As (Ⅲ)/As (Ⅴ) removal by microorganisms in a medium with an initial Fe (Ⅱ) concentration of about 3.5 mM (initial As concentration of 20 or 50 μM). The results showed that the removal rates of As (Ⅲ)/As (Ⅴ) by anaerobic Fe (Ⅱ) -oxidizing bacteria BoFeN1, photosynthetic Fe (Ⅱ-oxidizing bacteria SW2 and anaerobic Fe (Ⅱ-oxidizing bacteria KS were all higher than 96%.Li et al. Found that Sinorhizobium sp. GW3 had the ability to oxidize both Sb (Ⅲ) and Fe (Ⅱ), and the produced Sb (Ⅴ) was fixed on Fe (Ⅲ) minerals by adsorption/coprecipitation
[29][30]. It is believed that this process has great potential for bioremediation of Sb contaminated environment.
In the process of As/Sb fixation, the microbial oxidation of Fe (Ⅱ) may be accompanied by the oxidation of As (Ⅲ)/Sb (Ⅲ). As for As (Ⅲ), microbial As (Ⅲ) oxidation exists As a metabolic mechanism in a variety of heterotrophic and autotrophic microorganisms
[31,32]. However, there are relatively few studies on the mechanism of Sb (Ⅲ) microbial oxidation, and some studies suggest that Sb (Ⅲ) can also induce microbial As defense reaction, so the microbial pathway used for As (Ⅲ) oxidation can also be used for Sb (Ⅱ) oxidation
[33⇓~35]. Many studies have confirmed that the oxidation of arsenic and antimony in the process of microbial oxidation of Fe (Ⅱ) is widespread, which leads to the more complex process of arsenic and antimony fixation in iron minerals synthesized by microorganisms
[36,37]. Taking As as an example, Sun et al. Considered that in the presence of Fe (Ⅱ) -oxidizing microorganisms and Fe (Ⅱ), the removal mechanism of As is As follows: (1) Fe (Ⅲ) and As (Ⅲ) are oxidized to form Fe (Ⅲ) and As (Ⅴ); (2) As (Ⅴ) are adsorbed and coprecipitated by Fe (Ⅳ) minerals
[38]. Studies have shown that As (Ⅲ) is 60 times more toxic than As (Ⅴ), and is more soluble and mobile in water. However, the toxicity of Sb (Ⅲ) is 10 times higher than that of Sb (Ⅴ), so the microbial oxidation of As (Ⅲ)/Sb (Ⅲ) will help to reduce the toxicity of the two elements, and is also more beneficial to the fixation of arsenic and antimony in Fe (III) minerals
[22,39].
3 Release and Secondary Fixation of As/Sb by Microbial Involvement in Reductive Transformation of Fe (Ⅲ) Minerals
Under anaerobic conditions, many microorganisms can reduce and dissolve Fe (Ⅲ) minerals to Fe (Ⅱ), resulting in the release of As/Sb fixed in Fe (Ⅲ) minerals, and As (Ⅴ)/Sb (Ⅴ) can also be reduced by microorganisms. After reduction, Fe (Ⅱ) -bearing materials crystallize again to form secondary iron minerals, which also have the ability to fix As/Sb. The removal efficiency of As/Sb by Fe redox mediated by various microorganisms is summarized in Table 1. This chapter mainly introduces the microbial-mediated reduction of Fe (Ⅲ) minerals and the subsequent formation of secondary minerals under anaerobic conditions, and explains the release, reduction and secondary fixation of As/Sb caused by this process. As a large number of As/Sb bound to Fe (Ⅲ) minerals exist in nature, the release of As/Sb caused by the microbial reduction of Fe (Ⅲ) has an important environmental impact, which is the main cause of soil and groundwater As/Sb pollution, and the toxicity of As (Ⅲ)/Sb (Ⅱ) under reducing conditions is higher than that of As (Ⅴ)/Sb (Ⅴ)
[8,40]. Therefore, when the method of As/Sb fixation by microbial synthesis of Fe (Ⅲ) minerals is applied to practical wastewater treatment, the release of As/Sb caused by microbial reduction and dissolution of Fe (Ⅲ) minerals under anaerobic conditions should be avoided As far As possible.
3.1 Mineral reduction
Many microorganisms can reduce Fe (Ⅲ) to Fe (Ⅱ) to obtain energy by coupling organic or inorganic electron donors (acetate, lactate, H
2, etc.), and these microorganisms are called Fe (Ⅲ) -reducing bacteria
[41]. For example, Bretschger et al. Cultured MR-1 wild strain and mutant strain in 20 mM Fe (Ⅲ) medium, and detected a maximum increase of 492 ± 25.2 μM Fe (Ⅱ) after 48 H, indicating that these strains have Fe (Ⅲ) reduction ability
[42]. Microbial Fe (II) oxidation and Fe (III) reduction constitute the microbial iron cycle process (Fig. 2)
[43]. Microbial-mediated reduction of Fe (Ⅲ) (hydr) oxides can produce Fe (Ⅱ) -containing compounds in aqueous and solid phases, and the minerals produced by the re-crystallization of these compounds are called secondary iron minerals
[44]. Fe (Ⅲ) -reducing bacteria are ubiquitous and phylogenetically diverse in nature, which makes the study of microbial reduction of Fe (Ⅲ) of global significance
[44].
3.2 As/Sb release and secondary fixation
In the process of reductive dissolution of Fe (Ⅲ), when Fe (Ⅲ) (hydroxide) oxide contains As/Sb complex, the dissolution process may lead to the release of the two substances into the natural environment
[45]. Desorption and subsequent microbial transformation of As by reduction of iron oxides is considered to be the main mechanism of arsenic release by migration
[40]. Hockmann et al. Studied the migration and transformation process of Sb in soil microenvironment. By adding lactic acid to enhance microbial activity, they found that there was a significant positive correlation between Sb (Ⅲ) and Fe (Ⅱ) concentration in the solution, indicating that the reductive dissolution of iron minerals containing Sb (Ⅲ) led to a decrease in the number of adsorption sites, and the bound Sb (III) was gradually released into the solution
[46]. According to related studies, the release rate of As/Sb is usually affected by the volume of Fe (Ⅲ) mineral, crystallinity, cell-derived organic matter in the mineral and the concentration of pollutants
[47⇓~49].
Microbial Fe (Ⅲ) reduction dissolution is accompanied by the formation of secondary minerals, and the Fe (Ⅱ) produced by Fe (Ⅲ) reduction will crystallize again to form Fe (Ⅱ) minerals and mixed Fe (Ⅱ/ Ⅲ) minerals, which have the ability to isolate As/Sb again
[50]. Muehe et al. Studied the phenomenon that MR-1strain participated in the transformation of As during the formation of secondary iron minerals in the presence of phosphate, and detected the rapid release of As (Ⅲ) during the reduction of Fe (Ⅲ) minerals, but some As (Ⅲ) would be adsorbed and coprecipitated on the remaining minerals or secondary minerals again
[40]. However, the concentration change of As (Ⅴ) before and after reduction is not obvious, and most of As (Ⅴ) still exists in the minerals, indicating that the formation of secondary minerals has a fixing effect on both As (Ⅲ) and As (Ⅵ), and the effect on the latter is stronger. This phenomenon may be caused by the stronger adsorption capacity of As (Ⅴ) in neutral environment and the easier replacement of As (Ⅴ) with phosphate in vivianite (forming mixed mineral phase (Fe
3(PO
4/AsO
4)
2·8H
2O))
[51]. Fang et al. 'S study also proved a similar conclusion
[52]. In this study, Bacillus sp. D2201 was cultured in high arsenic sediment, and the microcosm experiment showed that the inoculation of Bacillus sp. D2201 led to the dissolution of Fe (Ⅱ) in the sediment and the increase of As (Ⅴ) concentration in the solution, while the concentration of As (Ⅴ) in the microenvironment decreased significantly after a period of treatment, indicating that As (Ⅵ) was refixed by the sediment.
For the secondary immobilization of Sb, Burton et al. Found that the microbial-mediated formation of secondary ferrihydrite minerals containing Sb (Ⅴ) would result in a significant decrease in both the Sb concentration in the solution and the Sb content on the mineral surface. The most likely reason is that Sb (Ⅴ) has been incorporated into the secondary iron minerals, resulting in a decrease in the Sb (Ⅵ) content in the solution and on the mineral surface
[53]. Because of the similarity of the ionic size of Sb (Ⅴ) and Fe (Ⅲ) and the bond distance between Sb — O and Fe — O octahedra, Sb (Ⅴ) has structural compatibility in Fe (Ⅲ) (hydroxide) oxides, so it is very likely that Sb (Ⅴ ′) can enter the interior of Fe (Ⅱ) minerals
[53,54]. Another study by Burton et al. Showed that Sb (Ⅴ) at low, medium and high concentrations could replace 0.1, 0.5 and 1.4 mol% of Fe (Ⅲ) in goethite during recrystallization
[55]. Mitsunobu et al. Obtained a similar conclusion
[56]. They found that the desorption of Sb (Ⅴ) was not observed during the transformation of ferrihydrite containing Sb (Ⅴ), and there was no separate phase of Sb and Fe. The study demonstrated by extended X-ray absorption fine structure (EXAFS) analysis that ferrihydrite mineral transformation results in the incorporation of Sb (Ⅴ) adsorbed on the mineral surface into the secondary mineral structure, so no Sb (Ⅴ) is released.
However, some studies suggest that the reductive dissolution of Fe (Ⅲ) is not a significant process. Lin et al. Established a coupled kinetic model describing the adsorption/desorption of As (Ⅲ) and As (Ⅴ) by Fe (Ⅲ) -reducing bacteria on iron minerals. The modeling results showed that the reductive dissolution of Fe (Ⅲ) was less than 8% of the total iron, which could be ignored
[57]. It is considered that the inhibition of Fe (Ⅲ) reduction is related to the adsorption of Fe (Ⅱ), As (Ⅴ) and other oxygen anions on Fe (Ⅲ) minerals in solution.
3.3 As/Sb valence conversion
The release and secondary fixation of As/Sb may also involve the valence transformation of the two elements. In addition to As (Ⅲ) microbial oxidation, As (Ⅴ) reduction has also been shown to act As a respiratory process for some anaerobic microorganisms, which are called heterotrophic arsenic-reducing microorganisms
[58]. Omoregie et al. Found anaerobic microorganisms that can simultaneously reduce As (Ⅴ) and Fe (Ⅲ) by culturing microorganisms in the microenvironment of groundwater sediments in Cambodia
[59]. As (Ⅴ) reducing microorganisms (including iron reducing bacteria and sulfate reducing bacteria which can participate in the reduction of Fe (Ⅲ)) can directly convert As (Ⅴ) released by the reduction of Fe (Ⅲ) and As (Ⅵ) in the solid phase of Fe minerals into As (Ⅲ)
[60]. Fig. 3 shows the reduction and dissolution of Fe (Ⅲ) minerals and the formation of secondary minerals accompanied by the transformation of As. It is known that the fixation intensity of iron minerals to As (Ⅲ) is generally less than that of As (Ⅴ), so the reduction of As (Ⅴ) may weaken the removal effect of secondary minerals on As
[61,62]. Compared with As (Ⅴ), studies on the bioreduction of Sb (Ⅴ) are relatively scarce. Abin et al. Added 1 mM Sb (Ⅴ) to sediments collected from an abandoned antimony mine in Idaho, USA, for anaerobic incubation, and Sb (Ⅴ) was completely reduced to Sb (Ⅲ) within 100 H, and proved by control that this process was not controlled by abiotic reactions
[63]. Kulp et al. Found the phenomenon of biological Sb (Ⅴ) reduction in anoxic freshwater environment. The reduction process uses lactic acid or acetic acid as electron donors, accompanied by the release of CO
2, indicating that the process is carried out by heterotrophic respiration
[64]. However, the molecular mechanism of microbial reduction of Sb (Ⅴ) is still unclear, and the enzyme involved in this reaction has not been identified
[37].
4 Mechanism of As/Sb Fixation by Microbial Synthesized Fe (Ⅲ) Mineral
The content of Fe (Ⅲ) mineral is the highest among the minerals fixing As/Sb in nature, and it is considered to be the most important mineral controlling the migration of As/Sb in the environment because of its strong affinity for As/Sb
[21,65]. In the actual wastewater treatment, Fe (Ⅲ) minerals are more widely used. Goethite, hematite, ferrihydrite and other materials are used to treat wastewater containing As/Sb, and they all show good As/Sb removal effect
[66]. Based on the universality of the application of Fe (Ⅲ) minerals, this section takes the Fe (Ⅲ) minerals synthesized by microorganisms under oxidizing conditions As the treatment materials, and expounds the mechanism of As/Sb fixation by Fe (III) minerals from three aspects of the mineralogical properties of Fe (III) minerals synthesized by microorganisms, the thermodynamics of As/Sb adsorption, and the complexation mechanism.
4.1 Mineralogy of Microbial Synthesis of Fe (Ⅲ)
In terms of mineral morphology, during the synthesis of microbial Fe (Ⅲ) (hydroxide) oxide, mineral crystals grow slowly due to the influence of cell-derived organic matter and ionic impurities, so the volume and crystallinity of microbial iron mineral crystals are smaller than those of abiogenic Fe (Ⅲ) (hydroxide) oxide. Sowers et al. Used TEM to compare the differences in the morphology of microbial and synthetic ferrihydrite (Fh)(Fe
10O
14(OH)
2+mH
2O)
[67]. The results showed that the former was irregular filamentous and spherical structure with particle diameter of 50 ~ 100 nm; The latter is mainly small spherical regular aggregates.
In terms of mineral composition, the composition of iron minerals synthesized by microorganisms is mainly affected by environmental media. As found by Posth et al., R. ferrooxidans sp in the presence of phosphate buffer medium
[47]. Ferrihydrite and iron phosphate were the main Fe (Ⅲ) minerals synthesized by strain SW2, while iron carbonate was the main Fe (Ⅲ) mineral in carbonate buffer. According to Kappler et al., the relative contents of acetate, Fe (Ⅱ) and nitrate will affect the composition of oxidation products of Fe (Ⅱ) -oxidizing strain BoFeN1, and acetate will promote the synthesis of goethite with better crystallinity by microorganisms
[68]. Wu Zijun et al. Selected the leachate precipitated iron mud with neutrophilic iron-oxidizing bacteria as the dominant population in the coastal zone of Zhoushan, Zhejiang Province for mineral phase and element analysis, and concluded that the iron mineralization products of the bacteria were mainly ferrihydrite with poor crystallinity, mainly composed of Fe and a small amount of Si (3% -5%)
[69].
Due to the obvious mineralogical differences between biogenic and abiogenic Fe (Ⅲ) (hydroxide) oxides, the adsorption capacity of iron minerals of different origins will be affected. Tyler et al. Compared the adsorption effect of As (Ⅴ) by microbial synthetic ferrihydrite and artificial synthetic ferrihydrite, and the results showed that the maximum adsorption capacity of the former was three times that of the latter. The reasons for the higher affinity of microbially synthesized ferrihydrite for As (Ⅴ) are speculated As follows: (1) As (Ⅴ) is mainly adsorbed through angle-shared sites, and microbially synthesized ferrihydrite may contain more angle-shared composite sites, so it has a stronger adsorption capacity for As (Ⅵ); (2) The tight binding of ferrihydrite and organic matter in the microbial synthesis of ferrihydrite may lead to physicochemical changes in the interfacial region, which promotes the accumulation of outer sphere complexes and leads to more As (Ⅴ) adsorption
[67].
In addition to microbial action, the presence of As/Sb can also affect the crystal growth during the formation of iron minerals, resulting in a decrease in crystal size and a delay in crystal growth
[29,70]. For example, Hohmann et al. Found that the increase of As (Ⅲ) and As (Ⅴ) concentration will lead to the formation of ferrihydrite with poor crystallinity, thus improving the potential bioavailability of Fe (Ⅲ) (hydroxide) oxide
[29,71]. According to the study of Wang et al., with the increase of As (Ⅲ)/Fe ratio from 0.8/1000 to 8/1000, the adsorption products of As (Ⅲ) on magnetite undergo mineral transformation, and the As-Fe atomic distances change from 3.28 ~ 3.30 ± 0.0404 Å and 4.54 ~ 4.57 ± 0.0505 Å (possibly
2C complexes) to 3.50~3.53±0.04Å(
3C complexes). For the effect of Sb, Mitsunobu et al.'s Research shows that under high concentration of Sb (Ⅴ), the particle aggregation of ferrihydrite will be disturbed and preferentially converted into goethite, so Sb (Ⅴ) will affect the type and quantity of secondary iron minerals
[56]. Burton et al. Considered that high concentration of Sb (Ⅴ) would reduce the rate of atomic exchange between Fe
2+ and Fe (Ⅲ) in goethite by inhibiting electron transfer, resulting in a decrease in the recrystallization level of goethite
[72].
4.2 Thermokinetics of As/Sb Adsorption on Fe (Ⅲ) Mineral
In most cases, the adsorption of As/Sb on iron minerals is more consistent with the Langmuir model than the Freundlich model
[73,74]. The results show that the adsorption process of As (Ⅲ/Ⅳ) by ferrihydrite and goethite and the adsorption process of Sb (Ⅲ/Ⅴ) by goethite, lepidocrocite, hematite and Hydrous ferric oxide (HFO) are more in line with the Langmuir isotherm, indicating that the adsorption of As (Ⅱ/Ⅴ) and Sb (Ⅲ/Ⅴ) is mainly monolayer adsorption
[75,76].
Zeng et al. Fitted the adsorption kinetics of As (Ⅴ) on synthetic magnetite, and the results showed that the fitting effect of the pseudo-second-order kinetic model was better than that of the first-order kinetic model at different initial concentrations of As (Ⅴ)
[74]. In this study, it is considered that the rate-limiting steps in the early and late stages of adsorption are intraparticle diffusion and film diffusion, respectively. In other studies, the adsorption kinetics of As (Ⅲ) and Sb (Ⅲ) on chitosan-supported iron nanomaterials and magnetic iron-based materials were fitted, and the pseudo-second-order model was also the best, indicating that the adsorption of As (Ⅲ) and Sb (III) was mainly a chemical adsorption process, and the effective active sites determined the adsorption capacity
[7,14,77]. D'Arcy et al. Regard the adsorption process of As (Ⅴ) on the D 'Arcy composite As a combination of two reactions according to the monodentate and bidentate surface complexation forms, and then obtain a two-channel kinetic model, which can more accurately fit the adsorption kinetic process
[78].
4.3 Complexation Mechanism of Fe (Ⅲ) Minerals with As/Sb
4.3.1 Complexation of Fe (Ⅲ) minerals with As
Early studies suggested that As (Ⅴ) and As (Ⅲ) could form bidentate and monodentate complexes with Fe
[76]. The EXAFS analysis of As (Ⅲ) adsorption on α-FeOOH by Manning et al. Showed that As (Ⅲ) adsorption mainly formed a bidentate dinuclear complex with an As (III) -Fe shell distance of 3.378 378 Å and an average fitting coordination number of 2.426 ± 0.099 Fe atoms
[79]. Ona-Nguema et al. Found that As (Ⅲ) adsorbed on second-line hydrated iron and hematite would form a bidentate mononuclear edge-sharing (
2E) complex [As (Ⅲ) -Fe = 2.9 ± 0.11 Å] and a bidentate binuclear corner-sharing (
2C) complex [As
[80]. Dutta et al. Used the synthesized polyaniline hollow microsphere /Fe
3O
4 material for As (Ⅲ)/As (Ⅴ) removal, and considered that the material adsorbed As through two complexation mechanisms of monodentate mononuclear and bidentate binuclear (Fig. 4)
[81]. According to the EXAFS analysis of Fendorf et al., the adsorption of As (Ⅴ) on α-FeOOH can form monodentate mononuclear complexes [As (Ⅴ) -Fe = 3.6060Å], bidentate dinuclear complexes [As (Ⅴ) -Fe = 3.24 ~ 3.2626Å] and bidentate mononuclear complexes [AsⅤ-Fe = 2.83 ~ 2.8585Å]
[82]. Venema et al. Used a modified Multi site complexation (MUSIC) model to analyze the main crystal planes of hematite, goethite and lepidocrocite participating in As (Ⅲ) adsorption, which were (110), (110) and (001), respectively
[83].
Later studies proposed a new coordination mode formed by the As removal process
[84]. The EXAFS analysis of As (Ⅴ) and As (Ⅲ) adsorbed on magnetite (Fe
3O
4) by Wang et al. Showed that AsO
4 mainly formed bidentate binuclear edge-sharing (
2C) complexes and exophore complexes on the (100) crystal face of magnetite
[85]; While AsO
3 will occupy the tetrahedral vacancy of magnetite (111) crystal face to form tridentate hexanuclear (
3C)As(Ⅲ) surface complex
[86]. It is believed that the coordination mode of As and Fe is affected by the interaction mode of As and Fe. The adsorption process will make As (Ⅴ) and magnetite form
2C complexes, while the coprecipitation process tends to form
3C complexes. Catalano et al. Used RAXR technique to analyze the adsorption mechanism of As (Ⅴ) on the (0 1 2) crystal face of hematite, and considered that there were two ways of complexation inside the sphere and outside the sphere in the adsorption process, and proposed that the combination of electrostatic attraction, hydrogen bonding, and interfacial water structure was an important mechanism for the stabilization of the outer sphere complex
[87]. Zhao et al. Used a 2:1 layered clay mineral NAu-1 containing Fe (Ⅲ) for the co-adsorption of As (Ⅲ) and As (Ⅴ), and considered that As (Ⅲ) was adsorbed on the octahedral layer through ligand exchange with OH, while As (Ⅴ) was adsorbed on the tetrahedral layer mainly through cation bridging, and the adsorption of different layers could effectively weaken the competition between the two
[84].
4.3.2 Complexation of Fe (Ⅲ) minerals with Sb
The space structure and Sb — O bond distance of Sb (Ⅲ) and Sb (Ⅴ) are different. The coordination of Sb (Ⅲ) is asymmetric, three coordination bonds away from the lone pair electrons form a trigonal pyramidal coordination structure, while Sb (Ⅴ) is six coordination bonds forming a symmetrical octahedral coordination structure. The bond distances of Sb (Ⅲ) — O bond and Sb (Ⅴ) — O bond are 1.92 ~ 2.0404 Å and 1.98 ~ 2.1010 Å, respectively
[88]. These two factors are the main reasons for the difference in the complexation mode between the two and iron minerals. For the immobilization of Sb (Ⅴ), it is believed that the adsorption of Sb (Ⅴ) on Fe minerals will form two complexes, bidentate mononuclear edge-sharing (
2E) complexation and bidentate binuclear corner-sharing (
2C) complexation, and the coordination structure model is shown in Fig. 5
[89]. Mitsunobu et al. Found that Sb (Ⅴ) adsorbed on synthetic ferrihydrite and goethite would form edge-sharing, corner-sharing and edge-sharing internal sphere complexes, respectively, by fitting the paths of Sb-O and Sb-Fe
[54]. However, the co-precipitation of Sb (Ⅴ) with the two minerals will cause the incorporation of Sb (Ⅴ) into the mineral structure, resulting in the increase of the intensity of the Sb-Fe peak. Because Sb (Ⅴ) and Fe (Ⅲ) have similar ionic sizes (0.6060 Å and 0.6565 Å, respectively) and the bond distances of the octahedral ligands are similar (Fe-O: 1.98 ~ 2.000 Å; Sb-O: 1.97 ~ 2.000 Å), this structural incorporation is reasonable. For the immobilization of Sb (Ⅲ), the EXAFS analysis of Scheinost and Leuz et al. Showed that Sb (Ⅲ) formed a bidentate corner-sharing complex on the surface of goethite [Sb (III) -Fe = 3.4646Å]
[88][90]. In the adsorption/coprecipitation process, Sb (Ⅴ) usually does not undergo valence conversion, while Sb (Ⅲ) is easily oxidized to Sb (Ⅴ). One study found that Sb (Ⅲ) adsorbed on goethite was oxidized to Sb (Ⅴ) under alkaline conditions (pH 9.9), resulting in a change in the total amount of Sb adsorbed on the mineral
[90]. Guo et al. Showed that Sb (Ⅲ) was oxidized in both HFO adsorption and coprecipitation processes, and Sb mainly existed in the form of Sb (Ⅴ) oxidation after the reaction
[75]. From the point of view of biological toxicity, the oxidation of Sb (Ⅲ) and the stability of Sb (Ⅴ) are beneficial to the reduction of Sb toxicity. The coordination mechanisms of different valence As/Sb with iron minerals are summarized in Table 2.
5 Factors affecting As/Sb fixation in microbially synthesized Fe (Ⅲ) mineral
In the application of microbial-led remediation technology, the influencing factors and mechanisms will be more complex because the microorganisms themselves are more sensitive to environmental changes. This section mainly introduces the reactant conditions (As/Sb species, iron mineral species) and environmental conditions (coexisting anions, organic matter, pH value) that affect the As/Sb fixation of Fe (Ⅲ) minerals synthesized by microorganisms. When the microbial synthesis of Fe (Ⅲ) minerals is applied to the actual wastewater treatment, the process parameters and environmental conditions can be adjusted according to the influencing factors and action modes to achieve the target As/Sb removal effect.
5.1 As/Sb and iron mineral specie
As/Sb species. Different oxidation States of As/Sb have different effects on microorganisms, and most of the studies on the effects of microorganisms focus on the oxidation state of As. Hohmann and Park et al. Suggested that high concentration of As (Ⅲ) would inhibit the growth of microorganisms, while As (Ⅴ) had no effect on microorganisms, and appropriate concentration of As (Ⅴ) could even promote the growth of related strains, which may be related to the metabolism of As (Ⅴ) As an electron acceptor by microorganisms
[29][91]. In addition, the valence state of As/Sb can affect the coordination complexation and electrostatic attraction in the process of Fe fixation. The effect of As/Sb on the coordination complexation may be caused by the difference of the existing form of As/Sb with different valence in solution and the structure of the surface complex formed with Fe
[90][79]. The electrostatic attraction is caused by the difference of acidity and charge of As/Sb with different valence States in solution
[7,92][76].
Iron mineral species. Fe crystallizes, dissolves and transforms continuously in the natural environment, forming different kinds of iron minerals, and the adsorption behavior of As/Sb on different iron minerals is different
[93]. Firstly, the adsorption capacity of iron minerals with large specific surface area (such as ferrihydrite) is stronger than that of iron minerals with small specific surface area (such as goethite)
[75]. Secondly, the structure of the crystal will affect the rate of mineral synthesis and reductive dissolution, thus indirectly affecting the rate of As/Sb fixation
[94]. Third, other substances in iron minerals (such As sulfate, phosphate and other metal minerals) can also promote or inhibit the adsorption of As/Sb
[76,95,96].
5.2 Coexisting anion
Nitrate. Under anaerobic conditions, microorganisms use nitrate as an electron acceptor for Fe (Ⅱ) oxidation, so nitrate is an important factor affecting Fe (Ⅱ) oxidation in anaerobic organisms. The addition of nitrate can promote the nitrate-dependent oxidation of Fe (Ⅱ) by microorganisms, thus accelerating the synthesis of Fe (Ⅲ) (hydroxide) and the removal rate of As/Sb
[46,59]. In addition, nitrate can also promote the anaerobic biological oxidation of As (Ⅲ)/Sb (Ⅲ), resulting in the change of As/Sb fixation behavior and efficiency of Fe minerals
[97,98].
Phosphate. Phosphate is an important factor affecting the microbial transformation of Fe (Ⅲ) (hydr) oxides and the formation rate of secondary minerals
[99,100]. Many studies suggest that phosphate can slow down the dissolution rate of iron minerals, thus inhibiting the mineral transformation process and slowing down the release rate of fixed As/Sb
[101,102]. However, some studies have come to different conclusions. For example, Deng et al. Proposed that the mode of action of phosphate varies with oxygen content. Under aerobic conditions, phosphate will inhibit the reduction of iron minerals, while under anoxic conditions, it will promote the reduction
[100]. In addition, phosphate has a strong adsorption capacity for iron minerals, so it will compete with As/Sb for surface adsorption sites, and the relative strength of competition is affected by pH value and As/Sb valence
[103][104][105,106].
Silicate. Silicates adsorb on the surface of iron oxides in the form of monomers and polymers, thus competing with As for adsorption sites, which is affected by the concentration of silicates and pH
[107,108]. However, the adsorption kinetic rate of silicate-iron oxides is much slower than that of As (Ⅲ) and As (Ⅴ), and it usually takes several hours to reach the equilibrium state, so the effect of silicate on As adsorption should be explored in a longer equilibrium time
[108].
Carbonate. The adsorption of carbonate on Fe (Ⅲ) (hydroxide) oxide can form an inner sphere complex to compete with As for adsorption sites, but the affinity is much lower than that of As (Ⅲ) and As (Ⅴ), and the competitive adsorption is pH-dependent
[109]. In addition, carbonate can also cause the change of solution pH, which indirectly affects the As adsorption behavior. For example, Brechb Brechbühl et al. Considered that under high CO
2 partial pressure, the dissolution of a large amount of CO
2 would reduce the pH value of the solution, which would lead to the change of the adsorption capacity of iron minerals for As
[110]. However, the experimental results of Radu et al. And Wei et al. Prove that under neutral conditions, the presence of CO
32- will compete with As for iron oxide adsorption sites, but the competition effect is relatively weak
[111][112]. The effects of several coexisting anions on As/Sb removal from iron minerals are summarized in Table 3.
5.3 Organic matter
Under natural conditions, iron minerals are usually formed in the presence of dissolved organic matter, resulting in mineral-organic bonding
[45,115]. The effects of organic matter on As/Sb adsorption of microbially synthesized iron minerals are various. First, organic matter can affect the mineral transformation of Fe by blocking adsorption sites and inhibiting mineral aggregation and growth
[48,116]. Secondly, organic matter can change the surface charge of minerals by blocking H
+ adsorption sites and deprotonating functional groups such as -COOH, thus affecting the adsorption of Fe
[117,118]. In addition, some organic compounds have catalytic effects on the redox of As and Sb. For example, Karimian et al. Suggested that Humic acid (HA) can be used as a redox activator and electron shuttle to promote the reduction of Sb (Ⅴ) to Sb (Ⅲ), thus changing the adsorption capacity of ferrihydrite for Sb.
5.4 pH value
The pH in the environment largely determines the type of metabolism in which the bacterial community appears in the environment and is an important parameter for biological activity
[119]. Each microorganism has a certain pH range beyond which it cannot survive. At the same time, microorganisms can also change the pH of their surrounding medium according to their physiological needs. In addition to bioactivity, pH also affects the As/Sb adsorption process of Fe minerals. The pH value can change the surface charge of the adsorbent and adsorbate, resulting in a change in electrostatic interaction. However, when specific adsorbents and adsorbates are discussed, the change rules are also different, because the effect of pH change on adsorption is different under different As/Sb valence States, As/Sb concentrations and iron mineral types (Table 4)
[74,79,90].
6 Summary and Prospect
The "synthesis-dissolution-transformation" cycle of iron minerals involving microorganisms exists widely in nature, and this process is also accompanied by the fixation and release of arsenic and antimony in the polluted environment. Under the oxidizing condition, Fe (Ⅱ) was oxidized by aerobic microorganisms to synthesize Fe (Ⅲ) minerals for As/Sb fixation. Under reducing conditions, anaerobic microorganisms reduce and dissolve Fe (Ⅲ) minerals, resulting in the release of fixed As/Sb, and the reduced iron-containing compound crystals can fix As/Sb again. The change of redox conditions can also lead to the occurrence of As (Ⅲ)/Sb (Ⅲ) microbial oxidation and As (Ⅴ)/Sb (Ⅴ) microbial reduction. Microbial synthesis of iron minerals for pollution remediation has the characteristics of high efficiency, low cost and environmental friendliness, so it has great potential in the removal of arsenic and antimony. At present, some progress has been made in the microbial recycling process of iron minerals, the mechanism and effect of arsenic and antimony removal by iron minerals, but most of the studies are still in the experimental stage, and there are few engineering examples applied in in-situ environmental remediation. According to the current research, in the wastewater treatment aiming at removing pollutants and reducing toxicity, the microbial synthesis of Fe (Ⅲ) minerals and the fixation of As/Sb and the oxidation of As (Ⅲ)/Sb (Ⅲ) under oxidizing conditions should be enhanced, while the release of As/Sb and the reduction of As (Ⅴ)/Sb (Ⅴ) under reducing conditions should be avoided.
In this paper, it is considered that the current research still has the following problems: First, there are many factors affecting the process of As/Sb fixation by microbially enhanced iron minerals, and the mechanism is complex, and As/Sb may have toxic effects on microorganisms, affecting the removal effect. Therefore, the stability and controllability of As/Sb removal still need to be improved in the promotion of this method. Secondly, the current research pays more attention to the process of As (Ⅲ/Ⅴ) removal by microorganism-iron minerals, but there is little in-depth discussion on Sb (Ⅲ/Ⅴ), which is of the same main group and also has high toxicity, and the complete process of antimony removal by microorganisms in iron minerals has not yet been summarized. Finally, the natural process of As/Sb fixation by iron minerals enhanced by microorganisms has been widely studied, while the engineering application based on the natural process is still in the exploratory stage. In the application process, how to enhance the As/Sb fixation and weaken the release effect to improve the removal efficiency remains to be explored.
In view of the above problems, future research can be carried out from the following four aspects.
(1) The mechanism of microbially enhanced As/Sb removal was discussed from the biological and chemical point of view. By integrating the studies of As/Sb in iron minerals involving microorganisms, a model was established to predict the speciation transformation of Fe/As/Sb, the flow and fixation of As/Sb, and the removal effect of As/Sb in the process of iron cycle involving microorganisms.
(2) Simulating the natural environment to explore the interaction between reactants and the environment in the removal of As/Sb from microbially enhanced iron minerals, to explore the impact factors and mechanisms of the removal process, and to improve the method according to the impact to ensure its sustainable and efficient removal of As/Sb;
(3) To further study the microbial toxicity and biological metabolism mechanism of Sb element, analyze the flow and transformation process of Sb with the biological iron cycle and the mechanism and influencing factors of iron mineral Sb fixation participated by microorganisms, and constantly establish microbial and chemical remediation methods for antimony contaminated environment;
(4) Small-scale and pilot-scale studies on As/Sb removal from iron minerals by microbial enhancement were carried out. Aerobic conditions are created and relevant process parameters are adjusted to strengthen As/Sb immobilization and As (Ⅲ)/Sb (Ⅲ) oxidation of Fe (Ⅲ) synthesized by microorganisms, and inhibit As/Sb release and As (Ⅴ)/Sb (Ⅴ) reduction of microorganisms at the same time, so As to achieve efficient and low-toxic As/Sb removal effect; An artificially controllable microbial-iron mineral arsenic-antimony remediation system is established by utilizing the metabolic advantages of microorganisms and the regulation function of an ecosystem.
National Natural Science Foundation of China(21976153)
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