Arsenate immobilization associated with microbial oxidation of ferrous ion in complex acid sulfate water

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Abstract

Chemical, XRD, SEM, RS, FTIR and XPS techniques were used to investigate arsenate immobilization associated with microbial Fe2+ oxidation in a complex acid sulfate water system consisting of a modified 9 K solution (pH 2.0) plus As, Cu, Cd, Pb, Zn and Mn. At a 1:12.5:70 molar ratio of As:Fe:S, schweretmannite formation was impeded. This was in contrast with the predominant presence of schwertmannite when the heavy metals were absent, suggesting that a schwertmannite binding model is not valid for explaining arsenate immobilization in the complex system. In this study, arsenate was initially immobilized through co-precipitation with non-Fe metals and phosphate. Subsequently when sufficient Fe3+ was produced from Fe2+ oxidation, formation of a mixed iron, arsenate and phosphate phase predominated. The last stage involved surface complexation of arsenate species. Pb appeared to play an insignificant role in arsenate immobilization due to its strong affinity for sulfate to form anglesite. Phosphate strongly competed with arsenate for the available binding sites. However, As exhibited an increased capacity to compete with P and S for available binding sites from the co-precipitation to surface complexation stage. Adsorbed As tended to be in HAsO42− form. The scavenged arsenate species was relatively stable after 2464-h aging.

Highlights

► Arsenate immobilization upon microbial Fe2+ oxidation in acid sulfate water was investigated. ► The schwertmannite model was invalid for explaining the process in the complex acid sulfate water. ► Arsenate was initially removed through co-precipitation of non-Fe metals, arsenate and phosphate. ► Followed by formation of mixed Fe, arsenate and phosphate phases after sufficient Fe3+ was produced. ► The last step involved surface complexation with HAsO42− being the dominant adsorbed As species.

Introduction

Arsenic is frequently associated with acid sulfate waters originating from mined areas where sulfidic rocks occur [1], [2]. Ferrous sulfate (FeSO4) is the major constitute of acidic mine water, and the mine water-borne Fe2+ tends to oxidize to Fe3+ and subsequently hydrolyse, resulting in the generation of H+ and formation of orchreous precipitates. It has been demonstrated that in some acid sulfate water systems, poorly crystalline schwertmannite (Fe8O8(OH)6SO4) is initially formed in a pH range of 2.8–4.5 [3], [4]. Schwertmannite is then gradually transformed into more stable mineral phases such as goethite (FeOOH) or/and jarosite (KFe3(OH)6(SO4)2) following aging [5], [6], [7], [8], [9], [10]. Due to their large specific surface area, ochreous precipitates (especially schwertmannite) possess a strong capacity to scavenge aqueous arsenate under acidic conditions [11], [12], [13]. This geochemical process is viewed to be responsible for the natural attenuation of mine water-borne arsenates [14], [15], [16]. Sorption of arsenate to pre-prepared schwertmannite has been investigated in the laboratory by some workers [17], [18]. However, such processes are not relevant to natural acid mine drainage scenarios that involve spontaneous formation of solid phases from mixed Fe, S and As systems where co-precipitation is likely to occur [11].

In abiotic co-precipitation experiments involving arsenate and other oxyanions (chromate and phosphate), Regenspurg et al. [19] found that the characteristic XRD peaks of schwertmannite disappeared when sufficient amounts of arsenate were present in the systems, suggesting the formation of other mineral phases rather than schwertmannite. The similar phenomena were also observed in the Fe–S–P system in their experiments. It is therefore likely that in more complex acid sulfate water systems, the immobilization of arsenate cannot be simply explained by a schwertmannite co-precipitation and adsorption model. This could be particularly true when the acid sulfate water contains sufficiently high concentration of heavy metals such as Cu, Cd, Pb, Zn and Mn, which can react with arsenate to form sparsely soluble or “insoluble” compounds. Moreover, the system can be further complicated due to the presence of other oxyanions such as phosphate, which may compete with arsenate for available binding sites. Also ferrous ion oxidation in actual mine water is predominantly mediated by iron-oxidizing bacteria since abiotic oxidation of ferrous ion using molecular oxygen as an oxidant is very slow under acidic conditions [20], [21]. It has been known that iron-oxidizing bacteria are not only responsible for the oxidation of ferrous ion but also have a role to play in the formation of iron precipitates [22], [23]. Therefore, laboratory experiments that use abiotic methods for Fe–S–As co-precipitation may not well simulate the process that takes place in the field conditions.

To the best of our knowledge, there has not been any detailed investigation into the microbially mediated oxidation of aqueous Fe2+ and the resulting arsenate immobilization by precipitate formation in acid sulfate water systems with complex chemical composition. In this study, a microcosm experiment was conducted to take into account a microbial factor and a multi-element scenario. The objectives of this study were to (a) understand the chemical behaviour of arsenic and other elements during the microbially mediated oxidation of aqueous Fe2+ in the complex acid sulfate water system; (b) characterize the precipitates formed during the period of incubation experiment; and (c) to depict the reaction mechanisms that were responsible for the observed arsenate immobilization in the investigated aqueous system.

Section snippets

Bacteria, culture conditions and inoculum preparation

A strain of Acidithiobacillus ferrooxidans was purchased from the Marine Culture Collection of China (MCCC). The bacterial culture was maintained at 4 °C in a 9 K nutrient medium containing 3.0 g of (NH4)2SO4, 0.01 g of Ca(NO3)2, 0.5 g of MgSO4·7H2O, 0.5 g of K2HPO4, 0.1 g of KCl and 44.3 g of FeSO4·7H2O in 1 L of distilled water with the pH adjusted to 1.6 with a H2SO4 solution.

The inoculum was prepared prior to the incubation experiment. An adequate amount of the bacteria required for the experiment

Water-borne element evolution in Phase I of the incubation experiment

The dynamics of various water chemical parameters during Phase I of the incubation experiment (996 h) are shown in Fig. 1. Ferrous ion (Fe2+) concentration decreased progressively over time and almost completely disappeared after approximately 686 h of the microbially mediated oxidation (Fig. 1a). This was accompanied by a sharp decrease in dissolved oxygen (DO) from the initial 6.4 mg L−1 to 1.0 mg L−1 at the 110th h of the experiment, followed by a gentler decrease, with marked fluctuation, to

Conclusion

The experimental results obtained from this study provide the insights into the mechanisms of arsenate immobilization associated with microbial oxidation of ferrous ion in a complex acid sulfate water system at initial pH of 2. The removal of arsenate from the acid sulfate water can be described as a three-stage process through which the aqueous arsenate species were progressively immobilized. Arsenate was initially immobilized through co-precipitation with non-Fe metals and phosphate.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (Project numbers: 40471067 and 40773058) and the Guangdong Bureau of Science and Technology (Project No. 2005A30402006).

References (40)

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