Mechanisms of arsenic immobilization in a biomat from mine discharge water

Abstract

We studied the mechanism of uptake of arsenic (As) in a bacterial microhabitat, a biomat, from discharged arsenic mine water at pH 4.7 in field and laboratory experiments. The mine water contained a yellowish biomat composed mainly of Fe-bearing compounds and associated microorganisms. Concentrations of As and Fe in the water decreased with increasing distance from the location of the discharge position. The spatial distributions of elements in the biomat collected at 0.3 m downstream from the discharge position, determined by scanning-particle-induced X-ray emission analysis (S-PIXE), indicated that As was co-located with Fe and S in the precipitates. Analyses of the precipitates by X-ray diffraction pattern (XRD) and transmission electron microscopy (TEM) revealed the presence of Fe–S-bearing minerals, such as schwertmannite and an unknown one, having a d-spacing of 3.3 Å. X-ray absorption near edge structure (XANES) analysis of the precipitates indicated that the oxidation states of As and Fe were V and III, respectively, whereas their corresponding oxidation states in the discharged mine water were V and II. After storing fresh mine water, collected 0.3 m downstream of the discharge position, for 17 days there was a decrease in As and Fe concentrations from 1.2×10 −6 and 1.8×10 −4 mol L −1 to 3.6×10 −7 and 2.0×10 −5 mol L −1, respectively, with the concomitant formation of a yellowish precipitate. In contrast, in filter-sterilized (0.2 μm) mine water there was no change in their concentrations. Molecular analysis, based on the 16S rDNA fragment extracted from the discharged mine water, indicated that Gallionella sp. was the predominant microorganism present. It is known to form stalk-like and sheath-like compounds that were distinguished in the precipitates by TEM observation. These results demonstrate that As(V) in the discharged mine water is co-precipitated with and/or adsorbed on Fe–S-bearing minerals in the biomat.