Immobilization of arsenite and ferric iron by Acidithiobacillus ferrooxidans and its relevance to acid mine drainage

Abstract

Weathering of the As-rich pyrite-rich tailings of the abandoned mining site of Carnoulès (southeastern France) results in the formation of acid waters heavily loaded with arsenic. Dissolved arsenic present in the seepage waters precipitates within a few meters from the bottom of the tailing dam in the presence of microorganisms. An Acidithiobacillus ferrooxidans strain, referred to as CC1, was isolated from the effluents. This strain was able to remove arsenic from a defined synthetic medium only when grown on ferrous iron. This A. ferrooxidans strain did not oxidize arsenite to arsenate directly or indirectly. Strain CC1 precipitated arsenic unexpectedly as arsenite but not arsenate, with ferric iron produced by its energy metabolism. Furthermore, arsenite was almost not found adsorbed on jarosite but associated with a poorly ordered schwertmannite. Arsenate is known to efficiently precipitate with ferric iron and sulfate in the form of more or less ordered schwertmannite, depending on the sulfur-to-arsenic ratio. Our data demonstrate that the coprecipitation of arsenite with schwertmannite also appears as a potential mechanism of arsenite removal in heavily contaminated acid waters. The removal of arsenite by coprecipitation with ferric iron appears to be a common property of the A. ferrooxidans species, as such a feature was observed with one private and three collection strains, one of which was the type strain.

FIG. 1.

Schematic map of the Carnoulès acid mine drainage system, showing the locations of the different sampling points.

FIG. 2.

Arsenic removal in biotic (A) and abiotic (B) conditions. (A) Growth of CC1 in Fe(II)-As(III) conditions and changes in the level of soluble arsenic, arsenite, arsenate, and ferrous iron in the culture medium. The arithmetic plots show relative concentrations of As (open squares), As(III) (solid squares), As(V) (solid diamonds), and Fe(II) (solid circles). The logarithmic plot shows growth curve of strain CC1 expressed as protein concentration (solid triangles). (B) Effect of growth conditions on the removal of arsenic from the medium. Concentrations of dissolved As (open symbols) and of As(III) (solid symbols) in abiotic As(III) conditions were determined with vigorous shaking (circles); with vigorous shaking and Fe(II) (triangles); and with vigorous shaking, Fe(II), and light (squares).

FIG. 3.

Effect of excreted products from CC1 cells on arsenic removal. Concentrations of soluble As (open symbols) and As(III) (solid symbols) in Fe(III) medium (squares) and in a filter-sterilized supernatant of strain CC1 grown on Fe(II)-As(III) medium supplemented with As(III) (circles).

FIG. 4.

XRD patterns and FTIR and As K-edge XANES spectra of the precipitates obtained from the Fe(II)-As(III)-CC1 (CC1), Fe(II)-As(III) abiotic [Fe(II)], and Fe(III)-As(III) abiotic [Fe(III)] experiments. As K-edge XANES spectra of reference compounds are reported for comparison, including 5% (wt/wt) As(III) adsorbed on schwertmannite [As(III)/schw], amorphous Fe(III)-As(III) mixed oxyhydroxide [am-Fe(III)-As(III)], and amorphous Fe(III)-As(V) mixed oxyhydroxide [am-Fe(III)-As(V)].

FIG. 5.

Arsenic removal by different A. ferrooxidans strains in Fe(II)-As(III) medium. Strains: type strain ATCC 23270 (triangles); ATCC 33020 (squares); ATCC 19859 (diamonds); and BRGM1 (circles). The concentrations of soluble As (open symbols) and As(III) (solid symbols) were determined.