Arsenic(V) Incorporation in Vivianite during Microbial Reduction of Arsenic(V)-Bearing Biogenic Fe(III) (Oxyhydr)oxides.

The dissolution of arsenic-bearing iron(III) (oxyhydr)oxides during combined microbial iron(III) and arsenate(V) reduction is thought to be the main mechanism responsible for arsenic mobilization in reducing environments. Besides its mobilization during bioreduction, arsenic is often resequestered by newly forming secondary iron(II)-bearing mineral phases. In phosphate-bearing environments, iron(II) inputs generally lead to vivianite precipitation. In fact, in a previous study we observed that during bioreduction of arsenate(V)-bearing biogenic iron(III) (oxyhydr)oxides in phosphate-containing growth media, arsenate(V) was immobilized by the newly forming secondary iron(II) and iron(II)/iron(III)mineral phases, including vivianite. In the present study, changes in arsenic redox state and binding environment in these experiments were analyzed. We found that arsenate(V) partly replaced phosphate in vivianite, thus forming a vivianite-symplesite solid solution identified as Fe3(PO4)1.7(AsO4)0.3·8H2O. Our data suggests that in order to predict the fate of arsenic during the bioreduction of abiogenic and biogenic iron(III) (oxyhydr)oxides in arsenic-contaminated environments, the formation of symplesite-vivianite minerals needs to be considered. Indeed, such mineral phases could contribute to a delayed and slow release of arsenic in phosphate-bearing surface and groundwater environments.