Reductive dissolution of scorodite in the presence of Shewanella sp. CN32 and Shewanella sp. ANA-3

Mining and mineral processing operations may generate a wide range of As-rich compounds including scorodite (FeAsO4·2H2O), a common secondary arsenate in near-surface environments and some gold mine tailings. Scorodite has a low solubility in a limited pH range, and it is relatively stable under near-surface conditions, but its behavior under the influence of bacteria is not well understood. Considering that reducing conditions are likely to develop in mine tailings with depth and under prolonged disposal conditions, the present study was undertaken to determine the influence of bacteria on the reductive dissolution of scorodite. Because the systematic studies on the microbial reduction of scorodite are lacking, we used two well characterized dissimilatory iron and arsenic reducing bacteria, i.e., Shewanella sp. ANA-3 and Shewanella putrefaciens CN32 in a chemically defined media, at circumneutral pH, containing various phosphate concentrations (i.e., 15, 40 and 400 μM). The initial rates of reduction and the plateau concentrations of reduced species formed in the aqueous phase were greater in the presence of Shewanella sp. ANA-3 than in the presence of Shewanella putrefaciens CN32. The initial rate of reduction was found to increase with increasing phosphate concentration, however, plateau concentrations of the dissolved reduced species, Fe(II) and As(III), formed in the aqueous phase were highest at the lowest phosphate concentration. The solid products of the post-reduction samples were characterized by synchrotron X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (XRD), scanning, transmission and high resolution electron microscopy (SEM/TEM/HRTEM) and energy dispersive spectrometry (EDS). The results indicate that the post reduction solids contained scorodite, a biogenic ferrous arsenite and parasymplesite (Fe3(AsO4)2·8H2O). Phosphate concentrations had an effect on the concentrations of released Fe(II) and As (III) during the microbial reduction and the formation of secondary phases. The influence of bacteria on the reductive dissolution of scorodite must be taken into consideration during mine waste management and disposal operations because of the potential of arsenic releases to terrestrial and aquatic environments as toxic and soluble As(III) species.