Selenite Stable Isotope Fractionation during Abiotic Reduction by Sodium Sulfide.

Reduction of Se(IV) by sulfur reducing bacteria (SRB) can remove Se from groundwater either by direct respiration or the production of HS and subsequent abiotic reduction. This study examined abiotic Se(IV) reduction by HS to determine the associated Se isotope fractionation. The extent of fractionation was compared to the results with studies of Se(IV) reduction in systems containing microorganisms to assess whether these processes could be distinguished. A solution containing NaS was added in increasing concentrations to solutions containing Se(IV) as SeO to reduce and precipitate Se. Precipitates with three distinct colors were observed. Powder X-ray diffraction (PXRD) results yielded three distinct spectra for each of the three colors of precipitate, which corresponded to SeS (orange) or Se(0) (red) and S(0) (yellow). The δSe values of the residual dissolved Se increased as the aqueous Se concentration decreased. The S/Se in solution affected the isotopic fractionation, with an ε of 10.1 ± 0.6‰ observed for solutions with S/Se < 1.7, and of 7.9 ± 0.3‰ for solutions with S/Se > 1.7.