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 H₂S_(g) and subsequent abiotic reduction. This study examined abiotic Se(IV) reduction by H₂S_(g) 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 Na₂S was added in increasing concentrations to solutions containing Se(IV) as SeO₃^2- 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_8- (orange) or Se(0) (red) and S(0) (yellow). The δ⁸²Se values of the residual dissolved Se increased as the aqueous Se concentration decreased. The S/Se in solution affected the isotopic fractionation, with an ⁸²ε 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.