ABSTRACT

The potential for Se removal from mine drainage water using permeable reactive materials was evaluated by a laboratory column experiment. The column materials, organic carbon and zero valence iron (ZVI), were exposed to mine drainage containing 630 mg L^-1 SO₄^2-. The influent water was spiked with 40 mg L^-1 Se(VI) to assess the potential for Se removal. This high Se(VI) concentration was selected to ensure that there would be a sufficient mass of Se-bearing reaction products available for mineralogical characterization. The experiment was conducted in an anaerobic chamber to replicate the anaerobic conditions that prevail in permeable reactive barrier systems. After loading 10.8 pore volumes of input solution, the column effluent contained <0.002 mg L^-1 Se and <300 mg L^-1 SO₄^2-. After the column experiments was complete the reactive materials were sampled in the anaerobic chamber and examined using scanning electron microscopy (SEM) coupled with energy dispersion X-ray analysis (SEM-EDAX), X-ray photoelectron spectroscopy (XPS) and by Raman spectroscopy. Sulfate was reduced to sulfide and elemental sulfur, which accumulated on the surfaces of the column materials. Se-bearing precipitates were observed at the base of column. Scanning electron microscopy (SEM) showed the presence of elemental Se, suggesting that Se(VI) was partly reduced to metallic Se(0). The XPS results revealed that selenate was reduced mainly to iron selenide (FeSe and/or FeSe₂) on the surface of the column substances. These observations suggest that both chemical reduction and biologically mediated reduction of Se(VI) occurred.