Sequential removal of nitrate and sulfate in woodchip and hematite - coated biochar bioreactor

Laboratory column experiments have been used to study the sequential removal of nitrate (NO3-) and sulfate (SO42-) from mine water, where NO3- was removed through denitrification and SO42- was removed through SO42- reduction and the subsequent precipitation of hydrogen sulfide (H2S) in a hematite-coated biochar (HCB) bioreactor. Denitrification and SO42- reduction were investigated in columns filled with pine woodchips and pine woodchips + biochar, both with and without the addition of lactate. Experimental results indicated that a >90% NO3- removal from 50 mg L-1 NO3--N was achieved at a hydraulic residence time of 5 days without lactate addition, but that SO42- reduction was minimal after an initial startup period. Lactate was added to stimulate SO42- reduction, producing H2S with >90% SO42- removal from an initial concentration of 361 mg L-1 SO42--S. Sulfate concentrations were reduced to a greater extent in the woodchip + biochar column, and NH4+ production was enhanced in both columns after lactate addition. After treatment in the HCB columns, H2S and NH4+ were removed to >95%. X-ray photoelectron spectroscopy (XPS) indicated that S2-, S-2(2-), S-0 and NH4+ were accumulating in the HCB columns and surface-bound iron was converted from Fe(iii) to Fe(ii). The XPS results suggested that the reductive dissolution of hematite preceded the precipitation of H2S as FeS, pyrite and elemental sulfur on the HCB surfaces.