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A field-scale experiment was conducted to evaluate various organic C sources as amendments for passive treatment of tailings pore water. Varied mixtures of peat, spent-brewing grain (SBG) and municipal biosolids (MB) were assessed for the potential to promote dissimilatory sulfate reduction (DSR) and metal-sulfide precipitation. Five amended cells and one control were constructed in the vadose zone of a sulfide- and carbonate-rich tailings deposit, and the geochemistry, microbiology and mineralogy were monitored for 4 a. Increases in pore-water concentrations of dissolved organic C (DOC) and decreases in aqueous SO 4 concentrations of >2500 mg L −1 were observed in cells amended with peat + SBG and peat + SBG + MB. Removal of SO 4 was accompanied by shifts in δ 34 S-SO 4 values of >+30‰, undersaturation of pore water with respect to gypsum [CaSO 4 ·2H 2 O], and increased populations of SO 4 -reducing bacteria (SRB). Decreases in aqueous concentrations of Zn, Mn, Ni, Sb and Tl were observed for these cells relative to the control. Organic C introduction also supported growth of Fe-reducing bacteria (IRB) and increases in Fe and As concentrations. Enhanced Fe and As mobility occurred in all cells; however, maximum concentrations were observed in cells amended with MB. Subsequent decreases in Fe and As concentrations were attributed to DSR and metal-sulfide precipitation. The common presence of secondary Zn-S and Fe-S phases was observed by field emission-scanning electron microscopy (FE-SEM) and energy dispersive X-ray (EDS) spectroscopy. Selective extractions indicated that large decreases in water-soluble SO 4 occurred in cells that supported DSR. Furthermore, amendments that supported DSR generally were characterized by slight decreases in solid-phase concentrations of extractable metal(loid)s. Amendment of tailings with organic C amendments that supported ongoing DOC production and DSR was essential for sustained treatment. |
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