Metal-ion-controlled sulfate reduction by sulfate-reducing bacteria under reaction equilibrium constraints

Abstract

Treatment of heavy metal-contaminated groundwater using sulfate-reducing bacteria (SRB) includes two subsequent reactions: SRB-mediated sulfate (SO42-) reduction to sulfide (S2-) and the association of S2- with metal ions to form poorly soluble metal sulfides. Although the potential factors affecting the biological activity of SRB have been largely explored in previous studies, the influence of S2- abundance in the system has never been systematically evaluated from the perspective of the overall reaction equilibrium. Here, the effect of co-existing metal ion concentrations—which control the residual S2- concentration in the system—on sulfate reduction was estimated through a series of batch experiments. Contaminated groundwater collected from a currently operating smelter was used, and a mixture of peat and calcite was employed as SRB carbon sources. Batch experiments were conducted for up to 20 days under controlled anaerobic conditions, in the presence of heavy metals at concentrations ranging 6.89–128.81 mM. SO42- reduction efficiencies reached approximately 72 % in the presence of Zn2+ at their highest concentrations. The experimental results clarified that sulfate reduction might be facilitated under the condition that the residual S2- could be continuously depleted by co-existing metal ions, indicating that the overall reaction efficiency is controlled by the reaction equilibrium. The current findings could provide critical insights for estimating the period and overall efficacy of remedial operation against heavy-metal-contaminated groundwater using SRB.