Synergistic effects of sulfate reducing bacteria and zero valent iron on zinc removal and stability in aquifer sediment

•Zn was efficiently removed in biotic and abiotic conditions.•Synergistically using Fe0 and SRB for metal removal is effective approach in forming more stable precipitates.•Change is ORP is more likely to cause leaching of Zn than change in pH in subsurface environments. We conducted a series of flow-through column experiments using aquifer sediment treated with zero valent iron (Fe0) with or without active microbial sulfate reduction. The aim of this study was to investigate the stability of Zn removed by Fe0 and impact of sulfate reducing bacteria (SRB). The leaching of Zn was assessed by flushing the aquifer in vertical glass columns with groundwater that was pre-treated with mixture of gases to simulate natural changes (pH and ORP) in subsurface environment (N2+CO2 for pH and N2+O2 for ORP) along with a control (N2 flushed). Zinc removed synergistically by Fe0 and SRBs was found to be more stable and did not leach from the aquifer sediment with pH or redox changes. We also observed that the Zn leaching in the aquifer treated only with Fe0 was more influenced by change in redox than by pH changes. These results were also confirmed by sequential metal extractions and scanning electron microscopy observations in the aquifer sediments. Mineral phases were predicted using geochemical modeling tool CHESS confirming that the metal removed in Fe0+SRB conditions formed more stable precipitates and did not show any signs of leaching back to groundwater upon change in pH and redox conditions.