Sulfate-Reducing Bacteria Enhance Abiotic Trichloroethene Reduction by Iron–Sulfur Mineral Precipitates
Erin Berns, Xueji You, Jilong Lin, Robert A. Sanford, Albert J. Valocchi, Timothy J. Strathmann, Charles E. Schaefer, Charles J. Werth
Abstract
Microbial metabolisms can impact abiotic mineral-promoted trichloroethene (TCE) reduction in groundwater environments, but mechanistic understanding of these coupled processes is limited. Here, we explore how sulfate-reducing bacteria (SRB) enhance TCE reactivity of iron sulfide minerals, specifically addressing how SRB maintain reactive iron sulfide surfaces after biogenic mineral formation. Iron sulfides were formed either abiotically (ferrous iron and sulfide) or biotically (ferrous iron and sulfate reduction by Desulfovibrio vulgaris) in batch systems. TCE was added, and reaction products were monitored under different ferrous iron:sulfur (Fe:S) ratios. With D. vulgaris present, higher Fe:S ratios showed over an order of magnitude increase in TCE transformation rates. These rates increased with lower reduction potentials (R2 = 0.66, p = 0.0014), as potentials decreased below −150 mV vs SHE. Mineral precipitate characterization indicated the presence of mackinawite (FeS), and pH and redox potentials confirmed experimental conditions in the FeS stability range. Filtered D. vulgaris media (SRB removed) showed similarly high rates to biotic experiments, implying the role of biogenic redox-active soluble microbial products (SMPs) in maintaining reducing conditions. From these results, we propose a reaction scheme, where iron sulfide surfaces reduce TCE, oxidizing mineral surface species, which are then “re-reduced” by SMPs from D. vulgaris.