Sulfate-Reducing Bacteria Enhance Abiotic Trichloroethene Reduction by Iron–Sulfur Mineral Precipitates

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 (R 2 = 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.

Kinetics of Hydroxyl Radical Production from Oxygenation of Reduced Iron Minerals and Their Reactivity with Trichloroethene: Effects of Iron Amounts, Iron Species, and Sulfate Reducing Bacteria

“…XRD spectra of the reduced iron minerals prepared under abiotic and biotic conditions were taken (Figure S6); sharp peaks indicate siderite (FeCO 3 ), while broad low peaks and shoulders suggest mackinawite (FeS). Mackinawite was expected to form from Fe and S and can have a weak XRD spectrum; , the siderite formed due to the addition of the bicarbonate buffer, even though FeCO 3 has a much larger solubility constant, K sp , than FeS (3.7 × 10 –11 vs 8.0 × 10 –19 , respectively). , Both mackinawite and siderite have shown reactivity for direct TCE reduction , and for oxygen reduction to create the •OH; , associated reaction rates are generally greater for mackinawite than siderite. The amounts of mackinawite and siderite in this work could not be distinguished, so they are collectively referred to as reduced iron minerals (RIM).…”

“…Results are similar for abiotic and biogenic RIM experiments, with small differences mainly apparent as fluctuations (e.g., for Fe–S = 50:1, total Fe­(II) concentration increases from 2.32 to 2.80 mM between 50 and 100 min) in the biogenic RIM experiments. This may be due to the biological re-reduction of oxidized iron to Fe­(II) that can be driven by SRB or electroactive exudates (e.g., electron shuttles) from SRB, − and this assertion is evaluated later in the paper.…”

“…Microbial activity can affect •OH production during the reaction of iron minerals with oxygen in several ways. Microorganisms produce electron-donating products like reduced flavins , and cytochromes; , these can directly reduce Fe­(III) minerals to Fe­(II) ,, and serve as extracellular electron shuttles (EES) to transfer electrons between reduced and oxidized species. , Iron reducing microorganisms can also re-reduce ferrous iron minerals that undergo oxidation upon reaction with O 2 , either through EES or direct electron transfer. , If sulfide is released during iron mineral oxidation and oxidized to sulfate, sulfate reducing bacteria can reduce this back to sulfide. , The effects of these biogeochemical conditions on a kinetic model for •OH production have not previously been considered.…”

“…24 Microbial activity can affect •OH production during the reaction of iron minerals with oxygen in several ways. Microorganisms produce electron-donating products like reduced flavins 25,26 and cytochromes; 27,28 these can directly reduce Fe(III) minerals to Fe(II) 25,29,30 and serve as extracellular electron shuttles (EES) to transfer electrons between reduced and oxidized species. 27,31 Iron reducing microorganisms can also re-reduce ferrous iron minerals that undergo oxidation upon reaction with O 2 , either through EES or direct electron transfer.…”

Reciprocal Interactions of Abiotic and Biotic Dechlorination of Chloroethenes in Soil

“…23 In addition, Desulfovibrio as sulfate-reducing bacteria (SRB) was recently identified to maintain low redox conditions for the OHRB by producing soluble microbial products and consequently facilitated TCE dechlorination. 24 Moreover, SRB-derived FeS and pyrite were recently identified to mediate the abiotic dechlorination of organohalides, suggesting reciprocal interactions of the abiotic and biotic processes. 25−27 Overall, the natural attenuation of CEs could be a combined biogeochemical process involving various abiotic and biotic processes in complex soil environments.…”