Synergistic Zerovalent Iron (Fe0) and Microbiological Trichloroethene and Perchlorate Reductions Are Determined by the Concentration and Speciation of Fe.

Trichloroethene (TCE) and perchlorate (ClO4-) are cocontaminants at multiple Superfund sites. Fe0 is often used during TCE bioremediation with Dehalococcoides mccartyi to establish anoxic conditions in the aquifer. However, the synergy between Fe0 abiotic reactions and microbiological TCE and ClO4- reductions is poorly understood and seldom addressed in the literature. Here, we investigated the effects of Fe0 and its oxidation product, Fe2+, at field-relevant concentrations in promoting microbial TCE and ClO4- reductions. Using semibatch microcosms with a Superfund site soil and groundwater, we showed that the high Fe0 concentration (16.5 g L-1) expected during Fe0 in situ injection mostly yielded TCE abiotic reduction to ethene/ethane. However, such concentrations obscured dechlorination by D. mccartyi, impeded ClO4- reduction, and enhanced SO42- reduction and methanogenesis. Fe2+ at 0.25 g L-1 substantially delayed conversion of TCE to ethene when compared to no-Fe controls. A low concentration of aged-Fe0 synergistically promoted microbiological TCE dechlorination to ethene while achieving complete ClO4- reduction. Collectively, these results illustrate scenarios relevant at or downstream of Fe0 injection zones when Fe0 is used to facilitate microbial dechlorination. Results also underscore the potential detrimental effects of Fe0 and bioaugmentation cultures coinjection for in situ treatment of chlorinated ethenes and ClO4-.