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 SO₄^2– 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^–.