Chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE), are one of the most prevalent pollutants at hundreds of Department of Defense (DoD) sites, and remain among the most difficult to remediate despite years of intense research and development. Biological degradation of PCE and TCE has been studied in some detail; however, there is still a significant knowledge gap in the understanding of how abiotic processes contribute to the degradation of PCE and TCE, in particular, the formation of reactive iron (Fe) mineral phases through biologically mediated pathways.

The primary objective of this project was to evaluate whether magnetite and Fe-containing clay minerals reduced PCE and TCE alone and then in the presence of ferrous iron (Fe(II)) OR sulfide (S(-II)). The goal was to evaluate pathways and factors controlling abiotic degradation of PCE and TCE by reactive minerals and evaluate which aquifer properties might be used as indicators for abiotic natural attenuation rates and products in anoxic PCE and TCE plumes.

The research team found that magnetite and reduced Fe-containing clay minerals alone did not reduce PCE and TCE under anoxic conditions. The findings suggest that it is unlikely that magnetite and reduced Fe-containing clay minerals alone reduce PCE and TCE fast enough to significantly contribute to the natural attenuation of PCE and TCE in contaminated, anoxic aquifer plumes. The research team also, somewhat surprisingly, found no reduced carbon products from PCE and TCE when sulfide was added to magnetite and Fe-containing clay mineral suspensions. They therefore suggest that PCE and TCE transformation by sulfide-reduced clay minerals is also likely not relevant in groundwater aquifers. They did, however, find that both magnetite and Fe-containing clay minerals reduced PCE and TCE in the presence of high concentrations of Fe(II). In both cases, color changes in the mineral suspensions, as well as spectroscopy analyses indicated that another, transient mineral phase was forming suggesting that dynamic conditions and high Fe(II) concentrations that favor active precipitation of minerals could help abiotically attenuate anoxic PCE and TCE plumes. Specifically, the research team observed the formation of ferrous hydroxide (Fe(OH)₂(s)) in the presence of magnetite and clay minerals that was associated with accelerated rates of PCE and TCE reduction. Reduction rates were slow with carbon products up to only 30% accumulating over several months. Acetylene was the primary product suggesting that reductive β-elimination of PCE and TCE was likely the primary mechanism for reduction. Reductive dechlorination products, including dichloroethenes and vinyl chloride, were measured, but not detected.

The findings suggest that the presence of either magnetite or Fe-containing clay minerals in aquifer sediments is not sufficient to suggest that abiotic natural attenuation is likely. Rather, it appears that conditions that favor active precipitation of reactive minerals may be necessary. The work suggests that zones of active Fe(II) precipitation in anoxic aquifers could result in PCE and TCE reduction that is sufficiently fast to help attenuate PCE and TCE plumes. Likely, there needs to be a source of Fe in the sediments and then conditions that are favorable to reducing and releasing that Fe into the dissolved phase. Those conditions could include a number of different Fe-reducing conditions driven by the microbiology or geochemical conditions. Some potential field protocols for abiotic degradation of PCE and TCE by Fe minerals include field screening methods for assessing extractable Fe and sulfide, as well as characterizing the electron accepting and donating capacity of the soils. Field screening methods for acid volatile sulfides targeting FeS and citrate-bicarbonate (CB) extractable Fe would provide a good indicator of readily available sources of Fe and sulfide.

Results from this project have significantly improved the understanding of pathways and factors controlling abiotic degradation of PCE and TCE as well as provided important insights on aquifer properties that are (and are not) important for predicting whether PCE and TCE plumes have the potential to be attenuated by abiotic mineral-based reactions. For this work, Dr. Scherer and her team received the 2018 SERDP Project-of-the-Year Award.

Culpepper, J.D., M.M. Scherer,  T.C Robinson, A. Neumann, D.M. Cwiertny, and D.E. Latta. 2018. Reduction of PCE and TCE by Magnetite Revisited. Environmental Science: Processes & Impacts, 20:1340-1349. doi.org/10.1039/c8em00286j

Entwistle, J., D.E. Latta, M.M. Scherer, and A. Neumann. 2019. Abiotic Degradation of Chlorinated Solvents by clay minerals and Fe(II): Evidence for Reactive Mineral Intermediates. Environmental Science & Technology.  53(24):14308-14318. doi.org/10.1021/acs.est.9b04665