Biologically Mediated Abiotic Degradation of Chlorinated Ethenes: A New Conceptual Framework

Dr. Michelle Scherer | University of Iowa



While biological degradation of tetrachloroethene (PCE) and trichloroethene (TCE) has been studied in some detail, there is still a significant knowledge gap in understanding how abiotic processes contribute to PCE and TCE degradation. The overall objective of this project is to apply a new conceptual framework based on solid-state mineral chemistry to understand biologically mediated abiotic degradation (BMAD) of PCE and TCE by magnetite, iron (Fe) sulfides, and Fe-bearing clays. While it has been long suspected that these minerals play an important role in BMAD of chlorinated solvents, BMAD performance has not been predictable or reproducible at the field scale, or even at the more controlled laboratory scale. The project team speculates that the variable field performances and erratic laboratory observations can be explained by the role of mineral solid state chemistry in controlling the rates and mechanism of PCE and TCE transformation during BMAD. Specifically, the team proposes that mineral stoichiometries as measured by their FeII to FeIII ratio may explain the variability in BMAD performance and may provide predictive value to engineers and site managers implementing natural attenuation strategies. Specific project objectives are to:

  1. Determine pathways and factors controlling abiotic degradation of PCE and TCE by reactive minerals formed from reaction of aqueous ferrous iron (FeII) with Fe-containing aquifer sediments.
  2. Understand pathways and factors controlling abiotic degradation of PCE and TCE by reactive minerals formed from reaction of aqueous sulfides (S-II) with Fe-containing aquifer sediments.
  3. Evaluate aquifer properties that can be used as indicators of BMAD rates and products in PCE and TCE plumes and identify strategies to accelerate BMAD processes.
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Technical Approach

The technical approach combines detailed measurements of FeII and sulfide-induced changes in the solid state chemistry (i.e., FeII to FeIII stoichiometry) of magnetite and Fe-containing clay mineral using isotope selective Mössbauer spectroscopy with batch PCE and TCE experiments to determine chlorinated solvent degradation rates and product distributions. The overall goal is to quantitatively correlate rates of PCE and TCE reduction and product yields to magnetite and clay mineral stoichiometry. Additionally, the team will react FeII and S-II with magnetite and Fe-containing clay minerals to determine if this is an effective strategy to enhance and sustain BMAD of PCE and TCE. In collaboration with Geosyntec Consultants, the team will identify and collect natural clay-rich aquifer soils/sediments. The sediments will be characterized for mineralogy and mineral stoichiometry and reacted with PCE and TCE to assess the potential use of mineral stoichiometry as a predictive tool for assessing the rates of natural attenuation at a site.

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Results from this work have tremendous potential to advance understanding of natural attenuation at contaminated Department of Defense sites by providing quantitative trends for the rates and product yields of PCE and TCE reduction as a function of mineral solid state chemistry (i.e., stoichiometry). The project will establish a framework by which characterization of select aquifer materials could be used by site managers to infer the likelihood of attenuation. Further, the project will provide tangible insights into how stoichiometry could be manipulated at the field-scale through the addition of specific reagents to accelerate degradation and sustain long-term degradation. Direct outcomes from this project include rate coefficients and product branching ratios for a range of mineral stoichiometries that should improve our ability to predict natural attenuation and advance the design of enhanced natural attenuation technologies. (Anticipated Project Completion - 2019)

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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.

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Points of Contact

Principal Investigator

Dr. Michelle Scherer

University of Iowa

Phone: 319-335-5654

Fax: 319-335-5660

Program Manager

Environmental Restoration