Chlorinated solvents are one of the most persistent classes of groundwater contaminants, with tetrachloroethene (PCE) and trichloroethene (TCE) detected at over 75% of Superfund sites in the USA. For sites which do not require urgent remediation, natural attenuation is viewed as an appropriate clean-up strategy due to its low operational costs. Whilst bioremediation is frequently used at sites, it can lead to the accumulation of incompletely dechlorinated products, such as vinyl chloride, that are more toxic than PCE and TCE. Alternatively, abiotic transformation of PCE and TCE generally results in more benign products, yet it is unclear to what extent naturally occurring minerals could contribute to PCE and TCE degradation. In particular, redox-active Fe-bearing clay minerals are seen as a promising solution due to their ubiquity in subsurface environments and resistance to dissolution. Here we reacted PCE and TCE with reduced Fe-bearing clay minerals that contained different amounts of structural Fe, and monitored PCE and TCE loss, as well as the accumulation of products. We observed no PCE and TCE degradation with Fe-bearing clay minerals that were reduced chemically to 80-90% Fe(II)/Fe(total), suggesting that structural Fe(II) in clay minerals cannot reduce PCE or TCE. In contrast, Fe(II)-addition to Fe-bearing clay minerals yielded a mixture of acetylene, ethane, and ethene, but only in reactors containing low Fe content clay mineral Wyoming montmorillonite (SWy-2, 2.2 wt% Fe) amended with 20 mM Fe(II). Mössbauer spectroscopy showed that electron transfer between aqueous Fe(II) and clay mineral structural Fe led to the formation of Fe precipitate(s) containing both Fe(II) and Fe(III), and that the amount and composition of the precipitate(s) changed with aqueous Fe(II) concentration and clay mineral Fe content. We are currently investigating how stoichiometry and total Fe(II) content of the precipitate(s) affect contaminant degradation.

Formation of mixed-valent nano-precipitates due to electron transfer: Impact on chlorinated solvent degradation

256th National Meeting (2018)

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