(248b) Abiotic Degradation of Chlorinated Solvents with Reactive Iron Minerals from Redox Transition Zones

Reactive iron (Fe) mineral coatings found in reduction-oxidation (redox) transition zones (RTZs) contribute to the attenuation of contaminants. This research focuses on the abiotic degradation processes of 1,4-dichlorobenzene (1,4-DCB), tetrachloroethylene (PCE), and trichloroethylene (TCE) with Fe mineral coatings from a contaminated site. An 18.3-m anoxic core was collected from the site, where constituents of concern (COCs) in groundwater included chlorinated solvents. In the earlier profile studies, RTZs were identified with a suite of analyses. Reactive Fe mineral coatings were abundant in RTZs, with distinct characteristics in surface composition, mineralogy, and morphology; sequential extraction was conducted for semi-quantifying speciation. In this research, batch studies were conducted to evaluate reaction kinetics with anoxic sediments bearing Fe(II) mineral nano-coatings spiked with either 1,4-DCB, PCE, or TCE. Control groups included pure pyrite and siderite. For 1,4-DCB treatment, although dechlorination was not observed over the time period of the study in the control groups, reaction kinetics with RTZ sediments followed second order rate expressions. Chlorobenzene and benzene were detected as byproducts, suggesting hydrogenolysis reduction. The second-order rate constants for the Fe(II) mineral nano-coatings in 1,4-DCB degradation resulted in 1.24´10^-3 L g^-1 h^-1for mackinawite (FeS), 1.73´10^-3 L g^-1 h^-1 for pyrite (FeS₂), 1.89´10^-4 L g^-1 h^-1 for siderite (FeCO₃), and 1.79´10^-4 L g^-1 h^-1 for magnetite (Fe₃O₄). The high reactivity of Fe mineral coatings expects to be due to large surface areas. PCE and TCE reduction were observed in the control and sediment groups, which also followed second-order rate expressions. For these three chlorinated solvents, the trend for dechlorination activity of Fe(II) mineral nano-coatings followed: Fe(II) sulfide minerals > magnetite > siderite. As a result, reactive Fe mineral nano-coatings are expected to play a significant role in the attenuation of chlorinated solvents in contaminated subsurface environments.