Mechanisms and Products of Surface-Mediated Reductive Dehalogenation of Carbon Tetrachloride by Fe(II) on Goethite

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Abstract
Natural attenuation processes of chlorinated solvents in soils and groundwaters are increasingly considered as options to manage contaminated sites. Under anoxic conditions, reactions with ferrous iron sorbed at iron(hyro)xides may dominate the overall transformation of carbon tetrachloride (CCl4) and other chlorinated aliphatic hydrocarbons. We investigated mechanisms and product formation of CCl4 reduction by Fe(II) sorbed to goethite, which may lead to completely dehalogenated products or to chloroform (CHCl3), a toxic product which is fairly persistent under anoxic conditions. A simultaneous transfer of two electrons and cleavage of two C−Cl bonds of CCl4 would completely circumvent chloroform production. To distinguish between initial one- or two-bond cleavage, 13C-isotope fractionation of CCl4 was studied for reactions with Fe(II)/goethite (isotopic enrichment factor ε = −26.5‰) and with model systems for one C−Cl bond cleavage and either single-electron transfer (Fe(II) porphyrin, ε = −26.1‰) or partial two-electron transfer (polysulfide, ε = −22.2‰). These ε values differ significantly from calculations for simultaneous cleavage of two C−Cl bonds (ε ≈ −50‰), indicating that only one C−Cl bond is broken in the critical first step of the reaction. At pH 7, reduction of CCl4 by Fe(II)/goethite produced ∼33% CHCl3, 20% carbon monoxide (CO), and up to 40% formate (HCOO-). Addition of 2-propanol-d8 resulted in 33% CDCl3 and only 4% CO, indicating that both products were generated from trichloromethyl radicals (•CCl3), chloroform by reaction with hydrogen radical donors and CO by an alternative pathway likely to involve surface-bound intermediates. Hydrolysis of CO to HCOO- was surface-catalyzed by goethite but was too slow to account for the measured formate concentrations. Chloroform yields slightly increased with pH at constant Fe(II) sorption density, suggesting that pH-dependent surface processes direct product branching ratios. Surface-stabilized intermediates may thus facilitate abiotic mineralization of CCl4, whereas the presence of H radical donors, such as natural organic matter, enhances formation of toxic CHCl3.