Abiotic Degradation of Chlorinated Solvents by Clay Minerals and Fe(II): Evidence for Reactive Mineral Intermediates

For decades, there has been evidence that Fe-containing minerals might contribute to abiotic degradation of chlorinated ethene (CE) plumes. Here, we evaluated whether Fe­(II) in clay minerals reduces tetrachloroethene (PCE) and trichloroethene (TCE). We found that structural Fe­(II) in both low (SWy-2) and high (NAu-1) Fe clay minerals did not reduce PCE or TCE under anoxic conditions. There was also no reduction of PCE or TCE after adding 5 mM dissolved Fe­(II) to the clay mineral suspensions. In the presence of high Fe­(II) concentrations (20 mM), however, PCE and TCE reduction products were observed in the presence of low Fe-content clay mineral SWy-2. Mössbauer spectroscopy results indicate that a mixed-valent Fe­(II)–Fe­(III) precipitate formed in the reactive SWy-2 suspensions. In contrast, in suspensions containing 20 mM Fe­(II) alone or Fe-free clay mineral (Syn-1), we observed a purely Fe­(II)-containing precipitate (Fe­(OH)2) and also PCE and TCE reduction products. Interestingly, the amount of CE products decreased in the order of Fe-free clay mineral Syn-1 > Fe­(OH)2 > low Fe-content clay mineral SWy-2, suggesting that clay mineral Fe controlled the formation of the reactive mineral phase. Additional experiments with hexachloroethane (HCA) revealed that faster HCA reduction occurred with decreasing clay mineral Fe content. Kinetic modeling yielded invariable second-order rate constants and increasing concentrations of reactive Fe­(II) as the Fe­(II)/Fe­(total) content of the precipitates increased. Our data suggest that clay mineral Fe­(III) is a sink for electrons from added Fe­(II) that otherwise might have reduced the CEs. Furthermore, our findings are consistent with the hypothesis that active precipitation of Fe­(II)-containing reactive mineral intermediates (RMI) may be important to CE reduction and suggest that RMI formation depends on clay mineral presence and Fe content.