Dichloroethene reduction by Fe(II): role of transient Fe(II) phases

Chlorinated ethenes (CEs) are some of the most commonly found groundwater contaminants, and their clean-up still relies heavily on energy intensive clean-up practices such as pump and treat. As a sustainable alternative, abiotic natural attenuation by Fe(II) species would be preferable. While data is available on reduction of some CEs by stable Fe(II) phases, these reactions appear to be slower than reduction by freshly precipitated, transient Fe(II) phases (i.e., reactive mineral intermediates, RMIs). Here, we evaluated cis-1,2-dichloroethene (cDCE) reduction by stable and transient Fe(II)-containing phases, and characterized the transient phases formed. In the absence of aqueous Fe(II) (Fe(II)_aq), magnetite, chloride green rust, hematite, mackinawite, and clay minerals did not reduce cDCE. When Fe(II)_aq was present with these minerals, reduction usually occurred when conditions favored precipitation of ferrous hydroxide (Fe(OH)₂). Additionally, we observed cDCE reduction by Fe(II) precipitates made from FeCl₂ and ferrous ammonium sulfate (FAS), but never with FeSO₄ present. Under no conditions, with or without Fe(II)_aq, was cDCE reduced by goethite, chukanovite, sulfate green rust, or aluminum oxide. Mössbauer spectra of the transient phases indicate that ferrous (oxy)hydroxides such as Fe(OH)₂ formed from FeCl₂ and FeSO₄ and a green rust-like precipitate formed from FAS. These spectra suggest that reduction is faster when the phases are less ordered, possibly because the Fe(II) precipitates are less crystalline or form smaller particles. Our work suggests that although most stable Fe(II) phases do not reduce cDCE sufficiently fast for significant abiotic natural attenuation, Fe(II) RMI phases may contribute to attenuation of cDCE plumes.