Abiotic Reductive Dechlorination of cis-Dichloroethylene by Fe Species Formed during Iron- or Sulfate-Reduction

This study investigated reductive dechlorination of cis-dichloroethylene (cis-DCE) by the reduced Fe phases obtained from in situ precipitation, which involved mixing of Fe(II), Fe(III), and S(-II) solutions. A range of redox conditions were simulated by varying the ratio of initial Fe(II) concentration ([Fe(II)]_o) to initial Fe(III) concentration ([Fe(III)]_o) for iron-reducing conditions (IRC) and the ratio of [Fe(II)]_o to initial sulfide concentration ([S(-II)]_o) for sulfate-reducing conditions (SRC). Significant dechlorination of cis-DCE occurred under highly reducing IRC and iron-rich SRC, suggesting that Fe (oxyhydr)oxides including green rusts are highly reactive with cis-DCE but that Fe sulfide as mackinawite (FeS) is nonreactive. Relative concentrations of sulfate to chloride were also varied to examine the anion impact on cis-DCE dechlorination. Generally, slower dechlorination occurred in the batches with higher sulfate concentrations. As indicated by higher dissolved Fe concentration, the slower dechlorination in the presence of sulfate was probably due to the decreased surface-complexed Fe(II). This study demonstrates that the chemical form of reduced Fe(II) is critical in determining the fate of cis-DCE under anoxic conditions.