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

This study investigated reductive dechlorination of <i>cis</i>-dichloroethylene (<i>cis</i>-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)]<sub>o</sub>) to initial Fe(III) concentration ([Fe(III)]<sub>o</sub>) for iron-reducing conditions (IRC) and the ratio of [Fe(II)]<sub>o</sub> to initial sulfide concentration ([S(-II)]<sub>o</sub>) for sulfate-reducing conditions (SRC). Significant dechlorination of <i>cis</i>-DCE occurred under highly reducing IRC and iron-rich SRC, suggesting that Fe (oxyhydr)oxides including green rusts are highly reactive with <i>cis</i>-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 <i>cis</i>-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 <i>cis</i>-DCE under anoxic conditions.