Reductive dechlorination of tetrachloroethylene and trichloroethylene by mackinawite (FeS) in the presence of metals: reaction rates.

Reductive dechlorination by mackinawite (FeS) is an important transformation pathway for chloroethylenes in anoxic environments. Yet, the impact of metals on reductive dechlorination is not well understood, despite their frequent cooccurrence with chloroethylenes. Fe(II), Co(II), Ni(II), and Hg(II) were evaluated for their impact on the dechlorination rates of PCE and TCE by FeS. Compared with unamended FeS batches, the dechlorination rates of both chloroethylenes decreased by addition of 0.01 M Fe(II). Relative to 0.01 M Fe(II)-added FeS batches, the dechlorination rates increased in FeS batches amended with 0.01 M of Co(II) and Hg(II), whereas the rates decreased in 0.01 M Ni(II)-added batches. While significantly impacting the dechlorination rates, the amended metals were quantitatively sequestered by FeS mainly because of formation of metal sulfides. Comparison of the dechlorination rates between metal-added FeS batches and metal sulfide batches suggests that discrete metal sulfides do not form in metal-added FeS batches. The observed exceptionally high reactivity of CoS suggests that it may be useful in reactive permeable barrier applications because of its stability in anoxic waters. The dechlorination rates of PCE and TCE significantly varied with Fe(ll) amendment concentrations (Fe(II)0), indicating the presence of different types of solid-bound Fe phases with Fe(II)o.