Reductive Dechlorination Pathways of Tetrachloroethylene and Trichloroethylene and Subsequent Transformation of Their Dechlorination Products by Mackinawite (FeS) in the Presence of Metals

Because of frequent co-occurrence of metals with chlorinated organic pollutants, Fe(II), Co(II), Ni(II), and Hg(II) were evaluated for their impact on the dechlorination pathways of PCE and TCE and the subsequent transformation of the initial dechlorination products by FeS. PCE transforms to acetylene via beta-elimination, TCE via hydrogenolysis, and 1,1-DCE via alpha-elimination, while TCE transforms to acetylene via beta-elimination and cis-DCE and 1,1-DCE via hydrogenolysis. Acetylene subsequently transforms in FeS batches, but little transformation of cis-DCE and 1,1-DCE was observed. Branching ratio calculations indicate that the added metals decrease the reductive transformation of PCE and TCE via beta-elimination relative to hydrogenolysis, resulting in a higher production of the toxic DCE byproducts. Nonetheless, acetylene is generally the dominant product. Production of highly water-soluble compound(s) is suspected as a significant source for incomplete mass recoveries. In the transformation of PCE and TCE, the formation of unidentified product(s) is most significant in Co(II)-added FeS batches. Although nearly complete mass recoveries were observed in the other FeS batches, the subsequent transformation of acetylene would lead to the formation of unidentified product(s) over long time periods.