Depending on the nature of the reducing agent and/or the catalyst, reductive dechlorination of trichloroethene (TCE) may occur via either hydrodechlorination (or hydrogenolysis, replacing a chloride with a proton and two electrons) or dichloro-elimination (eliminating two chloride ions and forming a carbon-carbon p bond with the addition of two electrons). The possible intermediates and products involved are shown in the pathway.
In biological systems, TCE reduction appears to proceed exclusively through hydrodechlorination; i.e., via the DCE isomers and vinyl chloride to ethene. Although complete dechlorination to non-toxic ethene is possible, vinyl chloride (VC, a known human carcinogen) often accumulates because of its slow biological reduction. Therefore, should microbial activities develop over time in the iron treatment zone, the formation of vinyl chloride may be detrimental to the performance of the iron barrier.