9

dechlorination. Reductive 3 elimination and hydrogenolysis play the most
important role in reduction of chlorinated ethylene compounds, whereas
reductive o elimination and dehydrohalogenation are not significant (Roberts et
al. 1996). The percentage of the reaction occurring via reductive 3 elimination by
zinc metal is 15% of PCE, 30% of TCE, 85% of cis-DCE, and 95% of trans-DCE
reaction (Arnold and Roberts 1998). For iron metal, less than a 5% of TCE is
degraded via hydrogenolysis (Orth and Gillham 1996, Sivavec et al. 1997).
Some dechlorinated byproducts from reduction reactions are more toxic
and less reactive compared to their parent compounds (Tratnyek et al. 1997,
Arnold and Roberts 1998). The hydrogenolysis pathway produces the most toxic
chlorinated byproduct, vinyl chloride (the MCL of VC is 2 jg/L). The percentage
of DCE isomers and VC generated by dechlorination is generally less than 5% of
the TCE that is degraded by iron metal (Orth and Gillham 1996, Sivavec et al.
1997). These low concentrations of byproducts (i.e., DCE isomers and VC) are
believed to be present because they are released from the iron surface before
complete reduction of TCE is attained (Orth and Gillham 1996).
The predominant pathway, reductive 3 elimination, does not favorably
produce DCE isomers and VC. The chlorinated acetylene compounds produced
from reductive f3 elimination reaction are toxic, but their short half-life usually
renders them unimportant in PRB design (Tratnyek et al. 1997, Arnold and
Roberts 1998).