Efficient design of zero-valent metal permeable 'barriers' for the reductio
n of organohalides requires information regarding the pertinent reaction ra
tes as well as an understanding of the resultant distribution of products.
In this study, the pathways and kinetics for reaction of polychlorinated et
hanes with Zn(0) have been examined in batch reactors. Reductive p-eliminat
ion was the only route through which hexachloroethane (HCA), 1,1,1,2-tetrac
hloroethane (1,1,1,2-TeCA), 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA), 1,1,2
-trichloroethane (1,1,2-TCA) and 1,2-dichloroethane (1,2-DCA) reacted. Pent
achloroethane (PCA) reacted via concurrent reductive beta-elimination (93%)
and hydrolysis (7%). As previously demonstrated, 1,1,1-trichloroethane (1,
1,1-TCA) and 1,1-dichloroethane (1,1-DCA) reacted predominantly via reducti
ve a-elimination. Attempts to correlate BET surface area-normalized rate co
nstants (k(SA-BET)) with one-electron reduction potential (E-1) met with li
mited success, as HCA, PCA, 1,1,1,2-TeCA, and 1,1,1-TCA reacted at nearly i
dentical rates despite substantial differences in E-1 values. Comparison of
the pseudo-first-order rate constants (k(obs)) for these species with rate
constants (k(L)a) obtained from a correlation for mass transfer to suspend
ed particles revealed that the reaction of these species was mass transfer
limited even though reaction rates were unaffected by mixing speed. Calcula
tions suggest that mass transfer limitations may also play a role in the de
sign of treatment systems for highly reactive species, with overall rate co
nstants predicted to increase with flow velocity. (C) 1999 Elsevier Science
B,V. All rights reserved.