Polychlorinated ethane reaction with zero-valent zinc: pathways and rate control

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.