Efficient design of zero-valent metal permeable 'barriers' for the reduction of organohalides requires information regarding the pertinent reaction rates as well as an understanding of the resultant distribution of products. In this study, the pathways and kinetics for reaction of polychlorinated ethanes with Zn(0) have been examined in batch reactors. Reductive β-elimination was the only route through which hexachloroethane (HCA), 1,1,1,2-tetrachloroethane (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. Pentachloroethane (PCA) reacted via concurrent reductive β-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 reductive α-elimination. Attempts to correlate BET surface area-normalized rate constants (k(SA-BET)) with one-electron reduction potential (E1) met with limited success, as HCA, PCA, 1,1,1,2-TeCA, and 1,1,1-TCA reacted at nearly identical rates despite substantial differences in E1 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 suspended particles revealed that the reaction of these species was mass transfer limited even though reaction rates were unaffected by mixing speed. Calculations suggest that mass transfer limitations may also play a role in the design of treatment systems for highly reactive species, with overall rate constants predicted to increase with flow velocity. Copyright (C) 1999 Elsevier Science B.V.
Bibliographical noteFunding Information:
This work was funded by an NSF Young Investigator Award (Grant BES-9457260) to A.L.R. and an EPA STAR Fellowship to W.A.A. Special thanks are extended to Ann Dennis and Alicia Groft for conducting preliminary experiments, and to Lisa Totten for sharing her calculated one-electron reduction potential values. We also thank Paul Roberts (Stanford University) and Peter Harriott (Cornell University) for valuable discussions and the anonymous reviewers whose comments greatly clarified this manuscript.
- Chlorinated hydrocarbons
- Groundwater remediation
- Mass transfer