Combined Zero- and First-Order Kinetic Model of the Degradation of TCE and cis-DCE with Commercial Iron

The design of a permeable iron wall depends to a great extent on the transformation kinetics of the chlorinated compounds. Therefore these degradation kinetics of TCE and cis-DCE with commercial iron and their dependence on the properties of the compounds and on the experimental conditions were studied in mixed-batch and column experiments. Since our data cannot sufficiently be described by a pseudo-first-order kinetics, we successfully applied an enhanced model accounting for both zero- and first-order kinetics. The fitted kinetic parameters, however, were found to depend on the experimental conditions and compound properties, which is interpreted in terms of different rate-limiting processes. The zero-order rate constant turned out to be twice as high for cis-DCE as for TCE in both experimental systems. Despite its slower transformation without transport control, the first-order rate constant was about 4 times higher for TCE than for cis-DCE in the mixed-batch vials. We attribute this to the lower water solubility and thus higher sorptivity of TCE at the polished iron surface. In the column experiments, transformation without transport control was twice as fast as in the batch experiments for both compounds. cis-DCE was degraded faster than TCE in the zero- and first-order region. At higher influent concentrations, the zero- and first-order rate constant of TCE decreased, which we assume to be due to the buildup of iron oxides, and transport to the reactive sites was found to depend a little on flow velocity. Due to the slow first-order kinetics of both compounds, we assume diffusion within micropores to be rate-limiting in flow-through systems. These variations in the kinetic parameters of the combined zero- and first-order model suggest that transport and sorption to reactive sites contribute to kinetic control of the degradation of chlorinated ethenes in addition to charge-transfer processes.