Kinetics of chlorinated ethylene reaction with zero-valent iron in column reactors

The kinetics of chlorinated ethylene reaction with zero-valent iron in column reactors was studied. Variations in the fractional removal encountered with changing influent concentration were reproducible, and did not reflect "aging" of the iron surface or alterations in transport properties of the column packing. These observations were consistent with intraspecies competition for reactive sites. The Langmuir-Hinshelwood-Hougen-Watson (LHHW) model allowed these effects to be quantified at various concentrations through two fitting parameters, a lumped kinetic constant and a constant for adsorption to reactive sites. Interspecies competitive effects were also observed with cis-dichloroethylene (cis-DCE). The presence of the trans-DCE inhibited the reaction of the cis-DCE. A potentially serious possibility for error in permeable barrier design arose from extrapolating a pseudo-first order rate constant obtained from an experiment performed at one initial concentration to a different initial concentration because the pseudo-first order rate "constant" was dependent upon concentration in the LHHW model. The calculated first-order rate constant underestimated the rate of TCE reaction, since it incorporates a larger intraspecies competitive effect than would be manifested at low influent concentrations. Such a rate constant might result in overdesign of a barrier. Such overdesign might make the use of a zero-valent iron barrier appear impracticable due to high costs of technical difficulties in emplacing a wide permeable barrier.