Kinetics of carbon tetrachloride reduction at an oxide-free iron electrode

To address some of the fundamental questions regarding the kinetics of reduction of contaminants by zero-valent iron (Fe⁰), we have taken advantage of the mass transport control afforded by a polished Fe⁰ rotating disk electrode (RDE) in an electrochemical cell. The kinetics of carbon tetrachloride (CCl₄) dechlorination at an Fe⁰ RDE were studied in pH 8.4 borate buffer at a potential at which an oxide film would not form. In this system, the cathodic current was essentially independent of electrode rotation rate, and the measured first-order heterogeneous rate constant for the chemical reaction (k(ct) = 2.3 x 10^-5 cm s^-1) was less than the estimated rate constant for mass transfer to the surface. Thus, for the conditions of this study, the rate of reduction of CCl₄ by oxide-free Fe⁰ appears to be dominated by reaction at the metal-water interface rather than by transport to the metal surface. Activation energies for reduction of CCl₄ and hexachloroethane by oxide-covered granular Fe⁰ (measured in batch systems) also indicate that overall rates are limited by reaction kinetics. Since mass transport rates vary little among the chlorinated solvents, it is likely that variation in k(ct) is primarily responsible for the wide range of dechlorination rates that have been reported for batch and column conditions.