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 (Fe 0 ), we have taken advantage of the mass transport control afforded by a polished Fe 0 rotating disk electrode (RDE) in an electrochemical cell. The kinetics of carbon tetrachloride (CCl 4 ) dechlorination at an Fe 0 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 × 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 CCl 4 by oxide-free Fe 0 appears to be dominated by reaction at the metal−water interface rather than by transport to the metal surface. Activation energies for reduction of CCl 4 and hexachloroethane by oxide-covered granular Fe 0 (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.