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 a dvantage of the mass transport control afforded by a polished Fe-0 rot ating disk electrode (RDE) in an electrochemical cell. The kinetics of carbon tetrachloride (CCl4) dechlorination at an Fe-0 RDE were studie d in pH 8.4 berate buffer at a potential at which an oxide film would not form. In this system, the cathodic current was essentially indepen dent of electrode rotation rate, and the measured first-order heteroge neous 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 redu ction of CCl4 by oxide-free Fe-0 appears to be dominated by reaction a t the metal-water interface rather than by transport to the metal surf ace. Activation energies for reduction of CCl4 and hexachloroethane by oxide-covered granular Fe-0 (measured in batch systems) also indicate that overall rates are limited by reaction kinetics. Since mass trans port rates vary little among the chlorinated solvents, it is likely th at variation in k(ct) is primarily responsible for the wide range of d echlorination rates that have been reported for batch and column condi tions.