Effects of carbonate species on the kinetics of dechlorination of 1,1,1-trichloroethane by zero-valent iron

The effect of precipitates on the reactivity of iron metal (Fe⁰) with 1,1,1-trichloroethane (TCA) was studied in batch systems designed to model groundwaters that contain dissolved carbonate species (i.e., C(IV)). At representative concentrations for high-C(IV) groundwaters (≈10^-2 M), the pH in batch reactors containing Fe⁰ was effectively buffered until most of the aqueous C(IV) precipitated. The precipitate was mainly FeCO₃ (siderite) but may also have included some carbonate green rust. Exposure of the Fe⁰ to dissolved C(IV) accelerated reduction of TCA, and the products formed under these conditions consisted mainly of ethane and ethene, with minor amounts of several butenes. The kinetics of TCA reduction were first-order when C(IV)-enhanced corrosion predominated but showed mixed-order kinetics (zero- and first-order) in experiments performed with passivated Fe⁰ (i.e., before the onset of pitting corrosion and after repassivation by precipitation of FeCO₃). All these data were described by fitting a Michaelis-Menten-type kinetic model and approximating the first-order rate constant as the ratio of the maximum reaction rate (V_m) and the concentration of TCA at half of the maximum rate (K_1/2). The decrease in V_m/K_1/2 with increasing C(IV) exposure time was fit to a heuristic model assuming proportionality between changes in TCA reduction rate and changes in surface coverage with FeCO₃.