The effect of precipitates on the reactivity of iron metal(Fe
0) with 1,1,1-trichloroethane (TCA) was studied in batchsystems designed to model groundwaters that containdissolved carbonate species (i.e., C(IV)). At representativeconcentrations for high-C(IV) groundwaters (
10
-2 M),the pH in batch reactors containing Fe
0 was effectivelybuffered until most of the aqueous C(IV) precipitated. Theprecipitate was mainly FeCO
3 (siderite) but may alsohave included some carbonate green rust. Exposure ofthe Fe
0 to dissolved C(IV) accelerated reduction of TCA, andthe products formed under these conditions consistedmainly of ethane and ethene, with minor amounts of severalbutenes. The kinetics of TCA reduction were first-orderwhen C(IV)-enhanced corrosion predominated but showedmixed-order kinetics (
zero- and first-order) in experimentsperformed with passivated Fe
0 (i.e., before the onset of pittingcorrosion and after repassivation by precipitation ofFeCO
3). 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 reactionrate (
Vm) and the concentration of TCA at half of themaximum rate (
K1/2). The decrease in
Vm/
K1/2 with increasingC(IV) exposure time was fit to a heuristic model assumingproportionality between changes in TCA reduction rateand changes in surface coverage with FeCO
3.