| Abstract
| - The effect of precipitates on the reactivity of iron metal(Fe0) 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 Fe0 was effectivelybuffered until most of the aqueous C(IV) precipitated. Theprecipitate was mainly FeCO3 (siderite) but may alsohave included some carbonate green rust. Exposure ofthe Fe0 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 Fe0 (i.e., before the onset of pittingcorrosion and after repassivation by precipitation ofFeCO3). 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 FeCO3.
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