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Kinetics of nitrite and Cr(VI) reduction by iron metal.pdf

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Kinetics of nitrite and Cr(VI) reduction by iron metal

Research Kinetics of Nitrate, Nitrite, and Cr(VI) Reduction by Iron Metal M I C H A E L J . A L O W I T Z ? A N D M I C H E L L E M . S C H E R E R * Department of Civil and Environmental Engineering, University of Iowa, 4105 Seamans Center, Iowa City, Iowa 52242 The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe 0 ) were studied in batch reactors for a range of Fe 0 surface area concentrations and solution pH values (5.5-9.0). At pH 7.0, there was only a modest difference (2-4×) in first-order rate coefficients (kobs) for each contaminant among the three Fe 0 types investigated (Fisher, Peerless, and Connelly). The kobs values at pH 7.0 for both nitrite and Cr(VI) reduction were first- order with respect to Fe 0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 × 10-3 and 2.2 × 10-1 L m-2 h-1 were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe 0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe 0 , which reduced nitrate at slower rates at higher Fe 0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe 0 decreased with increasing pH with apparent reaction orders of 0.49 ( 0.04 for nitrate, 0.61 ( 0.02 for nitrite, and 0.72 ( 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr- (VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h. Introduction The success of iron metal (Fe 0 ) permeable reactive barriers (PRBs) in treating groundwater plumes containing chlori- nated solvents (1-4) has stimulated significant interest in the application of Fe 0 PRBs to other compounds commonly present as co-contaminants in chlorinated solvent plumes (5-7). Three contaminants often associated with chlorinated solvent plumes i