Showing papers by "Paul G. Tratnyek published in 1995"

Dechlorination of carbon tetrachloride by iron metal: Effect of reactant concentrations

Abstract: Optimal application of zero-valent iron (Fe{sup 0}) in contaminant remediation requires an understanding of the mechanism by which the process occurs. An important first step towards developing such an understanding is determining the dependence of reaction rate on the concentration of each reactant. Previous efforts by Matheson and Tratnyek (1994) found that the first-order rate constant for reduction of CCl{sub 4} by Fe{sup 0} increased linearly with iron surface area. However, heterogeneous systems often exhibit more complex behavior, suggesting that the simple linear relationships previously reported may not fully reflect all of the potentially contributing processes. To investigate this possibility, further study of the effect of initial CCl{sub 4} concentration and iron surface area has been undertaken. Our findings suggest that site availability arguments can be applied to the effect of CCl{sub 4} concentration but that the effect of Fe{sup 0} surface area is more complex.

Dechlorination of carbon tetrachloride by iron metal: The role of competing corrosion reactions

Abstract: In waters contaminated with chlorinated solvents, three oxidants are available to drive corrosion of metals: water, dissolved oxygen, and the chlorinated contaminant. One goal of this work, accomplished through column experiments, was to access the relative significance of solvent dechlorination among these three possible pathways for corrosion. The unique design of these columns has allowed the rates and products of CCl{sub 4} degradation, as well as changes in column chemistry resulting from corrosion and precipitation, to be determined as a function of time and location along the length of the column. Before CCl{sub 4} was introduced to the column, very little iron dissolution was observed except at the up-gradient interface of the iron-bearing zone, where dissolved O{sub 2} was consumed. When CCl{sub 4} was added, a marked increase in iron dissolution was observed, which we attribute to corrosion by dechlorination of CCl{sub 4}. This interpretation is further supported by the correlation between dissolved iron and concentration of CCl{sub 4} across the length of the iron-bearing zone. The chlorinated contaminant may serve as the dominant oxidant of Fe{sup o} which results directly in reductive dechlorination.

Processes Affecting Nitro Reduction by Iron Metal: Mineralogical Consequences of Precipitation in Aqueous Carbonate Environments

Abstract: The effects of carbonate species on organic contaminant reduction by Fe{sup o} was investigated to assess field performance of groundwater remediation installation involving iron metal. First-order rate constants for nitrobenzene reduction, k{sub obs}, by Fe{sup o} was obtained in a well-mixed aqueous batch system buffered with CO{sub 2} that provided evidence for interaction between metal and carbonate species. Values of k{sub obs} declined with increased dissolved carbonates and with extended exposure of the metal to a particular carbonate buffer. SEM with energy-dispersive X-ray spectroscopy of iron samples exposed in aqueous bicarbonate medium revealed the temporal changes in the metal surface characteristics due to formation of a non-uniform film of FeCO{sub 3} crystalline aggregates. Based on these evidences, it can be concluded that the decreases in nitro reduction rate are due to accumulation of corrosion-resistant iron carbonate aggregates that inhibit mass-transfer of the reactants and products to and/or from reducing sites on the metal surface.

Interfacial phenomena affecting contaminant remediation with zero-valent iron metal

Abstract: The purposes of this paper are to discuss the importance of oxygen (or anoxia) in organic contaminant degradation by granular iron metal, and to explore the various ways in which corrosion, precipitation, and mass transport effects on iron reactivity reflect interfacial phenomena. Studies illustrating the importance of zone-scale and grain-scale interfaces are summarized. The effect of O{sub 2} on dechlorination rates is also briefly discussed, along with possible reaction mechanisms.