Dept. of Environmental Science and Engineering
Oregon Graduate Institute of Science & Technology
In-situ barriers of zero-valent iron (Fe[superscript 0]) have become one of the most attractive options for remediation of chlorinated solvents in groundwater due to their simplicity of application and ability to remediate a wide range of contaminants. This study clarifies the geochemistry associated with the reaction of Fe[superscript 0] and chlorinated aliphatic compounds. The focus (and chapters) proceeds from the macro scale to the molecular scale. Column experiments were used to simulate a cross-section of a permeable barrier with an iron-bearing zone, an up-gradient, and a down-gradient zone of sand. The columns showed that (i) water flowing from the treatment zone is anoxic, alkaline, and high in ferrous iron, (ii) the kinetics of dechlorination vary with flow rate, and (iii) there is a correlation between the initial concentration of CCl4 ([P]o) and the concentration of Fe2[superscript+] in solution. To further explore the mechanism of dechlorination by Fe[superscript 0], a combination of new and previously reported kinetic data was subjected to an analysis of factors effecting contaminant degradation rates. Rate constants from batch and column studies vary widely, but normalization to concentration of iron surface area yields a specific rate constant (k[subscript SA]) that varies by only 1 order of magnitude for individual halocarbons. Correlation analysis using k[subscript SA] reveals that dechlorination is generally more rapid at saturated carbon centers than unsaturated carbons, and high degrees of halogenation favor rapid reduction. It was concluded from this study that the unexplained order of magnitude variability in k[subscript SA] reflects differences in surface oxide composition. To evaluate the role of the surface oxide film in mediating the reduction of chlorinated solvents, CCl4 was used as a probe of degradation by Fe[superscript 0] under the influence of various anions, ligands, and [P]o. The reaction kinetics were pseudo-first order for CCl4 disappearance (k[subscript CCl4] and zero order for the appearance of Fe2[superscript+] in solution (k[subscriptFe2+]). Values of k[subscript CCl4] and k[subscriptFe2+] exhibit saturation kinetics with respect to [P]o, suggesting that CCl4 is transformed via reaction with a limited number of specific reactive sites, and that it is the dominant oxidant contributing to corrosion in these systems.
Johnson, Timothy Lee, "Surface mediated reduction of chlorinated solvents by zero-valent iron" (1997). Scholar Archive. 189.