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The Role of Metallic Iron in the Biotransformation of Chlorinated Xenobiotics

Principal Investigators
G.F. Parkin, L.J. Weathers, J.L. Schnoor, and P.J.J. Alvarez, University of Iowa


Goal: This research will investigate the hypothesis that both microbial and abiotic processes contribute to reductive dechlorination of xe nobiotics in methanogenic incubations with elemental metals, such as iron, serving as an ultimate electron donor.

Rationale: Polychlorinated compounds such as carbon tetrachloride (CT) are known to be transformed via sequential reductive de chlorination by both abiotic and microbial mechanisms under aerobic conditions. However, existing treatment processes which utilize reductive dechlorination suffer from several drawbacks including inefficient transfer of electrons from the ultimate electr on donor to the chlorinated compound and slow rates of reaction, thereby resulting in possible accumulation of transformation products of equal or even greater toxicity. Elemental metals in aqueous solution can act as an energy source for methanogens via production of hydrogen. Reductive dechlorination of chlorinated compounds could then proceed by three mechanisms: (1) abiotic processes whereby electrons are transferred directly from the elemental metal to the chlorinated compound, (2) microbial processe s whereby electrons from H2 that are involved in biosynthetic processes are diverted to the chlorinated compound, and (3) microbial-catalyzed abiotic processes whereby electrons from the elemental metal are transferred to the chlorinated compou nd via biological electron carriers.

Approach: Experiments will be conducted in batch and column-reactor systems. Initial studies will investigate iron and carbon tetrachloride (CT). Other metals, such as aluminum, tin, and zinc will be use d in later studies. Various chlorinated organics will also be assayed. A hydrogen-utilizing, mixed, methanogenic culture will be developed as an inoculum source for all experiments. Initial batch studies will be performed to determine the general time-course that the reactions will follow. Inhibition studies using 2-bromoethanesulfonate (BES), a specific methanogenic inhibitor, will address the role of methanogens. Analytes to be measured in headspace gas samples include CT, chloroform (CF), dichlorometha ne (DCM), chloromethane (CM), hydrogen, and methane. Subsequent, detailed, batch kinetic studies will be performed and, where appropriate, analytes will include ferrous iron, total soluble iron, CT, CF, DCM, CM, hydrogen, methane, and oxidation-reductionp otential. The stoichiometry and kinetics of all pertinent reactions will be determined. Electron balances will be conducted to provide insight into important abiotic and biotic processes. Flow-through column experiments using adjustable-bed-length, glass chromatographic columns packed with steel wool will be conducted to simulate long-term in situ treatment and to validate the kinetics determined in batch studies. A one-dimensional, finite-difference, numerical model will be developed to simulate t he performance of the column reactors. The model will include advection, dispersion, and sorption, and the appropriate degradation kinetics as determined from batch experiments.

Status: Initial experiments with carbon tetrachloride (CT), ch loroform (CF), iron, and hydrogen have been performed. Column reactors have been constructed and have been seeded with anaerobic bacteria. Aqueous-phase CT was rapidly dechlorinated in anoxic treatments containing Fe0 powder. Investigators found that deoxygenation is not a prerequisite for treatment of CT-contaminated water with Fe0. Recent work investigating the role of oxygen in Fe0/CT systems indicates DO may be beneficial in controlling product formation. The amount of CT transformed to CF under initially toxic conditions was less than the amount transformed under anoxic conditions, indicating the operation of an alternative reaction pathway involving molecular oxygen. These results show that the dechlorination pathway (s) for CT may be controlled to a certain extent by dissolved molecular oxygen, resulting in the formation of a higher proportion of innocuous products (e.g., chloride ion and carbon dioxide) compared to the reductive dechlorination products (chloroform a nd methylene chloride) which are both increasingly recalcitrant under anoxic conditions and are toxic. In mixed culture experiments, the following treatments were investigated: (1) iron and live cells, (2) iron and cell-free methanogenic supernatant, or ( 3) live cells. This project is in its first year.

Clients/Users: This research will be of interest to other researchers and to U.S. Department of Defense.

Key words: dechlorination, xenobiotics, heavy metals, iron.

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