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Transformation of Chlorinated Hydrocarbons Using Aquocobalamin or Coenzyme F₄₃₀ in Combination with Zero-Valent Iron

Soil Science Division, Univ. of Idaho, Moscow, ID 83844-2339.

^* Corresponding author (mmorra{at}uidaho.edu).

ABSTRACT

More effective methods are necessary for the remediation of soils, sediments, and ground waters contaminated with halogenated organic compounds. Our objective was to determine the feasibility and utility of using a tetrapyrrole-Fe(0) mixture for reductive dehalogenation of synthetic organic contaminants. Aquocobaiamin or coenzyme F₄₃₀ was combined with Fe(0) in aqueous systems containing either a single chlorinated compound or mixtures of chlorinated compounds, and substrate disappearance was monitored using gas chromatography-mass spectrometry (GC-MS). Zero-valent iron effectively dehalogenated CCl₄ at low to neutral pH values, while increases in CCl₄ dehalogenation resulting from inclusion of tetrapyrrole catalysts along with Fe(0) occurred only at basic pH values. Rates of CCl₄ disappearance increased with additional aquocobalamin, but reached a maximum and decreased at higher aquocobalamin concentrations. Overall dehalogenation rates may thus be a function of Fe(0)'s limited reactive surface area. There was a trend for both tetrapyrrole catalysts to promote the disappearance of halogenated compounds in a mixed substrate containing 20 compounds. Studies with five individual substrates likewise showed trends for increased substrate removal with F₄₃₀ beyond that for Fe(0) alone. This increase is most important for compounds such as 1,2-dichloroethane and 1,4-dichlorobenzene that are not readily dehalogenated by Fe(0). Chloride concentrations in the reaction mixtures indicated that reductive dehalogenation was the dominant process responsible for substrate disappearance. Use of a combination of aquocobalamin or coenzyme F₄₃₀ and Fe(0) may effectively promote dehalogenation, thus producing fewer products and more complete dehalogenation of the target substrates than can be achieved using only one of the abiotic reductants alone.