Environ. Sci. Technol. 2004, 38, 1045-1053
Carbon Tetrachloride Transformation on the Surface of Nanoscale Biogenic Magnetite Particles M I C H A E L L . M C C O R M I C K * ,† A N D PETER ADRIAENS Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109-2125
Iron-reducing conditions in subsurface environments promote dechlorination reactions via both biotic and abiotic pathways, the latter often mediated via biologically activated minerals formed by dissimilatory iron-reducing bacteria (DIRB). Here we report the major products and pathways associated with the abiotic transformation of carbon tetrachloride (CT) by nanoscale biogenic magnetite/ maghemite particles produced by the DIRB Geobacter metallireducens. Product formation and free radical/carbene trapping studies indicate that CT transformation occurs via three parallel pathways. The first pathway (hydrogenolysis) results in the formation of chloroform (45-50%) via a trichloromethyl free radical (‚CCl3) and possibly a trichloromethyl carbanion (:CCl3-). The second and third pathways involve a dichlorocarbene intermediate (:CCl2), which either hydrolyzes to form CO (∼38%) (carbene hydrolysis), or undergoes further reduction to yield methane (8-10%) (carbene reduction). The mechanism of methane formation from :CCl2 is not known, but is speculated to involve a sequence of surface coordinated carbenoid and free radical complexes. The large fraction of relatively benign products formed by the carbene-mediated pathways suggests that magnetite/maghemite particles may have a beneficial application in the remediation of CT contaminated environments.
Introduction Carbon tetrachloride (CT) is a common groundwater contaminant in the United States (1). Although recalcitrant in aerobic environments, it is susceptible to transformation in anaerobic environments by both biotic (2, 3) and abiotic mechanisms (4-7). The known pathways for the reductive transformation of CT in anoxic nonsulfidic environments result in an array of compounds that can be categorized as products of hydrogenolysis, radical addition, radical coupling, or carbene hydrolysis (Figure 1). While some of these products are relatively benign, others are of greater toxicological concern than CT itself, such as chloroform (CF) and dichloromethane (DCM). The factors that control the fate of CT in anaerobic environments are, therefore, of interest to environmental scientists and engineers. Because ferric iron (FeIII) is the predominant electron acceptor in many anoxic aquifer and lake sediments (8-10), and because dissimilative iron-reducing bacteria (DIRB) * Corresponding author. phone: (315)859-4832; fax: (315)8594807; e-mail:
[email protected]. † Present address: Department of Biology, Hamilton College, 198 College Hill Rd., Clinton, NY, 13323. 10.1021/es030487m CCC: $27.50 Published on Web 01/15/2004
2004 American Chemical Society
appear to be ubiquitous (11), interest has grown in the role that iron-reducing environments play in contaminant transformation. Recently, we reported that CT was readily transformed in a model iron-reducing system and that the transformation could be ascribed almost entirely to abiotic reactions occurring on the surfaces of nanoscale biogenic magnetite (Fe3O4) particles produced by the DIRB Geobacter metallireducens (12). Magnetite is a mixed-valence iron oxide (FeIIFeIII2O4) that has also been implicated in mediating CT transformation in systems that employ Fe0 either as a bulk reductant or as an electrode (13-15). Identifying the predominant products, intermediates, and transformation pathways of CT on the surface of magnetite may therefore enhance our understanding of the natural attenuation of CT in ironreducing environments and potentially improve the engineering design of permeable reactive barriers. Here, we report the reaction products and pathways associated with the abiotic transformation of CT in the presence of biogenic magnetite particles produced by G. metallireducens. The specific objectives of this work were: (1) to identify and quantify the principal products of biogenic magnetite-mediated CT transformation, and (2) to postulate plausible reaction pathways accounting for the observed product distribution.
Experimental Section Chemicals. All chemicals were of ACS reagent grade. Purified water was prepared using a Milli-Q plus system (Millipore Corp., Bedford, MA). All operations requiring strictly anaerobic conditions were performed in an anaerobic glovebag filled with 98% N2 and 2% H2 (Coy Laboratory Products, Ann Arbor, MI). Unless stated otherwise, the N2 used in establishing anaerobic conditions in media was pretreated to remove trace oxygen by passing the gas stream through a heated bed of copper metal (Cu0) turnings. Hydrous ferric oxide (HFO) was prepared by the neutralization of 0.17 M FeCl3‚6H2O (fw ) 270.3 g/mole) with 5 N NaOH in a well-mixed high-density polyethylene (HDPE) container. As circumneutral pH was approached, a thick red slurry of HFO formed. At pH 7.0, the slurry was allowed to ripen for 2-6 h, during which time the pH dropped slightly (usually
