Environ. Sci. Technol. 2004, 38, 4353-4360
Abiotic Degradation of Pentachloronitrobenzene by Fe(II): Reactions on Goethite and Iron Oxide Nanoparticles THEODORE P. KLUPINSKI Environmental Science Graduate Program, The Ohio State University, Columbus, Ohio 43210 YU-PING CHIN* Department of Geological Sciences, The Ohio State University, Columbus, Ohio 43210 SAMUEL J. TRAINA† School of Natural Resources, The Ohio State University, Columbus, Ohio 43210
Pentachloronitrobenzene is a fungicide that is degraded in anoxic soils and sediments through unknown processes that are often thought to be biologically mediated. The present research describes the kinetics for the abiotic reduction of this compound in aqueous Fe(II)/goethite systems at near-neutral pH values. The results provide evidence for a rate-affecting surface-association process rather than a direct (i.e., kinetically second-order) reaction with an adsorbed Fe(II) species. It is therefore likely that the iron oxide surface participates directly in the reaction. Furthermore, reduction is observed in the apparent presence of trace amounts of suspended iron oxide nanoparticles, formed in situ by the oxidation of Fe(II). Given that Fe(III) colloids and other nanoscale phases may occur in natural sediments, such abiotic reactions could significantly influence the environmental fate of nitroaromatic compounds.
Introduction The contamination of surface waters and sediments from nonpoint sources is a significant environmental concern. Due to their tendency to collect in surface runoff from agricultural lands, pesticides contribute substantially to nonpoint source contamination. A U.S. study (1) showed that pesticides were found in ∼40% of those watersheds identified as presenting the greatest risks to aquatic life or human health due to contaminated sediments. Pentachloronitrobenzene (C6Cl5NO2) is a fungicide that is used commonly in many European and Asian countries. In degradation studies conducted in soil and sediment environments (2-7), the reduction product pentachloroaniline (C6Cl5NH2) is commonly observed. In those publications, it was tacitly assumed that transformations occurred through biologically mediated pathways. Investigation of this possibility was further pursued in studies of C6Cl5NO2 degradation by isolated soil micromycetes (8, 9). * Corresponding author phone: (614)292-6953; fax: (614)292-7688; e-mail:
[email protected]. † Present address: Sierra Nevada Research Institute, University of California at Merced, Merced, CA 95344. 10.1021/es035434j CCC: $27.50 Published on Web 07/09/2004
2004 American Chemical Society
In the presence of mineral surfaces, Fe(II) promotes the reduction of nitroaromatic compounds (10). Wetland sediments are anoxic environments, often containing high levels of Fe(II) in porewaters (11). For example, at Old Woman Creek National Estuarine Research Reserve in northern Ohio, Fe(II) concentrations can reach 1 mM at sediment depths of 15-25 cm (12). Under such conditions, chemical (i.e., abiotic) reactions may contribute significantly to the degradation of nitroaromatic contaminants. In this paper, we investigate the degradation of C6Cl5NO2 in controlled laboratory experiments in order to characterize the abiotic reactions that may influence the fate of nitroaromatic herbicides in Fe(II)-rich sediments.
Materials and Methods Chemicals. C6Cl5NO2 (Sigma-Aldrich) and C6Cl5NH2 (Alfa Aesar) were each recrystallized from cold methanol (99.5%), MES (2-[N-morpholino]ethanesulfonic acid; SigmaAldrich, >99.5% as monohydrate), FeCl2‚4H2O (SigmaAldrich, 99.995%), ammonium acetate (Jenneile Enterprises, ACS Reagent, 99.1%), and 1,10-phenanthroline monohydrate (Fisher Certified ACS). Acid and base solutions were made from concentrated hydrochloric acid (Fisher Certified ACS Plus) and solid NaOH (Mallinckrodt AR, 99%), respectively. Solvents included Milli-Q water (Mill-Q UV Plus, Millipore) and methanol (J. T. Baker HPLC solvent). Minerals. Goethite (R-FeOOH) was synthesized by Meier (13) and stored in a concentrated aqueous suspension. The goethite has a surface area of 63 ( 2 m2/g, as determined by the BET method for N2 adsorption, and its integrity was verified by X-ray diffraction analysis (Cu KR radiation) of a random powder mount. Hectorite, a trioctahedral smectite, was obtained from the Source Repository of the Clay Mineral Society and fractionated to
