Environ. Sci. Technol. 2003, 37, 713-720
Reductive Dehalogenation of Halomethanes in Iron- and Sulfate-Reducing Sediments. 1. Reactivity Pattern Analysis JOHN F. KENNEKE AND ERIC J. WEBER* National Exposure Research Laboratory, U.S. Environmental Protection Agency, 960 College Station Road, Athens, Georgia 30605
The incorporation of reductive transformations into environmental fate models requires the characterization of natural reductants in sediments and aquifer materials. For this purpose, reactivity patterns (range and relative order of reactivity) for a series of 14 halogenated methanes were measured in iron- and sulfate-reducing sediments and two representative model systems: adsorbed Fe(II)/ goethite [Fe(II)ads/R-FeOOH] and iron sulfide (FeS). Both Fe(II)ads and FeS are naturally occurring reductants. The strong similarity in reactivity patterns between the iron- and sulfate-reducing sediments suggests that the two share a common reductant despite their different chemical compositions (i.e., the sulfate-reducing sediment contained FeS). An orthogonal regression analysis of the halomethane transformation rate data in the sediment and model systems supports the assumption that a common mechanism for halomethane transformation exists between the sediments and the Fe(II)ads/R-FeOOH system and further corroborates the conclusion that Fe(II) adsorbed to Fe(III)containing minerals is the dominant reductant in both sediment systems. Weak (0.5 N) and strong (6.0 N) acid extraction of the sediments indicated that solid-phase Fe(II) was 67% higher in the sulfate-reducing sediment than in the iron-reducing sediment, which is consistent with the observations that the halomethanes were transformed a factor of 3 times faster in the sulfate-reducing sediment and that Fe(II) was the dominant reductant.
Introduction In recent years, considerable effort has been directed toward increasing our understanding of the reductive transformation of organic pollutants in anoxic environments. Although the functional groups that are susceptible to reduction have been identified, our limited understanding of reaction mechanisms is currently a barrier to the prediction of absolute reduction rates and the manner in which reaction rates will vary from one environmental system to another (1-3). A growing number of reports in the literature concerning reductive transformations have focused on identifying and quantifying the dominant chemical reductants in anoxic sediments and aquifers (4-7). The occurrence of chemical reductants is a result of the reduction of inorganic electron acceptors coupled with the microbial oxidation of organic matter and H2 (8). From a thermodynamic point of view, a * Corresponding author phone: (706) 355-8224; fax: (706) 3558202; e-mail:
[email protected]. 10.1021/es0205941 Not subject to U.S. Copyright. Publ. 2003 Am. Chem. Soc. Published on Web 01/14/2003
sequence of redox zones (i.e., nitrate-, manganese-, iron-, and sulfate-reducing and methanogenic) can develop that are characterized by the respective dominant terminal electron-accepting processes. The mapping of redox zones in contaminated aquifers has been accomplished by the analysis of redox-active species in the pore waters [e.g., NO3-, Mn(II), Fe(II), SO42-, CH4, and H2] and associated solid phase (e.g., Fe and S minerals) (9-17). Knowing the identity and reactivity of chemical reductants as a function of redox zonation would be key to the development of reactive transport models describing the movement of redox-active organic contaminants through contaminated sediments and aquifers. Because Fe(III) and SO42- dominate the pool of electron acceptors in many aquifer systems, our initial focus has been the study of iron- and sulfate-reducing sediments. Laboratory studies in well-defined model systems have provided valuable insight into the chemical reductants that can potentially form under iron- and sulfate-reducing conditions. The microbially mediated reduction of iron oxides results in the formation of soluble Fe(II), as well as a number of biogenic and authigenic minerals including siderite (FeCO3), magnetite (Fe3O4), and vivianite [Fe3(PO4)2] (17-19). Although little is known about the ability of structural Fe(II) in these biogenic minerals to reduce organic pollutants, surface-complexed Fe(II) on iron oxides [Fe(II)ads] has been demonstrated, in a number of model studies, to mediate the reductive transformation of nitroaromatics (4-6) and halogenated aliphatics (19-21). The microbially mediated reduction of sulfate to sulfide and the subsequent reaction of sulfide with iron minerals and soluble Fe(II) leads to the formation of amorphous iron sulfide and mackinawite (FeS) (22, 23). Subsequent reactions with polysulfides can result in the formation of greigite (Fe3S4) and pyrite (FeS2) (24-26). Iron sulfides have been shown to mediate the reductive transformation of halogenated aliphatics (27-30). Soluble sulfide species can also play a role in reductive transformations by electron transfer (7, 31), X-philic attack at halogen (32), and nucleophilic attack at carbon (32, 33). The comparison of reactivity patterns (i.e., range and relative order of reactivity) measured for a series of related probe chemicals in natural and well-defined model systems is one approach to identifying predominant chemical reductants in the subsurface (6). Similarities in reactivity patterns are expected for those reaction systems in which the probe chemicals are reacting with the same reductant. This approach was previously used with a series of nitroaromatic probe compounds to demonstrate that Fe(II) associated with ferric iron minerals was the dominant reductant throughout the anaerobic region of a landfill leachate plume (4). In this work, we measured reaction kinetics for the reductive transformation of 14 halogenated methanes in iron- and sulfate-reducing sediments and Fe(II)ads/goethite (R-FeOOH) and FeS model systems. Reactivity patterns determined for these systems were compared to gain insight into the mechanisms of halomethane reduction under iron- and sulfate-reducing conditions. Halogenated methanes are particularly well-suited for this purpose because a wide range in reactivity can be achieved by varying the identity and number of the halogen substituents. Studies in well-defined model systems have demonstrated that halogenated methanes are susceptible to reduction by the chemical reductants thought to predominate in iron- and sulfate-reducing sediments (7, 19-21, 28). Furthermore, halogenated methanes are environmentally relevant chemiVOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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cals; through years of misuse and improper disposal, a number of the halogenated methanes have become ubiquitous environmental pollutants.
Experimental Section Materials. Hexanes (99.9% pure) was obtained from Fisher. Chloroform (CHCl3), carbon tetrachloride (CCl4), carbon tetrabromide (CBr4), tribromomethane (CHBr3), dibromodichloromethane (CBr2Cl2), bromotrichloromethane (CBrCl3), tribromofluoromethane (CBr3F), dibromochloromethane, (CHBr2Cl), dibromochlorofluoromethane (CBr2ClF), and 4-morpholineethanesulfonic acid monohydrate (MES) were obtained from Aldrich and were of >98% purity, with the exception of CBr2ClF, which was 90% pure. Nonstoichiometric, inhomogeneous, technical-grade iron sulfide (FeS) was obtained from Aldrich. R-FeOOH was a gift from the Bayer Corporation (Bayferrox 910 standard 86). All materials were used as received. Sediment Collection and Preparation. The sediment used in this study was collected at Cherokee Park Pond, Athens, GA, approximately 3 m from shore in 1-m deep water. Sediments were collected in 1-qt jars by scraping the top 5-10 cm of the sediment along with the immediate overlying water and were capped underwater to minimize exposure of the sediment to atmospheric oxygen. Site water was collected in a collapsible, 5-gal polypropylene jug. Sediment and site water were stored at 4 °C in the dark. Prior to use, 5 L of site water was sparged with argon for 1 h. The sparged water and sediment were then transferred to an anoxic chamber (Coy) containing an atmosphere of 4% H2 and 96% N2. Approximately 500 g of decanted, wet sediment was suspended in 2 L of site water and vigorously stirred for 2 min. The sediment slurry was passed through a 53-µm sieve (Fisher), and the resulting silt and clay fraction was collected in a 5-L vessel. The material retained on the sieve was returned to the original sediment slurry, and 500 mL of site water was added. This process was continued until 5 L of site water had been mixed with the sediment and passed through the sieve. The silt and clay slurry was then transferred to 60-mL serum bottles, leaving less than 1 mL of headspace, and sealed with Teflon-faced butyl septa and aluminum crimp seals. The bottles were removed from the chamber and stored at room temperature (19 °C) in the dark. The sediment slurries treated in this manner will be referred to as iron-reducing. Five 60-mL serum bottles were sacrificed from each slurry batch and used to determine sediment loadings. Typical sediment loadings were 0.016 ((0.0016) g/mL. To stimulate sulfate reduction, some of the sediment slurry bottles were spiked with 250 µL of an anoxic (argon-sparged for 1 h), aqueous solution containing 2.4 M sodium sulfate and 2.4 M lactate (each at a final concentration of 10 mM); the sediment slurries treated in this manner will be referred to as sulfate-reducing. After approximately 2 weeks, hydrogen sulfide (H2S) was detected by gas chromatography/mass spectrometry (GC/MS) in the headspace, and formation of a fine black precipitate consistent with iron sulfide was observed. Model Systems. Model systems were prepared in an anoxic chamber. Approximately 2.5 g of FeS was weighed into each of a series of 60-mL serum bottles, and the bottles were filled to nearly headspace free with argon-sparged, deionized water (Barnstead) and then sealed with Teflonfaced butyl septa and aluminum crimp seals. For the iron oxide system, 2.1 g of R-FeOOH (stored under house vacuum for 2 months to remove oxygen) was added to 4.3 L of argon-sparged water containing 27.15 g of MES buffer (final concentration, 30 mM); the buffer was used to compensate for the subsequent spiking of the R-FeOOH system with an acidic Fe(II) stock solution. The pH of the 714
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R-FeOOH slurry was adjusted with 1 N NaOH to pH 6.8. The solution was sparged with argon for 1.5 h, transferred to the anoxic chamber, and stirred overnight. The R-FeOOH slurry was transferred nearly headspace free to 60-mL serum bottles, and the bottles were sealed with Teflon-faced butyl septa and aluminum crimp seals. Sediment and Model Systems Solids Characterization. Samples of the whole sediment (
