Environ. Sci. Technol. 2008, 42, 4758–4763
Octachlorodibenzodioxin Formation on Fe(III)-Montmorillonite Clay CHENG GU, HUI LI, BRIAN J. TEPPEN, AND STEPHEN A. BOYD* Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824
Received November 29, 2007. Revised manuscript received March 17, 2008. Accepted April 15, 2008.
Polychlorinated dibenzo-p-dioxins (PCDDs) are ubiquitous and highly toxic environmental contaminants found in surface and subsurface soils and in clay deposits. Interestingly, the congener profiles of such PCDDs are inexplicably dissimilar to those of known anthropogenic (e.g., pesticide manufacture, waste incineration) or natural (e.g., forest fire) sources. Characteristic features of soil or clay-associated PCDDs are the dominance of octachlorodibenzo-p-dioxin (OCDD) as the most abundant congener and very low levels of polychlorinated dibenzofurans (PCDFs). These propensities led to the hypothesis of in situ PCDD formation in soils and geologic clay deposits. In this study, we demonstrate the formation of OCDD on the naturally occurring and widely distributed clay mineral montmorillonite under environmentally relevant conditions. When pentachlorophenol (PCP) was mixed with Fe(III)-montmorillonite, significant amounts of OCDD were rapidly (minutes to days) formed (∼5 mg OCDD/kg clay) at ambient temperature in the presence of water. This reaction is initiated by single electron transfer from PCP to Fe(III)-montmorillonite thereby forming the PCP radical cation. Subsequent dimerization, dechlorination, and ring closure reactions result in formation of OCDD. This study provides the first direct evidence for clay-catalyzed formation of OCDD supporting the plausibility of its in situ formation in soils.
Introduction It is now well established that both urban and rural soils are contaminated with polychlorodibenzodioxins (PCDDs) at levels that are in some instances potentially damaging to human and ecosystem health (1–4). Additionally, prehistoric clay deposits (e.g., ball clays) are contaminated with relatively high levels of PCDDs (5–11). In both instances a unique PCDD congener profile is present, which is dominated by octachlorodibenzodioxin (OCDD) and the absence of polychlorodibenzofurans (4). A major unanswered question is the source of such PCDDs in soils and clay deposits (2). A detailed accounting of PCDD emissions into the atmosphere and subsequent terrestrial deposition cannot reconcile the levels of PCDDs in soils owing primarily to the high levels of OCDD (2). This observation along with the unique PCDD congener profile in soils suggests some other unknown sources of PCDDs, such as atmospheric photochemical formation from precursor pentachlorophenol (PCP) (2). Holt et al. (4) convincingly argue that OCDD is formed in soils from pesticide derived precursors, most notably PCP. However, * Corresponding author phone: (517) 881-0579; fax: (517) 3550270; e-mail:
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the soil process by which OCDD is formed, especially in isolated rural soils and subsoils that have not received such pesticide formulations, and geologic deposits of clays, remains unknown. We hypothesize that PCDDs, especially OCDD, are synthesized in soils via clay-facilitated reactions with precursors such as chlorophenols, and that such synthesis in soils can help close the PCDD mass balance. Several studies have attempted global PCDD budgets (2, 12). In almost every instance, the concentrations or total mass of PCDDs in soils cannot be accounted for by the known sources of PCDDs. From some estimates, apparent air-borne PCDD deposition (based on measured PCDD concentrations in soils) was 10-20 times higher than annual emissions (12). A more recent detailed mass balance analysis of combustion as the major source of PCDDs in the environment, including rural soils, concluded that deposition of PCDDs exceeded emissions by a factor of at least 2 (2). This is especially important for rural soils, where the only logical known source of PCDD would be deposition. Clearly, these studies and others (11) suggest some sort of synthesis of PCDDs in the environment to account for the measured PCDD levels in soils. Pentachlorophenol volatilization with subsequent photochemical formation of OCDD has been hypothesized, a priori, to be a significant, and to-date the only, source of PCDDs formed in the environment, i.e., in addition to combustion which itself fails to account for PCDD levels in soils (2). However, this scenario cannot easily account for the presence of high levels of PCDDs, with unique OCDD dominated congener profiles, in prehistoric clay deposits, or in subsoils. Due to the extraordinary low water solubilities of PCDDs, it is impossible to attribute their presence in geologic clay deposits or subsoils to leaching from the very top few cm of soil where deposition occurs. So, atmospheric photochemical formation of PCDDs from chlorophenols is one likely (though not yet quantified) route to help account for dioxins in surface soils. We hypothesize that another plausible route is the in situ clay-facilitated formation of PCDDs in soils from precursor chlorophenols, especially PCP. The general idea of in situ formation of PCDDs in soils has been suggested (but not demonstrated) in several recent studies (4, 11, 13), though no known mechanisms exist or have even been suggested to account for formation of PCDD or the preferential formation of OCDD. Ball clay is a commercial term referring to various mixtures of clays that are mined and used to make ceramics, tile, bricks, etc. They have also been used widely as livestock feed additives. For the latter use, ball clays with relatively higher smectite contents are desired and selectively mined since they impart favorable properties to feeds (e.g., anticaking) and may promote health by adsorbing harmful fungal toxins for instance. Ball clays are known reservoirs for high levels of PCDDs; concentrations of PCDDs in samples were as high as ∼430,000 pg/g with toxic equivalents (TEQs) of 15,000 pg WHO-TEQ/g (9). These clays caused livestock contamination in several instances when used as feed additives (14). Characteristic features of clay-associated PCDDs are the dominance of OCDD as the most abundant congener (as is the case in soils), and very low levels of polychlorinated dibenzofurans. The concentrations and congener profiles of PCDDs in chicken eggs, farmed catfish, and even chickencontaining baby food showed strong correlations with those found in natural clays from geologic deposits. It was reported that about 5% of national poultry production, and at least 35% of farm-raised catfish in the United States, was contaminated by dioxin originating from clay added to animal feed (5, 14). It is noteworthy that the characteristic features 10.1021/es7029834 CCC: $40.75
2008 American Chemical Society
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of clay-associated PCDDs in ball clays, and those in soils (2), are similar and consistent with PCP as the primary precursor molecule for formation of OCDD in the environment, especially that produced via atmospheric photochemical reactions (2). However, the issue of precursor molecules (beyond PCP) involved in the in situ formation of PCDDs in soils, subsoils, and especially clays remains uncertain, and is beyond the scope of the present work. Rural soils are known to be contaminated with OCDD. In one recent study of 34 sampling sites, the mean OCDD concentration was ∼1500 pg/g with some individual samples showing concentrations g10,000 pg/g (3). In another study of 2081 soil samples (15), 371 soil samples were taken as background controls and they contained mean OCDD concentrations of 590 and 650 pg/g for the 0-1-in. and 1-6in. soil depths, respectively. These larger OCDD levels in the subsurface, despite lower soil organic matter in the subsurface, are consistent with the scenario of atmospheric deposition of PCP, PCP leaching as phenolate, and subsequent synthesis of OCDD in the soils. Smectites, including montmorillonite, are 2:1 layered aluminosilicate clay minerals that are widely distributed in soils, subsoils, sediments, and prehistoric clay deposits, and may comprise up to 78% of ball clays (16, 17). Due to isomorphic substitution in the tetrahedral Si and/or octahedral Al layers, smectites possess structural negative charges that are compensated by exchangeable cations in the interlayer regions. When exchanged with certain transition metal ions (e.g., Cu(II) and Fe(III)), smectites induce formation of radical cations from a variety of aromatic molecules including PCP (18) via single electron transfer to exchangeable interlayer cations, often followed by further reactions including dechlorination and polymerization (18–23). The reactions are quite facile, occur at room temperature, and have been suggested as a means to detoxify organic toxicants (18, 19, 21). However, such clay-facilitated reactions may also lead to the formation of more toxic compounds such as dioxins. Here, we demonstrate for the first time the formation of OCDD from precursor PCP on naturally occurring montmorillonite clay under environmentally relevant conditions. Formation of OCDD is initiated via formation of PCP radical cations followed by dimerization and ring closure reactions catalyzed by Fe(III)-montmorillonite.
Materials and Methods Chemicals. Pentachlorophenol was obtained from Aldrich (Milwaukee, WI) with purity >98%; GC-MS (see below) did not detect the presence of OCDD in PCP. Octachlorodibenzop-dioxin was purchased from AccuStandard (New Heaven, CT). Chloride (1000 mg/L) standard was obtained from Fisher Scientific (Fair Lawn, NJ). Acetone, hexane, dichloromethane, and toluene were HPLC grade. All other chemicals were reagent-grade and used as received. Clay Preparation. Smectite clay (Wyoming montmorillonite, SWy-2) was obtained from the Source Clays Repository of the Clay Minerals Society (Purdue University, West Lafayette, IN). The cation exchange capacity and theoretical surface area of SWy-2 are 82 cmolc/kg and 750 m2/g, respectively. The preparation of Fe(III)-montmorillonite followed the method of Arroyo et al. (24). Briefly, the clay suspension was first titrated to pH 6.8 with 0.5 M sodium acetate buffer (pH 5) to remove carbonate impurities. Claysized particles (65% of this was prime farmland (39). The PCDD congener profiles in ball clays are also dominated by OCDD, and Holmstrand et al. (11) proposed that some type of in situ OCDD formation by clay-mineral facilitated reactions must occur. In our research, dibenzop-dioxins, primarily OCDD, were formed from the reaction of Fe(III)-montmorillonite and PCP; no corresponding polychlorinated dibenzofurans (PCDFs) were detected. These results are consistent with the OCDD-dominated and PCDFdeficient signatures observed in soils and ball clay deposits. Radical cation formation from PCP, subsequent dimerization with molecular PCP, and dioxin ring closure catalyzed by 4762
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Fe(III)-montmorillonite demonstrate the plausibility of PCDD formation in situ in soils and clay deposits. The reaction mechanism proposed favors PCDD formation reactions involving highly chlorinated hence more oxidized chlorophenols, and this may help rationalize the unique PCDD profiles found in natural clays (i.e., dominance of OCDD), which occur as prehistoric deposits, and in soils, subsoils, and sediments.
Acknowledgments We thank the Mass Spectrometry Facility at Michigan State University for access to GC-MS instrument and Mr. Y. L. Pan for assistance in Cl- analysis. The project described was supported by grant P42 ES004911 from the National Institute of Environmental Health Sciences (NIEHS), NIH. The contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS, NIH.
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