ARTICLE pubs.acs.org/est
Clay Mediated Route to Natural Formation of Polychlorodibenzo-p-dioxins Cheng Gu,† Cun Liu,*,† Yunjie Ding,† Hui Li,† Brian J. Teppen,† Cliff T. Johnston,‡ and Stephen A. Boyd*,† † ‡
Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824, United States Crop, Soil and Environmental Sciences, Purdue University, West Lafayette, Indiana 47907, United States
bS Supporting Information ABSTRACT: Recent studies have documented the ubiquitous occurrence of polychlorodibenzo-p-dioxins and dibenzofurans (PCDD/Fs) of unknown origin in soils and clay deposits. Interestingly, the PCDD/F congener profiles do not match any known natural or anthropogenic source, and global PCDD/F budgets fail to account for the observed levels in soils. To reconcile these observations, clay minerals had been hypothesized to play a central role in the natural in situ synthesis of PCDD/Fs. We recently demonstrated the clay-mediated formation of the most prevalent PCDD congener in soils, octachlorodibenzo-p-dioxin (OCDD), supporting this hypothesis. Here we report the formation of the direct precursors (“predioxins”) of the most toxic PCDD congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), and of 1,2,4,7,8-pentachlorodizenzodioxin (1,2,4,7,8-PeCDD), and two additional dimers, from the reaction of 2,4,5-trichlorophenol (2,4,5-TCP) with Fe(III)montmorillonite clay. We propose plausible reaction pathways, each initiated by single electron transfer from 2,4,5-TCP to Fe(III)montmorillonite forming the 2,4,5-TCP radical cation. The operative reaction mechanisms, inferred from experimental results, are supported by quantum mechanical calculations. The key role of montmorillonite is apparently to stabilize the reactive radical cation intermediate. Fortuitously, PCDD formation reactions on clay surfaces are more facile for less toxic higher chlorinated congeners like OCDD, as predicted by the proposed reaction mechanism and consistent with the observed PCDD congener distributions in soils. Importantly, increasing the toxicity equivalency factor of OCDD would immediately cause many soils to exceed PCDD regulatory levels due to the predominance of this congener.
’ INTRODUCTION Polychlorodibenzo-p-dioxins are among the most toxic environmental contaminants due to their potency as aryl hydrocarbon receptor (AhR) ligands, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) is the prototypical AhR ligand.1 Polychlorinated dioxins are formed, and occur in the environment, as mixtures of congeners with different degrees of chlorination and substitution patterns. The recognized sources of PCDD/Fs, e.g., pesticide manufacture and waste incineration, can be identified from their known and unique congener distribution profiles.2 Toxicity equivalency factors (TEFs) have been assigned for all PCDD congeners with Cl-substitution in the 2,3,7,8 positions, and these are summed to generate an aggregate toxic equivalents quotient (TEQ) which is used as the regulatory basis for PCDD/Fs in soils. The 2,3,7,8TCDD congener is considered most potent (TEF = 1), and octachlorodibenzo-p-dioxin (OCDD) is considered to be a much less potent AhR ligand. The current preliminary remediation goal for soils contaminated with PCDD/Fs is 72 ppt TEQ.3 There is an established discrepancy between PCDD emissions versus actual PCDD levels in soils, and OCDD presents the greatest imbalance,4 viz. about 40 times greater than the expected amount, suggesting an in situ process of PCDD formation r 2011 American Chemical Society
favoring OCDD. The predominance of OCDD in residential and rural soils is illustrated by an EPA study where, of the 1585 ppt (average on mass basis) total dioxins, 1482 ppt were from OCDD.5 The inexplicably high PCDD levels in soils, and the unique congener profile dominated by OCDD, are also observed in certain clay deposits such as ball clays,6 with PCDD concentrations exceeding 15 000 pg WHO-TEQ/g.6 Although these clays impart certain favorable properties (e.g., anticaking) to feeds, and may promote animal health by adsorbing fungal toxins, they have caused several instances of livestock contamination; during one period, it was estimated that 5% of national poultry production, and at least 35% of farm raised catfish, in the USA was contaminated by PCDDs originating from ball clays added to animal feed.79 A survey of PCDD levels among a population living in proximity to dioxin-contaminated floodplain soils revealed that the individual with the highest dioxin body burden was a potter who fired ball clays in an in-home unvented kiln.10 Received: December 16, 2010 Accepted: March 15, 2011 Revised: March 11, 2011 Published: March 24, 2011 3445
dx.doi.org/10.1021/es104225d | Environ. Sci. Technol. 2011, 45, 3445–3451
Environmental Science & Technology These examples document the potential for dioxin to humans directly related to PCDD contaminated clays. Direct ingestion of contaminated soils by children is another potential route of human exposure to dioxins, as demonstrated for Pb and DDT in historically contaminated soils from former orchards.10,11 Recently, we provided the first direct evidence of claymediated PCDD formation.12 OCDD formed spontaneously when pentachlorophenol (PCP) was mixed with montmorillonite clay. This gave substance to the previously hypothesized in situ clay-facilitated formation of PCDDs in soils and clay deposits,13,14 and raised the question of whether the most toxic PCDD congener, 2,3,7,8-TCDD, could form on clays via the same mechanism that produced OCDD. The overall goal of this study was to expand our understanding of the reactions of chlorophenols with montmorillonite in the context of the natural in situ formation of PCDDs, and the underlying molecular scale mechanisms and forces governing these reactions. Recent quantum mechanical calculations for PCDD formation from 2,4,5-trichlorophenol (2,4,5-TCP) showed that dimerization could progress to form highly toxic PCDD congeners, i.e., 2,3,7,8-TCDD and 1,2,4,7,8-pentachlorodibenzo-p-dioxin (1,2,4,7,8-PeCDD).15,16 However, the estimated energy barriers indicated that, in the absence of an appropriate catalyst, the dioxin formation reactions can only proceed at high temperatures (∼1000 K).17 Here, 2,4,5-TCP, the logical precursor to 2,3,7,8-TCDD, was mixed with Fe(III)-montmorillonite clay and allowed to react at room temperature. We document the formation of the direct precursors (“predioxins”) of 2,3,7,8-TCDD and 1,2,4,7,8-PeCDD, as well as two other dimers, from this reaction. Quantum mechanical calculations were also employed to develop whole reaction energy profiles for PCDD formation from the reactions of 2,4,5-TCP with Fe(III)-montmorillonite. Plausible reaction mechanisms for the formation of these products are proposed, each initiated by single electron transfer of 2,4,5-TCP to Fe(III)-montmorillonite, forming the central reactive intermediate, viz. the 2,4,5-TCP radical cation. The proposed reaction pathways, inferred from experimental results, are supported by quantum mechanical calculations. The key role of montmorillonite, in addition to providing the necessary Fe(III)/Fe(II) couple, is apparently to stabilize the reactive radical cation intermediate.12
’ MATERIALS AND METHODS Chemicals. 2,4,5-Trichlorophenol was obtained from Alfa Aesar (Ward Hill, MA) with purity >98%. The 2,3,7,8-TCDD and 1,2,4,7,8-PeCDD standards were purchased from AccuStandard (New Heaven, CT). Trimethylsilydiazomethane (2 M in hexane) was from Sigma-Aldrich (Milwaukee, WI). Acetone and toluene were HPLC grade. All the chemicals were 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 surface area of SWy-2 are 82 cmolc/kg and ca. 750 m2/g, respectively. The preparation of Fe(III)-montmorillonite followed the method of Arroyo et al.18 Briefly, the clay suspension was first titrated to pH 6.8 with 0.5 M sodium acetate buffer (pH 5) to remove carbonate impurities. Clay-sized particles (200 °C), and formation of 2,3,7,8TCDD and 1,2,4,7,8-PeCDD decreased probably due to the evaporation and decomposition). Then, samples were extracted using the same method as described above. The extraction efficiency of 2,4,5-TCP spiked in control samples was 97 ( 2%. An experimental control consisted of Ca2þ-saturated montmorillonite, which lacks an exchangeable transition metal cation needed to induce single electron transfer from 2,4,5-TCP. Product Analysis. The reaction products were identified using a Hewlett-Packard gas chromatograph fitted with a mass spectrometer (GC-MS 6890/5973) operated on full scan mode (50800 amu). An HP-5 MS capillary column (length = 30 m; internal diameter = 250 μm; film thickness = 0.25 μm) was employed. Helium was the carrier gas at a flow rate of 1.2 mL/min with splitless injection at 300 °C. The oven temperature was programmed from 80 °C (2 min hold) to 260 °C (10 °C min1, 5 min hold), and then to 320 °C (15 °C min1, 5 min hold). Computational Methods. Density functional theory (DFT) calculations were carried out to evaluate the reaction energies and activation energies associated with the proposed reaction pathway with and without iron using the Gaussian software package.19 As suggested in earlier study,15,20,21 Fe(III)Oþ was used to represent the interlayer iron species responsible for the catalytic oxidation of 2,4,5-TCP to 2,4,5-TCP radicals and the following hydrogen abstraction during the dimerization process; Fe(II)(OH)þ was used in chlorine atom abstraction, since a reductant was needed in this catalytic reduction process to eliminate Cl. While gas-phase FeOþ/Fe(OH)þ is the simplest model for Fe(III) in the interlayer, it may overestimate Fe reactivity compared with the more networked Fe expected in smectite interlayers. The geometry optimizations of all intermediate and transition state structures had been performed using the Becke three-parameter exchange functional (B3)22 and the LeeYangParr correlation functional (LYP)23 with 6-31G* basis set for C, H, and O atoms and LANL2DZ ECP basis set for Fe and Cl. The total multiplicity of the system was assigned a value of 2 for the system containing only 2,4,5-TCP radicals, 6 for the system containing high spin Fe(III), and 5 for the system containing high spin Fe(II). To determine the activation energy of a specific path for 2,4,5-TCP reactions, the transition state that connects two immediate stable structures through a minimum energy path was identified by quadratic synchronous transit (QST3) search methods.24
’ RESULTS AND DISCUSSION The reaction of 2,4,5-TCP and Fe(III)-montmorillonite clay was conducted at room temperature and a relative humidity of 8%. The reaction occurred spontaneously, and within minutes 3446
dx.doi.org/10.1021/es104225d |Environ. Sci. Technol. 2011, 45, 3445–3451
Environmental Science & Technology
ARTICLE
Figure 1. GC-MS chromatograms of (a) 1:1 acetone/toluene extract of the Fe(III)-montmorillonite/2,4,5-TCP reaction mixture after 3 d reaction time (after methylation) and (b) zoom region between 20.60 and 21.90 min. Mass spectra of (c) product A (retention time of 20.78 min), (d) product B (retention time of 20.85 min), (e) product C (retention time of 21.03 min), and product D (retention time of 21.83 min). On the basis of peak areas the reaction was complete within ∼3 h.
the yellow color of Fe(III)-montmorillonite changed to bluish gray indicating formation of the organic radical cations via single electron transfer from 2,4,5-TCP to Fe(III) of Fe(III)-montmor illonite.12,25,26 Four major products were obtained by extraction of the clay with 1:1 acetone/toluene. These products were methylated then identified by GC-MS analysis (Figure 1). Product A is a dimer of 2,4,5-TCP (2TCP þ 2CH3) coupled by CC bond formation leaving the two phenolic OH groups intact. The three remaining major products are diphenylethers with the COC bond formed at various positions on the ring, leaving one phenolic OH group intact. Except for the mass of 372, the fragmentation pattern of product B was identical to
that of the 2,3,7,8-TCDD standard, so it was identified as 2,3,7, 8-predioxin. Both products C and D had the same molecular ion at m/z 406. Except for the mass of 406, the fragmentation pattern of product C was identical to that of the 1,2,4,7,8-PeCDD, so it is identified as 1,2,4,7,8-predioxin. On the basis of our previous results12 and similar structures reported in the literature,15 the proposed structures for products A, B, C, and D are 3,30 ,5,50 ,6,60 hexachloro-(1,10 -biphenyl)-2,20 -diol; 4,5-dichloro-2-(2,4,5-trichl orophenoxy)phenol; 3,4,6-trichloro-2-(2,4,5-trichlorophenoxy) phenol; and 3,4,6-trichloro-3-(2,4,5-trichlorophenoxy)phenol, respectively (Figure 1). The yield of product B, the predioxin (precursor) of 2,3,7,8-TCDD was about one-half that of the 3447
dx.doi.org/10.1021/es104225d |Environ. Sci. Technol. 2011, 45, 3445–3451
Environmental Science & Technology other three products (Figure 2). Heating the initial product mixtures at 140180 °C resulted in the formation of 2,3,7, 8-TCDD and 1,2,4,7,8-PeCDD in the range 0.0670.16 μmol/g
Figure 2. Formation of products A, B, C and D as a function of reaction time between Fe(III)-montmorillonite and 2,4,5-TCP (initial concentration 0.2 mmol/g clay). Experiments were conducted at room temperature (∼23 °C) and RH of 8%. Error bars are the standard deviations of triplicate analyses. Product A is (1,10 -biphenyl)-2,20 -diol, 3,30 ,5,50 ,6,60 -hexachloro-; product B is 4,5-dichloro-2-(2,4,5-trichlorophenoxy)phenol; product C is 3,4,6-trichloro-2-(3,4,6-trichlorophenoxy)phenol (predioxin 2); and product D is 2,4,5-trichloro-3-(3,4,6trichlorophenoxy)phenol.
ARTICLE
clay (2251 μg/g clay) and 0.0900.19 μmol/g clay (32 68 μg/g clay), respectively (Table SI 1, Supporting Information), confirming the initial formation of the corresponding predioxins; no polychlorinated dibenzofurans were detected. Processing of ball clays (120 °C) caused increases in concentrations of the more heavily chlorinated (penta and above) PCDDs, suggesting elevated levels of predioxins in the raw clay7 consistent with our results that dioxins were formed from predioxins upon heating. It is noteworthy that OCDD formed spontaneously at room temperature under the same reaction conditions.12 The reactions reported do not proceed with ferric salts, or in the absence of Fe-montmorillonite, and only minor yields of products (
