Environ. Sci. Technol. 1990, 24, 1575-1580
Photolysis of Chlorinated Dioxins in Organic Solvents and on Soils Somchal Kleatlwong, Long V. Nguyen, Vincent R. Hebert, Murray Hackett, and Glenn C. Miller"
Department of Biochemistry, University of Nevada, Reno, Reno, Nevada 89557 Michael J. M1111e and Robert Mltzel
Enseco-Cal Laboratory, 25544 Industrial Boulevard, Sacramento, California 9569 1 Photoreduction of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to 2,3,7-trichlorodibenzo-p-dioxin (TrCDD) in isooctane was found to account for approximately 10% of the loss of TCDD and is thus a minor photolysis pathway. The remainder undergoes conversion by other pathways, which may involve carbon-oxygen cleavage. One new photoproduct of TCDD, formed by a reductive rearrangement, is 4,4',5,5'-tetrachloro-2,2'-dihydroxybiphenyl, as demonstrated by mass spectrometry. Photolysis of TCDD on soils is slow relative to solution photolysis. Organic solvent added to the soil enhances the extent of photolysis. Evidence is presented that transport of TCDD to the surface in the organic solvent film is primarily responsible for the increased photolysis, rather than an effect from addition of a reducing hydrogen source. In the unamended soils, photochemical loss of TCDD was observed only for the first 5 days of a 15-day irradiation. In soils containing hexadecane, more than twice the amount of TCDD was lost, and photochemical loss continued at both 10 and 15 days of irradiation. This observation suggests that transport of the light-exposed soil/air interface is occurring. Octachlorodibenzo-p-dioxin(OCDD) undergoes photoreduction on soil surfaces to the lower chlorinated congeners. For the pentachloro and tetrachloro congeners, the toxic 2,3,7,8-tetrachlorinatedisomers were observed in greater yield than would be expected on the basis of the number of potential isomers.
H
Introduction Of the 75 possible polychlorinated dibenzo-p-dioxins (PCDDs), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic and has been the most extensively studied (1,2). PCDDs have become widespread in the environment from improper disposal of chlorinated chemical wastes, inefficient municipal and industrial incinerators, and industrial accidents (3). Because of their chemical stability and resistance to degradation, their presence in the environment poses a potential health hazard to humans and other organisms. The environmental fate and transport of TCDD may involve volatilization, bioaccumulation, biodegradation, oxidation, reduction, and photolysis. The physical properties controlling environmental transport of TCDD are water solubility (19.3 ng/L), octanol-water partition Torr at coefficient (1.4 X lo6),vapor pressure (7.4 X 25 "C), and molecular weight (321.974) (4-6). With its low vapor pressure and aqueous solubility, strong sorption to soils, and hydrophobicity, the mobility of TCDD in a soil environment is low (7). Nearly all TCDD will reside in biota, sediment, and soil (8). Although most biological and nonbiological transformation processes are slow, photolysis has been shown to be rapid (9-11). Photolysis is probably the most important transformation pathway in environmental systems in which sunlight can penetrate. Several reports (9, 11-14) have shown that photodechlorination is an important transformation pathway for dioxins. In fact, dechlorination products have been the only photoproducts detected in previous studies. Because 0013-936X/90/0924-1575$02.50/0
photodechlorination was assumed to be the major photochemical pathway, the rationale for the slow rate of photolysis on soil surfaces was thought to be due to the lack of a reducing hydrogen source. Support for this was provided when substantially increased rates of photolysis were observed when organic substances (Le., herbicide formulation) were added to the soil (11). Choudhry and Webster, in a recent article, suggested that reductive dechlorination is the usual process in the photolysis of chlorinated dioxins containing four or more chlorines (15), and for dioxins with three or less chlorines, carbon-oxygen cleavage is the preferred route of phototransformation (14). Epling and co-workers (16) found that in the presence of sodium borohydride the photolysis rates were increased, and that dechlorination gave principally the photoreduction products, although quantitative conversions were not presented. In another study, however, photodehalogenation of TCDD was found not to be a significant pathway (17). These workers were unable to detect 2,3,7-trichlorodibenzo-p-dioxin (TrCDD) as a photoproduct of TCDD in 5050 water-acetonitrile (17). They suggested that carbon-oxygen cleavage may be an important photolysis pathway. Particularly for the higher chlorinated dioxins, the relative contributions of dechlorination and the preferential dechlorination position(s) are important to determine due to the dramatically higher toxicity of the dioxin congeners that are chlorinated in the 2,3,7,8 positions. The first objective of this study was to determine the significanceof the photodechlorination as a transformation pathway and to investigate other photolysis mechanisms that may be involved. A second objective was to determine if the increased rate of photolysis of TCDD on soils containing solvents (i.e., hexadecane) was due to the presence of a hydrogen source for photodechlorination, or whether transport to the exposed soil surface in organic solvent was important. The photoreduction of octachlorodibenzo-pdioxin (OCDD) was also examined on soils to determine the distribution of photoproducts. Experimental Methods Octachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,7-trichlorodibenzo-p-dioxin,2,7-dichlorodibenzo-p-dioxin (DCDD), and dibenzo-p-dioxin were purchased from Cambridge Isotope Laboratory Inc. and used as received. Additional 2,7-dichlorodibenzo-pdioxin was synthesized by heating 9.7 g of 2,4-dichlorophenol and 3.4 g of potassium hydroxide in a 500-mL round-bottom flask attached to a condenser for 45 h at 200 "C. Material that sublimed on the condenser was dissolved in benzene and extracted with 1N KOH. The volume was reduced under nitrogen and recrystallized in benzene. The identity of this sublimate was established by comparison of the mass spectrum and chromatographic retention times with the purchased standard. Hexadecane was purchased from Sigma Chemical Co. Diazomethane was generated from Diazald (Aldrich Chemical Co.) by the procedures
0 1990 American Chemical Society
Environ. Sci. Technol., Vol. 24, No. 10, 1990
1575
Table I. Soil Properties of Kracaws, Montana Grain, and Tajunga Soil
Table 11. Photodechlorination of Octachlorodibenzo-pdioxin on Tajunga Soil Sunlight
%
soil designation
organic matter
pH
% clay
% silt
% sand
dioxin
Kracaws Montana Grain Tajunga
0.8 2.2 0.49
7.5 7.6 7.9
8
47 28 14
45 50 81
octa hepta total (1234678) hexa total (123478) (123678) (123789) penta total (12378) tetra total (2378)
22
4
supplied. Solvents were HPLC grade.
Irradiation of Dioxins in Solution. Isooctane solutions of TCDD (3.1 X lo* M) and TrCDD (1.4 X 10* M) were separately placed in (1.3-mm4.d.) borosilicate tubes. These solutions were irradiated simultaneously under a light bank composed of 16 Westinghouse FS40 (Amm 310 nm) preheat-rapid start sunlamps. The temperature was maintained at 31-33 O C by using six fans under the light bank during the exposure. Dark controls consisted of the same tubes covered with aluminum foil, and no loss was observed from these samples over the course of the experiment. The samples were analyzed at intervals of 0, 1, 3, 6, 10, 15, and 21 min by gas chromatography on a Hewlett-Packard 589OA gas chromatograph equipped with a B3Nielectron capture detector (GC-EC) connected to a Hewlett-Packard 3390A integrator (18). The column, injector, and detector were operated isothermally at 220,260, and 325 "C, respectively. Nitrogen was used as carrier and make-up gas. The compounds were resolved on a 5 m X 0.53 mm i.d. Hewlett-Packard HP1 column with a film thickness of 2.65 pm and quantitated by using unlabeled external standards. M) or TCDD For the product studies, DCDD (2.5 X (1.6 X lop3M) in toluene was placed in 1.8-mL vials and irradiated with the system described above. Samples were collected at various time intervals and analyzed. Following 25 min of irradiation, the samples were either directly injected onto the gas chromatograph-mass spectrometer (GC-MS) or methylated with 1mL of diazomethane solution and concentrated under nitrogen. The samples were analyzed for products by GC-MS using a Finnigan 4023 mass spectrometer with Incos data system. The compounds were resolved on a 30 m X 0.32 mm i.d. J&W DB-5 column with a film thickness of 0.25 pm. The gas chromatograph was held a t 60 "C for 1 min and then programmed at 10 "C/min to 180 "C and then at 5 OC/min to 260 "C. Irradiation of TCDD on Soils. Two agricultural soils, labeled Montana Grain and Kracaws, (Table I) were airdried for 24 h and sieved to 0.425 mm with a U.S.A. standard no. 40 sieve. The Montana Grain soil was obtained from an area 50 km south of Plentywood, MT, while the Kracaws soil was obtained from an area 70 km northwest of Winnemucca, NV. A 50-g portion of soil was placed into a 500-mL round-bottom flask along with 25 mL of methylene chloride containing respectively, 0,0.5, and 2.5 g of hexadecane, which corresponds to 0, 1, and 5% (w/w) hydrocarbon in the soil. The methylene chloride was removed by rotary evaporation under vacuum to dryness in a 45 O C water bath. The soils then received 500 pL of TCDD (20 pg/mL) in 100 mL of methylene chloride. The solvent was removed by rotary evaporation as before. Portions (2 g) of each soil were placed in separate Petri dishes, which were then covered with clear polyethylene film and fastened with rubber bands. The depths of the soils were calculated on the basis of bulk density and the surface area of the Petri dishes and were 0.29 mm for the Montana Grain soil and 0.27 mm for the Kracaws soil. The dark control samples were additionally covered with alu1578 Environ. Sci. Technol., Vol. 24, No. 10, 1990
concn' after sunlight exposure, rglk Odays 7 days 15 days 7000 16.3 4.1 ND ND ND ND ND ND ND ND
5010
231 82 28 ND 2.2 2.7 2.7 0.84 0.22 0.084
4930 246 93 28 2.1 3.0 2.9 1.1
0.95 0.22 0.11
ND, not detected.
minum foil. The soil samples were irradiated by using the system described above. Relative intensity of ultraviolet light was measured with a Spectroline DM-SOON ultraviolet meter. The soil samples were irradiated for 5-, lo-, and 15-day periods. The irradiated soils were quantitatively transferred to test tubes and extracted with 3 mL of 2-propanol/hexane (12238, v/v). The soil suspension was mixed on a pulser vortex text tube mixer for 30 s and sonicated in a water bath for 20 min. The soil suspension was again mixed for 10 s and centrifuged with a table-top centrifuge for 2 min. The supernatant was transferred into graduated centrifuge tubes. The extraction procedure was then repeated two additional times using 3 mL of the extraction solvent, omitting the sonication step during the final extraction. The combined extracts were then concentrated to 1.0 mL under nitrogen in a 45 "C water bath. Remaining TCDD was quantitated by 63Ni electron capture gas chromatography on a Hewlett-Packard 5890A gas chromatograph connected to HP 3390A integrator. The injector and detector temperatures were 275 O C , and the column temperature was 240 "C. Nitrogen was the carrier gas and make-up gas. The analytes were resolved on a Supelco SPB-1 wide-bore 30 m X 0.75 mm i.d. capillary column with a film thickness of 1.0 pm. Irradiation of OCDD on Soils. Samples of Montana Grain and Tajunga (CA) soils (2 g) were prepared as above containing 10 mg/ kg OCDD. Samples were irradiated in Petri dishes for up to 15 days in July sunlight. Following 0 , 7 , and 15 days of sunlight exposure, the samples were brought indoors and stored in the dark until analyzed. Dark samples covered with aluminum foil were placed in sunlight and brought indoors following 15 days of exposure. For each of the samples, a 1.0-g portion was extracted on a shaker for 4 h with hexane/methanol(80:20). '%-labeled 2,3,7,8-tetra-, penta-, hexa-, hepta-, and octachlorinated dioxin internal standards were added to the samples prior to extraction and used for all quantitation by isotope dilution methods. The specific 'W-labeled chlorinated isomers used as internal standards are those indicated in Tables I1 and 111. The extracts were passed through a traditional dioxin cleanup column (acidic and basic silica gel and acid alumina) and concentrated to a final volume of 50 pL in tetradecane. The extracts were analyzed by high-resolution gas chromatography-low-resolution mass spectrometry using a 60 m X 0.25 mm i.d. DB-5 fused-silica capillary column and a Finnigan 4500 mass spectrometer in SIM mode (EPA method 8280). Qualitative and quantitative confirmation of the 2,3,7,8-substituted tetrathrough heptachlorinated dioxin isomers was obtained by
4
Table 111. Photodechlorinationof Octachlorodibenzo-pdioxin on Montana Grain Soil in Sunlight
0
dioxin octa hepta total (1234678) hexa total (123478) (123678) (123789) penta total (12378) tetra total (2378) a
concn4 after sunlight exposure, Pdki 0 7 15 8640 17.2 4.0
ND ND ND ND ND ND ND ND
7840 236 60 27 1.2 1.4 1.7 1.7 0.62 0.16 0.059
0 \
7010 302 85 43 1.97 2.7 2.2 3.53 0.87 0.42 0.12
00
c -
5 Time (mi.)
Figure 1. FS40 sunlamp photolysis of TCDD and TrCDD in isooctane.
ND,not detected.
60
separation on a 60 m X 0.25 mm i.d. SP-2331 fused-silica capillary column. Due to the low concentration of the tetra isomers, they were quantitated on a Finnigan MAT-90 high-resolution mass spectrometer using the DB-5 column described above. Dark controls gave recovery of OCDD and heptachlorodibenzo-p-dioxin(HpCDD) contaminants similar to that of the nonexposed samples, and no other dioxins were observed. Recoveries of [2,3,7,8-lSC4]TCDD added at the beginning of the extraction varied from 55 to 71%.
Results and Discussion Irradiation of Dioxins in Solution. A major photochemical transformation pathway for TCDD has been suggested to be the dechlorination of TCDD to TrCDD (9,11).
c’m; LL C
CI
40 -
30
. . . . Predicted 2,3,7-TrCDD Formation
-
20 -
II-#
10 -
0
0
1
4
8
16
12
20
24
Irradiation (min)
Flgure 2. Production of TrCDD in isooctane during FS40 sunlamp photolysis of TCDD.
I
In order to determine the fraction (f) of TCDD photoreduced to TrCDD, a simple kinetic scheme was used that was based on the assumption of first-order reaction rates. By measurement of concentrations of TCDD and TrCDD during the photolysis of TCDD, a point will be reached when the TrCDD concentration is maximized. At this point, the rate of formation of TrCDD is exactly equal to its loss, and the fraction (f) of conversion of TCDD to TrCDD multiplied by the photolysis rate of TCDD is equal to the photolysis rate of TrCDD. This type of analysis requires that the amount of light absorbed by the solution during the irradiation is low (
