Polychlorinated Biphenyls (PCBs): Environmental Chemistry

Transcription

1 ES/RP 532 Applied Environmental Toxicology Page 1 of 17 October 13, 2004 Lecture 13 Polychlorinated Biphenyls (PCBs): Environmental Chemistry I. Introduction: History & Use A. Polyhalogenated aromatic compounds actually encompass 4 classes of chemicals, PCBs, PCTs (polychlorinated terphenyls), PCNs (polychlorinated naphthalenes), PBBs (polybrominated biphenyls). Owing to the large volume of PCBs produced in the U.S. compared to the other groups, the lecture will focus on that group. B. History-PCBs 1. Commercial production of PCBs in U.S. commenced during the late 1920s (Monsanto Co.); the products were marketed under the name AROCLOR, and there were a number of different formulations that differed by the weight percentage of chlorine in the product. a. Production continued until 1971 when Monsanto, practically the only producer, ceased production for open-ended and nominally closed system uses. 1. Annual production in 1957 was 32.3 million pounds; in 1970 it had reached 85 million pounds. 2. By 1975, production was back down to 34.1 million pounds. 2. In 1966, PCBs were reported as environmental contaminants, unleashing a flurry of basic and applied studies. 3. In 1968, rice oil contaminated with PCBs caused acute toxicity in over 600 people in Yusho, Japan. 4. PCB production in the U.S. ceased by the end of C. Uses of PCBs 1. Uses were either open-ended (26%), nominally closed (13%), or closed (61%) System categories for uses of PCBs Category End-Use Aroclor 1200 Series osed system Capacitors Transformers 16, 21, 54 42, 54, 60 Nominally closed system Hydraulic fluids Heat-transfer fluids Lubricants , Open-ended application Plasticizer in rubbers Plasticizer in resins Carbonless copy paper Adhesives Wax extenders Dedusting agents Inks 21-54, , 54, 68 54, II. Chemistry A. The general empirical structure for PCBs is 1. C 12 H 10-n n with n being Theoretically there are 209 different PCB structures, but in practice not all of them were present in commercial AROCLORS (~20 structures were known to be absent in technical PCB mixtures)

2 ES/RP 532 Applied Environmental Toxicology Page 2 of 17 X X X X 3 2 2' 3' X 4 A 1 1' B 4' X X 5 6 X X 6' 5' X a. The generic structure of a PCB consists of two directly linked benzene (phenyl) rings; the biphenyl structure and numbering of carbon atoms in each ring is as follows: is the numbering of carbon atoms in aromatic ring A; is the numbering of carbon atoms in aromatic ring B 3. X represents either H or depending on the level of chlorination b. PCB congeners are named officially by their IUPAC designation and by a system of sequential numbering developed by Ballschmiter and Zell, 1980 (known as the BZ number). The nomenclature system can be viewed at the EPA website (PCB ID-Table of PCB Congeners & Other Species). 1. For example, if each of the rings has a chlorine in the 4 position, then the IUPAC name is 4,4 dichlorobiphenyl, and the BZ number is 15. B. Production process--batch chlorination of biphenyl using ferric chloride or iron filings as a catalyst; temperatures raised to above melting point of biphenyl but not greater than 150 C. Degree of chlorination controlled by the contact time of the reactants varying from hours. 1. Contaminants arising from the manufacturing process include ppm levels of polychlorinated dibenzofurans (PCDFs) and PCNs. C. The use of the term AROCLOR refers strictly to the PCB formulations prepared by Monsanto; the Japanese and Germans used their own commercial names--kanechlor and ophen, respectively. D. Each formulation of Aroclor was designated by a 4-digit number, for example 12nn (where 12 refers to biphenyl). The nn refers to the weight percentage of chlorine. For ex., Aroclor 1242 had 42% by weight chlorine. 1. The higher the formulation number, the more heavily chlorinated the PCB rings (i.e., the more total chlorine substituents. a. Each commercial Aroclor was a mix of isomers (congeners) containing various amounts of chlorine; for ex., Aroclor 1242, the most heavily produced formulation mix, had 45% by weight compounds with 3 chlorines, 31% with 4 chlorines, 13% with 2 chlorines, 10% with 5 chlorines, and 1% with 1 chlorine (see next table). b. Formulation Aroclor 1221 had the lowest weight percentage of chlorine with 50% by weight of PCBS having only one chlorine substituent. Proportion of Aroclor Formulations with Indicated Number of Chlorine Substituents on Phenyl Rings n in Empirical C 12 H 10-n n Formula 0 C 12 H C 12 H C 12 H

3 ES/RP 532 Applied Environmental Toxicology Page 3 of 17 3 C 12 H C 12 H C 12 H 5 l C 12 H C 12 H C 12 H C 12 H 9 1 Average Molecular Weight A comparison of different batches of Aroclor 1242 (nominally 42% chorine by weight) gave variable weight percentages of different chlorinated isomers. Comparison of analyses for PCB mixtures containing approx. 42 wt. % of chlorine. Data expressed as wt. % of chlorine in each congener of the indicated chlorine numbers within the PCB mixture known as 1242 PCB mix Aroclor Pyralene Kanechlor E. Major isomers of PCBs (i.e., congeners) 1. An estimated different chlorinated biphenyl isomers (congeners) are present in each of the PCB mixtures. PCB Empirical formulas, molecular weight, and number of corresponding isomers PCB Empirical Molecular Weight No. of Isomers Formula chlorobiphenyl C 12 H dichlorobiphenyl C 12 H trichlorobiphenyl C 12 H tetrachlorobiphenyl C 12 H pentachlorobiphenyl C 12 H hexachlorobiphenyl C 12 H heptachlorobiphenyl C 12 H octachlorobiphenyl C 12 H nonachlorobiphenyl C 12 H decachlorobiphenyl C Distribution of individual isomers in the major Aroclors is shown below [Rappe and Buser (1980)]. a. Note that each Aroclor will have different amounts of the various isomers b. Each Aroclor can be characterized by the relative amounts of the various isomers; the characteristic kinds of isomers and their amounts forms a fingerprint of that Aroclor 1. For example, in the table you will note that biphenyl and the monochloro substituted isomers are very dominant in Aroclor 1221

4 ES/RP 532 Applied Environmental Toxicology Page 4 of Aroclor 1242 can be distinguished from 1016 by the presence of low amounts but numerous pentachloro-substituted isomers Congener Distribution of PCBs in Different Aroclor Formulations F. Structures 1. Individual isomers can be characterized as symmetrically substituted or unsymmetrically substituted depending on the position of the chlorines on the rings 2. Note that the unsymmetrical isomers have different numbers of chlorines on each of the rings 3. Depending on whether the ortho sites are substituted with chlorines or not, the three dimensional conformation of PCBs will either be coplanar (non-ortho substituted congeners) or non-coplanar. 4. The reason is that chlorine atoms, being significantly larger than hydrogen atoms, will not permit free rotation of the rings when substituted on the ortho positions owing to steric hindrance (schematically shown in the right hand structure below). H H H H H H 5. Examples of co-planar and non-coplanar PCB structures are shown below:

5 ES/RP 532 Applied Environmental Toxicology Page 5 of 17 biphenyl 2,4,4',6'-tetrachlorobiphenyl 2',4',6'-trichlorobiphenyl 2,5,3',5'-tetrachlorobiphenyl 3',4',6'-trichlorobiphenyl 2,3,5',6'-tetrachlorobiphenyl 3',4',5',6'-tetrachlorobiphenyl 2,6,2',6'-tetrachlorobiphenyl 2',3',5'6'-tetrachlorobiphenyl 3,5,3',5'-tetrachorobiphenyl 6,4',5'-trichlorobiphenyl 4,5,4',5'-tetrachlorobiphenyl

6 ES/RP 532 Applied Environmental Toxicology Page 6 of 17 Aroclor G. Physical Characteristics of Aroclor mixtures 1. Appearance a. ear mobile oil (up to Aroclor 1248) (i.e., the mixture flows ) b. Light-yellow viscous liquid or resin (Aroclor ) c. Off-white powder (Aroclor 1268) 2. Other characteristics are shown in the table below a. Note that density, viscosity, fire point, and boiling point increase with the number of chlorines b. PCBs make excellent insulating fluids because of the high dielectric constants Density (@20 C ) Viscosity (@98.9 C ) Flash Pt. ( C) Fire Pt. ( C) Pour Pt. ( C) Distillation Range ( C) Vaporization Rate g/cm 2 /h x C ntb ntb ntb ntb * ntb ntb ntb ntb ntb ntb ntb ntb ntb *ntb = none to boiling Dielectric 100 C H. Analytical Chemistry--as a result of the bewildering number of possible isomers and congeners in an Aroclor, analysis becomes a virtual nightmare! 1. Analysis of a matrix depends on extraction with apolar solvents and extensive cleanup to get the lowest limit of detection 2. Analysis by capillary gas chromatography (GC) with confirmation by GC-mass spectrometry 3. ean-up techniques are quite important in separating DDT residues from PCBs; a. Indeed, prior to the realization of the widespread distribution of the PCBs, many detections of DDT may have been confused with PCB isomers b. Now PCB isomers can be separated from DDT residues prior to GC I. Note in the gas chromatograms on the next page, however, where the DDT elutes relative to the various PCB isomers J. The gas chromatogram is a plot of the signal from an analyte (it s actually the microvolts of potential elicited across a detector) relative to the time the compound elutes into the detector after injection of the sample extract matrix. K. The top chromatogram is a reference standard of a PCB mixture; the bottom chromatogram is the PCB congener distribution in a sample of blood.

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8 ES/RP 532 Applied Environmental Toxicology Page 8 of 17 L. Physicochemical Properties 1. Because Aroclors represent mixtures of various chlorinated isomers, properties represent average values for a combination of the most abundant isomers in any one mixture 2. PCB congeners tend to have low water solubilities and are very stable. These two descriptions should allow an estimation of other properties. However, the degree of C w (a.k.a. as S in units of g/m 3, i.e., ppm--see table below) or T 1/2 (half-life in hours, days, weeks, months, or years--see table below) will depend on the amount of chlorination of any particular congener. 3. Physicochemical Properties of PCB Isomer Groups (Mackay et al. 1992) a. Note that water solubility and vapor pressure decrease as chlorine substitution increases. b. Pertinently, the K H of the more highly chlorinated congeners stays pretty high even though VP goes down. PCB Isomer Group by # of chlorine atoms S (Water Solub.) [ppm or g/m 3 ] Vapor Pressure, P, as Pa, liquid state Henry s Law Constant, H, Pa m 3 /mol log K ow, octanolwater partition coeff. log K oc (sorption coeff.) biphenyl (0) mono (1) di (2) tri (3) tetra (4) penta (5) hexa (6) hepta (7) octa (8) nona (9) nd nd deca (10) Physicochemical Properties of Aroclor Mixtures (Mackay et al. 1992) Aroclor Mixture S (ppm) V. P. (P), Pa H (Pa m 3 /mol) log K ow log K oc Physicochemical Properties of Individual Isomers (Mackay et al. 1992) Isomer S (ppm) P (Pa) H log K ow log K oc biphenyl chlorobiphenyl ,2 -dichloro ,2,5-trichloro

9 ES/RP 532 Applied Environmental Toxicology Page 9 of 17 2,2,5,5 -tetrachloro ,2,4, 5,5 -penta ,2,3,3,4,4 -hexa ,2,3,3;,4,4,6-hepta ,2,3,3,5,5,6,6 -octa ,2,3,3,4,4,5,5,6-nona Influence of dissolved organic matter on water solubility a. Smith et al (Rev. Environ. Contam. Toxicol. v.103) reported that the water solubility of p,p -DDT, 2,4,4-trichloro-biphenyl and 2,4,5,2,5,5 -pentachlorobiphenyl increased as the amount of dissolved soil humic acid (DOM or dissolved organic matter) increased; 1. Thus, at a dissolved humic acid concentration of 20 mg/l, the WS of DDT, 2,4,4 -tetrachloro-biphenyl and 2,4,5,2,5 -pentachlorobiphenyl was ~12, 140, and 18 ppb, respectively; a. At 80 mg/l DOM, the WS was ~35, 255, and 47 ppb, respectively. M. Dissipation (Mechanism of loss from a compartment not specified or known): Estimated Half-Life asses for PCB Isomer (Congener) Groups in Various Media (Mackay et al. 1992)* PCB Isomer Group by # of Air Water Soil Sediment chlorine atoms biphenyl (0) mono (1) di (2) tri (3) tetra (4) penta (5) hexa (6) hepta (7) octa (8) nona (9) deca (10) *Key for Half-Life asses ass Mean Half-Life (hours) Range (hours) 1 5 < (~ 1 day) (~2 days) (~1 week) (~3 weeks) 300-1, ,700 (~2 months) 1,000-3, ,500 (~8 months) 3,000-10, ,000 (~2 years) 10,000-30, ,000 (~6 years) >30,000

10 ES/RP 532 Applied Environmental Toxicology Page 10 of 17 III. Chemistry Degradation Potential A. Photodegradation is characterized by a series of reductive dechlorinations; most of this work has been done under laboratory conditions using a variety of solvents and artificial sunlight (UV lamps, for ex.). hv 300 nm hv B. Degradation in Soil 1. The persistence of PCBs is related to the degree of chlorination; for ex., Fries and Marrow (1984) showed that 14 CO 2 evolution decreased as the numbers of chlorines in the molecule increased. This study does show that lower chlorinated PCBs have the potential for mineralization. 2. Fries and Marrow (1984) also showed that the largest number of metabolites were organosoluble and with the lower chlorinated compounds, bound residues were significant. They also hypothesized volatilization as a significant route of loss. Recovery of original 14 C after incubating congeners in soil for 98 days (Fries & Marrow (1984) Compound CO 2 Hexane Aqueous Methanol Bound Total biphenyl monochloro monochloro monochloro ,2,5,5 -tetra ,2,4,4 -tetra ,2,3,3,5,5 -hexa ,2 4,4,5,5 -hexa C. Activated Sludge--Combined Biochemical and Photochemical Degradation 1. Experiment reported by Baxter and Sutherland (1984, ES&T 18: ) a. Resting suspension of a pseudomonad bacterium isolated from activated sludge and grown with biphenyl as the sole carbon and energy source converted 2,4 - dichlorobiphenyl to a variety of products, including two monochlorobenzoic acids, two monohydroxydichlorobiphenyls probably produced by dehydration of the corresponding dihydrodiols, and a yellow hydroxyoxochlorophenyl chlorohexadienoic acid. Irradiation of the metabolite mixture led to the disappearance of the yellow compound and appearance of two monochloroacetophenones. The same or very similar compounds, and analogous more highly chlorined compounds, were produced from Aroclor 1242, a commercial PCB mixture. It was suggested that successive biochemical and photochemical processes may contribute to the degradation of chlorinated biphenyls in the environment. b. See metabolic pathways below

11 ES/RP 532 Applied Environmental Toxicology Page 11 of 17 D. Potential for degradation of aged residues in sediments 1. Work from the PCB-contaminated Hudson River in NY shows a historical decrease in highly chlorinated congeners, indicating biodegradation in an aqueous sediment system. Bush et al. (1994) found that in a >120 km stretch of the Hudson River, PCB residues in sediment had declined by an average of 3.7% per annum since Microbial cultures have been developed which can biodegrade PCBs. a. Prospects for bioremediation: field application of these cultures may be impractical at present; incineration still remains the main process for disposing of PCBs and decontaminating soil/sediments. IV. Environmental Distribution A. Principal Sources of PCBs include atmospheric inputs, tributary loads, and direct discharges. 1. PCBs are volatilizing to the atmosphere from various sources, and during the 1980 s fluxes of µg/m 2 /yr were calculated for this process. B. Potential for Plant Uptake 1. Plants grown in PCB contaminated soil will have low amounts of PCBs detected in the tissue, but early work by Fries and Marrow (1981, J. Agric. Food Chem. 29: ) indicated that the residues may result from volatilization from soil and deposition on the leaf surfaces. 2. Fries and Marrow treated a sandy loam soil with three PCBs (2,2,5-trichlorbiphenyl (Tri), 2,2,5,5 -tetrachorobiphenyl (Tetra), and 2,2,4,5,5 -pentachorobiphenyl. The soil was layered in the pots, with a surface treated or nontreated section separated by an impermeable barrier from a treated or nontreated subsurface area. The soybean plants were rooted in the untreated subsurface zone.

12 ES/RP 532 Applied Environmental Toxicology Page 12 of 17 a. Results for Treated Surface Soil (untreated subsurface soil): % of Soil Concentration for Indicated Compound Plant Part Tri Tetra Penta Upper stem <MDL (0.4%) 0.1 (=MDL, 0.1%) <MDL (0.2%) Lower stem Upper leaves <MDL (0.4%) Lower leaves Seed pods <MDL (0.4%) 0.5 <MDL (0.2%) Whole plant Note that the lower leaves had the highest levels of PCBs, and the amount increased as the chlorine content of the congener increased. 2. Note how the upper bound of the Henry s Law Constant (H) for the congeners studied increased with chlorine content (see section II.L.3.) a. Corresponding to an expected increase in K H, the lower leaves had more pent chlorinated PCBs than tri- and tetrachlorinated PCBs. 3. Most importantly, soybeans grown in untreated surface layer soil had NO DETECTABLE residues. 4. When the plants were harvested for residue determination after 52 days, the surface and subsurface soils were analyzed for PCBs. The subsurface soils contained about 15% less PCBs than the surface soils, suggesting volatilization of the PCB residues. C. PCBs can be found in U.S. environments that one might think of as pristine, i.e., remote from industrial activity (Hawthorne et al J. Environ. Qual. 25: ). 1. However, the concentrations are very low compared to areas impacted by industrial activity (Panshin and Hites, 1994, Environ. Sci. Technol. 28: ). a. For example, a study in N. Dakota showed that the highest concentrations were found for the tetra-, penta- and hexachlorobipheny PCBs during the spring, and summer at levels were up to 50 pg/m 3. At Bloomington, IN, the concentration ranged between 0.65 to 2.53 ng/m 3 (nearly 100-fold greater). b. Panshin and Hites noted that the concentrations of PCBs in air had not changed in a 6 year period (they compared their 1993 data to those collected in ). 1. They calculated a atmospheric residence time for PCB congeners of 49±16 days. The residence time is the time a PCB congener spends in the atmosphere before being either degraded or removed to another environmental compartment. D. Considerable research on PCB fluxes in the Great Lakes and eastern coastal areas has been conducted owing to concern about bioaccumulation in the food web. 1. Thus, many studies have examined precipitation deposition, particle-phase deposition, gas absorption (from air to water), and volatilization to understand net flux of PCBs. 2. Steven J. Eisenreich and coworkers have been assessing the fluxes of PCBs into and out of the Great Lakes. Lakes Superior, Michigan, and Huron are the most susceptible to contamination with Lake Michigan being the most contaminated system. The reasons for this and the sensitivity of the lake are a. Large surface areas b. Extreme depths c. Low suspended sediment loads per unit volume d. Low production of particulates e. Highly sensitive biota f. Extreme hydraulic retention times E. A recent study has examined the major sources of input of PCBs to urban and background areas of the mid-atlantic States (Van Ry et al., 2002, ES&T 36(15):3201; Totten et al. 2004, ES&T 38(9):2568.

13 ES/RP 532 Applied Environmental Toxicology Page 13 of Wet deposition (volume weighted mean concentration), particle phase (dry deposition), and gas absorption (air to water transfer) were calculated for different land use sites in the New Jersey region from the coastal areas to the eastern edge boarded by the Delaware River. 2. Urban areas had significantly higher PCB net fluxes into the sites than more suburban areas or a forested area ( background area) (Totten et al. 2004) Site (Land use) Gas Absorption (ng/m 2 /day) Dry Deposition (ng/m 2 /d Volume Weighted Mean Concentration in Rainfall (ng/l) Wet Deposition Flux (ng/m 2 /day) Camden (urban industrial Jersey City (urban industrial) Sandy Hook (coastal urban) Tuckerton (Coastal light residential) New Brunswick (suburban) Pinelands (forested=background) Chester (light suburban)

14 ES/RP 532 Applied Environmental Toxicology Page 14 of Wet deposition fluxes at the New Jersey sites (as measured by volume weighted mean concentration in rain) were similar or less than in other areas of the U.S. (Van Ry et al. 2002) 4. An analysis of the proportions of the various chlorinated homologues in precipitation showed that the industrialized sites had a predominance of more chlorinated congeners. (Van Ry et al. 2002) Relative Percent Contributions of Different Congener Groups Found in Rainfall Homolog Group Camden Jersey City Pinelands Chester (i.e., number of chlorines in congener group) Seasonal fluxes tended to be higher in the winter and spring in the urban areas (Camden and Jersey City) but variable in the more pristine areas (Pinelands and Chester) F. Time Trend Analysis Using Limnological Sediment Cores

15 ES/RP 532 Applied Environmental Toxicology Page 15 of Owing to sedimentation processes in large lakes and established rates of sedimentation, cores can be collected and dated. One pathway (or sink) for PCB (and other recalcitrant, i.e., persistent, chlorinated hydrocarbons) will be sedimentation. 2. A core representing times dating to 1882 was analyzed for PCBs (Kjeller and Rappe, 1995, Environ. Sci. Technol. 29: ). PCBs were detected at very low levels in the core slice (i.e., disks) corresponding to However, the concentrations stayed at a very low level until about In Sweden, where the study was conducted, large scale PCB use started around 1950 and was totally banned for open use in The data shown in the graph are more suggestive of a variation in time associated with use of PCBs rather than a variation much affected by increased combustion processes, although PCBs was detected above the detection level in older sediments. 5. The graph also suggests a leveling off of PCB deposition. G. A more recent trend analysis was published for PCB levels in the northern Great Lakes (Schneider et al ES&T 35(19:3809) (next two figures)

16 ES/RP 532 Applied Environmental Toxicology Page 16 of A recent PCB mass balance study in lake Michigan reported that the average mean PCB gross gas depositional fluxes were 30 times higher than the average PCB dry deposition flux. The magnitude and direction of the fluxes varied seasonally; Miller et al.,2001, Environ. Sci. Technol. 35: ). a. Since the 1980 s, average concentrations in the water are believed to have fallen from 1 ng/l to 0.1 ng/l in H. Studies of the Baltimore Harbor and Chesapeake Bay show a net positive volatilization and thus a net flux into the atmosphere. This positive net flux was also associated with being a source for contamination of rural locations with PCBs (Bamburg et al Seasonal and

17 ES/RP 532 Applied Environmental Toxicology Page 17 of 17 annual air-water exchange of polychlorinated biphenyls across Baltimore harbor and the Northern Chesapeake Bay. Environ. Sci. Technol. 36:4245). 1. Volatilization losses from the Chesapeake Bay have been estimated as being 10 times greater than inputs from wet and dry deposition. I. Overall, numerous studies of PCB concentrations in sediment, precipitation, air, and fluxes indicate that PCB levels, while still easily detectable, have gone done since their ban in the U.S.