Environ. Sci. Techno/. 1995, 29, 836-840
Catalytic DechioAdon of Polychlorinated Bqhemyls Y U h l I N LIU, J E F F R E Y S C H W A R T Z , ' A N D CULLEN L. CAVALLARO
Department of Chemisty, Princeton University, Princeton, New Jersey 08544-1009
Introduction Polychlorinated biphenyl (PCB) mixtures have been used worldwide as nonflammable oils and as lubricants for hightemperature applications ( I ) , and PCB contamination of soils and water is nowwidespread. Due to potential adverse human health and environmental effects, the use of PCBs has ceased, and remediation methodology for existing PCB contamination has become of substantial interest (1). We report herein a new method for the conversion of PCBs to readily biodegradable biphenyl, which succeeds in simple solution and, importantly, on contaminated soils. It involves reductive dechlorination and is accomplished through homogeneous catalysisunder mild conditions. The dechlorination catalyst is prepared i n situ from the simple titanium complex titanocene dichloride,and easy-to-handle sodium borohydride is the ultimate reducing agent. Chemically, PCBs are a group of 209 simple, chlorinated aromatic congeners ( I ) , but many proposed chemical remediation processes for PCB contamination involve reactions that are typically not efficient for aromatic halides. Such procedures include, for example, substitution of chloride (2) or photolysis (3). In addition, although incineration or other oxidative procedures are widely discussed methodologies for PCB destruction (41, PCBs derive their widespread industrial use and environmental persistence from their resistance to oxidative degradation: The more highly chlorinated the congener, the greater its resistance to oxidation, and the longer it will persist in the environment. Furthermore, oxidation can lead to the formation of toxic dioxins (1). In contrast, reduction of a PCB congener should yield no dioxins and should be facilitated by high chlorine content. In fact, several reduction methodologies for PCB remediation have been described, including treatment with dissolving or molten metals (5, 6), through heterogeneous catalysis ( 3 ,or by electrolysis (8). However their success with pure PCBs, these techniques may be impractical for remediation of PCB-contaminated materials such as soils.
Results and Discussion Reaction between titanocene dichloride (Cp2TiCln) and sodium borohydride (NaBH4)gives Cp2TiBH4(1) (91, but 1 had only low activityfor reduction of a simple aryl halide, 4-bromochlorobenzene (10). However, rapid reduction of 4-bromochlorobenzene occurred (10) in diglyme using a catalyticamount of in situ prepared 1, a near stoichiometric amount of NaBH4,and a tertiary amine. We note here that reduction rates depend on the structure of the added amine, and these rates show an adverse effect of steric hindrance at nitrogen. Simple aliphatic amines are less effective catalysis promoters than are unhindered pyridines, and a
836 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 3, 1995
synergistic effect is noted when both an aliphatic amine and a pyridine are used. I l B NMR analysis of the reaction of 1 and pyridine shows formation of pyridineBH3, and 1 and N,N-dimethyloctylamine give N,N-dimethyloctylamineBH3 (11, 12). When a 1:l mixture of N,N-dimethyloctylamine and pyridine is added to a solution of 1, only N,N-dimethyloctylamineBH3is produced (11, 12); no pyridine.BH3 is detected (11, 12). In each experiment, cleavage of the borohydride ligand of 1 apparently occurs to give a coordinatively unsaturated moiety, CpzTiH (13). Since observed catalytic rates depend strongly on the structure of the added pyridine (121, we propose that the observed synergism derives from coordination of the pyridine to this Cp2TiH species to give 2, the coordinatively saturated, active catalyst (Scheme 1). Relative rates for the catalyzed reduction of several substituted bromobenzenes correlated well (10)with their relative electrochemical reduction potentials (14),consistent with a rate-determining electron transfer process: the less negative the reduction potential, the faster the chemical reduction. Since 2 is coordinatively saturated, this correlation (10) further suggests that the electron transfer process is outer sphere. In the context of PCB treatment, phenyl group substitution in an aryl halide should facilitate reduction by electron transfer (but not by nucleophilic addition (15)): p-phenyl or p-C1 group substitution have similar effects on the reduction potential of substituted bromobenzenes (14) (-1.56 and -1.61 Vvs AglAgBr). In fact, both substrates are reduced at comparable rates by the Cp2TiC12-NaBH4-amine system, and rates for reduction of p-dichlorobenzene and 4-chlorobiphenyl (pphenylchlorobenzene) are also similar. A definitive relationship between relative rates of PCB congener reduction and relative e e d o x values would be expected for rate-determining electron transfer-based reduction (IO),but it is difficult to determine ,$,do, for aryl halides (16). The measured value (14) for an aryl halide reduction potential (E1,*)does depend on e e d o x , but it is also affected by the rate of electron transfer from the electrode to the substrate and the rate of halide loss from the radical anion intermediate (16). Reversible reduction potentials have been determined for only a few PCB congeners (17 ) . These potentials become less negative as the chlorine content per congener increases (17 ) , and orthosubstitution, in which steric factors can hamper microbial degradation (18),has little effect. A meaningful relative rate correlation with relative E112 data might be expected within agroup of structurallystronglysimilar PCB congeners for which data have not yet been determined, assuming that rates of electron transfer from an electrode to each congener in this group and of halide loss from the resulting radical anions were similar (19). However, structural grouping of PCB congeners is not easily done; therefore, with the exception of congeners for which Eo has been determined, only broad correlations between measured El / 2 values (20)and relative rates for PCB congener reduction can be reasonably expected. We find that rates for reduction of individual PCB congeners do broadly correlate with measured reduction potentials (Table 1). Therefore we suggest a general mechanism for PCB
0013-936X/95/0929-0836$09.00/0
E 1995 American Chemical Society
SCHEME 1
Proposed Mechanism for PCB Reduction Based on Electron Transfer
+ I
Ti"', 2
\
.
BH3
Note:
7
1
BH3-
I product 1
TABLE 1
TABLE 2
PCB Congener Reduction Rates and Measured Reduction Potentials
Reduction of Aroclor 1248 CpZTiClZ-NaBH4-amine
*
reduction potential
PCB congener 4322,32,42,53,42,2'2,c2,4,6,2',4',6'-
(V vs SCElb
reduction rate'
-2.26 -2.11 -2.10 -1.96 -1.98 - 1.94 -1.87 -2.23
0.66 0.45 0.80 1.03 1.07 0.93 1.15 0.57 0.68 1.24
-2.04
-1.91
"Turnover number;defined asmolesofstarting materialconsumed per mole of catalyst per hour. *Ref 20.
reductive dechlorination that is based on electron transfer (16,211 (Scheme 11, analogous to the one we reported (10) for substituted bromobenzenes.
CI,
CI,
processing time (125 "C) ~~~
xSy
0 (biphenyl) 1 (mono-CI) 2 (di-CI) 3 (tri-CI) 4 (tetra-CI) 5 (penta-CI) 6 (hexa-CI) av no. of CI % CI remaining
Aroclor1248 12min 30min
2% 18 40 36 4 4.22 100
80% 2
0 0 0
2.20 52
6h
~
24h
50% 80% 100% 50 20 0 0 0 0
35% 65 0
2h
0
0
0 0 0
0 0 0
1.65 39
0.5 12
0
0 0 0
0.2 2
0
0 0 0 0 0
Aroclor 1248 is a common PCB pollutant mixture that contains a substantialfraction of environmentallypersistent VOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
837
a
b
C
d
FIGURE 1.
Reduction of Aroclor 1248. (a) gas chromatogram of Aroclor 1248; (b) after 12 min processing; (c) after 2 h; (d) after 24 h.
tetra-, penta-, and hexachlorinated congeners (11, and therefore it is an especially significant mixture for reductive dechlorination studies. As expected, we find that the more heavily chlorinated congeners are most rapidly reduced: The complex mixture of Aroclor PCBs not only undergoes dechlorination in generalbut congener mixtures remaining after even short processing times (Table 2) also fall within the distribution of easily aerobically biodegradable species ( 1 ) . Complete reduction to biphenyl occurs in less than 24 h (Figure Id). Significantly, amine-promoted titanium 838 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 3, 1995
complex-catalyzed dechlorination by NaBH4 is effective for the treatment of PCBs in soil; soil treatment may require some additional NaBH4 beyond that required for dechlorination of pure PCBs because of residual water and/or reducible humic components that might be present (22).
Experimental Section Reduction of Pure Aroclor 1248. In a typical procedure, a solution of Aroclor 1248 (Figure l a (23);1.0 g; 13.7 mmol
chlorine) in triglyme (20 mL) was heated at 125 "C with CpzTiCln(171 mg; 0.687 mmol; 0.05 equiv per Cl), NaBH4 (622 mg; 16.44 mmol; 1.2 equiv per Cl), pyridine (0.68 mL; 8.4 mmol; 0.61 equiv per Cl), and N,N-dimethyloctylamine (1.73 mL; 8.4 mmol; 0.61 equiv per Cl). After 12 min, a mixture consisting only of dichlorobiphenyls (cu.80%)and trichlorobiphenyl (cu.20%)was obtained (Figure lb). After further heating (total time 2 h), the product mixture contained cu. 50% biphenyl and cu. 50% monochlorobiphenyls (primarily 3-chlorobiphenyl; Figure IC). After 24 h, complete reduction to biphenyl was observed (Figure Id) (24). Reduction of Aroclor 1248 from Spiked Soil. High "organic content" garden soil (10.2 g) was heated at 90 "C to remove superficial water (25) and was then spiked with Aroclor 1248 (1.05 g; 14.8 mmol C1; 9.3% PCB by weight). The soil sample was then suspended in 20 mL of diglyme and treated similarly as described above for pure Aroclor 1248. In particular, the suspended soil was heated at 125 "C with CpPTiClz(171 mg; 0.687 mmol; 0.05 equiv per Cl), NaBH4 (822 mg; 21.7 mmol; 1.47 equiv per Cl), pyridine (0.68 mL; 8.4 mmol; 0.57 equiv per Cl), and N,N-dimethyloctylamine (1.73 mL; 8.4 mmol; 0.57 equiv per Cl). A mixture containing cu. 50% biphenyl and cu. 50% 3-chlorobiphenyl was obtained after 24 h of treatment. Acknowledging the possible presence of residual water and reducible soil components (22) that could consume borohydride, a freshly dried and spiked soil sample (10.2 g of soil; 1.02 g of Aroclor 1248; 14.4 mmol C1; 9.1% PCB by weight) was heated at 125 "C in 20 mL of diglyme in the presence of CppTiClz (366 mg; 1.47 mmol; 0.10 equiv per Cl), NaBH4 (1.335g; 35.28 mmol; 2.4 equiv per Cl), pyridine (1.43 mL; 17.7mmol; 1.2 equiv per Cl), and N,N-dimethyloctylamine (3.63 mL; 17.7 mmol; 1.2 equiv per Cl). After 12 min, a mixture consisting only of dichlorobiphenyls (cu.70%)and monochlorobiphenyls(cu.30%)was obtained. After further heating (total time 2 h), only biphenyl was observed.
Conclusions We have demonstrated that a simple titanium catalyst system based on sodium borohydride can accomplishfacile reduction of complex mixtures of PCBs to biphenyl under mild conditions. Amines potentiate the reactivity of Cp2TiBH4 toward aryl halide reduction, and comparative PCB congener reduction rates can be estimated on the basis of available electrochemical data (20). Rates for PCB congener dechlorination using spiked soil are comparable to those found for the pure materials. Hydrolyzed reduction reaction mixtures contain only readily biodegradable organic components (26)and NaC1, Ti02, and sodium borate byproducts. Therefore, this new process has the potential to rid soils of harmful PCBs with a net positive environmental benefit.
Acknowledgments
The authors acknowledge support for this work provided by the Texas Eastern Gas Pipeline Transmission Company and the National Science Foundation. They also thank Prof. A. B. Bocarsly for help with electrochemical and thermal programmed determinations and Ms. Nanying Bian for assistance with GClMS measurements.
Literature Cited (1) Fora general overview, see (a) PCBsand theEnvironment; Waid, J. S., Ed.; CRC Press: Boca Raton, FL, 1986. (b) PCBs: Human
and Environmental Hazards; D'Itri, F. M., Kamrin, M. A., Eds.; Butterworth Publishers: Boston, 1983. (2) For an example of substitution of chloride by strong base assisted by FeC13, see (a) Wilwerding, C. M. Chem. Abstr. 1990, 113, 158131e. (b) U.S. Patent 4 931 167, 1990. For an example of the use of strong base in polyethylene glycol, see (a) Peterson, R. L.; New, S . L. Proc. Natl. Con$ Hatard. Waste Hazard. Mater. 7th 1990,207. (b) Wentz, J. A.; Taylor, M. L. Proc. Natl. Con$ Hazard. Waste Hazard. Mater. 7th 1990, 392. (3) For example, see (a) Hawari, J.; Demeter, A.; Samson, R. Environ. Sci. Technol. 1992, 26, 2022. (b) Sustar, E.; Nowakowska, IM.; Guillet, J. E. J. Photochem. Photobiol. A: Chem. 1992, 63, 357. (4) For example, see Evans, D. H.; Pirbazari, M.; Benson, S . W.; Tsotsis, T. T.; Devinny, J. S. J. Hazard. Mater. 1991, 27, 253. (5) For an example of the use of molten iron, see (a) Shultz, C. G. European Patent 170 714, 1986. (b) Chem. Abstr. 1986, 105, 48445n. For an example using sodium sand, see Lalancette J. M.; Belanger, G. Chem. Abstr. 1988, 110, 179014~. (6) For an example of the use of sodium naphthalenide, see Smith, J. G.; Bubbar, G. L. 1. Chem. Technol. Biotechnol. 1980,30,620. (7) For examples using nickel salts and sodium borohydride, see (a) Roth, J. A.; Dakoji, S. R.; Hughes, R. C.; Carmody, R. E. Environ. Sci. Technol. 1994, 28, 80. (b) Dennis, W. H., Jr.; Chang, Y. H.; Cooper, W. J. Bull. Environ. Contam. Toxicol. 1979, 22, 750. Because the catalyst formed under the reported reaction conditions is insoluble, its removal from treated soils or other solids would be problematic. Contamination of the treated materials with nickel would likely result, and many nickel compounds are classified as hazardous. See Handbook ofToxic and Hazardous Chemicals and Carcinogens, 2nd ed.; Sittig, M., Ed.; Noyes Publications: Park Ridge, NJ, 1985; p 639. (8) For example, see Zhang, S.; Rusling, J. F. Environ. Sci. Technol. 1993, 27, 1375. (9) Noth, H.; Hartwimmer, R. Chem. Ber. 1960, 93, 2238. (10) Liu, Y.; Schwartz, J. J. Org. Chem. 1994, 59, 940. (11) Noth, H.; Wrackmeyer, B. Nuclear Resonance Spectroscopy of Boron Compounds. In NMR Basic Principles and Progress;Diehl, P., Fluck, E., Kosfled, R., Eds.; Springer-Verlag: New York, 1978; Chapter 7, p 88; Table LXV, p 311. (12) IlB NMR data for reactions of 1 with amines are R3N = pyridine (75 "C, 5 min), 6 -10.4 (cr pyridineBH3ll); R3N = N,Ndimethyloctylamine (75 "C, 5 min), 6 -8.4 (c$ N,N-dimethyloctyla m i r ~ e B H ~R3N ~ l ) ;= N,N-dimethyloctylamine and pyridine (1: 1),6-8.4. Reduction reactions were run using simple aryl halides (0.5 M), NaBH4 (0.6 MI, amine (0.6 M), and titanium catalyst (0.025 M) in diglyme at 125 "C. Reduction rates for 4-bromochlorobenzene were (amine added, rate [kobs h-'1): pyridine, 1.01; 2-picoline, 0.67; 2,6-lutidine, 0.03; 3,5-lutidine, 0.84; N,Ndimethyloctylamine, 0.06;N,N-dimethyloctylamine (0.3M) and pyridine (0.3 M), 1.4. Reduction rates for other bromobenzenes under similar conditions (pyridine, 0.6 M) are 4-bromobiphenyl, 1.11; p-dichlorobenzene, 0.20; 4-chlorobiphenyl, 0.22. (13) For Zr analogs, see (a) Shoer, L. I.; Gell, K. I.; Schwartz, J. J. Organomet. Chem. 1977, 136, C19. (b) James, B. D.; Nanda, R. K.; Wallbridge, M. G. H. Chem. Commun. 1966, 849. (14) Sease, J. W.; Burton, F. G.; Nickol, S . L. J. Am. Chem. SOC.1968, 90, 2595. (15) March, J. Advanced Organic Chemistry. Reactions, Mechanisms, and Structure; John Wiley & Sons: New York, 1992; p 280. (16) Saveant, J. M. Adv. Phys. Org. Chem. 1990, 26, 1. (17) Rusling, J. F.; Miaw, C. L. Environ. Sci. Technol. 1989, 23, 476. (18) For example, see Reed, G. Y.; Bush, B.; Bethony, C. M.; DeNucci, A.; Oh, H. M.; Sokol, C. Environ. Toxicol. Chem. 1993, 12, 1025. (19) Bockris, J. 0. M.; Khan, S. U. M. Surface Electrochemistry. A Molecular Level Approach; Plenum Press: New York, 1993; Chapter 6, pp 627-628. (20) Wiley, J. R.; Chen, E. C. M.; Chen, E. S. D.; Richardson, P.; Reed, W. R.; Wentworth, W. E. J. Electroanal. Chem. 1991, 307, 169. (21) Reduction of PCB congeners (0.05 M) used NaBH4 (0.1 M), pyridine (0.08MI, and the titanium catalyst (0.02 M) in diglyme at 125 "C. (22) For example, carbonyl-containing species have been reported in humic soils. See Hempfling, R.; Zech, W.; Schulten, H.-R. Soil Sci. 1988, 146, 262. (23) Individual PCB congeners have been identified. See Ballschmiter, K.; Zell, M. Fresenius Z. Anal. Chem. 1980, 302, 207.
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(24) The chlorine content of each component in these mixtures was determined by G U M S analysis. ( 2 5 ) Simple temperature-programmed desorption measurements indicate this is adequate for rapid removal of superficial water. No attempts to dehydrate the soil, in toto, were made. Since soil heating preceded treatment with the titanium catalyst system, removal of these heavily chlorinated PCB congeners cannot be ascribed to the physical processing of the soil.
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(26) Rapid biodegradation of pyridine or 3-methylpyridine in soils has been noted. See Sims, G. K.; Sommers, L. E. 1.Enuiron. Qual. 1985, 14, 580.
Received for review September 8, 1994. Revised manuscript received November 29, 1994. Accepted December 12, 1994. ES940562G
