Hydrothermal Decomposition of Polychlorinated Biphenyls Nakamichi Yamasaki' The Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 780, Japan
Takaji Yasui and Kiyoshi Matsuoka Department of Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 780, Japan
Decomposition of polychlorinated biphenyls (PCBs) by dechlorination was studied with a microautoclave. PCBs were decomposed completely in the presence of a methanol and sodium hydroxide solution under hydrothermal conditions of 300-320 "C and 180 kg/cm2 pressure. A similar dechlorinating decomposition process using 4-chlorobiphenyl as a simple model compound was tested. A possibility was suggested for the industrial treatment of PCBs with a continuous pipeline system. The decomposed mixture was easily treated with activated bacteria. Environmental contamination by polychlorinated biphenyls (PCBs),which are stable compounds, is a widespread social problem. Some methods (1-3) have been investigated for the decomposition of PCBs, including open systems using radiant rays or an oxygen flame of temperature >2000 "C. Authors have confirmed that the chlorine atoms of PCBs are substituted by hydroxyl groups under the hydrothermal condition of a sodium hydroxide solution. A closed system with an autoclave for the hydrothermal dechlorination of PCBs was studied. The treatment proposed here is a safe, simple, and rapid method for the decomposition of PCBs. The released chloride from PCBs forms a chloride ion or sodium chloride, and the dechlorinated organic compounds are safely burned or are treated easily by activated sludge processes. A pipeline system such as the Zimmermann process ( 4 ) may be suitable rather than a large-volume autoclave for the treatment of PCBs on an industrial scale. In this study, various effects were examined to find out the most suitable conditions for the hydrothermal dechlorination of PCBs, and a possibility of industrial treatment was suggested with a pipeline system.
Experimental Apparatus. Figure 1 shows the microautoclave that was used in this study. The stirring ball in the reaction chamber can be rolled up and down with the rocking motion of the heater. The induction heating method as shown in Figure 2 was used. The heating system is convenient for rapid temperature increasing (100 OC/min). Test Sample. The sample of PCBs used in this study was KC-400, which was produced a t Kanegafuchi Chemical Co. It was confirmed using a modified Carius method ( 5 )that the PCB sample contained four chlorine atoms per molecule of PCB on the average. Procedure. PCB (1-15 g) and 15-30 mL of solvent (a sodium hydroxide solution and dispersing agents) were poured into the microautoclave (inner volume: about 60 mL). The total volume of the mixture in the autoclave was adjusted to 30 mL (half the volume of the reaction chamber in Figure 1). The mixture in the autoclave was heated with an induction heater, and then cooled with a fan. The mixture was run off with water and hexane in a separating funnel. The aqueous and organic layers were separated. The aqueous solution was washed with hexane, and the organic layer was washed with a 0.25 M sodium hydroxide solution repeatedly. All aqueous solutions were mixed and decolored with 1 g of activated charcoal. The amount of chloride ion in the aqueous solution was determined with Mohr's titrimetric method. On the other 550
Environmental Science & Technology
Figure 1. Microautoclave: (1) lid: (2) cone packing: (3) reaction chamber (60 mL); (4) bail for stirring; (5) thermocouple well; (6) socket of steel rod for taking out the autoclave
1
Figure 2. Induction heater: (1) lead wire for alternating current; (2) cooling water: (3) lead wire of thermocouple: (4) copper tube (induction coil); (5) steel blades of ferrosilicon: (6) microautoclave: (7) reduction gear
hand, all hexane solutions were mixed and dried with 20 g of anhydrous calcium chloride and then evaporated to 10 mL. The amount of undecomposed PCBs in the hexane solution was determined using gas chromatography and mass spectrometry. Results and Discussion Dechlorination Conditions. The pressure in the reaction chamber was measured with a specially made autoclave equipped with a pressure gauge. The pressure decreased as the concentration of the sodium hydroxide solution increased. In this study, the pressure was -170-200 kg/cm2 a t 300 "C.
0013-936X/80/0914-0550$01 .OO/O
@ 1980 American Chemical Society
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Alkaline methanolic soh. ( methanol / 5M NaOH 1 )
Content of PCB
o 0 0
ll20glml 1/10 1/3
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2
4
6 8 1 NaOH concentration(M)
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Figure 3. Effects of NaOH concentration on the dechlorination: total volume of PCB and NaOH solution, 30 mL; reaction time, 20 min; temperature, 350 OC
Figufe 5. Effect of methanol: total volume of PCB (3 g) and 5 M NaOH solution or alkaline methanolic solution (equal volume mixture of methanol and 5 M NaOH solution), 30 mL; reaction temperature, 250 "C
h-:
Starting 4-chlorobiphenyl
0 Methanol 6 : Phenol
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8 : Glycol 0 : EMAZOL 8 : Ethanol C3 : Glycerol
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: No addition
325O.C
.
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200 250 300 Temperature P C )
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Figure 4. Effects of dispersing agents: total volume of PCB (3 g) and 5 M NaOH solution, 15 mL; dispersing agent, 15 mL; reaction time, 0 min
Figure 3 shows the effects of the concentration of the sodium hydroxide solution on the dechlorinating decomposition of PCBs in the absence of dispersing agents such as alcohol or glycol. A large amount of sodium hydroxide was needed for the complete dechlorination of PCBs even a t 350 "C. PCBs are generally insoluble in water, but their solubility is expected to increase under hydrothermal conditions. High temperatures are found to be more conducive to PCB decomposition than low temperatures under any conditions. A homogeneous condition may also be effective for the hydrolysis reaction. The effects of the following dispersing agents were tested: methanol, ethanol, ethylene glycol, glycerol, phenol, and Emazol (produced by KAO ATRAS Co.). Of
n
L
2 4 6 8 1 Retention time (min.)
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Flgure 6. Gas chromatograms of hexane extracts from the 4-chlorobiphenyl solutions treated at various temperatures: reaction chamber volume of autoclave, 10 mL; reaction time, 3 min; 4-chlorobiphenyl, 0.2 g; 5 M NaOH solution, 5 mL
these, methanol was the best agent for the PCB decomposition. The results are summarized in Figures 4 and 5 . The recommended conditions for the dechlorination of PCBs with the microautoclave are: temperature, 300-320 "C; PCB, 1 g; methanol, 12.5 mL; and 5 M sodium hydroxide solution, 12.5 mL. PCBs were completely decomposed under these conditions within a few minutes. The decomposition of PCBs was confirmed using gas chromatography and mass spectrometry. Mechanism of Dechlorination. The hydrolysis mechanism of chlorobenzene has been studied by Luttringhaus and Ambros (6). Bottini and Roberts ( 7 ) have proposed a mechanism for the hydrolysis of halotoluene. 4-Chlorobiphenyl was used as a simple model compound for PCB dechlorination, treated in the same way as PCBs Volume 14, Number 5, May 1980
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under hydrothermal conditions. The results are shown in Figure 6. The peaks in Figure 6 were assigned using infrared and mass spectrometry: (a) 2-chlorobiphenyl; (b) 4-chlorobiphenyl; (c) biphenyl-4-01; (d) 4-phenoxybiphenyl; (e) 2phenoxybiphenyl; (f) biphenyl-2-01; (g, h, and i) di(4-biphenyl), di(2-biphenyl), or 2-biphenyl 4-biphenyl ether. The following mechanism is proposed for the dechlorinating decomposition of 4-chlorobiphenyl under hydrothermal conditions:
F
C
Product
The main reaction a t low temperature was suggested to be stage I in the reaction process. Reactions 11-X, as well as reaction I, were presumed to occur a t 325-400 “C. The decomposition mechanism of 4-chlorobiphenyl is a useful model for the dechlorination of PCBs, though the reaction of PCBs may be more complex than that of 4-chlorobiphenyl. Bottini and Roberts proposed a benzyne mechanism for the hydrolysis of halotoluene ( 7 ) .The formation of chlorinated dibenzofurans is presumed. However, these were not detected in the decomposition products of 4-chlorobiphenyl. In the case of PCBs, the analysis of products was not performed because the starting PCBs were mixtures of many isomers. Possibility of Industrial Treatment. The continuouspipeline capillary system seems to be the most suitable for the treatment of PCBs on an industrial scale. PCBs were decomposed within a few minutes above 300 “C, and a long pipeline may be needless. The authors assembled a “mini”test plant, as shown in Figure 7 . The Zimmermann process, using similar equipment, had already been applied to a waste disposal process ( 4 ) .About 20 mL of PCBs per minute was able to be decomposed successively by the test plant. Biological treatment was studied as a method for the decomposition of the PCB solution. A PCB solution dechlorinated by the proposed method was diluted with water to 2350
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Figure 7. Flow diagram of a continuous pipeline system for industrial treatment. N and M are the injection systems for PCB treatment only, and the raw material tank is filled with an alkaline methanolic solution only. in the treatment of industrial waste solutions containing PCBs, N and M are not necessary and the raw material tank is filled with waste solution: (A) inlet pipe; (B) ball valve; (C) injection pump; (D) ball valve; (E) reactor column inlet; (F) reactor column; (G) reactor column outlet; (H) ball valve; (I)back pressure pump; (J) ball valve; (K) pressure control valve; (L) outlet; (0)heat exchanger
ppm of the biochemical oxygen demand (BOD) value. The solution was treated with a mixture of six Enzobac species (produced by the American Enzyme Co.). The BOD value was decreased by about 98% after 9 days. These results support the view that highly toxic materials such as chlorinated dibenzofuran do not exist. Furthermore, the waste solutions containing dechlorinated PCBs can be easily treated with activated sludge without hazard.
Literature Cited (1) Nasu, H., Sangyo Kogai, 11(3), 221 (1973). (2) Imamura, A,, Kagaku, 28(7), 581 (1973). (3) Komamiya, K., Morisaki, S., Enuiron. Sei. Technol., 12(10j, 1205 (1978). (4) Zimmermann, F. J., Chem. Eng., 25, 117 (1958). (5) Carius, G. L., Ann. Chem. Pharm., 136, 129 (1865); 2. Anal. Chem., 4, 451 (1865). (6) Luttringhaus, A,, Ambros, D., Chem. Rer., 89,463 (1956). (7) Bottini, A. T., Roberts, J. D., J . Am. Chem. Soc., 79, 4458 (1957).
Received for reuieu January 29, 1979. Accepted January 29, 1980.
