Environ. Scl. Technol. 1993, 27, 1375-1380
Dechlorination of Polychlorinated Biphenyls by Electrochemical Catalysis in a Bicontinuous Microemulsion Shlplng Zhang and James F. Rusllng’
Department of Chemistry, Box U-60, University of Connecticut, Storrs, Connecticut 06269-3060
A stable, conductive, bicontinuous microemulsion of surfactant/oil/water was evaluated as a medium for catalytic dechlorination of PCBs at constant current on lead cathodes. Biphenyl and its reduced alkylbenzene derivatives were the major products. Zinc phthalocyanine provided better catalysis than nickel phthalocyanine tetrasulfonate. Maximum current efficiency was 20 % for 4,4’-DCB but increased to 42 % overall for the most heavily chlorinated PCB mixture. Nearly complete dechlorination of 100 mg of Aroclor 1260 (60% C1) in 20 mL of microemulsion was achieved in 18 h, equivalent to the best previous result for Aroclor 1016 (48% C1) on Hg cathodes in buffered surfactant dispersion. Advantages of the method include avoidance of added salts and buffers, decreased toxicity of the medium compared to polar organic solvents, and the use of lead cathodes in place of environmentally unacceptable Hg. Introduction
New technology for permanent decomposition of pollutants is considered a critical factor for timely cleanup and future protection of our environment ( I ) . Reductive dechlorination by electrochemical catalysis selectively removes halogen from polychlorinated biphenyls (PCBs) to yield biphenyl (2, 31, which is much less toxic than PCBs. Chemical ( 4 ) and photochemical (5) dehalogenations, although quite effective, require 2 mol of chemical reductant for each mol of C1removed. Electrolysis requires 2 mol of electrons per mol of C1 removed, but this can be achieved by using small amounts of catalysts. Electrochemical catalytic dehalogenation is also feasible in waterbased surfactant media (6-111, providing a lower-cost,safer alternative (12)to toxic organic solvents. Micelles of the surfactant cetyltrimethylammonium bromide (CTAB) in water gave large enhancements in rates of electrochemical catalytic dechlorination (6, 11). Although such micellar solutions increase the solubility of nonpolar organohalides over that in water, PCBs still have millimolar solubilities which limit practical applications. Significant improvements in solubilities and dehalogenation rates of PCBs were achieved (9, 10) by using dispersions of the more hydrophobic surfactant didodecyldimethylammonium bromide (DDAB). Dechlorination rates 10-fold larger than in CTAB micelles were caused by improved reactant-DDAB coadsorption on carbon electrodes (9, 10, 13). A disadvantage was that pH of the dispersions needed to be controlled, and added salt was necessary to lower viscosity and increase conductivity. More recently, we have been exploring conductive bicontinuous microemulsions as media for catalytic dehalogenations at electrodes (10,141. Microemulsions are clear, thermodynamically stable mixtures of water, oil, and surfactant. Bicontinuous microemulsions feature intertwined, dynamic networks of oil and water with surfactant residing at interfacial regions (12,15). Mass 0013-936X/93/0927-1375$04.00/0
0 1893 Amerlcan Chemlcal Society
transport in bicontinuous microemulsions is much faster than predicted from the bulk viscosity of the medium because molecules and ions travel along the respective oil and water conduits in the network (16). Fast mass transport is an important consideration for electrolysis, in which reactants must be transported to the electrode and products transported away. Although analytical voltammetric studies of these media have appeared we are unaware of reports of bulk catalytic electrolysesin bicontinuous microemulsionsother than our own preliminary work (10). In those studies, rates of dehalogenation of 4,4’-dichlorobipheny1(4,4’-DCB) were somewhat better in microemulsions of DDAB, water, and dodecane than in DDAB dispersions at optimum pH. Addition of salt and control of pH were not necessary in the microemulsion. In this paper, we report a more extended study employing a bicontinuous microemulsion as a medium for dechlorination of PCBs by electrochemical catalytic reduction. Reaction and catalyst performance were evaluated for constant current electrolysis of 4,4’-DCB with activated lead cathodes. For PCB mixtures, complete dechlorination of 75-100 mg in 20-mL microemulsionswas achieved in 12-18 h.
(In,
Experimental Section
Chemicals and Solutions. Nickel phthalocyaninetetrasulfonic acid, tetrasodium salt (NiPcTS), 4,4’-dichlorobiphenyl, 4-chlorobiphenyl, biphenyl, l-cyclohexenylbenzene, and cyclohexylbenzene (all >98%) were from Aldrich Chemical Co. Aroclor 1221,1232, and 1260 were gifts from the US.EPA. Didodecyldimethylammonium bromide (DDAB, 99%+) and zinc phthalocyanine (ZnPc) were from Eastman Kodak. Acetonitrile and absolute methanol (both HPLC grade) were from Baker Chemicals. Dodecane was ACS certified from Fisher Co. Water was purified with a Sybron-Barnstead Nanopure System to specific resistance >15 MQ-cm. All other chemicals were reagent grade. Microemulsions (DDAB/dodecane/water,21/57/22 wt% ) were prepared by mixing DDAB and oil, while stirring the mixture, and titrating it with water as described previously (10,14,16). The DDAB/oil mixture was chosen from the phase diagram (15). Optical clarity of the fluid with a simultaneous increase in conductivity signaled that the bicontinuous microemulsionregion had been reached. This gave clear, stable fluids with specific conductivity 21.7 X l W 3 Q-1 cm-l. PCBs, ZnPc, and NiPcTS were added to microemulsions with stirring for at least 4 h. Apparatus and Procedures. Electrolyses were done in a thermostated cell with anode and cathode in the same compartment, using 20 mL of microemulsion. Working electrodes were 1 X 5.6 X 0.05 cm lead foil electrode (Aldrich gold label, 99+%) or a 8.5 X 1 X 0.5 cm carbon felt (type VDG, National Electrical Carbon Corp.). The . counter electrode was 7 X 0.1 cm platinum coil, and the Envlron. Sci. Technol., Vol. 27, No. 7, 1983
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reference was Ag/AgBr. Current or potential of the cell was controlled by a EG&G Princeton Applied Research Model 363potentiostat/galvanostat. Purified nitrogen was bubbled through the stirred solution at all times. Mass transport was assisted with ultrasound (3)by using a Fisher sonic dismembrator (Model 300) with titanium microtip (15-338-41) inserted in the center of the cell and set at 30% power. Temperature was controlled at 25 “C. Pretreatment of the lead cathode immediately before each experiment was very important in maintaining high rates of electrolysis. The cathode was first placed in 1M HC1until ita surface became mirror bright, removed, rinsed copiously with distilled water, and dried. For electrolyses of PCB mixtures, this electrode treatment was repeated every 2 h. The current efficiency (CE) of an electrolysis is the fraction of the current (or charge) passed through the cell that is used in converting starting material to desired products. Current efficiency of dechlorination was obtained from
Figure 1. Current-potential profiles In pure DDAB mlcroemulsion In cells wlth carbon felt (CF) and lead (Pb) cathodes.
CE = (Q required for products found)/&passed (1) where Q passed = applied current X time. For 4,4’-DCB electrolyses:
Table I. Influence of Applied Current on Electrolysis Products for Catalytic Reduction of 4,4‘-Dichlorobiphenyl on Lead ElectrodesP
Q required = (mmol of DCB initial)F[(2 mol of e-) X (% 4-CB) + (4 mol of e-)(% BP)1/100 For PCB mixtures: Q required = [F(2mol of e-)(mg of PCBsinitmg of PCBs,,,) 7%Cll/[lOO X at. wt of C11 Electrolyzed solutions were analyzed by high-pressure liquid chromatography (HPLC) by using a Spectra Physics SP8810 isocratic pump and a SP8450 UV/VIS detector at 254 and 210 nm for most product analyses or a HewlettPackard Series I1 1090 LC system in selected cases. UV spectra of HPLC peaks were obtained in the second case with a Hewlett-Packard 1040A diode array spectrophotometer controlled by a Hewlett-Packard 85 computer. HPLC employed 4.5 X 150 mm, 5 mm 0.d. IBM 6402633 column, 90/10 or 70/30 (v/v) acetonitrile/water with flow rate of 1.0 mL min-1 at ambient temperature. All solutions were diluted with methanol and filtered with 0.22-pm filters. Concentrations (except reduced biphenyls) were determined by calibration curves with standards. Reduced biphenyls were referred to cyclohexylbenzene. Each major peak eluted by HPLC was collected, extracted with methylene chloride, and analyzed on a Hewlett-Packard 5890 GC with 12 m X 0.20 mm, 0.33 mm i.d. methyl silicone column, and 5970 quadrupole mass spectrometric detector. Results and Discussion
In preliminary work using DDAB microemulsions, lead cathodes gave somewhat better rates than carbon felt for dechlorination of 4,4’-DCB using zinc phthalocyanine (ZnPc) as catalyst (10). We confirmed this and also found that current vs potential profiles in cells containing the pure microemulsions showed a flatter profile on lead electrodes than on carbon felt (Figure 1). The more rapid negative of -2 V on upward curvature at potentials (Eapp) carbon felt was accompanied by rapid bubble formation on the cathode. The rate of bubble formation on lead 1376 Environ. Scl. Technol., Vol. 27, No. 7, 1993
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Y 0 0
-1
- 3
- 2 E, V vs AgIAgBr
ilA (mA/cm2) BP 0.89 1.07 1.34 1.79
35 38 33 23
mol% found 4-ClBP red BPb 10 13 12
I
10 6 11 28
4,4’-DCB
-E, V/SCE (hO.1V)
46 44 46 46
2.1 2.2 2.6 3.7
aElectrolysis for 1 h of 8.9 mg of 4,4’-DCB in 20 mL of microemulsion (DDABldodecaneIwater:21/57/22 w t % ) containing 1 mM ZnPc with ultrasonic mass transport. Combined reduced biphenyls as cyclohexylbenzene.
cathodes in this potential range was much smaller than on carbon felt. The upward curvature in current at negative potential on carbon felt is probably caused by reduction of water to form hydrogen gas. Although this may increase the pH of the medium, no obvious adverse effects were observed on bulk electrolysis rates. The better performance of lead is due to less competition from reduction of water. For this reason, activated lead electrodes were used exclusively in subsequent work. Constant current operation of the cell provided faster dechlorination than constant potential when about the same potentials were generated at the cathode. This is an advantage since industrial electrolyses (18)are usually done under the easier to use constant current conditions. Catalytic Reduction of 4,4’-DCB. Using ZnPc as catalyst, current densities of 0.89-1.8 mA cm-2on an 11.2cm2lead cathode gave similar conversions of 4,4’-DCB in 1-h electrolyses. Although product distributions (Table I) were similar between 0.89 and 1.34 mA cm-2,much more reduced biphenyl was found at 1.8 mA cm-2, and the potential of the cathode became quite negative. Very negative potentials are undesirable, since energy will be wasted in reducing biphenyl and in other side reactions such as the reduction of water. Side reactions probably also cause conversion of 4,4’-DCB to remain relatively invariant with increasing current density in the range studied. Thus, we decided on an applied current density of 1.07 mA cm-2 (12 mA) for further experiments.
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Flgure 2. HPLC analyses with UV detector at 254 nm (MeCN:H20 = 9010): (a) 2 mM 4,4’-DCB In microemulsion before electrolysls; (b) same microemuision after 4-h electrolysis at 1.07 mA cm-2 using Pb cathode and 1 mM ZnPc. Numbers on graph are retention tlmes in min.
Flgure 3. HPLC analyses with UV detection at 210 nm (MeCN:H20 = 70:30); (a) 2 mM 4,4‘-D@B in microemulsion before electrolysls: (b) same microemulsion after 4-h electrolysis at 1.07 mA cm-2 using Pb cathode and 0.5 mM ZnPC. Numbers on the graph are retention times In mln.
HPLC with UV detection at 254 nm (9010MeCN:water) for microemulsions before electrolysis shows (Figure 2a) only a major peak at retention time ( t ~4.0) rnin for 4,4’DCB, with small peaks at 1.8-2.0 min presumably caused by scattering from microemulsion components. After electrolysis for 4 h with ZnPc as catalyst (Figure 2b), new major peaks are found for biphenyl (BP, 2.8 min) and 4-chlorobiphenyl (CClBP, 3.4 rnin). Similar chromatograms were obtained for solutions electrolyzed without catalysis, but 4,4’-DCB and 4-C1BP peaks were larger for a given electrolysis time. Results are consistent with stepwise dechlorination of 4,4’-DCB as in organic solvent (3,4)and aqueous surfactant dispersions (6-10). However, mass balances were 4 min with ,A, between 204 and 210 nm are most probably reduced biphenyls either with a saturated cyclohexane substituent or with no conjugation between the remaining benzene ring and the double bond of possible cyclohexenyl substituents. The peak a t t~ = 6.3 min was identified as ’ correlation. Consideringthe cyclohexylbenzenewith 85% three peaks a t 4.1,5.3, and 6.3 min as reduced biphenyls gave mass balances very close to 100% (cf. Table I). The large HPLC peak at t~ < 2 min clearly contains no biphenyl derivatives. This peak was collected, extracted with methylene chloride, and analyzed by GC-MS. Results were as follows: GC t~ (mid
rnlz
9.2 9.9 22.1 24.8
213,184,142,128,114,84,58
248,135,57 366,310,212,140 394,241,212,58
All of these GC peaks had parent rnlz values that were larger than for chlorobiphenyl, biphenyl, or reduction products of biphenyl. This is consistent with the good mass balances obtained for 4,4’-DCB electrolyeses based on chlorinated biphenyls,biphenyl, and reduced biphenyls. Dimers of biphenyl are ruled out because of the absence of absorbance in the 248-260-nm region. Injection of pure DDAB into the GC-MS resulted in no detectable signals. rnlz peaks ranging from 366 to 213 (f1)suggestdegradation of DDA+ ions (MW 382) during electrolysis. Reduction of tetraalkylammonium salts on lead cathodes is acomplex 1378
Envlron. Scl. Technol., Vol. 27, No. 7,
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1
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Time (Hr)
Figure 5. Reactant and product concentrations during electrolysis at 1.07 mA cm-* on Pb cathode of 2 mM 4,4’-DCB in microemuislon with 0.5 mM ZnPc.
process that often yields tertiary amines as the initial products (23). The peak at t~ 9.2, mlz 213 could result from an amine formed by loss of one dodecyl chain from DDA+. The peak at t R 22.1, rnlz 366 could result from an amine formed by loss of a methyl group from the nitrogen of DDA+. Thus, the GC-MS pattern is consistent with some surfactant decomposition. The extent of such decomposition has not been assessed. Collection,extraction, and GC-MS analysesof the major HPLC peaks attributed to biphenyl derivatives confirmed assignments in Table 11. Influence of Catalyst. ZnPc was found to be the best of a series of organic and metal complex catalysts (9) in previous work using DDAB dispersionsfor dechlorinations. Catalytic reduction of PCBs involved the addition of three or four electrons to the phthalocyanine ring (17). During constant current electrolysis of 4,4’-DCB using ZnPc in aDDAE! microemulsionat alead cathode, amounts of 4,4’-DCB decrease and BP increase with time, and 4-ClBP reaches a broad maximum between 1 and 2 h (Figure 5). This indicates stepwise dechlorination of 4,4’DCB (2, 3). Similar behavior was found using constant potential, but twice the time was needed for equivalent conversion of 4,4’-DCB (10) at the same cathode potential. Microemulsions have water and oil microphases. The water-insoluble ZnPc resides mainly in the oil phase. We
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on Pb cathode of 8.9 mg of Flgure 6. Electrolysis at 1.07 mA 4,4'DcB in 20 mL of microemulslon with two different catalysts and with no catalyst: (a) current efficiencies (b) and rate of 4,4'-DCB reduction.
Flgure 7. HPLC analyses of 100 mg of Aroclor 1260 In 20 mL of microemulsion using a total 4.5 mM ZnPc at 1.07 mA cm-2 on Pb cathode: (a) before electrolysis; (b) 8-h electrolysis; and (c) 184 electrolysis. UV detection at 254 nm, MeCN:H20 = 9010.
compared its catalytic properties to water-soluble nickel phthalocyanine tetrasulfonate (NiPcTSP) which has similar electrochemical properties (24) but should reside predominantly in the water microphase or in the interface bound to DDAB head groups. ZnPc was the best catalyst based on current efficiency (Figure 6a) and bulk dechlorination rate of 4,4'-DCB (Figure 6b). The current efficiency and reaction rate with ZnPc was better over the first 2.5 h of the reaction. Also, reduction without catalyst, which gives the same products, still proceeds a t fairly respectable current efficiencies and rates. The current efficiencies are lower than could be desired. At the maximum current efficiency for dechlorination of 4,4'-DCB (Figure 6a), about 80% of the charge is being used by side reactions, most likely reduction of water and surfactant. Dechlorination of PCB Mixtures. ZnPc was used as catalyst for commercial PCB mixtures (Aroclors) in the microemulsion. HPLC with 254-nm detection shows many peaks at t R 2-14 min for the PCB congeners of Aroclor 1260 (60% C1) before electrolysis (Figure 7a). After 8-h electrolysis, the biphenyl peak at 2.8 min is a bit larger, and the main cluster of peaks are found at t~ between 2
Table 111. Results of Catalytic Electrolysis of Aroclorsa wt%
Aroclor
C1
amt time [ZnPcl mgof dechlor CE (mg) (h) (mM) ClBPfound (%) (%)
1221 21 72 10 2.5 0.9* -99 19 1232 32 69 12 3.5 0.01c >99.8 23 1260 60 100 18 4.5 0.02c >99.8 42 Applied current of 1.07 mA on 11.2 om2 lead cathode, 20 mL microemulsion. HPLC analysis showed monochlorobiphenyla as the only remaining chlorinated products. HPLC analysis showed small amounts of mono- anddichlorobiphenyls as the only chlorinated products.
and 8.5 min (Figure 7b). Since the more heavily chlorinated biphenyls elute with longer t~ in this method (3), these results are consistent with stepwise dechlorination of the higher PCBs to less heavily chlorinated congeners. After 18-helectrolysis, the biphenyl peak is the major peak in the chromatogram (Figure 7c), with very small peaks attributed to mono- and dichlorobiphenyls. Quantitative analyses show >99.8% dechlorination of the original mixture (Table 111). Overall current efficiency was a respectable 42 % . Environ. Scl. Technol., Vol. 27, No. 7, 1993
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The results above are comparable in rate of electrolysis to results with 100 mg of Aroclor 1016 (42% C1) in a buffered DDAB dispersion using ZnPc with Hg as a cathode at constant potential -2.3 V (IO, 13). Thus, by using the microemulsion, an activated lead cathode, and constant current, we achieve comparable or better results than the previous best effort. Also, solid lead is an environmentally more acceptable electrode than liquid mercury. For less heavily chlorinated Aroclors, we arbitrarily used less catalyst. Less time was required for nearly complete dechlorination of Aroclors with smaller % C1than Aroclor 1260. Overall current efficiencies were better than that for 4,4’-DCB alone. Possible reasons are (i) Aroclors contain many more heavily chlorinated PCBs which are easier to reduce than 4,4’-DCB or (ii) more catalyst was used in Aroclor experiments. Conclusions This study demonstrates the feasibility of using bicontinuous microemulsions for catalytic electrolytic dechlorination of PCB mixtures at constant current on activated lead cathodes. Dechlorination of 100 mg of Aroclor (60% C1) was achieved in 18 h, equivalent to the best previous result on Hg cathodes in a buffered surfactant dispersion. Advantages of the present method include (i) the use of lead in place of the environmentally unacceptable mercury; (ii) the avoidance of components of previous systems such as additional supporting electrolyte, buffer, or organic solvents more toxic than dodecane (2,3,6-10);and (iii) the future possibility of using microemulsions for releasing PCBs from soils and sediments, with subsequent electrolysis. The latter approach showed promise in preliminary experiments (IO). Several future improvements can be envisioned. Current efficiencymight be increased by using better catalysts and inhibiting side reactions. If surfactant content of the microemulsion can be decreased 10-fold, the medium would be less expensive, and decomposition of surfactant might be less competitive. Inhibiting the reduction of water would also be beneficial. Recycling of components will also be important. Research along these lines is presently underway. Acknowledgments This work was supported by U.S. PHS Grant ES03154 awarded by the National Institute of Environmental Health Sciences and in part by a grant from the CT Department of Environment Protection. The authors thank Albert J. Kind (University of Connecticut Microchemistry Lab) for assistance with HPLC-UV analyses.
1380 Envlron. Scl. Technol., Vol. 27, No. 7, 1993
Literature Cited Daley, P. S. Environ. Sci. Technol. 1989,23,912. Connors, T. F.;Rusling, J. F. J. Electrochem. SOC.1983, 138,1120. Connors, T.F.;Rusling, J. F. Chemosphere 1984,13,415. Oku,A.; Yasufku, K.; Kataoka, H. Chem.Znd. 1978,21,841. Epling,G.A.;Florio,E.;Bourque,A. J.;Qian,X.-H.;Stuart, J. D. Environ. Sci. Technol. 1988,22,952 and references therein. Ruling, J. F.; Shi, C.-N.; Gosser, D. K.; Shukla, S. S. J. Electroanal. Chem. 1988,240,201. Rusling, J. F.;Shi, C.-N.; Suib, S. L. J . Electroanal. Chem. 1988,245,331. Shi, C.-N.; Rusling, J. F.; Wang, Z.; Willis, W. S.; Winiecki, A. M.; Suib, S. L. Langmuir 1989,5,650. Iwunze, M. 0.; Rusling, J. F. J . Electroanal. Chem. 1989, 266, 197. Couture, E.; Rusling, J. F.; Zhang, S. Trans. Znst. Chem. Eng. (U.K.)1992,70B, 153-157. Rusling, J. F. Acc. Chem. Res. 1991,24,75. Friberg, S.E. Adv. Colloid Interface Sci. 1990,32,167. Couture,E. C. Ph.D. Thesis, University of Connecticut,1991. Kamau, G. N.; Hu, N.; Rusling, J. F. Langmuir 1992,8, 1042. Blum, F.D.; Pickup, S.; Ninham, B. W.; Chen, S. J.; Evans, D. F. J . Phys. Chem. 1985,89,711. Iwunze, M. 0.; Sucheta, A. S.; Rusling, J. F. Anal. Chem. 1990,62,644. Rusling, J. F.In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1993;Vol. 18. Danley, D. E. In Organic Electrochemistry, 2nd ed.; Baizer, M.M.: Lund,. H.,. Eds., Marcel Dekker: New York. 1983:.-. VD 959-994. (19)Hutzinger, 0.; Safe, S.; Zitko, V. The Chemistry of PCBs; CRC Press: Cleveland, OH, 1974. (20) Silverstein, R. M.; Bassler, G. C. Spectrometric Zdentification of Organic Compounds, 2nd ed.; Wiley: New York, 1967;pp 149-152. (21) Mann, C. K.; Barnes, K. B. Electrochemical Reactions in Nonaqueous Systems; Marcel Dekker: New York, 1970;p 40. (22) Hill, D. W.; Kelley, T. R.; Langner, K. J. Anal. Chem. 1987, 59,350. (23) Homer, L. In Organic Electrochemistry, 2nd ed.; Baizer, M. M., Lund, H. Eds.; Marcel Dekker: New York, 1983;pp 729-733. (24) Lever, A. B. P.; Licoccia, S.; Magnell, K.; Minor, P. C. Adu. Chem. Ser. 1982,No. 201,237-251.
Received for review October 13, 1992.Revised manuscript received February 18,1993.Accepted February 25, 1993.
