Environ. Sci. Technot. 1988, 20, 1056-1058
NOTES Radical Formation and Polymerization of Chlorophenols and Chloroanisole on Copper( 11)-Smectite Stephen A. Boyd* and Max M. Mortland
Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824- 1114 -
Chlorinated phenols and anisoles formed radical cations on copper(I1)-smectite in mild reaction conditions. Electron spin resonance was used to demonstrate the formation of radical cations for pentachlorophenol, 4chlorophenol, and 3-chloroanisole. Electron transfer from the aromatic species to Cu(I1) resulted in the formation of the aromatic radical cation and Cu(1). The electronwithdrawing properties of C1 substituents did not prohibit oxidation of the aromatic species. The pentachlorophenol and 3-chloroanisole radicals formed dimers as revealed by mass spectrometry. The formation of dimers when 3chloroanisole is reacted with Cu(I1)-smectite was supported by changes in the UV spectrum. Pentachlorophenol was also dechlorinated forming 2,3,5,6-tetrachlorophenol. The formation of radical cations on Cu(I1)-smectite may provide the basis for a new detoxication technology in which recalcitrant halogenated aromatic molecules are converted to less toxic products through a variety of potential reactions (e.g., polymerization or dechlorination) involving the reactive radical species. ~
Introduction The formation of radical cations of aromatic species when reacted with copper(II)-smectite is well documented (1-7). In addition, further reactions of these unstable species has been observed as, for example, the polymerization of benzene to form p-polyphenyl polymers (3, anisole to form 4,4'-dimethoxybiphenyl (5),and dimethylphenol (8)and dioxins (9)to form dimers and trimers. The important point about these reactions is that they occur under very moderate conditions, as compared with the conditions usually required to form radicals in homogeneous solution. For example, refluxing these compounds in n-hexane (69 "C) together with the Cu(I1)-smectite is usually sufficient to create the radical cations. Probably the refluxing process partially dehydrates the Cu(I1) ion, and an aromatic molecule then interacts with the ion at a vacant ligand position via its n electrons. At that point an electron is completely transferred to the copper ion, reducing it to Cu(1) and creating the radical cation of the aromatic species. It is important to understand that the silicate surface facilitates this reaction and that radical cation formation in homogeneous solution requires much more severe reaction conditions such as concentrated acids and strong oxidizing agents (10-12). It has been proposed that such clay-based systems may be generally applicable to the alteration and detoxication of halogenated aromatic molecules that comprise an important class of environmental pollutants including chlorinated dioxins and dibenzofurans (9). Although radical cation formation on Cu(I1)-smectite has been demonstrated for many aromatic molecules, this reaction has not been shown for chlorinated aromatics. Formation of the 1056 Environ. Sci. Technol., Vol. 20,
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reactive radical species on Cu(I1)-smectite may allow a variety of subsequent reactions, e.g., polymerization or dechlorination, resulting in the formation of less toxic products. The work presented here is an extension of that performed previously on dioxins (9). The primary objective was to demonstrate that chlorinated aromatic compounds would form radical cations on Cu(I1)-smectite. Formation of radical cations by electron transfer to Cu(I1) may be more difficult for chlorinated aromatics due to the electron-withdrawing properties of C1 substituents [positive Hammett u values (23)]. Materials and Methods The clay used in this work was smectite (montmorillonite) from Wyoming bentonite (American Colloid). It was dispersed in water and coarse material removed by settlement. The clay suspension was then treated with a CuC1, solution in an amount 5 times as great as the cation-exchange capacity [90 mequiv (100 g)-l]. The material was allowed to stand overnight, then placed in dialysis tubing, and dialyzed against distilled water until no chloride appeared in the dialysate as indicated by negative tests with AgNO,. The Cu(I1)-saturated clay was then freeze-dried and stored in glass bottles until used. Smectite with a 101ratio of Ca2+to Cu(I1) was prepared by treating the natural clay in excess of its cation-exchange capacity with a single solution which was 0.1 M in Ca2+and 0.01 M in Cu(I1). After being washed, this clay was purified by dialysis and freeze-dried as described above. The following compounds were studied: 4-chlorophenol, pentachlorophenol (PCP), and 3-chloroanisole. All compounds were obtained from Aldrich and used without further purification. The octachlorodibenzo-p-dioxin standard was obtained from Ultra Scientific (Hope, RI). The electron spin resonance (ESR) measurements were made on a Varian E-4 spectrometer. For these analyses, 0.1 g of the appropriate aromatic compound was added to 0.1 g of the Ca2+/Cu(II)-smectite in 50 mL of n-hexane and refluxed (69 OC) for 4 h. The refluxed clay was then transferred to quartz ESR tubes fitted with vacuum stop cocks, evacuated with a mechanical pump for an hour, and then placed in the spectrometer for analysis at room temperature. Material for mass spectrometric analysis was obtained by refluxing 1 g of the appropriate aromatic compound with 1 g of Cu(I1)-smectite in 100 mL of n-hexane for 6 h. The clay material was then removed from the flask, the hexane evaporated off, and the clay extracted with methanol. The methanol was then removed by evaporation and the resulting residue analyzed on a Hewlett-Packard 5985A mass spectrometer using a solid probe. The sample was held at 30 "C for 1.5 min and then heated at the rate of
0013-936X/86/0920-1056$01.50/0
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606
.
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, II 11
Figure 1. ESR spectra for radical cations of (A) standard pitch (g = 2.0028),(E) pentachlorophenol, (C) 4-chlorophenol, and (D) 3-chloroanisole on Cu(II)/Ca2+ ( i : i O ratio of Cu(II)/Ca*+)-smectite.
30 OC min-l to a maximum temperature of 280 "C. Methanol extract from the PCP reaction was analyzed by high-performance liquid chromatography (HPLC) for the presence of lower chlorinated phenols. A C-18 reverse-phase column (LiChrosorb, 10 pm, EM Science) was used for separation. The mobile phase was a 1:l mixture of acetonitrile and 5 % aqueous acetic acid with a flow rate of 2.0 mL mi&. Detection was by UV absorbance at either 280 or 300 nm. Retention times and peak areas of standards and unknowns were measured with a Waters data module. Retention times of the extracted compounds were within f0.01 min of the standard compounds. Detection b i t a for PCP and octachlorodibenzo-p-dioxinwere 0.5 ppm. Ultraviolet-visible differential spectra of the radical of 3-chloroanisole on Cu(I1)-smectite were obtained by evaporating water suspensions of the clay on one internal surface of matched sample cells. The sample cell was then placed in n-hexane containing 3-chloroanisole and refluxed until a deep blue color developed. The cell still full of n-hexane was then removed from the refluxing equipment and placed in the sample side of a Perkin-Elmer 320 spectrophotometer. The reference cell with its unreacted clay film was also filled with hexane and placed in the reference beam.
Results Figure 1 shows electron spin resonance (ESR) signals for the radical cations, of PCP, 4-chlorophenol, and 3chloroanisole. These all appeared on Cu(II)/Ca2+smectite after refluxing the given compound with the clay in hexane. The signal for paramagnetic Cu(I1) was eliminated and replaced by the free electron of the organic radical cations. Loss of the Cu(I1) signal demonstrated that paramagnetic Cu(I1) was completely reduced to Cu(1) with concomitant oxidation of the organic molecule. The lack of hyperfine structure is consistent with ESR spectra of other aromatic molecules that form radical cations on transition metal layer silicates and has been attributed to rapid electron exchange on the smectite surface (6). A chromatogram of the methanol-extractable material remaining after PCP was refluxed in hexane with Cu(11)-smectite revealed both unreacted PCP and a reaction product identified by cochromatography as 2,3,5,6-tetrachlorophenol (data not presented). These data showed that some dechlorination occurred under the conditions of these experiments. The presence of unreacted PCP was expected because PCP was added in excess of the Cu(I1) present as Cu(II)-smectite. Reoxidation of Cu(1) to Cu(I1) with air would allow the reaction of additional PCP. Residues extracted from the 3-chloroanisole and PCP experiments were analyzed by mass spectrometry (Figure 2). Formation of a dimer from 3-chloroanisolewas clearly evidenced by the intense mass at m l e 282. Relative intensities of the masses at 282 (A),284 (A + 2), and 286 (A + 4) were consistent with that predicted for the dimer with two chlorine atoms. The mass spectral data from the PCP residue also indicated the formation of a dimer. Masses were observed at m l e 460,424, 390, and 356. The mass at m l e 266 was PCP (A + 2). The highest mass observed at m / e 460 (A + 4) corresponds to an oxidized PCP dimer. Either a diphenoquinone or dibenzodioxin structure could be assigned to the observed molecular weight. The diphenoquinone structure seems most likely because the Environ. Sci. Technoi., Vol. 20, No. 10, 1986
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Environ. Sci. Techno/. 1986, 20, 1058-1060
detoxlcation reactions: oolvmerization. dechlorination
...
Flgure 3. Detoxication scheme based on the use of Cu(I1)-smectite as a catalyst.
PCP reaction product was highly colored (purple) and because dimerization previously observed in Cu(I1)smc Aite occurred at the para position, as in the case of anisole which formed 4,4'-dimethoxybiphenyl (5). HPLC analysis of the PCP reaction products showed no peaks corresponding to the retention time of an authentic standard of octachlorodibenzo-p-dioxin.The masses at mle 424 ( A 2), 390 ( A + 2), and 356 ( A + 2) correspond to PCP dimers having seven, six, and five chlorine atoms, respectively. The relative intensities of the A , A + 2, A + 4, A + 6, and A + 8 masses were consistent with these numbers of chlorine atoms. It is not clear whether the lower chlorinated dimers were separate reaction products or if they appeared as fragmentation products from the molecular ion at mle 460. A visible spectrum'of 3-chloroanisole radical cations formed on Cu(I1)-smectite showed two main peaks, one in the red region at 625 nm and the other at 465-475 nm in the blue. These two peaks represent two species, the one at 625 nm being the monomer radical cation and the one at 465-475 nm a dimer radical cation. The rationale for this statement is that the clay initially turns blue which suggests an absorption band in the red region. This would be the first reaction to occur. Then with time the color changes to more green, which would signal the dimerization process.
+
Discussion The results demonstrate that chlorinated phenols and anisoles form radical cations and polymerize upon refluxing in hexane together with Cu(I1)-smectite. Thus, the presence of chlorine on the aromatic ring does not prevent the oxidation reaction from occurring. For phenol, this is true even when the ring is fully chlorinated as demonstrated for PCP. In addition to the polymerization reaction, the observation of a dechlorination reaction taking place, as in the case of PCP going to a tetrachlorophenol, is of interest. Both dechlorination and polymerization
reactions may be important steps in the ultimate detoxication of organochlorine pollutants. The work reported here suggests the possibility of using Cu(I1)-smectite as a catalytic material to alter or degrade chlorinated aromatic molecules that may be present in industrial wastes or elsewhere. This material could easily be reused by oxidation of Cu(1) to Cu(I1) with air (Figure 3). This work supports the earlier observation of radical cation formation and polymerization of some dioxins (9) via the same reaction. Once the reactive radical cation is formed, it may be subject to a variety of detoxication reactions (e.g., polymerization, dechlorination) resulting in the formation of less toxic products (Figure 3). Advantages of this system are that the Cu(I1)-smectite is easily prepared from natural products such as Wyoming bentonite, the reaction temperature is low (69 "C, the boiling point of hexane), and the process is relatively inexpensive compared with other disposal operations (e.g., incineration). Registry No. PCP, 87-86-5;p-HOCsH4CI,106-48-9; 3935-95-5. chloroanisole, 2845-89-8;2,3,5,6-tetrachlorophenol,
Literature Cited (1) Doner, H.E.;Mortland, M. M. Science (Washington,D.C.) 1969, 166, 1406. (2) Mortland, M. M.; Pinnavaia,T. J. Nature Phys. Sci. 1971, 229, 75. (3) Pinnavaia, T.J.; Mortland, M. M. J. Phys. Chem. 1971,75, 3957. (4) Rupert, J. P.J. Phys. Chem. 1973, 77,784. (5) Fenn, D.; Mortland, M. M.; Pinnavaia, T. J. Clays Clay Miner. 1973,21, 315. (6) Pinnavaia,T.J.; Hall, P. L.; Cady, S. S.; Mortland, M. M. J. Phys. Chem. 1974, 78,994. (7) Mortland, M. M.;Halloran, L. J. Soil Sci. SOC.Am. Proc. 1976, 40, 367.
(8) Sawhney, B.L.;Kozloski, R. K.; Jackson, P. J.; Gent, M. P. N.Clays Clay Miner. 1985, 32, 108. (9) Boyd, S. A,; Mortland, M. M. Nature (London) 1985,316, 532. (10) Tomita, M.; Ueda, S. Chem. Pharm. Bull. 1964, 12, 33. (11) Shine, H.J.; Piette, L. J. Am. Chem. SOC.1962,84, 4798. (12) Yang, G.C.; Pohland, A. E. In Chlorodioxins-Origin and Fate; Blair, E., Ed.; American Chemical Society: Washington, DC, 1973; pp 33-43. (13) March, J. Advanced Organic Chemistry; McGraw-Hill: New York, NY, 1977; Chapter 9.
Received for review October 30, 1985. Accepted May 9, 1986. Contribution of the Michigan Agricultural Experiment Station, East Lansing, MI, 48824-1114. MAES J. Ser. No. 12028. This work was supported by a Toxicology Research Grant from the MAES and the Michigan State University Center for Environmental Toxicology and the U S . Environmental Protection Agency (Grant CR813215).
Platinum and Palladium in Roadside Dust Vernon F. Hodge" and Martha 0. Stallard Scripps Institution of Oceanography, University of California-San
converters significant quantities of platinum and palladium to the roadside environment. Dust samples collected from broad-leaved plants contained as high as 0.7 ppm of platinum and 2.5 times less palladium. Rains may wash this concentrate into local water systems or the ocean.
Diego, La Jolla, California 92093 brication of catalytic converters has approximately doubled the use of metals in the United States, and consequently, the autOIIIobile industry consumes as rf~uch of these rare metals as all other industries combined ( I ) . In 1977, researchers at General Motors reported that Platinum was emitted at the rate of 1-3 pgfmile (0.8-1.9 .ualkm) in the exhaust from cars equipped with catalytic converters (2). On the basis of thismeasurement, the authors of an excellent report on the platinum group I,
Since 1975, the exhaust systems of millions of new cars have been equipped with catalytic converters. The fa1058
Environ. Sci. Technol., Vol. 20,
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0013-936X/86/0920-1058$01.50/0
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