Environ. Sci. Technol. 1902, 28, 2524-2527
Flash PhotolysWHigh-Performance Liquid Chromatography Method for Studying the Sequence of Photochemical Reactions: Direct Photolysis of Phenol Ewa Llpczynska-Kochany*,+and James R. bolt on'^^
Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 587
The application of flash photolysis followed by highperformance liquid chromatography (HPLC) analysis of photoproducts is extended to the investigation of the photodegradation of phenol (I) and p-benzoquinone (11) in undegassed (1.0 X M) aqueous solution. Analysis after a single flash indicated that I1 is the principal primary product of the photolysis of I. Small amounts of hydroquinone are also detected. After several flashes had been executed, the quantity of hydroquinone (III) increased and formation of 2-hydroxy-p-benzoquinone(IV) was also observed. These compounds are concluded to be the products of the secondary photolysis of I, since the analysis of a solution of I1 after a single flash revealed these same two photoproducts in equimolar amounts. The photochemistry of I was found to be independent of pH in the range 3.0-8.0, but both the amount of photodegradation of I and the product distribution changed markedly on alkalization (pH >lo). It is concluded that photochemical removal of I from alkaline solutions is more efficient than from neutral and acidic ones.
Introduction The photochemical transformations of organic compounds present in natural waters and wastewaters is currently a subject of considerable interest, especially as regards an understanding of the detailed mechanism of these reactions. There have been studies of the photochemistry of some of these organic compounds; however, most investigations have been carried out in nonpolar solvents and in degassed solutions. Those carried out in water used steady-state UV sources and relatively high concentrations of substrate. Organic pollutants are usually present at very low concentrations in the aquatic environment. Thus photochemical studies designed to simulate environmental conditions must be carried out in dilute aqueous solutions. However, this presents two difficulties: (1)Steady-state photolysis of dilute solutions results in low concentrations of products, and hence long irradiation times are necessary to obtain a detectable amount of product. (2) Secondary photochemical processes (i.e., photochemistry of primary photoproducts) are quite likely. The net effect is a mixture of photoproducts, both of the primary and secondary (and even tertiary) steps. In our previous work (1-5), we have employed flash photolysis followed by analysis of products by high-performance liquid chromatography (HPLC). This approach is advantageous since it gives a significant conversion in a short period of time without permitting the photolysis 'On leave from Warsaw Technical University, Department of Chemistry, ul. Koszykowa 75,OO-662 Warsaw, Poland. Present address: University of Waterloo, Waterloo Centre for Ground-water Research, Waterloo, ON, Canada N2L 3G1. Present address on leave: Solarchem Environmental Systems, 40 West Wilmot #5, Richmond Hill, ON, Canada L4B 1H8. 2524
Envlron. Sci. Technol., Vol. 26, No. 12, 1992
of products. Analysis by HPLC allows the detection of small amounts of photoproducts and thus permits studies at much lower concentrations, where the possibility of bimolecular reactions is minimized. The method has been successfully applied to the study of the progress of the direct photolysis of 4-chlorophenol(1,2) and its photolysis in the presence of hydrogen peroxide (3). In this paper the technique is extended to the investigation of the direct photolysis of phenol (I), another common pollutant, in dilute undegassed aqueous solution. Since we have found that p-benzoquinone (11)is the major photoproduct from the photolysis of I, we have also employed the flash photolysis/HPLC technique to investigate the primary steps of the photochemical reaction of 11. The photochemical reaction of aqueous solutions of I was previously investigated by Audureau et al. (6) using a steady-state light source and spectrophotometricdetection. They reported a complex mixture of products: trihydroxybiphenyls, tetrahydroxybiphenyls, quinones, and diphenols.
Experimental Details Materials. Acetonitrile (for liquid chromatography) was from Omnisolv. Other chemicals were obtained from Aldrich Chemical Inc. and recrystallized using standard methods. Apparatus. Samples were irradiated using a PRA International Model FP lo00 flash photolysis system. The light was provided by two 100-5Model FX 141C-3.5 xenon lamps (EG&G Inc., Electro-Optics Division) located on either side of the sample chamber. The flash duration was -10 (fwhm) at 49-5discharge (7 kV and 2 pF capacitor), but the lamp profile showed a long tail extending to -30 p s . The emission spectrum of the flash lamp (7) closely matched that of sunlight. High-performance liquid chromatography (HPLC) was performed with a Waters Model 501 solvent delivery system, equipped with a p Bondapak CIScolumn, a Waters Model 441 absorbance detector set at 254 nm, a Waters Lambda-Max Model 481 LC spectrophotometer set at 280 nm, and a Fisher Recordall Series 5000 dual-channel recorder. UV-visible absorbance spectra were measured on a Hewlett-Packard Model 8450A diode-array spectrometer. The pH was adjusted using a Model E520 pH meter (Methrohm Herisau). Methods. Irradiations were performed using a cylindrical quartz cell (length 10.0 cm and diameter 1.0 cm). The progress of the photochemical degradation of phenol (I) and of p-benzoquinone (11)in undegassed aqueous solutions was observed with the concentration maintained M, while the number of flashes (16 J) was at 1.0 X varied. Any pH effect (in the pH range 3.0-11.3) on the photochemical reaction of aqueous I and I1 (1.0 X lo4 M) was studied using a phosphate buffer (25 mM) prepared as recommended in ref 8. Variation of the buffer concentration around this concentration did not significantly
0013-936X/92/0926-2524$03.00/0
0 1992 American Chemical Society
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Number of flashes (45 J) Figure 1. Mole fractions (%) of I reacted and products formed in an undegassedaqueous solutlon (lnltially 1.O X lo4 M) of I subjected to a number of flashes (16 J): phenol (I); p-benzoquinone (11); hydroqulnone (111); 2-hydroxy-p-benzoqulnone (IV).
affect the results. The pH of unbuffered aqueous solution of I was 6.7. All flashed solutions were analyzed promptly by HPLC (2-3 min) after irradiation using the method described previously for the photochemical reaction of 4-chlorophenol (2). Standards were used to establish the retention times and to calibrate the areas. Corrections were made for the different extinction coefficients at the detection wavelength of the compounds studied, using the spectrophotometer. We have varied the flash energy up to 100 J and have found that the results are linear in the flash energy except at the highest energy, where some slight change in product yields was observed.
Results and Discussion Effect of Number of Flashes on the Photochemical Reaction of Phenol in Water. HPLC analysis of the solution of I, following a single flash, indicated that pbenzoquinone (11) was the principal primary photoproduct. Small amounts of hydroquinone (111) were also formed. After several more flashes had been executed, the amount of I11 increased and 2-hydroxy-p-benzoquinone(IV)was also detected. Traces of a compound with a retention time identical to that for catechol (V)were also observed. The overall yield of formation of the detected products II-IV is smaller than that of degradation of I, suggesting that further degradations are occurring. Plots of the depletion of I and the formation of the photoproducts II-IV as a function of the number of flashes are shown in Figure 1. They are very similar to those obtained previously for direct photolysis of 4-chlorophenol (1, 2). Results of investigations using the flash photolysis/ HPLC method are in a good agreement with those performed by Audureau et al. (6), employing a more conventional technique. However, in the present studies a much less complex mixture of photoproducts is observed, since the applied method avoids secondary and bimolecular processes. The present results verify the mechanism of
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Number of flashes (45 J) Figure 2. Mde fractions (%) of I1 reacted and products being formed in an undegassed aqueous solutlon (lnltlally 1.0 X lo-' M) of I1 subjected to a number of flashes (16 J): p-benzoquinone (11); hydroquinone (111); P-hydroxy-p-benzoqulnone(IV). The yields of 111 and I V were virtually the same at all points; thus these two curves superlmpose on this figure.
direct photolysis of I proposed earlier (6). According to this mechanism, the primary photochemical step is the formation of phenoxy radical and a hydrated electron, followed by the attack of oxygen. The photoionization of aqueous phenol is a well-known process, studied using variety of techniques (10-14).The results of our previous work (15) on the photochemistry of I and 4-chlorophenol using a spin-trapping EPR technique are also in a good agreement with this assumption. Effect of Number of Flashes on the Photochemical Reaction of p-Benzoquinone in Water. The photochemical reaction of I1 in aqueous solutions has been studied in the past (9,16-23), and it was suggested that I11 and IV are the photoproducta (20). Joschek and Miller (18) and Kurien and Robins (19) proposed a radical mechanism for the primary process, in which the excited triplet state of I1 abstracts a hydrogen atom from a water molecule giving p-benzosemiquinone and a hydroxyl radical; the hydroxyl radical then reacts with either the pbenzosemiquinone radical or another molecule of 11, resulting in formation of IV. Ononye and Bolton (9) and Ononye et al. (22) confirmed the formation of the hydroxyl and p-benzosemiquinone radicals using flash photolysis coupled with both electron paramagnetic resonance (EPR) and optical spectroscopic detection of intermediates and determined rate constants of some of the intermediate steps. At the same time, independently, Rossi et al. (23) used a flash photolysis technique with transient absorbance detection to demonstrate generation of the p-benzosemiquinone radical as well as formation of I11 and IV as the ultimate stable photoproducts. Thus, it was expected that I11 and IV,observed during the present investigations of direct photolysis of I and in previous studies of 4-chlorophenol (1,2), are the products of a secondary reaction. To verify this, the flash photolysis/HPLC technique has been employed to observe the progress of the photochemical reaction of 11. Envlron. Sci. Technoi., Vol. 26, No. 12, 1992 2525
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pH values higher than 8, I11 appears at higher levels, and I the formation of IV is also observed. The amount of pbenzoquinone detected after two flashes (16 J) decreases at pH values higher than 10, with a corresponding rise in the concentration of I11 and IV. Thus, the removal of I by photoirradiation in alkaline solution appears to be more efficient than that in neutral solution, not only because of a faster degradation of the parent molecule but also because the process results in the formation of less stable / products.
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PH Flguro 3. Effect of pH on the photoreaction of I (1.0 X lo-' M) subjected to two l6J flashes: phenol (I); p-benzoquinone (11); hydroquinone ( I II); 2-hydroxy-p-benzoquinone (IV).
When an undegassed solution of I1 (1.0 X lo4 M) was subjected to a single flash in a manner identical to that described for I, two photoproducts (111 and IV) were observed in equimolar amounts (see Figure 2). After several flashes further degradation processes occurred, since the overall yield of the detected products is lower than that of I1 reacted. In addition to I11 and IV, the formation of 1,2,4-trihydroxybenzene(VI) and 2,5-dihydroxy-p-benzoquinone (VII), not shown in Figure 2, were also detected after several flashes. The results of the above experiments are in good agreement with those reported previously (2e23)and also verify the expectations that I11 and IV are the products of the secondary photochemical processes, occurring concurrently with direct photolysis of phenol and 4-chlorophenol. Effect of pH on the Progress of Direct Photolysis of I. As shown above, the direct photolysis of aqueous undegassed solutions of phenol results in the formation of molecules known to be less stable than the parent compound. In order to evaluate a possible pH influence on the efficiency of direct photolysis of I, as well as on the product distribution, the pH effect on the reaction (in the range 3.0-11.3) was investigated utilizing the flash photolysis/HPLC method. The pH effect on the chemical yield of the photodegradation of I (occurring during two flashes of energy 16 J) as well as on the formation of the main photoproducts is shown in Figure 3. No significant pH effect on the photodegradiation of I was observed in the pH range 3.0-8.0. At pH values above 8, both the rate of the degradation and the product distribution change. As can be seen from Figure 3, the chemical yield of photodegradation increases rapidly between pH 9 and 10, Le., close to the pK, (9.89) (ref 8, p D-172) of I in the ground state. The phenolate anion absorbs more strongly than phenol in the wavelength range 250-350 nm (24),where the flash lamp output is high (7) and thus the overall yields are higher. Also, the primary products are much less stable at high pH and thus we see formation of some secondary products. For example, for 2528
Envlron. Scl. Technol., Vol. 26, No. 12, 1992
Conclusions Results of the investigations using the flash photolysis/HPLC method show that the photolysis of phenol in undegassed aqueous solution gives I1 as the predominant primary photoproduct, with some small amounts of 111. After several flashes, the amount of I11 increases and the formation of IV is also observed. The fact that under identical conditions the photolysis of p-benzoquinone yields I11 and IV (in equimolar amounts) indicates that I11 and IV, observed during the direct photodegradation of I, arise from a secondary process, namely, the photolysis of 11. The effect of pH is not significant in the pH range 3.0-8.0. However, on alkalization (pH >lo), the chemical yield of photolysis of I increases and the photoproduct distribution changes markedly, yielding less stable compounds. Therefore, the photochemical removal of phenol from alkaline solutions should be more efficient than that from neutral and acidic ones. Acknowledgments We thank Dr.J. Kochany from the Institute for Environmental Protection in Warsaw for helpful suggestions and Miss Linda Kozo for her contribution to some preliminary experiments. Literature Cited (1) Lipczynska-Kochany,E.; Bolton, J. R. J. Chem. Soc., Chem. Commun. 1990, 1596. (2) Lipczynska-Kochany, E.; Bolton, J. R. J. Photochem. Photobiol. 1991, 58, 315. (3) Lipczynska-Kochany, E.; Bolton, J. R. Environ. Sci. Technol. 1992, 26, 259. (4) Lipczynska-Kochany, E. Chemosphere 1992,24, 911. (5) Lipczynska-Kochany, E. Environ. Pollut., in press. (6) Audureau, J.; Filiol, C.; Boule, P.; Lemaire, J. J. Chim. Phys., Phys. Chim. Biol. 1976, 73, 613. (7) EG&G Electro-Optics, Flashlamps Manual; 1988; p 2 (Figure 2). (8) Weast, R. C.; Astle, M. J., Eds. CRC Handbook of Chemistry and Physics, 63rd ed.; CRC Press: Boca Raton, FL, 1982; p D-154. (9) Ononye, A. I.; Bolton, J. R. J. Phys. Chem. 1986,90,6271. (10) Lipczynska-Kochany, E.; Bolton, J. R., unpublished work. (11) . . Land, E. J.; Porter,. G.;. Strachan, E. Trans. Faraday SOC. 1961,.57, 1885. (12) Jortner, J.; Ottolenghi, M.; Stein, G. J. Am. Chem. SOC. 1963,85, 2712. (13) Mialocq, J.-C.;Sutton, J.; Goujon, P. J. Chem. Phys. 1980, 72, 6338. (14) Grabner, G.; Kohler, G.; Zechner, J.; Getoff, N. J. Phys. Chem. 1980,84, 3000. (15) Lipczynska-Kochany, E.; Kochany, J.; Bolton, J. R. J. Photochem. Photobiol. 1991, 62, 229. (16) Leighton, P. A.; Forbes, G. S. J. Am. Chem. SOC.1929,51, 3549. (17) Poupe, F. Collect. Czech. Chem. Commun. 1949,12, 225. (18) Joschek, H. I.; Miller, S. I. J. Am. Chem. SOC. 1966,88,3273. (19) Kurien, K. C.; Robins, P. A. J. Chem. SOC.1970, 855.
(20) Hashimoto, S.; Kano, K.; Okamoto, H. Bull. Chem. SOC. Jpn. 1972, 45, 966. (21) Shirai, M.; Awatauji, T.; Tanaka, M. Bull. Chem. SOC.Jpn. 1975, 48,1329. (22) Ononye, A. I.; McIntosh, A. R.; Bolton, J. R. J. Phys. Chem. 1986, 90, 6266. (23) Rossi, A.; Guyot, G.; Boule, P. C.R. Acad. Sci. Paris, Ser. 2 1986, 303,1179.
(24) Phillips, J. P.,Bates, D., Feuer, H., Thyaganajau, B. S., MS. Organic Electronic Spectral Data; John Wiley & Sons: New York, 1972. Received for review May I, 1992. Revised manuscript received August 27, 1992. Accepted September 2, 1992. This research was supported by a research grant from the Ontario Ministry of the Environment, for which we give grateful thanks.
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