Environ. Sci. Technol. 1992, 26, 262-265
Harbour, J. R.; Chow, V.; Bolton, J. R. Can. J. Chem. 1974, 52, 3549-3553. Ononye, A. I.; McIntosh, A. R.; Bolton, J. R. J. Phys. Chem. 1986, 90, 6266-6274. Kochany, J.; Bolton, J. R. J . Phys. Chem. 1991, 95, 5116-5120. Kochany, J.; Bolton, J. R. Enuiron. Sci. Technol., following paper in this issue.
(28) Lipczynska-Kochany, E.; Kochany, J.; Bolton, J. R. J . Photochem. Photobiol., in press.
Received f o r review November 21, 1990. Revised manuscript received April 24, 1991. Accepted September 17, 1991. This research was supported by a Grant f r o m the Ontario Ministry of the Environment (Project 4876).
Mechanism of Photodegradation of Aqueous Organic Pollutants. 2. Measurement of the Primary Rate Constants for Reaction of 'OH Radicals with Benzene and Some Halobenzenes Using an EPR Spin-Trapping Method following the Photolysis of H202 Jan Kochanyt and James R. Bolton" Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7
rn The technique of spin trapping, with EPR detection of spin adducts, has been applied to the study of the photodegradation of benzene, chlorobenzene, bromobenzene, iodobenzene, o-dichlorobenzene, m-dichlorobenzene, and p-dichlorobenzene sensitized by the photolysis of HzOzin aqueous solution. By employing a competition kinetic scheme and relative initial slopes or signal amplitudes, plus published rate constants for the reaction of *OHradicals with the spin trap (DMPO), it has been possible to obtain rate constants for the reaction of *OH radicals with benzene and its halo derivatives. Rate constants obtained by this method at neutral and acidic solutions are similar to published methods using pulse radiolysis, where available. Significant differences were found for measurements carried out in basic solutions. The rate constants do not vary much among the halobenzene derivatives studied, and an average rate constant of -5.0 X lo9 could be used for any of these compounds. M-l Introduction
Many organic chemicals discharged into the aquatic environment are not only toxic but also only partly biodegradable, in that they are not easily removed in biological wastewater treatment plants. That is why there is a need to develop effective methods for the degradation of organic pollutants, either to less harmful compounds or to their complete mineralization. During the last few years a series of new methods for water and wastewater purification, called advanced oxidation processes, have received increasing attention. They rely mainly on the use of short-lived oxidative species (often hydroxyl radicals) generated by photolysis (1-6) and radiolysis (7-10). The main advantages of these methods are high rates of pollutant oxidation, flexibility concerning water quality variations, and small dimensions of the equipment (11-13). The main disadvantages are high operating costs and special safety requirements because of the use of very reactive chemicals (ozone, hydrogen peroxide, etc.) and high-energy sources (Wlamps, electron beams, radioactive sources). In spite of several reviews (14-17) reporting degradation rates for a variety of pollutants, very little is known about +On leave from the Institute for Environmental Protection, Warsaw, Poland. 262
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the detailed mechanism of oxidation of organic pollutants in water, except that hydroxyl radicals are important intermediates. Understanding the mechanism is very important if the optimization of these processes is to be achieved. The aim of our investigations is to contribute to an understanding of the photodegradation mechanisms of organic pollutants in homogeneous systems. We have focused our interests on the photolysis of hydrogen peroxide in aqueous solution as a source of hydroxyl radicals. This reaction is known to produce hydroxyl radicals cleanly (18), and it is practical, since H202 is used in commercially produced equipment for water and wastewater purification (10-1 3). Most available data (19) concerning the rate constants of the reactions of hydroxyl radicals in water toward different organic compounds have been obtained from systems where the hydroxyl radicals were generated by pulse radiolysis. The photolysis of HzOzis an alternative source of 'OH radicals, and some rate constants (20) have been obtained using this source. However, here one must take account of the possibility of direct photolysis of the organic substance, a process not important in radiolysis. It follows that one may encounter differences in the apparent rate constants of *OH reacting with the same compound obtained using either pulse radiolysis or photolysis. We have developed a reliable procedure for determining the primary rate constants for the attack of 'OH radicals on pollutant molecules using a competition method based on the technique of spin trapping (21) with detection by electron paramagnetic resonance (EPR) spectroscopy. This method uses a spin trap molecule, e.g., 5,5'-dimethylpyrrolineN-oxide (DMPO), that reacts rapidly with a variety of reactive radicals, such as 'OH and HOz', to produce relatively long-lived spin-adduct nitroxide radicals that can be detected readily by EPR. Although this method is relative, we have validated it by reproducing several published rate constants that have been determined previously using pulse radiolysis (21). In this paper we apply our method to the determination of primary *OHradical rate constants for benzene and some of its halo derivatives. Experimental Section
The spin trap used was 5,5'-dimethylpyrroline N-oxide (DMPO), which reacts with free radicals, such as 'OH (22), to form moderately stable nitroxides (spin adducts). This
001 3-936X/92/0926-0262$03.00/0
0 1992 American Chemical Society
-.-2a
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e
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.=9
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-fa2
-300 344.0
346.0
350.0
348.0
352.0
354.0
magnetic field /mT
T
0,
300
/
c
concentration /pM
~~
-20
0
20
40
60
80
100
120
140
160
180
time /s.ec Flgure 1. (a, top) EPR spectrum of the DMPO-OH spin adduct recorded during UV irradiation of an aqueous solution (pH = 7.0)of H,O, and DMPO (both at a concentration of 0.20 mM); EPR spectrometer settings: center field, 349.505 mT; sweep width, 10.0 mT, scan time, 5.243 s; number of scans, 10; microwave frequency, 9.780 GHz; microwave power, 20 mW. (b, bottom) Time course of the DMPO-OH signal maximum (low-field central peak) before and during UV irradiation of the same solution as in (a); EPR spectrometer settings the same as in (a) except for the following: center field, 348.755 mT; sweep width, 0 mT; scan time, 167.50 s; number of scans, 1.
method has been successfully applied to develop a protocol, based on a competition reaction with formate, to test for the presence of free *OH radicals in solution (23,24). The following compounds have been investigated: benzene, chlorobenzene, bromobenzene, iodobenzene, odichlorobenzene, m-dichlorobenzene, and p-dichlorobenzene. Stock solutions of the above compounds in 1.0 mM phosphate buffer (pH = 7.0, 3.5, and 11.0) were diluted with the buffer solution to obtain the final concentrations in the range 5-50 pM. To each sample solution so prepared were added sufficient volumes of 1.0% H202 and 10 mM DMPO buffer solutions so that each had a final concentration of 0.20 mM. Sample solutions were pumped into an EPR quartz flat cell (WG-814 Wilmad) and irradiated for 1min with the UV lamp (150-W mercury-xenon lamp, Conrad Hanovia 901 B001). EPR spectra were obtained using a Bruker Model ESP 300 EPR spectrometer, coupled with a computer for data acquisition and instrument control. A TM102 cavity fitted with an aqueous solution sample holder was used. Details of the experimental procedure are described elsewhere (25). Both the full DMPO-OH spin-adduct EPR spectrum and its signal increase were recorded during irradiation. To allow comparison of the results of the various experiments, control runs were carried out in the same manner with solutions of H202and DMPO (both at 0.20 mM) in phosphate buffer solution. Photolysis of a solution containing only DMPO gave no detectable EPR signals. The initial slope (initial rate Ito),measured according to a mirror method described by Bolton et al. (26),of the EPR signal and the maximum value of the EPR signal was recorded and normalized using experimental values obtained from control runs. The Sigma-Plot computer
0
5
10
15
20
25
30
35
40
45
50
concentration /@M
Figure 2. (a) Inverse amplitude ratio AOIA and (b) inverse initial slope ratio RooIRovs substrate concentration for the photooxidation of benzene (O), chlorobenzene (A),bromobenzene (O), and iodobenzene (V)wlth H,O, at pH = 7.0. Error bars are f l standard deviation.
program, version 3.1 (Jandel Scientific, Sausalito, CA), was used for the analysis of experimental errors and for graphic presentations.
Results and Discussion In each experiment, the DMPO-OH EPR spectrum was recorded after the same time of irradiation and after the same number of scans (see Figure la) [The four-line EPR spectrum of the DMPO-OH spin adduct (Figure 1) was the only spectrum observed. DMPO reacts rapidly with other radicals, such as H02'. However, if this radical were present, the six-line EPR spectrum of the DMPO-H02 spin adduct would have appeared (22). Thus we can rule out the presence of the H02' radical as a significant intermediate in this system.]. The field was then set at the maximum of the second peak and the amplitude of the EPR signal was recorded as a function of time before and during the W-light exposure of the sample (see Figure lb). For control runs ([pollutant] = 0), the maximum amplitude of the EPR signal A" and the initial slope (initial rate) Roowere used as reference values. The inverse relative signal intensity A"/A was calculated for every sample Environ. Sci. Technol., Vol. 26, No. 2, 1992 263
Table I. Rate Constants k3for the Reaction of 'OH Radicals with Benzene and Halobenzenes at pH = 7.0 k3/(109 M-' s-' ) compound
a
b
C
d
e
benzene chlorobenzene bromobenzene iodobenzene o-dichlorobenzene m-dichlorobenzene p-dichlorobenzene
7.7 f 0.4 4.6 f 0.3 5.0 f 0.3 5.5 i 0.3 3.9 f 0.3 5.9 f 0.4 5.3 f 0.5
7.3 f 0.3 4.3 f 0.2 4.9 f 0.3 5.3 f 0.3 3.7 f 0.2 5.8 f 0.4 5.0 f 0.5
7.5 f 0.4 4.3 f 0.3 4.8 f 0.3 5.3 f 0.3 3.9 f 0.3 5.4 f 0.5 5.3 f 0.4
7.2 f 0.4 4.4 f 0.2 4.8 f 0.3 5.4 f 0.3 4.0 f 0.3 5.5 f 0.5 5.1 i 0.4
7x4 5.58 5.0h
"Calculated from a plot of A o / A vs concentration with Pyrex filter. bCalculated from a plot of A o / Avs concentration without Pyrex filter, Calculated from a plot of Roo/Rovs concentration with Pyrex filter. Calculated from a plot of Roo/Rovs concentration without Pyrex filter. "Literature values. 'Average of four values measured at pH = 7.0 by pulse radiolysis (30). gAverage of two values measured at pH = 9.0 and 10.7 by pulse radiolysis (31). hValue obtained by pulse radiolysis at pH = 9.0 (32).
Table 11. Rate Constants for the Reaction of 'OH Radicals with Benzene and Halobenzenes at Different pH Values" k3/(109 M-l
s-l)
compound
pH = 3.5
pH = 7.0
pH = 11.0
lit. values
benzene chlorobenzene bromobenzene iodobenzene o-dichlorobenzene m-dichlorobenzene p-dichlorobenzene
7.7 f 0.4 4.3 f 0.3 4.9 f 0.3 5.7 f 0.4 3.9 f 0.3 5.8 f 0.4 5.3 f 0.4
7.6 f 0.3 4.3 f 0.3 4.8 f 0.3 5.3 f 0.3 4.0 f 0.3 5.7 f 0.4 5.4 f 0.4
5.9 f 0.3 4.1 f 0.3 4.2 0.3 4.4 f 0.3 3.3 f 0.3 4.5 f 0.4 4.3 i 0.4
6.3;b7.8;' 6.8d 6.!je
nValues in the table are the average calculated from both amplitude and slope measurements carried out at the given pH. *Value obtained by pulse radiolysis at pH = 3.0 (33). 'Average of four values obtained by pulse radiolysis at pH 7.0 (30). dValue obtained by pulse radiolysis at pH = 10.5 (33). eValue obtained by pulse radiolysis at pH = 10.7 (33).
and measurement for each series of experiments. Corresponding calculations were carried out to obtain the inverse relative initial slopes Roo/Ro.A sample of the results of this analysis is shown in Figure 2a,b. We have analyzed our results within the following kinetic model (25): H202
m a
2'OH
+ DMPO 5 DMPO-OH 'OH + pollutant products
*OH
k3
pollutant
OYN:
products
(1) (2) (3)
(4)
where N, and N,' are the rates of photon absorption for H202and pollutant, respectively, and 4 and 4' are the corresponding quantum yields. A steady-state analysis of this mechanism (25) yields the following relation:
Roo/Ro= 1 + k,[pollutant]o/k2[DMPO]o
(5)
where Roo= 24N,. Thus plots of the inverse initial rate ]~ be straight lines with ratio Roo/Rovs [ p o l l ~ t a n tshould an intercept of 1 and a slope of k3/(k2[DMPOIo). If we assume that the decay kinetics of the DMPO-OH spin adduct are unaffected by the presence of the substrate, the same information on rate constants can be obtained from plots of the inverse ratio of the maximum signal amplitude A , that is A " / A = 1 + k,[p0ll~tant]o/k2[DMPO]o (6) where A" is the amplitude in the absence of scavenger. Since k2 (the rate constant for the reaction of hydroxyl radicals with DMPO) is well-known (27-29), we can obtain values for k, (the rate constants for the reaction of 'OH with a given compound) from the slopes. For all calculations, we have assumed that k2 = 4.3 X lo9 M-l s-l; this 264
Environ. Sci. Technol., Vol. 26, No. 2, 1992
is an average literature value calculated from various experiments in the pH range 6.5-10.0 (29). As is shown in Figures 2a,b and 3a,b, the inverse analysis plots for benzene and halobenzenes are good straight lines, with standard deviations that varied from 5% to 9%. Reaction rate constants, calculated from both A o / A and Ro"/Roplots, are collected in Table I. To examine the question of the influence of direct photolysis of the investigated compounds on the 'OH radicals reaction rate constants, we have performed a series of experiments using a Pyrex filter, which blocks light radiation for h < 300 nm. For h > 300 nm, the absorbance of the compounds under consideration is negligible. When the values for the experiments done with and without a Pyrex filter are compared (see Table I), the results agree within experimental error. This shows that the rate of reaction of 'OH radicals with benzene and its derivatives is much greater than their rates of photolysis (34). The above explanation is also supported by experiments carried out with each investigated compound (at concentrations of -10 1M) irradiated under the same conditions (but without H202and DMPO) during the same time (1min) as in the EPR experiments. No significant changes in both HPLC signals and UV spectra of the compounds before and after irradiations were observed. On the other hand, the same solutions irradiated with 0.2 mM hydrogen peroxide showed a considerable decrease (8-15 %) in the HPLC signals and changes in their UV spectra. It follows that under the experimental conditions, the influence of direct photolysis on the hydroxyl radical reactions can be neglected. We have also performed a series of experiments with the candidate compounds in buffers at pH = 3.5 and pH = 11.0 to find out the effect of pH on the 'OH radical reaction rate constants. The results of these experiments are given in Table I1 and show that in acidic solutions the reaction rate constants are the same (within experimental error) as those at neutral pH and close to the literature values.
However, in basic solutions rate constants appear to be slightly lower, This phenomenon can be explained in terms of the ionization of Hz02 HzOz + HO2- + H+ (7) which has a pK, of 11.6 (35). The rate constant (7.5 X lo9 M-l s-l) for the reaction of 'OH radicals with HOC is nearly 300 times that of HzOz (29) and reacts almost as fast as with benzene, Thus, as the pH is increased above -9, the presence of this alternate scavenger should be considered. This hypothesis is also supported by the control runs (EPR spectra of a photolyzed aqueous solution of H202and DMPO) carried out at pH = 3.5, 7.0, and 11.0. The maximum of DMPO-OH signal amplitude (which is related to the 'OH radical concentration) was similar for pH = 3.5 and 7.0 (4200 and 4100, respectively, in arbitrary units), but for pH = 11.0 it was significantly smaller (3300).
(11)
(12)
(13)
(14)
Conclusions
The spin-trapping technique with EPR detection can be successfully used for the study the reaction of hydroxyl radicals, generated by H202photolysis, with halobenzenes. The method can give both relative reactivity information and reliable values of the rate constants. Since scavenger concentrations can be varied over a wide range, we estimate that this method should be applicable for the determination of rate constants in the range 1O6-10l1 M-l s-l; however, impurities may interfere for slowly reacting scavengers. In addition, it should be possible to determine apparent rate constants for complex mixtures, such as humic acids, which would be useful in assessing competition effects in natural waters or wastewaters. The data for halobenzenes show that the reactivity of 'OH radicals with these derivatives does not vary much among the compounds studied. It would be safe to use an average rate constant of -5.0 X lo9 M-l for any of the halobenzene derivatives in the pH range 3-8. Acknowledgments
We are grateful to the reviewers for a number of very useful comments and suggestions. Registry No. 'OH, 3352-57-6;H202,7722-84-1;benzene, 7143-2; chlorobenzene, 108-90-7; iodobenzene, 591-50-4; o-dichlorobenzene, 95-50-1; m-dichlorobenzene, 541-73-1; p-dichlorobenzene, 106-46-7; bromobenzene, 108-86-1.
Literature Cited (1) Koubek, E. Ind. Eng. Chem. Process. Des. Dev. 1975, 14, 348. (2) Malaiyandi, M.; Sadar, M. H.; Lee, P.; O'Grady, R. Water Res. 1980, 14, 1131. (3) Peyton, G. R.; Huang, F. Y.; Burleson, J. L.; Glaze, W. H. Environ. Sci. Technol. 1982, 16, 448. (4) Prat, C.; Vicente, M.; Esplugas, S. Water Res. 1988,22,663. (5) Peyton, G. R.; Glaze, W. H. Environ. Sci. Technol. 1988, 22, 761. (6) Glaze, W. H.; Kang, J. W. Ind. Eng. Chem. Res. 1989,28, 1573. (7) Savel'eva, 0. S.; Shevchuk, L. G.; Vysotskaya, N. A. J. Org. Chem. USSR (Engl. Transl.) 1972,8, 283. ( 8 ) Proksch, E. P.; Gehringer, P.; Szinovatz, W.; Eschweiler, H. Appl. Radiat. Isot. 1987, 38, 911. (9) Getoff, N.; Solar, S. Radiat. Phys. Chem. 1988, 31, 121. (10) Cooper, W. J.; Nickelsen, M. G.; Waite, T. D.; Kurucz, C. N. Proceedings: A Symposium on Advanced Oxidation
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Processes for the Treatment of Contaminated Water and Air; June 4-5, 1990, Toronto, ON, Canada, Wastewater Technology Centre, Burlington, ON, Canada, 1990. Leitzke, 0.;Whitby, G. E. Proceedings: A Symposium on Advanced Oxidation Processes for the Treatment of Contaminated Water and Air; June 4-5,1990, Toronto, ON, Canada, Wastewater Technology Centre, Burlington, ON, Canada, 1990. Zeff, J. D.; Barich, J. T. Proceedings: A Symposium on Advanced Oxidation Processes for the Treatment of Contaminated Water and Air; June 4-5,1990, Toronto, ON, Canada, Wastewater Technology Centre, Burlington, ON, Canada, 1990. Cater, S. R.; Bircher, K. G.; Stevens, R. D. S. Proceedings: A Symposium on Advanced Oxidation Processes for the Treatment of Contaminated Water and Air; June 4-5, 1990, Toronto, ON, Canada, Wastewater Technology Centre, Burlington, ON, Canada, 1990. Oliver, B. G.; Carey, J. H. In Homogeneous and Heterogeneous Photocatalysis;Pellizzetti, E., Serpone, N. Eds.; Reidel Publishing Co.: Dordrecht, The Netherlands, 1986; p 625. Glaze, W. H.; Kang, J. W.; Chapin, D. H. Ozone Sci. Eng.
1987, 9, 335. (16) Paillard, H.; Brunet, R.; Dore, M. Water Res. 1988,22,91. (17) Peyton, G. R. Proceedings: A Symposium on Advanced Oxidation Processes for the Treatment of Contaminated Water and Air; June 4-5, 1990, Toronto, ON, Canada,
Wastewater Technology Centre, Burlington, ON, Canada, 1990. (18) Haber, F.; Weiss, J. Proc. R. SOC. London 1934, A147, 332. (19) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 676. (20) Podzorova, E. A.; Bychkov, N. V. High Energy Chem. (Engl. Transl.) 1979, 13, 88. (21) Mason, R. P.; Morehouse, K. M. In Free Radicals-
Methodology and Concepts;Rice-Evans, C., Halliwell, B., Eds.; Richelieu Press: London, 1988; pp 157-67. (22) Harbour, J. R.; Chow, V.; Bolton, J. R. Can. J. Chem. 1974, 52, 3549. (23) Ononye, A. I.; McIntosh, A. R.; Bolton, J. R. J. Phys. Chem. 1986,90,6266. (24) Harbour, J. R.; Bolton, J. R. Photochem. Photobiol. 1978, 28, 231. (25) Kochany, J.; Bolton, J. R. J. Phys. Chem. 1991,95, 5116. (26) Bolton, J. R.; Clayton, R. K.; Reed, D. W. Photochem. Photobiol. 1969. 9. 209. (27) Castelhano, A.; Perkins, M. J.; Griller, D. Can. J. Chem. 1983, 61. 298. (28) Neta; P.iSteenken, S.; Janzen, E. G.; Shetty, R. V. J. Phys. Chem. 1980,84, 532. (29) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 717. (30) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988,17,701. (31) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988,17, 707. (32) Shevchuk, L. G.; Zhikharev, V. S.; Vysotskaya, N. A. J. Org. Chem. USSR (Engl. Transl.) 1969,5, 1606. (33) Matthews, R. W.; Sangster, D. F. J. Phys. Chem. 1965,69, 1938. (34) Choudhry, G. G.; Webster, G. R. B.; Hutzinger, 0. Toxicol. Enuiron. Chem. 1986, 13, 27. (35) Anbar, M.; Meyerstein, D.; Neta, P. J. Phys. Chem. 1966, 70, 2660.
Received for review January 23,1991. Revised manuscript received June 25,1991. Accepted August 22,1991. This work was supported by a Strategic Research Grant from the Natural Sciences and Engineering Research Council of Canada.
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