J . Phys. Chem. 1991, 95, 51 16-5120
5116
successive shots. They found lower rate constant values. They concluded that their latter lower values were to be preferred on account of smaller contributions to the decay of atomic carbon by products resulting from the photolysis of reactants. However, they did not say whether these contributions became negligible in these latest experiments and it is unfortunate that the rate constant values have not been extrapolated to a zero flash energy. Our results provide strong evidence for the presence of a photolytic interference in the flash-lamp experiments and suggest that the slightly larger values observed by Husain and Young are due to this effect. We find no agreement, in any manner, with the laser photolysis experiments of Bccker et aLS In their experiments, atomic carbon was generated from the multiphoton dissociation of CH2Br2,at 248 nm, with the focused KrF radiation from an excimer laser. The radiation energy in the tiny focal volume of the laser is several orders of magnitude greater than in the large volume irradiated by a flash lamp. Husain and Young have demonstrated that C NO and C O2rate constants depend on the flash-lamp energy, providing evidence that such determinations were affected by the contribution of reactant photolysis products to the atomic carbon decay. Such a problem must be even greater in laser photolysis experiments since the photon-energy density allows multiphoton
+
+
processes to affect the reactants more severely and in a hard to predict way. Unfortunately, no information is given by Becker et al. about the influence of laser intensity on their atomic carbon decays. Our experiments with 02,NO, and N20allowed the contradictions between flash lamp and laser photolysis experiments to be cleared up. Conclusion The fast flow technique has allowed us to determine the rate constants of atomic carbon reactions with OCS, SO2,and H2S for the first time. For reactions with 02,NO, and N,O,where a rather large scatter of rate constant values had been given by either flash-lamp or laser-photolysis experiments, our results have been found to be close, but a t a lower value, to those given by flash-lamp experiments at the lowest flash energy, More generally, our study provides a new example of the danger of significantly underestimating rate constant values when assuming plug flow in flow experiments combining fast homogeneous gas-phase reactions and efficient wall removal,
Acknowledgment. We thank C . Lalaude for technical assistance with the flow reactor.
Mechanlsm of Photodegradation of Aqueous Organic Pollutants. 1. EPR Spin-Trapping Technique for the Determination of 'OH Radical Rate Constants in the PhotooxIdation of Chlorophenol8 following the Photolysls of H202 Jan Kochanyt and James R. Bolton* Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 587 (Received: August 22, 1990; In Final Form: January 15, 1991)
A technique for the determination of the rate constants for the reaction of 'OH radicals with organic substrate is described. This technique is based on the use of spin trapping with electron paramagnetic resonance (EPR) detection of spin adducts. The desired rate constants are obtained by measuring either the initial rate of production of the EPR signal of the spin adduct or its amplitude after a fixed time, as a function of the concentration of the substrate. By using the known rate constant for the reaction of 'OH radicals with the spin trap (DMPO),rate constants for the reaction of *OH radicals with a given substrate can be obtained within a competition kinetic scheme. The method has been validated by determining the rate constants for the reaction of 'OH radicals with phenol and with formate, two reactions well studied by using pulse radiolysis. Our results agree with literature values within experimental error. The method is then applied to several chlorophenols. The rate constants for 3-chloro-substituted phenols are significantly less than for 4-chloro or 2-chloro-substituted phenols. Some rate constants are significantly larger than the diffusionantrolled rate constant. This is explained by proposing a Grotthus-type mechanism for the movement of 'OH radicals through water.
Introduction Most of the earth's surface is covered by water in the form of oceans, seas, rivers, lakes, and ponds. Many chemicals are entering the aquatic environment because of human activities, including chemical discharges from the production, processing, use, and disposal of industrial and agricultural products. The total amount of these discharges has increased dramatically during the last forty years and is now the subject of considerable concern regarding their impact on both the aquatic environment and human health. Many organic chemicals discharged into the 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, more desirable, to their complete mineralization. 'On leave from the Institute for Environmental Protection, Warsaw, Poland.
0022-3654/91/2095-5116302.50/0
Lately a series of new techniques, called advanced oxidation processes, has received increasing attention. They rely on the use of short-lived oxidative radicals (often hydroxyl radicals) generated by photolysis or radiolysi~.'-~ In spite of several reports and reviewsb* reporting degradation rates for a variety of pollutants, very little is known about the detailed mechanism of these degE.Ind. Eng. Chem. Process. Des. Deu. 1975, 14, 348. (2) Malaiyandi, M.; Sadar, M. H.; Lee,P.; Grady, R. 0. Water Res. 1W, (1) Koubek,
4, 1131.
(3) Mansour. M. Bull. Enuiron. Confam. Toxicol. 1985, 34, 89. (4) Jacob, N.; Balakrishnan. I.; Reddy, M. P. J . Phys. Chem. 1977, 81,
I.
(5) Prat, C.; Vicente, M.; Esplugas, S. Water Res. 1988, 22, 663. (6) Langford, C. H.;Carey, J. H. ACS Symp. Ser. 1987, 327, 225. (7) Ollis. D. F.; Pelizzetti, E.; Serpone. N . In Photocatalysis, Fundameaals and Applicarlons; Serpone, N., Pelizzetti, E., Eds.; John Wiley: New York, 1989; Chapter 18. (8) Metthews, R. W. J. Phys. Chem. 1987, 91, 3328.
0 1991 American Chemical Society
Photodegradation of Aqueous Organic Pollutants radation processes, except that hydroxyl radicals are important intermediates. Understanding their mechanism is, however, very important if improvements in the yield and efficiency of these processes are to be achieved and thus permit their practical application. The aim of our investigations is to contribute to an understanding of the photodegradation mechanism of organic pollutants in a homogeneous system. We have focused our interest on the photolysis of hydrogen peroxide in aqueous solution as the source of hydroxyl radicals, which then attack pollutant molecules in an oxidative degradation mechanism. It has been long known that hydrogen peroxide photolyzes cleanly to produce hydroxyl radicals? which then react with many organic compounds to produce low-molecular-weight oxygenated substances that are easily biodegradable in the aquatic environment. In the studies presented here we have developed a method for determining the primary rate constants for attack of 'OH radicals on pollutant molecules using a competition method based on the technique of spin trappinglo with detection by electron paramagnetic resonance (EPR) spectroscopy. Our method is a modification of that proposed by Castelhano et al." Traditionally, 'OH radical rate constants have been measured by using a pulse radiolysis method;I2 however, this requires a very expensive facility. The EPR method we propose here should be more widely applicable.
Experimental Section The spin trap used was 5,5'-dimethylpyrroline N-oxide (DMPO), which reacts with free radicals, such as 'OH,"to form relatively stable nitroxides (spin adducts). This method has been successfully applied to develop a protocol, based on a competition reaction with formate, to test for the presence of 'OH radicals in ~ o l u t i o n . ' ~ J ~ The following compounds have been investigated: phenol, formate, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol, 2,4dichlorophenol, and 3,5-dichlorophenol, with the first two compounds being used to test the method. Stock solutions of the above phenols in 1.OmM phosphate buffer (pH = 7.0) were diluted with the buffer solution to obtain the final concentrations (in the range 0.01-0.20 mM) of the studied compounds. To each sample solution so prepared, sufficient volumes of 1.0% H202and 10 mM DMPO buffer solutions were added so that each had a final concentration of 0.50 mM. Sample solutions were freshly prepared in the dark just a few minutes before experiments and purged with prepurified argon for 20 min before and throughout the experiment. The solutions were pumped through Teflon tubing into an EPR quartz flat cell (WG-814 Wilmad) and irradiated for 1 min with the UV lamp. Both the full DMPO-OH spin-adduct EPR spectrum and its signal increase were recorded during irradiation. Experiments for each concentration of a given candidate compound were repeated several times (at least four) for every stock solution. For every compound, at least three different stock solutions were prepared and investigated by the above procedures. To compare the results of the various experiments, control runs were conducted in the same manner with solutions of H202and DMPO (both at 0.50 mM) in phosphate buffer solution. These control runs were carried out both at the beginning and after each series of experiments. F.;Weiss, J. J. Proc. R. Soc. London 1934. A147, 332. (IO) Mason, R. P.; Morehouse, K. M.In Free Radicals-Methodology and Conceprs; Rice-Evans, C., Halliwell, B.,Eds.; Richelieu Pres: London, (9) Habcr,
1988; p 157. (1 1) Castelhano, A.; Perkins, M. J.; Griller, D. Can. J . Chem. 1983, 61, 298. (1 2) Buxton, G. V.;Greenstock, C. L.; Helman, W.P.;Ross, A. B. J. Phys. Chem. Re/. Dura 1988,17, 517. (13) Harbour, J. H.; Chew, V. S.F.;Bolton, J. R. Can. J . Chem. 1974, 52, 3549. (14) Ononye, A. J.; McIntosh, A. R.; Bolton, J. R. J . Phys. Chem. 1986, 90.6266. (15) Harbour, J. H.; Bolton, J. R. Photochcm. Phorobiol. 1978, 28, 231.
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5117
340.0
346.0
344.0
352.0
350.0
354.0
magnetic field ImT cp
I
I
-20
0
20
40
60
80
100
120
140
160
180
time /r.ec
Figure 1. (a, top) EPR spectrum of the DMPO-OH spin adduct recorded during UV irradiation of an aqueous solution (pH = 7.0) of H202 and DMPO (both at a concentration of 0.50 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. (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 center field = 348.755 mT; sweep width = 0 mT; scan time = 167.50 s; number of scans = 1.
All chemicals used in the experiments were ACS reagent grade. All solutions were prepared by using doubly distilled water. Fresh bottles of DMPO (Aldrich Chemical Co.) were stored at -20 O C in a freezer under nitrogen. Open bottles of DMPO were also stored in the freezer and only used if the liquid was clear and colorless. Buffer solutions were prepared from 1.O mM potassium phosphate monobasic solution by adding orthophosphoric acid to obtain a final pH = 7.0. Experiments conducted using phenol, with buffer concentrations between 0.5 and 2.0 mM, show that there is no significant buffer effect on either the signal amplitude or the initial slope values. We have chosen a standard buffer concentration of 1.0 mM, as such a buffer solution was used previously in similar experiments.14J5 EPR spectra were obtained with a Bruker Model ESP 300 electron paramagnetic resonance (EPR) spectrometer, coupled with a computer for data acquisition and instrument control. A TM102 cavity fitted with an aqueous solution sample holder was used. A 150-W mercury-xenon lamp (Conrad Hanovia 901 B001) was used as the light source. The lamp was connected to a Model 8500 Oriel power supply. During irradiation a quartz water filter was placed in the light beam to protect the cavity from heating. The initial slope (initial rate) Ro of the EPR signal, measured according to a mirror method described by Bolton et a1.,16 and the maximum amplitude of the EPR signal were recorded and normalized by using experimental values obtained from control runs (with no substrates present). The SIGMA-PLOTcomputer program, version 3.1 (Jandel Scientific, Sausalito, CA), was used for the analysis of experimental errors and also for graphic presentations. A weighted least-squares method was used to obtain the slopes of the graphs. The lines of the best fit include all points within 90-95% of confidence interval. Weighting factors varied (16) Bolton, J. 1969, 9, 209.
R.; Clayton, R. K.;Red, D. W. Phorochem. Phorobiol.
5118 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
Kochany and Bolton
TABLE I: Rate Cowtmb k 3 for the Reaction of *OH Radicals with Phenol and Formate kal(lO'O M-'s-')
compound phenol formate
a
b
C
1.70 f 0.20 0.33 i 0.03
1.40 f 0.15 0.29 f 0.03
1.60 f 0.20 0.30f 0.03
d 1.45 f 0.15 0.27 f 0.03
e
1.46
0.32,r0.26h
"Calculated from a plot of Ao/A vs [SIwith Pyrex filter. *Calculated from a plot of Ao/A vs [SI without Pyrex filter. 'Calculated from a plot of Roo/% vs [SIwith Pyrex filter. dCalculated from a plot of &O/& vs [SI without Pyrex filter. 'Literature values. /Reference 20;there are other valuessee text. #Reference 21;value obtained by using a pulse radiolysis method. hReference 13; value obtained by using a photolysis method. from 10.9 (for concentrations about 0.01-0.03 mM) to 2.8 (for concentrations about 0.15-0.2 mM). Two different methods were used to measure the DMPO-OH EPR signal in each experiment: ( I ) recording spectra taken after the same time of irradiation and after the same number of scans; (2) recording the change of the signal amplitude at the field maximum with time before and during irradiation. Examples of the recorded spectra and a signal time profile, are shown in Figure la,b. The second method is better for the following reasons: (1) it provides more information about the system under investigation, since one can calculate not only the maximum amplitude of the EPR signal but also the initial slope; (2) it gives the real maximum value of the signal for every sample-this is not always the case for the recorded spectrum method (the maximum amplitude of the signal occurs at slightly different times of recording and varies from compound to compound and with concentration). However, to obtain reliable results, the second method requires careful sample handling and a stable laboratory environment, because the exact position of the signal field maximum can shift during the experiment and therefore render the results not comparable. That is why recording EPR spectra for every sample, which gives exact field position of the EPR signal maximum, is also necessary. Ononye at al." have examined this system and have shown that in the absence of either H 2 0 2 or DMPO no EPR signals are obtained. They also varied the concentration of DMPO and H202 and found that the rate constants are independent of concentration in the range 0.2-0.7 mM for DMPO and 0.5-2.0 mM for H202. The intensity of the light source can change from day to day; therefore, for each series of experiments a solution of H 2 0 2and DMPO (without any substrate) was used as a control and the results (signal maximum Ao and initial rate &O) used as reference values. The inverse relative signal intensity Ao/A was calculated for every sample and measurement for each series of experiments. Corresponding calculations were carried out to obtain the inverse relative initial rates Roo/Ro.
The Kinetic Model We have analyzed our results within the following kinetic model Hz02 'OH
w.
2'OH
+ DMPO 2 DMPO-OH k3 *OH + S products
-
s
-
(1) (2)
(3)
0"'.
products (4) where N, and N', are the rates of photon absorption by HZO2and the substrate (S),respectively, and 4 and 4' are the corresponding quantum yields. We expect that, under most conditions, the rate of step 4 will be much slower than that of step 1, since it is known that the direct photolysis of phenol and chlorophenols is slower (k = M-' s-I)I7 than the reaction of hydroxyl radical with DMPO (k = (2.7-4.3) X lo9M-I s-l).I8 This is why competitive reactions of hydroxyl radicals with both DMPO and phenols are decisive for the whole system. (17) Moa,P.N.; Fytianos, K.;Samanidou, V.;Korte, F. Bull. Enuiron. Contam. Toxicol. 1988, 4I,678. (18) Neta, P.; Steenken, S.; Janzen, E. G.; Shetty, R. V. J. Phys. Chem. 1980,84, 532.
On applying a steady-state analysis to the above mechanism, it follows that ['OH] =
24Na k,[DMPO] + k,[S]
The rate (R) of formation of DMPO-OH (proportional to the rate of change of the DMPO-OH EPR signal) is
R = d[DMPO-OH]/dt = k2['OH][DMPO]
(6)
or R=
2k24"MPOI kZ[DMPO]
+ k,[S]
(7)
If we define R" as the value of R when [SI = 0, then
Ro = 24N,
(8)
On taking the inverse of eq 7 and rearranging, we obtain (9)
If initial rates (Rooand Ro) are measured then
-Roo- - 1 + RO
k3[S]o k2[DMPOIo
where [SIo and [DMPOIo are the initial concentrations. Thus plots of the inverse initial rate ratio &O/& vs [SIoshould be straight lines with an intercept of 1 and a slope of k3/(k2[DMPOIo). Since k2 (the rate constant for the reaction of hydroxyl radicals with DMPO) is well-known,"-19 we can obtain absolute values for k, (the rate constants for the reaction of *OH with a given phenol) from the slopes. For all calculations, we have assumed that k2 = 4.3 X IO9 M-'s-I; this is an average literature value calculated from various experiments in the pH range 6.510.8.19 Since 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 maximum signal amplitude Ao/A.
Results and Discussion 1. Validation of the Metbod. To test the method we have used phenol and formate as substrates, since the rate constants for the reactions of *OH radicals with these substrates have been well established from many pulse radiolysis studies. Figure 2, a and b, shows plots of the inverse amplitude ratio (Ao/A)and the inverse initial rate ratio (bo/&), respectively, vs substrate concentration for these two substrates. Table I summarizes the rate constants obtained. Clearly, the Ao/A and Roo/Roplots are very similar for each compound. This shows that secondary reactions involving product spin adducts are insignificant. Also the rate constants for 'OH reaction with each substrate agree very well with those obtained from pulse radiolysis experiments. In the case of phenol we found differences in the literature, even when the authors described the same experimental conditiom2' The values (in (19) Buxton, G.V.;Greenstock, C. L.; Helman, W.P.;Ross,A. E.J. Phys. Chem. Ref. Dafa 1988, 17, 717. (20) Podzorova, E. A.; Bychkov, N. V. High Energy Chem. (Engl. Trcmrl.) 1979, 13, 88.
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5119
Photodegradation of Aqueous Organic Pollutants
TABLE II: Rate Coastants k , for tbe Reaction of *OHRadials with Chloropheaob k,/(10I0 M-I s-I)
compound 2-chlorophenol 3-chlorophenol 4-chlorophenol 2,4-dichlorophenol 3,5-dichlorophenol
b
C
1.74 f 0.14
1.65 0.14 0.86 f 0.08 2.82 0.21 3.10 f 0.20
a
1.92 0.21
* 0.10 3.20 * 0.24 3.82 f 0.26 0.90 * 0.1 1
0.88 f 0.07 2.45 f 0.18 2.65 i 0.18
1.04
0.82
* 0.07
d
** 0.11 * 0.06 0.12 0.14 * 0.06
1.69 0.84 2.65 2.95 A 0.72
0.08
0.78
e
1.26 0.72r 0.76h
aCalculated from a plot of Ao/A vs [phenol] with a Pyrex filter. *Calculated from a plot of Ao/A vs [phenol] without a Pyrex filter. 'Calculated from a plot of Roo/Rovs [phenol] with a Pyrex filter. dCalculated from a plot of R t / R 0 vs [phenol] without a Pyrex filter. eLiterature values. /Reference 22; value obtained by using a pulse radiolysis method at pH = 6.5-7.0. #Reference 28; value obtained by using a pulse radiolysis method at pH = 9.0. *Reference 27; value obtained by using a pulse radiolysis method at pH = 9.0. 9.08.0 ..
7.0 -.
7.0
I
noJ 0.00
0.05
0.15
0.10
0.20
-.-
concentration/mM
0.00
:I
10.0,
1
,
".-
7.0 8.0
0.00
0.05
0.05
0.10
0.15
0.20 2.0 1 .o
concentration/mM
Figure 2. (a, top) Inverse amplitude ratio Ao/A and (b, bottom) inverse initial slope ratio &O/& vs substrate concentration for photooxidation of phenol (-0-) and formate (--A--) with H202at pH = 7.0. Error bars are f 2 standard deviations.
units of M-I s-l) vary from 6.6 X to 1.44 X 101023*24 and 1.88 X 1010.20For comparison with our data we used the value 1.44 X 1O1O M-I s-I obtained for the reaction of hydroxyl radicals with phenol, since in this study the hydroxyl radicals were generated both by photolysis and by radiolysis under the same (phosphate buffer, pH = 7.0) reaction conditionsaZo 2. Application of the Metbad to cblorophenols. Inverse plots for various chlorophenols are shown in Figure 3a.b. These plots are good straight lines, with standard deviations that varied from 5-8% (at lower phenols concentrations) to 8-1596 (for higher concentrations). Reaction rate constants, calculated from the slopes of both Ao/A and Roo/Roplots, are collected in Table 11. Again we found that the rate constants calculated using both methods are almost the same, within experimental error. 3. A Possible Role for Direct Photolysis? To exclude the influence of direct photolysis as a possible side reaction, we have (21) Bwton, 0.V.;Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chcm. Ref. Data 1988,17,721. (22) Buxton, G. V.;Greenstock, C. L.; Helman, W. P.; Ross,A. B. J . Phys. Chcm. Ref. Dora 1988, 17, 707. (23) Field, R. J.; Raghavan, N. V.;Brummer, J. G. J . Phys. Chem. 1983, 86, 2443. (24) Podzorova, E. A.; Plotnikova, V. P.; Bychkov, N. V. High E m f 0 Chem. (Engl. Transl.) 1976, 10, 374.
0.10
0.15
0.20
concentration/mM
7.0
-.-
t
-.-
t
/I
w
0.00
0.05
0.10
0.15
0.20
concentration/mM
Figure 3, (a, top) Inverse amplitude ratio Ao/A and (b, bottom) inverse initial slope ratio ROO/& vs substrate concentration for photooxidation of 2-chlorophenol (-0-), 3-chlorophenol (--A- -), 4-chlorophenol (--O--), 2,4-dichlorophenol (--V--), and 3,5dichlorophenol(-- 0--) with H202at pH = 7.0. Error bars are *I standard deviation.
performed a series of experiments using a Pyrex filter, which eliminates light radiation for A < 300 nm. For A > 300 nm the absorbance of most of the compounds under consideration is negligible. When the values for the experiments done with and without a Pyrex filter (seeTable 11) are compared, the values agree within experimental error; however, the values obtained with a Pyrex filter are almost always slightly smaller than those without the filter. If this is significant, it may arise from a small contribution from reaction 4 (Le,, direct photolysis). This idea is supported by experiments done with phenol, 4-chlorophenol, and 2,4-dichlorophenol irradiated under the same conditions (but without HzOz) during the same time (1 min) as in the EPR experiments. Some decreases (5% for phenol and 5-98 for 4chlom and 2,edichlorophenol) were observed in the HPLC signals of these compounds. Detailed studies on the stepwise mechanism of the photodegradation of these phenols have been carried out by Lipczynska-Kochany and Bolton.zs (25) Lipczynska-Kochany,E.; Bolton, J. R. Submitted to J . Phorochcm.
Photobiol.
5120 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
0.00
0.05
0.10
0.15
0.20
concentration/mM
7.0
Kochany and Bolton calculated by us differ, however, from those obtained from pulse radiolysis. The differences may be the result of different reaction conditions, especially different pH values (pulse radiolysis measurements for 3-chlore2' and 4-chlorophen01~~ were carried out at pH = 9.0). We have examined the reaction of hydroxyl radicals with phenol at different pH values and have found differences for both the inverse relative signal amplitude ( A o / A ) (Figure 4a) and its relative initial rate (&O/R0) (Figure 3b). The rate constants at pH 3.0 (k = 1.8 X 1O'O M-'5-I) and at pH 7.0 (k = 1.4 X lolo M-'s-*) were the same within experimental error, whereas at pH 11.0, the rate constant was significantly lower (k = 4.5 X lo9 M-' s-]). These values were calculated by using rate constants for the reaction of hydroxyl radicals with DMPO obtained at the similar pHZ1(for calculations at pH = 3.0 we used the literature value, 3.6 X lo9 M-'s-l, obtained at pH = 5.6; and for calculations for pH = 1 1.0, we used the value 4.9 X 1O'O M-'s-I obtained at pH = 10.8, since for calculations at pH = 7.0 we used the average value from the literature data, which was almost equal to that for pH = 7.0). However, when investigating the reaction of 'OH radicals with phenol at different pH we should take under consideration that at pH > 11.O phenol exists mainly as the phenolate ion and 'OH radicals undergo dissociation to the conjugate base *0-.29
0.0 1
0.00
0.05
0.10
0.19
I
0.20
concmtrotlon/mY
Figure 4. (a, top) Inverse amplitude ratio Ao/A and (b, bottom) inverse initial slope ratio Roo/Rovs concentration for photooxidation of phenol at pH = 3.0 (-0-), pH = 7.0 (--A--),and at pH = 11.0 (---V---). Error bars are f2 standard deviations.
4. Discussion of the Rate Constants. The data from Table I1 indicate a higher reactivity for 2,4-dichlorophenol and 4-chlorophenol with 'OH radicals, as compared with 3-chloro- and 3 3 dichlorophenol. This suggests an effect of chlorine substitution in the phenol compounds on their reactivity with hydroxyl radicals, in that 3-chloro-substituted compounds are less reactive than 4-chloro-substituted phenols. These results generally agree with data published for reactions of some chlorophends with hydroxyl radicals generated by pulse radiolysis.26 The exact values (26) Getoff, N.; Solar, S. Radiat. Phys. Chem. 1986, 28, 443.
It is interesting that many of our rate constants are significantly larger than the diffusion controlled rate constant (0.7 X lotoM-' s-I) in aqueous solution.30 One explanation for this is that 'OH radicals can move through water with little or no movement of atoms employing a mechanism similar to that of the Grotthus mechanism for the transport of H+and OH- ions through watera3'
Conclusions The spin-trapping technique with EPR detection has been shown to be effective in the study the reaction of hydroxyl radicals with phenolic compounds. This method can provide not only relative reactivity information but also reliable values of the absolute reaction rate constants for the reaction of 'OH with a variety of pollutant substrates. (27) Shetiya, R. S.; Rao, K. N.; Shankar, J. Indian J. Chem. 1976, IIA, 575. (28) Savel'eva, 0. S.; Shevchuk, L. G.; Vysotskaya, N. A. J. Org. Chem. USSR (Engl. Transl.) 1972, 8, 293. (29) Anbar, M.; Meyerstein, D.; Neta, P. J . Phys. Chem. 1966,70,2660. (30) Levine, I. N . Physical Chemistry; McGraw-Hill: New York, 1988; 'D 560. ( 3 1 ) Adamson, A. W. A Textbook of Physical Chemistry; Academic Press: New York, 1979; p 448.
