J. Phys. Chem. 1994,98, 6343-6351
6343
Radiolytic and TiO2-Assisted Photocatalytic Degradation of 4-Chlorophenol. A Comparative Study Ulick Stafford,b.l Kimberly A. Gray,*)?and Prashant V. Kamat'vs Department of Chemical Engineering, Department of Civil Engineering and Geological Sciences, and Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: December 27, 1993; In Final Form: March 29, 19946
Mechanistic and kinetics details of 4-chlorophenol (4-CP) oxidation have been elucidated by radiolytic and photocatalytic techniques. When 4-chlorophenol is oxidized by y-radiolysis in conditions favoring hydroxyl radical oxidation ('OH), significant concentrations of 4-chlorocatechol (4-CC) and hydroquinone (HQ) are formed as intermediates. Phenol is the only major intermediate when conditions favoring reduction by hydrated electrons are employed. 4-CC and H Q are not detected when 4-chlorophenol is oxidized with azide radicals. Hydroxyl radical mediated oxidative degradation rates of 4-CP are similar at pH 3.0 and pH 6.1. The 4-CP degradation rate is relatively slower at pH 9.1, and no aromatic intermediates are detected. These results confirm the proposal that hydroxy-mediated 4-CP oxidation followsat least three separate degradation pathways, forming 4-CC, HQ, and unidentified mostly nonaromatic compounds, as reaction intermediates enroute to complete mineralization. Pulse radiolysis experiments have also been carried out to characterize the radical transient species formed during the oxidation of 4-CP (pH 6 and 10) and 4-chlorocatechol (pH 6). The second-order rates for scavenging of 'OH radicals a t pH 6 are measured as 9.3 X lo9 M-l s-l for 4-CP and 7.0X 109 M-1 s-l for 4-CC. Photocatalytic degradation produces intermediates consistent with hydroxyl radical oxidation, but the concentration of aromatic intermediates is lower than in the radiolysis experiments, especially at higher concentrations of TiOz. This indicates that the course of photocatalytic transformation of 4-CP does not involve hydroxyl radical oxidation exclusively. Direct electron transfer and surface chemical reactions also contribute significantly to the disappearance of 4-CP and its reaction intermediates in Ti02 slurries.
Introduction Photocatalytic degradation and mineralization of 4-chloropheno1 (4-CP) using titanium dioxide and ultraviolet light have been the topicof many recent investigations.1-9 In some photocatalytic studies hydroquinone (HQ) has been found to be the predominant aromatic intermediate,2.3*7 while in others 4-chlorocatechol (4CC) was pred0minant.5.~A variety of mechanisms and factors has been proposed to explain theseobserved resultsand differences. Yet, these efforts have proven inadequate, and a comprehensive picture describingthe intermediate courseof 4-CP photocatalysis is lacking. Radiolytic techniques have been found to be convenient in elucidatingradical reaction mechanisms; a few radiolytic studies pertaining to the mechanisticdetails of Ti02 photocatalysishave been reported.l"12 Free radicals are formed when water is irradiated with ionizing radiation, such as y raysor a high-energy electron beam.13 In the absence of scavengers, the majority of these radicals are hydroxyl radicals ('OH) and hydrated electrons (e-as), although smaller quantities of other radicals such as 'H are also formed. Some preliminary studies of y-radiolysis of 4-CP have been reported.4J4J5 In these investigations G values (yield) for 4-CP consumption and choride formation were reported, but aromatic intermediate yields were not well characterized. Second-order rates for the reaction of 4-CP with hydroxyl free radicals have been reported using pulse r a d i o l y s i ~ ~ ~ and competitive kinetic methods,16J7 but the reported rate constants vary. Some effort has also been made to characterize the transient intermediates of aqueous 4-CP solutions formed by pulse radiolysis under oxidizing and reducing conditions.14J5The simultaneous production of phenoxy1 and semiquinone radicals Authors to whom correspondence should be addressed. Department of Civil Engineering and Geological Sciences. t Department of Chemical Engineering. Notre Dame Radiation Laboratory. Abstract published in Aduunce ACS Absrrucrs, June 1, 1994.
0022-3654/94/2098-6343$04.50/0
and *OH adducts complicates the characterization of individual transient species in such radiolytic studies. In order to establish the free radical pathways involved in the photocatalytic mineralization of 4-CP in Ti02 suspensions, the degradation of 4-CP has been investigated by Ti02 photocatalysis and radiolysis. We have used y-radiolysis and appropriate scavengers to selectively degrade 4-CP with hydroxyl radicals or other free radical species, to identify the aromatic intermediates produced, and to measure reactant and product yields. By comparingthe intermediatesformed using yradiolysis with those produced duringTi02 photocatalysis,we can assess the importance of the different free radicals in the photocatalytic degradation of 4-CP. We have used pulse radiolysis to identify transient intermediates formed when 4-CP scavenges the various free radicals and to measure the rates of transient formation, in order to restablish the first steps of intermediateformation. In addition, we have examined the effect of increased Ti02 loading on intermediate concentrations during photocatalytic experiments, to see if the concentration of surface sites has an influence on the concentration of intermediates and the degradation pathway. Experimental Section Chemicals. 4-Chlorophenol(4-CP) (Aldrich, 99%+), 4-chlorocatechol (4-CC) (Tokeo Kasei), 4-chlororesorcinol (4-CR) (Aldrich, 98%), and phenol (Fluka, >99.5%) were used without further purification. Hydroquinone (HQ) (Aldrich, 99%+) was recrystalizedfrom water/ethanol prior to use, and benzoquinone (Alfa) was purified by sublimation. Borax, potassium phosphate mono- and dibasic, and hydrochloric acid were used to prepare buffers (all Fisher A.C.S grade). Fumed titanium dioxide (P25) was obtained from Degussa Corp. The BET surface area of P25 was 50 f5 m2/g (measured using a Quantasorb Quantchrome). Ferrous sulfate (Fisher) and sulfuric acid (Malinckrodt) were used to prepare solutions for Fricke dosimetry.lS Sodium azide (Fluka) and tert-butyl alcohol (Fisher) were used in some
0 1994 American Chemical Society
6344 The Journal of Physical Chemistry, Vol. 98, No. 25, 1994
Stafford et al.
TABLE 1: Preliminary y-Irradiation Results reactions conditions radical9 G(4-CP)' G(4-CC)c GWQY 'OH 3.45 & 0.26d N20 saturated 2.02 0.14d 0.38 0.044 0 2 saturated 'OH, '023.19 i 0.2Sd 1.65 i 0.074 0.42 f 0.05" 0.77 f 0.1 lef 0.03 i 0.02'f not detected e-aq 1% rert-butanol, N2 saturated 0.13 i 0.02' not detected e-aq,'020.33 f 0.07' 1% rert-butanol, 02 saturated a All reactions involved the y-irradiation of unbuffered 1 mM khlorophenol solutions (pH -6 dropping to -4 as HCl is formed). The radicals here are the predominant radicals. Other radicals such as *H are also present but in smaller concentrations (G('H) = 0.5) and are assumed not to have a major effect on the overall situation. G ( 4 C P ) is the yield of degradation of 4-CP. G(4-CC) and G(HQ) are yields of formation of 4-CC and HQ. G( ) is the number of molecules reacting per absorption of 100 eV of energy. The values represent the yields after the absorption of 250 Gy. The values represent the yields after the absorption of 690 Gy. /Under these reducing conditions phenol was the major intermediate formed, G = 0.37 f 0.03.
*
y-radiolysis experiments. The nitrogen and oxygen used were high purity, and the nitrous oxide was U.S.P. grade. Milli-Q water was used to prepare all samples. y-Radiolysis. y-Radiolysis experiments were conducted in a Gammacell 220 (Atomic Energy of Canada, Ltd.). Solutions for irradiation were prepared by mixing reagents and buffer stock solutions and saturating them with either nitrous oxide, oxygen, or nitrogen. HPLC sample vials (4 mL) were filled with the solutionsand sealed with gas-tight polytetrafluoroethylenesepta. To study the degradation over time, a sufficient number of samples were placed in the Gammacell to allow one to be taken out at regular intervals for later HPLC analysis. To measure initial rates, three samples were irradiated for 10 min. The output of the Gammacell was measured using Fricke dosimetry (1 mM F & O ~ - ~ H Z 0.4 O , M H2SO4) to be 8.68 f 0.40 Gy/min.l* Pulse Radiolysis. Pulse radiolysis experimentswere conducted using a linear accelerator at the Radiation Laboratory, University of Notre Dame, with an output of 1-5 Gy per p~1se.l~ Absorbance-time profiles at various wavelengths were recorded immediately after aqueous solutions of 4-CP and 4-CC were pulsed and saturated with nitrous oxide at different pH's. For direct oxidation of 4-CP, 0.1 M sodium azide was added to the solutions. The pH was adjusted to either 6 or 10 using dilute hydrochloric acid or ammonium hydroxide. The measured absorbance values werecalibrated against theextinction coefficientof the thiocyanate radical at 472 nm (7580 M-1 cm-l).,'J The rates of formation of the hydroxyl radical adducts of 4-CP and 4-CC were measured at pH 6 by varying the concentrations of the substrate (25-250 pM) and analyzing the first-order growth curves. Photocatalysis. An annular photoreactor (800-mL capacity) was used for photocatalytic degradation reactions, the details of which are described elsewhere.21 The reaction mixture was illuminated using an 8-W black lamp (New England Ultraviolet Products, Inc., A, = 350 nm) within the borosilicate glass thimble. Short-wavelength ultraviolet light and lamp heat were removed by a filter solution (60 g/L CuSO4 (Fisher), 0.25 g/L 2,7-dimethyl-3,6-diazacyclohepta1,6-dieneperchlorate (Eastern Chemical, 330-nm cutoff)) constantly circulated through this thimble from a reservoir that was cooled by cold water circulating through copper coils. The reaction temperature was 25 f 0.5 OC. The dissolved oxygen concentration, pH, and temperature were monitored constantly. The amount of light emitted by the lamp and absorbed by a potassium ferrioxalate actinometric solution in the reactor was 111 fiE/min.21.22 Solutions of 0.250 mM 4-CP were prepared by diluting a stock solution with Milli-Q water and saturating with oxygen. Ti02 addition made no significantchange to either the 4-CPor dissolved oxygen concentration. Samplesfor HPLC analysis were removed from the reactor with a syringe at regular intervals and filtered (Gelman 0.2-mm PVDF syringe filters) to remove the catalyst particles. Comparisons of light transmission were made using a long-wave UV meter (by Ultraviolet Products, Inc.). Analysis. HPLC (Waters 600E system controller, 712WISP auto sample injector, 990 Photodiode array detector) was used to analyze for 4-CP and aromatic intermediates in the samples. A reverse-phase C-18 column (Waters Nova-Pak or Supelco
Supelcosil) and water-methanol eluant (60:40 with 1 mL/L of acetic acid) wereused. Absorbance at 280 nm was used to measure the concentrations of 4-CP and aromatic intermediates (except benzoquinone, whose concentration was measured at 254 nm). Analyses of total organic carbon and mass spectrometry were also used to identify the nature of intermediates produced during oxidatioe of 4-CP with azide radicals in y-radiolysis. Results y-Radiolysis. Oxidative and Reductive Attack. 4-CP in aqueous solution is readily degraded by y-radiolysis. Oxidizing conditions are achieved by saturating with N 2 0 or 0 2 . The hydrated electrons formed in the spur (by reactions such as reaction 1) are scavenged by N20 (reaction 2) or 02.The full series of reactions is reported elsewhere.13.23 Hydroxyl ('OH) radicals can then react with 4-CP (reaction 3). The products H,O e-an
--
+ N,O + H,O 4-CP
+ 'OH
-
'OH, e-a,, 'H
N, + 'OH
+ OH-
products
(2)
(3)
formed as a result of 'OH attack were identified as 4-CC and HQ, with traces of 4chlororcsorcino1, benzoquinone, 1,2,4trihydroxybenzene, and other as yet unidentified compounds (Table 1). Yields of 4-CP disappearance and formation of intermediates are measured in terms of the G value, which is the number of molecules reacting per 100 eV of absorbed energy.13 The yield of 'OH generated in a N20 saturated system, G('OH), is -5.5. Yields of 4-CP disappearance, G(-4-CP), greater than 3, and 4-CC formation, G(4-CC), greater than 1.5 were measured (Table 1). The initial G values may be slightly higher than the values reported here because of competition with intermediates during the long irradiations. To study the effect of reductive radical attack, experiments were carried out in a nitrogen saturated solution with 1% tertbutyl alcohol added to scavenge hydroxyl free radicals (reaction 4). This ensured that the predominant reacting radicals in solution (CH,),COH
+ 'OH
-
'CH,(CH,),COH
+ H,O
(4)
were reducing. Phenol was the only major aromatic intermediate formed during reductive degradation of 4-CP. A trace of 4-CC was also detected due to competitive scavenging of the hydroxyl radicals by 4CP.z4 The reaction was conducted in a 1% tertbutyl alcohol solution saturated with oxygen, to investigate the effect of attack by superoxide radicals or other reduced oxygen species. Insignificant degradation was seen. Effect ofpH. To investigate the effect of pH on the reaction rate and intermediate composition, solutions at pH -3, pH -6, and pH 9.1 were irradiated for 1 h and sampled at intervals of IO min. The results are shown in Figure la,b. The rates of 4-CP degradation at pH 3 and 6 are practically identical. The rates
Photocatalytic Degradation of 4-Chlorophenol
The Journal of Physical Chemistry, Vol. 98, No. 25, 1994 6345 0.26
. p.
0.20
-
0.18
I
0.16 0
0.10
100
, 0
z
E
A
1 0
0
pH3,4-CC pH6,4-CC pH3,HQ pH6,HQ
.. .
P
% , -
100
m
400
300
500
Figure 1. 7-Radiolytic degradation of 4chlorophenol in a nitrous oxide saturated reaction mixture (mostly 'OH radicals) at pH 3, 6, and 9.1: (a, Top) decline in 4-CP concentration; (b Bottom) formation of intermediates.
of degradation of 4-CP and formation of 4-CC and HQ decrease with time over the course of a radiation dose of 500 Gy. This can be accounted for by competition with intermediates for hydroxyl radicals. In these mildly acidic conditions, -60% of the 4-CP is converted initially to 4-CC and 10%to HQ, during the first 10 min of irradiation. However, this leaves a balance of -30% that is degraded via other intermediates not detectable by the HPLC method used in the present study. The rateof 4-CP degradation is slower at pH 9 relative to more acidic pH (Figure la), and negligible concentrations of intermediates are detected. However, the rate of 4-CP degradation slows over time, in a manner consistent with neither zero- nor first-order kinetics. This indicates that some undetected species are competing successfully for hydroxyl radicals. Effect of 4-CP Concentration on Yield. To assess the effect of 4-CP concentration on yield (Gvalues of 4-CP disappearance and intermediate formation), the initial rates of 4-CP degradation by hydroxyl radical attack and formation of intermediates were measured over 10 min (total dose 85.6 Gy f 3%) for the degradation of 4-CP solutions ranging in concentration 64-2000 gM at pH's -3, -6, and 9.1. The G(4-CP) values were 3.5 0.5 at pH 3 and pH 6, and 2.7 f 0.5 at pH 9. G(4-CC) was 1.8 0.35 at pH 3 and pH 6, and G(HQ) was 0.35 f 0.10 at pH 3, and 0.45 f 0.15 at pH 6. There was little variation in these G values over the 4-CP concentration range 64-2000 pM, indicating a zero-order rate dependence on [4-CP]. Reaction with Azide Radicals. Azide radicals are generated by scavenging hydroxyl radicals with azide ions (reaction 5). The azide radicals so formedcan then be used to examine theoxidation
-
*
.
I
.
200
I
.
300
I
,
400
I
500
Figure 2. y-Radiolytic degradation of 4-chlorophenol (250 pM) in a nitrous oxide saturated reaction mixture with 0.05 M sodium azide at pH 6. No aromatic intermediates were detected using HPLC.
of a substrate S by direct electron transfer (reaction 6). When
-
0
Dose, Gy
*
100
Dose, Gy
I
*
0.08
500
400
300
Dose, Gy
0
0.02
ZOO
N3-+ *OH
+
ON3 OH-
(5)
the 4-CP was degraded in a buffered (pH 6) 0.05 M azidesolution, it was degraded at a nearly zero-order rate over 500 Gy (Figure 2). No aromatic intermediates weredetected using HPLC. After 1 h of radiation of 0.1 mM 4-CP, TOC was reduced by 25% concomitant with a 90% reduction in [4-CP]. Analysis by mass spectrometryof a hexane extract of the irradiated reaction mixture revealed many nonaromatic fragments and a small quantity of polymerized phenols (up to tetramers). These results suggest that many of the unidentified organic species are likely to be aliphatic compounds, although some polyaromatic species were detected. The failure to detect 4-CC or HQ indicates that the reaction path due to direct electron transfer to azide radicals is different from hydroxyl radical attack at pH 3-6. PuLPeRadiolysis. In order toobtain insight into the mechanism of 4-CP oxidation, time-resolved transient absorbance spectra were recorded in pulse radiolysis of 4-CP under various experimental conditions. Oxidationof 4-CPatpHlO. Thespectraof the radicalspecies formed 550 ns after the pulse radiolysis of N20 saturated 2 mM 4-CP aqueous solution at pH 10 in the absence and presence of 0.1 M azide are shown in Figure 3a and 3b, respectively. The inserts show the growth of transient absorbance at A,, (400 nm for 3a, 417.5 nm for 3b). The main contribution of both spectra is made by the 4-chlorophenoxyl radical (4-CP') which has maxima at 400 and 4 17111111' and is formed when the 4-chlorophenoxideion scavenges either the hydroxyl or azide radical (reaction 7). Because N20 C6H&lO-
+ ON3
+
C&C10'
+ N3-
(7)
scavenges eaq-and azide ions scavenge 'OH and *H radicals,14 only species resulting from reaction with 'Ns contribute significantly to spectrum 3b. In Figure 3a, the additional contribution over the broad range 340-440 nm is attributed to the semiquinone radical, the 'OH radical adduct of the phenoxide ion, and the cyclohexadienyl radical that results from the 4-CP scavenging *Hradicals.14 Theseassignments areconsistent with the reported spectrum of the transient species fromed when a 4-CP solution is pulsed at pH 11.3.14 In that study, Ye and Schuler14reported a spectrum with a lower extinction coefficient for the broad peak at 400 nm. This is accounted for by lower 'H and *OH concentrations produced at higher pH (pK,(*OH) = 11.9)23 and
jrl
6346 The Journal of Physical Chemistry, Vol. 98, No. 25, 1994
a
m
Stafford et al. 5000
-
4000
loo0 0
1.a
0.5
0.0
Time.
5
LIS
;aEi 300
350
400
500
450
lo00
550
--
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----
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0
I
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400
450
500
.
.
.
.
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~
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'
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'
.
.
'
'
'
UY)
' . 450
~
'
~
'
500
"
.
' ' 550
w m ,
Figure 3. Differenceabsorbancespectra of transient intermediatespecies generated following the pulse radiolysis of N20 saturated aqueous 4-CP solution at pH 10. (a, Top) 2 mM 4-CP solution saturated with NzO recorded 550 nsafter irradiation by an electron pulse. (b, Bottom) 2 mM 4-CP, 0.1 M sodium azide solution saturated with NzO recorded 550 ns after irradiation by an electronpulse. Thedashed lineshowstheextinction coefficient of 4-CP' corrected by adding the extinction coefficient of phenoxide (A, = 296 nm) to the difference ~pectrum.2~ The inserts show the rise in absorbance at, ,A 400 nm in part a and 417.5 nm in part b.
F i g m e l Differcnceabsorbancespectraoftransient intermediatespecies generated following the pulse radiolysis of N z 0 saturated aqueous 4-CP solution at pH 6. (a, Top) 2 mM 4-CP solution saturated with NzO recorded 2 ps after irradiation by an electron pulse. (b, Bottom) 2 mM 4-CP, 0.1 M sodium azide solution saturated with NzO recorded 20-25 ps after irradiation by an electron pulse. The inserts show the rise in absorbance at 305 and 420 nm in part a, and 420 nm in part b.
by the higher phenoxide ion concentration (99% of 4-CP dissociates at pH 11.3 compared with 80% at pH 10.1; pK,(4CP) = 9.4).25 Oxidationof4-CPatpH6. The spectrumof the radical species formed 2 ps after the pulse radiolysis of N2O saturated 2 mM 4-CP aqueous solution at pH 6 is shown in Figure 4a. The top insert shows the growth of transient absorbance at A, (305nm) over 1 ps. The spectrum is similar to a reported ~ p e c t r u m l ~ recorded 2 ps after an 02 saturated solution of 0.5 mM 4-CP at pH 8 was pulsed, which was attributed to the formation of the 'OH adduct.15 The absorbance-time profiles in the lower insert show the slow decay of the 'OH adduct over 200 ps (304 nm) and the slight rise in the absorbance at 420 nm, indicating the formation of 4-CP'. The transient spectrum recorded 20-25 ps after the pulse radiolysis of NzO saturated aqueous solution containing 0.1 M azide and 2 mM 4-CP at pH 6 is shown in Figure 4b. The insert shows the growth of transient absorbance at 420 nm. The rate of formation of this transient is 2 orders of magnitude slower than in the absence of azide, and the spectrum is considerably different. The peak with maxima at 400 and 417 nm is similar to the 4-chlorophenoxyl radical (4-CP') recorded in basic conditions (pH 10) in Figure 3b. This transient is formed when either 4-chlorophenoxide (reaction 7) or neutral 4-CP scavenges an azide radical (reaction 8).
On the basis of reported values for redox potentials, there is not a large driving force for the scavenging of azide radicals by phenol (Eo(Ph0,H+/PhOH) = Eo('N3/N3-) = 1.33V].2c27This also appears to be true for the scavenging of azide radicals by 4-CP (reaction 8). Therefore, it is expected that the scavenging of azide radicals by 4-CP is slow. At pH 6.1 the concentration of chlorophenoxideions is approximately 4 orders of magnitude less than the concentration of chlorophenol ([CaH4ClO] = 0.0005[C~H4ClOH],pK. = 9.4).25 This low concentration of dissociated 4-CP and the slow scavenging by the neutral 4-CP account for the slow formation of the transient. The peak at 305 nm is also attributable to the 4-chlorophenoxyl radical.28 The 417-nm peak of 4-CP' is lower than in Figure 3b due to decay of azide radicals over the long time scale that is needed for the reaction (8) to complete. Oxidation of 4-CC by 'OH a? pH 6. The spectrum of the radical species formed after the pulse radiolysis of N2O saturated 2 mM 4-CC (4-chlorocatechol)aqueoussolution at pH 6 is shown in Figure 5. We attribute this transient spectrum to the 'OHadduct of 4-CC, whose absorbance features with ,A, at 3 17 nm are similar to those of the *OH adduct of 4-CP in Figure 4a. Second-Order Rates of Hydroxyl Adduct Formation. The pseudo-first-order rates of adduct formation (rate of scavenging of hydroxyl radicals by the substrate monitored at 305 nm for 4-CP and 3 17 nm for 4-CC) were measured over the range 25250 MMand give the bimolecular rate constants of formation of
C,H,ClOH
+ ON,
-
C,H,ClO*
+ N; + H+
(8)
.
Photocatalytic Degradation of 4-Chlorophenol 5000 r
7
-6'
4000
F .-$
The Journal of Physical Chemistry, Vol. 98, No. 25, 1994 6347 300
a
-
3000
-
0
2000
f
Time, ps
5
Ys
-
3.-
100
c
low
x
0
-
o
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A
1.00 g/l TiO,
I
A O
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-
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;
-
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: i
-
~ ' ' " ' ' ' ' L ' ' ' 0 250 300 350
0 400
0
500
450
50
100
150
BO
150
200
150
200
Time, min
Wavelength, nm m
Figure 5. Difference absorbance spectrum of 4-chlorocatechol radical species recorded 2 fis after electron pulse irradiation of N20 saturated aqueous solution of 2 mM 4-CC at pH 6. The insert shows the rise in absorbance at hmx, 317 nm.
40
TABLE 2 Photocatalytic Degradation initial rate of Ti02 transmittance! 4-CP6reaction, conc (g/I) through slurry, T k X 106 (M min-l) 0.050 0.47 1.53 0.125
0.250 0.500 1.000
0.17 0.05 0.00 0.00
1.57 1.70 1.93 1.96
initial rate of 4-CF reaction, k/(l
- i")
5, 30 5
0 I
20
2.88
1.89 1.79 1.93
10
1.96 The ratio of the reading from a long wavelength ultraviolet meter through the slurry to the reading with no Ti02 in solution. The initial zero order rate for 4-CP disappearance which is the slope of the initial straight part of the 4-CP curves in Figure 6a. The initial rate corrected for light absorption. Calculated by dividing the previous column values by (1 - T ) , where T is transmittance from column 2. 4-CP-*OH (CCDHCHD) and 4-CC-'OH adducts at pH 6 as 9.3 X IO9 and 7.0 X IO9 M-l s-I, respectively. The value 9.3 X IO9 M-l s-l for formation of the 4-CP adduct is similar to the value measured by Shetiya et al. by competitive means a pH 9 (7.3 X lo9 M-1 s-l).l'j It is less than the values of 1.5 X 1OIo M-l s-l reported by Getoff and Solar (pulse radiolysis in neutral conditions)l5or2.45 X 101OM-1 s-1 reported by Kochany and Bolton (measured by an ESR spin trapping technique).17No reported values of 4-CC-OOH adduct formation were found in the literature for comparison. The pseudo-first-order rates for the scavenging of 'N3 radicals by 4-CP were monitored at both 303 and 417 nm over the range 50-2000 pM to give a bimolecular rate constant of 4.0 X lo7M-l s-l at pH 6.1. This value is similar to a reported value for the scavenging of azide radicals by phenol (5.0 X lo7M-l s-l)Z9 and is 2 orders of magnitude slower than the rate of scavenging of *OHradicals by 4-CP. Since the concentration of neutral 4-CP exceeds that of 4-chlorophenoxideions by a factor of 2 X 103 at pH 6.1, the calculated scavenging rate of "3 radicals by the 4-chlorophenoxideion is 8.0 X 1010 M-1 s-1. This rate is an order of magnitude faster than the reported values for scavenging of azide radicals by phenoxide ions,29 verifying that scavenging of azide radicals by neutral 4-CP (reaction 8) occurs slowly. Photocatalysis. Photocatalytic degradation of 250 pM 4-CP solutions was carried out with Ti02 catalyst loadings of 0.0501.00 g/L under UV irradiation for 3 4 h. A drop in the pH (from pH 6 to pH 4) was seen as a result of HCl formation. HQ and 4-CC are the major reaction intermediates in the photocatalytic oxidation process. The concentrations of 4-CP and the reaction intermediates, HQ and 4-CC, during some experiments are shown in Figure 6. The values for the initial rates (zero order) of 4-CP disappearance are listed in Table 2. The initial
0 0
50
100
Time, min "- I
E
30/ 20
t 0
50
100
Time, min
Figure 6. Photocatalytic degradation of 4-chlorophenol(a) with catalyst loadings of 0.050-1 .OOO g/L, and the aromatic intermediates formed, 4-chlorocatechol (b) and hydroquinone (c).
rate of 4-CPdisappearance shows a small increase with increasing Ti02. At lower catalyst loadings much of the light is transmitted through the slurry in the annular reactor, while at higher catalyst loadings all the incident photons are absorbed by the slurry. To correct for the amount oflight actually absorbed (not transmitted), the initial rate values are divided by the absorptance,30 and the corrected zero-order rate values are listed in Table 2. There is little difference in this corrected rate at different Ti02 loadings, with the exceptionof the lowest Ti02 loading. The faster corrected rate at this loading (0.050 g/L) is accounted for by an increase in quantum yield when more energetic photons are uniformly absorbed by the small amount of Ti02 in the annular reactor (0.026 vs 0.018 obtained by dividing k by 111 pE/min).ZI At
6348 The Journal of Physical Chemistry, Vol. 98, No. 25, 1994
Stafford et al.
TABLE 3: Maximum Intermediate Concentrations for 4-CP Photocatalytic Degndation initial 4-CP conc [~-CPIO (mM) 0.2506 0.2506 0.2506 0.2506 0.2506 0.1w O.1SSd
0.06Y 0.2sor 0.25or
max 4-CC' fraction, [4-CCI,/ [4CPIo 0.19 0.14 0.12 0.10 0.03 0.183 trace trace
max H@ fraction, [HQI-/[~-CPIO 0.04 0.04 0.04 0.04 0.03 0.063 0.012 0.039 0.200
0.3W
OM@
ratio of columns 2 and 3, [HQlmu/[4-CClmu 0.21 0.28 0.33 0.40 1 .o 0.34 >5 >5 >10 0.2
Ti02 conc (g/L) 0.05 0.125 0.250 0.500 1.OOo
0.5 g/L (P25) 2.0 g/L (P25) immobilized P25 0.5 g/L
Calculated by dividing the maximum concentrations in Figure 6 by the initial 4-CP concentration. Illuminated from the inside of annular photoreactor of path length 6 mm by one 8-W black lamp. Initial pH ~ 6 . ~Illuminated ' by six 8-W low-pressure Hg lamps around reactor. Initial pH 2.' Illuminated from below by one 125-W medium-pressure Hg lamp. Initial pH 3.4-6.0.3 e Recirculated tubular Pyrex reactor with Ti02 immobilized on the inside illuminated from the outside by six 15-W low-pressure Hg lamps. Initial pH -6.*fFalling recirculated film illuminated by a 2.5-kW medium-pressure Hg lamp. Initial pH ~ 6 . y-Radiolysis ~ 8 (Figure 1). Initial pH 6. The last point measured was the maximum. It is possible that a higher maximum value would be recorded at later reaction times.
*
SCHEME 1: Conduction Band (CB) and Valence Band Ti02 at Neutral pH and (VB) Energy Levels of AMRedox Potentials of the Various Couples Considered in This Study 4 I
I
0.0v
--- ----L----
-0.3V 0 $Oi
-
0.8 v 4-CP
Eg -3.2 eV
2.5 V
+
1.33 V*Ng/Ni 1.9 V *OH/OH
I
E vs. NHE
higher Ti02 loadings the uniform absorbance of energetic light becomers limited as most of it is absorbed near the inner wall of the reactor. Ti02 loading has a significant effect on the formation and decay of the reaction intermediate, 4-CC, measured during the reaction (Figure 6b). However, this effect is less pronounced on HQconcentration (Figure6c). Theratiosofthe maximum4-CC and HQ concentrationsrelative to the initial 4-CP concentration are reported in Table 3. The ratio of these two values is also reported. It is evident that, as the loading of Ti02 is increased, the ratio of [HQ]- to [4-CC],, also increases.
Discussion The various redox couples considered in both radiolysis and photocatalysis are shown in Scheme 1. In a photocatalytic reaction, photogenerated valence band holes are capable of oxidizing both OH- (to produces 'OH radicals; Eo = 1900 mV29 and 4-CPdirectly (Eo = 800 mV3I). The 'OH radical production is favored because of the abundance of hydroxyl groups on the Ti02 surface. These surface-bound hydroxyl radicals arc capable of subsequently oxidizing 4-CP. Similarly, the *OH and "3 radicals generated by radiolysis are also capable of oxidizing 4-CP, thereby providing insight into the radical mechanisms in Ti02 photocatalysis. The *OH radical oxidation simulates aqueous-phase 'OH oxidation that occurs in a photocatalytic degradation. The *N3 radical simulates oxidation by direct electron transfer, similar to that which may occur with the
photogenerated hole on the surface of TiO2. The redox couple for the proposed electron scavenging role of oxygen' in the conduction band is also shown in Scheme 1. Reaction Pathways in Radiolysis. Reaction pathways for oxidative degradation of 4-CP are shown in Scheme 2. When 4-CP is oxidized by hydroxyl free radicals in neutral or weak acidic conditions, the 4-chlorodihydroxycyclohexadienylradical (4-CDHCHD) is the most abundant short-lived intermediate formed (reaction a). A small quantity of the 4-chlorophenoxyl radical (4-CP') is also formed initially (reaction e), and via decay of 4-CDHCHD (reaction d; k 2 X 104 s-I, from Figure 4a). In pulse radiolysis, 4-CDHCHD decays following second-order kinetics (k = 3 X 108 M-1 s-I).I5 The dismutation reaction (reaction c) has been proposed as a pathway to form 4-CC.3.15 We propose that reaction b also occurs producing 4-CC and HQ. Dechlorination has been reported to occur during enzymecatalyzedoxidativecouplingofchlorophenols.32Reaction b occurs to a lesser extent than reaction c because the presence of chlorine at the para position directs more of the expected para-ortho attack to the ortho position, accounting for the greater yield of 4-CC over HQ. Other decay mechanisms of 4-CDHCHD to HQ in photocatalysishave been proposed but require reductiveconditions absent under oxidative radi~lysis.~ 4-CC and HQ were also the main intermediates detected by HPLC after flash photolysis of an H202/4-CP solution, conditionsthat favor hydroxyl attack.33 However, under low radiolysis dose rates such as those found in y-radiolysis, the second-order dismutationreactions (reactions b and c) are kinetically less favored than first-order water elimination to more stable phenoxy1 radicals (reaction d).34But in conditions where 4-CP' is the predominant initial reaction intermediate (e.g. high pH, *N3),neither 4-CC nor HQ is formed during the oxidative degradation of 4-CP (see below). This indicates that 4-CDHCHD is an essential intermediate in the formation of 4-CC and HQ by 'OH oxidation. Therefore, we suggest that 4-CC and HQ are formed by reactions g and h, in which 4-CP'reacts with 4-CDHCHD. In addition to these dismutationreactions, some C C P is further oxidized, accounting for 30% destruction of 4-CP via intermediates not detectable by the HPLC method used (reactioni). The 4-CP coupling reaction that can Occur under pulse radiolysisand during enzyme-catalyzed oxidation reactions occurs to a slight extent under y-radiolysis conditions.32.35 When 4-CP is oxidized at higher pH by either 'OH or *N3, 4-CP' is the predominant initial reaction intermediate detected during pulse radiolysis. At high pH more 4-CP is dissociated and oxidation of the 4-chlorophenoxide ion by direct electron transfer to form 4-CP' is predominant (reaction f). When 4-CP is oxidized by azide radicals at pH 6,4-CP' is formed, but much more slowly, by a combination of reactions e and f. 4-CDHCHD
-
Photocatalytic Degradation of 4-Chlorophenol
The Journal of Physical Chemistry, Vol. 98, No. 25, I994 6349
SCHEME 2: 4-Chloropbenol (4-CP) Oxidative -dation
Pathways Due to Oxidative y-Radiolysis. See Text for Discussion
IIQm 4-cp
4-CDHCm
OH
OH
61
\
OH Hli.-
61
H OH
T
6 +
c1
Cl
p
H
2
0
c1
4-CP 4 - c c
+
f$F76+@
6 ++)G+e;.. c1
Cl
+
+ HCI
Cl OH
10 is not formed under these conditions. The absence of stable aromatic intermediates, apart from a small quantity of polymeric species, after y-radiolysis and the reduction in TOC suggests that further oxidation of 4-CP' is the major next step (reaction i). It is likely that more of the unidentified organic intermediates are oxidative cleavage products of the aromatic ring rather than polymer ~ r o d u c t s . 3 ~The * ~ ~resonance structure of benzene is destabilized with the removal of an additional electron, such that the ring is broken, forming nonaromatic intermediates. Diaromatic species can be formed by pulsing an azide-phenol solution due to a second-order oxidative coupling reaction, but under y-radiolysis conditions, yields are low.35 Discrepancies in the Product Distribution. The similarity in the observed aromatic intermediates in the *OH-mediated y-radiolysis experiments and in photocatalytic degradation experimentssuggestsa common oxidativepathway to be operative in the oxidation of 4-CP. These results support the notion that hydroxyl radical oxidation is a major reaction pathway in the photocatalytic degradation of 4-CP. The absence of phenol as an intermediate, in the photocatalytic studies, suggests that reduction of 4-CP does not occur in Ti02 slurries. However, we have observed significantdifferencesin the relative concentrations of intermediates formed by radiolysis and photocatalysis. Table 3 includes values for intermediateconcentrations relative to the initial 4-CP concentration for photocatalytic experiments reported here and in the literature, and from our y-radiolysis experiments. The concentrationsand proportionsof intermediates relative to the initial 4-CPconcentrationvary considerably. Much higher concentrationsof 4-CC were detected for the photocatalytic reaction at lower catalyst loadings, but in all photocatalytic reactions, [4-CC] was lower than in the yradiolysis of 4-CP solutions at pH 3-6 with conditions favoring *OH radical oxidation. Theconcentrations of HQ were similar over the range of Ti02 loadings. At the highest Ti02 catalyst loading (1 g/L) the maximum concentration of HQ formed was the same as the maximum concentration of 4-CC. Differences are also seen among the values of the previously reported studies (Table 3). The pH has been proposed as an
c1
+A
OH
- Ai.
C1
non-aromatic intermediates
explanation for these discrepancie~.~ No aromatic intermediates were detected after y-radiolysis of 4-CP solutions at pH 9 in this study. However, during the hydroxyl radical mediated degradation of 4-CP over the range pH 3-6, the nature and yields of intermediates were not dependent on the pH. All results listed in Table 3 were conducted in unbuffered systems with initial pH 2-6, so pH does not adequately account for the discrepancies. Reactor configuration and catalyst loading have a significant influence on the product distribution and account for most of the differences in the reported yields of reaction intermediates. Significant concentrations of hydroquinone with only trace amounts of 4-chlorocatechol were detected in several previous s t u d i e ~ . ~The ? ~ . study ~ by Al-Ekabi et a1.2 was conducted in a tubular reactor illuminated from the outside with Ti02 immobilized on the inside wall. It is possible that in this study all the light was absorbed by Ti02near the reactor wall, and so few radical species were able to migrate away from the solid-liquid interface. A diffuse reflectance FTIR study of dry Ti02 powder has shown that 4-CP adsorbed to the surface is degraded to adsorbed hydroquinone? and so the only reaction taking place in the study by Al-Ekabi et al.2 is likely to be that of adsorbed 4-CP, undergoing oxidation to form hydroquinone. The study by Al-Sayyed et al.3 was conducted with a high catalyst loading in a deep reactor. In this case it is likely that most light would be absorbed by Ti02 particles near the wall. There may be significant adsorption of 4-CP in the dark region and subsequent photodegradation on the surface. In addition, much of the 4-CC formed may also have been adsorbed in this dark region and thus was not detected (see below). In the study by Yatmaz et ale7the reaction was conducted in a falling film reactor, in which the reaction mixture was briefly exposed to the light prior to a long period in the dark. This time in the dark may allow equilibration of surface species with those in solution, not normally possible in uniformly illuminated reactors. However, because of the high levels of HQ detected in this study, and the powerful lamp used with no apparent filtration of short-wavelength ultraviolet light (A < 290 nm), it is possible that significant direct photodegradation of 4-CP also occurred.38
6350 The Journal of Physical Chemistry, Vol. 98, No. 25, 1994 Importance of Surface ChemicalReactions, The concentrations of intermediates formed in semiconductor photocatalyzed degradation, especially at high catalyst loadings, are lower than those formed in y-radiolysis. The amount of HQ formed relative to the amount of 4-CC ([HQlm/[4-CC],), however, is greater in photocatalysis (Table 3). As the loading of Ti02 particles in photocatalytic degradation reactions is increased, the concentration of 4-CC decreases (Figure 6). These results suggest that solution-phase hydroxyl radical oxidation, although a major contributing factor, does not completely account for the observed product distribution. It also highlights the fact that surface reactions on Ti02 particles play an important role in the photocatalyticdegradation of 4-CP. Previous studies have shown that very small amountsof 4-CPare adsorbed by titaniumdioxide from aqueous solution because it has to compete with water for adsorption sites.S~~-3~ However, even such a small amount of adsorbed 4-CP may be important in long-term irradiation experiments, especially at high surface loadings. As previously stated, when the photocatalytic reaction was carried out in a gas/solid system, 4-CP was chemisorbed to the Ti02 surface and was degraded under ultraviolet irradiation to adsorbed HQ.9 The increased ratio of [HQ]/[4-CC] (Table 3) with higher Ti02 loadings illustrates the possible contribution from such a surface reaction. The longer time the catalyst particles spend in thedark after illumination would allow more time for adsorption and desorption to It has been shown that direct oxidation of several substrates with photogenerated holes on the Ti02 surface is a possibility.40 In this case an electron is transferred directly from the substrate to a surface-trapped hole. These conditions were simulated in solution when y-radiolysis-generated hydroxyl radicals were scavenged by azide ions to facilitate direct oxidation by azide radicals. Under these conditions the predominant reaction pathway was electron transfer from 4-chlorophenoxide ions (or 4-CP) to the azide radicals, which produced predominantly nonaromatic intermediate^.^^ The lower concentration of aromatic intermediates observed at higher Ti02 loadings in the photocatalyticdegradation suggeststhe possibilityof an increased amount of degradation occurring via the direct electron-transfer pathway at the surface. Adsorption of Reaction Intermediates. While adsorption of 4-CP to Ti02 has been examined,s*6J9little study has been made of the adsorption of 4-CC and HQ to the surface. Langmuir isotherm data show that 4-CC is adsorbed strongly to Ti02 (K = 0.13 pM-I, C,, = 45 pmol g1).21 Much less HQ is adsorbed under similar circumstances.2' It is possible that low concentrations of 4-CC seen in some of the studies may be the result of unaccounted 4-CC that is adsorbed to the surface via a bidentate complex formation. Because more 4-CC than 4-CP is adsorbed to TiO2, oxidation by valence band holes is likely to be more important for subsequent steps than it was for the initialoxidation step of 4-CP. Initial Oxidation Step. Most studies of TiO2-assisted photocatalytic degradation have proposed that hydroxyl radical attack on the substrate is the primary step in an oxidative process. And in a study comparing Ti02 photocatalysis with y-radiolysis by Mao et al.,I2 it was shown that the oxidative degradation of chloroethanes proceeded by hydroxyl radical attack in Ti02 photocatalysis. However, in the same study it was shown that organic acids (trichloroacetic and oxalic acid) were oxidized primarily by valence band holes.12 In a diffuse reflectance laser flash photolysis study of several compounds, including 2,4,5trichlorophenol, over Ti02 surfaces,no expected hydroxyl adducts were detected.40 This suggests that, in some surface-promoted photocatalytic degradation reactions, oxidation occurs by direct electron transfer and not by hydroxyl radical mediated attack. The pulse radiolysis of an aqueous solution of 4-CP and azide has shown that the 4-chlorophenoxide ion and, to a lesser extent,
Stafford et al. 4-CP are directly oxidized by the azide radical to form 4-chlorophenoxyl radicals. In y-radiolysis under similar conditions, no aromatic intermediates are detected.41 The concentration of phenoxide ions is dependent upon the pH. At higher pH more of the 4-CP will be dissociated, increasing the likelihood of direct oxidation. The compounds mentioned above that are oxidized directly by valence band holes are stronger acids than 4-CP (trichlorophenol,pK, 7; trichloroaceticacid, pK. = 0Sl;oxalic acid, pK, = 1.2).25 Therefore, more of each compound will be present in the dissociated form, increasing the likelihood of direct oxidation. It is evident from present studies of 4-CP degradation in Ti02 slurries that oxidationoccurs by a combinationof hydroxyl radical and direct hole oxidation processes. For the poorly adsorbed 4-CP, the hydroxyl radical reaction will dominate when less Ti02 surface is available. The preferred site of 'OH attack is the ortho position on 4-CP to yield mostly 4-CC ([HQ]/[4-CC] = 0.2 in y-radiolysis). When the contribution from surface processes is promoted with increased Ti02 loadings, more 4-CP degradation occurs by direct reaction with surface holes and the adsorption of byproducts such as 4-CC increases. Intermediate compound distributions are a function of the relative importance of these phenomena: 'OH attack, direct hole oxidation, and surface adsorption.
-
conclusion The complete photocatalytic degradation of 4-CP in Ti02 slurries proceeds via a combination of hydroxyl radical oxidation and surface oxidation by valence band holes. The contribution of these reactions in general will depend on the surface loading, pH, and substrate propertiessuch as pK, and structure. Aromatic intermediates, such as 4-chlorocatechol and hydroquinone, are detected during hydroxyl radical oxidation but not during direct oxidation. This suggests that increased Ti02 loadings favor more direct ring ~leavage.~'Solution-phase reduction makes no contribution. The effect of increased catalyst concentration is to reduce the concentration of aromatic intermediates detected in solution. The dependenceon Ti02 concentration is being further investigated to determine whether the rate of overall mineralization and quantum yield can be improved.
Acknowledgment. The authors gratefully acknowledge the supportofNSF [Grant No. BCS91-57948,K.A.G.] andtheoffice of Basic Energy Sciences of the US. Department of Energy [P.V.K.] (this is contribution no. NDRL-3669 from the Notre Dame Radiation Laboratory). The authors thank the Center for Bioengineering and Pollution Control at the University of Notre Dame for the use of analytical equipment. The authors thank Kim McAuliffe and David Griffiths for running TOC and MS analysis. The authors thank G. N. R. Tripathi for helpful discussions and Degussa Corp. for the gift sample of TiO2. References and Notes (1) Barbcni, M.;Raumero, E.; Pelizzctti, E.; Borgarello, E.; Grittztl,
M.: Scrpone. N. Now. J. Chim. 1984,8,547.
(2) (a) Al-Ekabi, H.; Serpone, N. J. Phys. Chem. 1988,92,5726. (b) AI-Ekabi, H.; Serpone, N.; Peliuetti, E.; Minero, C.; Fox, M.A.; Draper, R. B. Lonamuir 1989, 5, 250. (3) AI-Sayyed,G.;D'Oliveira, J . C ; Pichat, P. J. Photochem.Photobiol. A: Chem. 1991, 58.99. (4) (a) Matthews, R. W. Wuter Res. 1986,20,569. (b) Matthews, R. W. J. Carol. 1988,111, 264. ( 5 ) (a) Mills, A.; Morris,S.; Davics, R. J. Photochem. Photobiol. A: Chem. 1993, 70, 183. (b) Milis, A.; Moms, S. Ibid. 1993, 71, 75. (6) (a) Cunningham, J.; Sedlak, P. In Phorocutulytic Purification und Treutment of Wurcr und Air; Ollir, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993; p 67. (b) Cunningham, J.; Sedlak, P. J. Photochem. Photobiol. A: Chem. 1994, 77, 255. (7) Yatmaz, H. C.; Howarth, C. R.; Wallis, C. In Phorocatulyric Puriflcorion und Treatment of Wurer und Air; Ollis, D. F., Al-Ekabi. H., Eds.; Elsevier: Amsterdam, 1993: p 795.
Photocatalytic Degradation of 4-Chlorophenol (8) Sehi1i.T.; Boule, P.; Guyon, C.; LeMaire, J. 1.Photochem.Photobiol. A: Chem. 1989, 50, 117. (9) Stafford, U.; Gray. K. A.; Kamat, P. V.; Varama, A. Chem. Phys. Lett. 1993, 205, 55. 110) Lawless. D.: Serwne.N.: Meisel. D. J.Phvs. Chem. 1991.95.5166. (1 1) Terzian,' R.f Se;pone; N.'; Draper, R. B.;Fox, M. A.; Pelizzctti, E. Longmuir 1991, 7, 3081. (12) Mao, Y.; Schheich, C.; Asmus, K.-D. J. Phys. Chem. 1991, 95, 10080. (1 3) Buxton, G. V. In The Study of Fast Processes and Transient Species
by Electron Pulse Radiolysis; Baxendale, J. H., Busi, F., Eds.; D. Reidel: Dortrecht, The Netherlands 1981; p 241. (14) (a) Ye, M. Y.; Schuler, R. H. J. Liq. Chromatogr. 1990, 13, 3369. (b) Ye, M. Y. Ibid. 1992,15,875. (c) Ye, M. Ph.D. Dissertation. University of Notre Dame, 1989. (15) (a) Getoff, N.; Solar, S.Radiat. Phys. Chem. 1986, 28, 443. (b) Getoff, N.; Solar, S.Ibid. 1988, 31, 121. (16) Shetiya, R. S.;Rao, K. N.; Shankar, J. Indian J . Chem. 1976,14A, 575. (17) Kwhany, J.; Bolton, J. R. J. Phys. Chem. 1991, 95, 5116. (18) (a) Fricke, H.; Hart, E. J. In Radiation Dosimetry, VolumeII; Attix, F. H., Roesch, W. C., Eds.; Academic Press: New York, 1966; Chapter 12. (b) McLaughlin, W. L.; Boyd, A. W.; Chadwick, K. H.; McDonald, J. C.; Miller, A. Dosimetryfor Radiation Processing,Taylor and Francis: London, 1989; p 144. (19) (a) Patterson, L. K.; Lilie, J. Int. J. Radiat. Phys. Chem. 1974, 6, 129. (b) Janeta, E.; Schuler, R. H. J . Phys. Chem. 1982,86, 2078. (20) Schuler, R. H.; Patterson, L. K.; Janeta, E. J . Phys. Chem. 1980,84, 2088. (21) Stafford, U.; Gray, K. A.; Kamat, P. V.; Varma, A. Manuscript in preparation. (22) Murov, S.Handbookof Photochemistry;Marcel Dekker: New York, 1973; p 119. (23) Pikaev, A. K. TheSolvated Electron in Radiation Chemistry; Israel Program for Scientific Translations Ltd.: Jerusalem, 1971; p 48. (24) The rate of scavenging of 'OH by 1% rert-butyl alcohol is -5.4 X 107 s-1 (4 X 10' M-1 s-1 X 135 mM). The rate of scavenging of 'OH by 1 mM 4-CP is -9.3 X 106 s-1. At most this competitive interaction would account for 15%of 'OH-4-CP interaction. The tert-butyl alcohol rate was
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The Journal of Physical Chemistry, Vol. 98, No. 25, 1994 6351 obtained in Farhataziz: Ross, A. B. Selected Specific Rates of Reactio'ns of Transientsfrom Water in Aqueous Solution 1977, 111, 50. (25) Serjeant, E. P.; Dempscy, B.Ionisation Constants of Organic Acids in Aqueous Solution; IUPAC Chemical Data Series, No. 23; Pergamon: Oxford, U.K., 1979. (26) Wardman, P. J. Phys. Chem. Ref Data 1989, 18, 1637. (27) Surdhar, P. S.;Armstrong, D. A. J . Phys. Chem. 1987, 91, 6532. (28) The ground-state absorbances at pH 6 and pH 10 are different as 4-CP exists in the protonated (X,= 296 nm) and unprotonated (A, = 280 nm) forms, respectively. The similar extinction coefficients of ground-state phenoxide ion and the 4 - C P radical at pH 10 make the Ar small in the 305-nm region. However, at pH 6.4-CP has negligible absorbance at 305 nm as a result of which the phenoxy1 radical absorbance dominates. The transient spectrum corrected for ground-state absorbance confirms the validity of our 305-nmassignmentto thephenoxylradical (dashed linein Figure 3b): Tripathi, G. N. R. Private corrtspondence. (29) Alfassi, Z. B.; Schuler, R. H. J. Phys. Chem. 1985, 89, 3359. (30) Absorptance is the amount of light absorbed relative to the incident light. Its value is 1 - T, where T, the transmittance, is the amount of light transmitted through the reactor. (31) Vincdgopal, K.; Stafford, U.; Gray, K. A.; Kamat, P. V. J . Phys. Chem., in press. (32) Dac, J.; Bollag, J.-M. Enuiron. Sci. Technol. 1994, 28, 484. (33) Lipczynska-Kwhany, E.; Bolton, J. R. Enuiron. Sci. Technol.1992, 26, 259. (34) Land, E. J.; Ebert, M. Trans. Faraday Soc. 1967, 63, 1181. (35) Ye, M.; Schuler, R. H. J. Phys. Chem. 1989, 93, 1898. (36) March, J. Aduanced Organic Chemistry;John Wiley and Sons: New York, 1992; p 1181. (37) Lowry, T. H.; Richardson, K. S . Mechanism and Theory in Organic Chemistry; Harper & Row: New York, 1981; p 731. (38) Boule, P.; Guyon, C.; Lemaire, J. Chemosphere 1982, 11, 1179. (39) Tunesi, S.;Anderson, M.J. Phys. Chem. 1991, 95, 3399. (40) Draper, R. B.; Fox, M. A. Longmuir 1990,6, 1396. (41) It was discussed above that while some polymerization of 4-CP' to oligomers was seen using mass spectrometry after y-radiolysis of 4-CP with azide radicals, the reduction in TOC suggests that most azide oxidation of 4-CP results in cleavage of the aromatic ring.
