250
Langmuir 1989,5, 250-255
Kinetic Studies in Heterogeneous Photocatalysis. 2. Ti02-MediatedDegradation of 4-Chlorophenol Alone and in a Three-Component Mixture of 4-Chlorophenol, 2,4-Dichlorophenol, and 2,4,5-Trichlorophenol in Air-Equilibrated Aqueous Media Hussain Al-Ekabi and Nick Serpone* Department of Chemistry, Concordia University, 1455 deMaisonneuue Blud. West, Montrbal, Qubbec, Canada H3G 1M8
Ezio Pelizzetti and Claudio Minero Dipartimento di Chimica Analitica, Universitd di Torino, 10125 Torino, Italy
Marye Anne Fox and R. Barton Draper Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 Received May 2,1988. I n Final Form: September 9, 1988 The kinetics of the Ti02-mediatedphotocatalyzed degradation of 4-chlorophenol(4-CP),2,4-dichlorophenol (2,4-DCP),and 2,4,5-trichloropheno1(2,4,5-TCP) have been investigated. The hetergeneous photodegradation of 4-CP proceeds through formation of intermediateswhich are subsequently rapidly and totally mineralized to COPand HCl. Hydroquinone (HQ) is the major intermediate. The dependence of the photodegradation rate of 4-CP in an annular photoreactor on the initial 4-CP concentration, [4-CP], shows that saturation coverage of the TiOzparticle surface-active sites is achieved, as no further increase in the rate of degradation is obtained with further increase in [4-CP]. This indicates that the Langmuir-Hinshelwood (L-H) kinetic model appropriatelydescribes the results; as required by this model, degradation occurs on the TiOz particle surface. The photocatalytic degradation of 63 and 194 rM 4-CP follows apparent zero-order kinetics to -5040% conversion; saturation coverage of the Ti02surface by 4-CP occurs. By contrast, in an equimolar mixture (63 rM in each phenol; total [CPs] = 189 rM) of 4-CP, 2,4-DCP,and 2,4,5-TCP, the degradation of each component in the mixture follows apparent first-orderkinetics to -75% conversion. It is suggested that there is no saturation coverage of the surface of TiOz particles by 4-CP inasmuch as 2,4-DCP and 2,4,5-TCP compete effectively with 4-CP for the active sites, thereby substantially decreasing the "activen concentration of 4-CP. Comparable percent degradation (53-55%) is achieved in the multicomponent mixture and for the single-component 4-CP, under identical conditions and overall concentration.
Introduction The photocatalyzed mineralization of many chlorinated aliphatic and aromatic hydrocarbons, as well as other organic contaminants, in drinking water has been successfully Although the photocatalytic (1) Carey, J. H.; Lawrence, J.; Tosine, H. M. Bull. Enuiron. Contam. Toxicol. 1976, 16, 697. (2) (a) Hsiao, C.-Y.; Lee, C.-L.; Ollis, D. F. J. Catal. 1983,82,418. (b) Pruden, A. L.; Ollis, D. F. Enuiron. Sci. Technol. 1983, 17, 628. (c) Pruden, A. L.; Ollis, D. F. J . Catal. 1983,82,404. (d) Ollis, D. F.; Hsiao, C.-Y.; Budiman, L.; Lee, C.-L. J. Catal. 1984,88,89. (e) Nguyen, T.; O b , D. F. J . Phys. Chem. 1984,88,3386. (fJO b ,D. F. Enuiron. Sci. Technol. 1985, 19, 480. (3) (a) Matthews, R. W. J . Chem. Soc., Faraday Trans.1 1984,80,457. (b) Matthews, R. W. J. Catal. 1986,97,565. (c) Matthews, R. W. Water Res. 1986,20,569. (d) Matthews, R. W. J. Phys. Chem. 1987,91,3328. (e) Matthews, R. W. Solar Energy 1987, 38, 405. (0 Matthews, R. W. Aust. J . Chem. 1987, 40, 667. (4) (a) Al-Ekabi, H.; Serpone, N. J . Phys. Chem., in press (1988). (b) Barbeni, M.; Minero, C.; Pelizzetti, E.; Borgarello, E.; Serpone, N. Chemosphere 1988, 16, 2225. (c) Borello, R.; Minero, C.; Pramauro, E.; Pelizzetti, E.; Serpone, N.; Hidaka, H. Chemosphere, submitted. (5) (a) Pelizzetti, E.; Borgarello, M.; Minero, C.; Pramauro, E.; Borgarello, E.; Serpone, N. Chemosphere, submitted. (b) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Vincenti, M.; Borgarello, E.; Serpone, N. Chemosphere 1987, 16, 47. (c) Hidaka, H.; Kubota, H.; Gratzel, M.; Pelizzetti, E.; Serpone, N. J . Photochem. 1986,35, 219. (d) Borgarello, E.; Serpone, N.; Barbeni, M.; Minero, C.; Pelizzetti, E.; Pramauro, E., Chim. Ind. (Milano), 1986, 68, 53. (e) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello, E.; Serpone, N.; Jamieson, M. A., Chemosphere 1986,15,1913. (f) Pelizzetti, E.; Barbeni, M.; Pramauro, E.; Serpone, N.; Borgarello, E.; Jamieson, M. A,; Hidaka, H. Chim.Ind. (Milano) 1986, 67, 623. (9) Hidaka, H.; Kubota, H.; Gratzel, M.; Serpone, N.; Pelizzetti, E. Nouu. J. Chim. 1985,9,67. (h) Barbeni, M.; Pelizzetti, E.; Borgarello, E.; Serpone, N. Chemosphere 1985,14, 195. (i) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello, E.; Gratzel, M.; Serpone, N. Nouu. J . Chim. 1984, 8, 547. (j) Serpone, N.; Borgarello, E.; Harris, R.; Cahill, P.; Borgarello, M.; Pelizzetti, E., Sol. Energy Mater., 1986, 14, 121. (k) Borgarello, E.; Serpone, N.; Liska, P.; Erbs, W.; Gratzel, M.; Pelizzetti, E. Gatz. Chim. Ztal. 1985, 115, 599.
degradation of several chlorinated phenols has been reported, particularly that of 4-chlorophenol (4-CP), an indepth study of their degradation seems important, as they represent an important class of pollutants. These compounds are largely used as herbicides and fungicides." The total mineralization of these compounds to innocuous C02and HC1 (two produch from human metabolism) via heterogeneous photocatalysis mediated by illuminated Ti02is preferred; direct photolysis of these compounds by UV light generally only leads to dechlorination'Jl and to partial degradation.12 In some cases, direct photolysis has led to compounds more toxic than the parent systems.13 The determination of the effects of various kinetic factors on the photocatalytic degradation and the deter(6) (a) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M. Bull. Chem. SOC.Jpn. 1985,58,2015. (b) Okamoto,K.; Yamamoto, Y.; Tanaka, H.; Itaya, A. Bull. Chem. SOC.Jpn. 1985,58, 2023. (7) (a) Kraeutler, B.; Bard, A. J. J . Am. Chem. SOC.1978, 100, 5985. (b) Izumi, I.; Dunn, W. W.; Wilbourn, K. 0.;Fan, F.-R. F.; Bard, A. J. J . Phys. Chem. 1980,84, 3207. (8) Oosawa, Y. J. Phys. Chem. 1984,88, 3069. (9) Herrmann, J.-M.; Mozzanega, M.-N.; Pichat, P. J . Photochem. 1983, 22, 333. (10) Hustert, K.; Kotzias, D.; Korte, F. Chemosphere 1983, 12, 55. (11) Jones, P. A. "Chlorophenolsand their Impurities in the Canadian Environment"; Report No. EPS 3-EC-81-2,March 1981, Environmental Impact Control Directorate, Environmental Protection Service, Environment Canada, Ottawa, Canada. See also: 1983 Supplement Report, No. EPS 3-EP-84-3, Environmental Protection Programs Directorate, March 1984. (12) (a) Wong, A. S.; Crosby, D. G. In Pentachlorophenol: Chemistry, Pharmacology, and Environmental Toxicology; Rao, K. R., Ed.; Plenum Pres: New York, 1978; pp 19-25. (b) Wong, A. S.;Crosby, D. G. J.Agric. Food. Chem. 1981,29, 125. (13) Plimmer, J. R.; Klingebiel, U. I.; Crosby, D. G.;Wong, A. S. Adu. Chem. Ser. 1973, 120,44.
0 1989 American Chemical Society
Kinetic Studies in Heterogeneous Photocatalysis. 2.
Langmuir, Vol. 5, No. 1, 1989 251
mination of the nature of the principal intermediates form part of the focus of this study. Another aspect is the determination of whether the degradation of 4-CP occurs on the surface of the Ti02 semiconductor particles or in solution. The Langmuir-Hinshelwood (L-H) kinetic model is applied to discriminate between the two possibilities. Most of the earlier studies on the Ti02-mediated degradation of organic contaminants have dealt with the degradation of single organic compounds. It was important, therefore, to also investigate the kinetics of the heterogeneous photocatalyzed degradation of a mixture composed of 4-CP, 2,4-dichlorophenol (2,4-DCP), and 2,4,5-trichlorophenol (2,4,5-TCP). Ti02 was used in a stationary phase supported on a glass matrix forming an annular p h o t o r e a ~ t o r . ~ ~
Experimental Section Chemicals. 4-Chlorophenol and 2,4-dichlorophenol (Anachemia)and 2,4,5-trichlorophenol(Eastman) were laboratory reagent grade and were used without further purification. Acetonitrile (HPLC grade, Caledon Laboratories Ltd..), doubly distilled water, and milli-Q water were used as the solvents throughout this study. Photochemical Reactor. The reactor was composed of a Pyrex glass tube (1.5 m long and 0.8-cm i.d.) wound 13 times to form a coil, which supported 165mg of Ti02Degussa P-25 evenly distributed throughout.48The surface area of the internal surface of the coil was 374 cm2;the coating of Ti02on the surface of the coil was -0.44 mg cm-2. The volume of the coil was 75 cm3. The Ti02-coated coil was illuminated with six UV (black light) fluorescent tubes of 15 W each (lifetime -7500 h; maximum emission at 366 nm) surrounding the glass coil and mounted on a setup with back aluminum reflectors. Details about this light source are found el~ewhere.~'A short Tygon tubing was fitted to the inlet and outlet ports of the reactor to allow circulation of the solution throughout the reactor. In the continuous recirculation mode experiments, 250 mL of an air-equilibrated solution was pumped at a flow rate of 250 mL min-' (throughout). A control experiment under nearly identical conditions, but without Ti02,showed no evidence of direct photolysis, in spite of the possibility of a red-shift in the absorption spectrum of 4-CP when adsorbed on metal oxide surfaces. Analyses. The kinetics of the degradation of 4-CP (and its reaction intermediates), 2,4-DCP, and 2,4,5-TCP were followed by HPLC techniques using a Waters Associates HPLC instrument; the column was a Whatman reversed-phase octadecyl c-18. The detectionwavelengthswe-e254 and 280 nm. The eluent consisted of a mixture of water, acetonitrile, and acetic acid (70:300.2 by volume). One-milliliter aliquots were taken from the reservoir at certain time intervals during each run. The solution was analyzed by HPLC after adding enough benzoic acid (internal reference) to give a concentration of 123-164 wM. Conversion for an arbitrary set of conditions was reproducible to within 110% over the period required to complete a set of experiments.
Figure 1. Degradation of 4-CP ( 0 ) and the formation and as a function of irradiation time. Initial degradation of HQ (0) [4-CP] = 63 rM. Other conditions: initial pH 5.85; flow rate, 250 mL min-'; temperature, 30 & 2 "C.
UV illumination of an aqueous slurry of 4-CP and Ti02 produces only six detected intermediates, all of which do degrade further to C02 and HC1. Benzoquinone and dihydroxychlorobenzeneintermediates, as well as HQ, were identified, and unlike direct photolysis, the Ti02-mediated photodegradation yields no biphenyl interrnediate~.'~In the present work, other intermediates, in addition to HQ, were also detected; however, their rapid mineralization and their low concentrations made their identification difficult. The initial decrease in pH upon illumination is more pronounced than was expected from the amount of HCl produced from the degradation of 4-CP as per reaction 1. This suggests the formation of some acidic intermediate(s) (probably of the type RCOOH), which is subsequently converted to other intermediates or to the final products C02 and HC1. Langmuir-Hinshelwood Kinetics. The LangmuirHinshelwood kinetic treatment has been used widely to quantitatively describe solid-gas reactions. Recently, they have successfully been used as a qualitative model to describe solid-liquid r e a c t i ~ n s , ~In ~ Ja~previous we modified the Langmuir-Hinshelwood (L-H) equation to include the solvent which, in principle, can also compete with the organic substrate for the active sites of the semiconductor particle surface. Accordingly, the rate of a surface unimolecular reaction, where the reactant is significantly more strongly adsorbed than the product, will follow eq 2: ro = k,KC0/(1 + KCo K,C,) (2)
+
of 4-CP (63 pM) and the formation and subsequent degradation of its major intermediate, hydroquinone (HQ), with irradiation time, mediated by illuminated Ti02in the stationary phase. Direct photolysis of 4-CP by UV light (2250 nm) yields 11detected organic intermediates, some of which undergo no further degradati~n.'~By contrast,
where ro is the reaction rate, k, is the reaction rate constant, K is the adsorption coefficient (binding constant) of the reactant, Co is the initial concentration of the reactant, K, is the adsorption coefficient of the solvent, and C, is the concentration of the solvent (for water, C, 55.5 M). The effect of the initial concentration of 4-CP on the rate of degradation mediated by Ti02in a stationary phase is portrayed in Figure 2. Clearly, saturation coverage (above -150 pM) of the TiOz surface active sites is achieved, inasmuch as no further increase in the rate of degradation is obtained with further increase in [4-CP]. This supports the notion that the L-H kinetic model is appropriate, and, as required by this model, the degradation reaction proceeds on the semiconductor particle surface.
(14) Pichat, P. In New Trends and Applications of Photocatalysis and Photoelectrochemistry to Environmental Problem; Schiavello, M., Ed.; Reidel: Dordrecht, The Netherlands, in press, 1988.
(15) (a) Al-Ekabi, H.; de Mayo, P. J. Phys. Chem. 1986,90,4075. (b) Al-Ekabi, H.; de Mayo, P. Tetrahedron 1986,42,6277. (c) Hasegawa, T.; de Mayo, P. Langmuir 1986,2, 362.
Results and Discussion Photocatalyzed Degradation of 4-CP.Figure 1 illustrates the course of the degradation reaction (eq l) ClC6H40H+ (13/2)02
6C02 + 2H20 + HCl (1)
-
Al-Ekabi et al.
252 Langmuir, Vol. 5, No. 1, 1989 200
bo.
I-I
V
I
.I
0
.2 .3 .4 14-CP1, mM
.5
Assuming that the solvent competes with the reactant substrate for the surface-active sites, i n t e g r a t i ~ nof~the ~ L-H eq 2 yields
CO
\
\\-
I
'0
Figure 2. Effect of 4-CP concentration on the rate of degradation of 4-CP ( 0 )and the rate of formation of HQ (0). Conditions: initial pH 5 . 8 flow rate, 250 mL min-'; temperature, 30 h 2 O C .
K k& c + 1 + K,C, (C, - C) = 1 + K,C, t
In -
0
(3)
This expression represents the sum of zero-order and first-order rate equations, and their contribution to the overall reaction depends on the initial concentration C,. We have already d e m ~ n s t r a t e dthat ~ ~ for small concentrations of 4-CP (513 pM), the degradation reaction, conducted in the same photoreactor, follows apparent first-order kinetics. At the higher concentrations, where saturation coverage of the TiOz active surface sites is achieved (i.e., KCo >> 1 + K,C,), eq 3 reduces to the simple zero-order rate equation Co - C = k,t (4) Additional features of eq 2 are worth consideration.16 A plot of r0-l versus C0-' from eq 5 should yield a straight line with positive slope (kJPP)-l and intercept (k,)-l; here KaPP = K/(1 + K,C,), suggesting that competitive adsorption by a solvent present at constant concentration (5)
leads to a diminution of the true binding constant K to an apparent value KaPP. Analysis of the data depicted in Figure 2 yields k , = 1.2 pM min-' and a KaPP of 1.9 X lo4 M-l. Reaction intermediates (for example, hydroquinone) can also bind competitively to the catalytic active sites, and their concentrations will also change with time, inasmuch as they are eventually and completely mineralized. In this instance, eq 2 may be written in the general form 1 + KCo + CKiCi (i = 1, n ) i=l
where i is the number of intermediates formed during the photodegradation reaction (note that the summation term may also include solvent and/or other reactants). Again, eq 6 reduces to the form of where KCo >> 1 + C:&C, eq 4 and the degradation reaction will follow zero-order kinetics. However, where one or more intermediates (or reactants) strongly bind (large Ki)to the surface-active (16) Ollis, D. F.; Pelizzetti, E.; Serpone, N. In PhotocatalysisFundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley: New York, Chapter 18, in press.
Irmdiation Time, hr
Figure 3. Plots showing the degradation of 4-CP (194 pM) without previous addition of HCl (0, initial pH 5.8) and with a previous addition of HCl ( 0 , initial pH 2.5) as a function of irradiation time. This figure also shows the formation and degradation of HQ as a function of irradiation time (0). Conditions: flow rate, 250 mL min-'; temperature, 30 & 2 "C.
sites, the apparent reaction order can rise from zero to first order. Thus a reaction may initially follow zero-order kinetics and subsequently show partial-order kinetics (0 < order < 1) to first order (see below). Because of this kinetic variation, we made no further attempts to fit our data to determine the parameters of eq 2. The degradation of 4-CP shows a linear dependence of [4-CP] with time to -60% conversion (Figure 1). The reaction follows apparent zero-order kinetics; the rate constant = 0.64 pM min-' (tl12 50 min) within this conversion range.17 Complete mineralization is achieved after -2.5 h of irradiation. Similarly, when [4-CP] is 194 pM, the reaction also follows apparent zero-order kinetics to -50% conversion (Figure 3); the rate constant is 0.87 pM min-l (tl12 111 min). The pH of the solution, initially 5.8, decreased to 3.5 at this conversion. The larger rate constant obtained at the higher [4-CP] further implicates zero-order kinetics. The early negative deviation (inhibition) from apparent zero-order kinetics (Figure 3), in spite of the 4-CP (-97 pM)remaining unconsumed, must be caused by some reaction intermediate(s) and/or mineralization product(s). To the extent that the concentration of the intermediate(s) is small (however, see below) compared with the unreacted 4-CP concentration, inhibition by the intermediate(s) cannot be precluded if Ki is large. Ollis and co-workers2 have noted that C02 does not inhibit the Ti02-mediated degradation of chlorinated aliphatics. Formation of HC1 (eq 1) remains a possible cause for this inhibition, in keeping with Ollis' notion2 that HCl inhibits the Ti02mediated degradation of chlorinated hydrocarbons via two routes: (i) by competition of Cl- ions with the hydrocarbon for the active adsorption sites of the TiOz surface at low acid concentrations and (ii) by protonation of the Ti02 particle surface at high acid concentrations when the pH of the solution is IpK, of the TiOz surface.
-
-
(17) In heterogeneous catalysis, it is customary to report rates in units of rate per gram of catalyst. In photocatalysis, however, rate units should also include (if accessible) the number of catalytic sites as well as the surface area of the catalyst. Under the experimental conditions used here, we do not report rates per gram of catalyst since the catalyst was in this case supported on a glass matrix and thus not all the particles took part in the reaction. Also, the number of active sites is at present an unknown parameter in heterogeneous photocatalysis, and the exposed surface area of the photocatalyst is undetermined. To simplify, we used rate units commensurate with homogeneous phase kinetics.
Kinetic Studies in Heterogeneous Photocatalysis. 2.
120
5 0 0 k
Langmuir, Vol. 5, No. 1, 1989 253 lo/\
I
I
0 O0
I
O
* Irrad.Tirne,hrfj
8
1"
Figure 4. Plots showing the degradation of 4-CP ( 0 )and the formation of HQ ( 0 )as a function of irradiation time. Initial [4-CP] = 513 WM. Other conditions: pH 5.7; flow rate, 250 mL mi&; temperature, 30 A 2 "C.
That HC1 does indeed inhibit the photocatalyzed degradation of 4-CP appears possible as the degradation rate decreases apreciably when enough HC1 is added to bring the pH of a solution 194 p M in 4-CP from 5.8 to 2.5. Under these conditions, the reaction also follows apparent zeroorder kinetics (Figure 3), but the rate constant is 0.22 p M min-l; this is 4 times less than that obtained without previous addition of HC1 to the 4-CP. Importantly, the degradation rate of 4-CP (194 pM)is not affected when 58 mg of KC1 is added to the 250-mL solution (pH 5.8) of 4-CP to make the initial [Cl-] (=3.16 X lo9 M) equivalent to the C1- ion concentration at pH 2.5. The surface inhibition by C1- ions at low acid concentration does not appear significant. Quenching of the degradation process by C1- ions via eq 7 with OH radicals is also not significant.18
+
-
+
C1- *OH C1' OH(7) In agreement with Ollis' suggestion: protonation of the Ti02 surface at low pH may be responsible for the inhibition of Ti02-mediated degradation of chlorinated hydrocarbons. However, the details are not yet clear how protonation of the Ti02 surface inhibits the reactions. Changes in adsorption/desorption characteristics of the various surface species with changes in pH may play an important role. In contrast to the above, the situation appears different when [4-CP] = 513 p M . A t this concentration, zero-order kinetics are evident only after -30% of the 4-CP is consumed (Figure 4). At such a high concentration of 4-CP, this behavior suggests the establishment of a steady state between the reactant, the intermediates, and the products on the particle surface after -30% of 4-CP has been consumed. The maximum concentrations of hydroquinone formed from 63 and 194 p M solutions of 4-CP are 2.5 and 7 p M after 1 and 2.5 h of irradiation, respectively. After these times, -60% of 4-CP is consumed, producing -6% of HQ. In the degradation of 513 p M of 4-CP after 8 h of irradiation, the quantity of HQ produced for 60% degradation is -17 p M (-6%) at this conversion. The nearly constant yield of HQ (-6%), at all the 4-CP concentrations used, reflects the relative stabilities of 4-CP vs HQ under the photocatalytic conditions. The low concentration of HQ, as a major intermediate, implies its rapid conversion to other intermediates. Since consumption of (18) Schwarcz, H. A. J. Phys. Chem. 1986, 66, 255.
2t
4
io
I
I
I
50
Irrad. Time,min
I
60
Figure 5. Degradation of HQ as a function of irradiation time: initial [HQ], 10 wM;initial pH 3.5; flow rate, 250 mL m i d ; temperature, 30 A 2 "C.
60% of 194 p M of 4-CP (initial pH 5.8) produces 7 p M of HQ and reduces the pH of the solution to -3.5, an experiment was carried out to test the stability of HQ under these conditions. Figure 5 illustrates the rapid degradation of 10 p M of HQ in the photoreactor used in this work (apparent first-order kinetics; k = 0.034 min-') initial pH 3.5. This behavior explains why low concentrations of HQ result from the photocatalyzed degradation of 4-CP in the stationary phase photoreactor. Mechanism of HQ Formation. The formation of HQ as the major intermediate, along with benzoquinone and dihydroxychlorobenzene,under photocatalytic conditions suggests the involvement of 'OH radicals in the photodegradation of 4-CP. Hydroxyl radicals have been implicated as the reactive species in the photocatalyzed degradation of many organic c o m p o ~ n d s . ~ - ~ J ~ ~ ~ ~ The formation of *OH radicals can be achieved by two routes: (a) via reaction of the valence band holes with either adsorbed H20 or with surface OH- groups (reactions 8-10) Ti02
hv . )
hVB+
hVB+ + H20(ads)
+ eCB-
-
hm+ + OH-(suif)
'OH
-
+ H+
'OH
(8) (9) (10)
or (b) via H202from 0;-. It is generally accepted that surface-adsorbed oxygen delays the electron/hole recombination process by trapping the conduction band electron as a superoxide ion (02*-). 0 2
+ eCB-
-
02'-
(11)
Subsequent to reaction 11, Hz02can be formed from 02'via reactions 12-15.3a,2&23 02'-
+ H+
-
--*
HOP'
+ HO2' H202 + 0 2 0 2 ' - + HO2 HO2- + 0 2 H02- + H+ H202
HO2'
+
-
(12)
(13) (14) (15)
(19) Fujihira, M.; Satoh, Y.; Osa, T. Bull. Chem. SOC.Jpn. 1982,55, 666. (20) Izumi, I.; Fan, F.-R. F.; Bard, A. J. J.Phys. Chem. 1981,&5,218. (21) Harvey, P. R.; Rudham, R.; Ward, S. J. Chem. SOC.,Faraday Trans. I 1983, 79, 1391. (22) Cundall, R. B.; Rudham, R.; Salim, M. S. J. Chem. SOC.,Faraday Trans. 1 1976, 72, 1642.
Al-Ekabi et al.
254 Langmuir, Vol. 5, No. I , 1989 Scheme I OH
OH
OH I
I
I
-.
d i hydroxychlorocyclohexadienyl
4-CP
U
HQ
radical
I
\
L
OH
OH
I
I
I
I CI
8-
CI-
OH
I
Ll
I
.02
I
i
OH
0
I
It
CI'
Cleavage of HzOzby any of reactions 14-16 yields 'OH radicals.W~1922 HzOz HzOz
+ eCB-
+ 02'-
-+
HzOz
-+
+ OH+ OH- + 0 2
'OH
'OH
2'OH
(16) (17) (18)
Whatever the route for *OH radical formation, the degradation of 4-CP implicates reaction of the 'OH radical with the phenyl ring in the manner illustrated in Scheme The C1' radicals undergo subsequent reduction by the conduction band electrons eCB-(reaction 19). The rate I.24325
C1'
-
+ eCB- c1-
60
120
180
Irrad.Time, rnin
OH (HQ)
a/
i
OH
*Ob
' CI
I
\
-O5I,
Scheme I1 OH
I
\
(19)
constant for 'OH radical addition to the analogous 2chlorophenol to produce the dihydroxychlorocyclohexadienyl radical is 1.2 X 1O1O M-' s-1;z6 the radical subsequently decays either by elimination of water (k lo3 - lo5s-l)= or by dimerization/disproportionationreactions. Second-order rate constants for decay of various substituted hydroxycyclohexadienyl radicals have been reported by Buxton et al.? k 4-10 X los M-l s-l. Mechanistic and kinetic details from pulse radiolytic studies of the reaction of 'OH, N3*, and H' radicals with 2,4,5-trichlorophenol are reported elsewhere.29 The details of the degradation of HQ to COz and HzO mediated by illuminated TiOz in air-equilibrated aqueous media have yet to be established. By contrast, the formation of HQ by direct photolysis (no TiOz) of 4-CP in water14 may follow two different pathways (a and b, Scheme 11). The conditions through which these two pathways operate have recently been
-
-
(23)Herrmann, J.-M.; Pichat, P. J . Chem. SOC., Faraday Trans. 1 1980,76,1138. (24)Metelitas, D. J. Russ. Chem. Rev. 1971,40, 563. (25)Walling, C.; Camaioni, D. M.; Kim, S. S. J. Am. Chem. SOC.1978, 100,4814. (26)Getoff, N.;Solar, S. Radiat. Phys. Chem. 1986,28, 443. (27)Land, E. J.; Ebert, M. Trans. Faraday SOC.1967,63,1181. (28)Buxton, G. V.;Langman, J. R.; Smith, J. R. J.Phys. Chem. 1986, 90, 6309. (29)Draper, R. B.; Fox, M. A.; Pelizzetti, E.; Serpone, N. J. Phys. Chem. 1988,in press.
Figure 6. Semilogarithmic plot of the degradation of a mixture of equimolar quantities (63 rM each) of 4-CP, 2,4-DCP, and 2,4,5-TCP. The plots show normalized [CPs] against irradiation 4-CP; ( X ) 2,4-DCP; ( 0 )2,4,5-TCP. The data for the time: (0) photodegradation of 63 WMof pure 4-CP (taken from Figure 1) are shown here for comparison as In normalized [CCP] vs time (0). Other conditions: pH natural, -5.6-5.8; flow rate, 250 mL min-'; temperature, 30 f 2 "C. discussed.30 Pathway a operates only in the absence of oxygen and under acidic conditions; it is a minor pathway. By contrast, the homolytic cleavage of the C-C1 bond (pathway b) yields the hydroxyphenyl radical, which upon reaction with oxygen and subsequent disproportionation gives HQ and p-benzoquinone (p-BQ). This radical could, in principle, also be a precursor of biphenyl derivatives. Photocatalyzed Degradation of Chlorinated Phenols in the Multicomponent Mixture. Most of the reports so far published, regarding the Ti02-mediated degradation of organic contaminants, have dealt with the degradation of single organic compounds. It was therefore desirable to test heterogeneous photocatalysis in the degradation of a multicomponent mixture. The degradation of an equimolar mixture of 4-CP, 2,4-DCP, and 2,4,5-TCP (63 FM each), as a function of irradiation time, was carried out in a continuous recirculation mode. The results are illustrated in Figure 6. Plots of the natural logarithm of normalized concentrations of solutes versus irradiation time show good linearity to -75% conversion. The degradation of these three chlorinated phenols follows apparent first-order kinetics with nearly identical rates (4.0 x min-l). We have noted earlier in this work that the photocatalyzed degradation of 63 /IM of pure 4-CP initially follows zero-order kinetics (to -60% conversion; Figure 1). The change from apparent zero-order to first-order kinetics,31with respect to the 4-CP degradation, results (see also above) from the lack of surface saturation coverage by 4-CP when equimolar quantities of 2,4-DCP and (30)(a) Boule, P.; Guyon, C.; Tissot, A.; Lemaire, J. In Photochemistry of Enuironmental Aqwtic Systems; Zika, R. G., Cooper, W. J., Eds.; ACS Symposium Series 327;American Chemical Society: Washington, D.C., 1987;Chapter 2,pp 10-26. (b) Boule, P.; Guyon, C.; Lemaire, J. Chemosphere 1982,11, 1179. (31)The resulta reported here (and earlier") were obtained under similar experimental conditions and thus are to be considered internally consistent; the conclusions and the discussions are not affected. A problem with the values presented here for k,, P , and the apparent rate constants throughout is that these resulted from photodegradation of organic substrates in a continuous recirculation mode in a photoreactor in which TiOBwas present in a stationary phase. Consequently, external mass-transfer kinetics may have partially masked (in the extreme case, mass transfer could be rate-determining) the true intrinsic surface kinetics. That is, the values reported here may embody both surface kinetics and mass-transfer kinetics. Work is in progress to establish the extent to which the latter contributes to the apparent rate constants.
Langmuir 1989,5, 255-260 2,4,5-TCP are also present with 4-CP in the solution. These two chlorinated phenols, DCP and TCP, compete effectively with 4-CP for the active sites on the semiconductor surface, leading to a significant decrease in the concentration of “activen 4-CP on the surface. Finally, it must be pointed out that while the degradation follows apparent first-order kinetics in the equimolar mixture of 4-CP, 2,4-DCP, and 2,4,5-TCP (total initial concentration = 189 pM) OP irradiation for 2 h, -55% (104 pM) of these pollutants is decomposed. For comparison, the degradation of 4-CP (concentration = 194 pM) with apparent zero-order kinetics also achieves -53% (103 pM) decomposition after irradiation for 2 h under similar conditions. Thus, the photocatalytic capacity of the stationary phase photoreactor is the same for the degradation of 4-CP alone and in a three-component mixture of 4-CP, 2,4-DCP, and 2,4,5-TCP.
Conclusions In contrast to the photochemical route (direct photolysis), the photocatalytic degradation of 4-CP is associated with fewer intermediates which are rapidly and totally mineralized. This confirms and gives additional support to the potential interest of heterogeneous photocatalysis in water detoxification. The degradation of 63 and 194 pM of pure 4-CP in our photoreactor follows apparent zeroorder kinetics to -50-60% conversion. The results are
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explained in terms of the saturation coverage of the surface by 4-CP. However, for an equimolar mixture of 4-CP, 2,4-DCP, and 2,4,5-TCP (63 pM each; total initial concentration = 189 pM) the degradation of each component of the mixture follows apparent first-order kinetics to 75% conversion. These results are understood in terms of the lack of saturation coverage of the surface by 4-CP as 2,4-DCP and 2,4,5-TCP compete effectively with 4-CP for the active sites. Overall, a comparable quantity of the pollutants is degraded in the mixture. The future challenge will be to examine the degradation of ”real” industrial mixtures in which numerous organic contaminants are present.
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Acknowledgment. We are grateful to the following agencies for generous support of our work Natural Science and Engineering Research Council of Canada (NS); Consiglio Nazionale delle Ricerche, Roma, (EP); Army Research Office (MAF); and NATO through Grant No. 843184 that provided funding for the collaboration between the three laboratories. We also thank Bert Patterson for allowing us the use of the HPLC chromatograph in the course of this work and Prof. David Ollis (Chemical Engineering Department, North Carolina State University) for very useful discussions regarding mass-transfer kinetics. Registry No. 4-CP, 106-48-9;2,4-DCP,120-83-2;2,4,6-TCP, 95-95-4;HQ, 123-31-9;Ti02, 13463-67-7.
Luminescence Probe Studies on Naked and Cadmium Sulfide Modified Cellulose Matrices Krishnan Rajeshwar* Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065
Masao Kaneko* The Institute of Physical and Chemical Research, Wako, Saitama 351 -01, Japan Received July 1, 1988. In Final Form: September 21, 1988 Steady-state and transient luminescence measurements were carried out on two probe molecules, namely, pyene and Ru(bpy)s2+(bpy = 2,2’-bipyridyl),in a cellulose matrix. To assess microenvironmental effects within this matrix, the R ~ ( b p y )probe ~ ~ + was employed in neat form and as a polymer-bound complex. Alterations in the photophysicalbehavior of the two probes were also examined after dispersion of CdS semiconductor particles in the cellulose matrix. Sensitization of the -520-nm luminescence from CdS was observed with parallel quenching of the yrene emission from the cellulose matrix. CdS was also efficient in quenching luminescence from R ~ ( b p y )within , ~ this matrix, although the 520-nm emission was absent in this case. This was attributed to the mediation of holes from CdS in the quenching process. Mirroring trends observed in previous studies, quenching efficiencies for molecules such as O2were drastically lowered for both the probes in cellulose relative to the situation in homogeneous media. Transient data from the analyses of luminescence decay curves corroborated the trends observed in the steady-state measurements.
Introduction Previous work by one of the present authors (M,K,)1as well as studies by Thomas a d collaborators2 have clearly established cellulose as a useful matrix for performing (1) (a) Kaneko, M.; Motoyashi, J.; Yamada, A. Nature 1980,285,468. (b) Kaneko, M.; Yamada, A. Makromol. Chem. 1981, 182, 1111. (c) Kaneko, M.; Yamada, A. Photochem. Photobiol. 1981,33, 793. (2) (a) Milosavljevic, B.; Thomas, J. K. J. Phys. Chem. 1983,87, 616. (b) Milosavljevic, B.; Thomas, J. K. Inst. J.Radiat. Chem. 1984,23,237.
(c) Milosavljevic, B.; Thomas, J. K. Macromolecules 1984,17,2244. (d) Milosavljevic, B.; Thomas, J. K. Chem. Phys. Lett. 1985, 114, 133. (e) Milosavljevic, B.; Thomas, 3. K. J. Chem. Soc., Faraday Trans. 1 1985, 81, 735. (0Milosavljevic, B.; Thomas, J. K. J. Phys. Chem. 1985, 89, 1983. (g) Milosavljevic, B.; Thomas, J. K. J . Am. Chem. SOC.1986,108, 2513.
0743-7463/89/2405-0255$01.50/0
photophysical and photochemical experiments. For example, mechanical constriction Of the active mOleCUleS largely precludes diffusion complications in the study of electron-transfer phenomenae2Immobilization matrices such as cellulose have the useful attribute of excluding extraneous quenchers such as O2 from interaction with thi photoluminescent molecules. This was demonstrated in an early study3 via room temperature phosphorescence emission from arenes adsorbed onto cellulose. In this paper, we present the steady-state and transient luminescence of two probe molecules, namely, pyrene and Ru(bpy),Cl2(bpy = 2,2’-bipyridyl),in a cellulosic medium. (3) Schulman, E. M.; Walling, C.
J.Phys. Chem. 1973, 77,902.
0 1989 American Chemical Society
