Environ. Sci. Technol. lSS3, 27, 1681-1689
Photocatalytic Degradation of Pentachlorophenol on Ti02 Particles: Identification of Intermediates and Mechanism of Reaction German Mlllst and Michael R. Hoffmann' Environmental Engineering Science, W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 9 1125
The photoassisted oxidation of pentachlorophenol (PCP) in Ti02 particle suspensions was investigated. Complete dechlorination of 47 pM PCP was achieved after 3 h of illumination at high intensity with apparent quantum efficiencies (@pep, (Pel-, a"+,a1-1~0~) ranging from 1to 3%. p-Chloranil, tetrachlorohydroquinone, HzOz, and o-chloranilwere formed as the principal intermediates. Formate and acetate were formed as products during the latter stages of photooxidation. The mechanism for photooxidation of PCP proceeds via hydroxyl radical attack on the para position of the PCP ring to form a semiquinone radical which in turn disproportionates to yield p-chloranil and tetrachlorohydroquinone. Under high-intensity illumination, the reaction intermediates are attacked further by 'OH to yield HC02-, CH&02-, C02, H+, and C1-. Introduction Photocatalytic oxidation of organic compounds is of considerable interest for the elimination of hazardous wastes (1-3). The complete mineralization (Le., oxidation to C02 and H20) of a variety of aliphatic and aromatic chlorinated hydrocarbons via heterogeneous photooxidation on Ti02 has been reported (2-27). For some systems, intermediate reaction products have been identified, and reaction mechanisms have been proposed ( I , 3,5-12,1625,27-30). Pentachlorophenol (PCP) has been used widely as a pesticide and a wood preservative (28). The photooxidation of PCP in the presence of Ti02 particles has been reported previously (18-24, 31): 2HOC,C1,
+ 70,
-
hv,Ti02
4HC0,H
+ 8C02 + lOHCl
(1)
In homogeneous solution, photolysis of PCP has been shown to produce toxic byproducts (32, 33) such as tetrachlorodioxins (32). In this paper, we report on the results of a kinetic investigation of the photodegradation of PCP in aqueous Ti02 suspensions which was aimed at providing mechanistic insight into semiconductor photocatalysis. This information is important for an assessment of the feasibility of semiconductor photocatalysis as an alternative method for the treatment of hazardous wastes. Experimental Section
Apparatusand Analysis. The irradiation source was a Kratos 450-W Xe arc lamp. A combination of a CuSO4 water filter, a glass IR filter, and a Corning GS-7-60-1 filter (bandpass 330-370 nm) was employed to limit heating of the sample and to prevent direct photolysis of the substrate. Actinometry was performed using (E)-2-[1-
* Author to whom correspondence should be addressed.
Present address: Department of Chemistry, Auburn University, Auburn, AL 36849. 0013-938X/93/0927-1881$04.00/0
0 1883 Amerlcan Chemical Society
(2,5-dimethyl-2-furyl)ethylidenel-3-isopropylidenesuccinic anhydride in toluene (34, 35). Light intensities were varied using neutral density filters (34). No corrections were made for light losses resulting from light scattering in the Ti02 suspensions or for the lower sensitivity of the actinometer at X > 370 nm (an uncertainty of 12 % in the light intensity was estimated from the transmission spectra of the Corning filter). All irradiations were carried out with air-saturated suspensions under constant stirring; the suspensions were equilibrated for at least 15 m prior to irradiation. In all experiments the [Ti021 was 0.2 g L-I, and the ionic strength was maintained constant at p = 0.001 M with NaC104. Determination of [H+] was carried out in a 70-mL suspension with a radiometer autotitration system. Chloride ion concentrations were monitored in situ with a radiometer K601 mercurous sulfate reference electrode and chloride ion-selective electrode. Potential measurementa were performed after interrupting the light to avoid artifacts induced by direct illumination of the pH and reference electrodes. The equilibrium potential (in the dark) was established within 1min. The potentiometic [Cl-I was verified by ion chromatography. Aliquots were extracted for the simultaneousanalysis of several products. Illuminated suspensions (35 mL) were divided as follows: 20 mL was extracted with ether, and 100pL of the extract was analyzed by HPLC (Hewlett-Packard Model 1084B). In addition, the ether extract was analyzed spectrophotometrically. The remaining 15mL were used for analysis of H+, C1-, organic acids, and H202. PCP and p-tetrachlorobenzoquinone (p-chloranil) concentrations were determined by HPLC using an Alltech Spherisorb ODs-2 (5 pm) reversed-phase column with an eluent of 60% methanol and 40% water. Organic acids and C1- were measured with a Dionex 2020i ion chromatograph using a Dionex AS4-A column with a 5 mM Na2B40, solution or a mixture of 2.8 mM NaHC03 and 2.2 mM Na2C03 as eluents. Hydrogen peroxide concentrations were determined spectrophotometrically with iodide or by enzyme fluorescence techniques (34). Fluorescenceanalyses were performed on a ShimadzuRF-54Q,and UV-VIS absorption spectra were measured on a Shimadzu MPS-2000 spectrophotometer. Materials.Ti02 was obtained from Degussa (P-25; pH,,, = 6.8, D, = 30 nm, A, = 50 m2 g-l). All other chemicals were of the highest purity reagent grade available. The purity of the chlorinated quinones and hydroquinones was verified by melting point determination. Water employed in all preparations was purified by a Milli-Q/RO system (Millipore) to a resistivity of 18 MOScm. Stock solutions of PCP were prepared by heating (-50 "C) 0.5g of PCP in 2 L of water for 4 days in the dark. The resulting solutions were filtered twice through 0.2pm Nuclepore filters. The absence of degradation products in the stock solutions was confirmed by UV-VIS, IC, and HPLC analyses. Extinction coefficients of the pentachlorophenoxide ion measured in 20 mM NaOH were deterEnviron. Scl. Technoi., Vol. 27, No. 8, 1893 1881
PCP DEGRADATION
PCP DEGRADATION
HPLC 254 and 290 nm PCP 50
A
54 pM PCP,
P-MORANIL
I
[H'I
pH=5,
1,=190 pM(hv)/min A
[CHLORIDE]
20 1
I
O.?\
"I \ 20
0
IRRADIATION TIME (rnin)
Flgure 1. Degradation of PCP and formation of pchloranllas function of irradlatlontime in a suspenslonof 0.2 g L1T102at pH = 5 with [PCP] = 47 pM and I, = 1.3 X lo4 M hu min-I. (0)PCP; (A)pchioranil.
mined to be 6249 = 1.0 X lo4 M-l cm-l at X = 249 nm and €320 = 5.12 X 103 M-1 cm-1 at X = 320 nm; these values are in good agreement with previously reported extinction coefficients (32).A limiting solubility of PCP in water of 60 pM was determined spectrophotometrically. Stock solutions of p-chloranil were prepared by heating (=50 "C) 0.02 g of the solid in 1L H2O for 4 h in the dark. The twice-filtered solutions were used within 3 days of preparation and stored at 4 "C in the dark to avoid decomposition. An extinction coefficient of e = 1.74 X lo4 M-' cm-l at h = 292 nm and a solubility of 13 pM were determined for p-chloranil in water. Results Illumination of a solution containing 47 pM PCP and 0.2 g L-l Ti02 at pH = 5 resulted in a rapid initial decrease in the concentration of PCP as shown in Figure 1. The concentration of PCP varied linearly with irradiation time between 0 and 15 m. From the linear portion of this plot, an apparent quantum efficiency (@pep) of 1.3% for the loss of PCP was determined; apparent quantum efficiency was defined as initial rate of degradation divided by theoretical maximum rate of photon absorption (assuming that all photons are absorbed by the Ti02 and that actual light-scattering losses out of the reaction cell are negligible) as determined by chemical actinometry. This definition for species i can be expressed as follows:
where @xi= the apparent quantum efficiencyfor chemical species, Xi; d[Xil/dt is either the initial rate of formation or loss of chemical species, Xi; and d[hv]/dt is the incident photon flux per unit volume. Sufficient Ti02 is present at t = 0 in a reactor cell with a 10-cmpath length to totally absorb all incident photons; however, it should be noted that some of the incident photons are lost by light scattering in the turbid Ti02 suspensions. At long irradiation times, the concentration vs time profiles as shown in Figure 1 became nonlinear. The complete disappearance of PCP was observed after 3 h of irradiation. These results agree fairly well with previous observations (18-24). As shown in Figure 1,p-chloranil was detected as an intermediate of the degradation of PCP. 1682 Envlron. Scl. Technol., Vol. 27. No. 8, 1993
TIME (rnin)
Flgure 2. Formation of CI and H+ as a function of illumination tlme In a TiOn suspension contalning 47 pM PCP at I, = 1.9 X lo4 M hu mini with the same conditions as in Figure 1. (A)Chloride Ions; (0) H+.
However, the data points (A) of Figure 1 represent the sum of the concentrations of p-chloranil and tetrachlorohydroquinone (TCHQ),the reduced form ofp-chloranil. The reasons for combining the two concentrations are as follows (a) spectrophotometric analysis of samples extracted with ether immediately after irradiation failed to show a strong absorption at 284 nm (e = 1.77 X 104 M-1 cm-9, characteristic of p-chloranil. Instead, a weak and broad absorption between 280 and 320 nm, corresponding to PCP, TCHQ, and other products, was observed. The high extinction coefficient for p-chloranil in ether allowed its detection at concentrations 110 pM in the control samples containing PCP, but not in irradiated samples (a similar concentration was detected after 20 min of illumination, see Figure 1). (b) As will be shown below, TCHQ decayed in water by air-oxidation to p-chloranil and also by dechlorination. (c) After storage of the irradiated samples a t low temperature for several days and extraction with ether, the characteristic absorption of p-chloranil at 284 nm was observed. Under these conditions, the decomposition of TCHQ through the loss of C1-ions was negligible. This method was used to obtain the concentrations of p-chloranil shown in Figure 1. A qualitative determination of TCHQ by HPLC was not possible because of the partial decomposition of this compound in the eluent. An additional complication arose from the similarity in absorption wavelengths and retention times of TCHQ and the isomer tetrachlorocatechol (TCC). An apparent quantum efficiency of @ = 0.4% was obtained from the linear increase of [p-chloranill/ [TCHQI with respect to time between 0 and 20 m. The concentration of p-chloranillTCHQ reached a maximum of 17 p M at 50 m of illumination and decreased at longer irradiation periods. These compounds disappeared after about 2 h of illumination. Weak absorption signals at 350 and 450 nm gave an indication for o-tetrachlorobenzoquinone (0-chloranil),and a HPLC peak was detected for TCC. However, their low steady-state concentrations and the instability of both compounds in water precluded a precisequantitative analysis. Other possible intermediates such as the tetrachloro- and trichlorophenols were not detected. Figure 2 shows the formation of chloride and hydrogen ions upon illumination of Ti02 suspensions with PCP.
pH = 5
[PCP] = 47 pM FORMATE
A
+ IODIDE
IODIDE TEST
PCP PHOTODEGRADATION
A
TEST
FLUORESCWCE
TEST
ACETATE
20
15
10
5
0
10
20
30
40
50
60
70
J
80
TIME (mid
0
2.00
TIME (mini Fbwo 3. Formatlonof acetateand formate as a functionof illumination time in a Ti02 suspension containing 47 pM PCP at Io = 1.3 X lo4 M hv mini (vMe supra Figure 1). (A)Acetate: (0)formate.
Apparent quantum efficiencies of @cl-= 2% and @H+ = 1.4% were determined from the initial slopes of these curves. The ratio of C1- ions produced per PCP molecule degraded increased gradually from an initial value of 2 to about 4 in the first 2 h of illumination. Chloride ion concentrations equivalent to a complete dechlorination of PCP were detected only after 3 h of irradiation. In contrast to the decay of PCP, the formation was zeroorder only up to 4 min of illumination; a t longer reaction times, C1- and H+ increased with an apparent first-order fashion. However, as shown in Figure 2, [H+l< [Cl-I. On the other hand, illumination of PCP solutions a t pH values lower or higher than 5 led to ratios of [H+l to [Cl-I equal to 1. These results suggest that PCP with a pKa = 4.8 was able to provide some buffering capacity at short irradiation times. At longer irradiation times, the products from the degradation of PCP appeared to provide some additional buffering capacity. For example, acetate and formate (Figure 3) were detected as products after an induction period of 40 m with the attainment of apparent steadystate concentrations of 17 and 12 pM,respectively, after 90 m of illumination. Fumaric, maleic, oxalic, and chloroacetic acid were not detected as reaction products. Hydrogen peroxide was formed as an intermediate during the photooxidation of PCP on TiO2. Figure 4 illustrates the formation of H202 at two different light intensities. The results presented in Figure 4a were obtained with two independent analytical methods (34). This double determination was necessary to avoid interferences from the other products, especially o-chloranil, p-chloranil, and organic peroxides. Both quinones reacted with I-, while o-chloranil oxidized iodide very fast and quantitatively. The reaction between p-chloranil and Itook several hours to go to completion. In contrast, the fluorescence method is insensitive to the quinones. Organic peroxides may also be formed during the TiO2catalyzed degradationof PCP. The fluorescencetechnique responds to both H202 and organic peroxides. Hydrogen peroxide reacts rapidly (-30 s) with I-, whereas the formation of Is- from organic peroxides requires approximately 1 h to go to completion (34). The concentrations of H2Oz shown in Figure 4 (a and b) were determined from the amount of 13- formed within the first 2 min after mixing the filtered sample aliquots and the I- reagent solution. No further increase of 113-1 a t longer reactions times was
f 3
TIME (hr) Figure 4. Hydrogen peroxide formation in a TiO2 suspension with [PCP] = 47 pM, (a) Curve 1: I,, = 1.9 X lo4 M hv minl. (0)Iodide method; (A)fluorescence method. Curve 2: Io = 1.3 X lo4 M hv minl. (b) Single-wavelengthirradiation at 366 nm, 1, = 7.3 X lo6 M hv mini.
noticed. These results indicate that organic peroxides were absent. Good agreement between the two alternative methods shown in Figure 4 provides further indication that o-chloranil and p-chloranil did not have significant lifetimes as intermediates. An apparent quantum efficiency of @ H ~ o=~1 % was calculated from the initial slopes of the curves in Figure 4a. The concentration of H202 reached a maximum of 15 p M after 20 min of high-intensity illumination (curve 1 of Figure 4a). However, further irradiation resulted in lower peroxide concentrations. At lower light intensity (curve 2 of Figure 4a), smaller [H2021s were observed at all irradiation times. Hydrogen peroxide concentrations reached a maximum of 10.3 pM at 10 m of illumination, followed by a steep decrease with further illumination. Figure 4b illustrates the formation of H202 of Ti02 suspensions illuminated at a single wavelength of irradiation at X = 366 nm. A relatively slow generation (@H~o~ = 0.04% of hydrogen peroxide was observed under these conditions. Higher concentrations of hydrogen peroxide than those shown in Figure 4a (curve 1) were detected when preirradiated samples were stirred (“aged”) in the dark for various lengths of time. Figure 5 shows an apparent “postirradiation” formation of H202 vs time from PCPcontaining Ti02 suspensions illuminated for a total of 10 min. The concentrations of H202 shown in Figure 5 were calculated by subtraction of the [H~021obtained immediately after 10 m of illumination (11.5 p M , Figure 4a) from the [H2021 measured after “aging”. A maximum of [HzOzl was detected after 60 m of “aging”, followed by a Envlron. Scl. Technol., Vol. 27, No. 8. 1993 1683
10 I
PHOTODEGRADATION OF PCP
I
I
I
[PCP] = 47 pM, 0
Ti0,
0.2 gll
-I
o
60
120
180
Io (pMm i d )
@PCP (% )
@a-(% )
@H+ ( % )
@H,o,( % )
1906 130b 6.7‘
1.0 1.3 1.3
2.0 2.7
1.4
1.0 1.0 0.04
1.6
\ I
’I
01 0
L1 TiOz, pH = 5.
\
2t
4
8
12
16
20
24
28
I
32
l o ( 1 0 ~ M/ rnin) Figure 6. Initial apparent quantum efficiency for Ci ion formation as a function of light intensity. [PCP] = 48.5 pM and 0.2 g Li TiOp at pH 5.
drop in [H202] with increasing “aging” times. Postirradiation effects were not observed at lower light intensities. Results obtained from experiments with different light intensities are summarized in Table I. With the exception , is small at low I, but did not change at of @ H ~ o ~which higher light intensities, the apparent quantum efficiencies for consumption of PCP or generation of H+and C1- ions decreased with increasing I, from 1.3 X 10-4 to 1.9 X 10-4 M min-1. As shown in Figure 6, @cl-was found to be independent of I, at low light intensities (I,I 5.9 X min-1) and inversely dependent on I, at higher intensities. @c1-was not found to be proportional to (d1,I-las would be predicted for photo-inefficiencies that are dominated by electron-hole recombination (36, 37). Parlar and Korte (38)have suggested that the photochemistry of PCP is enhanced as a result of the red shift of its absorption spectrum upon sorption to metal oxide surfaces. In order to address this possibility, we studied C1- formation by photolysis of 47 pM PCP in the presence of suspensions of 0.2 g L-’ Si02 (Aerosil380 from Degussa, 1684 Envlron. Scl. Technol., Vol. 27, No. 8, 1993
[NaCIO,] + SiQ 0.2 g/i
= 1 rnM A
p-chloranil 13 UM
300
Table I. Experimentally Determined Apparent Quantum Efficiencies for Pentachlorophenol Degradation.
-
No Tt02
I
240
TIME (mid Figure 5. Postirradiation effect observed in a TiO2 suspenslon preirradiatedfor 10 m with [PCP] = 47 pM and I,,= 1.9 X lo4 M hv mini.
“47 pM PCP, 0.001 M NaC104, 0.2 g Multiwavelength irradiation. C X = 366 nm.
A
TIME (mini Figure 7. Formation of chloride ions as a function of irradiation time in solutions containing PCP or pchioranii. (A)47 pM PCP, no T102, I,, = 1.0 X I O 4 M hv minl; (+) 47 pM PCP, 0.2 g L1 Si02, I,, = 1.0 X lo4 M hv mini; (0)46 pM PCP, 0.2 g L1 T102, I,, = 1.2 X lo4 M hv min’; (A)13 pM pchloranii, 0.2 g L1 T102, pH = 5, I,, = 7 X 10’ M hv min’.
D, = 7 nm, BET surface area = 380 m2 g-l) as shown in Figure 7. In both cases, the concentration of C1- ions increased linearly with time with 0~1-= 0.2%. For comparison, the C1- production rate in a irradiated Ti02 suspension ([PCPI, = 47 pM) is also shown in Figure 7. In this case, @c1-= 2.8 76 for short irradiation times. From these results, it appears that surface-enhanced direct photolysispathway for PCP degradation is small compared to the semiconductor-driven Ti02 pathway. The photocatalytic degradation of the intermediate product, p-chloranil, at pH 5 is also shown in Figure 7. In this case, C1- was released rapidly, with (d[Cl-l/dt,) comparable to that observed for PCP, even though it was exposed to a lower incident light intensity (I,, = 7 X 106 M min-l). In order to evaluate the competitive effect of p-chloranil on the heterogeneous photodegradation of PCP, a solution containing TiOz, 30 pM PCP, and 7.5 pM p-chloranil was illuminated at Io= 7.5 X M min-l, and @cl-= 6.2 % was obtained. Similar experiments performed with PCP alone and p-chloranil alone led to apparent quantum efficiencies of @pa- = 3.1% for 30 pM PCP and @cl-= 3.9% for 7.5 pM chloranil, respectively. Only small concentrations of o-chloranil were observed in irradiated solutions of PCP. These small concentrations were due, in part, to the spontaneous decomposition of o-chloranil in water. Chloride and hydrogen ions were generated spontaneously when solid o-chloranilwas added to a Ti02 suspension containing 47 pM PCP to make a 20 pM solution of the chloroquinone. Chloride formation as a function of dark reaction time is illustrated in Figure 8 (curvea). After an induction period of about 15min, [Cl-I increased following apparent first-order kinetics. Complete dissolution of o-chloranil was observed after 2 h of reaction. A faster dissolution of o-chloranil in water was achieved by adding 200 pL of a CC4 solution containing 32.5 mM o-chloranil to 500 mL of a Ti02 suspension with PCP; the final concentration of the chloroquinone was 13 pM (Figure 8b). After the dissolution of the CC4 solution was completed (-4 min), an exponential increase of [Cl-I was observed. This latter process was completed after 15 min of reaction. A first-order rate constant of 121 = 1.3 X 10-2 min-’ was obtained assuming that one C1- ion was
2.00 c
3
-s1
u
1.50
1.00
I
o-chloranil +
o-chloranil A
o- 12+M A
20
13
with HzOz
w
w
TETRACKOROHYDROQUINONE
p- i3p.4 with H Z 0 2
I
'
I
c
t
u
A
TETRACKOROCATECHOL
d
l 4 Y
0.50
-
0
2
0
20
40
60
80
100
1 10
TIME (mid
TIME (hr) Flgure 8. Thermal decay of tetrachioroquinones In Ti02 suspenslons containing 47 pM PCP at pH = 5. (0)c-chloranil; (+) 13 pM solutlon of o-chioranll; (A)13 pM c-chloranll, 20 pM H202;(A)13 pM pchloranii, 20 pM H202.
Flgure 9. Chloride Ion formation from the thermal decay of tetrachlorohydroquinones in T102 suspensions with [PCP] = 47 pM. (0) 13 pM tetrachlorohydroquinone; (A)13 pM tetrachlorocatechol.
produced by the decay of the chloroquinone. At longer reaction times, the decompositionof o-chloranilproceeded via a different first-order process (Le., decomposition appears to be biphasic) with a rate constant of k2 = 4.3 X 1O-amin-1. Similar results were obtained when o-chloranil was dissolved in plain water. Thus, other possible sources for C1- ions such as the reaction of o-chloranil with PCP or the TiOz-mediateddechlorination of o-chloranil can be excluded. In the absence of PCP and TiO2, o-chloranil was followed spectrophotometrically as a function of reaction time by monitoring the disappearance of the absorption bands at 450 and 350 nm. At short reaction times, the absorbance changes were too small to allow a reliable determination of kl but a t longer times the decrease in o-chloranil was first-order with a rate constant of k2 = 3.6 X 1o-Smin-l. This value is in agreement with the value obtained from potentiometric data. An even faster generation of C1- ions was detected when o-chloranil was dissolved in the presence of TiO2,47 pM PCP, and 20 pM H202; results of these experiments are presented in Figure 8 (curve c). The increase of [Cl-I was exponential up to 42 min with a first-order rate constant of 3.6 min-l. No simple kinetic relationship was followed at reaction times longer than 42 min. Curve d of Figure 8 shows the slow generation of C1- (with a lag time of 1.6 h) as a function of time in a Ti02 suspension containing 47 pM PCP, 13 pM p-chloranil, and 20 pM H202. After the kinetics of the thermal decay of o-chloranil in water were determined, experimentson the photooxidation of this compound in the presence of Ti02 particles were performed. Accelerated dissolution of the chloroquinone was achieved by gently heating (-50 OC) an aqueous suspension in the dark for 15 min. Illumination of the resulting solution (12 pM o-chloranil) with Ti02 produced a rapid decomposition of the chloroquinone. An initial apparent quantum efficiency for C1- production (@c1-= 6.7%) was calculated after correction for the thermal decay of o-chloranil. The stability of other compounds produced as intermediates during the photocatalyzed oxidation of PCP in water was investigated. TCHQ is partially oxidized by 0 2 to p-chloranil as shown by a decrease in the absorption band of TCHQ at 308 nm with a simultaneous increase in the absorption band characteristic of p-chloranil at 284 nm (in ether). Chloride ions were detected during the decay of TCHQ as shown in Figure 9 (curve 1)for a 13 pM
solution of TCHQ containing 47 pM PCP and TiOz. Spectrophotometric analysis of the ether extract after 18 min of reaction indicated a 25% conversion of TCHQ to p-chloranil based on the absorption spectrum of the chloroquinone. Small amounts of C1- ions were released after dissolving TCC in water with the same method used for TCHQ (Figure 9). However, decay of TCC appeared to be relativelyrapid in that a blue-black solid precipitated after 5 min of reaction with a corresponding decrease in the absorption band of TCC at h = 296 nm (in ether) and with the appearance of a broad absorption band overlapping the o-chloranil absorption (A = 340 nm in ether). The decay of organic compounds by photolysis of Ti02 particles has been described in terms of reactions between photogenerated trapped holes, surface-bound 'OH (presumably as >TiOH;) or freely-diffusing 'OH radicals and the organic substrates (I). Several experiments were conducted in the presence of known hole or 'OH radical scavengers with the aim of exploring reaction pathways which do not involve oxidizing species. Ti02 suspensions containing 200 mM sodium formate or 2-propanol and PCP andp-chloranil were used for this purpose. An initial apparent quantum efficiency of @c1-= 1.2 % was obtained for the irradiation of a 20 pM solution of PCP with Ti02 at pH = 5 in the presence of 200 mM formate. A higher apparent quantum efficiency for C1- ion liberation of @cl= 1.6 % was obtained when a similar solution was irradiated at pH = 3. Illumination of a 13pM solution of p-chloranil with formate produced chloride ions with an initial apparent quantum efficiency of 0.6 % In addition, TCHQ was detected spectrophotometrically during the course of this reaction. An illuminated Ti02 suspensionwith HC02as an electron donor released C1- ions from TCHQ faster than from the spontaneous decay of TCHQ in water. After correcting the experimental results for the latter reaction, an initial apparent quantum efficiency of ipcl- = 1.1% was obtained. Similar amounts ofp-chloranil to those detected during the thermal decay of TCHQ in water were observed in this experiment. Illumination of 13 p M TCC with formate and Ti02 yielded C1- ions with an initial apparent quantum efficiency of 1% after correction for the thermal decay of TCC in water. Traces of o-chloranil and a broad band centered a t 275 nm (in ether) were detected during this reaction. Similar results were obtained when formate was replaced by 2-propanol. [Cl-I vs time was found to be linear with irradiation time in all of these experiments.
.
Environ. Scl. Technol., Voi. 27, No. 8, 1993 1686
Discussion The photocatalytic degradation of PCP on Ti02 particles is an apparent zero-order reaction for more than one halflife at [PCPI 123 pM. Likewise, the formation of the intermediatesp-chloranil and TCHQ, which are the direct oxidation products of PCP, is zero order for the first 15 min of illumination. When [PCPI 123 pM or at illumination times longer than 15min, the intermediate reaction products compete with PCP for the photogenerated oxidizers and simple zero-order kinetics are no longer observed. This conclusionis supported by the experiment in which a mixture of 30 pM PCP and 7.5 p M p-chloranil was irradiated in the presence of TiO2. In that experiment the initial apparent quantum efficiency for C1- ion formation of Ocl- = 6.2 % was found to be close to the sum of apparent quantum efficiencies obtained by irradiation of each component separately (3.1% 3.9% = 7 % ). The lower apparent quantum efficiency for the illuminated mixture can be explained in terms of preferential adsorption of PCP on Ti02 and the lower apparent quantum efficiency for PCP degradation compared to p-chloranil. The formation of C1- and H+ ions was found to increase in a linear fashion during the first 5 m of illumination as shown in Figures 2 and 8;the linear increases were followed by an exponential increase at longer periods of illumination. The average number of C1- ions released per PCP molecule degraded was initially 1.8; similar to the ratio of initial apparent quantum efficiencies, Ocl-/Opcp = 2 (as shown in Table I). This impliesthe formation of additional chloride ions by the fast decay of some unstable products, since only one C1- ion would be released when PCP is transformed to p-chloranil, o-chloranil, TCHQ, or TCC. The initial apparent quantum efficiency of unstable products (0-chloranil and TCC) is calculated after subtraction of O ~ c ~ o r ~ i l ( O . from 4 % ) Opcp (1.3 %) to give @, = 0.9% at I, = 1.3 X 10-4M min-l. If the formation of one C1- ion per each PCP molecule oxidized plus one C1- ion from each molecule of unstable product is assumed, the estimated initial apparent quantum efficiency is Ocl- = 1.3% + 0.9% = 2.2 % . The fact that the calculated Oclis lower than the experimental value of Ocl- = 2.7% is reasonable since additional C1- ions were formed via the parallel thermal decay of o-chloranil and TCC. At extended illumination times, the ratio of C1- ions produced per PCP molecule destroyed increased to 4 at 60 min of total illumination. The gradual degradation of several yet unidentified intermediates carrying 2-3 chlorine atoms is suggested by these results. Formation of significant amounts of formate and acetate was observed only after 40 min of illumination. Thus, ring fragmentation appears to be a slowprocessand probably occurred between carbon atoms of the ring which had no chlorine atoms, since chloroacetic acid was not found among the products. The formation of acetate appears to involve a reduction of carbon centers and probably proceeds via disproportionation reactions of free radical intermediates as proposed for a likely ring-fragmentation biradical:
+
1686 Envlron. Scl. Technol., Vol. 27, No. 8, 1993
Hydrogen peroxide formation during the illumination of Ti02 has been reported for several reactions (21,12,34, 37,391. Irradiation of PCP solutions with Ti02 results in initial apparent quantum efficiencies for H202 formation of 1%at high light intensities. However, the peroxide concentration vs time profile is dependent on I, as shown in Figure 4a. At I, = 1.9 X lo4 M min-l, the maximum [HzOz] was observed after 20 m of irradiation; whereas at lower light intensity (Figure 4a) the time required to achieve [H2021maxwas increased by 10min. Furthermore, a decrease of [HzOzl at longer illumination times was evident in experiments where I, was lower (e.g., 1.3 X 10“‘ M min-1). Single-wavelength irradiation at 366 nm resulted in a smaller apparent quantum efficiency O H ~ O ~ = 0.04%. In contrast, Ocl- and ‘Ppcp decreased with increasing light intensity (Figure 6 and Table I). The decrease of Ocl- with increasing light intensity appears to be unrelated to losses of charge carriers due to faster recombination at high I, (37). Hydrogen peroxide is formed on illuminated Ti02 surfaces via dioxygen reduction by a conduction-band electron in the presence of a suitable electron donor such as acetate or PCP according to the following mechanism (34):
-
n
hv
(4)
>Ti-O,’-
+ H,O
-
+ HO,’
-
+ RR”CH’
>Ti-OH
(5)
H,O, + 0, (6) 2H0,’ + 2H’ In the presence of organic scavengers,organic peroxides and additional H202 may be formed through the following generalized sequence (34):
e-
+ RCH,R” RR”CH* + 0,
>Ti-OH’
-
>Ti-OH,
RR”CH0,’
(8) (9)
+
RR”CH0,’ + R’H RR’CHOOH R’* (10) RCHR”O0H RR”C=O + H,O, (11) where RR”CHp is a general organic electron donor with an abstractable hydrogen atom and RR”CH* is the freeradical intermediate produced by oxidation of RR”CH2. In Figure 4b, the initial slow rise in [HzOzl that is followed by a sharper increase in [H2021 with time is consistent with a pathway dominated by reactions 7-11. In the specific case of PCP, RR”CH2 of eq 8 represents products of the initial oxidation of PCP such as o-chloranil (or products of the reaction of these intermediates with the solvent) rather than PCP. Under the experimental conditions of Figure 4b, only very small concentrations of products existed at short irradiation times; therefore, the
rate of formation of H202 was slow. As the intermediates accumulated, they were able to compete successfully with PCP for the photogenerated oxidants (Le., >TiOH*, 'OH, and htr+)and concomitantly increase the rate of peroxide formation with increasing irradiation time. Under the conditions used in the experiments shown in Figure 4b, the reactions of o-chloranil, TCC, or TCHQ with H202 can be discounted because of the low Ioand the short illumination times. In addition, the concentrations of the chloro compounds and H202 (via reaction 4) were expected to be very low. On the other hand, high lightintensity illuminations (Figure 4a) produced higher concentrations of o-chloranil and other intermediates which then led to a higher frequency of surficial reactions with trapped holes or surface-bound hydroxyl radical and thus a faster rate of H202 generation. The faster rate of decrease of [HzOzl at longer illumination times when Io = 1.3 X 10-4 M min-1 (curve 2, Figure 4a) resulted from faster reactions between the degradation products and H202 (vide supra). The higher steady-state concentration of hydrogen peroxide measured at Io = 1.9 X 10-4 M min-l resulted from a faster decay of the unstable intermediates via eq 11. The highest concentrations of hydrogen peroxide were observed when a preirradiated sample was "aged" in the dark (Figure 5 ) . The delayed formation of H202 suggests a slow decay of the precursors of H202. A slow formation of hydrogen peroxide is consistent with organic peroxides or hydroperoxide radicals as precursors (34) as follows: OH
OH
I
I
OH
I
RiR2C-02.
--C
RjR2C=O
+ HO;
(13)
pentachlorophenolate. The azide radical reacts with aromatic compounds via direct electron transfer. In the case of 'OH + pentachlorophenolate, Terzian et al. (41) found spectroscopic evidence for the pentachlorophenol'OH adduct which was characterized by a transient absorption band at 320 nm. Their kinetic transient results are consistent with our experimentalobservationsobtained from steady-state photolyses. Addition of *OH radicals to the aromatic ring of PCP is expected to occur preferentially a t the ortho and para positions. Both positions are preferred by the orientation tendency of the ring hydroxyl and by the meta orientation of two ring chlorines. From statistical considerations, it is reasonable to assume that 'OH addition in the ortho position is more frequent than in the para position. An apparent quantum efficiency of 0.9% was obtained for the formation of products o-chloranil and TCC, which are produced by 'OH radical attack on the ortho position. On the other hand, the apparent quantum efficiency for the formation of p-chloranil and TCHQ, which result from the attack of 'OH at the para position, is @ = 0.4%. Furthermore, the ratio of these apparent quantum efficiencies is about 2.3, which is the same ratio of products from the reaction of 'OH radicals with 4-chlorophenoland phenol (42-44). In a study of the reaction between hydroxyl radicals and 2,4,5-trichlorophenol, kinetic evidence for a preferential attack of 'OH at the unsubstituted carbon in position 6 of the ring has been presented (5). However, similar reactivities toward *OH radicals are expected for PCP and phenol from symmetry considerations. Our experimental observations confirm this assumption. The following mechanism for the formation of p-chlorani1 and TCHQ by 'OH attack on the para position of PCP is proposed:
The direct electron-transfer reaction between a surfacetrapped hole and a surface-bound PCP molecule is expected to yield a phenoxyl radical as follows:
+
&$CI c CI
b,,+
4
;;& CI
+ >TiOH2 CI
(15)
CI
CI
+ H+
(14)
CI II
HO' 'CI
The resulting pentachlorophenoxylradical is most likely a strong oxidant which should be reduced by electrons from the conduction band or by peroxide radicals to regenerate PCP. Addition of oxygen to some of the resonance forms of the phenoxyl radical is considered to be a slow process (40). In the specific case of phenol, 0 2 addition to the phenoxyl radical is 100 times slower than the self-reaction between phenoxyl radicals (40). The formation of 'OH radicals by illumination of aqueous Ti02 suspensions has been verified by EPR (37), while direct chemicalevidencefor the reactions of benzoic acid and phenol is available (11-13). Given the observed reaction intermediates in the photooxidation of PCP on illuminated TiO2, the overall photo-assisted reaction appears to proceed primarily via oxidation by surface bound 'OH radical pathway and secondarily by direct electron transfer to a surface-trapped hole. Terzian et al. (41)have shown in a series of pulse radiolysis experiments that the pentachlorophenoxyl radical can be formed via the homogeneous reaction of the azide radical (N$) with
n
-
0
+ HO;
_c_
CI
CI
0
(1 7)
0
This mechanism predicts p-chloranil and H202 (via eqs 17 and 6) as the products. However, as mentioned above,
TCHQ is the initial product in the photocatalyzed degradation of PCP. Similar studies with phenol and 4-chlorophenolhave identified hydroquinone as the main product of 'OH attack in para position of the aromatic ring (5,12,44). The semiquinone radical of eq 18is known to decay by disproportionation to form p-chloranil and TCHQ (45) as shown: Environ. Sci. Technol., Vol. 27,
No. 8, 1993 1887
demonstrate that a photocatalyzed degradation of PCP is possible even in the presence of large amounts of radical scavengers as would be the case in practical applications for wastewater treatment. Acknowledgments
p-Chloranil is degraded faster than PCP in illuminated Ti02 suspensions (Figure 7). In the presence of high concentrations of PCP, efficient dechlorination of the chloroquinone was observed. However, the concentration of p-chloranil and TCHQ increased linearly during the photolysis of PCP as illustrated in Figure 1. These results are consistent with the assumption that 'OH radicals react at least 10 times faster with TCHQ than with p-chloranil as in the case of hydroquinone and p-benzoquinone (46):
C "#I I
CI
+ >TIOH'
Literature Cited
-
CI + >TiOH*
I $"CI
OH
(20)
OH
The semiquinone radicals will eventually regenerate TCHQ as shown above. TCHQ decays slowly by dechlorination (Figure 9) and by oxidation to p-chloranil. As long as the concentration of TCHQ is higher than the concentration of p-chloranil, both species will be interconverted. Over longer time frames they disappear via the thermal degradation of TCHQ. Formation of o-chlorani1 and TCC from 'OH radical attack on the ortho position of the aromatic ring is expected to proceed through similar reaction pathways. Both compounds are unstable in water, which explains their low steady-state concentrations in irradiated solutions of PCP. p-Chloranil cam be reduced rapidly (k = 9.8 X loa M-l s-l) by 02'- to the semiquinone radical (47), while the corresponding hydroquinone can be oxidized slowly (k = 1.7 x 104 M-I 8-1) by HO2' (pK, = 4.8). Thus, the preferential formation of TCHQ over p-chloranil appears to arise from the large difference ( lo6) in the relative rates of reduction and oxidation by superoxide (hydroperoxyl radical) at the surface of illuminated TiO2. When formate is present as acompetitive electron donor, the following pathway is likely to become important: N
>TiOH'
+ HCO;
--+
>TiOH,
+ C0,'-
+ HCO; H+ + C02'c0;- + 0, o,*-+ CO,
h,+
With are grateful to the U.S.Environmental Protection Agency (Grant R813326-01-0) for the financial resources to pursue this project and to the Degussa Corporation for a gift of the titanium dioxide. Glossary: PCP, pentachlorophenol; p-chloranil, p-tetrachlorobenzoquinone; TCHQ, p -tetrachlorohydroquinone; o-chloranil, o-tetrachlorobenzoquinone;TCC, tetrachlorocatechol.
(21) (22)
(23)
Adsorption studies of PCP on Ti02 performed in this laboratory indicate that only 5 % of the initial concentration (20 pM) of PCP is adsorbed on the semiconductor particles [Ti021 = 0.2 g L-1. Under these conditions, PCP should be displaced from the surface by formate. Thus, the photocatalytic dechlorination of PCP, p-chloranil, TCHQ, and TCC in the presence of formate or 2-propanol can proceed by either oxidation with *OH (ha+)or by reduction with C02'- or 'ROH. Given the experimental evidence obtained in this study, we cannot distinguish between these alternative pathways in the presence of radical scavengers;although,in our studies, the scavenging of C02'- radicals by PCP was minimized by using solutions that were 520 pM in PCP. This concentration is 12times smaller than the concentration of 0 2 in an air-saturated solution. However, these results are important since they 1688 Envlron. Sci. Technol.. Vol. 27, No. 8, 1993
(1)Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Enuiron. Sci. Technol. 1991,25, 494. (2) Pelizzetti, E.;Minero, C.; Carlin, V.; Borgarello, E. Chemosphere 1992,25,343. (3) Ollis, D. F.;Pelizzetti, E.; Serpone, N. Enuiron. Sci. Technol. 1991,25, 1522. (4) Hidaka, H.; Nohara, K.; Zhao, J.;Serpone, N.; Pelizzetti, E. J. Photochem. Photobiol. A 1992,64,103. (5) Al-Ekabi, H.; Serpone, N.; Pelizzetti, E.; Minero, C.; Fox, M. A.; Draper, R. B. Langmuir 1989,5,250. (6) AI-Ekabi, H.; Serpone, N. J . Phys. Chem. 1988,92,5726. (7) Matthews, R. W. Aust. J . Chem. 1987,40,667. (8) Matthews, R. W. J . Phys. Chem. 1987,91,3328. (9) Matthews, R. W. J. Cutal. 1986,97,565. (10) Matthews, R. W. Water Res. 1986,20,569. (11) Matthews, R. W. J . Chem. SOC., Faraday Trans. 1 1984,80, 457. (12) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M. Bull. Chem. SOC.Jpn. 1985,58,2015. (13) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Itaya, A. Bull. Chem. SOC.Jpn. 1985,58,2023. (14) Ossawa, Y. J. Phys. Chem. 1984,88,3069. (15)Hermman, J.-M.; Morranega, M.-N.; Pichat, P. J . Photochem. 1983,22,333. (16) Zumi, I.; Dunn, W. W.; Wilbaum, K. 0.;Fan, F.-R. F.; Bard, A. J. J . Phys. Chem. 1980,84,3207. (17) Krautler, B.; Bard, A. J. J.Am. Chem. SOC.1978,100,5985. (18) Pelizzetti, E.;Borgarello, M.; Minero, C.; Pramauro, E.; Borgarello, E.; Serpone, N. Chemosphere 1988,17,499. (19) Barbeni, M.; Minero, C.; Pelizzetti, E.; Borgarello, E.; Serpone, N. Chemosphere 1988,16,2225. (20)Barbeni, M.; Morello, M.; Pramauro, E.; Pelizzetti, E.; Vicenti, M.; Borgarello,E.; Serpone, N. Chemosphere 1987, 16,1165. (21) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Vicenti, M.; Borgarello, E.; Serpone, N. Chemosphere 1987,16,47. (22) Hidaka, H.; Kubota, H.; Graetrel, M.; Pelizzetti, E.; Serpone, N. J . Photochem. 1986,35,219. (23) Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello, E.; Serpone, N.; Jamieson, M. A. Chemosphere 1986,15,1913. (24)Barbeni, M.; Pramauro, E.; Pelizzetti, E.; Borgarello, E.; Serpone, N. Chemosphere 1985,14,195. (25) Chun, H. L.;Ratcliff, M.; Posey, F. L.; Turner, J. A,; Nozik, A. J. J . Phys. Chem. 1983,87,3089. (26) Tunesi, S.;Anderson, M. A. Chemosphere 1987,16, 1447. (27) Harada, K.; Hisawga, T.; Tanaka, K. New J. Chem. 1987, 11) 597. (28) Keith, L.H.; Telliard, W. A. Enuiron. Sci. Technol. 1979, 13,416. (29) Al-Sayyed, G.; D'Oliveria, J.; Pichat, P. J . Photochem. Photobiol. A 1991,58, 99. (30) Mamlal, V. B.; Haridas, A.; Alexander, R.; Surender, G. D. Water Res. 1992,26,1035. (31)Terzian,R.; Serpone, N.; Draper,R. B.;Fox, M. A.; Pelizzetti, E. Langmuir 1991,7,3081.
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Environ. Scl. Technol., Vol. 27,
No. 8, 1093 1889
