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KINETICS, CATALYSIS, AND REACTION ENGINEERING Effect of the Radiation Wavelength on the Rate of Photocatalytic Oxidation of Organic Pollutants Gianluca Li Puma*,† and Po Lock Yue‡ School of Chemical, Environmental and Mining Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom, and Department of Chemical Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
The effect of the radiation wavelength upon the rate of oxidation of a model organic pollutant, 2-chlorophenol (2-CP), on titanium dioxide (TiO2) aqueous suspensions was investigated using UV-A radiation alone and concomitant UV-A, -B, and -C radiation. The rates of degradation and mineralization of 2-CP were significantly enhanced with UV-ABC radiation compared to the use of UV-A radiation alone. This rate enhancement was found to be caused by the combined effect of photolysis, photocatalysis, and synergistic effects due to concomitant photolysis and photocatalysis. In the initial stage of the oxidation reaction, 2-CP is preferentially photolyzed to yield catechol as the main intermediate which is mineralized further to CO2 and water via a photocatalytic process. A significant effect of temperature was found in the experiment using UV-ABC radiation. The rate of mineralization of 2-CP was found to double with a temperature increase from 20 to 40 °C when UV-ABC radiation was used. The effect of temperature was found to be negligible when the suspension was irradiated with UV-A radiation alone. Introduction Heterogeneous photocatalysis is one of the advanced oxidation processes which has been shown to be very effective in oxidizing a large variety of toxic organic compounds in water and wastewater.1-3 To conduct this photoactivated redox reaction, the photon source must possess energy greater than the band gap of the semiconductor. Among the different semiconductors investigated, including MgO, ZnO, WO3, CdS, and TiO2, the white paint pigment titanium dioxide has been shown to have the largest potential for application in water treatment and purification.1-4 The vast majority of studies quoted in the literature have been carried out with suspensions of TiO2 in water which were irradiated with radiation in the UV-A region of the electromagnetic spectrum, i.e., 320-380 nm.1,3 In a photocatalytic reactor, UV-A radiation is normally provided by fluorescent low-pressure mercury lamps emitting low-intensity UV-A radiation. Medium-pressure mercury lamps have also been utilized which emit high-intensity UV light in the short-, medium-, and long-UV region of the electromagnetic spectrum. However, the short- and medium-UV radiation (UV-C and UV-B) emitted by mercury lamps is usually cut off by the material of construction of the photoreactor, unless quartz windows are used. Surprisingly, there is very * Corresponding author. E-mail: gianluca.li.puma@ nottingham.ac.uk. Tel: +44 (0) 115 9514170. Fax: +44 (0) 115 9514115. † The University of Nottingham. ‡ The Hong Kong University of Science & Technology.
little work with suspensions of TiO2 irradiated by UV-C radiation alone or by concomitant UV-A, -B, and -C radiation. An indication that UV-C radiation may be more effective than UV-A in promoting the photocatalytic degradation of selected organic compounds has been reported by Matthews and McEvoy5 and more recently by Li Puma and Yue.6,7 The authors explained the enhanced effectiveness with the short UV in terms of contribution of direct photolysis and higher probability of trapping of electron-hole pairs with shorter wavelength excitation. For all practical applications, UV-C radiation in a photocatalytic reactor can be provided either by lowpressure mercury lamps, which emit low-intensity radiation predominantly at 253.7 nm, or by medium/ high-pressure mercury lamps enclosed in quartz tubes, which emit high-intensity radiation in the UV-A, -B, and -C radiation of the electromagnetic spectrum. In this paper a study of the effect of the radiation wavelength on the rates of degradation and mineralization of a model organic pollutant, 2-chlorophenol (2-CP), on titanium dioxide (TiO2) aqueous suspensions is presented. At radiation wavelengths higher than approximately 310 nm, the oxidation of 2-CP is carried out by photocatalysis alone; however, at wavelengths below 300 nm, 2-CP will undergo oxidation by concomitant photolysis and photocatalysis. In this latter case, photolysis was found to play a crucial role in the enhancement of the oxidation rates of 2-CP, and as a result, the oxidation mechanism was found to be different from that occurring by photocatalysis alone. It is well documented that the effect of the temperature on the rates of photocatalytic oxidation of organics
10.1021/ie0203274 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/15/2002
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5595 Table 1. Spectral Distribution for a 250 W Medium-Pressure Mercury Arc Lamp As Supplied by the Lamp Manufacturer wavelength (nm)
power (W)
energy (kcal einstein-1)
photon flux (×106 einstein s-1)
less than 254 254 257 265 270-275 280-289 296-302 313 334 366 404 435 546 578
negligible 1.077 0.344 0.323 0.129 0.215 0.425 1.078 0.196 1.634 1.274 1.830 2.06 0.818
negligible 112.56 111.25 107.89 105.89 102.11 96.59 91.35 85.6 78.12 70.77 65.73 52.36 49.47
negligible 2.29 0.74 0.72 0.29 0.51 1.06 2.82 0.55 5.00 4.30 6.65 9.4 3.95
under UV-A radiation is negligible.8,9 However, the present study shows that the effect of temperature can be significant when the oxidation is carried out with concomitant UV-A, -B, and -C radiation. Experimental Section Reagents and Materials. All of the chemicals used in the experiments and analyses were reagent grade or higher and were used as received without further purification. 2-CP (99.5%), ethyl acetate and methanol (pesticide residue analysis grade), phosphoric acid, hydrochloric acid, ferric trichloride anhydrous, sodium acetate, and 1,10-phenantroline hydrate (AnalR) were supplied from BDH Merck. Acetic anhydride (99%) and potassium oxalate monohydrate (ACS reagent) were supplied by Aldrich. Potassium persulfate was supplied by Sigma, while sulfuric acid (about 1.84 specific gravity) was supplied by Fisons. Ultrapure water was supplied by an Elga Maxima Ultrapure Water system [total organic carbon (TOC) less than 0.5 ppb; inorganics, 18 MΩ cm-1 resistivity at 25 °C]. Titanium dioxide P25 powder was obtained from Degussa (70:30% anatase to rutile; average particle size 30 nm in 100 nm aggregates; surface area 50 ( 15 m2 g-1). Photoreactor. The experiments were performed using a 3.6 L stirred multilamp batch photoreactor shown elsewhere.10 The reactor vessel consisted of a QVF borosilicate glass tube (300 mm high and 152 mm diameter) sealed at both ends with stainless steel 316 plates. The photoreactor was fitted with three ultraviolet lamps arranged 40 mm from the reactor center at a pitch of 120°. The lamps used were 250 W mediumpressure mercury arc lamps of 152 mm in length and 19 mm in diameter, housed inside Pyrex or quartz tubes with an external diameter of 38 mm. The spectral power distribution of the lamp (Table 1) extended into the short, medium, and long wavelength of ultraviolet light. Oxygen was supplied to the suspensions through a porous Teflon plate at the bottom of the reactor. The reactor was fitted with a mixer using a standard 2 in. Rushton turbine impeller, a glass cooling coil placed close to the internal walls of the photoreactor, and probes for temperature and pH measurements. The temperature of the suspension was controlled by circulating water from a thermostatic water bath. The reactor was accommodated inside a safety box for protection from UV radiation. Experimental Procedures. Suspensions were prepared in ultrapure water by mixing TiO2 with appropri-
ate solutions of 2-CP to give the selected concentrations to a final volume of 3.6 L. All experiments were conducted at a natural unadjusted pH of the solution/ suspension, which was in the acidic range from 4.2 to 4.5. Irradiation commenced after the suspensions had been equilibrated in the dark for 40 min with constant mixing and an oxygen sparge of 1 L min-1. The oxygen sparging was maintained at the same rate throughout the irradiation experiments. Samples collected at appropriate time intervals were analyzed for 2-CP concentration and TOC. Analyses. The degradation of the 2-CP substrate and the formation of intermediate products were followed by a Hewlett-Packard model 5890 series II gas chromatograph equipped with a Ni-63 electron capture detector and on-column injection system (GC-ECD). The instrument was fitted with a Hewlett-Packard model 7673 autosampler and SGE capillary column BP-X5, 25 m, 0.22 mm i.d., and 0.25 µm film. For these analyses the samples were acetylated with acetic anhydride according to the method described in Mathew and Elzerman11 and concentrated by solid-phase extraction in a VAC-Elut SPS-24 (Analytichem International). Ethyl acetate was used as the eluting solvent, and Bond Elut CH sorbent (Varian), as the stationary phase. The GC-ECD analytical method used was the same as that described in the paper by Li Puma and Yue.12 The course of mineralization of 2-CP to carbon dioxide was traced with a Dohrman DC-180 carbon analyzer. TOC was calculated as the difference between the total carbon (TC) and inorganic carbon (IC). The sensitivity of the analyzer was in the range of 10 ppb to 30 000 ppm carbon. Chemical Actinometry. The photon flux from the ultraviolet lamps was measured by potassium ferrioxalate actinometry.13 Actinometric experiments were performed individually for each of the three lamps using both quartz and Pyrex tubes as lamp housing material. The combined radiant power for the three lamps was measured to be 4.30 × 10-5 einstein s-1 when the lamps were inside quartz tubes using an average quantum yield for a Fe2+ production of 1.185 and 2.74 × 10-5 einstein s-1 when the lamps were inside Pyrex tubes using an average quantum yield for a Fe2+ production of 1.159. These values are slightly lower than that calculated using the photon flux data supplied by the lamp manufacturer (Table 1); however, their ratio is approximately the same. The radiant power per unit reactor volume was calculated to be 1.19 × 10-5 einstein s-1 L-1 with the lamp housed inside quartz tubes and 0.76 × 10-5 einstein s-1 L-1 with the lamp housed inside Pyrex tubes. In the presence of a TiO2 water suspension, absorption of photons would be less than that in these estimations because of (i) the dispersive effect of light scattering, (ii) the absorption of photons from the walls of the photoreactor and from the lamp bulbs, and (iii) the fact that a ferrioxalate actinometer solution absorbed photons up to 436 nm, while TiO2 photocatalytic reactions are driven by photons of wavelength less than 380 nm. Results and Discussion The procedure of continuous sparging of oxygen in the reactor could have resulted in losses of volatile organic carbon (VOC) from the liquid phase. To assess this effect, a number of control experiments were conducted
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Figure 1. Photodegradation of 2-CP in the absence of TiO2 at 20 °C. Irradiation with UV-ABC radiation.
with the outflow gas from the reactor being bubbled through a cold trap system. The cold trap system consisted of a cylinder containing 200 mL of ethyl acetate maintained at a temperature of 0 °C. After the trapping experiments, the total volume of ethyl acetate was reduced to 10 mL by evaporation under slight vacuum (3 kPa) at 0 °C, to avoid the release of an appreciable amount of VOC, and the sample was analyzed by GC-ECD and GC-MS. In all of the experiments, no VOC peaks were found. Parallel experiments were conducted using Pyrex and quartz as lamp housing material for the study of the effect of the radiation wavelength upon the kinetics of photooxidation of 2-CP in an aqueous TiO2 suspension. For the reactor with Pyrex lamp housing, only UV-A radiation of wavelength higher than 320 nm was transmitted. For the configuration with quartz lamp housing, the wavelength cutoff was 220 nm; thus, the TiO2 suspension received UV-A, -B, and -C radiation. These two experimental situations will be referred to as UV-A and UV-ABC, respectively. The total photon flux with UV-ABC radiation was 1.56 times higher than that with UV-A radiation. Photodegradation of 2-CP in the Absence of TiO2. Initially, the effect of UV alone in the absence of TiO2 was evaluated. Figure 1 shows the results of direct photolysis of 2-CP with UV-ABC radiation, in the absence of TiO2 and at a natural unadjusted pH of the solution. A fast and almost complete degradation of 2-CP was observed following apparent first-order kinetics. This was accompanied by the release of chloride ions in solution, and as a result, the pH of the solution was found to decrease as the reaction proceeded. Direct photolysis of 2-CP at pH < 7 has been shown to yield primarily catechol.14,15 Four intermediate products were detected by GC-ECD under the conditions indicated. Catechol was found to be the main intermediate, and when at its peak, it accounted for approximately 60%
Figure 2. Photooxidation of 2-CP in the presence of TiO2 with UV-A alone and UV-ABC radiation: (a) degradation profiles; (b) mineralization profiles; (c) pH profiles. TiO2 ) 0.5 g L-1. Temperature ) 20 °C.
of the initial concentration of 2-CP based on chromatographic peak areas. Once formed catechol was photolyzed further to yield other intermediates. However, it was observed that photolysis alone does not mineralize 2-CP significantly because there was no appreciable decrease in the TOC of the solution as the reaction proceeded. Indeed, the intermediates after catechol are unlikely to absorb the wavelength emitted by the lamp. In a separate experiment, 2-CP was not degraded to any appreciable extent when it was irradiated with UV-A radiation alone and in the absence of TiO2. Photooxidation of 2-CP in the Presence of TiO2. The effect of catalyst loading was first established to determine the optimal loading of the photocatalyst for the geometry of the reactor. The results, presented elsewhere,10 showed that the concentration of TiO2 (Degussa P25) at which the oxidation rates reached a plateau was 0.5 g/L. As a consequence, this concentration of photocatalyst was used in the experiments concerned with this study. Parts a and b of Figure 2 show a direct comparison of the kinetics of degradation and mineralization of 2-CP in suspensions of TiO2 in water, with UV-A and with UV-ABC radiation and at the initial pH of the solution. The adsorption of 2-CP on TiO2 (Degussa P25) has been reported to be poor, and for the range of initial concentration chosen for these experiments (about 45 µM) the amount of 2-CP in equilibrium absorbed on the catalyst
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was approximately 1.5 µM/g of TiO2.16 The amount of 2-CP absorbed on TiO2 at the beginning of the present experiments was therefore estimated to be approximately 0.75 µM or 1.6% of 2-CP in solution. The results show that the initial rate of degradation of 2-CP was found to be 1.8 times faster when the suspension was irradiated with UV-ABC radiation and the degradation of 2-CP was 98% complete in 20 min. Figure 2b shows that the initial rate of mineralization was approximately the same with UV-A and UV-ABC radiation; however, after 8 min the rate of mineralization with UV-A declined compared with that using UVABC radiation. Figure 2c shows that the reduction in pH is more pronounced with UV-ABC radiation rather than with UV-A alone, which suggests that direct photolysis of 2-CP with UV-ABC might have occurred. These results indicate different initial reaction mechanisms of 2-CP oxidation for the two cases. Some intermediate products were detected by GC-ECD for the experiment conducted with UV-ABC radiation only; however, their quantification was difficult. The above experiments were repeated at an initial concentration of 2-CP of 1 order of magnitude higher (approximately 400 µM). This allowed for a more precise detection of the intermediates to be obtained. At this value of initial concentration, the equilibrium amount of 2-CP adsorbed on TiO2 (Degussa P25) has been reported to be approximately 11 µM/g of TiO2.16 This means that approximately 5.5 µM of 2-CP or 1.4% of 2-CP in solution was absorbed on TiO2 under the conditions indicated. Therefore, a slight decrease in the photocatalytic reaction rate with UV-A radiation should be expected compared with the run in Figure 2a. Conversely, the rate of photolysis of 2-CP with UV-ABC radiation was expected to increase because the rate of photolysis should be proportional to the local volumetric rate of energy absorption (µI) which in turn depends on the concentration of 2-CP in solution. The results presented in Figure 3a confirmed what was expected, the rate of degradation of 2-CP with UV-A radiation was found to decrease 10% and the rate with UV-ABC radiation increased by 43% compared with the results of Figure 2a. The degradation rate with UV-ABC radiation was found to be 2.8 times faster compared with the case in which UV-A radiation was used. Figure 3b shows that the rate of mineralization of 2-CP with UV-ABC was twice that with UV-A. Again, a decrease in the rate with UV-A radiation and an increase in the rate with UV-ABC radiation, compared with the former case with a lower initial 2-CP concentration, were found. However, the increase in the mineralization rate with UV-ABC was 25% lower compared with the rate of degradation. This suggests that under UV-ABC radiation mineralization of 2-CP is controlled by the rate of photocatalytic oxidation rather than by the rate of photolysis. Figure 3c shows that a much faster dechlorination of 2-CP was observed with UV-ABC radiation, and a sharper decrease in pH was found as the reaction proceeded (Figure 3c). Three intermediates were found by the GC-ECD analysis under the conditions indicated for the experiment performed with UV-ABC radiation (Figure 3d). Catechol was found to be the main intermediate, and the other two minor intermediates exhibited the same chromatographic retention times as those shown in Figure 1b. However, catechol was formed at a slower
Figure 3. Photooxidation of 2-CP in the presence of TiO2 with UV-A alone and UV-ABC radiation: (a) degradation profiles; (b) mineralization profiles; (c) pH profiles; (d) intermediates for run with UV-ABC radiation. TiO2 ) 0.5 g L-1. Temperature ) 20 °C.
rate than previously (Figure 1b) because a fraction of the available photon flux was adsorbed by the TiO2 particles and a small percentage of 2-CP was adsorbed on the catalyst. Catechol accounted for approximately 40% of the initial concentration of 2-CP after 60 min of irradiation based on chromatographic peak areas. Conversely, no intermediates were detected by GC-ECD analysis in any of the experiments performed with UV-A radiation under the conditions indicated presumably because of the slow initial oxidation rate and different reaction mechanism. Effect of the Temperature. Figure 4 shows the results of the experiments performed when the temperature of the suspension was doubled to 40 °C. The results of the experiment performed with UV-A radiation are essentially identical with that at 20 °C and in agreement with the literature (low apparent activation
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Figure 4. Photooxidation of 2-CP in the presence of TiO2 with UV-A alone and UV-ABC radiation: (a) degradation profiles; (b) mineralization profiles. TiO2 ) 0.5 g L-1. Temperature ) 40 °C.
energy).8,9 Thus, temperature has a negligible effect on the oxidation rates of 2-CP when the TiO2 suspension is irradiated with UV-A radiation. However, a significant effect of temperature was found when UV-ABC radiation was used. The rate of mineralization at 40 °C using UV-ABC radiation was found to be 1.4 times higher compared with the experiment conducted at 20 °C and almost 4-fold higher than that with UV-A at 40 °C. The apparent energy of activation in the range of temperature from 10 to 40 °C, obtained from an Arrhenius-type plot, was found to be 18.4 ( 3.0 kJ mol-1 for the degradation of 2-CP and 36.0 ( 13.8 kJ mol-1 for the mineralization of 2-CP as shown elsewhere.10 One reason for the enhanced oxidation rates when UV-ABC was used was that the photon flux with UV-
ABC radiation was 1.56 times higher than that with UV-A radiation. Another reason is the possibility that recombination of electrons and holes in TiO2 using UVABC may have a lower rate than that in the case of UV-A alone. This effect may be due to an excess energy present in the semiconductor well above the band-gap energy and to a shorter penetration distance of highenergy photons into the particle of TiO2, which might have caused an increase in the concentration of active oxidizing species at the surface of the catalysts.5 Because the magnitude of the observed rate enhancement was always more than twice higher, the possible decrease in the electron-hole recombination rate and the difference in photon flux do not entirely explain the enhancement in the oxidation rate when UV-ABC radiation was used. The magnitude of this effect and the large change in the apparent activation energy when UV-ABC radiation was used can more convincingly be explained by an increase in the rate of photolysis of substrates in a solution of TiO2, effectively resulting in faster observed oxidation rates of 2-CP. Reaction Mechanism. The above results suggest a reaction mechanism for the photooxidation of 2-CP in a suspension of TiO2 when irradiated with UV-ABC radiation (Figure 5). As reported earlier, under this situation concomitant photolysis and photocatalysis occur. Photolysis is clearly responsible for the initial breakdown of the 2-CP molecule, yielding catechol and other minor intermediates. Catechol in this case is formed by a homogeneous reaction from a nucleophilic displacement of chloride from the singlet excited state of 2-CP. Conversely, the mechanism of photocatalytic oxidation of 2-CP with UV-A radiation should have yielded primarily 2-chlorohydroquinone (not detected by GC-ECD), which arises from the hydroxylation of the benzenic ring of 2-CP preferentially in the para position, and 2-hydroxyhydroquinone following the attack of the carbon-chlorine bond by a hydroxyl radical as a second step.15 This heterogeneous reaction occurs on or very near to the surface of the photocatalyst and involves the adsorption of 2-CP on the catalyst followed by reaction
Figure 5. Reaction scheme of photooxidation of 2-CP in the presence of TiO2 with concomitant photolysis and photocatalysis. The main path is indicated with thick arrows.
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on the surface. Given that 2-CP is poorly absorbed on TiO2,8,16-18 the photocatalytic reaction is in this case clearly much slower than the photolytic reaction of 2-CP with UV-ABC radiationl thus, the photocatalytic reaction pathway is minor compared with that from direct photolysis. The intermediates generated by photolysis may be further photolyzed in the initial stage of the reaction when their concentration in solution is high; however, their mineralization to carbon dioxide, water, and mineral acids is essentially carried out through a photocatalytic mechanism involving hydroxyl radicals because photolysis alone does not mineralize 2-CP in any significant extent as was shown in Figure 1. Thus, the acceleration of the oxidation process of 2-CP when irradiated with UV-ABC radiation in the presence of TiO2 takes place through a synergistic effect of photolysis and photocatalysis. The production of intermediates by photolytic reactions accelerates the oxidation process of 2-CP that is indeed completed by the hydroxyl radicals generated at the surface of TiO2 reacting primarily with weakly UV-absorbing intermediates adsorbed onto the surface. These results are somewhat in contrast with the results by D’Oliveira et al.,15 in which they found that photolysis has a detrimental effect on the photocatalytic degradation of 3-chlorophenol; however, they are in agreement with the results of Matthews and McEvoy,5 in which 254 nm radiation was found to be more effective than 356 nm radiation in the degradation of phenol in the presence of TiO2. D’Oliveira et al.15 explained their results in terms of the competition of the intermediates (chlorohydroquinone and resorcinol) with 3-chlorophenol for the adsorption sites. An alternative explanation to the findings of D’Oliveira et al.15 is that their results may have been affected by the experimental conditions used (a very high value of optical thickness and an irradiation wavelength higher than 290 nm) which may have impeded effective photolysis of 3-chlorophenol when in the presence of TiO2. This synergistic effect of photolysis and photocatalysis is augmented further, in the case of the oxidation of 2-CP with UV-ABC radiation, by the possible surface photosensitization of TiO2 nanoparticles due to the adsorption of catechol. Because catechol absorbed onto TiO2 (Degussa P25 and Aldrich) has been shown to yield an absorbance band from 400 to 440 nm,19,20 the radiation emitted at 435 nm by the UV lamp (Table 1) could have also been used to induce charge-transfer reactions and accelerate the photooxidation of the intermediates in the early stage of the reaction when catechol is present. This augmentation was not obtainable in the UV-A system because catechol was not detected. The main pathways of the reaction scheme shown in Figure 5 may be represented as two parallel/series reactions as follows: k1
k2
photolytic pathway A 98 B 98 C k3
k4
photocatalytic pathway A 98 D 98 C where B and D are different intermediates, k1 > k3 and k2 > k4, and k2 and k4 may be controlling the rate. The effect of photolysis should be greater for organic compounds that adsorb poorly on TiO2 such as chlorophenols. In these cases, the use of concomitant UV-A and UV-C radiation should accelerate their oxidation
rates. Previous results by Li Puma and Yue6,7 on the photocatalytic oxidation of salicylic acid in a pilot-plant falling film photoreactor using UV-A and UV-C radiation further confirm that simultaneous photolysis and photocatalysis can be a much more effective process than photocatalysis or photolysis alone for the oxidation of organics. Conclusions The effect of the radiation wavelength upon the rate of oxidation of a model organic pollutant, 2-CP, on TiO2 aqueous suspensions was investigated using UV-A radiation alone and concomitant UV-A, -B, and -C radiation. With UV-ABC radiation the rates of degradation and mineralization of 2-CP were significantly enhanced compared with the case in which UV-A radiation alone was used. This rate enhancement was found to be at least 2-fold and caused by the combined effect of photolysis, photocatalysis, and synergistic effects due to concomitant photolysis and photocatalysis. In the initial stage of this oxidation reaction, 2-CP is preferentially photolyzed to yield catechol as the main intermediate. This intermediate and others are mineralized further to CO2 and water via a photocatalytic process. When using UV-ABC radiation, a significant effect of the temperature was found and the rate of mineralization of 2-CP was found to double with a temperature increase from 20 to 40 °C. The effect of temperature was found to be negligible when the suspensions were irradiated with UV-A radiation alone. Possible synergistic effects due to concomitant photolysis and photocatalysis have been suggested. Irradiation of a solution of 2-CP with UV-ABC radiation in the absence of TiO2 produces a fast degradation rate, but photolysis alone does not mineralize 2-CP to a significant extent. Literature Cited (1) Legrini, O.; Oliveros, E.; Braun, A. M. Photochemical processes for water treatment. Chem. Rev. 1993, 93, 671. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69. (3) Alfano, O. M.; Bahnemann, D.; Cassano, A. E.; Dillert, R.; Goslich, R. Photocatalysis in water environments using artificial and solar light. Catal. Today 2000, 58, 199. (4) Bahnemann, D.; Bockelmann, D.; Goslich, R. Mechanistic studies of water detoxification in illuminated TiO2 suspensions. Solar Energy Mater. 1991, 24, 564. (5) Matthews, R. W.; McEvoy, S. R. A comparison of 254 nm and 350 nm excitation of TiO2 in simple photocatalytic reactors, J. Photochem. Photobiol. A: Chem. 1992, 66, 355. (6) Li Puma, G.; Yue, P. L. Enhanced photocatalysis in a pilot laminar falling film slurry reactor. Ind. Eng. Chem. Res. 1999, 38, 3246. (7) Li Puma, G.; Yue, P. L. Comparison of the effectiveness of photon-based oxidation processes in a pilot falling film photoreactor. Environ. Sci. Technol. 1999, 33, 3210. (8) Al-Sayyed, G.; D’Oliveira, J. C.; Pichat, P. Semiconductor sensitized photodegradation of 4-chlorophenol in water. J. Photochem. Photobiol. A: Chem. 1991, 58, 99. (9) Rideh, L.; Wehrer, A.; Ronze, D.; Zoulalian, A. Photocatalytic degradation of 2-chlorophenol in TiO2 aqueous suspensions: modeling of reaction rate. Ind. Eng. Chem. Res. 1997, 36, 4712. (10) Li Puma, G.; Yue, P. L. Photocatalytic oxidation of chlorophenols in single-component and multicomponent systems. Ind. Eng. Chem. Res. 1999, 38, 3238. (11) Mathew, J.; Elzerman, A. W. Gas-liquid chromatographic determination of some chloro and nitrophenols by direct acetylation in aqueous solution. Anal. Lett. 1981, 14, 1351.
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(12) Li Puma, G.; Yue, P. L. Photodegradation of pentachlorophenol. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, The Netherlands, 1993. (13) Hatchard, C. G.; Parker, C. A. A new sensitive actinometer. II. Potassium ferrioxalate as a standard chemical actinometer. Proc. R. Soc. London 1956, 235A, 518. (14) Shi, Z.; Sigman, M. E.; Ghosh, M. M.; Dabestani, R. Photolysis of 2-chlorophenol dissolved in surfactant solutions. Environ. Sci. Technol. 1997, 31, 3581. (15) D’Oliveira, J. C.; Al-Sayyed, G.; Pichat, P. Photodegradation of 2- and 3-chlorophenol in TiO2 aqueous suspensions. Environ. Sci. Technol. 1990, 24, 990. (16) Cunningham, J.; Sedlak, P. Initial rates of TiO2-photocatalyzed degradations of water pollutants: influences of adsorption, pH and photon-flux. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, The Netherlands, 1993.
(17) Tseng, J. M.; Huang, C. P. Removal of chlorophenols from water by photocatalytic oxidation. Water Sci. Technol. 1991, 23, 377. (18) Serrano, B.; de Lasa, H. Photocatalytic degradation of water organic pollutants: pollutant reactivity and kinetic modeling. Chem. Eng. Sci. 1999, 54, 3063. (19) Rodriguez, R.; Blesa, M. A.; Regazzoni, A. E. Surface complexation at the TiO2 (anatase)/aqueous solution interface: chemisorption of catechol. J. Colloid Interface Sci. 1996, 177, 122. (20) Liu, Y.; Dadap, J. I.; Zimdars, D.; Eisenthal, K. B. Study of interfacial charge-transfer complex on TiO2 particles in aqueous suspension by second-harmonic generation. J. Phys. Chem. B: Chem. 1999, 103, 2480.
Received for review May 1, 2002 Revised manuscript received August 28, 2002 Accepted September 5, 2002 IE0203274
