Enhancement of Photocatalytic Degradation of Phenol and

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Ind. Eng. Chem. Res. 2002, 41, 5958-5965

Enhancement of Photocatalytic Degradation of Phenol and Chlorophenols by Ultrasound Yi-Chuan Chen and Panagiotis Smirniotis* Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171

The synergistic effect due to the application of ultrasound on the photocatalytic degradation of phenol and chlorophenols was demonstrated in the presence of Hombikat TiO2 suspensions in our sonophotocatalytic reactor. A noticeable enhancement of the reaction rate was found when ultrasound was combined with the photocatalysis of phenol in a volume of 100 mL as compared with UV light photocatalysis alone. By reducing the reaction volume or increasing the average ultrasonic power density within a reasonable range, the enhancement could be intensified considerably. The average ultrasonic power density of around 0.7 W/mL seemed to be the optimal usage of ultrasonic energy in our system for achieving efficient sonophotocatalytic degradation of phenol. The influence of ionic strength and anions on sonophotocatalysis was evaluated. Our results show that chloride anions (Cl-) inhibit the action of sonophotocatalysis at low pH values. Chlorophenols, namely, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol, were involved in this investigation as well, leading to the suggestions that (1) the generation of chloride radicals (Cl•) might assist the degradation of the organic compounds before substantial chloride anions (Cl-) were formed to inhibit the photocatalysis and (2) the solubility of the examined organic compounds has a significant influence on the performances of the photocatalytic and sonochemical reactions. 1. Introduction Recently, considerable interest has been shown by researchers all over the world in the application of photocatalysis for the destruction of organic contaminants in aqueous streams. Many reports in the literature have noted that a number of toxic or hazardous industrial chemicals can be destroyed by this novel technique. The technique has proven to be effective for the oxidative destruction of the most recalcitrant organic compounds such as azo dyes,1 TNT,2 and paraquat3 and for the reduction of several heavy metals.4 However, even faster decomposition is needed for the oxidation to be carried out at the commercial level. In this study, we applied ultrasound to improve the performance of photocatalytic degradation of some organic compounds and investigated some parametric effects of the enhancement due to ultrasonication. The technique of ultrasound has received much attention as an advanced oxidation process for treating wastewater.5-7 The use of ultrasound has been recognized for many years in a wide variety of processes such as cleaning, sterilization, flotation, drying, degassing, defoaming, soldering, plastic welding, drilling, filtration, homogenization, emulsification, dissolution, deaggregation of powder, biological cell disruption, extraction, and crystallization8 and as a stimulus for chemical reactions.9 By using ultrasound, some complicated reactions can be performed with inexpensive equipment and often in fewer steps than with the conventional methods.10 The destruction of hazardous chemicals, one of these applications, has been widely studied and reported, including the degradation of chlorofluorocarbons,11 volatile organic compounds,12 pesticides13 such as parathion, and 1-methylhydantoin polychlorinated * Corresponding author. Tel.: (513) 556-1474. Fax: (513) 556-3473. E-mail: [email protected].

biphenyls.14 The interest in the application of ultrasound is primarily due to its relative inexpensiveness, simple apparatus requirements, and low-severity conditions, i.e., the destruction can be achieved at ambient conditions. The chemical effects of ultrasound enhance the chemical reactivity through the phenomenon of cavitation, which involves the nucleation, growth, and collapse of bubbles in a liquid. Cavitation occurs whenever a new surface, or cavity, is created within a liquid.15 The collapse of the bubbles induces localized supercritical conditions: high temperature, high pressure, electrical discharges, and plasma effects. It has been reported that the gaseous contents of a collapsing cavity reach temperatures of 5500 °C and the liquid immediately surrounding the cavity reaches 2100 °C.16 The localized pressure has been estimated to be about 500 atm, resulting in the formation of transient supercritical water. Thus, cavitation serves as a means of concentrating the diffuse energy of sound into microreactors. Even though the local temperature and pressure conditions created by the cavity implosion are extreme, one can exert good control over the sonochemical reactions. The intensity of cavity implosion, and hence the nature of the reaction, is controlled by factors such as acoustic frequency, acoustic intensity, bulk temperature, static pressure, and the choice of solvent or dissolved gas.17 The consequences of these extreme conditions are the cleavage of dissolved oxygen molecules and water molecules (into •H atoms and •OH radicals). From the reactions of these entities (•O, •H, •OH) with each other and with H2O and O2 during the rapid cooling phase, HO2• radicals and H2O2 are formed. In this molecular environment, organic compounds are decomposed, and inorganic compounds are oxidized or reduced. In view of the above, we studied the possible enhancement of the reactant transformation due to ultrasoni-

10.1021/ie020415o CCC: $22.00 © 2002 American Chemical Society Published on Web 10/26/2002

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Figure 1. Schematic of the sonophotocatalytic reactor employed in the present study.

cation in the photocatalytic degradation of phenol. One of the main purposes in this research was to investigate how some factors such as the intensity of ultrasonic energy and the effects of ionic strength influence the degradation efficiency under sonophotocatalysis. Moreover, to obtain a better understanding of the influences of the presence of chloride radicals and ions and the chemical properties, chlorophenols with increasing number of chlorine atoms per molecule were considered in this study. 2. Experimental Section Materials. The characteristics and performances of different commercial titanias on the destruction of salicylic acid were compared in a previous work of our research group.18 The titania powders utilized in the previous study were Aldrich anatase, Hombikat UV-100, Ishihara ST-21, and Degussa P25. In the present study, we selected the commercial TiO2 powder Hombikat UV100 as our experimental photocatalyst because it exhibited the highest enhancement under ultrasound among these photocatalysts. The catalyst concentration of 0.25 g/L was utilized for the present study because it had been found earlier18 that this value provides optimum conditions. Phenol (Fisher Scientific) was chosen as the probe compound because it is one of the most common and recalcitrant pollutants present in industrial wastewaters. Its high stability and solubility in water are the main reasons that the degradation of this compound to acceptable levels is a relatively difficult process. For evaluating the effect of ionic strength, sodium chloride (Fisher Scientific) and sodium sulfate (Fisher Scientific) were utilized. The chlorophenols examined in our system were 4-chlorophenol (Aldrich), 2,4-dichlorophenol (Alfa Aesar), and 2,4,6-trichlorophenol (Aldrich). Photocatalytic Studies. The photoreactor (ACE Glass) consisted of a working volume of 50-300 mL and

was surrounded by a glass jacket to allow circulation of cooling water to maintain the reaction temperature at 30 ( 2 °C (Figure 1). The photocatalytic activity of the selected titania powder (Hombikat) was evaluated using phenol in oxygenated aqueous suspensions. A highintensity UV lamp (450-W, medium-pressure, mercury vapor quartz lamp) hosted in a Pyrex glass filter (cutoff wavelength of 320 nm) was positioned on the bottom of the reacted solution. Pure oxygen gas (Wright Brothers, 99.5%) was sparged through the vessel with a glass bubbler operating at a rate of 500 cm3/min (excess amount). The agitating action from the oxygen flow as well as ultrasound ensured adequate mixing of the suspension of titania particles inside the reactor. The sonophotocatalytic reactor was housed inside a UV safety cabinet. The experiments were performed with an aqueous solution of phenol having an initial concentration of 1 mM. The catalyst concentration in the solution was 0.25 g/L. In the pretreatment of the solutions in all of our experimental runs, the titania particles in the solution were dispersed for 10 min by an ultrasonic bath (model Labline LC20H) and magnetic-bar stirring for 40 min. Prior to the addition of the slurry solution into the reactor, another 5-min ultrasonic dispersion was conducted to ensure that none of the TiO2 powders deposited on the container. The ultrasonic field generated from the ultrasonic bath provided very satisfactory dispersion of the titania particles. The UV lamp was warmed up for 5 min before the start of the experiments. In the experiments involving the usage of ultrasound in the reaction, an ultrasound transducer was dipped into the solution from the top of the reactor and was powered by a UWR ultrasonic processor at an amplitude of 50%, corresponding to the power input of 65-75 W at the frequency 20 kHz. Samples (1 mL each) were withdrawn from the reactor by a syringe attached to extended flexible tubing through the wall of the cabinet. To facilitate the analytical process, the samples were

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Figure 2. Photocatalytic degradation of phenol in the presence of 0.25 g/L Hombikat titania with and without ultrasound [initial phenol concentration C0, 1 mM; reaction volume, 100 mL; reaction time, 3 h; oxygen flow rate, 500 mL/min; 450-W UV lamp; 70-W ultrasound (US)].

diluted three times with deionized water and then passed through 0.2-µm membrane filters contained in a plastic filter holder (Gelman Science) to remove suspended titania particles. In addition, an experiment was conducted to demonstrate that the magnetic stirring was not required in our photocatalytic reaction system without ultrasound because the reaction volume was only 100 mL and the oxygen flow was sufficient to mix the solution efficiently. In this experiment, extra mixing power provided by a motor-driven mixer provided no beneficial increase in the reaction rate. The purpose of this test was to smooth the progress of the experimental operation because generally the magnetic power was not strong enough to reach the magnetic bar in the solution through the thick barrier of the UV lamp, the Pyrex filter, and the cooling device (Figure 1). Experimental runs involving ultraviolet light (UV), ultrasound (US), and the combination of UV light and ultrasound (UV + US) were performed separately to evaluate each factor influencing the enhancement of photocatalytic degradation by ultrasound. The accuracy of all of the data was within (5%, and the experimental data presented in this paper are the average values. The reliability of the results has been confirmed with good experimental reproducibility for our reaction systems. Analytical Methods. The concentrations of diluted samples were determined by measuring the solution absorbance using a UV/vis spectrophotometer (Shimadzu UV-2501PC) equipped with quartz cuvettes of 1-cm light path. The wavelengths used for phenol, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol were 270, 274, 284, and 293 nm, respectively. Necessary calibrations were done for spectrophotometric determination of these compounds. 3. Results and Discussion Enhancement of Photocatalysis by Ultrasound. First, we evaluated the effect of ultrasound on the rate of photocatalytic degradation of phenol in our custommade photoreactor with 100-mL reaction volume (Figure 1). All of these experiments were undertaken in the presence of 0.25 g/L Hombikat titania and an excess amount of oxygen. One can observe in Figure 2 that no degradation took place in the absence of UV light and ultrasound (the top curve). This was expected, because the rate of thermal heterogeneous catalysis in titania slurries19 and

the amount of phenol stripped by the oxygen flow are negligible. However, under the presence of ultrasound with 50% amplitude (∼70 W) at the frequency of 20 kHz, a certain degree of conversion (about 10%) of phenol occurred within 3 h. It is known that sonolysis of water produces active radicals (H• and •OH) via cavitation that are capable of attacking organic compounds in solution.5 In addition, the presence of a highly heterogeneous environment (TiO2 particles) in the reaction mixture enhances this phenomenon,20 as the microbubbles tend to break up into smaller ones, thus increasing the total number of regions of high temperature and pressure. Meanwhile, dissolved gases (oxygen) serve as a source of nuclei for cavitation. This increases the number of hydroxyl radicals produced by the system and lead to the oxidation of the reactant in the absence of light. In addition to the production of radicals, the sonolysis can also occur through the pyrolysis of vaporized molecules and shear stress.7 In the present study, a high-intensity UV lamp with the power of 450 W (about 5 times the ultrasound power) was used for the purpose of highlighting the effect of ultrasonication. Figure 2 shows 76% photocatalytic decomposition of phenol within 3 h using UV only (2).The decomposition of phenol took place by the mechanism composed of the well-accepted steps of photocatalysis.21 One can see in the figure that phenol was not completely removed by photocatalysis within 3 h. However, when the application of ultrasound was combined with the above photocatalytic degradation of phenol, the result showed that a significant enhancement of the degradation rate occurred in our reaction system and the elimination of phenol was achieved in about 150 min. The reaction rate in the photocatalytic decomposition of phenol with ultrasound (7.0 µM/min) increased by about 63% in comparison with that without ultrasound (4.3 µM/min). Evidently, a synergistic effect due to the combination of UV and ultrasound on the degradation of phenol exists, because, from the chart, the degradation efficiency of UV + US is higher than that resulting from the addition of the individual effects of UV and US. This study was undertaken because ultrasound is known for its ability to disperse agglomerated particles, form hydrogen peroxide in the aqueous phase, and generate hydroxyl radicals during sonication of the aqueous phase. These actions are essential for the photodegradation process. Most importantly, catalyst deagglomeration can increase the overall surface area of the solid particles and, hence, provide more active sites to generate reactive oxygen species. One preliminary experiment was performed to demonstrate this effect (not shown in the figures). In this experiment, the photocatalytic reaction was interrupted periodically by turning the UV light off and adding ultrasound in the dark to break up particles. The period of ultrasonic interruption in the dark was only 5 min every 25 min of the photocatalytic reaction. When comparing this experiment with the one without interruption, we found that there was a certain enhancement (about 15%) in conversion in 3 h. Considering that ultrasonic breaking of the catalyst particles is the only controlling parameter in the test and that the influence of sonolysis is very limited, it could be concluded that the aid of deagglomeration due to ultrasound plays an important role in the enhanced rate of photocatalysis. In addition to providing more accessible sites for photocatalytic substrates, the increased overall surface area of TiO2 particles resulting

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from this effect can also improve the absorption of photons as the catalyst concentration is low. Accordingly, hydroxyl radicals and other reactive oxygen species can be produced more efficiently. However, the enhancement extent of the test was barely as much as the UV + US set in Figure 2 in which UV and US were implemented simultaneously along with the whole process. This implies particle dispersion is not the only factor, although it could be a major factor, in the synergistic effect of UV + US. In addition to deagglomeration, the other major reason could be the ability of ultrasound to prevent the deactivation by removing adsorbed substances on the catalyst surface. The action of surface cleaning (defined here as removal of adsorbed poisons on the TiO2 surface, produced during the photodegradation, by means of the ultrasonic phenomena of microstreaming and microbubble eruptions) is essential for reactivating the active sites on the catalyst surface. Yet, this effect can contribute to the reaction only through the simultaneous implementation of UV and US, because microstreaming and microbubble eruptions only occur in the solution during the instantaneous moment when ultrasound is involved. Thus, the catalyst used in the photocatalytic degradation can be subjected to continuous cleaning under ultrasonic treatment. Accordingly, we suggest that surface cleaning/ depoisoning of catalyst particles is another major reason for the synergistic effect. Although sonolysis itself cannot degrade phenol as effectively as photocatalysis in our system (as shown in Figure 2), it can assist the photocatalytic reaction significantly. Taking other minor factors into account, it has been found that externally adding hydrogen peroxide can enhance photocatalytic degradation.22 Hence, hydrogen peroxide generated by ultrasound in the solution can also aid the degradation process. On the other hand, it is well-known that hydroxyl radicals are the primary source of photocatalytic degradation and that sonication of the aqueous phase generates these radicals. However, a further study of radical quantification is needed to measure the influence of such factors. Additional tests were conducted in this study to demonstrate that the enhancement of the photocatalytic degradation rate due to ultrasound was not solely attributed to the better mixing provided by ultrasound. First, we supposed that the excess amount of oxygen flow (500 sccm) could offer enough agitation to form a uniform TiO2 powder distribution in the absence of magnetic stirring and ultrasound in a solution with a small reaction volume (100 mL). The extra mixing was employed using a motor-driven mixer. We found that there was no enhancement in the rate of degradation in the presence of the mixer. Also, the use of an oxygen flow rate of 1000 sccm, which increased the mixing of the suspension as well, did not lead to any additional enhancement of the reaction rate (this also demonstrated that no significant amount of phenol was stripped from the solution during the reaction). These results illustrate that, even if additional agitation in the system can be provided by ultrasound, it will not enhance the reaction rate. Therefore, we suggest that the enhancement of the photocatalytic degradation rate due to ultrasound does not mainly rely on the better mixing provided by ultrasound. Instead, the enhancement should be attributed to the supportive functions of ultrasound in the photocatalysis, i.e., mainly deagglomeration and surface cleaning.

Figure 3. Effect of reaction volume on photocatalytic degradation of phenol in the presence of 0.25 g/L Hombikat titania with and without ultrasound [initial phenol concentration, 1 mM; reaction time, 3 h; oxygen flow rate, 500 mL/min; 450-W UV lamp; 50% amplitude of power output for ultrasound (US)].

Improvement of the Sonophotocatalytic Degradation Rate by Maximizing the Utilization of Ultrasonic Energy (Volume Effect). By examining the effect of volume on the efficiency of sonophotocatalytic degradation (UV + US) of phenol, we found that reducing the reaction volume without changing other parameters gave a dramatic promotion of the synergistic effect from the combination of UV and US (Figure 3). The results show that all of the reaction rates for US, UV, and UV + US increased with decreasing reaction volume, which provides a strong demonstration that the combination of UV and US can generate a synergistic effect rather than an additive effect on the degradation of phenol in our system, regardless of the change in reaction volume. Apparently, in the experimental set of US in the figure, the decrease in volume from 300 to 50 mL did not provide a significant increase in the reaction rate when compared with the set of UV. Yet, it was expected that a considerable improvement of the photocatalytic degradation of phenol would occur as a result of the reduction in reaction volume, because the decrease in thickness of the irradiated region can minimize the attenuation of the UV intensity through the solution.21 However, it is remarkable to note that the decrease in the reaction volume significantly promoted the synergistic effect of UV and US and, consequently, the conversion. One can observe that the difference between the reaction rates of UV + US and UV in Figure 3 becomes larger as the volume decreases. This implies that the increase in the ultrasonic intensity boosts the enhancement. Let us define the relative enhancement as the absolute difference between the reaction rates of the UV + US and UV sets divided by the rate of UV alone. A relative enhancement of 29% was found at a volume of 300 mL; the enhancements observed for 200 and 100 mL were 70 and 82%, respectively. As only the smallest volume (50 mL) was addressed in this figure, the experimental set of US presented a relatively low reaction rate (0.5 µM/min) compared with the experimental set of UV (5.8 µM/min). Nevertheless, in the set of UV + US, an increase of 4.4 µM/min over UV alone was achieved. This increase was about 9 times more than the reaction rate of the US set. In other words, for a 50-mL reaction volume in our system, the synergistic effect from the combination of UV and US achieved 9 times greater effectiveness than US alone in terms of the usage of ultrasonic energy. The same wattage of ultrasonic energy was utilized in the experi-

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Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 Table 1. Conversions of Phenol under Sonophotocatalytic Degradation with and without Induced Salts

Figure 4. Dependence of the reaction rate and the relative enhancement of the reaction rate (defined as the absolute difference between the reaction rates of UV + US and UV divided by the rate of UV alone) as functions of average ultrasonic power density [Hombikat titania concentration, 0.25 g/L; initial phenol concentration, 1 mM; reaction time, 3 h; oxygen flow rate, 500 mL/ min; 450-W UV lamp; 50% amplitude of power output for ultrasound (US)].

mental sets of US and UV + US, but its effect was magnified by nearly 1 order of magnitude when coexisting with photocatalysis. Figure 4 shows that, even though the reaction rate of UV + US increases with the augmentation of average ultrasonic power density (defined as the applied ultrasonic power divided by the reaction volume), it gradually reaches a plateau. This can be attributed to the eventual absorption of ultrasonic energy by the surrounding apparatus, i.e., the reactor wall and cooling water. It can also be noticed in the figure that a maximum in the relative enhancement was found to be about 82% at the power density of 0.7 W/mL. The relative enhancement was not improved beyond this point even though the power density increased to above 1.3 W/mL. This is because, as the reaction volume was being reduced, the effect of UV irradiation began to dominate over the effect of the synergistic enhancement from UV + US. In other words, the effect of UV light attenuation through the solution was minimized as reaction volume was decreased while more and more ultrasonic energy was absorbed by the surrounding apparatus. Thus, the relative enhancement could reduce even though the reaction rate increased. Moreover, after examining the two curves in Figure 4 closely, we found that the region around the average ultrasonic power density of 0.7 W/mL seemed to give the optimal usage of ultrasonic energy in our system in terms of achieving the efficient sonophotocatalytic degradation of phenol. Influence of the Ionic Strength and the Presence of Anions. Seymour and Gupta23 reported that large salt-induced (NaCl) enhancements were observed using 20-kHz ultrasound: 6-fold for chlorobenzene, 7-fold for p-ethylphenol, and 3-fold for phenol oxidation. On the other hand, it has also been reported that the presence of a massive amount of Cl- (anions) negatively affects the photocatalytic degradation rate at low pH.24,25 Although some studies of this factor have been done for sonochemical and photocatalytic reactions individually, the effect of induced salts on sonophotocatalytic degradation of organic contaminants has not been investigated. Hereby, the combination of UV and US irradiations was applied to phenol-TiO2 slurries to perform a few experiments with the intent of understanding

induced salt

experimental set

conversion at 140 min (%)

0.5 M NaCl 0.5 M NaCl 0.5 M NaCl 1.0 M NaCl 0.25 M Na2SO4 0.25 M Na2SO4 0.25 M Na2SO4 none

US UV UV + US UV + US US UV UV + US US UV UV + US

4 28 39 33 6 52 86 7 58 96

whether the ionic strength and the presence of anions would affect the degradation efficiency. In this study, sodium chloride (NaCl) and sodium sulfate (Na2SO4) were used as sources of salt to increase the ionic strength in the phenol-TiO2 slurries. All of the pH values before and after reactions were found to be between 6.0 and 7.5. The conversions at the reaction time of 140 min for different experimental configurations are provided in Table 1. When only ultrasound was applied for the degradation, there seemed to be a certain degree of adverse effect (in both cases of NaCl and Na2SO4) instead of improved degradation rates. One of the reasons might be the presence of TiO2 particles in the solution, which reduces the sonolysis of organic compounds.28 However, a detailed investigation of the interaction between salts and TiO2 particles in the solution was not included in the scope of our research, and it requires further study. The decreased rates might also be associated with the change of vapor pressure and the number of cavities generated at the same acoustic power. In the experimental set of UV + US with 0.25 M Na2SO4, a conversion of 86% was achieved, which is lower than that obtained without any salt (96%). Nevertheless, under the same conditions but with 0.5 M NaCl, the conversion was dramatically reduced to 39%. It was also found that the increase in NaCl concentration led to a decrease of the conversion. From the table, when UV alone was applied with salts in the reaction, the conversion decreased as well. The addition of salt increases the ionic strength of the aqueous phase, which drives the organic pollutants toward the bubble-bulk interface under ultrasonication. It is well-known that the majority of degradation takes place at the bubble-bulk interface.26 The surface tension affects the nucleation process and the cavitational threshold. Thus, it was expected that enhancing the interfacial concentration of pollutants could enhance the overall degradation rate. However, the existence of Clanions imposed the major negative effect on the photocatalytic processes in our reaction system. A possible explanation is discussed later. This observation also provided support for the results presented in next section that the production of Cl- anions could inhibit the performance of the photocatalytic degradation at low pH. Comparison of the Degradation of Chlorophenols under Sonophotocatalysis. With the assumption that both the chloride radicals produced during photocatalytic degradation of chlorophenol species and the chemical properties of these organic compounds could significantly affect the degradation efficiencies of photocatalysis with and without ultrasound, experiments were conducted with three chlorophenols, namely,

Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 5963 Table 2. Comparison of Some Chemical Properties with the Reactivity in Each Tested System of the Chosen Organic Compounds chemicals

phenol 4-CP DCP TCP

solubilitya in water 3 number of Cl atoms per molecule 0 10.0 dissociation constant, Ka vapor pressure (25 °C) pH (after 3-h reaction)b US UV UV + US

7.1 7.2 7.4

half-life (min)

UV 130 UV + US 78

initial reaction ratec (µM/min)

UV UV + US

4.3 7.0

conversion (after 3-h reaction)

blank US UV UV + US

9% 76% 98%

1 2 1 2 9.1 7.9 high r low 7.4 4.4 4.1 >180 98

7.3 3.8 3.7 75 42

4.6 6.9 10.2 11.0 1% 47% 68%

26% 34% 74% 88%

2 3 6.0 6.6 3.6 3.4 35 25 14.4 17.8 11% 23% 87% 96%

a According to the CRC Atlas of Spectral Data and Physical Constants for Organic Compounds, 2nd ed. (CRC Press: Boca Raton, FL, 1975), 1 indicates insoluble, 2 slight soluble, and 3 soluble. b pH value in the beginning of reaction for each experimental set was between 6.5-7.2. c Evaluated for the first 30 min.

Figure 5. Photocatalytic degradation of (a) 4-chlorophenol, (b) 2,4-dichlorophenol, and (c) 2,4,6-trichlorophenol in the presence of 0.25 g/L Hombikat titania with and without ultrasound [initial concentration C0, 1 mM; reaction volume, 100 mL; reaction time, 3 h; oxygen flow rate, 500 mL/min; 450-W UV lamp; 70-W ultrasound (US)].

4-chlorophenol (4-CP), 2,4-dichlorophenol (DCP), and 2,4,6-trichlorophenol (TCP). During the course of the investigation, we found that the adsorption of these organics on Hombikat titania particles was insignificant by comparing the concentrations of the contaminants in the bulk solution before and after the addition of TiO2 powders under good mixing. At first, the results show that the photocatalytic degradations of 4-CP, DCP, and TCP in the presence and absence of ultrasound roughly followed a first-order reaction (Figure 5), which is different from the behavior of phenol, which is characterized by a zero-order reaction (Figure 2). From Langmuir kinetics

rate ) kKaCa/(1 + KaCa) this observation might imply that phenol adsorbs to TiO2 more strongly than chlorophenols. Therefore, one could expect that the photocatalytic degradation of

phenol could be zero-order if all of the active sites were occupied throughout the reaction. In contrast, the weakly adsorbed chlorophenols would exhibit first-order behavior. In addition, these results indicate that, for each chlorophenol, there was a certain degree of enhancement taking place due to the application of ultrasound on the photocatalysis. This is in agreement, despite some other factors, with the observations of Shirgaonkar and Pandit27 because a high-intensity UV lamp (450-W) and lower catalyst concentration were utilized in our experiments and, therefore, the adverse effect of turbidity due to ultrasound was reduced. Comparing the degradation efficiencies of TCP (Figure 5c) and phenol (Figure 2), we can see that the decomposition of TCP in our photocatalytic system was much faster than that of phenol. Under the presence of UV alone, the half-life of TCP was 35 min, compared to 130 min for phenol; under the coexistence of UV and US, the half-life of TCP was only 25 min, compared to 78 min for phenol. The above divergences in the same reaction system could possibly be attributed to the presence of chlorine radicals (Cl•) generated during the photocatalytic or sonophotocatalytic degradation process of TCP. Thus, the “autocatalysis” resulted in an increase in the initial reaction rates because Cl• itself can attack TCP similarly to a hydroxyl radical. However, the accumulation of chloride radicals seemed not to cause expected chain reactions throughout the entire 3-h degradation process. Instead, the reaction rate decreased largely after about the first 30 min. A possible explanation is incorporated below. Table 2 summarizes some chemical properties and the reactivity for each system of the chosen organic compounds. It was noticed that the pH of the TCP-TiO2 slurry solutions before reaction was 6.5, but it declined to 3.6 and 3.4 after 3 h of reaction for the sets of US and UV + US, respectively. This observation can be explained by the accumulating production of HCl during the mineralization process. Nevertheless, this was not the case for the sets of US and blank experiments in Figure 5c, i.e., their final pH values were 6.6 and 6.8, respectively. This indicates that the degradation was far from completion. It has been known that both the

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environment of low pH and the presence of chloride anions (Cl-) can result in adverse effects on the photocatalysis of TCP.25 The decrease in reaction rate along with decreasing solution pH can be attributed to the decrease in the number of hydroxyl ions in the solution. Because fewer hydroxide ions (OH-) can donate their electrons to the photoproduced holes on the surface of the TiO2 particles to form hydroxyl radicals (•OH), less photodegradation of TCP will be achieved via radical reactions. In addition, it has been reported that the dissociation of TCP might be another factor in changes in its reactivity,25 and we know that the dissociation constant (pKa) of TCP is 6.0 (Table 2). On the other hand, inhibition by Cl- on the photocatalysis at low pH might be the other reason. The pI (isoelectric point) of TiO2 is around 6.3, so the TiO2 particles carry positive charges when the solution pH is lower than 6.3. Thus, Cl- ions can be adsorbed onto the positively charged TiO2 particle surface at low pH (below the pI of TiO2) through ionic forces. Accordingly, the reaction of surface holes with the undesirable Cl- ions can decrease the formation of hydroxyl radicals, resulting in a low photocatalytic efficiency. Furthermore, we can observe from Figure 5c that no synergistic effect occurred for TCP, i.e., the enhancement was even lower than what one would expect by considering the additive effect. This is because the effect of the inhibition by Cl- ions on photocatalysis was much more significant than the effect of ultrasonic enhancement. In other words, the production of Cl- ions in the set of UV + US was much faster than that in the set of UV because of the ultrasound, which diminished the additional enhancement of degradation due to the synergistic effects from the combination of UV and ultrasound. It was noticed that some characteristics or physicochemical properties of the examined organic compounds can have dramatic effects on the performance of photocatalytic and sonochemical reactions. One of these could be the solubility of the solutes. By comparing the charts in Figures 5 and 2, it can clearly be seen that, in our system, the photocatalytic reactivity of 4-CP was lower than those of DCP, TCP, and phenol. Literature information indicates that the solubility of the chosen organic compounds in water is in the order phenol > TCP = DCP > 4-CP (Table 2). Lower solubility in water also means higher hydrophobicity. As a result of its high hydrophobicity, the 4-CP added to the suspension had fewer opportunities to be attacked by reactive oxygen species (mainly •OH) in the aqueous solution. Therefore, 4-CP, which is hardly soluble in water, gains the least benefit from photocatalysis, whereas phenol, DCP, and TCP can obtain the advantages through more frequent contacts with •OH because of their better solubilities. The effect of solubility was suggested because we found that, even without the aid of chloride radicals, the photocatalytic reactivity of phenol was still much higher than that of 4-CP. Moreover, although low-solubility and volatile organic compounds, such as 4-CP, tend to partition into the collapsing cavitation bubbles and degrade mainly by direct thermal decomposition, the pyrolytic effect due to ultrasound was relatively insignificant as compared to the effect of reactive radical attacks in the liquid phase.18 Thus, almost no conversion was observed when only ultrasound was applied to 4-CP (Figure 5a). However, a synergistic effect from the combination of UV and US was obtained in the sono-

photocatalytic degradation of 4-CP owing to the reasons described previously. In contrast to the stability of 4-CP under sonolysis, a certain degree of degradation was achieved by the application of US to DCP and TCP (Figure 5b and c). These two chlorophenol species even underwent oxidation during the 3-h reaction period by the dissolved oxygen in the blank experiments (conducted with Hombikat titania and oxygen flow but without UV and US). Another reason could be that the reactants were stripped from the solution by the oxygen flow. Considering the numbers of chlorine atoms per molecule in these three kinds of chlorophenols, more chlorine radicals could be generated by the attack of reactive oxygen species in the photocatalytic degradation process of TCP than of DCP and 4-CP, leading to the higher efficiency of degradation (despite the fact that the solubility could affect the degradation in the cases of 4-CP and phenol). These chlorine radicals could assist the degradation of the organic compounds before substantial numbers of chloride anions were formed to inhibit the photocatalysis. Also, the decrease in solution pH after 3-h reactions for 4-CP, DCP, and TCP in the experimental sets of UV and UV + US (Table 2) suggested the production of HCl, which was not the case for phenol. 4. Concluding Remarks A dramatic enhancement of the degradation rate of phenol via the synergistic effect of UV light and ultrasound irradiation has been achieved in our sonophotocatalytic reactor with a relatively small volume. By reducing the reaction volume or increasing the average ultrasonic power density within a reasonable range, the enhancement can be intensified considerably. A possible explanation for the enhancement due to ultrasound in our study lies in the effects of deagglomeration and surface cleaning from ultrasound. The influence of ionic strength and the presence of anions on sonophotocatalysis were evaluated. The results show that chloride anions (Cl-) apparently inhibit the action of photocatalysis at low pH, which corresponds to the observations in the degradation of chlorophenols. With the testing of three chlorophenols, namely, 4-CP, DCP, and TCP, we propose that the production of chloride radicals (Cl•) can assist the degradation of the organic compounds before substantial numbers of chloride anions are formed to inhibit the photocatalysis. Because of the accumulating production of HCl during the mineralization of the chlorophenols, the solution pH decreases, which is an adverse effect for photocatalysis. In the case of TCP, we found that the effect of the inhibition by Clions on photocatalysis was much more significant than the enhancement from the synergistic effects of the combination of UV and ultrasound. In addition, it is suggested in this study that the solubility of the examined organic compounds has a significant influence on the performance of the photocatalytic and sonochemical reactions. Acknowledgment This research is based on work supported in part by the U.S. Army Research Office (Young Investigator Award to P.G.S.) under Grant DAAD 19-00-1-0399. The authors are also grateful to the NSF (Grant CTS0097347) for partial assistance.

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Received for review June 3, 2002 Revised manuscript received September 10, 2002 Accepted September 13, 2002 IE020415O