Environ. Sci. Technol. 2003, 37, 1926-1932
Synergy of Combining Sonolysis and Photocatalysis in the Degradation and Mineralization of Chlorinated Aromatic Compounds J U L I E P E L L E R , †,‡ O L A F W I E S T , ‡ A N D P R A S H A N T V . K A M A T * ,† Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556-0579
Merits of using advanced oxidation processes such as sonolysis and photocatalysis as well as a combination of the two have been explored using model herbicides such as 2,4-dichlorophenoxy acetic acid and 2,4-dichlorophenoxypropionic acid and the chlorinated phenols 2,4-dichlorophenol and 2,4,6-trichlorophenol. Whereas sonolysis is quite effective in the initial degradation of chlorinated aromatic molecules, complete mineralization is difficult to achieve. Photocatalysis is selective toward the degradation of polar compounds but causes the build up of undesirable chemical intermediates. In contrast to sonolytic degradation, photocatalysis is very effective toward achieving complete mineralization. By simultaneously carrying out highfrequency sonolysis and photocatalysis we have succeeded in achieving faster and complete mineralization with no build up of toxic intermediates even at very low catalyst loadings. The synergy of combining the two advanced oxidation processes is discussed.
Introduction A great deal of attention has been focused in the past several years on advanced oxidation processes (AOPs) for the remediation of environmental contaminants (1-4). These processes involve the strong hydroxyl radical as the common oxidant, along with additional oxidation species or mechanisms. In many cases, an advanced oxidation process may be effective in certain steps or with certain compounds in the remediation but may invoke formation of undesirable or toxic chemical intermediates. In other cases, the oxidation does not proceed to complete mineralization of the organic compounds. High-frequency ultrasound is an advanced oxidation process that provides a very efficient means for the breakdown of nonpolar materials such as hydrocarbons and aromatic contaminants (5-7). The degradative processes involved in the sonolysis of aqueous solutions include •OHmediated oxidation, pyrolitic degradation, and, to a much lesser extent, oxidation via other species such as H2O2 and HO2•-. As compounds become more polar through oxidation reactions, the ultrasonic environment becomes less effective in further degradation. In fact, studies highlight the limitations of high-frequency ultrasound in achieving complete mineralization of organic contaminants (5, 8-11). * Corresponding author phone: (574)631-5411; fax: (574)631-8068; e-mail:
[email protected]. † Radiation Laboratory. ‡ Department of Chemistry and Biochemistry. 1926
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Another AOP that has been extensively studied is TiO2 photocatalysis (12). It is effective in the remediation of many organic compounds and most efficient in the later stages of degradation of more polar components. While hydroxyl radicals play a role in TiO2 photocatalysis, direct hole induced oxidation on the catalyst surface opens up an alternative route in the degradation mechanism (13-16). Many organic compounds are readily broken down, but others are not readily transformed by this heterogeneous oxidation system (17). The TiO2 photocatalysis process is most useful in its ability to rapidly degrade polar compounds. When highly polar compounds are formed during the oxidation of organic contaminants, complete breakdown to CO2 and H2O (mineralization) is quickly realized. Since the strengths of sonolysis and photocatalysis are complimentary, a system that employs them together seemed a worthwhile endeavor. Our laboratory tested a combination of sonolysis and photocatalysis in both a simultaneous and a sequential manner using the dye naphthol blue black as the model compound (9). In both cases, an additive effect was found on the rate of degradation of the dye. Additionally, the complete degradation to carbon dioxide and water was enhanced using the combination of sonolysis and photocatalysis. Other researchers have also explored the synergy of combining photocatalysis with ozonolysis (18), hydrogen peroxide (19, 20), or sonolysis (8, 21-24) for the degradation of organic compounds. In addition to increasing the rate of degradation and mineralization of environmental contaminants, the combinative approaches are implemented to solve the problem of toxic intermediates that often accumulate as chemical intermediates during the degradation process. Four model compounds that fall under the general category of chlorinated aromatic compounds were selected to further understand the utility of the combination system. Two of the compounds are widely used herbicides: 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-(2,4-dichlorophenoxy)propionic acid (2,4-DP) that are known to oxidatively transform into 2,4-dichlorophenol (2,4-DCP), chosen as the third model compound. Since chlorophenols are also common drinking water contaminants (25, 26), 2,4,6-trichlorophenol (2,4,6-TCP) was chosen as the fourth model compound. Many studies on 2,4-D have shown that advanced oxidation systems transform 2,4-D into smaller components, with the prevalent and longer-lasting intermediate 2,4-dichlorophenol (2,4-DCP) formed in the process (5, 27-32). This intermediate is more toxic than the herbicides. We have shown that the combination system effectively addresses this issue of toxic, longer lasting intermediates in addition to providing an effective route to the complete mineralization of organic compounds.
Experimental Section Materials and Reagents. 2,4-Dichlorophenoxyacetic acid (Aldrich, 99%), 2,4-dichlorophenol (Aldrich, 99%), 2-(2,4dichlorophenoxy)propionic acid (Aldrich, 95%), 2,4,6trichlorophenol (98%), TiO2 (Degussa-Huls P25). All chemicals were used without any further purification. Unbuffered aqueous solutions were prepared using Milli-Q purified water. High purity O2 gas was supplied by Mittler Supply Co., South Bend, IN. Sonolysis. Sonolysis experiments were performed using an ultrasound of 660 kHz frequency (Ultrasonics Energy Systems, Panama City, FL). The sonication cell has a volume capacity of approximately 700 mL, while actual sample volumes ranged from 475 to 500 mL. The ultrasound cell consists of a polyethylene base (PE window), which is 10.1021/es0261630 CCC: $25.00
2003 American Chemical Society Published on Web 04/02/2003
FIGURE 2. Structures of the model compounds used in the experiment. 2,4-D and 2,4-DP are herbicides. 2,4,6-trichlorophenol is also classified as a herbicide. 2,4-DCP is a prominent intermediate in the breakdown of 2,4-D and 2,4-DP.
FIGURE 1. Experimental setup employed for sonolysis, photolysis, photocatalysis, and combination experiments. The transducer and lamp were turned on as needed to conduct the individual experiment. mounted about 4 cm above the transducer (Figure 1). This configuration allows for the separation of the transducer from the reaction mixture. The power output as determined by the calorimetric method is approximately 50 W. The cell was set horizontally on a fitted plastic base, which held it in a consistent position. Throughout the experiments, the transducer and the lower portion of the cell remained submerged in an ice-water bath, to maintain the temperature inside the glass vessel around 308 K. The solutions were sparged with oxygen gas throughout the experiments. Photocatalysis and Photolysis. Photocatalysis and photolysis experiments were performed in the same vessel as described in the sonolysis experiments. A Hanovia (medium pressure) quartz, mercury-vapor immersion lamp distributed by Ace Glass, Inc. was placed next to the reaction vessel. The lamp was set inside a jacketed glass cylinder. This allowed for the circulation of water-cooled copper sulfate solution, which removed the light with wavelengths lower than 315 nm. The solution cell was the same as the one described in the sonolysis experiments and illustrated in Figure 1. During the various photolysis and photocatalysis experiments, the ice-water bath was surrounded by foil to maximize the light exposure. As with the sonolysis experiments, the solutions were sparged with oxygen gas throughout the experiments. Combination Photocatalysis/Sonolysis. The setup for the combination system was the same as that used in the photocatalysis experiments with the addition of ultrasonic waves. Both the lamp and transducer were turned on during these experiments. Adsorption Isotherms. Solutions of 2,4-D and 2,4-DCP ranging from 1.0 × 10-4 M to 7.0 × 10-4 M of 100.0 mL volumes were used along with 0.20 g of TiO2. All solutions were covered with aluminum foil to prevent light exposure and were set on stirring plates to mix for 24 h. The TiO2 was removed by centrifugation, and the solution concentrations were determined using HPLC. The equilibrium concentrations were graphed against the number of moles of compound adsorbed per gram of TiO2, and the slopes were used to calculate the equilibrium adsorption constant. Analyses. HPLC. The samples were taken out at different time intervals during the oxidation experiments. For the solutions containing TiO2, the solids were removed by centrifugation before analysis. Compound degradation and intermediate formation were monitored using a Waters HPLC system (Millennium 2010, Waters 717 plus Autosampler, Waters 600 Controller Solvent Pump) with an Alltech Econosphere C8 column, 5 mm (250 × 4.6 mm). A solvent gradient consisting of methanol, water, and water containing 1% acetic
acid was utilized. The solvent mixture started as 6% methanol, 92% water, and 2% of the acetic acid solution. By 18 min, the mixture was 70% methanol, 27% water, and 3% acetic acid solution, and it returned back to the original composition at the 30 min mark. A Photodiode Array detector monitored the 200-400 nm range. Intermediates were identified by comparison to the retention times and spectra of authentic samples. TOC. Mineralized carbon was measured using a Shimadzu Total Organic Carbon Analyzer, model TOC-5050 equipped with an ASI-500A autosampler. All experiments were carried out at least in duplicate. The reported values are within the experimental error of (5%.
Results and Discussion Sonolytic Degradation of 2,4-D and Other Model Compounds. The herbicide 2,4-D undergoes quick degradation when subjected to sonolysis in Ar or O2 saturated solutions. Hydroxyl radicals generated during the cavitation process are the primary oxidants responsible for 2,4-D degradation, reactions 1 and 2. By comparing the products with those obtained from selective radiolytic degradation we were able to demonstrate the role of •OH radicals in sonolytic degradation of aromatic compounds (5).
•
H2O ⊃⊃⊃f •OH + H•
(1)
OH + 2,4-D f products
(2)
In addition to 2,4-D, three other model compounds were also tested in the present study (2,4-DP, 2,4-DCP, and 2,4,6TCP) to compare the effectiveness of sonolysis and photocatalysis in overall degradation. The structures of these compounds are shown in Figure 2. 2,4-DCP is the major chemical intermediate formed during the initial oxidation of 2,4-D and one of the major intermediates in the breakdown of 2,4-DP. Hence, the accumulation/degradation of 2,4-DCP during the breakdown of 2,4-D/2,4-DP is crucial in deciphering the overall rate of degradation and mineralization. In the sonolysis of O2-saturated 2,4-D and 2,4-DP solutions, we observed quick degradation of the herbicides. The decay of the herbicides follows a pseudo-first-order decay with degradation lifetimes of 12 and 11 min for 2,4-D and 2,4-DP, respectively. Figure 3 shows the decrease of 2,4-D upon exposure to high frequency ultrasound, and the small amount of 2,4-DCP that is detected and subsequently degraded. A key observation in the degradation of 2,4-D is that no real build-up of the main intermediate, 2,4-DCP, occurs, revealing the main strength of the sonolytic degradation process. The sonolytic degradation of 2,4-DCP in solution indicated a degradation rate that is similar (lifetime ) 11 min) to 2,4-D. Thus, in the high frequency (660 MHz) ultrasonic degradation of 2,4-D, effective oxidation takes place VOL. 37, NO. 9, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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degradation of 2,4-D (reaction 6).
TiO2 + hυ f TiO2 (e- + h)
(3)
TiO2 (e-) +O2 f TiO2 + O2-
(4)
TiO2(h) + OH- f TiO2 + •OHsurf
(5)
TiO2(h) or •OHsurf + 2,4-D f oxidative transformations (6) FIGURE 3. Sonolysis of 0.22 mM 2,4-D solution using O2 as the sparging gas. The breakdown of (a) 2,4-D (9) and (b) formation/ degradation of the intermediate 2,4-DCP (b).
to form 2,4-DCP. This, in turn, is quickly oxidized to various other intermediate compounds. The additional compound under study, 2,4,6-TCP, reacted in the same manner under sonolytic conditions, with a lifetime of 12 min. Whereas the sonolytic degradation lifetimes for the four chlorinated aromatic compounds are very similar and relatively short, the decrease in total organic carbon occurs at a remarkably slow rate. The decrease in TOC in the sonolysis experiments is depicted in Figure 4A. Even though the parent compounds were short-lived, mineralization was typically less than 50% after 4-5 h of high frequency ultrasound. After subjecting a 0.20 mM 2,4-D solution to high frequency ultrasound for 5 h, only about 50% reduction in the total organic carbon was seen. As indicated in our previous study,5 low molecular weight organic acids (e.g., oxalic acid) accumulate in solution during sonolysis, and their degradation occurs with an extremely slow rate. Being polar in nature, these carboxylic acids are repelled from the hydrophobic cavitation bubble surface, which decreases the probability of interaction with OH radicals. Their resistance to sonolytic oxidation makes the overall process of mineralization of organic compounds very slow. Photocatalytic Degradation. The photocatalytic degradations of 2,4-D and other model compounds were carried out in the same cell that was employed for sonolysis. The illumination was done externally by placing the mercury lamp close to the solution cell that was used for sonolysis. Compared to sonolysis, a different pattern of reactivity emerged for the chlorinated aromatic compounds in the photocatalytic studies using TiO2 and UV light. Photoinduced charge separation in TiO2 is followed by charge transfer at the interface, depicted in reactions 3-5. Both valence band holes and surface hydroxyl radicals contribute to the
The herbicide 2,4-D was transformed to oxidation products during photocatalysis, and the initial degradation rate was relatively fast (lifetime ) 8 min), slightly faster than the one observed in the sonolysis experiments. The degradation lifetime for the herbicide 2,4-DP was 6 min. The clear difference in comparison with the sonolysis experiment can be seen in the formation of the chemical intermediate 2,4DCP, which accumulates during the photocatalysis of 2,4-D and 2,4-DP solutions and remains in solution for a substantial period of time. The formation/breakdown of 2,4-DCP during the photocatalysis of 2,4-D is shown in Figure 5. The photocatalytic degradation lifetime of 2,4-DCP is 20 min, which is more than twice the degradation lifetime of 2,4-D. The 2,4,6-trichlorophenol molecule, with a lifetime of 18 min, is also less prone to photocatalytic degradation than the herbicides. These experiments clearly demonstrate that certain organic compounds, which show a higher degree of resistance to TiO2 photocatalysis, tend to accumulate in solutions. Such longer survival of chemical intermediates in a water treatment process can be problematic if they turn out to be more toxic than the parent compounds. Whereas TiO2 photocatalysis is slow in its ability to efficiently break down chlorinated phenols, it is quite powerful in its ability to induce complete mineralization in a short period of time. As soon as 2,4-DCP is oxidized to more polar compounds, further oxidation to the point of mineralization is fairly rapid. The change in total organic carbon in the photocatalytic breakdown of the model compounds, illustrated in Figure 4B, supports this argument. All four compounds transformed by TiO2 photocatalysis were completely mineralized in 2-3 h, compared to the incomplete mineralization (less than 50% during the same period) noted in the sonolysis experiments. To achieve effective oxidation in the photocatalysis experiments, it is necessary for substrates to interact with the TiO2 surface. Such surface interactions lead to a faster oxidation by the photogenerated holes and surface hydroxyl
FIGURE 4. (A) Change in total organic carbon content (TOC, ppm) of model compound solutions when subjected to 660 kHz ultrasonic waves in O2-sparged system over a 4 h period. (B) Change in TOC (ppm) of model compound solutions using TiO2 photocatalysis (500 mg/L solution and CuSO4 filter solution). (a) 2,4-D; (b) 2,4-DP; (c) 2,4-DCP; and (d) 2,4,6-TCP. 1928
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TABLE 1. Degradation Lifetimes of Chlorinated Aromatic Compounds degradation lifetime, ((1) mina compound
sonolysis
sonolysis + UV photolysis
TiO2 photocatalysis
combined sonolysis and photocatalysis
2,4-dichlorophenoxyacetic acid (2,4-D) 2,4-dichlorophenol (2,4-DCP) 2,4-dichlorophenoxypropionic acid (2,4-DP) 2,4,6-trichlorophenol (2,4,6-TCP)
12 11 11 12
11 11 10 10
8.0 20.0 6.2 18
6.6 7.7 5.9 8.2
a
Degradation lifetimes (τ ) 1/k) were obtained from the first-order kinetic fit of the 2,4-decay.
FIGURE 5. Photocatalysis of 0.24 mM 2,4-D solution with O2-sparged system. The breakdown of (a) 2,4-D (9) leads to the longer-lasting (b) 2,4-DCP (b), with a lifetime (20 min) more than twice that of 2,4-D (8 min).
FIGURE 6. Adsorption of (a) 2,4-D and (b) 2,4-DCP (moles × 10-6) in the dark for 24 h per gram of TiO2 plotted against the equilibrium concentration (µm). radicals. The type of chemistry that ensues may also be dependent upon the extent of adsorption (14). Earlier studies have employed Langmuir-Hinshelwood analyses to follow the degradation of chlorinated phenols in UV-irradiated slurries (14, 33, 34). Such analyses point to the fact that the adsorption of molecules to the TiO2 surface is the limiting factor in dictating the overall rate of degradation. Other studies indicate that no strong correlation exists between the strength of adsorption and the initial rate of degradation (35, 36). Since the reactivities of 2,4-D and 2,4DCP in the photocatalysis experiments proved to be blatantly different, we probed the adsorption behaviors of these two compounds. Various concentrations in 100 mL sample volumes of 2,4-D and 2,4-DCP were stirred with 200 mg of TiO2 in the dark for 24 h. After removal of the TiO2 by centrifugation, the equilibrium concentrations were determined and graphed against the number of moles of solute adsorbed per gram of TiO2. The adsorption results are shown in Figure 6. The adsorption isotherm for 2,4-DCP yielded an equilibrium constant value, K, of 110 L/mol, while the K value for 2,4-D was calculated at 360 L/mol. While both compounds are considered to be weakly adsorbing species (36), the herbicide 2,4-D exhibits a three times greater adsorption ability than the 2,4-DCP. The greater adsorption of 2,4-D to the catalyst surface, though moderate, provides an explanation for the higher degradation rate observed for 2,4-D in the photocatalytic experiments.
FIGURE 7. Oxidative degradation of 2,4-DCP using (a) photocatalysis (9), (b) sonolysis (2), and (c) the combined photocatalysis and sonolysis, PS (b). TiO2 loading was 125 mg/L for photocatalysis and for the combination system. Combined Sonolysis and Photocatalysis. The experimental results obtained from the sonolytic and photocatalytic degradation of 2,4-D, 2,4-DP, and chlorinated phenols (Figures 3-5) demonstrate both the strengths and weaknesses of the individual processes. Whereas slow mineralization rates are established in sonolysis experiments, photocatalysis experiments exhibit a build-up of a toxic intermediate (chlorinated phenol) during the course of 2,4-D and 2,4-DP degradation. The obvious strategy would be to overcome these deficiencies by combining the two advanced oxidation processes. The merits of such an advanced oxidation approach were demonstrated in our preliminary study on the degradation of an azo dye, Naphthol Blue Black (9). We have employed simultaneous sonolysis/photocatalysis to establish the benefits of combining these two advanced oxidation processes in the degradation of the four chlorinated aromatic compounds. In the case of all four compounds, the oxidative degradation was significantly enhanced when photocatalysis and sonolysis were carried out simultaneously. The degradation lifetimes of all the compounds decreased as compared to the lifetimes obtained in either of the individual photocatalysis or sonolysis experiments. Table 1 summarizes the degradation lifetimes of the chlorinated aromatic compounds in the sonolysis, photocatalysis, and combined oxidation processes. The degradation lifetimes for the chlorinated phenols are significantly shorter in the combination experiments than the ones observed in either the photocatalysis or sonolysis experiments. For example, 2,4-DCP, the toxic intermediate of the 2,4-D breakdown, has a lifetime of only 7.7 min in the combination system compared to its lifetimes of 11 and 20 min in the sonolysis and photocatalysis separately. The degradation enhancement of 2,4-DCP is exemplified in Figure 7. The observed degradation rate follows approximately an additive rate enhancement of the individual rates according to eq 7
kobsd ) ksono + kphotocat
(7)
where the k values are pseudo-first-order rate constants determined from the reciprocal lifetimes of the corresponding degradation processes. VOL. 37, NO. 9, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. Mineralization of 2,4-D solutions using photocatalysis (9), sonolysis (b), and the combination system (2) as change in total organic carbon in ppm.
TABLE 2. Effect of TiO2 Loading on the Degradation Rate of 2,4-Dichlorophenol degradation lifetime, ((1) min TiO2, g/L 0.5 0.25 0.125 0.064 0.034 0.020 0.010 0
photocatalysis 20 21.5 25.6 49.0 >60 only 9% degradation after 90 min
combined sonolysis and photocatalysis 7.7 7.4 7.4 8.1 8.2 8.8 8.8 11.0
An interesting observation in these experiments is the fact that the decrease in total organic carbon was expedited in the combination experiments. Although photocatalysis is quite effective in mineralizing organic compounds, the addition of ultrasonic waves augments the speed of mineralization. Figure 8 shows the decrease in organic carbon of aqueous solutions of 2,4-D during the individual and combined systems. A significant benefit in mineralization rate is noted when the combined system is compared to the high-frequency ultrasound results. We also independently assessed the role of direct UVirradiation in the combined experiment. The degradation lifetimes of the four model compounds were determined using sonolysis under ultraviolet irradiation, without any added TiO2 catalyst. No difference was realized by the presence of UV light during the sonolysis experiments. The absorption band of these compounds lies in the range of 281-287 nm and tails off around 310. Since the light filtering solution used in the photolysis experiments cuts off wavelengths below 315 nm, the tested compounds should not be affected directly by the UV light. 2,4,6-Trichlorophenol showed a slight decrease in lifetime of the compound. This small difference, noted in Table 1, is within the experimental error of ( 1 min. These control experiments confirm that the additive effect seen in the combination experiment arises from the photocatalytic activity of the TiO2 nanoparticles and not the UV light. Earlier efforts have been made to combine sonolysis with either ozonolysis (8, 37) or photocatalysis (38) to improve
the effectiveness of remediation processes. In the case of phenyltrifluoromethyl ketone, the researchers failed to observe synergy using simultaneous photocatalysis and highfrequency sonolysis of TiO2 suspensions (38). The combination approach was only effective at the lower ultrasound frequency (30 kHz). By combining 500-kHz sonolysis with ozonolysis, Hoffman and co-workers (8) succeeded in improving the mineralization rate of azobenzene and methyl orange solutions from 20% to more than 80% in the presence of ozone. The results described in the present study demonstrate the advantage of combining photocatalysis with highfrequency sonolysis for the mineralization of chlorinated aromatic compounds. It is well-known in the literature that high-frequency ultrasound is useful in the destruction of nonpolar compounds (5, 7, 39, 40). The behavior of the four chlorinated aromatic compounds chosen in the present study strengthens our argument that these compounds are oxidatively transformed at the cavitation bubble interface. Chlorinated aromatics all have similar affinities toward the bubble surface, which provides a hydrophobic environment. The hydroxyl radicals produced during cavitation at the bubble interface initiate the oxidation reactions, producing more polar compounds. Since the ultrasound environment becomes less effective in further oxidative transformations, we do not observe complete mineralization. The more highly oxidized compounds formed in sonolysis have little attraction for the hydrophobic cavitation bubbles where the transformations take place. Hence, sonolysis is not a useful technique for the complete mineralization of organic compounds. The complimentary nature of TiO2 photocatalysis becomes apparent, since it is able to transform polar compounds to the point of complete mineralization. Suggestions have been made in the past that the sonication prevents the catalyst from aggregation and facilitates mass transfer of molecules to and from the catalyst surface (7, 41). While this argument is certainly valid in a general sense, it alone cannot explain the synergetic effects seen in combination experiments. We tested the hypothesis that sonolysis facilitates the break-up of TiO2 aggregates in aqueous suspensions. For this purpose, a photocatalysis experiment was performed using sonolytically treated TiO2 catalyst. First, the TiO2 suspension (no solute) was subjected to high frequency ultrasound waves for 30 min with the oxygen gas flowing to finely disperse the TiO2. The ultrasound was then turned off, solute was added, and the independent photocatalysis experiment was completed. The degradation lifetime of 2,4-DCP in this case was 18 min as compared to a degradation lifetime of 20 min (Table 2) using the catalyst without sonolysis treatment. These results show that pretreatment of the catalyst with sonolysis has limited beneficial effects but cannot account fully for the enhancement observed in the combined photocatalysis/sonolysis experiment. Similar findings were also noted at different TiO2 loadings. We further evaluated the synergy effects by carrying out photocatalysis and combination experiments at different TiO2 loadings. These results are consistent with the hypothesis that the main explanation for the enhancement in rate of degradation at the higher TiO2 loadings is the additive effect
TABLE 3. Comparison of Salient Features of Different Advanced Oxidation Processes feature
sonolysis
photocatalysis
combined sonolysis and photocatalysis
degradation of nonpolar compounds degradation of polar compounds mineralization avoidance of toxic intermediate need for a catalyst
good poor poor good no
variable good good poor yes
very good very good very good very good yes, in smaller quantity
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tocatalysis simultaneously. More importantly, no build up of toxic intermediates such as chlorinated phenols is detected in the combination experiment.
Acknowledgments We thank Dr. K. Vinodgopal for many useful discussions and the Department of Chemistry, Indiana University Northwest, for providing the facilities to perform HPLC and TOC analysis. The work described herein was supported by the Office of the Basic Energy Sciences of the U.S. Department of Energy. This is contribution no. 4409 from the Notre Dame Radiation Laboratory. FIGURE 9. Change in the total organic carbon of 2,4-DCP solutions in photocatalysis or the combined photocatalysis/sonolysis experiments using various loadings of TiO2: (a) photocatalysis, 10 mg/L; (b) photocatalysis, 64 mg/L; (c) photocatalysis, 500 mg/L; (d) photocatalysis + sonolysis, 10 mg/L; (e) photocatalysis + sonolysis, 250 mg/L; (f) photocatalysis + sonolysis, 500 mg/L. of the two processes, as described above. Additional factors, such as improved mass transfer, are less important under standard conditions but may play a larger role in the degradation at lower TiO2 loadings. The degradation results for 0.2 mM solutions of 2,4-DCP with varying TiO2 loading are summarized in Table 2. In these experiments, the amount of catalyst was systematically decreased in half, over several cycles. With small catalyst loading, a major difference is prevalent between the independent photocatalysis and the combination process. TiO2 loadings as small as 10 mg/L of TiO2 are sufficient to maintain an effective degradation rate in the combination experiment. Conversely, in the photocatalysis experiments, (no sonolysis) the lifetimes of 2,4-DCP lengthen at TiO2 loadings smaller than 125 mg/L. With a loading of 34 mg/L, the performance is maintained for the combination process (lifetime ) 8.2 min) and diminishes substantially in the photocatalysis experiment (lifetime ) 49 ( 2 min). Thus, it is clear that the combination approach remains very effective for degradation of 2,4-DCP even at very low levels of TiO2, a trend not present in the independent photocatalysis. The combination process also offers effective mineralization rates with low catalyst levels. At TiO2 loadings of 10 mg/L or greater in the combination process, the extent of mineralization of 0.2 mM 2,4-DCP remains relatively insensitive to catalytic loadings. Mineralization in the combination process is complete at 3 h, even though the rate of organic carbon depletion is somewhat slower (Figure 9). At very low TiO2 loadings in the independent photocatalysis experiments (10 mg/L or less), the change in organic carbon is drastically diminished. More than 60% of the organic carbon still remains after 3 h of photocatalysis with the low level of catalyst. At low TiO2 loadings (e65 mg/L), the photocatalysis process becomes ineffective since the rates of both mineralization and degradation are too slow. The presence of the high-frequency ultrasound waves under these conditions must offer more than just an additive process through a cavitation-induced degradation since the combination approach displays a degree of synergy at levels of TiO2 that are ineffective in the separate photocatalysis process. If the combination system functioned only by an additive effect, the photocatalysis would offer no enhancement of degradation or mineralization at these very low TiO2 levels. While analyzing the merits and deficiencies of sonolysis and photocatalysis, we came across several complimentary aspects of these two processes. Table 3 summarizes salient features of the two advanced oxidation processes and highlights the benefits of the combined approach. Quick degradation and mineralization of the chlorinated organic compounds is induced by carrying out sonolysis and pho-
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Received for review September 17, 2002. Revised manuscript received February 12, 2003. Accepted February 18, 2003. ES0261630
