A Sonophotochemical Reactor for the Removal of Formic Acid from

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A Sonophotochemical Reactor for the Removal of Formic Acid from Wastewater Parag R. Gogate, Sukti Mujumdar, and Aniruddha B. Pandit* Chemical Engineering Section, U.I.C.T., Matunga, Mumbai 400 019, India

The efficacy of a sonophotochemical reactor coupling ultrasonic irradiation with photocatalytic oxidation has been evaluated using formic acid degradation as the model reaction. The reactor used in the present work is a hexagonal flow cell irradiated with multiple-frequency transducers placed at the wall of the reactor and by ultraviolet radiation positioned at the center of the reactor. The various operating parameters studied in the present work include the concentration of the photocatalyst, initial concentration of the pollutant, and effect of aeration for the case of photocatalytic oxidation. The sonophotochemical operation has been studied in two ways: simultaneous irradiation and sequential irradiation with the aim of understanding the expected synergism between the two modes of energy transmission. The effect of the addition of hydrogen peroxide on the extent of degradation has also been studied. The hybrid technique of sonophotochemical degradation in the presence of aeration and hydrogen peroxide has been compared with the individual techniques of photocatalysis, ultrasonic irradiation, and addition of hydrogen peroxide alone, and recommendations about the effective techniques have been made for wastewater treatment applications. 1. Introduction Pollution is the contamination of water, air, and land with materials that distract from their ability to support the ecosystem or provide some human need. Human activities, industrial processes, and agricultural usage are the big contributors to the pollution problem. With new developments in almost all of the fields and the invention of new molecules which can withstand biological degradation, it is important to invent and apply new treatment procedures to keep the balance of the ecosystem intact. One of the new generation technologies, quite well established on a laboratory scale, is acoustic cavitation, i.e., cavitation generated using ultrasound. During the cavitation phenomenon, very large magnitudes of temperature and pressure are generated locally, also releasing large amounts of free radicals by the dissociation of water, which are capable of oxidizing/degrading a variety of organic and inorganic compounds, specifically biorefractory chemicals. The use of sonochemical reactors for wastewater treatment applications is not new to researchers.1-10 Excellent reviews are also available on the use of sonochemical reactors depicting in detail the mechanism of generation of cavitation as well as its action, the types of reactors used, the optimum operating conditions, and different applications.11-15 It should be noted here that the majority of the studies are on a laboratory scale, with the capacity of the reactor on the order of a few milliliters. It is very unreliable to use this knowledge to design large-scale reactors for wastewater treatment applications because of very high scale-up ratios and also nonsuitability of ultrasonic (US) transducers to operate at high frequencies and high power dissipation rates efficiently. In an earlier work,5 a triple-frequency flow cell was designed with operating conditions of a * To whom correspondence should be addressed. E-mail: [email protected]. Phone: 91-22-414 5616. Fax: 91-224145614.

power input of 900 W and frequencies of 20, 30, and 50 kHz operating in multiple combinations and was used for the degradation of formic acid. It was observed that, although the equipment was able to destroy formic acid at a reactor capacity of 7 L, the rate of degradation was quite less (only 6-7% destruction was observed in 2 h of time) and, hence, the time spans required for achieving complete degradation of chemical contaminants will be quite large (the rate of degradation will be further reduced for complex real effluents where a large variety of chemicals will be present that would interfere with the main reaction pathways). Further, it is also important to consider the cost of using US irradiation for the destruction process on an industrial scale. The current costs for the cleaning of contaminated groundwater with ultrasound are an order of magnitude higher than those by an air stripping/active carbon process.7 Thus, it is important to either find an alternative means for using cavitation energy efficiently or use acoustic cavitation (generated by US irradiation) in combination with other methods, e.g., photocatalytic oxidation, etc., with the aim of enhancing the rates of degradation. A promising aspect of photocatalysis with semiconductor materials is the near-complete mineralization of environmental pollutants in the waste stream under ambient conditions. Various chalcogenides (oxides such as TiO2, ZnO, ZrO2, CeO2, etc., or sulfides such as CdS, ZnS, etc.) can been used as photocatalysts; however, it should be noted that the best photocatalytic performances with maximum quantum yields have always been reported with titania (Degussa P-25 type, which is a combination of rutile and anatase grade). There has been extensive literature on the application of photocatalysis to wastewater treatment,16-25 and excellent reviews are also available on this subject.26-30 In the case of photocatalytic oxidation, the most common problem is the reduced efficiency of the photocatalyst with continuous operation possibly due to the adsorption

10.1021/ie010711l CCC: $22.00 © 2002 American Chemical Society Published on Web 06/07/2002

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of contaminants on the catalyst surface and blocking of the sites, which makes them unavailable for the destruction. Thus, the aim should be to devise a technique for proper cleaning of the catalyst surface during the photocatalytic operation. US irradiation can be one such technique, and it also provides an additional surface area for the reaction due to fragmentation or deagglomeration of the catalyst particles. Ultrasound will play a profound role in the situations where the adsorption of pollutants at the specific sites is the rate-controlling step. Moreover, photocatalytic oxidation is also affected by severe mass-transfer limitations especially in the supported catalyst types of reactors, which are generally preferred over slurry reactors to avoid separation problems. Turbulence created by the acoustic streaming produced by ultrasound should eliminate or decrease the mass-transfer resistance considerably. One more factor suggesting that the two techniques will give better results when operated in combination is the fact that, for both of the techniques, the basic reaction mechanism is the generation of free radicals and subsequent attack of these on the pollutant species. If the two irradiations are operated in combination, a larger number of free radicals will be available for the reaction, thereby increasing the rates of reaction, as well as the degree of oxidation conditions will be comparatively higher and hence the hybrid method should be able to degrade a wider range of pollutants. There have been many studies depicting the observed synergism and the enhanced rates of degradation for the combinatorial operation of sonochemical reactors and photocatalytic oxidation.31-42 Recently, Toma et al.43 have given a brief overview of different studies pertaining to the effect of ultrasound on photochemical reactions concentrating on the chemistry aspects, i.e., mechanisms and pathways of different chemical reactions. It has also been observed that the majority of studies are restricted to volumes of less than 1 L, and hence the scale-up of these reactors for actual wastewater treatment application will be extremely difficult because the scale-up ratio required is very high. With a view of moving one step ahead in this direction, it was thought desirable to study the synergism between sonochemical and photocatalytic reactors at a large scale of operation (7 L) in a triple-frequency hexagonal sonophotochemical flow cell. The system chosen was the destruction of formic acid because in the majority of the degradation schemes of organic compounds the end products are usually lower acids which control the overall rate of reduction of total organic carbon (TOC) or chemical oxygen demand. It should be also noted that the destruction of formic acid leads to a complete mineralization if operated for longer times (oxalic acid and hydrogen peroxide, which may be formed in the course of the reaction, are also susceptible to both US and ultraviolet (UV) irradiation and hence should be degraded; the other oxidation products can be carbon dioxide, water, CO, and H2). Because the main aim of the present work is to highlight the engineering aspects of the large-scale operation of sonophotochemical reactors, not much effort was put forward to perform a detailed kinetic analysis. Much of the information is available regarding the kinetic studies in the open literature,33,34,36,37,41 and all of the studies have reported that the degradation using the combination of US and UV irradiation follows first-order kinetics with respect to the pollutant, irrespective of its type.

Figure 1. Experimental setup used for studying synergism between the US and UV irradiations.

The effect of the addition of hydrogen peroxide (it is an additional source of free radicals) has not been studied in the literature (except for some papers by the Fung group33,34,37 at an operating capacity of 4.5 L) for the combination technique of US and UV irradiation, and hence some experiments with the addition of hydrogen peroxide have been performed in the present work at enhanced capacities. 2. Experimental Section The experiments were performed in a hexagonal flow cell depicted schematically in Figure 1. The reactor has a total capacity of 7.5 L and can be operated in either batch or continuous mode. In the present work, 7 L of aqueous formic acid has been used and the operation was in batch mode. Each side of the hexagonal flow cell is provided with three transducers, each with a power dissipation of 150 W/side. The opposite faces of the hexagon have transducers with irradiating frequencies of 20, 30, and 50 kHz and can be operated either individually or in combinations. The total maximum power that can be dissipated in the system is 900 W when all of the transducers (a combination of 20 + 30 + 50 kHz) are operated. In the annular space provided at the center of the reactor, one tube of UV light was used with a power dissipation of 8 W. The UV light source was fitted in a quartz tube placed inside the reactor, and the entire volume of the reactant is enclosed in an annular space surrounding the quartz tube and enclosed by the hexagonal faces. A stirrer was fitted in the reaction zone to ensure a good dispersion of TiO2 catalyst along with continuous renewal of the film of the reactants around the quartz tube for UV irradiation as well as around the wall of the reactor for US irradiation. Different concentrations of formic acid, in the range 100-1000 ppm, have been used to study the effect of the initial concentration on the extent of degradation. Anatase-grade TiO2 was used as the photocatalyst, and its concentration was also varied in the range 100-1000 ppm to check for optimum concentration of the same. The temperature of the reaction mixture was always maintained in the range of 28-30 °C, and continuous aeration using a sintered multipoint sparger, at a rate of 1.02 cm3/s, was carried out in all of the experiments. Some experiments were done in the presence of H2O2

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Figure 2. Degradation of formic acid using US irradiation alone.

(concentration in the range 0.1-0.5 mL/L of formic acid) alone and also in the combination of UV/US/H2O2 to examine the extent of enhancement observed due to hydrogen peroxide. The amount of the formic acid remaining in the solution was determined using titration with the standard alkali (NaOH). The concentration of the standard alkali (NaOH) was so adjusted that the readings of titration were in the range of 20-25 mL with the least count of the buret used as 0.1 mL. Moreover, the titration was repeated two to three times to get accurate readings, and hence attempts have been made to keep the errors below 0.1%. The experiments were continued for up to 2 h of irradiation time, and analysis was done at intervals of 30 min. 3. Results and Discussion The experiments with degradation of formic acid consisted of three parts: US irradiation alone, photocatalytic oxidation alone, and a combination of US and UV irradiation (in two parts: sequential operation, i.e., US irradiation followed by UV irradiation and simultaneous irradiation of US and UV irradiation). The aim of comparing the simultaneous and sequential operations was to check whether any synergism exists between the two processes or only the individual effects are contributing to the overall effect. The sequential operation of UV irradiation followed by US irradiation was not considered in the present work. This can be attributed to the fact that the most important contribution of US irradiation is to regenerate the deactivated catalyst as well as to increase the surface area of the catalyst particles available for the adsorption of the pollutant and subsequent reaction due to the photocatalytic oxidation. If UV irradiations were to be used first, with fresh or the recycled catalyst in the actual operation, this contribution would not be there and may result in an even lesser extent of degradation as compared to the operation of US irradiation followed by UV irradiation. 3.1. US Irradiation. The results with only US irradiation of formic acid have been discussed in detail in the earlier work,5 and only important findings have been depicted here to give readers better insight into optimum parameters. Figure 2 gives the variation of percentage degradation with time of irradiation for the different combinations studied in the work. It can be seen from the figure that the extent of degradation increases with an increase in the frequency of operation; further, multiple frequencies give better degradation as compared to single frequencies. The detailed explanation for this observation and also more studies with

Figure 3. Variation of percentage degradation of formic acid with initial concentration during an irradiation time of 90 min at a catalyst concentration of 500 ppm.

individual sonochemical degradation of formic acid can be obtained in the earlier work.5 Moreover, the presence of air increases the extent of irradiation. The enhancement in the presence of air was observed for all of the seven combinations studied in terms of frequency; results are only shown for the multiple-frequency operation to avoid repetition. 3.2. Photocatalytic Oxidation. The experiments for individual photocatalytic oxidation studies were performed in two different sets for investigating the effect of the initial concentration of formic acid and the concentration of photocatalyst TiO2. Aeration was found to be essential for the photocatalytic oxidation, and hence in all of the experiments aeration was used. 3.2.1. Effect of the Initial Concentration of Formic Acid. Figure 3 shows the variation in the percentage reduction in the concentration of formic acid with time for different initial concentrations. It can be seen that the percentage degradation of formic acid is found to be inversely proportional to the initial concentration of formic acid (in the range of 100-1000 ppm at a catalyst loading of 500 ppm). Topalov et al.16 have also reported that the rate of degradation of metalaxyl (a typical fungicide) decreases with an increase in the initial pollutant concentration although the total volume treated was only 20 mL. Sakthivel et al.,24 with experiments on a leather dye, acid green 16, in a reactor with a capacity of 90 mL, have obtained similar results. In the present case, the total volume treated was 7 L, and thus it can be said that the total volume to be treated does not have much effect on the trends observed for the variation in rate constants with the initial concentration of the pollutant. Further, it was also shown in the earlier work5 that the rate of sonochemical destruction of formic acid also shows similar trends, a fact that is very much important and essential when designing reactors with the combination of the two modes of irradiation. One must have similarities in terms of the variation with respect to the different operating parameters, when the combination of two techniques has to be used for synergism. It should be, however, also noted that, even if the percentage degradation of formic acid is found to be inversely proportional to the initial concentration of formic acid, the net number of moles degraded (calculated as percentage degradation multiplied by the initial concentration of the pollutant) is greater for the higher initial concentrations. This may be attributed to the fact that the variation in the rate of degradation is perhaps little hampered by the effect of pH of the solution. As the initial concentration of acid increases, the pH


Figure 5. Parity plot for the model equation.

Figure 4. Variation of the percentage degradation of formic acid with catalyst concentration during 90 min of irradiation and at 100 ppm initial concentration of acid.

decreases, thereby causing an increase in the expected rates of degradation (acidic conditions are always preferred for the photocatalytic oxidation of the pollutants as observed by Tanaka et al.21 and Andreozzi et al.23). Thus, the observed degradation in the present case will be somewhat lower as compared to the one observed with constant pH. Thus, one should always aim at having a lower initial concentration of the pollutant for having higher rates of degradation; no doubt some compromise will have to be done because of the problems associated with the treatment of higher volumes of pollutant. 3.2.2. Effect of the Catalyst Concentration. The effect of the catalyst concentration on the extent of degradation is shown in Figure 4 (the experimental conditions are a formic acid initial concentration of 100 ppm and a total time of irradiation of 90 min). It can be seen from the graph that the percentage degradation increases with an increase in the catalyst concentration until an optimum value of 500 ppm, beyond which the degradation is observed to decrease. The initial increase in the rate of degradation can be attributed to the fact that a greater number of electron-hole pairs are formed because of the large amount of the photocatalyst, resulting in a greater number of free radicals that attack the pollutant molecules. Above 500 ppm, however, it is the opacity of the solution due to the presence of solid particles which inhibits the transmission of UV light as well as results in the scattering of the incident UV light and the enhanced rate of electron-hole pair recombination which results in reduced rates of degradation. Another factor affecting the degradation might be the poor dispersion characteristics of the solid photocatalyst at higher concentrations though the contribution of this might be quite low as compared to the earlier two factors. Similar results have been obtained in the past with respect to variation of percentage degradation with the catalyst concentration although for different operating and geometric conditions. Sakthivel et al.24 have also reported an optimum value of 250 000 ppm for the catalyst concentration in the destruction of acid green 16 dye, whereas Andreozzi et al.23 have shown that the photocatalytic degradation of 4-nitrophenol increases with an increase in the catalyst concentration until an optimum value of 40 000 ppm. It can be seen that, although the trends in the observed variation are same, the optimum values of the catalyst concentration are 2-3 orders of magnitude different. This can be attributed to the fact that the optimum catalyst concentration value will be a strong function of the geometry

of the system (affecting in terms of blockage of the light) and the operating conditions viz., incident intensity of UV light and concentration and type of the pollutant (deciding the optimization in terms of relative rates of generation and recombination of the electron-hole pairs). Thus, it is of utmost importance to perform laboratory-scale experiments for the pollutant in question unless data are available in the existing literature with similarity in the operating conditions and geometry of the reactor. 3.2.3. Modeling Approach Required for Design. The data obtained should be modeled to study the effect of various parameters, and the fitted equation should give the net degradation of the pollutant. To give an indication about the type of equations to be developed, the variation of the extent of degradation with the catalyst concentration (only until the optimum value) and the initial concentration of the acid for 30 min of irradiation has been modeled using regression analysis and the resulting equation is given as

Ct/C0 ) 0.6631(catalyst concentration)-0.0474 × (initial concentration of acid)0.0955 (1) where Ct is the concentration remaining after time t and C0 is the initial concentration of acid. The equation requires that the catalyst as well as the initial concentration of acids be in units of ppm and is valid in the range of catalyst concentration of 0-500 ppm (500 ppm is the optimum concentration found in the present work) and in the range of acid concentration of 100-1000 ppm. The constant 0.6631 will be a strong function of the time of irradiation, whereas the exponents may or may not be dependent on the time of operation. The resultant model should be checked for accuracy by comparison with the experimental data, and parity plots (graph of predicted Ct/C0 against experimentally observed Ct/C0) as shown in the Figure 5 should be made. It must be noted at this stage that the present equation has been developed only to give an indication about the design strategy and is valid only in a small range of operating parameters. The generalized equations to be used for design procedures must be developed considering the entire range of operating parameters and must contain a large number of data points to enhance the confidence in the design. 3.3. Combination of US and UV Irradiation. As was said earlier, two sets, viz., sequential and simultaneous irradiation, were performed in order to investigate the mechanism of the synergism for the two techniques under consideration. The simultaneous irradiation technique was studied under different conditions of catalyst concentration and frequencies of irradiation. 3.3.1. Comparison of Sequential and Simultaneous Operation. In sequential operation, the photocatalyst was predispersed using ultrasound, resulting in an increase in the catalyst surface area by fragmen-

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Figure 6. Comparison of sequential and simultaneous irradiation techniques for sonophotochemical reactors.

Figure 7. Comparison of the combination method with the individual techniques for degradation of formic acid.

tation/deagglomeration and also cleaning of the catalyst, and the resultant solution was irradiated by UV light. The experimental conditions were 100 ppm initial concentration of the formic acid and 500 ppm catalyst concentration. In the simultaneous irradiation technique, as the name suggests, both ultrasound and UV light were put on simultaneously. The results for these two techniques are shown in Figure 6. It can be seen from the graph that the simultaneous irradiation technique gives better results as compared to the sequential technique, confirming the fact that not only an increase in the surface area and precleaning of catalyst are the important factors but also production of an additional number of hydroxyl radicals as well as continuous cleaning of the catalyst surface during the process of photoactivation under the continuous action of ultrasound are equally contributing to the expected enhancement using the combination. To give a quantitative idea, the extent of degradation of formic acid by the simultaneous technique is 30% more as compared to that of the sequential technique. Similar results have also been obtained by Stock et al.36 for the degradation of an azo dye, naphthol blue black (NBB). In their work (640 kHz sonoreactor with a power input of 240 W and a capacity of 600 mL has been used), the simultaneous technique resulted in a decrease of TOC by 75% as compared to only 50% with the sequential technique. Thus, simultaneous use of UV irradiation and ultrasound is recommended for getting better results in terms of the extent of degradation. 3.3.2. Comparison of Combination with the Individual Techniques. The results for experimentation with 100 ppm initial concentration of formic acid in the presence of air for different techniques are shown in Figure 7. The combination gives about 4 times more degradation as compared to sonication alone, whereas

the comparison with photocatalytic oxidation alone follows a somewhat complex nature. The percentage degradation initially appears to be reduced in the presence of ultrasound, which can be explained on the basis of relative contributions of the different factors underlying the expected synergism between the two techniques. Initially, fragmentation of the catalyst is the dominant factor due to large catalyst size as compared to cleaning as well as enhancement in the formation of free radicals. The formation of a large number of solid particles increases the available surface area, but, on the other hand, it also causes scattering of the UV light as well as the ultrasound and also inhibits efficient transmission of both through the medium. This was also observed visually by way of the appearance of high turbidity in the initial periods of irradiation. As the time progresses, a greater number of free radicals are generated because of the combined action (UV + US), resulting in the enhancement, and also continuous cleaning of the catalyst particle takes place which is absent in the case of photocatalytic oxidation alone, resulting in a higher extent of degradation for the combination. Moreover, as the catalyst size reaches a certain lower limit (an equilibrium value), further fragmentation does not take place. It appears that fouling of the catalyst surface is the controlling factor in the later stages of the degradation of formic acid by UV light alone as the extent of degradation almost reaches a constant value within 2 h of irradiation. In contrast, the extent of degradation increases continuously in the presence of ultrasound possibly because of the continuous cleaning of the catalyst surface. To validate the above hypothesis of relative contributions of different effects of ultrasound, the experiments were repeated with a catalyst concentration of 300 ppm. For this set, the percentage degradation for the combination was 12.24%, whereas for photocatalytic oxidation alone, it was 14.23% in the first 30 min of irradiation time. Thus, the combination technique gives 14% less degradation as compared to only photocatalytic oxidation. However, for the case of 500 ppm catalyst concentration, the same technique gives 28% less degradation as compared to the photocatalytic oxidation. This can be attributed to the fact that the negative contribution due to the fragmentation of photocatalyst will be less in the case of 300 ppm concentration as compared to 500 ppm catalyst concentration. In other words, the time required for the combination to give better results as compared to photocatalytic oxidation alone is less as soon as the fragmentation is reduced. Further confirmation of this fact was obtained when the US irradiation was restricted to 20 kHz alone instead of a 20 + 30 + 50 kHz combination for the 300 ppm catalyst concentration. The percentage degradations were found to be 12.24% (for a combination of UV light and 20 + 30 + 50 kHz ultrasound), 15.75% (for a combination of UV light and 20 kHz ultrasound), and 14.23% (for UV light alone) in the first 30 min of irradiation time. In this case, the combination technique shows better results for 30 min of irradiation time as the extent of fragmentation in the case of 20 kHz ultrasound is considerably lowered (because of a lower intensity of cavitation and hence the turbulence for the 20 kHz irradiating frequency as compared to a combination of 20 + 30 + 50 kHz ultrasound). It must also be noted at this stage that all of these results are only at initial stages and


indeed a 20 + 30 + 50 kHz combination gives better results than 20 kHz alone over extended time periods. The negative contribution resulting from the scattering of the waves due to solid particles makes a certain contribution throughout the irradiation time. This is evident also from the fact that, in 90 min of irradiation, a combination gives 15% more degradation as compared to photocatalytic oxidation for 300 ppm concentration, whereas for 500 ppm concentration of catalyst, the enhancement is just 10%, indicating a higher contribution of fragmentation at higher solid loadings. It was also observed in the study that 300 ppm concentration of catalyst in the presence of ultrasound gives almost the same amount of degradation as compared to 500 ppm catalyst in the absence of ultrasound. Thus, it can be said that the presence of ultrasound decreases the required catalyst loading to achieve the same degradation, which is likely to reduce separation problems and give some cost benefits. It should also be noted that the optimum value of the catalyst concentration (500 ppm for photocatalytic oxidation as observed in this case) will be different for the combination of US and UV irradiation because the relative contributions of different factors (fragmentation, cleaning, and production of free radicals) will be strongly dependent on the catalyst concentration. 3.4. Effect of the Addition of Hydrogen Peroxide. It can be seen from the above results that the production of the free radicals and further enhancement in the rate of generation of the same in the presence of ultrasound make a crucial impact on the overall rates of degradation. Thus, with the aim to increase the extent of degradation further, it was thought of supplying an additional source for the free radicals in the form of hydrogen peroxide. It is well-known that hydrogen peroxide in the presence of US as well as UV irradiation undergoes a dissociation reaction, forming free hydroxyl radicals which are strong oxidizing agents.44-48 It must also be noted here that if one tries to estimate the dissociation rates alone in the presence of US irradiation, it may not be possible to get measurable rates because the recombination of the formed free radicals to form hydrogen peroxide will be the dominant mechanism.49 The loading of hydrogen peroxide was restricted in the range of 0.1-0.5 mL/L of formic acid to be treated. This can be attributed to the fact that if the concentration of H2O2 is increased beyond certain limits, additional hydrogen peroxide usually recombines with the dissociated hydroxyl radicals, decreasing the availability for the pullutant degradation, and it also acts as an additional pollutant if present in large quantities. 3.4.1. Addition of Hydrogen Peroxide Alone. To see the contribution of hydrogen peroxide as an oxidizing agent, experiments were performed with the addition of 0.5 mL/L of hydrogen peroxide to 7 L of a 100 ppm formic acid solution. It was, however, observed that the hydrogen peroxide has almost no effect on the removal of formic acid with a percentage removal of just 1.65% in 90 min of treatment time. Thus, it can be said that hydrogen peroxide alone is ineffective in the treatment of formic acid. Further addition of 500 ppm catalyst (TiO2) with 0.5 mL/L of hydrogen peroxide with continuous stirring increases this degradation to 6.3% in the same time of operation. Thus, the rates of degradation are comparable with US irradiation alone5

Figure 8. Results for the combination of hydrogen peroxide and US irradiation.

but are still quite lower considering the final aim of designing reactors for effluent treatment. 3.4.2. Combination of Hydrogen Peroxide and US Irradiation. The results of experiments with the combination of US irradiation (20 + 30 + 50 kHz operating simultaneously; power input of 900 W) and hydrogen peroxide (loading of 0.5 mL/L) are shown in Figure 8. The degradation for the combination is almost double as compared to both individual techniques of US irradiation and the addition of hydrogen peroxide with the catalyst. Thus, it is now confirmed that, in the presence of US irradiation, hydrogen peroxide does dissociate, forming highly reactive hydroxyl radicals and resulting in an expected enhancement in the overall rate of degradation. Moreover, the acoustic streaming produced by US-induced cavitation phenomena50 also helps in decreasing the effect of mass-transfer limitations which otherwise severely affect the processes for treatment with hydrogen peroxide alone. Teo et al.8 have also reported that the initial rate of degradation of a 0.4 mM p-chlorophenol aqueous solution increases substantially (by about 3 times for a concentration of hydrogen peroxide of 15 mM as compared to that in the absence of hydrogen peroxide) but observed that the addition of hydrogen peroxide is only facilitated until an optimum concentration of hydrogen peroxide (40 mM for the initial concentration of pollutant as 0.4 mM). It can also be seen from the figure that the extent of degradation is marginally higher for the catalyst loading of 500 ppm as compared to 300 ppm, which can be attributed to the enhanced contribution of surface cavitation at the surface of solid particles.51 Shirgaonkar and Pandit32 have also reported that the presence of TiO2 increases the extent of degradation of 2,4,6trichlorophenol for the operation with 22 kHz frequency and a power input range of 0.4-0.12 W/mL. In the present case, the operating power density is 0.13 W/mL and the reactor is irradiated by 20 + 30 + 50 kHz frequencies operating simultaneously. 3.4.3. Hybrid Technique of UV/US/Hydrogen Peroxide. Figure 9 shows the extent of degradation of formic acid for different catalyst loadings at different times of operation. It can be seen that the presence of hydrogen peroxide significantly enhances the extent of degradation (an almost 120% increase is observed for hydrogen peroxide loading of 0.5 mL/L over the UV/US combination). This can be very well explained on the basis of enhanced formation of free radicals from the dissociation of hydrogen peroxide under the combined action of US as well as UV irradiation. Fung et al.37 have also shown that the extent of degradation of CI reactive red 120 dye increases with an addition of hydrogen peroxide until loading of 0.2

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Table 1. Comparison of the Results Obtained for Different Approaches Used in the Present Work approach

experimental conditions

results and comments

US irradiation

seven different combinations of frequencies, experiment with the presence of air

hydrogen peroxide

concentration of 0.5 mL/L, additional experiment with TiO2 catalyst and stirring

US irradiation + hydrogen peroxide

all of the frequencies operating and a hydrogen peroxide loading of 0.5 mL/L

photocatalytic oxidation

different initial concentrations of acid and catalyst TiO2 in the range of 100-1000 ppm

US irradiation + UV irradiation

100 ppm acid concentration, 500 ppm of catalyst, all frequencies operating for US irradiation and UV irradiation; two different methods of sequential and simultaneous irradiations

US + UV + hydrogen peroxide

100 ppm acid concentration, 500 ppm of catalyst, all frequencies operating for US irradiation and UV irradiation (simultaneous way); addition of hydrogen peroxide in the range of 0.1-0.5 mL/L)

multiple frequencies are better as compared to single frequencies; the presence of air also increases the extent of degradation by providing additional nuclei; the maximum degradation achieved by US alone in the presence of air is around 6.5% hydrogen peroxide does not oxidize formic acid (1.65% destruction in 90 min); stirring in the presence of TiO2 (500 ppm concentration) increases degradation to 6.3% percentage degradation is almost double as compared to individual techniques, confirming the release of free radicals due to dissociation of hydrogen peroxide rate decreases with an increase in the initial concentration of pollutant and shows optima with respect to the catalyst concentration at 500 ppm; maximum percentage degradation is observed at 500 ppm catalyst loading and 100 ppm acid initial concentration results better than the individual techniques for larger treatment periods; simultaneous irradiation gives better results as compared to sequential operation (30% more degradation); dependency of synergism between the two techniques on time of operation, more synergistic at higher treatment times best treatment approach with maximum percentage degradation achieved as 55% in 90 min of treatment time; more enhanced generation of free radicals due to faster dissociation of hydrogen peroxide under the combined action of ultrasound and UV light

is a strong function of the pollutant studied and also of the reaction conditions. Thus, it has been conclusively established that the hybrid technique of sonophotochemical destruction along with the addition of hydrogen peroxide as a source of hydroxyl radicals gives excellent results as compared to individual techniques. Table 1 gives at a glance a look at the various results obtained in the work and helps the comparison of the different approaches used in the study. 5. Conclusions Figure 9. Results for the hybrid technique of UV/US/hydrogen peroxide.

mL/L, whereas for the degradation of Cuprophenyl Yellow RL, the optimum loading of hydrogen peroxide was reported to be 0.1 mL/L.33 The operating conditions used in the experimentation of Poon et al.33 and Fung et al.37 are US transducers operating at 340 kHz frequency and UV radiations with a power input of 66 W (six tubes of 11 W each), whereas in the present case, the frequencies of operation are 20 + 30 + 50 kHz and a power input of 8 W for UV light. Thus, the rate as well as the number of formation of hydroxyl radicals will be much higher in the case of Poon et al.33 and Fung et al.37 because of more severe conditions as compared to the present case, and hence the optimum concentration of hydrogen peroxide (beyond which the scavenging action of hydrogen peroxide and the recombination of free radicals are dominant52) will be lower. In the present case, we have found no decrease in the extent of degradation with an increase in the hydrogen peroxide loading in the range of 0.1-0.5 mL/L. Thus, it can be said that the optimum loading of hydrogen peroxide

Destruction of formic acid was found to be more for the operation with multiple frequencies for US irradiation alone as compared to operation with a single frequency and was further enhanced by aeration. Photocatalytic oxidation of formic acid was observed to increase with an increase in the catalyst concentration until an optimum value of 500 ppm, whereas a lower initial concentration of formic acid gives better results. It must be kept in mind that this optimum concentration will be different for the hybrid technique. Net degradation (Ct/C0) has been correlated with these two parameters, indicating the type of correlations to be developed using the data available in laboratory-scale studies, which will then be used in the design of largescale reactors. The common optimum conditions for the two techniques of US irradiation and photocatalytic oxidation coupled with the similarity in the mechanism of destruction and cleaning of photocatalyst due to the turbulence generated by acoustic streaming make way for the development of sonophotochemical reactors. Three important factors, viz., fragmentation of the catalyst under the action of ultrasound leading to an


increased surface area but at the same time resulting in more scattering of incident UV and US waves, cleaning of the fouled catalyst as time proceeds, and enhancement in the number of free radicals generated, decide the overall synergism between the two techniques. Initially, fragmentation of the catalyst is the controlling mechanism, which results in lowering of degradation rates as compared to the photocatalytic oxidation alone, but as time progresses, the later two mechanisms take over, resulting in enhanced degradation of formic acid. These governing mechanisms have been conclusively established with an observed increased degradation as soon as the turbulence using ultrasound is reduced (comparison between 20 kHz and 20 + 30 + 50 kHz operation), lower contribution of fragmentation for reduced initial concentration of the catalyst (comparison between 300 and 500 ppm catalyst concentration), and almost constant percentage degradation at higher time periods in the case of photocatalytic oxidation alone indicating fouling of the catalyst. Moreover, better results with simultaneous operation as compared to those of the sequential one have also confirmed that the role of an enhanced amount of free radicals and continuous cleaning in the reaction period is crucial. Addition of hydrogen peroxide increases the extent of degradation because of the enhanced dissociation of hydrogen peroxide into hydroxyl radicals under the action of US and UV irradiation. In the present work, although it has been observed that the increase is continuous in the range of 0.1-0.5 mL/L loading, it must be kept in mind that addition of large quantities of hydrogen peroxide results in detrimental effects, possibly because of the recombination of free radicals with hydrogen peroxide. The optimum value is strongly dependent on the operating conditions (frequency of irradiation as well as total power input by both US transducers and UV irradiation), which decides the rate of formation of free radicals and also the type of pollutant studied. Overall, it can be said that a hybrid technique of UV/ US/hydrogen peroxide gives excellent results as compared to all of the individual techniques and the future research should be concentrated in evaluating the efficacy of this hybrid method for a variety of pollutants and, most importantly, for complex mixtures and real effluents. It should be also kept in mind that the major factor controlling the overall efficiency of destruction will be the stability of the photocatalyst under the effect of ultrasound, and efforts are required in terms of new designs, which will protect the catalyst but at the same time will give enhanced effects. Acknowledgment Authors acknowledge the funding of the Department of Science and Technology, New Delhi, India, for the research work. Nomenclature Ct ) concentration of the pollutant at time t, ppm C0 ) initial concentration of the pollutant, ppm t ) time of irradiation, min

Literature Cited (1) Price, G. J.; Matthias, P.; Lenz, E. J. The use of high power ultrasound for the destruction of aromatic compounds in aqueous solutions. Trans. Inst. Chem. Eng. 1994, 72 (part B), 27.

(2) Catallo, W. J.; Junk, T. Sonochemical dechlorination of hazardous wastes in aqueous systems. Waste Manage. 1995, 15, 303. (3) Hung, H.-M.; Hoffmann, M. R. Kinetics and mechanism of the sonolytic degradation of chlorinated hydrocarbons: Frequency effects. J. Phys. Chem. A 1999, 103, 2734. (4) Nagata, Y.; Nagakawa, M.; Okuno, H.; Mizukoshi, Y.; Yim, B.; Maeda, Y. Sonochemical degradation of chlorophenols in water. Ultrason. Sonochem. 2000, 7, 115. (5) Gogate, P. R.; Mujumdar, S.; Pandit, A. B. Sonochemical reactors for wastewater treatment: Comparison using formic acid degradation as model reaction. Adv. Environ. Res. 2001, in press. (6) Dewulf, J.; Van Langenhove, H.; De Visscher, A.; Sabbe, S. Ultrasonic degradation of trichloroethylene and chlorobenzene at micromolar concentrations: kinetics and modeling. Ultrason. Sonochem. 2001, 8, 143. (7) Peters, D. Sonolytic degradation of volatile pollutants in natural ground water: conclusions from a model study. Ultrason. Sonochem. 2001, 8, 221. (8) Teo, K. C.; Xu, Y.; Yang, C. Sonochemical degradation of toxic halogenated organic compounds. Ultrason. Sonochem. 2001, 8, 241. (9) Gaddam, K.; Cheung, H. M. Effects of pressure, temperature, and pH on the sonochemical destruction of 1,1,1-trichloroethane in dilute aqueous solution. Ultrason. Sonochem. 2001, 8, 103. (10) Sivakumar, M.; Pandit, A. B. Ultrasound enhanced degradation of Rhodamine-B: Optimisation with power density. Ultrason. Sonochem. 2001, 8, 233. (11) Keil, F. J.; Swamy, K. M. Reactors for sonochemical engineering - present status. Rev. Chem. Eng. 1999, 15, 85. (12) Mason, T. J. Sonochemistry: current uses and future prospects in the chemical and processing industries. Philos. Trans. R. Soc. London A 1999, 357, 355. (13) Thomson, L. H.; Doraiswamy, L. D. Sonochemistry: Science and Engineering. Ind. Eng. Chem. Res. 1999, 38, 1215. (14) Adewuyi, Y. G. Sonochemistry: Environmental Science and Engineering applications. Ind. Eng. Chem. Res. 2001, 40, 4681. (15) Gogate, P. R. Cavitation: An Auxiliary technique in Wastewater Treatment schemes. Adv. Environ. Res. 2001, in press. (16) Topalov, A.; Molanar-Gabor, D.; Csanadi, J. Photocatalytic oxidation of the fungicide metalaxyl dissolved in water over TiO2. Water Res. 1999, 33, 1371. (17) Canela, M. C.; Alberici, R. M.; Sofia, R. C. R.; Eberlin, M. N.; Jardim, W. F. Destruction of malodorous compounds using heterogeneous photocatalysis. Environ. Sci. Technol. 1999, 33, 2788. (18) Xu, N.; Shi, Z.; Fan, Y.; Dong, J.; Shi, J.; Hu, M. Z.-C. Effects of particle size of TiO2 on photocatalytic degradation of methylene blue in aqueous suspensions. Ind. Eng. Chem. Res. 1999, 38, 373. (19) Mazzarino, I.; Piccinini, P. Photocatalytic oxidation of organic acids in aqueous media by a supported catalyst. Chem. Eng. Sci. 1999, 54, 3107. (20) Serrano, B.; de Lasa, H. Photocatalytic degradation of water organic pollutants: pollutant reactivity and kinetic modeling. Chem. Eng. Sci. 1999, 54, 3063. (21) Tanaka, K.; Padermpole, K.; Hisanaga, T. Photocatalytic degradation of commercial azo dyes. Water Res. 2000, 34, 327. (22) Subramanian, V.; Pangarkar, V. G.; Beenackers, A. A. C. M. Photocatalytic degradation of PHBA: Relationship between substrate adsorption and photocatalytic degradation. Clean Prod. Process. 2000, 2, 149. (23) Andreozzi, R.; Caprio, V.; Insola, A.; Longo, G.; Tufano, V. Photocatalytic oxidation of 4-nitrophenol in aqueous TiO2 slurries: an experimental validation of literature kinetic models. J. Chem. Technol. Biotechnol. 2000, 75, 131. (24) Sakthivel, S.; Neppolian, B.; Arabindoo, B.; Palanichamy, M.; Murugesan, V. TiO2 catalysed photodegradation of leather dye, Acid Green 16. J. Sci. Ind. Res. 2000, 59, 556. (25) Yawalkar, A. A.; Bhatkhande, D. S.; Pangarkar, V. G.; Beenackers, A. A. C. M. Solar-assisted photochemical and photocatalytic degradation of phenol. J. Chem. Technol. Biotechnol. 2001, 76, 363. (26) Mills, A.; Davies, R. H.; Worsley, D. Water purification by semiconductor photocatalysis. Chem. Soc. Rev. 1993, 22, 417.

3378


(27) Venkatadri, R.; Peters, R. W. Chemical oxidation technologies: Ultraviolet light/hydrogen peroxide, Fenton’s reagent and Titanium dioxide assisted photocatalysis. Hazard. Waste Hazard. Mater. 1993, 10, 107. (28) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Behnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 93, 69. (29) Blake, D. M. Bibliography of work on photocatalytic removal of hazardous compounds from water and air; NREL/TP430-22197; National Renewable Energy Laboratory: Golden, CO, 1997. (30) Herrmann, J.-M. Heterogeneous photocatalysis: fundamentals and applications to removal of various types of aqueous pollutants. Catal. Today 1999, 53, 115. (31) Toy, M. S.; Carter, M. K.; Passell, T. O. Photosonochemical decomposition of aqueous 1,1,1-trichloroethane. Environ. Technol. 1990, 11, 837. (32) Shirgaonkar, I. Z.; Pandit, A. B. Sonophotochemical destruction of aqueous solution of 2,4,6-trichlorophenol. Ultrason. Sonochem. 1998, 5, 53. (33) Poon, C. S.; Huang, Q.; Fung, P. C. Degradation kinetics of cuprophenyl Yellow RL by UV/H2O2/Ultrasonication (US) process in aqueous solution. Chemosphere 1999, 38, 1005. (34) Fung, P. C.; Huang, Q.; Tsui, S. M.; Poon, C. S. Treatability study of organic and color removal in desizing/dyeing wastewater by UV/US system combined with hydrogen peroxide. Water Sci. Technol. 1999, 40, 153. (35) Naffrechoux, E.; Chanoux, S.; Petrier, C.; Suptil, J. Sonochemical and photochemical oxidation of organic matter. Ultrason. Sonochem. 2000, 7, 255. (36) Stock, N. L.; Peller, J.; Vinodgopal, K.; Kamat, P. V. Combinative sonolysis and photocatalysis for textile dye degradation. Environ. Sci. Technol. 2000, 34, 1747. (37) Fung, P. C.; Sin, K. M.; Tsui, S. M. Decolorisation and degradation kinetics of reactive dye wastewater by a UV/ultrasonic/ peroxide system. J. Soc. Dyers Colour. 2000, 116, 170. (38) Fung, P. C.; Poon, C. S.; Chu, C. W.; Tsui, S. M. Degradation kinetics of reactive red by UV/H2O2/US process under continuous mode operation. Proceedings of the IWA conferences Managing water + waste in the New Millennium: Challenges for developing areas, Midrand/Johannesburg, South Africa, 2000; Paper 3C-1. (39) Sohmiya, H.; Kimura, T.; Fujita, M.; Ando, T. Simultaneous irradiation of ultrasound and UV light. Ultrasonic acceleration of the photochemical disappearance of 4,4′-dihalogenated benzils in 1,4-dioxane. Ultrason. Sonochem. 2001, 8, 7. (40) Harada, H. Sonophotocatalytic decomposition of water using TiO2 photocatalyst. Ultrason. Sonochem. 2001, 8, 55.

(41) Ragaini, V.; Selli, E.; Bianchi, C. L.; Pirola, C. Sonophotocatalytic degradation of 2-chlorophenol in water: kinetic and energetic comparison with other techniques. Ultrason. Sonochem. 2001, 8, 251. (42) Kado, Y.; Atobe, M.; Nonaka, T. Ultrasonic effects on electroorganic processessPart 20. Photocatalytic oxidation of aliphatic alcohols in aqueous suspension of TiO2 powder. Ultrason. Sonochem. 2001, 8, 69. (43) Toma, S.; Gaplovsky, A.; Luche, J.-L. The effect of ultrasound on photochemical reactions. Ultrason. Sonochem. 2001, 8, 201. (44) Clarke, N.; Knowles, G. High purity water using hydrogen peroxide and ultraviolet radiation. Effluent Water Treat. J. 1982, Sept, 335. (45) Glaze, W. H.; Kang, J. W.; Chapin, D. H. The chemistry of water treatment processes involving ozone, hydrogen peroxide and ultraviolet radiation. Ozone Sci. Eng. 1987, 9, 335. (46) Eul, W.; Scherer, G.; Helmling, O. Practical applications of hydrogen peroxide for wastewater treatment. In Proceedings of the First International Symposium: Chemical Oxidation, Technology for the ’90s, Nashville, TN, 1991; Eckenfelder, W. W., Bowers, A. R., Roth, J. A., Eds.; 1991; p 68. (47) Bull, R. A.; Zeff, J. D. Hydrogen peroxide in advanced oxidation processes for treatment of industrial process and contaminated groundwater. In First International Symposium: Chemical Oxidation, Technology for the ’90s, Nashville, TN, 1991; Eckenfelder, W. W., Bowers, A. R., Roth, J. A., Eds.; 1991; p 26. (48) Ince, N. H. “Critical” Effect of Hydrogen Peroxide in Photochemical Dye Degradation. Water Res. 1999, 33, 1080. (49) Jyoti, K. K.; Pandit, A. B. Hybrid methods for water disinfection. Biochem. Eng. J. 2001, forwarded for publication. (50) Vichare, N. P.; Gogate, P. R.; Dindore, V. Y.; Pandit, A. B. Mixing time analysis of a sonochemical reactor. Ultrason. Sonochem. 2001, 8, 23. (51) Pandit, A. B.; Gogate, P. R.; Mujumdar, S. Ultrasonic degradation of 2,4,6-trichlorophenol in the presence of TiO2 catalyst. Ultrason. Sonochem. 2001, 8, 227. (52) Legrini, O.; Oliveros, E.; Braun, A. M. Photochemical processes for water treatment. Chem. Rev. 1993, 93, 671.

Resubmitted for review March 7, 2002 Revised manuscript received March 7, 2002 Accepted May 3, 2002 IE010711L

A Sonophotochemical Reactor for the Removal of Formic Acid from

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