Intensification of Degradation of 2,4,6-Trichlorophenol Using

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Intensification of Degradation of 2,4,6-Trichlorophenol Using Sonochemical Reactors: Understanding Mechanism and Scale-up Aspects Akshaykumar K. Shriwas and Parag R. Gogate* Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai-40019, India ABSTRACT: Degradation of 2,4,6-trichlorophenol (TCP) has been investigated using two types of sonochemical reactors viz. ultrasonic horn and ultrasonic bath reactors with an objective of investigating the effect of different operating parameters and intensifying the extent of degradation using different additives. Effect of different operating parameters such as operating temperature, power input, operating pH, and use of different additives such as solid particles, air, and hydrogen peroxide has been investigated. Experiments have been performed at two different scales of operation with an objective of illustrating the guidelines for possible scale-up strategies for cost-effective operation. As the extent of degradation was significantly lower in large scale reactor, intensification studies have been carried out by combining sonication with UV light and ozone. Possible mechanisms for intensification due to the use of different additives and combining oxidation techniques have also been illustrated based on the observed results. Maximum degradation has been obtained in the presence of air in the horn type reactor and in the presence of TiO2 as catalyst in larger capacity reactor. It has been observed that the use of additives which enhance the overall cavitational activity is more recommended due to the dominant pyrolytic mechanism of degradation.

1. INTRODUCTION In recent years, concentrated efforts have been made to harness the positive effects of cavitational reactors in different physical and chemical processing applications.1,2 Cavitation refers to formation, growth, and subsequent collapse of micro cavities causing highly turbulent motion in liquid bulk with pressures around a few thousand bars and temperatures up to a few thousand Kelvins.3 These conditions are extremely suitable for breaking the chemical bonds of complex organic compounds. Also, under cavitating conditions hydroxyl radicals are formed which can attack the organic pollutant molecules. Especially considering the organic chemical industries, where the effluents might contain hazardous pollutants such as phenol or substituted phenols, there has been great attention to develop alternate treatment schemes that would result in achieving the desired objectives of effluent treatment.4,5 The literature reports various categories of pollutants which could be degraded by ultrasound; these include dyes,6 1,4-dioxane,7 phenol,8 substituted phenols,2,9,10 and pesticides,11,12 etc. But the degradation trends and mechanisms by which different pollutants can be degraded are not the same and different dominant pathways of oxidative breakage for different organic pollutants are evident.5 There are many configurations of cavitational reactors which can have different effects for any particular application, e.g., degradation of formic acid.13 Furthermore only sonolysis is not effective many times and its effect can be intensified using some additives.14 The other way to obtain better degradation is to couple cavitation with other advanced oxidation schemes viz. use of ozone, photocatalysis, etc. The present work has been devoted to study the degradation of 2,4,6-trichlorophenol (TCP) using ultrasound. TCP is used in the leather industry, wood preservatives, and glue preparations, and also as an intermediate in the preparation of pesticides. It is an anticipated human carcinogen r 2011 American Chemical Society

according to the U.S. EPA (U.S. Environmental Protection Agency) and sufficient cases of carcinogenicity have been reported during its production.15 There have been some investigations reporting the destruction of TCP by different advanced oxidation processes including the use of ultrasonic irradiations.2,9,1618 Shirgaonkar and Pandit9 have investigated the use of a combination of ultrasound and photochemical oxidation for degradation of TCP, and reported that the use of ultrasonic irradiations alone results in much lower extents of degradation as compared to the combination approach. In another work, Pandit et al.2 have investigated the sonochemical degradation of TCP in the presence of TiO2 as a catalyst and showed that the contribution of adsorption of the pollutant to the overall extent of removal depends on the concentrations levels. Tiehm and Neis17 have investigated the effect of frequency of irradiation on the extent of degradation of TCP using ultrasonic irradiations at 41, 206, 360, 618, 1068, and 3217 kHz and reported that most efficient ultrasonic dechlorination was achieved at 360 kHz. In another recent work, Park et al.18 have investigated the degradation of different chlorophenols and showed that the partition coefficient of the pollutants strongly affects the extent of distribution due to the dependency on the availability of the pollutant molecules at the sonochemical reaction sites. A careful analysis of the earlier work related to the degradation of TCP using sonochemical reactors indicates that the work has been concentrated to investigate the degradation in a specific system using a limited set of operating conditions in terms of variation of the different operating parameters (usually only one operating parameter has been investigated in detail). Received: April 16, 2011 Accepted: July 13, 2011 Revised: July 9, 2011 Published: July 14, 2011 9601

dx.doi.org/10.1021/ie200817u | Ind. Eng. Chem. Res. 2011, 50, 9601–9608

Industrial & Engineering Chemistry Research For example, the work of Shirgaonkar and Pandit9 is based only on the use of TiO2 based processes of ultrasonic irradiation and combination of ultrasound/ultraviolet irradiations. The main focus of the present work is on understanding the dependency of extent of degradation over a broader set of different important operating parameters and also to intensify the degradation by using different additives, such as air, hydrogen peroxide, carbon tetrachloride, and ozone. The presence of additives in the sonochemical reactors results in intensification due to different mechanisms1 such as provision of additional nuclei to enhance the cavitational activity, alteration of physicochemical properties or the distribution of pollutants and enhanced generation of free radicals, or generation of additional oxidizing species in the system. It should also be noted here that depending on the additive used, it might be possible that only traces of additives remain in the final solution, so deciding the optimum concentrations of additives is a crucial factor. The expected effects are also dependent on the type of pollutant and hence it is important to investigate the effects of additives for different pollutants. Because none of the earlier works on TCP have been directed to investigating the effect of using additives, the novelty of the current work is clearly established. The study also undertakes the work at different volumes (using two different reactors) to examine the effectiveness of ultrasound as a function of scale of operation.

2. MATERIALS AND METHODS 2.1. Materials. TCP was purchased from Central Drug House,

Delhi. All other chemicals used were purchased from SD. Fine Chem. Pvt. Ltd., Mumbai, India. Distilled water was used for preparation of solutions. All the chemicals were used as received from the supplier. 2.2. Experimental Setup. Two cavitational reactors with varying capacity were used for the study. Basic configuration of both the reactors involves ultrasound transmission into the reactant mixture using a transducer, and the ultrasound generators were separate units. A schematic representation of the reactor assembly for ultrasonic horn and ultrasonic bath is depicted in Figure 1. The ultrasonic horn, procured from M/s Dakshin, Mumbai, had an operating frequency of 20 kHz and maximum power rating of 270 W. The ultrasonic bath with operating frequency of 20 kHz and power rating 230 W was procured from M/s Supersonic Ltd., Mumbai. In the case of experiments with ultrasonic bath in combination with photocatalytic oxidation, direct exposure of solution to ultraviolet radiations was achieved with a UV tube with power of 11 W and peak emission wavelength of around 365 nm. A fish pond aerator with flow rate of 0.0418 m3/h was used to bubble the air through the solution being sonicated when the study of sonodegradation in presence of air was carried out. 2.3. Experimental Methodology. Two different initial concentrations of TCP solution was used in the work, as 100 ppm and 500 ppm, to investigate the effect of initial concentration on the extent of degradation. The selected concentrations were based on the commonly found concentrations of TCP in industrial wastewaters. However, it should be also noted that, based on the type of industry, the concentration levels may be higher in which case some dilution would be essential for the applicability of the ultrasonic reactors. The operating volumes of the ultrasonic horn and ultrasonic bath reactors were 100 mL and 3.2 L, respectively. Depth of

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Figure 1. Schematic representation of the sonochemical reactors: (A) ultrasonic horn; (B) ultrasonic bath.

horn-tip was 1 cm from the liquid surface in case of the ultrasonic horn, whereas for ultrasonic bath, transducers were located at the bottom of the reactor. Temperature was varied in the range of 2540 °C to investigate the effect of operating temperature but all other reactions were carried out at a constant temperature of 30 ( 1 °C. The operating pH was varied from 1.7 to 6 to investigate the dependency of the operating pH. The power dissipation of the sonochemical reactor (ultrasonic horn) was varied over the range of 150 to 240 W in terms of the supplied power. Calorimetric studies were also undertaken to investigate the actual power dissipation into the system and the energy efficiency (ratio of actual power measured using calorimetry to the rated power dissipation) was observed to be equal to 14% over the entire range of rated power dissipation. Air flow rate was fixed at 0.0418 m3/h and air was sparged using a porous fish pond aerator for experiments related to effect of aeration. H2O2 (30% w/v) was used in the range of 0.051 wt % of solution. TiO2 loading was changed from 0.001 to 1 wt % of solution. TiO2 used in the present work was a mixture of anatase and rutile grades and was available in the powder form. The size and structure of the catalyst was not analyzed in detail due to the fact that the focus of the work was on intensification of degradation of TCP and understanding the scale-up aspects. The experiments on the large-scale bath reactor were carried out at optimized values of parameters as obtained using laboratory-scale experiments. The degradation values reported in this work are based on sonication time of 150 min. 2.4. Analysis. The samples of aqueous solution of TCP were analyzed on a UVvis-spectrophotometer (Shimadzu UV-1800) at wavelength of 293 nm. The concentration of the solution was determined from the calibration charts prepared earlier with known concentrations. Centrifugation of samples at approximately 10 000 rpm was carried out to separate the solid particles prior to the analysis where the catalytic schemes were used. 9602

dx.doi.org/10.1021/ie200817u |Ind. Eng. Chem. Res. 2011, 50, 9601–9608

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Figure 2. Effect of temperature on TCP degradation using ultrasonic horn at operating power of 240 W and initial concentration of TCP at 100 ppm.

To quantify the extent of mineralization, total organic carbon content of the samples for optimized schemes was also measured by a TOC analyzer (TOC ANATOC Series II, SGE International, PTY Ltd. Australia).

3. RESULTS AND DISCUSSION 3.1. Effect of Initial TCP Concentration. Sonodegradation of TCP was investigated at two initial concentrations, 100 ppm and 500 ppm, and it was observed that the extent of degradation was almost the same, at 24.6% and 23.3%, respectively. TOC reductions in both the cases were about 15%. It can be said that the degradation products of TCP are more resistant to the sonochemical oxidation and hence result in lower rates of removal as compared to the parent compound. The observation regarding independency of sonodegradation on the initial pollutant concentration points toward the fact that the generation of cavitational events is the rate controlling step as compared to the availability of the pollutant molecules. The hypothesis is also confirmed with the experiments involving use of air to intensify the cavitational activity in the reactor as discussed later. The observed variation is in accordance with that reported by Shirgaonkar and Pandit.9 Also, Serpone et al.19 have reported degradation of 4-chlorophenol to follow zero-order kinetics at low substrate concentrations at 20 kHz irradiation and the mechanism becomes similar to that predicted by Langmuir adsorption for higher substrate concentration. 3.2. Effect of Temperature. Operating temperatures were varied over the range of 25 to 40 °C with fixed operating power at 240 W. The extent of degradation was found to be a strong function of operating temperature. As shown in Figure 2, the degradation increased with an increase in the operating temperature. The possible reason for the observed effect can be that at higher temperatures a higher amount of pollutant is present in the collapsing cavity leading to enhanced degradation. Also at higher temperatures, the kinetic rate constants for reaction of the pollutant with free radicals are expected to be higher leading to higher extents of degradation. Based on the obtained results for the effect of operating temperature, it can be said that the main

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Figure 3. Effect of power input on TCP degradation with ultrasonic horn using initial concentration of 100 ppm at operating pH of 4.8.

reaction site for the degradation of TCP seems to be the cavitating bubble and pyrolytic mechanism is dominating over the free radical mechanism. Benitez et al.20 reported that variation in the oxidation rate of TCP with temperature is in accordance with Arrhenius dependence. Shirgaonkar and Pandit9 have also reported that higher temperatures assist sonochemical degradation of TCP. 3.3. Effect of Operating Power. Operating power is another important parameter affecting the cavitational activity and hence the extent of degradation. The operating power was varied over the range 150 to 240 W with an energy efficiency of 14%. The degradation of TCP was observed to be the maximum at power input of 180 W as depicted in Figure 3. All further investigations on the horn-type reactor were carried out at 180 W operating power. The possible reason for the observed optimum power dissipation can be the phenomenon of cavitational blocking (acoustic decoupling) at higher power inputs which arises because of very high power densities close to the delivery point.1 The obtained results also indicate that it might be a good idea to dissipate same power over larger areas of irradiation such that the problem of acoustic decoupling is avoided. Due to the use of higher transducer areas, it is expected that there will not be any accumulation of cavitating bubbles near the vicinity of the transducer as local ultrasonic intensity would be lower. Sivakumar and Pandit21 have also reported similar existence of optimum power dissipation levels for the degradation of rhodamine B. 3.4. Effect of pH. The experiments were carried out at different solution pH values adjusted by H2SO4 and NaOH solutions. The typical range considered was from pH 1.7 to pH 6. The results are shown in Figure 4. It can be seen from the figure that the sonochemical degradation of TCP is maximum at pH 2.5. A careful evaluation of the results indicated that the enhancement was quite marginal (27.7% from 24.6%) when pH was changed from 4.8 to 2.5. As 4.8 was the natural pH of the prepared solution of TCP, it was decided to investigate further treatment schemes at operating pH of 4.8. It can be also seen from the figure that there is a significant decrease in the extent of degradation when the pH was increased to 6. This can be attributed to the fact that it is important that TCP remains in molecular state, as the main mechanism for sonochemical degradation is pyrolysis. 9603



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Figure 4. Effect of pH on TCP degradation using ultrasonic horn at ultrasonic power of 180 W and initial concentration of TCP at 100 ppm.

Figure 6. Effect of H2O2 concentration on TCP degradation with 100 ppm initial TCP concentration using ultrasonic horn at 180 W power dissipation and operating pH of 4.8.

Figure 5. Effect of initial TCP concentration on TCP degradation in presence of air using ultrasonic horn at 180W power dissipation and operating pH of 4.8.

respectively. The obtained results can be attributed to two reasons viz. availability of additional oxidizing species and increased cavitational instances in the reactor leading to enhanced cavitational activity. Petrier and Casadonte26 have reported that solution saturated with air results in additional radicals, •NO2 and •NO3, which also act as oxidizers resulting in enhanced degradation of the pollutant species. Chakinala et al.27 have reported that the presence of air increases the cavitational activity as indicated by the quantification of hydroxyl radical formation using the salicylic acid dosimetry. Presence of air creates deformities in the liquid medium and hence eases the process of generation of cavitation events. Thus at any given instance there would be higher number of cavitational events occurring in the presence of air also eliminating the controlling resistance of the cavitational events and giving a first-order reaction kinetics. 3.6. Effect of Hydrogen Peroxide. Hydrogen peroxide is known to assist oxidative degradation of several organic compounds but the effect is dependent on pollutant type and the mechanism of degradation. Hydrogen peroxide has been used in some previous studies as an additional source of hydroxyl radicals with beneficial results up to certain optimum concentrations.2830 In this study, the concentration of H2O2 was varied over the range 0.05 to 1% by weight of solution (Figure 6). It has been observed that the overall effect was scavenging action of hydrogen peroxide leading to lower extent of degradation in the presence of H2O2 as compared to that obtained in the absence of H2O2. Similar results were obtained for both the initial concentrations of TCP investigated in the work. The observed results can be attributed to the fact that pyrolytic degradation is the controlling mechanism in this case. As hydrogen peroxide is more volatile, it undergoes ready dissociation in the cavitating bubbles leading to the formation of hydroxyl radicals. However due to this easy dissociation, the pyrolytic dissociation of TCP is hampered leading to lower rates of degradation. The hypothesis is confirmed by the fact that with an increase in the concentration of hydrogen peroxide, the extent of reduction in the extent of degradation of TCP also increases (about 12% reduction at loading of 0.05% to about 50% reduction at hydrogen peroxide loading of 1%).

The pKa value for TCP is 6.2 and hence it is likely that at the operating pH of 6, large quantum of TCP would be present in ionic form as compared to the natural pH of 4.8 or even stronger acidic conditions used in the work. Many researchers have also reported that acidic conditions are better for sonochemical degradation of pollutants.6,2224 Hamdaou and Naffrechoux25 have shown pH 5.5 to be better than pH 12.7 for 4-chlorophenol degradation by ultrasound. 3.5. Effect of Air. Experiments were carried out using a fish pond aerator with an air flow rate of 0.0418 m3/h with two initial TCP concentrations. The results are shown in Figure 5 and it can be seen that presence of air assists sonodegradation of TCP remarkably; 52.6% and 65% degradation of TCP was obtained by bubbling air along with sonolysis for 100 ppm and 500 ppm initial TCP concentrations, compared against 24.6% and 23.3% without the presence of air. The extent of TOC reduction was 45% and 58.8% for 100 and 500 ppm initial TCP concentration,

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3.7. Effect of Solid Particles. Presence of solid particles helps sonodegradation of organic pollutants as it can increase cavitational activity by providing additional deformities in the liquid medium which can act as additional nuclei for the cavitation events.31 TiO2 was used in the range from 0.001 to 0.05 wt %. For all loadings, presence of TiO2 was found to be considerably effective, but maximum effect was with 0.001 wt % TiO2 loading as shown in Figure 7. At this optimum concentration of solid particles, TOC removal was observed to be 20%. The increased degradation of TCP in the presence of solids can be attributed to the fact that the solid particles provide additional nuclei for cavitation. Similar observations of increased extents of TCP degradation by sonocatalysis were reported by Shirgaonkar and Pandit9 and Pandit et al.2 Adsorption of TCP on TiO2 particles may play a vital role in making the cavitational degradation ineffective since adsorption is only physical removal of pollutant which is not the desired objective. Adsorption studies were carried out and it was

observed that adsorption of TCP on TiO2 is negligible perhaps due to much less loading of TiO2 used in the current work. The observed optimum concentration of the solid additive can be explained on the basis of screening effect of the presence of solid particles. Due to the presence of excess solid particles in the system, the incident sound waves are scattered resulting in lower amount of energy dissipated into the system due to the ultrasonic irradiation. It should be also noted here that the extent of intensification due to the use of solid particles will be dependent on the structure and size of the particles used, especially when catalyst flakes or lumps are used. It is expected that under the influence of ultrasonic irradiations, there might be a change in the structure of the solids that would have an effect on the rates of degradation. As the TiO2 used in the present work is in the powder form, not much change in the structure is expected and the effect of size/ structure would be more dependent in the case of solid lumps/ flakes or in the case of supported catalysts. 3.8. Effect of Scale of Operation and Type of Reactor. Experiments were carried out at larger scale operation (3.2 L using ultrasonic bath) to investigate the dependency of the degradation process on the scale of operation. It has been observed that the extent of degradation was much lower (68 times) in ultrasonic bath as compared to ultrasonic horn under otherwise similar operating conditions except in case of sonocatalysis where degradation was 45% lower in bath than that obtained in horn (Table 1). Figure 8 gives the obtained data for ultrasonic horn and ultrasonic bath (operation using only ultrasound). The possible reason for the observed results is lower power density which results in lesser cavitational activity in the system leading to lower degradation rates. Lower number of cavitational events occur in the reactor leading to lower rates of pyrolytic degradation of pollutant. The extent of degradation was again found to be independent of the pollutant concentration confirming the earlier hypothesis of rate controlling action of the cavitational events. Because of lesser extent of degradation in the ultrasonic bath reactor, an attempt was made to investigate the different hybrid strategies to intensify the degradation process such as using combination of horn and bath, using CCl4 as an additive, and combining advanced oxidation process with ultrasonic irradiations.

Figure 7. Effect of catalyst (TiO2) at different loadings on TCP degradation using ultrasonic horn at 180 W power dissipation, operating pH of 4.8, and initial concentration of TCP at 100 ppm.

Table 1. Comparison of Ultrasonic Horn and Ultrasonic Bath for TCP Degradation and TOC Removal (a) scheme

% TCP degradation

% TOC removal

sonolysis of 100 ppm TCP solution

24.6

14.9

sonolysis of 100 ppm TCP solution in presence of air

52.6

45

sonolysis of 500 ppm TCP solution in presence of air sonolysis of 100 ppm TCP solution with TiO2 catalyst

65.0 37.9

58.8 20

% TCP degradation

% TOC removal

(b) scheme sonication of 100 ppm TCP solution on ultrasonic bath reactor

4.1

2.2

sonication of 100 ppm TCP solution in presence of air on ultrasonic bath reactor

8.3

6.0

22.5 7.0

17.5 5.1

sonication of 100 ppm TCP solution in presence of TiO2 on ultrasonic bath reactor sonication of 100 ppm TCP solution on ultrasonic bath reactor in combination with horn sonication of 100 ppm TCP solution in presence of CCl4 on ultrasonic bath reactor

6.7

2.5

sonication of 100 ppm TCP solution with UV and TiO2 on ultrasonic bath reactor

20.9

12.8

sonication of 100 ppm TCP solution in presence of ozone on ultrasonic bath reactor

21.9

17.9

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Figure 8. Comparison of the extent of degradation in ultrasonic horn and ultrasonic bath for initial concentration of TCP at 100 ppm and operating pH of 4.8.

3.8.1. Combination of Ultrasonic Horn and Bath Reactor for Sonodegradation. To study the effect of combination of both the reactors, the tip of the ultrasonic horn reactor was kept immersed 1 cm into the solution in the ultrasonic bath during sonication. The aim of this scheme was to investigate the degradation of TCP with ultrasound irradiation from opposite planes, which can result in interference patterns and generation of standing waves. It has been observed that the extent of degradation of TCP increased to about 7.0% from 4.1% obtained in bath. TOC reduction obtained in this case was 5.1%. The possible reason for the observed effects can be enhanced generation of cavitational events and higher cavitational intensity in the system due to interference and presence of standing waves. The obtained results also point toward scale-up strategy that it is better to use multiple transducers in the reactor to get beneficial results at larger scales of operation. 3.8.2. Effect of CCl4. Sonodegradation, in the presence of CCl4, may be enhanced since CCl4 increases radical formation to help degradation of organic pollutants.32 CCl4 was added to the system at a loading of 1 g/L, however it was observed that the effect was not pronounced since only 6.7% degradation was obtained in the presence of CCl4 as against 4.1% without CCl4. A 2.5% reduction in TOC was observed with this scheme. The observed results can be attributed to the fact that the presence of CCl4 in the cavitating bubble competes with the pollutant molecule for degradation and it is likely that large concentrations of CCl4 are present in the cavitating bubble as compared to TCP due to much higher vapor pressures of CCl4. 3.8.3. Effect of Ozone. Ozone is considered to be an excellent oxidizing agent though the cost associated with usage is a limiting factor, especially in wastewater treatment application.33 The ozone generator used in this work was from Amsons Technologies with an output of 10 g/h. The presence of ozone has been observed to assist sonodegradation remarkably since 21.9% degradation of TCP was obtained in presence of O3 (Figure 9) as against only 4.1% without bubbling ozone through solution. TOC reduction was 17.9% using combined operation of ozone and ultrasound. The observed results in terms of enhancement in

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Figure 9. Effect of sonication in the presence of ozone using ultrasonic bath reactor for initial concentration of TCP at 100 ppm, operating pH of 4.8, and ozone loading of 10 g/h.

the extent of degradation can be attributed to enhanced cavitational activity by similar mechanism as discussed for the effect of presence of air. In addition there is enhanced radical generation for combination of ozone and sonication as illustrated in following reaction schemes:34,35 O3 þ OH f 2HO þ O2 2HO þ H þ f H2 O2 O3 þ 2HO f 2O þ 3 OH þ O2 2O þ H þ f HO2 3 O3 þ 3 OH f O2 þ HO2 3 O3 þ 2O f O2 þ 3O 3O þ H2 O f OH þ 3 OH þ O2 Also, the presence of ozone in the system can give direct oxidation of TCP due to the oxidizing capacity of ozone molecule. Graham et al.36 have reported the aqueous reactivity of 2,4,6-trichlorophenol (TCP) with ozone using a simple gas bubble/liquid contacting system. 3.8.4. Sonophotocatalytic Degradation of TCP. Combined use of sonochemistry and photocatalytic oxidation has also been reported to be efficient for the degradation of chloroderivates of phenol.37 Due to this fact, direct exposure of the TCP solution containing TiO2 to UV-radiation was maintained in the present study in order to study the efficiency of sonophotocatalysis. The extent of removal of TCP was found to be 20.9%, which is significantly more than that obtained with only sonication (4.1%) but similar to that obtained by using a combination of sonication and TiO2 (21.9%). The extent of TOC removal with sonophotocatalysis was observed to be 12.8%. Shirgaonkar and Pandit9 have reported similar observation as compared to the present 9606


Industrial & Engineering Chemistry Research study; the degradation of TCP was lower for sonophotocatalysis as compared to sonocatalysis at operating temperatures of 30 ( 2 °C. It can be said that the use of UV light does not help in increasing the extent of mineralization and the same effect can be achieved using a combination of ultrasound and TiO2. The observed results are attributed to the controlling dominance of pyrolytic mechanism over the free radical mechanism in the overall degradation of TCP. The observed enhancement in sonophotocatalytic oxidation is mainly attributed to the controlling effects of action of the hydroxyl radicals and photogenerated holes in the case of photocatalysis.33,3840

4. CONCLUSIONS The present work has clearly established the dependency of TCP removal rates on the different important parameters and the use of different additives. Based on the investigations related to removal of TCP under different operating conditions, the following important conclusions can be established: (1) The dominant mechanism for sonochemical degradation of TCP is pyrolytic degradation as clearly established by higher rates for additives leading to a larger number of cavitational events. (2) Additives can be used to enhance the degradation of pollutants and the extent of intensification depends on the controlling mechanism of the pollutant. When the dominant mechanism is pyrolytic, it is better to use additives (air, solid particles) which give higher cavitational activity rather than additives which give a larger number of free radicals (hydrogen peroxide, carbon tetrachloride or combining sonication with photocatalysis). It is also expected that the efficacy of the used additives will be similar for real wastewaters as the main mechanism is in terms of the enhanced cavitational activity. In the case of pollutants where the controlling mechanism is free radical attack, some pretreatment will be required especially when radical scavengers are present in the real industrial wastewater. (3) It is imperative to use higher areas of irradiation to avoid acoustic decoupling effects. Also combined irradiation using different transducers leads to enhanced cavitational activity and hence more degradation of pollutants. Thus, the scale up of sonochemical reactors should be based on the use of multiple transducers with larger areas of irradiation. (4) Due to the use of additives favoring enhanced cavitational activity, there is a substantial increase in the extents of degradation and the use of additives does not have any effect on the energy consumption. Thus, for these additives, the extent of degradation per unit of supplied energy will increase and hence the applicability of the sonochemical reactors will be enhanced. (5) Further work can be concentrated on investigating the sonochemical degradation of TCP in larger capacity reactors and also using different additives such as gases or nanoparticles which can enhance the overall cavitational activity in the reactor. As pyrolytic mechanism is controlling use of salts which can alter the partition coefficient of TCP and ensure more presence at the cavity collapse site can be investigated to further enhance the rates of degradation.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: +91-22-33612024; fax: +91-22-33611020.

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