ARTICLE pubs.acs.org/IECR
Ultrasonic Degradation of p-Nitrophenol in the Presence of Additives at Pilot Scale Capacity Kashyap P. Mishra† and Parag R. Gogate*,† †
Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai-40019, India ABSTRACT: In the present work, the efficacy of the ultrasonic reactors for the degradation of p-nitrophenol, one of the most refractory compounds, has been investigated at pilot scale operation. An attempt has also been made to intensify the degradation process using different additives. The effect of air sparging, CCl4 addition, addition of solid particles such as TiO2 and CuO, and addition of ozone on the extent of degradation of p-nitrophenol has been studied. The sonochemical reactor used in the work is an ultrasonic longitudinal horn type reactor operating at 25 kHz frequency, power output of 1 kW, and capacity of 7 L. It has been observed that the extent of degradation increased for all the additives used in the present study possibly attributed to enhanced cavitational activity and generation of additional free radicals or oxidizing species. Maximum degradation of 93.5% was obtained using CCl4 as an additive at an optimum loading of 1 g/L. It appears that among different additives, additives giving additional free radicals and parallel oxidation mechanisms give more intensification as compared to additives which only enhance the cavitational activity in the system. It has been established that the use of additives at optimum concentration, which is dependent on the inherent cavitational activity in the reactor, results in significant intensification of the degradation process.
1. INTRODUCTION Increasingly stringent water quality regulations require better removal of organic pollutants from sewage and industrial effluents. With the continuous advent of new refractory chemicals, research into alternate treatment schemes has gained considerable momentum over the past few years1 as the effectiveness of the conventional treatment schemes is not able to meet the demands of the stringent regulations. One of the new promising technologies has been based on the use of power ultrasound for wastewater treatment.2 The advantage of this process is based on the fact that the oxidation reactions can be carried out under ambient global conditions avoiding the use of rigorous conditions such as high temperature and pressure in some of the alternate treatment schemes. In this technique, the free radicals are generated through transient collapse of cavitation bubbles driven by an ultrasound wave.3 The sonochemical degradation can proceed via direct pyrolysis, which occurs in and around the collapsing bubbles as well as via the production of OH• radicals. There have been many illustrations in the open literature where acoustic cavitation has been used for the wastewater treatment applications,2,4,5 but the majority of the work available in open literature is restricted to laboratory scale operation. The required scale up ratio for translating the available literature information to large scale operation would be very large inducing a degree of uncertainty. The current work tries to bridge the gap by reporting the results at a pilot scale operation using a model pollutant as p-nitrophenol and also illustrates the designs of the sonochemical reactors and approaches (using combination with additives) to be used for successful application in the field of wastewater treatment. p-Nitrophenol is one of the most refractory substances present in industrial wastewaters because of their high stability and solubility in water. p-Nitrophenol is present in the wastewaters from a number of industries such as textiles, paper and pulp, plastics, etc. Nitrophenols, in general, pose significant health risks r 2011 American Chemical Society
since they are carcinogenic, and p-nitrophenol is also listed on the US Environmental Protection Agency’s priority pollutants list. The treatment times for removal of nitrophenols with chemical or biological methods may be quite high and total mineralization of the effluent stream may not be possible. Considering these issues, p-Nitrophenol has been selected as an industrially important pollutant for investigating the application of sonochemical reactors for wastewater treatment. The different additives used in the work to intensify the sonochemical degradation of p-nitrophenol include air sparging, CCl4 addition, addition of solid particles such as TiO2 and CuO, and combined treatment using ozone. The presence of additives in the sonochemical reactors2,69 can typically provide additional nuclei or alter the physicochemical properties of the liquid medium so that the number of cavitation events in the system increases leading to enhanced effects. Also some of the additives can promote generation of free radicals or generation of additional oxidizing species in the system.
2. MATERIALS AND METHODS 2.1. Materials. p-Nitrophenol has been obtained from HiMedia Laboratories Pvt. Ltd., Mumbai, India. Carbon tetrachloride (CCl4), titanium dioxide (TiO2) powder (mixture of anatase and rutile grade), and CuO powder all of AR grade were obtained from SD Fine Chem. Pvt. Ltd., Mumbai, India. All the chemicals were used as received from the suppliers. Distilled water was used for the preparation of solutions. Received: July 5, 2011 Accepted: December 20, 2011 Revised: December 13, 2011 Published: December 20, 2011 1166
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Figure 2. Sonochemical degradation of p-nitrophenol in the absence and presence of air sparging.
Figure 1. Schematic representation of ozonation setup with reactor.
2.2. Experimental Setup. The ultrasonic reactor configuration used in the present work is basically an ultrasonic bath equipped with single transducer, procured from Roop Telesonic Ultrasonic Limited, India. The reactor has an operating frequency of 25 kHz and a rated power output of 1 kW. The internal of the bath consists of a transducer fitted at the bottom of the bath horizontally along the length of the bath and generally vibrates in a perpendicular direction away from the reactor bottom. The energy to this transducer is provided by a generator which is a separate unit. A drainage valve is also provided at the bottom of the reactor. The reactor body is made up of stainless steel. Calorimetric studies have been undertaken to quantify the actual power dissipation into the system.10,11 It has been observed that the actual power dissipation at an operating capacity of 7 L is 195 W giving an energy efficiency of 19.5%. For combinations with chemical oxidation based on the use of ozone, an ozone generator was used (PCI Ozone Corporation, West Caldwell, New Jersey, Model: GL-1, Capacity: 1 lb/day). An oxygen cylinder was used to provide oxygen to the ozonator. The experimental setup for the combined operation (sonication and ozonation) has been depicted schematically in Figure 1. 2.3. Experimental Procedure. The reactor was filled with 7 L of aqueous solution of p-nitrophenol. During the study, samples were collected after every 15 min to monitor the progress of degradation. 240 min was kept as the base time for each experiment (except for ozonation) as the objective was to mainly quantify the intensification obtained due to the use of different additives. For all the runs, pH was initially set around acidic (2.5) and was not controlled during the experimental runs. The adjustment of the pH was achieved using 0.2 M sulfuric acid to get the desired acidic conditions. The reaction mixture was cooled using an external cooling agent such as ice to counter the heat generated into the system due to the action of ultrasound. The operating temperature of the solution was maintained at 30 ( 2 °C. All the experiments were repeated two times, and the reported values are an average of the two runs. The experimental errors were within 2% of the reported average value of the extent of degradation.
2.4. Analysis. Analysis of the samples was done using SHIMADZU-1800 UVvis spectrophotometer at λ = 401 nm.5,12 Before analysis of the samples, alkaline borate buffer solution (pH 9) was used to make the samples alkaline to ensure correct determination of p-nitrophenol using spectrophotometer.5 Samples of sonocatalysis experiments were centrifuged at 10000 rpm, and the supernatant was carefully removed prior to the analysis. The concentration of p-nitrophenol was calculated by measuring the absorbance of p-nitrophenol (all samples at constant pH adjusted using buffer solution) with the help of the calibration chart prepared earlier with known concentrations. As the main aim was to maximize the extent of degradation using different additives, quick analysis methodology based on a UV spectrophotometer has been used. Sivasankar and Moholkar13 have also reported a similar approach of analysis for investigations related to the sonochemical degradation of p-nitrophenol. The detailed discussion on the degradation mechanism based on more sophisticated analysis techniques such as HPLC and the various primary and secondary products is readily available in the open literature.12,14 The detailed analysis of the intermediates has been done for the case of intensification using carbon tetrachloride as an additive using LC-MS. For other approaches based on the use of ultrasonic irradiation alone or using additives which intensify the hydroxyl radical formation, detailed analysis of the intermediates has not been performed as related information is readily available.12,14 The present work concentrated only on engineering aspects related to intensification using different additives at a large scale of operation. Under optimum conditions for each approach (where maximum removal of the main pollutant has been observed), samples were also analyzed to determine the total organic carbon (TOC) content using a ANATOC II total organic carbon analyzer from SGE International Pvt. Ltd., Australia.
3. RESULTS AND DISCUSSION 3.1. Degradation of p-Nitrophenol Using Ultrasound Alone. Initially, degradation of p-nitrophenol has been investi-
gated using ultrasonic irradiations alone to quantify the basic extent of degradation achieved due to the generation of free radicals by the cavitational events in the reactor. The obtained results have been shown in Figure 2. It can be seen that using ultrasonic irradiations alone, 23.2% degradation is obtained, which also seems to remain more or less constant after 4 h of treatment. The initial rate constant for the degradation was found 1167
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Industrial & Engineering Chemistry Research to be equal to 8.75 103 min1. TOC measurements indicated that the extent of TOC reduction due to the action of ultrasonic irradiations alone was only 7.4%. Comparing the extent of degradation of p-nitrophenol using a dual frequency flow cell as used in the work of Sivakumar et al.,5 it can be said that the extent of degradation is higher in the present case (around 20% degradation in 150 min) as compared to that obtained in the case of dual frequency flow cell (11% degradation was observed in 150 min of treatment at a single frequency operation at 25 kHz). It is more important to know that the power density in the present case is also lower (27.85 W/L) as compared to that used in the dual frequency flow cell5 (43.2 W/L). The beneficial results in the present work can be attributed to the higher irradiation area which leads to much larger cavitational activity and hence the extent of generation of free radicals is expected to be larger. This leads to an important conclusion that in the design of large scale sonochemical reactors, it is better to dissipate the ultrasonic energy through larger area of transducers which gives enhanced cavitational activity as well as more intense collapse of the cavities. 3.2. Effect of Air Sparging. Experiments have been carried out in order to check the efficacy of air sparging during the sonication process by introducing air at a flow rate of 0.07 m3/h in the reactor. Air was introduced using the simple fish pond aerator commonly available in the local market and sparged into the reactor just above the transducer using a ceramic sparger. Introduction of air was made continuously in the system. The obtained results have been shown in the Figure 2, which also gives a ready comparison with the extent of degradation using only sonochemical oxidation. It can be seen from the figure that the degradation is enhanced due to the presence of air, and about 34% degradation was obtained using ultrasound in combination with air sparging against 23.2% for the case of ultrasound alone in similar treatment times. Not only the extent of removal of p-nitrophenol increases but also the extent of TOC reduction increased from a value of 7.4% in the absence of air to 13.3% in the presence of air. A careful observation of the figure also indicates that the initial rates of degradation are higher and the rate of degradation decreases marginally especially in the case of operation of only ultrasonic irradiations. This can be attributed to the ease of cavity generation initially due to the presence of dissolved gases. During the treatment using ultrasonic irradiations, the dissolved gases are slowly degassed so that the generation of cavitation is hampered and hence there is a decrease in the rate of degradation. In the case of continuous aeration, the extent of degassing and its effect on the cavitation process is not strongly felt, and hence the extent of decrease in the rate of degradation is lower as observed from Figure 2. Senthilkumar and Pandit15 have also reported similar effects of higher extents of degradation during the initial treatment times in the case of cavitational reactors using KI oxidation as the model reaction. The observed increase in the extent of degradation can be attributed to the fact that the presence of the dissolved gases in the liquid increases the cavitational effect by supplying nuclei for the process. It should be noted here that even though the sizes of possible air bubbles and cavity generated during the cavitation phenomena are an order of magnitude different, the passage of air does introduce discontinuity in the system which leads to facilitation of the cavity generation process leading to an enhanced number of cavitation events and hence the enhanced generation of hydroxyl radicals. This has also been observed with
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Figure 3. Degradation of p-nitrophenol in the presence of ultrasound and TiO2.
the salicylic acid dosimetry for quantification of hydroxyl radicals.16 Also studies with the introduction of air in the absence of ultrasound indicated that no degradation of p-nitrophenol occurred, which confirms that the possible oxidation of p-nitrophenol using air does not occur under the set of operating conditions investigated in the work. Sivakumar et al.5 have reported similar effects for the enhanced ultrasonic degradation in the presence of air. Comparing the extent of intensification obtained due to the presence of air can lead us to important conclusions regarding the scale up aspects of sonochemical reactors. It has been observed that the presence of air leads to about 15% enhancement in the rate constant for degradation in the case of a dual frequency flow cell as compared to about 50% enhancement in the present work. The obtained results can be explained on the basis of counteracting the effects of the introduction of air. Due to the introduction of air, the energy released at the end of the cavity collapse decreases, but at the same time the number of such cavitation events increases in the presence of air, due to the presence of gas bubbles as additional nuclei. The proportions of these two effects will decide the net effect on the extent of degradation. Due to higher irradiation area and higher volume of operation, it is expected that the presence of air leads to a substantial increase in the number of cavitation events without any coalescence effects leading to overall enhancements in the intensification. In our earlier work with the degradation of formic acid,17 similar dependence of the introduction of air on the scale of operation and geometry of the sonochemical reactors has been observed. 3.3. Combination of Sonication with TiO2 Particles. The presence of solid particles such as TiO2 can aid in increasing the surface cavitation as well as can have catalytic action on the sonochemical oxidation.18 With this background, degradation of p-nitrophenol has been investigated using a combination of ultrasound and TiO2. The experiments were performed over different solid loading in the range of 1 to 4 g/L. Initially adsorption studies were also carried out to examine the possibility of adsorption of p-nitrophenol on the solid particles used in the study. It has been observed that the contribution of adsorption was negligible (less than 2% removal of the pollutant) over the range of concentrations as investigated in the current work. The obtained results related to the ultrasound induced degradation in the presence of TiO2 particles have been shown in Figure 3. It has been observed that the extent of degradation increased initially with an increase in the catalyst loading, and maximum degradation of 37.2% was obtained at a TiO2 1168
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Figure 4. Degradation of p-nitrophenol in the presence of ultrasound and CuO.
concentration of 3 g/L. At this optimum loading, the corresponding TOC removal was 21.3%. The initial rate constant for the degradation process at the optimum loading was 1.08 102 min1. The increased degradation in the presence of TiO2 can be attributed to the fact that the presence of solid particles provides additional nuclei for cavitation by introducing discontinuity in the continuous liquid medium. Quantification of the hydroxyl radicals measurements using salicylic acid dosimetry,16 though in the case of conventional laboratory scale ultrasonic horn reactor, has also confirmed this observation of enhanced cavitational activity leading to the enhanced formation of hydroxyl radicals in the irradiated solutions. Similar quantification of enhanced hydroxyl radicals have also been reported using KI oxidation in the earlier work of Katekhaye and Gogate.19 The generated hydrogen peroxide by recombination of 3 OH radicals may also interact with the surface of TiO2 and produce a number of oxidizing agents, which can facilitate the pollutant degradation. Wang et al.18 have investigated sonochemical degradation of methyl orange using TiO2 particles and reported that the use of TiO2 results in almost 100% increase in the extent of degradation of the dye. The decrease in the extent of degradation above 3 g/L loading as obtained in the present work may be attributed to the dominant screening effect of excess TiO2 particles in the solution on the incident sound waves, thereby decreasing the net transfer of ultrasonic energy. It should be also noted here that the existence of optimum solid particle concentration would be strongly dependent on the ultrasonic reactor configuration, and hence this effect or the magnitude of optimum loading cannot be generalized. 3.4. Intensification Using CuO Particles. Cupric oxide has also been used as an additive to intensify the sonochemical degradation of p-nitrophenol. The logic behind using CuO as a solid particle was just to introduce discontinuity in the liquid medium to facilitate the formation of cavitation nuclei. The experiments have been conducted to investigate p-nitrophenol degradation at different loadings of CuO in the range of 1 to 4 g/L, and the obtained results have been depicted in Figure 4. It can be seen from the figure that 30.5% degradation was obtained at CuO loading of 3 g/L, and beyond this concentration there was not any increase in the extent of degradation. At the loading of 3 g/L, the corresponding TOC removal was found to 10.5%. The degradation rates were higher as compared to sonication alone, which can be attributed to increase in the cavitational activity due
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Figure 5. Effect of addition of CCl4 on the extent of sonochemical degradation of p-nitrophenol.
to provision of additional nuclei in the presence of CuO leading to an enhanced number of active radical species in the reactor. Comparing the obtained results for TiO2 and CuO in combination with ultrasound, it can be said that TiO2 gives better degradation of 37.2% as against 30.5% using CuO at similar loading. The observed trends can be attributed to the fact that the presence of CuO only provides additional nuclei for cavitation to occur, whereas TiO2 provides an activated surface area in addition to providing nuclei for cavitation; these activated surface areas can accelerate the reactivity and generation of hydroxyl radicals by possible decomposition of the generated hydrogen peroxide in the system.18,20 A recent study on quantification of hydroxyl radicals using KI oxidation as the model reaction19 has also confirmed that the presence of TiO2 particles leads to enhanced generation of hydroxyl radicals as compared to similar loading of CuO particles. Another possible mechanism for the difference in the observed intensification due to the use of two solid particles with the same loading can be attributed to the possible differences in the particle size distribution of the solid particles though this could not be exactly quantified due to lack of facilities. 3.5. Intensification Using CCl4. To enhance the rate of degradation of p-nitrophenol, CCl4 as an additive has been used at different concentrations of 0.25, 0.4, 0.5, 1, and 2 g/L. It has been observed that as the concentration of CCl4 increases the extent of degradation of p-nitrophenol increases up to a certain point and then remains almost constant. Maximum degradation of 93.5% was obtained at a CCl4 concentration of 1 g/L as depicted in Figure 5. The initial rate constant for the degradation process at the optimum loading was 3.45 102 min1. Similar trends have been observed in the literature for the degradation of phenol where CCl4 concentration used was 76 ppm and 413 ppm.21 The increase in the rate of p-nitrophenol degradation can be explained by the fact that carbon tetrachloride being a volatile compound enters the cavitation bubbles formed during ultrasonic irradiation. On the collapse of these cavitation bubbles, Cl radicals are generated due to the decomposition of carbon tetrachloride molecules inside the cavity due to the conditions of high temperatures and pressures attained during its collapse (pyrolytic cleavage). The generation of •Cl radicals leads to a series of reactions leading to the formation of additional active species, such as HClO, Cl2, and chlorine-containing radicals (•Cl, • CCl3, and :CCl2), having a strong oxidizing property. Monitoring of pH was done for the experiments involving CCl4 as an additive and reduced pH of 2.0 also confirmed the formation of 1169
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Figure 6. Degradation of p-nitrophenol using combination of ultrasound and O3.
acids in the system. The overall reaction mechanism can be written as2124 CCl4 f 3 CCl3 þ 3 Cl
ð1Þ
CCl4 f : CCl2 þ Cl2
ð2Þ
3 CCl3
f : CCl2 þ 3 Cl
ð3Þ
3 CCl3
þ 3 CCl3 f CCl4 þ : CCl2
ð4Þ
3 CCl3
þ 3 CCl3 f C2 Cl6
ð5Þ
CCl2 þ : CCl2 f C2 Cl4 3 Cl
þ 3 Cl f Cl2
Cl2 þ H2 O f HClO þ HCl
ð6Þ
observed that the extent of intensification obtained in the present work is higher as compared to that obtained in the work of Mahamuni and Pandit21 which can be attributed to the enhanced cavitational activity in the present case due to the use of a longitudinal horn with much higher dissipation area as compared to a conventional horn used in the earlier work.21 Due to higher cavitational activity, it is expected that the extent of dissociation of additive is increased leading to the generation of enhanced quantum of the radicals and other oxidizing species leading to enhanced extent of degradation of the pollutant. 3.6. Combination of Ultrasound and O3. Experiments related to combining sonolysis and ozonation were performed at 10 ppm p-nitrophenol concentration and O3 mixture (O3+O2) flow rate of 25 mL/s. The concentration of ozone in the ozone oxygen mixture was 10% by volume. It has been observed that the combined operation results in 98.3% degradation (Figure 6) which clearly confirms that the combination approach is much better as compared to the use of ultrasonic irradiations alone. The initial rate constant for the degradation process using the combination approach was 4.58 102 min1. Additional experiments were also done to check the extent of degradation of p-nitrophenol using ozone only, and it was observed that the extent of degradation in similar treatment time was 76.5% indicating that ozone also has strong oxidizing action on the pollutant. The extent of mineralization for 10 ppm p-nitrophenol was obtained to be 36% for the combined operation. During acoustic cavitation, water decomposes into hydroxyl and hydroperoxyl radicals. In the bulk aqueous phase, ozone can be decomposed to yield HO2 3 and 3 OH radicals. The combination of sonolysis with ozonolysis increases 3 OH radical formation because of thermolytical ozone decomposition which occurs in the cavitation bubble.25 The detailed reaction mechanism for the combination of ozone and ultrasound can be given as below
ð7Þ
O3 þ OH f HO2 þ O2
ð8Þ
HO2 þ Hþ f H2 O2
ð10Þ
O3 þ HO2 f O2 þ 3 OH þ O2
ð11Þ
O2 þ Hþ f HO2 3
ð12Þ
O3 þ 3 OH f O2 þ HO2 3
ð13Þ
O3 þ O 2 f O 2 þ O 3
ð14Þ
O3 þ H2 O f OHþ þ 3 OH þ O2
ð15Þ
The additional oxidizing species attack the p-nitrophenol molecules present in the bulk of the solution or at the cavity/ water interface. This attack is in addition to the attack by hydroxyl radicals generated due to the sonolysis of water vapor in the cavitation bubble. This combined attack increases the rate of p-nitrophenol degradation. Additional detailed studies were undertaken to identify the degradation products in the present case and compare it with that observed in the earlier work of Mahamuni and Pandit21 related to phenol degradation. The degradation products in the present work, as observed using LC-MS studies, were chlorophenol, chloronitrophenol, catechol, hydroquinone, and resorcinol. At low concentrations of CCl4, some amount of H2O and some amount of CCl4 goes into the cavitating bubble. The amount of H2O in the cavitating bubble in this case is less than in the case in which no CCl4 is used. Whatever small amounts of Cl radicals are generated will combine with the H+ radicals present in the bubble to produce HCl. So there are fewer amounts of OH radicals available at the cavity interface and in the bulk to react with p-nitrophenol to degrade it. At higher concentration of CCl4, the quantum of 3 OH radicals generated will be minimized, and hence the existence of optimum concentration of CCl4 can be explained. It is also worthwhile to compare the obtained intensification due to the addition of CCl4 in the case of earlier work related to degradation of phenol21 and the present work. It has been
ð9Þ
Similar results of enhancement of the extent of degradation of organic pollutants using a combination of ozone and ultrasound can be observed in the literature. Weavers et al.26 studied the degradation of different aromatic compounds viz. nitrobenzene, 4-nitrophenol, and 4-chlorophenol using the combination technique of sonolysis and ozonolysis. It has been reported that the degradation rates for the combination technique for nitrobenzene, 4-nitrophenol, and 4-chlorophenol were all higher as compared to the individual operation. Weavers et al.27 have also obtained similar results with the degradation of pentachlorophenol. A detailed analysis of the intermediates reported in the earlier work26,27 also confirms the formation of hydroxyl radicals and subsequent chain reaction as illustrated in the reaction mechanism given by eqs 9 to 15. 1170
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Industrial & Engineering Chemistry Research 3.7. Energy Consumption Analysis. In the case of sonochemical reactors, the main energy consuming component is the generator used for introducing the ultrasonic irradiations into the system. The operating power rating of the sonochemical reactor was 1 kW, and based on the actual calorimetric studies the net power transferred into the system and hence available for generation of cavitation events was 195 W. Considering the actual dissipated power and the operating volume of 7 L, the power density for the reactor has been found to be 27.85 W/L. Considering the operation using ultrasound alone, the net extent of degradation has been observed to be 23.2% in 4 h of treatment time for an initial concentration of pollutant as 10 ppm. The net cavitational yield (defined as moles degraded per unit dissipated power) is obtained to be equal to 4.195 106 mol/(W/L).
4. CONCLUSIONS The present work has shown that p-nitrophenol can be effectively removed using sonochemical reactors operating at a capacity of 7 L, and different additives can be used to intensify the extent of degradation. From this study, following important conclusions can be established: 1. For a large scale operation, it is better to use a large transducer area for dissipating similar power into the system as it leads to enhanced cavitational activity leading to formation of enhanced quantum of free radicals. 2. Air sparging in combination with ultrasound gives better degradation as compared to only sonication, which can be attributed to the enhanced generation of cavitational events. Also the presence of air helps in minimizing the degassing action of ultrasound which otherwise can give lower degradation rates due to detrimental effects on the process of cavitation inception. 3. The presence of solids in the system also leads to enhanced cavitational activity and hence an enhanced extent of degradation. Use of TiO2 is better as compared to CuO, possibly due to some role in enhancing the extent of generation of free radicals due to catalytic action on the dissociation of generated hydrogen peroxide. 4. Maximum degradation of 93.5% was obtained at a CCl4 concentration of 1 g/L along with a combination of ultrasound as against 23.2% degradation using only ultrasound. The effectiveness of the additive also depends on the inherent cavitational activity in the reactor. 5. The combination of ozone and ultrasound results in 98.3% degradation which is much more than ultrasonic irradiations alone but marginally higher as compared to the use of ozone alone. 6. It has been conclusively established that the large scale operation of sonochemical reactors is benefited by use of additives which gives additional oxidation mechanisms in the system (use of carbon tetrachloride and ozone in the present case) as against those additives which gives only enhanced cavitational activity by providing additional nuclei (air and solid particles). ’ AUTHOR INFORMATION Corresponding Author
*Phone: +91-22-33612222. Fax: +91-22-3361 1020. E-mail:
[email protected].
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’ REFERENCES (1) Gogate, P. R.; Pandit, A. B. A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions. Adv. Environ. Res. 2004, 8, 501–551. (2) Adewuyi, Y. G. Sonochemistry: environmental science and engineering applications. Ind. Eng. Chem. Res. 2001, 40, 4681–4715. (3) Colussi, A. J.; Weavers, L. K.; Hoffmann, M. R. Chemical bubble dynamics and quantitative sonochemistry. J. Phys. Chem. A 1998, 102, 6927–6934. (4) Gogate, P. R. Cavitation: An auxiliary technique in waste water treatment schemes. Adv. Environ. Res. 2002, 6, 335–352. (5) Sivakumar, M.; Tatake, P. A.; Pandit, A. B. Kinetics of p-nitrophenol degradation: effect of reaction conditions and cavitational parameters for a multiple frequency system. Chem. Eng. J. 2002, 85, 327–338. (6) Chand, R.; Ince, N. H.; Gogate, P. R.; Bremner, D. H. Phenol degradation using 20, 300 and 520 kHz ultrasonic reactors with hydrogen peroxide, ozone and zero valent metals. Sep. Purif. Technol. 2009, 67, 103–109. (7) Ince, N. H.; Belen, R. Aqueous phase disinfection with power ultrasound: Process kinetics and effect of solid catalysts. Environ. Sci. Technol. 2001, 35, 1885–1888. (8) Kubo, M.; Matsuoka, K.; Takahashi, A.; Shibasaki-Kitakawa, N.; Yonemoto, T. Kinetics of ultrasonic degradation of phenol in the presence of TiO2 particles. Ultrason. Sonochem. 2005, 12, 263–269. (9) Tuziuti, T.; Yasui, K.; Sivakumar, M.; Iida, Y.; Miyoshi, N. Correlation between acoustic cavitation noise and yield enhancement of sonochemical reaction by particle addition. J. Phys. Chem. A 2005, 109, 4869–72. (10) Ratoarinoro; Contamine, F.; Wilhelm, A. M.; Berlan, J.; Delmas, H. Power measurement in sonochemistry. Ultrason. Sonochem. 1995, 2, S43–S47. (11) Bhirud, U. S.; Gogate, P. R.; Wilhelm, A. M.; Pandit, A. B. Ultrasonic bath with longitudinal vibrations: a novel configuration for efficient wastewater treatment. Ultrason. Sonochem. 2004, 11, 143–147. (12) Kotronarou, A.; Mills, G.; Hoffmann, M. R. Ultrasonic irradiation of p-nitrophenol in aqueous solution. J. Phys. Chem. 1991, 95, 3630–3638. (13) Sivasankar, T.; Moholkar, V. S. Physical insights into the sonochemical degradation of recalcitrant organic pollutants with cavitation bubble dynamics. Ultrason. Sonochem. 2009, 16, 769–781. (14) Hoffmann, M. R.; Hua, I.; Hochemer, R. Application of ultrasonic irradiation for the degradation of chemical contaminants in water. Ultrason. Sonochem. 1996, 3, S163–S172. (15) Senthilkumar, P.; Sivakumar, M.; Pandit, A. B. Experimental quantification of chemical effects of hydrodynamic cavitation. Chem. Eng. Sci. 2000, 55, 1633–39. (16) Chakinala, A. G.; Gogate, P. R.; Burgess, A. E.; Bremner, D. H. Intensification of hydroxyl radical production in sonochemical reactors. Ultrason. Sonochem. 2007, 14, 509–514. (17) Gogate, P. R.; Mujumdar, S.; Pandit, A. B. Sonochemical reactors for waste water treatment: comparison using formic acid degradation as a model reaction. Adv. Env. Res. 2003, 7, 283–299. (18) Wang, J.; Guo, B.; Shang, X.; Shang, Z.; Han, J.; Wu, J. Sonocatalytic degradation of methyl orange in the presence of TiO2 catalysts and catalytic activity comparison of rutile and anatase. Ultrason. Sonochem. 2005, 12, 331–337. (19) Katekhaye, S. N.; Gogate, P. R. Intensification of cavitational activity in sonochemical reactors using different additives: Efficacy assessment using a model reaction. Chem. Eng. Proc. 2011, 50, 95–103. (20) Matsuzawa, S.; Tanaka, J.; Sato, S.; Ibusuki, T. Photocatalytic Oxidation of Dibenzothiophenes in Acetonitrile Using TiO2: Effect of Hydrogen Peroxide and Ultrasonic Irradiation. J. Photochem. Photobiol., A 2002, 149, 183–189. (21) Mahamuni, N. N.; Pandit, A. B. Effect of additives on ultrasonic degradation of phenol. Ultrason. Sonochem. 2006, 13, 165–174. 1171
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Industrial & Engineering Chemistry Research
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(22) Bhatnagar, A.; Cheung, H. M. Sonochemical destruction of chlorinated c1 and c2 volatile organic compounds in dilute aqueous solution. Environ. Sci. Technol. 1994, 28, 1481–1486. (23) Hua, I.; Hoffmann, M. R. Kinetics and mechanism of the sonolytic degradation of CCl4: intermediates and byproducts. Environ. Sci. Technol. 1996, 30, 864–871. (24) Francony, A.; Petrier, C. Sonochemical degradation of carbon tetrachloride in aqueous solution at two frequencies: 20 kHz and 500 kHz. Ultrason. Sonochem. 1996, 3, S77–S82. (25) Hart, E. J.; Henglein, A. Sonolysis of ozone in aqueous solution. J. Phys. Chem. 1986, 90, 3061–3062. (26) Weavers, L. K.; Ling, F. H.; Hoffmann, M. R. Aromatic compound degradation in water using a combination of sonolysis and ozonolysis. Environ. Sci. Technol. 1998, 32, 2727–2733. (27) Weavers, L. K.; Malmstadt, N.; Hoffmann, M. R. Kinetics and mechanism of pentachlorophenol degradation by sonication, Ozonation and sonolytic Ozonation. Environ. Sci. Technol. 2000, 34, 1280–1285.
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