Degradation of Low Concentration Methyl Orange in Aqueous

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Degradation of Low Concentration Methyl Orange in Aqueous Solution through Sonophotocatalysis with Simultaneous Recovery of Photocatalyst by Ceramic Membrane Microfiltration Peng Cui,* Yazhong Chen, and Guijuan Chen School of Chemical Engineering, Hefei University of Technology, No. 193 Tunxi Road, Hefei, 230009, Anhui, P. R. China

bS Supporting Information ABSTRACT: Photocatalysis is a promising technology for wastewater treatment, particularly for mineralization of nonbiodegradable and toxic components in wastewater. TiO2 is usually utilized as photocatalyst in slurry reactors in order to overcome mass transfer limitations. The difficulty in recovering TiO2 photocatalyst from treated water hindered its wide application. In this work, a novel process efficiently integrating sonophotocatalysis for methyl orange degradation and ultrasonic-enhanced ceramic membrane microfiltration for TiO2 separation was proposed and demonstrated. The results indicated that ultrasonic introduction could enhance photocatalysis reaction rate through cavitation effect, a synergetic effect between sonolysis and photocatalysis was found, and that is closely related with working conditions. Ceramic membrane microfiltration could efficiently recover TiO2 photocatalyst with a mean granular size of 0.33 μm from slurry reactor, achieving 99.9% recovery rate. Ultrasonic introduction into microfiltration process efficiently increased transmembrane permeation flux, suppressing membrane fouling under optimal working conditions. However, due to the problems associated with conversion efficiency of ultrasonic energy and the uncertain synergistic effect of sonolysis and photocatalysis, there is still much work before application of this process for wastewater treatment.

1. INTRODUCTION Photocatalysis has been a focus of intensive research since it was first proposed.1 TiO2 is the most frequently utilized photocatalyst due to its considerable activity, high stability, nonenvironmental impact, and low cost. Currently, TiO2 photocatalyst in the form of powder was either suspended in the contaminated water, or fixed on supporting material such as glass, concrete, and ceramics. Although supporting TiO2 on support may actually provide finer TiO2 particles, mass transfer limitation dominated photocatalytic process over particle size effect. Thus, slurry reactors exceed fixed-bed reactors in photocatalytic degradation efficiency due to a much shorter mass transfer path.2 For slurry reactors, there are also shortcomings such as difficulty of TiO2 separation from the treated water after detoxification. Thus, for the development of photocatalytic technologies for detoxification of wastewater, solid-liquid separation was very important, particularly when the size of the solid particles was in the range 0.1-5.0 μm. To the best of our knowledge, the probable way for TiO2 recovery should meet the requirements such as no significant decrease of TiO2 concentration in the reactor and no washout of submicrometer TiO2 particles causing a nonacceptable secondary pollution. Microfiltration has emerged as a very promising solid-liquid separation technology when the particle size was in the size range 0.1-5.0 μm.3 The most widely utilized TiO2 photocatalyst has similar aggregate particle size range, and its separation from treated wastewater attracted considerable attention recently. Ceramic membrane microfiltration is a physical process without phase change and interphase mass transfer, which could completely recover utilized TiO2 photocatalyst from aqueous solution. Furthermore, dissolved compounds with high molecular weight r 2011 American Chemical Society

and germs in the treated water could be rejected by the membrane which increases the overall efficiency of the photocatalytic process. However, during the microfiltration process, membrane fouling has always been a problem which directly resulted in an “irreversible” decline of transmembrane permeation flux. Although some chemical or electrical methods have been investigated to reduce membrane fouling, problems associated with “secondary pollution”, electrode corrosion, and high power cost prevented their wide application.4 Sonochemistry enhances or promotes chemical reactions and mass transfer through cavitation.5 It offers the potential for shorter reaction cycles, cheaper reagents, and milder working conditions. Cavitation is the formation, growth, and sudden collapse of bubbles in liquids. Ultrasonic vibration could reduce the thickness of liquid films and enhance gas mass transfer, reduce bubble coalescence, and increase the interfacial area for gas mass transfer. For example, the diffusion of liquids through porous media could be enhanced by ultrasound, which is closely related to the membrane microfiltration process. Literature results showed that transmembrane permeation flux during membrane filtration could be significantly enhanced through ultrasonic introduction.6-10 Moreover, problems associated with membrane fouling could be reduced. Recently, the combined utilization of sonolysis and photocatalysis, e.g., sonophotocatalysis in degradation of organic water pollutants, has been proposed and investigated.11-13 Sonochemical oxidation Received: April 6, 2010 Accepted: February 8, 2011 Revised: January 17, 2011 Published: March 01, 2011 3947

dx.doi.org/10.1021/ie100832q | Ind. Eng. Chem. Res. 2011, 50, 3947–3954

Industrial & Engineering Chemistry Research

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Figure 1. (a) Schematic diagram of the integrated process and components: (1) photocatalytic reactor, (2) ultrasonic transducer, (3) cold trap, (4) low pressure mercury lamp, (5) centrifugal pump, (6) membrane separation module, (7) flowmeter, (8) flowmeter, (9) gas container, (10) air compressor, (11) backflushing system and controlling valves V0—V10. (b) Membrane separation module: (12) inlet, (13) ceramic membrane, (14) membrane module, (15) ultrasound generator and transducer, (16) permeate outlet, (17) flange, and (18) retentate outlet.

techniques involve the use of sonic or ultrasonic waves to produce an oxidative environment via cavitation that yields local microbubbles and supercritical regions in the aqueous phase. The collapse of these bubbles leads to surprisingly high local temperatures and pressures, which may reach up to and over 5000 K and 1000 bar, respectively. Sonophotocatalysis has emerged as a very young advanced oxidation process for aqueous contaminant degradation. However, the overall degradation rate could be higher than, or equal to, the sum of the individual degradation rates for photocatalysis and sonolysis. Whether there was an obvious promoting effect depended upon the specific ultrasonic power when photocatalysis was combined with low frequency ultrasound.14 Those results indicated that the relationship between the promoting effects in sonophotocatalysis and working conditions is uncertain and merits further investigation. In this work, a novel process integrating sonophotocatalysis and ceramic membrane microfiltration for photocatalyst recovery was proposed and initially demonstrated for degradation of methyl orange (MO). The results turned out that such a process could operate continuously for the degradation of aqueous contaminant and ultrasonic introduction promoted the reaction rate for MO degradation. Simultaneously, ultrasonic introduction into ceramic membrane microfiltration process could enhance transmembrane permeation flux, reducing membrane fouling.

However, problems associated with the optimized ultrasonic field distribution in sonophotocatalysis and microfiltration, and ultrasonic energy efficiency in the aqueous solution, are complex and key problems to be solved before such a process could be actually applied.

2. EXPERIMENTAL SECTION Figure 1a shows the self-made apparatus and schematic diagram of the process to perform sonophotocatalysis for oxidative degradation of MO using TiO2 as catalyst, and recovering photocatalyst through ceramic membrane microfiltration at the same time. The process mainly consists of a slurry photocatalysis reactor, two ultrasonic generators, and ceramic membrane microfiltration module and backflushing equipment. In the slurry photocatalysis reactor, MO solution with an initial concentration of 25.0 mg/L with a whole volume of 5.0 L underwent sonophotocatalysis oxidative degradation by bubbling air into the reactor. The ultrasonic generator and transducer was provided by Kunshan Ultrasound Instrument Co. Ltd. China with nominal 28.0 kHz vibrating frequency and variable nominal power in the range 0-300.0 W. Low pressure mercury lamp with a nominal power of 30.0 W and central wavelength of 254.0 nm was inserted into a quartz cold trap to provide light energy. The cold 3948

dx.doi.org/10.1021/ie100832q |Ind. Eng. Chem. Res. 2011, 50, 3947–3954


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100 °C overnight, and calcined at 500 °C for 3.0 h to make it a constant weight. The weight of the bottle and TiO2 photocatalyst was determined using an electronic balance with accuracy of 0.1 mg.

3. RESULTS AND DISCUSSION

Figure 2. XRD pattern of TiO2 powder.

trap was utilized to maintain reaction temperature by circulating water through the reactor, while the ultrasonic transducer was positioned below the cold trap. The outlet of the reactor was connected to a ceramic membrane microfiltration module, which mainly included a single-channel ceramic membrane with a length of 500 mm and an average pore diameter of 0.2 μm (Membrane Science and Technology Research Center of Nanjing University of Technology) and a stainless steel shell, as shown in Figure 1b. An ultrasonic horn with a frequency of 20.0 kHz and maximum power of 100.0 W provided ultrasonic field during ceramic membrane microfiltration process. The backflushing equipment involves a gas container and an air compressor. The process was operated in the following way: After reaction for a period of time, the water suspension was pumped to the membrane separation module for microfiltration, and the permeation flux was also circulated to the feed tank to keep a constant whole volume of the solution. Analytical grade fine TiO2 was purchased from Nanjing Titanium Dioxide Chemical Co. Ltd. Its phase structure was determined using a powder X-ray diffraction instrument (Rigaku D/max-rB). The particle size of TiO2 was measured by ZETASIZER-3000HSA and H-800 TEM (Philips) characterizations. The initial concentration and that of MO after a certain period of time was determined by a UV-722 spectrometer (Shanghai Precision and Scientific Instrument Co., Ltd., China). The degradation rate (R%) of MO was calculated using adsorption intensity at 463.0 nm according to eq 1 Rð%Þ ¼ ðAð463, IÞ - Að463, FÞ Þ=Að463, IÞ 100%

ð1Þ

where A(463, I) was the absorbance of initial MO solution (without suspended TiO2 in the aqueous solution), while A(463, F) was the absorbance of final MO solution (after a period of reaction time and after TiO2 separation). The crossflow rate was defined as the ratio between the fluxes (Q, m3/s) across the membrane to the section area of the membrane (A, m2). The rejection rate of membrane for TiO2 was calculated using eq 2 CP γ ¼ 1 100% ð2Þ CR where CP was the TiO2 concentration (g/L) in the water suspension of the permeation side, and CR was the TiO2 concentration (g/L) in the water suspension of the retention side. Concentration of TiO2 was determined using a weight method. Water suspension (10.0 mL) in the photocatalytic reactor or in the transmembrane permeation flux after complete mixing was transferred to a constant weight bottle, dried at

Characterizations of TiO2 Photocatalyst. Figure 2 shows the powder XRD pattern of the commercial TiO2 photocatalyst, indicating an anatase structure for TiO2 with obvious reflection peak at 2θ values of around 25°, 38°, and 48°, etc., which agreed with the supplier’s specification. The particle size distribution of the commercial powder determined by TEM as well as ZETASIZER was shown in Figure 3a,b, respectively. It turned out that particle size of TiO2 was in the range 0.2-0.7 μm, which was determined by ZETASIZER and agreed with the TEM characterization results. Thus, we chose ceramic membrane with an average pore size of 0.2 μm for TiO2 photocatalyst recovery in this work to reduce initial loss of TiO2. Although nanosized Degussa P25 TiO2 showed high activity for photocatalysis, ceramic membrane with an average pore size around 10.0 nm tends to block soon during microfiltration. Furthermore, nanosized P25 showed no obvious advantage due to the conglomeration of nanosized particles through ultrasonic standing wave effect. Thus, in our initial work in this integrated process, we chose a kind of stable TiO2 photocatalyst (against conglomeration in ultrasonic field during sonophotocatalysis and fouling during membrane microfiltration). Ultrasonic-Enhanced Microfiltration of TiO2. Figure 4 shows the rejection rate of TiO2 photocatalyst and change of transmembrane permeation flux with operation time during microfiltration. The initial rejection rate of TiO2 photocatalyst was 98.0%, increasing to over 99.5% after 120 min continuous operation. Simultaneously, transmembrane permeation flux was enhanced by around 10.0%, as shown in Figure 4b. The permeation flux quickly decreased with operation time-onstream when no ultrasonic irradiation and backflushing were applied. Although backflushing recovered transmembrane permeation flux close to the initial one in the short period time of 100 min, a gradual decrease of transmembrane permeation flux after 120 min occurred when only backflushing was applied (Figure S1 in Supporting Information), indicating that compaction of the filtration cake during the microfiltration process took place. However, in the ultrasonic-enhanced membrane microfiltration process, this phenomenon was efficiently prevented (Figure S2 in Supporting Information). The permeation flux showed less fluctuation under ultrasonic irradiation and increased by around 10.0% in comparison with the one without sonication. This could be closely related to the four specific effects of ultrasound irradiation.15 First, sonication can cause agglomeration of fine particles in solution, thus efficiently reducing pore blockage and cake compaction. Second, sonication can supply sufficient mechanical vibration energy to the aqueous media to keep particles partly suspended and therefore leave more free channels for solvent elution. Third, cavitation gas bubbles can scour membrane surfaces and can reach the crevices in the membrane that are not easily accessible by conventional cleaning methods. Finally, acoustic streaming causes turbulence in the water suspension which results in bulk fluid movement toward and away from the membrane cake layer, with velocity gradients near the cake layer that may again scour particles from the surface. Thus, enhanced transmembrane permeation flux and 3949



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Figure 3. Particle size and the distribution of TiO2 powder. (a) Characterization by ZETASIZER. (b) Characterization by TEM.

Figure 4. (a) Change of rejection rate during ceramic microfiltration with time-on-stream. (b) Change of transmembrane permeation flux with only backflushing (b) and those with backflushing and ultrasonic irradiation (4). (Working conditions: T = 308.0 K, TiO2 concentration = 1.0 g/L, pH = 7.0, nominal ultrasonic power = 100.0 W, P = 0.2 MPa.)

increased rejection rate of TiO2 particles were observed during microfiltration under ultrasonic irradiation. The capacity of promoting transmembrane permeation flux during the

microfiltration process could be closely related to the maintenance of a clean and stable membrane surface through ultrasonic introduction. As an example, Kobayashi et al.10 found that ultrasonic irradiation promoted membrane cleaning when peptone and milk solution were subjected to microfiltration or ultrafiltration. During the membrane cleaning process with deionized water, an ultrasonic wave with 28 kHz frequency showed the best performances and recovered transmembrane permeation flux to its initial level compared with the one of 45.0 kHz or 100 kHz frequency. During the continuous filtration of 1.0 wt % milk, the permeation flux was enhanced by over 400.0% after 30 min operation, showing that ultrasonic irradiation could be applied not only for membrane cleaning after fouling, but also for the continuous microfiltration process to keep the membrane uncontaminated. Chai et al.7 also found ultrasonic irradiation increased permeation flux during ultrafiltration of different molecular weight dextran solutions using polyacrylonitrile (PAN) membrane. Despite the significant effect for cleaning the fouled membrane and improved permeation flux, ultrasonic introduction into the membrane filtration process should be performed with care due to its potential damage to membrane. Masselin et al.16 investigated the effect of 47.0 kHz ultrasonic wave on polymer membranes constructed by polyethersulfone (PES), polyvinylidenefluoride (PVDF), and PAN in aqueous media. It turned out that the PES membrane was affected by the ultrasonic treatment over its entire surface. PVDF and PAN membranes were more resistant and present less damage. Results also showed that the degradation of membrane under ultrasonic stress led to an increase in pore radius for large pores, an overall increase in pore density and porosity, and formation of large cracks preferentially at the edges of the membrane samples. Any increase in pore density, especially when large pores are concerned, may favor the formation of cracks resulting from the interconnection of neighboring pores. These large cracks could bring a large contribution to the increase in permeability and porosity observed for most membranes. Also, in our work for the ceramic membrane, ultrasonic effect on the membrane was investigated. The results indicated that sonication has little effect on the ceramic membrane even after 100.0 h of discontinuous operation. We attributed this to the ceramic membrane preparation process, which includes sintering the membrane at 3950



Figure 5. Degradation rate of methyl orange under sonolysis, photocatalysis, and sonophotocatalysis (working conditions: pH = 7.0, C0 = 25.0 mg/L, fultrasonic = 28.0 kHz, Pultrasonic = 240.0 W, Plight = 30.0 W, TiO2 = 0.5 g/L, T = 308 K, and air bubbling rate = 500.0 mL/min).

temperatures over 1300 °C. Thus, for practical application of ultrasonic-enhanced membrane process, ceramic membrane has obvious superiority over polymeric membrane due to its structure stability under sonication. Sonophotocatalysis Degradation of Methyl Orange. Wastewater from the textile industry has been an enormous environmental concern. It was estimated around 15.0% of dye was lost during dyeing process and eventually released into wastewater streams. These dyeing effluents can have toxic effects on the ecosystem, particularly on microorganisms. MO is a typical azo dye and is known to be carcinogenic and mutagenic. Thus, complete degradation of MO in wastewater is necessary. The topic has been a focus of investigation,11-13 and techniques concerning sonophotocatalysis in slurry reactors were proposed. However, these works did not cope with the recovery of TiO2 photocatalyst from the treated water suspension for realizing continuous operation, which was our aim in this work. Thus, an integrated process which could be operated continuously or intermittently was proposed and demonstrated in this work. Ultrasonic irradiation in photocatalysis does not necessarily improve the photocatalytic activity,14 and whether there was a synergistic effect between sonolysis and photocatalysis depended upon specific working conditions. In this work, three kinds of advanced oxidation processes (AOPs), sonolysis, photocatalysis, and sonophotocatalysis, for MO degradation were investigated, and the results were shown in Figure 5. Under specific working conditions of initial CMO 25.0 mg/L, ultrasonic power of 240.0 W, ultrasonic frequency of 28.0 kHz, air bubbling rate of 500 mL/ min, TiO2 concentration of 0.5 g/L, ultraviolet light power of 30.0 W, and pH value of water suspension 7.0, for sonolysis (without photocatalyst and ultraviolet light irradiation), the degradation rate of MO was low, achieving only 15.1% after 100 min reaction time-on-stream. Photocatalysis only achieved a higher degradation rate than sonolysis at any time interval during reaction time-on-stream, obtaining 28.0% degradation rate of MO at the end of the reaction. Sonophotocatalysis achieved the highest degradation rate, but it was lower than the sum of those from sonolysis and photocatalysis. Analysis of the experimental results using linear regression based upon the assumption of the first order kinetics for sonolysis, photocatalysis, and

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Figure 6. Degradation rate of methyl orange under sonolysis, photocatalysis, and sonophotocatalysis (working conditions: pH = 4.0, C0 = 25.0 mg/L, fultrasonic = 28.0 kHz, Pultrasonic = 240.0 W, Plight = 30.0 W, TiO2 = 0.5 g/L, T = 308 K, and air bubbling rate = 500.0 mL/min).

sonophotocatalysis,17-19 and plotting ln(C0/C) as a function of reaction time t (min), gave the results that the reaction rate constants for sonophotocatalysis, photocatalysis, and sonolysis were 0.003 88, 0.003 23, and 0.001 55 min-1, respectively (Figure S3 in Supporting Information). Although the reaction rate constant for sonophotocatalysis was the highest, no obvious synergistic effects were observed under these working conditions, due to the fact that the reaction rate constant for sonophotocatalysis was still less than the sum of those for photocatalysis and sonolysis. However, under acid working conditions, e.g., when the pH value of the water suspension was 4.0, while the other conditions were kept the same, degradation of MO proceeded faster, as shown in Figure 6. Obvious synergistic effect was observed because under those circumstances the reaction rate constants for sonolysis, photocatalysis, and sonophotocatalysis were 0.001 55, 0.0050, and 0.009 38 min-1, respectively (Figure S4 in Supporting Information). The results suggested a strong synergy effect under those working conditions. Comparison of the experimental results under working conditions with pH value of 7.0 and those at pH value of 4.0 suggested that an acidic working condition benefited degradation of MO under sonophotocatalysis and photocatalysis, while little effect was observed for sonolysis. At lower pH values, TiO2 would be positively charged, while MO existed as the quinone structure and was negatively charged, which benefited the adsorption of MO on the surface of TiO2 photocatalyst. The effect of TiO2 dosage on sonophotocatalysis for the degradation of MO was also investigated, and results are shown in Figure 7. It was found that photocatalyst concentration of 0.5 g/L gave the best results in the range 0.01-1.0 g/L. The reaction rate constants for sonophotocatalysis were 0.0026, 0.0032, 0.003 89, and 0.003 54 min-1 when TiO2 dosage was 0.05, 0.10, 0.5, and 1.0 g/L, respectively (Figure S5 in Supporting Information). When TiO2 dosage was too low, sonophotocatalysis was dominated by absorption and photocatalysis, and ultrasonic irradiation could be utilized to generate HO• and H• free radicals and keep the photocatalyst well dispersed in water suspension. However, when TiO2 dosage was enough for establishment of adsorption equilibrium, further increase in TiO2 dosage resulted in slightly lower degradation rate probably due to the following two reasons. On one hand, higher TiO2 dosage was 3951



Figure 7. Degradation rate of methyl orange under sonophotocatalysis with different TiO2 photocatalyst concentrations (working conditions: pH = 7.0, C0 = 25.0 mg/L, fultrasonic = 28.0 kHz, Pultrasonic = 240.0 W, Plight = 30.0 W, T = 308 K, and air bubbling rate = 500.0 mL/min).

Figure 8. Degradation rate of methyl orange under sonophotocatalysis at different ultrasonic power densities (working conditions: pH = 7.0, C0 = 25.0 mg/L, fultrasonic = 28.0 kHz, Plight = 30.0 W, T = 308 K, TiO2 = 0.5 g/L, and air bubbling rate = 500.0 mL/min).

unfavorable for light transmitting in water suspension; on the other hand, ultrasonic irradiation generates a kind of force acting upon TiO2 particles and tending to agglomerate them in the pressure node or antinode of the ultrasonic wave, which is also called the ultrasonic standing wave effect.20 The effect of ultrasonic power on the MO degradation was also investigated. Under working conditions of pH value of water suspension 7.0, initial CMO 25.0 mg/L, ultrasonic frequency 28.0 kHz, ultraviolet power 30.0 W, temperature 308 K, TiO2 dosage 0.5 g/L, and air bubbling rate 500.0 mL/min, intermediate ultrasonic power gave better results, as shown in Figure 8. The calculated reaction rate constants for sonophotocatalysis were 0.002 98, 0.0038, and 0.003 39 min-1 when the nominal ultrasonic power was 150.0, 240.0, and 300.0 W, respectively (Figure S6 in Supporting Information). Despite the advantage of high ultrasonic power density for cavitation bubble formation, only those with transient collapse are effective for MO degradation.5 Most of the cavitation bubbles generated under high ultrasonic pressure conditions only resonate with the solution and will not

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collapse instantly, or rise to the surface of solution and collapse, which causes serious energy loss. Furthermore, too high ultrasonic pressure will cause gas bubbles to grow too big in the negative acoustic pressure phase and result in acoustic shielding. In the following positive acoustic pressure phase, the cavitation bubble could not completely collapse, which resulted in low degradation efficiency of MO. The MO degradation rate in sonophotocatalysis agreed with results reported by Selli,17 Theron,18 and Davydov.19 Selli17 and Theron18 observed a significant increase of degradation rate with ultrasonic introduction. Davydov et al.19 found that the effect of sonolysis upon degradation rate had a close relationship with the kinds of photocatalyst applied. TiO2 with small particle size led to a synergistic effect between sonolysis and photocatalysis, while negligible enhancement was observed for commercial anatase TiO2 with large particle size from Aldrich. In this work, the results strongly suggested the synergistic effect between photocatalysis and sonolysis depended upon specific working conditions such as pH value of water suspension and catalyst dosage. Thus, much effort has to be taken for a detailed understanding of the interaction between sonolysis and photocatalysis, particularly on the level of interaction mechanism. Ultrasonic introduction may bring out the following influences upon photocatalysis. First, mass transfer rates for the reagents between bulk solution and photocatalyst surface could enhance through sonolysis, as reported by Stock et al.21 and Chen et al.22 Second, sonolysis may efficiently prevent the aggregation of TiO2 particle during reaction and photocatalytic utilization of the species generated by sonolysis in water suspension, as demonstrated by Davydov19 in the ultrasonic-enhanced photodegradation of salicylic acid on four commercial TiO2 powders. Ragaini23 also found that specific surface area of TiO2 increased after 6 h of ultrasonic irradiation. However, this was not certain. Ultrasonic introduction may benefit the aggregation of fine particles through the standing wave effect. Third, sonolysis could decompose water into hydrogen and hydroxyl free radicals over the surface of photocatalyst, which could be the important intermediate species for the photocatalysis. Selli17 found obvious synergy effects between the sonolysis and photocatalysis in the degradation of acid orange 8, which was attributed to an increased concentration of free radicals. Theron18 also reported that the reaction rate constant for phenyltrifluoromethylketone decomposition by sonophotocatalysis was higher than the sum of the reaction rate constant for photocatalysis and that for sonolysis: this was attributed to the formation of OH free radical by sonolysis. The reason for the enhancement effects of ultrasound on photocatalysis could not be agreed upon, which could be attributed to the complexity of the reaction process closely associated with free radical. Most work17,18,21-23 reported an enhancement effect of sonolysis upon photocatalysis, though the enhancement degree differed. The difference in the enhancement degree could be closely related with the energy conversion efficiency of ultrasound into the photocatalysis reaction, which was a problem that has attracted much less attention currently. Almost all the literature work used nominal power density and frequency to describe the ultrasonic effect, but actually only part of the energy could be efficiently transferred to the photocatalysis reaction or membrane microfiltration process. Muthukumaran15 found that the energy conversion efficiency of nominal power into the ultrasonic bath through transducer was only about 30.0% in a microfiltration process using Minitan S unit, which was determined calorimetrically by removing the Minitan unit and 3952


Industrial & Engineering Chemistry Research observing the temperature change with time-on-stream. 24,25 Kobayashi et al.10 found that the ultrasonic power intensity reaching membrane separation module was further reduced by a factor of approximately 10 by transmission through the acrylic and stainless steel membrane holder in a similar microfiltration device. Thus, the actual energy put into the microfiltration could be as low as only 3% of nominal power. As the energy conversion efficiency depended upon the detailed design of reactor or separation module, the difference in experiments makes it very hard to compare the results of literature work and reach a consensus upon the mechanism of sonolysis enhancement for photocatalysis. Another question concerns the cavitation intensity: although H2O2 concentration determination and luminol luminescence have been tentatively applied to quasiquantitatively determine the cavitation intensity, accuracy remains a problem.15,26 However, in this integrated process, another factor to be cautiously considered was the adaptability of the particle size and membrane pore size distribution. To sum up, incorporation of ultrasonic-enhanced ceramic membrane microfiltration into sonophotocatalysis could potentially provide a continuous wastewater treatment, in situ regenerating TiO2 photocatalyst. Currently, to understand the interaction between sonolysis and photocatalysis on the molecular level is a key problem before such an integrated process could be applied to industrial wastewater treatment.

4. CONCLUSIONS A novel process that efficiently integrated a photocatalytic reactor, ceramic membrane microfiltration, and ultrasonic irradiation, for both ceramic membrane microfiltration and photocatalysis, was proposed and demonstrated for the degradation of low concentration MO in water suspension. The results indicated that such a process could efficiently recover TiO2 photocatalyst in slurry reactors, realizing a continuous operation for wastewater treatment. A synergy effect between sonolysis and photocatalysis was found to be closely related with specific working conditions such as pH value of water suspension, photocatalyst type, and concentration in sonophotocatalysis. Intermediate photocatalyst concentration and ultrasonic power density achieved higher degradation efficiency. An obvious but not significant enhancing effect of ultrasonic irradiation for ceramic membrane microfiltration was found, which could be closely related with the low conversion efficiency of ultrasonic energy. Contrary to polymeric membranes, the ceramic membrane showed good initial stability under ultrasonic irradiation of 28.0 kHz frequency. However, in this initial investigation, due to the lack of energy efficiency data and direct measurement of cavitation intensity, there is still much work to do before application. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional figures.This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: þ86-551-2904653. Fax: þ86551-2901450.

’ ACKNOWLEDGMENT The authors greatly acknowledge the National Natural Science Foundation of China (20876030) and International

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Cooperation in Science and Technology between Hungary and P.R. China (2010-5-28) for financial support.

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Degradation of Low Concentration Methyl Orange in Aqueous

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