Degradation performance of a Keggin type Zn–Mo–Zr catalyst for acidic green B with ultrasonic waves

Abstract

A new heteropolyacid salt, Na₆[Zn(Mo₁₁ZrO₃₉)]·20H₂O, with a Keggin structure of the 1 : 1 : 11 series was synthesized via a classic acidification method, and characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet (UV) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) and scanning electron microscopy (SEM). Acidic green B (AGB) was applied to examine the degradation performance of the catalyst which was promoted by ultrasonic waves. The effects of the operating parameters, such as the catalyst dosage, ultrasonic power, medium ultrasonic frequency, initial dye concentration and pH value on the degradation are discussed. In the experiments, the degradation rate reached up to 95.72% under the optimum conditions, i.e. a catalyst dosage of 0.8 g L⁻¹, an AGB concentration of 10 mg L⁻¹, an ultrasonic frequency of 45 kHz and an ultrasonic power of 100 W in just 60 minutes.

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1. Introduction

With the deterioration of the environment, increasingly serious pollution problems have greatly attracted the attention of human beings. Water resources, especially available water resources, are fewer day by day because of pollution, so it is essential to find some effective methods for wastewater treatment. Textile effluents contain a large quantity of organic dye compounds, which can cause serious environmental problems due to their poor biodegradability, toxicity and carcinogenicity.^1–5 Physical adsorption and microbiological discoloration are two master traditional methods to reduce contamination by synthetic dye molecules,^5–9 but both of them have certain limitations.^5,6,10–13

Heteropolyacids (salts) belong to a prodigious class of chemical substances which behave pre-eminently in some areas. The excellent transmission and reserve capacities of electrons and protons, high activity of “lattice oxygens” and great proton acidity are features which contribute to the versatile properties of heteropolyacids (salts), such as their acidic and redox properties. Due to these splendid properties, heteropoly compounds have received much attention for many years.¹⁴ Many works have been done to correlate these properties with catalytic actions. Recently, significant efforts have addressed the application of such compounds in organic synthesis,¹⁵ pharmaceutical chemistry, electrolyte and surface promoters for low temperature fuel cells,^16–18 wastewater treatment and other fields. In our experiment, the anion of the catalyst, [Zn(Mo₁₁ZrO₃₉)]⁶⁻, was made by the replacement of molybdenum with zinc in the peripheral metal atom positions of the 1 : 12 series [ZrMo₁₂O₄₀]⁴⁻, the structure remains consistent after the replacement of molybdenum, Fig. 1 shows the structure of [Zn(Mo₁₁ZrO₃₉)]⁶⁻, the purple atom at the center of the anion is zirconium, the center of the octahedral atoms are molybdenum.

Fig. 1 Structure of [Zn(Mo₁₁ZrO₃₉)]⁶⁻.

Better operating conditions, stronger oxidation ability, and the capability of completely degrading pollutants without secondary pollution are all advantages belonging to the heteropolyacid (salts) photocatalytic oxidation method. In view of the advantages mentioned above, heteropolyacids (salts) have been considered as favourable catalysts for the catalytic degradation of organic wastewater.^19,20 Nevertheless, due to the deep chromaticity and high turbidity of dye wastewater, the effect of light transmission should be considered, which may lead to a drop in the degradation rate. Many researchers have concentrated on the actual application of ultrasonic waves in the degradation of organic pollutants,^21–26 however, currently, few have concentrated on the combination of heteropolyacids (salts) and ultrasonic waves to degrade organic wastewater, this research is still in the initial stage. The weakness of poor light transmission during photocatalytic methods mentioned above does not exist for ultrasonication, even in a water medium, where the penetration of ultrasonic waves is generally up to 15–20 cm. In general cases, those chemical reactions catalyzed by light can also be managed by ultrasonic waves. Through access to large amounts of literature, the degradation principle of ultrasonic waves can be classified into the following two mechanisms: one is the sonoluminescence mechanism, sonoluminescence is a phenomenon that occurs when a small gas bubble is acoustically suspended and periodically driven in a liquid solution at ultrasonic frequencies, resulting in bubble collapse, cavitation, and light emission. The thermal energy released from the bubble collapse is so great that it can cause weak light emission,²⁷ so the light generated may allow heteropolyacids (salts) to operate as photocatalysts. The other is the high excitation mechanism, acoustic cavitation can cause the formation, growth and implosive collapse of bubbles in the liquid, the collapse of the bubbles generates localized “hot spots” with transient temperatures of about 5000 K and pressure levels of about 1000 atm,²⁴ under such extreme conditions, water molecules inside the bubbles dissociate into ˙OH, yet the efficiency is very low. If heteropolyacids (salts) get energy from the ultrasonic waves, this will lead to the escape of oxygen atoms from the lattice and the appearance of “holes”, the presence of “holes” can result in the generation of more ˙OH. The degradation mechanism of the catalyst is described in Fig. 2, it is easy to see that this degradation mechanism is similar to the mechanism of TiO₂ photocatalysts .

Fig. 2 Degradation mechanism of the catalyst.

Acidic green B (AGB) (Fig. 3) was selected as the objective pollutant to test the ultrasonic degradation performance of the catalyst owing to its functional groups, the benzene rings, naphthalene ring, and azo-bonds, which are difficult to degrade. In the experiments, the degradation of organics is not a direct effect of the ultrasonic waves or catalyst, but the influence of the ˙OH generated in the reaction, ˙OH can destroy the refractory functional groups, and accordingly, the dye molecules are degraded.

Fig. 3 Structural formula of AGB.

In this paper, a novel heteropolyacid salt, ZnMoZr, has been synthesized, the structure and ultrasonic catalytic degradation performance under different conditions are reported, and the degradation kinetics of AGB were also studied in this work.

2. Experimental

All the chemicals were purchased from Keluo (China), of analytical grade and used as received without further purification. Deionized (DI) water was used throughout the whole study.

2.1. Synthesis of ZnZrMo

Na₂MoO₄·2H₂O (0.044 mol) was dissolved in H₂O (200 mL) and the pH value of the solution was adjusted to 6.5 with acetic acid. Then a solution of ZnCl₂·8H₂O (0.004 mol) in H₂O (20 mL) was added drop wise under magnetic stirring. When a white precipitate appeared, the pH of the solution was readjusted to 5.5 and stirring was continued until the solution clarified at a temperature of 70 °C. A solution of ZnCl₂ (0.004 mol) in H₂O (20 mL) was added drop wise with boiling-reflux for 1–2 h. The product was purified using a recrystallization method and dried at room temperature. The ZnZrMo heteropolyacid salt was a colorless powder.

3. Results and discussion

3.1. Elemental analysis

The molar ratio of the elements was examined by HK-2000 inductively coupled plasma atomic emission spectroscopy (ICP-AES). As shown in Table 1, the ICP-AES analysis shows that the mole ratio of Zn : Zr : Mo is close to 1 : 1 : 11 (data in the brackets is theoretical), which indicates the catalyst belongs to the 1 : 1 : 11 series of heteropolyacid salts.

Table 1 Chemical analysis of the catalyst

HPS	Zr	Mo	Zn	Zn : Zr : Mo
ZnZrMo	3.82 (3.90)	44.75 (45.24)	2.73 (2.78)	1.00 : 1.00 : 11.10

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3.2. TGA/DSC analysis

The thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) study was carried out using Mettler Toledo TGA/DSC 1 equipment in a dynamic argon atmosphere in the range of 25–600 °C, and the heating rate was 15 °C per minute. As shown in Fig. 4, the weight of the catalyst decreases from 74 °C to 400 °C, but after 400 °C, the weight of the catalyst remains constant. There are four endothermic peaks (74, 105, 325 and 400 °C) in the DSC curve, three major weight loss stages are divided by these four peaks. At the first stage, the weight loss is 5.4%, and the lost components are mainly small molecules such as physically absorbed water and acetic acid. In the second stage, the weight loss is 9.4%, equivalent to the loss of thirteen crystallized waters. In the third stage, the weight loss is 4.8%, corresponding to the loss of seven crystallized waters. The endothermic peaks at 470 °C and 580 °C are assigned to the structural collapse of the catalyst. It can be concluded from Fig. 4 that the sample examined is an outstanding catalyst which possesses favorable thermal stability. Besides, the molecular formula of the catalyst can be summarized as Na₆[Zn(Mo₁₁ZrO₃₉)]·20H₂O.

Fig. 4 TGA/DSC curves of the catalyst.

3.3. FT-IR and UV analyses

The Fourier transform infrared (FT-IR) spectroscopy was recorded on a Nicolet Nexus spectrometer in the region of 700–4000 cm⁻¹ using KBr pellets. The characteristic vibrational bands of heteropolyacid salts with a Keggin structure appear in the range of 700–1100 cm⁻¹. As shown in Fig. 5, four characteristic bands can be observed: 1020 cm⁻¹ (Zr–Oa), 943 cm⁻¹ (Mo=Od), 877 cm⁻¹ (Mo–Ob–Mo) and 771 cm⁻¹ (Mo–Oc–Mo) (where Oa = oxygen in the central ZrO₄ tetrahedron, Od = terminal oxygen bonding to a Mo atom, Ob = edge-sharing oxygen connecting Mo atoms and Oc = corner-sharing oxygen connecting Mo₃O₁₃ units), all of which can be detected for a Keggin anion. Besides, the band at 910 cm⁻¹ is attributed to the symmetric stretching of Mo=O bonds, and the band at 1410 cm⁻¹ could be attributed to the stretching vibration of C–O bonds or in-plane bending vibration of O–H bonds. In the high frequency region, the band at 1340 cm⁻¹ might be assigned to the bending vibration of the C–H bonds. In addition, the band at 1560 cm⁻¹ could be caused by a carbonyl group. Meanwhile, there are other bands at 1640 cm⁻¹ and 3430 cm⁻¹, which were respectively assigned to the bending vibration of the O–H bonds and the stretching vibration of the H–O–H bonds of absorbed water. The appearance of a carboxyl group and C–H bonds is due to residual glacial acetic acid or its salts. The split of the stretching vibration band is mainly due to the large size of the zirconium ions, which may cause the relaxation and small cohesion of the molecule, so that changes to the relative chemical bond vibrations are obvious.

Fig. 5 FT-IR spectrum of the catalyst.

The ultraviolet (UV) spectrum was measured on a Shimadzu UV-2500 spectrophotometer from 200 to 400 nm. The UV spectra of the 1 : 1 : 11 series of Keggin type heteropolyacids (salts) generally possess two strong characteristic absorption peaks, the absorption peak with higher energy is a result of the double bond character, which is in the vicinity of 200 nm, and the absorption peak with lower energy is attributed to the single bond character, which is at about 260 nm. As shown in Fig. 6, the peaks at 208 nm and 253 nm are respectively assigned to the Od → Mo and Ob/Oc → Mo charge transfers, and both of them are in accordance with the UV spectra of the 1 : 1 : 11 series of Keggin type heteropoly compounds.

Fig. 6 UV absorption spectrum of the catalyst.

3.4. XRD and SEM analyses

X-ray powder diffraction (XRD) analysis was perfomed on a Shimadzu XRD-6000 X-ray diffractometer. The instrument was equipped with a Cu tube operated at 40 kV and 30 mA, and the diffraction data were collected in the 5–40° 2θ range with a step size of 0.02 at a rate of 4° per minute. There are four characteristic peaks (2θ = 7–13°, 16–23°, 25–30° and 31–38°) in the XRD pattern of a 1 : 1 : 11 series of Keggin type heteropolyacid (salt). Fig. 7 depicts the XRD pattern of the catalyst, the unfolded peaks (2θ = 10.18°, 18.86°, 29.12° and 34°) are consistent with the characteristic peaks for the 1 : 1 : 11 series of Keggin type heteropoly compounds. By integrating the results from TGA/DSC, FT-IR spectroscopy, UV spectroscopy and XRD, the catalyst is demonstrated to be a kind of Keggin type heteropoly compound.

Fig. 7 XRD pattern of the catalyst.

The surface morphology of the catalyst was obtained using a KY-AMRAY 1000B scanning electron microscope (SEM) (Fig. 8a and b). Fig. 8 shows that the sample particles are honeycomb at low magnification, and appear as regular hexahedron shapes at high magnification. Although the size of the sample particles is not consistent, the shape of the sample particles is almost the same. In addition, it can be clearly seen that the particle size is basically less than 3 μm as presented.

Fig. 8 SEM images of the catalyst: (a) low magnification, and (b) high magnification.

3.5. Ultrasonic degradation properties analysis

Analysis method: the maximum absorbance of AGB occurs at 605 nm, the absorbances in the following experiments were measured at this wave number. The degradation rate can be described by the following equation according to the Beer–Lambert law:

Degradation rate (%) = [(A₀ − A)/A₀] × 100% (1)

where A₀ is the absorbance of the initial AGB dye wastewater before ultrasonic catalytic treatment, and A is the absorbance of the AGB dye wastewater irradiated with ultrasonic waves after t minutes.

3.5.1. The effect of different catalyst dosages on the catalytic activity. The dosage of the catalyst was tested in a range from 0.0 g L⁻¹ to 1.0 g L⁻¹, the initial concentration of the AGB dye wastewater was kept constant (100 mL, 10 mg L⁻¹) and the reactions were irradiated with 45 kHz and 80 W ultrasonic waves. As shown in Fig. 9, in the first 20 minutes, the highest degradation rate was achieved when the dosage was 0.6 g L⁻¹, but after 20 minutes, the degradation rate of the 0.8 g L⁻¹ dose was the highest. As a result, if the reaction time is less than 20 minutes, a dosage of 0.6 g L⁻¹ should be taken into the consideration, while if the reaction time is more than 20 minutes, a dosage of 0.8 g L⁻¹ would be the best choice. The following experiments were conducted for more than 20 minutes, therefore, a 0.8 g L⁻¹ dose was selected. Fig. 9 shows that the degradation rate increased up to a tip-top 95.72% after 60 minutes when the dosage was 0.8 g L⁻¹. Although the experiment found that when the dosage of catalyst is controlled within a certain range, the degradation rate increases with the increasing dosage, above a certain value, the increase in the degradation rate is not obvious, which is mainly because as more catalyst is added into the solution, side reactions might be more easily accessed. At the same time, superfluous catalysts might shield each other, which would diminish the acoustic effect of the ultrasonic waves, and finally, the degradation rate might be affected.

Fig. 9 Effect of catalyst dosage.

3.5.2. The effect of different initial concentrations of dye wastewater on the catalytic activity. The initial concentration of the wastewater was tested in a range from 10 mg L⁻¹ to 50 mg L⁻¹, and the solutions were irradiated with 45 kHz and 80 W ultrasonic waves for 60 minutes, the corresponding final degradation rates were 95.72%, 88.60%, 75.07%, 60.65% and 57.24%, the catalyst shows the highest catalytic activity when the initial concentration is 10 mg L⁻¹, therefore all the following experiments were conducted at this concentration. Fig. 10 shows that with increasing initial concentration, the degradation rate decreases, but the degradation quantity increases relatively. This is mainly because when the dosage, ultrasonic frequency and power remain the same, and the concentration of the system is kept low, the number of cavitation bubbles, ˙OH and other free radicals will remain relatively stable, and the possibility of the degradation reaction occurring and the concentration of the reactant are proportional, therefore, the total removal of AGB dye molecules increases with the increasing initial concentration of the wastewater per unit volume and per unit time. However, as the initial concentration increases, dye molecules and transient products on the cavitation bubble surfaces tend to be saturated, which may hinder the spread of free radicals outside of the bubbles and reduce the region in which the reaction occurs. In the meantime, the reaction region may be occupied by transient products, and the reverse reaction to degradation is probably expedited due to the presence of excessive transient products.²⁸

Fig. 10 The effect of the initial AGB concentration.

3.5.3. The effect of the pH of the dye wastewater on the catalytic activity. Due to the fact that the pH value has a very important effect on catalytic activity,²⁹ the influence of different pH values on the ultrasonic catalytic activity was investigated. The pH values of the solutions (1, 3, 5, 7 and 9) were adjusted using sodium hydroxide and hydrochloric acid solutions, and the solutions were irradiated with 45 kHz and 80 W ultrasonic waves for 60 minutes, the final corresponding degradation rates (52.63%, 73.81%, 92.17%, 91.86% and 76.87%) appear in Fig. 11. Obviously, the degradation rate was the highest when the initial pH value of the dye wastewater was 5.0, thus the pH values of the following experiments were controlled at 5.0. It can be clearly seen that when the pH value of the solution was neutral or weakly acidic, the degradation rate was relatively high. This is mainly because the pH value can affect the form of the heteropolyacid salts and the dissociation of the dye molecules. Dye molecules exist in a molecular form in acidic solution, and they can accumulate on the cavitation bubble surfaces or volatilize into the cavitation bubbles, however, a strongly acidic environment may lead the H⁺ in solution to expend some of the ˙OH, and the degradation reaction would be affected. Dye molecules only exist in their ionic form in alkaline solution, and it is difficult for ions to accumulate on the cavitation bubble surfaces or volatilize into the cavitation bubbles, reactions only take place at the gas–liquid interface, which is unbeneficial to the degradation reaction. On the other hand, heteropolyacid salts only exist in neutral or weakly acidic solutions, they will decompose in other conditions and lose their catalytic activity. Under the experimental conditions, when the added amount of catalyst was 0.8 g L⁻¹, the acidity of the solution was just within the best range, therefore, it was unnecessary to adjust the acidity of the solution.

Fig. 11 Effect of the initial pH.

3.5.4. The effect of different ultrasonic frequencies on the catalytic activity. The ultrasonic power was controlled at 80 W and the ultrasonic frequency was adjusted to 28 kHz, 45 kHz, 80 kHz and 100 kHz. As shown in Fig. 12, the highest degradation rate attained was 95.72%, when the ultrasonic frequency was 45 kHz, thus the ultrasonic frequency of the following experiments was fixed at 45 kHz. It can be concluded from Fig. 12 that, when other conditions remain unchanged, there is an optimum frequency, but this frequency is not the highest or the lowest. This is mainly because the radii of the cavitation bubbles at low frequency are larger than that at high frequency and, as a result, the collapse time at low frequency is longer than that at high frequency. However, a high frequency leads to a shorter time of collapse, and there will be more chance for ˙OH to escape from the cavitation bubbles, which may avoid the free radicals recombining with each other in the bubbles, thus a high frequency can promote the degradation. However, a high frequency can also lead to a drop in cavitation intensity and accelerate the decay of liquid energy, which may make it difficult for the cavitation process to occur and decrease the amount of ˙OH, therefore the degradation rate may be affected. The best frequency for degradation activity emerges by considering both of the actions mentioned above.

Fig. 12 Effect of ultrasonic frequency.

3.5.5. The effect of different ultrasonic powers on the catalytic activity. The power of the ultrasonic waves was set to 40 W, 60 W, 80 W and 100 W. Fig. 13 shows that when the power of ultrasonic waves was 100 W, the greatest effect appeared, the degradation rate rose up to 95.72%. Apparently, it also can be seen that the degradation rate increases with the increasing power. This is mainly because the reinforcing effect of the ultrasonic power adds to the number of “cavitation nuclei” and strengthens the effect of cavitation pyrolysis on the pollutants.^30,31 At the same time, the oxidation of ˙OH and other free radicals is also enhanced, therefore, ultrasonic degradation is promoted. Nevertheless, an excessively strong power intensity may be counterproductive. When the system accepts excessively high acoustic energy, the cavitation bubbles will get very big in the sonic negative phase and cause acoustic shielding, which may decrease the available acoustic energy in the reaction.

Fig. 13 Effect of ultrasonic power.

3.5.6. Degradation kinetics. Ultrasonic catalytic oxidation tests were carried out with different concentrations of AGB dye wastewater to investigate the kinetics of the ultrasonic catalytic degradation of AGB by the catalyst. The experimental data can be described by the pseudo-first order equation:where C₀ is the initial concentration of AGB (mg L⁻¹), C_t is the concentration of AGB (mg L⁻¹) at t minutes and k (min⁻¹) is the pseudo-first order rate constant. 0.8 g L⁻¹ of catalyst was added to the AGB dye wastewater, the ultrasonic frequency was 45 kHz and the power of the ultrasonic waves was 100 W.

Fig. 14 shows the degradation kinetics curves for AGB over the catalyst during the irradiation time. Obviously, the degradation rate of the AGB dye decreases with increasing initial concentration. It can be concluded from Table 2 that the linear coefficients (R²) of the curves obtained by plotting ln(C₀/C_t) against time are almost more than 0.99, which indicates that the degradation process is basically in accord with the pseudo-first order kinetic model.

Fig. 14 Kinetic characteristics of the ultrasonic degradation of AGB at different initial concentrations, with a pseudo-first order model fit.

Table 2 Kinetic parameters of the pseudo-first order equation

Initial concentration (mg L^−1)	Rate constant (min^−1)	Half-life period (min)	Linear coefficients (R^2)
10	0.02374	29.20	0.98179
20	0.01822	30.05	0.99558
30	0.01850	37.47	0.99011
40	0.01615	42.92	0.99450
50	0.01436	48.27	0.99580

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4. Conclusions

We have successfully synthesized a novel heteropolyacid salt Na₆[Zn(Mo₁₁ZrO₃₉)]·20H₂O. The catalyst possesses a complete Keggin structure and has good thermal stability. The molar ratio of the elements in the catalyst is 1 : 1 : 11. In the experiments, the degradation performance of the catalyst promoted by ultrasonic waves was excellent, the degradation rate reached 95.72% under the optimum conditions. Additionally, the degradation of AGB dye in wastewater by the catalyst with ultrasonic waves adhered well to the pseudo-first order kinetic model.

Acknowledgements

The authors are grateful to Xi’an Technological University (XAGDXJJ0911).

References

1. S. Sathian, M. Rajasimman, G. Radha, V. Shanmugapriya, V. Shanmugapriya and C. Karthikeyan, Alexandria Eng. J., 2014, 53, 417.
2. K. Zhao, G. Zhao, P. Li, J. Gao, B. Lv and D. Li, Chemosphere, 2010, 80, 410.
3. R. G. Saratale, G. D. Saratale, J. S. Chang and S. P. Govindwar, Biodegradation, 2010, 21, 999.
4. E. E. Baldez, N. F. Robaina and R. J. Cassella, J. Hazard. Mater., 2008, 159, 580.
5. E. Forgacs, T. Cserhátia and G. Orosb, Environ. Int., 2004, 30, 953.
6. L. Lucarelli, V. Nadtochenko and J. Kiwi, Langmuir, 2000, 16, 1102.
7. M. T. Yagub, T. K. Sen, S. Afroze and H. M. Ang, Adv. Colloid Interface Sci., 2014, 209, 172.
8. V. K. Gupta and M. Suhas, J. Environ. Manage., 2009, 90, 2313.
9. E. J. Almeida and C. R. Corso, Chemosphere, 2014, 112, 317.
10. M. Tichonovas, E. Krugly, V. Racys, R. Hippler, V. Kauneliene, I. Stasiulaitiene and D. Martuzevicius, Chem. Eng. J., 2013, 229, 9.
11. S. U. M. Khan, M. Al-Shahry and W. B. Ingler Jr, Sci., 2002, 297, 2243.
12. C. Bauer, P. Jacques and A. Kalt, J. Photochem. Photobiol., A, 2001, 140, 87.
13. I. Justicia, P. Ordejón, G. Canto, J. L. Mozos, J. Fraxedas, G. A. Battiston, R. Gerbasi and A. Figueras, Adv. Mater., 2002, 14, 1399.
14. P. Souchay, Ions minéraux condenses, Masson, Paris, 1969.
15. K. Nowińska, A. Wącław, M. Sopa and M. Klak, Catal. Lett., 2002, 78, 347.
16. N. Giordano, P. Staiti, S. Hocevar and A. S. Arico, Electrochim. Acta, 1996, 41, 397.
17. A. S. Aricò, E. Modica, I. Ferrara and V. Antonucci, J. Appl. Chem., 1998, 28, 881.
18. A. S. Aricòa, H. Kim, A. K. Shuklab, M. K. Ravikumarc, V. Antonuccia and N. Giordanoa, Electrochim. Acta, 1994, 39, 691.
19. I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171.
20. S. Antonaraki, E. Androulaki, D. Dimotikali, A. Hiskia and E. Papaconstantinou, J. Photochem. Photobiol., A, 2002, 148, 191.
21. P. Ning, H. J. Bart, Y. J. Jiang, A. D. Haan and C. Tien, Sep. Purif. Technol., 2005, 41, 133.
22. L. Jiang, S. Y. Guo and X. N. Li, Polym. Degrad. Stab., 2005, 89, 6.
23. S. G. Ostlund and A. M. Striegel, Polym. Degrad. Stab., 2008, 93, 1510.
24. X. K. Wang, Y. C. Wei, C. Wang, W. L. Guo, J. G. Wang and J. X. Jiang, Sep. Purif. Technol., 2011, 81, 69.
25. X. Wang, L. G. Wang, J. B. Li, J. J. Qiu, C. Cai and H. Zhang, Sep. Purif. Technol., 2014, 122, 41.
26. A. Ghauch, H. Baydoun and P. Dermesropian, Chem. Eng. J., 2011, 172, 18.
27. M. P. Brenner, S. Hilgenfeldt and D. Lohse, Rev. Mod. Phys., 2002, 74, 425.
28. I. Hua and M. R. Hoffmann, Environ. Sci. Technol., 1997, 31, 2237.
29. T. E. Agustina, H. M. Ang and V. K. Vareek, J. Photochem. Photobiol., C, 2005, 6, 264.
30. G. J. Price, P. Matthias and E. J. Lenz, Process Saf. Environ. Prot., 1994, 72, 27.
31. Y. Jiang, C. Pétrier and T. D. Waite, Ultrason. Sonochem., 2002, 9, 317.

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