ZnO-TiO₂ Composite Material Prepared by Sol-Gel Method to Treat Pharmaceutical Wastewater

Abstract:

This research aims to treat pharmaceutical wastewater with the ZnO-TiO₂composite material prepared by the sol-gel method. It examines and analyzes the causes and effects of ZnO‘s doping ratio, calcination temperature calcination time and irradiation time on ZnO-TiO₂ composite photocatalyst material’s degradation of pharmaceutical wastewater. Both the undoped TiO₂ test and the blank test are done for its reference and contrast.

• Periodical: Applied Mechanics and Materials (Volumes 361-363)
• Pages: 736-739
• DOI: https://doi.org/10.4028/www.scientific.net/AMM.361-363.736
• Online since: August 2013
• Authors: Xun Wang, Wei Wei Li, Yang Fan Xiao, Lei Zhu, Xiao Yan Liu
• Keywords: Pharmaceutical Wastewater, Photocatalytic Degradation, Sol-Gel Process, ZnO-TiO₂

References

[1] In this test, the ZnO-TiO2 composite photocatalyst material are prepared by the sol-gel method. And it evaluates ZnO-TiO2 material's photocatalytic activity with pharmaceutical wastewater as the target pollutant. By changing the conditions of tests, the ZnO-TiO2 composite material's optimum conditions to degrade pharmaceutical wastewater and the causes to improve its catalytic efficiency are all analysed. The test Water samples. The sample of the wastewater is taken from a pharmaceutical factory in Hubei Province. Its water quality is shown as follows: COD 300, 000mg • L-1, chromaticity colour 600 times, pH 5-6. Reagents and instruments. The reagents in this test are: tetrabutyl titanate(TBT)(chemical pure reagent); anhydrous ethanol, zinc nitrate hexahydrate, nitrate, mercuric sulfate, potassium dichromate, silver sulfate, sulfuric aci( analytical reagents); distilled water. The main instruments are: UV-6100S UV-visible spectrophotometer, TG16-WS desktop high-speed centrifuge, GXZ-9070MBE digital blast oven, HJ-3 digital thermostat magnetic stirrer, SX2-4-10 box-type energy-saving electric resistance furnace, UV lamp, etc. The preparation for the ZnO-TiO2 composite material. By certain mole ratio, mix TBT, anhydrous ethanol, nitric acid, water and zinc nitrate together to Prepare ZnO-TiO2 sol. Sufficiently mix TBT with anhydrous ethanol which takes up 2/3 of its total volume to make the raw liquid. Weigh certain zinc nitrate and dissolve it in other 1/3 of the anhydrous ethanol and distill them with water so that the dropwise solution is made. Drip the solution dropwise and slowly into the raw liquid, still the liquid for a moment and then the ZnO-TiO2 sol becomes the gel. By drying, calcinating and grinding the gel, the nano ZnO-TiO2 powder is obtained. In addition, change the amount of zinc nitrate and the ZnO-TiO2 composite material of different dopings (molar ratio) can be prepared. Test method. Use the ZnO-TiO2 powder prepared by the author to photocatalytically degradate the pharmaceutical wastewater diluted with distilled water of 5 times amount of the wastewater. Add in 0. 15mg of the ZnO-TiO2 powder into 20ml of the diluted wastewater and stir it in the dark. Then irradiate the wastewater with an 18W-UV lamp above its surface with a vertical of 15cm for continuous irradiation. At set intervals, take out certain amount of the solution for centrifugation. Then get its supernatant to measure the absorbance of the solution before and after the reaction with the UV-visible spectrophotometer by the wastewater's maximum absorption wavelength (600nm). According to the formula, the decolorization rate can be given as below: Decolorization rate =（A0－A）/ A0×100% (1) where: A0, A－respectively representing the absorbance(Abs) before and after the reaction of pharmaceutical wastewater. COD is determined by the potassium dichromate method [2]. Test results and discussions This essay mainly discusses the effects of 4 factors, namely doping amount of ZnO, calcination temperature, calcination time and irradiation time, on the ZnO-TiO2 photocatalytic material's catalytic efficiency. Effects of ZnO's doping ratio. Doped with appropriate amount of ZnO in TiO2, the photocatalyst material can take advantage of the coupling interaction between the nanoparticles not only to improve the stability of ZnO-TiO2, but also to increase the efficiency of ZnO-TiO2 to degradate organism in the wastewater. Dope the TiO2 gel with ZnO according to the ratios of 0. 5%, 1%, 3%, 5%, 6%, 8%, 10% (mole ratio) to prepare the composite gel. Then calcinate each type of the gel at 600°C for 2h to prepare photocatalytic materials with different doping amount of ZnO. Add them respectively in the wastewater for the degradation reaction under the UV lamp for 2h The catalysts' dosage is 0. 15mg. Their degradation effects are shown in Table. 1. Table. 1 Catalytic effects of different doping amounts of ZnO ZnO/ TiO2(mole ratio).

5% 1% 3% 5% 6% 8% 10% Decoloration rate(%) COD removal rate(%).

[21] 4.

[31] 8.

[25] 6.

[42] 1.

[31] 0.

[57] 1.

[36] 5.

[69] 0.

[32] 3.

[56] 6.

[29] 1.

[33] 6.

[20] 7.

[30] 8 Table. 1 shows that after the pharmaceutical wastewater is degradated by ZnO-TiO2 composite photocatalytic materials of different doping amounts under the UV lamp for 2h, its decolorization rates are all around 30%. Its COD removal rate increases at first with the growth of the doping amount of ZnO, when the doping amount of ZnO reaches 5%, its COD removal rate achieves the top of 69. 0%; when the the doping amount of ZnO goes beyond 5%, its COD removal tend to decrease gradually. Doped with too low amount of ZnO, the ability of TiO2 to capture the electrons and holes is so insufficient that electron-hole pairs can not be efficiently separated, while too large doping amount of ZnO causes too small of the TiO2 activity center, so that ZnO is easy to accumulate on the surface of ZnO, unfavorable of the photocatalyst material's sufficient absorption of UV light[3]. In this test, 5% (molar ratio) is the optimum doping amount of ZnO. Effects of different calcination temperatures. The calcination temperature can have effect on the particle size, form and specific surface area of the catalyst. The dried gel ZnO-TiO2 power under different calcination temperatures, will have different crystallines which own different photocatalytic effects. Calcinate equal amount of the dried gel ZnO-TiO2 power in the resistance furnace for 2h respectively at temperatures of 450°C, 500°C, 550°C, 600°C, 650°C and 700°C, and series of ZnO-TiO2 (5%) composite photocatalyst materials of different calcination temperatures are prepared. Add the catalysts respectively into the wastewater with an equal dosage of 0. 15mg to degradate the wastewater under UV lamp for 2h. Their degradation effects are shown in Table. 2. Table. 2 Different calcination temperatures' effects on the catalytic activity of ZnO-TiO2(5%) Calcination temperature(°C) 0 450 500 550 600 650 700 Decoloration rate(%) COD removal rate(%).

[10] 9.

[11] 6.

[20] 3.

[50] 9.

[23] 4.

[58] 3.

[35] 0.

[60] 8.

[37] 0.

[69] 2.

[30] 0.

[53] 1.

[29] 7.

[44] 7 It is known from Table. 2 that non-calcinated ZnO-TiO2 has the least catalytic effect, and the optimum calcination temperature is 600°C. The temperature that is either higher or lower than 600°C can decrease the catalytic effect to some extent. A too-low calcination temperature makes TiO2 unable to have a good crystalline thus affecting its photocatalytic activity, while an excessively high temperature tends to cause the catalyst to lose its activity for the catalytic material is easy to be converted into rutile phase [4]. Effects of calcination time. Calcination time can also affect the catalytic performance of catalysts. Calcinate equal amount of the dried gel powder of ZnO-TiO2 (5%) under 600°C for 0. 5, 1, 1. 5, 2, 2. 5 and 3h respectively so as to produce different series of ZnO-TiO2 composite light catalytic materials. Then, under the UV lamp, respectively add 0. 15mg of catalyst into the wastewater for the degradation reaction for 2h. Their degradation effects are shown in Table. 3. Table. 3 Different calcination time's effects on ZnO-TiO2（5%） Calcination time（h）.

5 1.

[1] 5 2.

[2] 5 3 Decoloration rate(%) COD removal rate(%).

[21] 0.

[44] 6.

[36] 7.

[68] 6.

[45] 8.

[79] 0.

[37] 0.

[69] 2.

[36] 8.

[57] 4.

[37] 1.

[54] 8 Table. 3 shows that the pharmaceutical wastewater's decolorization rates are all about 40% if the dried gel powder of ZnO-TiO2 (5%) is calcinated for at least 1h, and when the the calcination time is 1. 5h, the COD removal rate reaches the highest of 79. 0%, almost twice of that being calcinated for 0. 5h. When the calcination time is prolonged from 2h, the degradation effect takes a downward trend. Therefore, if the calcination time is too short, the reaction of ZnO-TiO2 particles is not sufficient; if too long, part of the particles' sizes would get larger and tend to agglomerate and form the rutile, which is not favorable of improving the catalytic activity [5]. Effects of irradiation time. The longer irradiation time is, the fuller reaction will be. However, the degradation cost grows with the reaction time increasing. Use the photocatalytic ZnO-TiO2 (5%) powder calcined for 2h under 600°C to degradate the pharmaceutical wastewater. Each interval of 30min, sample and analyse the wastewater, and the results are shown in Table. 4. Table. 4 Different irradiation time's effects on the catalytic performance of ZnO-TiO2(5%) Irradiation time（h）.

5 1.

[1] 5 2.

[2] 5 3 Decoloration rate(%) COD removal rate(%).

[29] 0.

[33] 2.

[32] 7.

[55] 4.

[36] 1.

[60] 2.

[37] 0.

[69] 2.

[37] 2.

[69] 7.

[37] 7.

[69] 9 From Table. 4, it is found that the degradation effect of ZnO-TiO2 on the pharmaceutical wastewater improves with the extension of irradiation time. From the duration of 0. 5-2h, the decoloration rate and the COD removal rate of the wastewater show a rapid growth trend; and after 2-3h, the decoloration and the COD removal rates increase marginally. Taken the economic reasons into consideration, the optimum irradiation time is determined to be 2h. Blank and contrast tests. The test adopts both the unmodified TiO2 and ZnO-TiO2 (5%) for the same water sample being processed and illuminating with the UV lamp for 2h. And the blank test is done for contrast and comparison. Then get these samples for analysis. The test results are shown in Table. 5. Table. 5 Blank and Contrast Tests Items Pure TiO2 ZnO-TiO2 (5%) Irradiation without the photocatalyst The photeocatalyst without irradiation Decoloration rate(%) COD removal rate(%).

DOI: 10.1016/j.matpr.2021.12.229

[12] 5.

[25] 8.

[37] 0.

[69] 2.

[8] 8.

[9] 7.

[9] 4.

[12] 9 It's seen from Table. 5 that in the case of no photocatalyst, the UV lamp has relatively less effective degradation for the wastewater, the decolorization and COD removal rates are both less than 10%. Without using the UV lamp, the photocatalyst has comparatively small effect on the degradation of wastewater, though its adsorption property can make the wastewater decolored by 9. 4% and remove its COD by 12. 9%. Thus it is obvious that the ZnO-TiO2 photocatalytic composite material has much better catalytic effect than pure TiO2. Conclusions Through tests, the optimum process conditions for ZnO-TiO2 is determined: 0. 15mg ZnO-TiO2, with a doping amount of 5%, calcined under 600°C for 1. 5h, is added into 20 ml of the wastewater, then irradiate the wastewater for 2h, finally obtain the resluts that the decolorization rate and the COD removal rate of the pharmaceutical wastewater are repectively 45. 8% and 79. 0%. Therefore, it is concluded that ZnO-TiO2 composite photocatalyst material has good photocatalytic activity and catalytic effects on degradation of the pharmaceutical wastewater. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51272189). Refernces.

[1] Marci G, Augugliaro V, Lopez Munoz MJ, et al. Preparation Characterization and Photo-catalytic Activity of Polycrystalline ZnO/TiO2 Systems [J]. J Phys Chem B, 2001, 105: 1033.

[2] GB l1914-89 Water Quality in the Determination of the Chemical Oxygen Demand: Dichromate Method [S]. (In Chinese).

[3] Gang Cheng, Xiaode Zhou, et al. Preparation of ZnO-TiO2 Composite Semiconductor and Its Photocatalytic Properties [J]. Journal of Xi'an University of Technology, 2009, 25 (1): 10. (In Chinese).

[4] Yang Li. Preparation and Properties of Nano-TiO2 and TiO2-ZnO Photocatalytic Materials [D]. Lanzhou University of Technology, 2008. (In Chinese).

[5] Lian Gao, Shan Zheng, Qinghong Zhang. Study on Nano-TiO2 photocatalytic material and Its Application [M]. Beijing: Chemical Industry Press, 2002. (In Chinese).