Application of MoO3 as an efficient catalyst for wet air oxidation treatment of pharmaceutical wastewater (Experimental and DFT study)

Abstract

In this work, a highly effective catalyst (MoO3) is synthesized and applied for catalytic wet air oxidation (CWAO) treatment of pharmaceutical wastewater. The catalyst is systematically characterized to investigate the morphology, crystal structure and chemical composition, and the findings demostrated that MoO3 catalyst is successfully synthesized. The degradation mechanism is also illustrated by the density functional theory (DFT) calculation. The degradation experiments confirm that MoO3 catalyst exhibits excellent catalytic performance in CWAO, and the removal rate of TOC (Total Organic Carbon) and COD (Chemical Oxygen Demand) is achieved to more than 93%. The catalyst doses, reaction temperature and reaction time have a significant impact on the removal of pollutants. The degradation process of pollutants in CWAO could be satisfactorily fitted by the second-order kinetics. Besides, MoO3 displays a favorable stability as CWAO catalyst. DFT calculation illustrates that MoO3 catalyst is a typical indirect band gap semiconductor. Moreover, the high temperature environment provides the thermal excitation energy, which favors to the free electrons nearing Fermi level to escape the material surface, and excites them to the conduction band, then directly reduces the pollutants in CWAO. These findings demonstrate that MoO3 can be used as an efficient and excellent catalyst for CWAO of pharmaceutical wastewater. 48 C. Chen, T. Cheng, L. Wang, Y. Tian, Q. Deng, Y. Shi produces hydroxyl radical (OH) with strong oxidation ability under the reaction conditions of high temperature and high pressure, electricity, sound, light irradiation, and catalyst. By utilizing the advanced oxidation technology, the refractory organic substances of macromolecules could be oxidized into low toxic or non-toxic small molecular substances. Schrank et al. use diverse advanced oxidation techniques of UV (Ultraviolet Light), TiO2/UV, O3 and O3/UV to degrade pollutants in tannery wastewater; it demonstrates that the biodegradation of wastewater is enhanced through oxidation, and the toxicity of pollutants is also decreased (Schrank et al, 2004). Hofman-Caris et al. removed pharmaceuticals from wastewater effluent and dissolved non-biodegradable organic matter by advanced oxidation technique. The results suggest that advanced oxidation techniques could remove multiple different pharmaceuticals and humic acid to a large extent (Hofman-Caris et al., 2017). Wet air oxidation (WAO) technology is a kind of advanced oxidation technology. It can oxidize organic substances in wastewater to small molecules or inorganic matter under the condition of high temperature (120–320°C) and high pressure (0.5 ~20 MPa). Gaseous oxygen is utilized as oxidant in this reaction. Moreover, it can treat wastewater with refractory substances, and be highly evaluated by many researchers due to its simple operation and easy industrialization. However, the WAO technology requires high temperature, and the cost to treat wastewater is very high. To solve this problem, the existing idea is to add suitable catalysts on the basis of WAO, which calls catalytic wet air oxidation (CWAO), so that the reaction could be carried out under mild conditions. In this way, it could reduce the cost of wastewater treatment, and improve the removal efficiency (Kang et al, 2011). The development of catalysts is crucial to the treatment of pollutants in catalytic wet oxidation technology. So far, the research of catalytic wet oxidation has focused mainly on the synthesis of novel catalysts and investigating the effect of materials on wastewater treatment in CWAO. Researchers have developed a series of catalysts in CWAO, such as TiO2 (Lunagomez Rocha et al, 2015), Al2O3 (Sushma et al, 2018), CeO2 (Parvas et al, 2019), MoO3 (Li et al, 2009, Wang et al, 2017, Wang et al, 2020b) and so on. Among them, MoO3 exhibits favorable catalytic performance in the degradation of organic pollutants. Li et al. synthesized one-dimensional Ce-doped MoO3 nanofibers with different doping amounts by combining sol-gel method and electrospinning technology. It was found that 11.86wt% CeO2-doped nanofibers exhibited excellent catalytic activity for the fast degradation of organic dye (Li et al., 2009). Wang et al. synthesized MoO3 catalyst by hydrothermal method, and the catalytic material demonstrated good catalytic performance at 400°C for the degradation of dye wastewater (Wang et al., 2017). Wang et al. synthesized highly active and stable nano-hybrid bimetallic catalysts, and applied to the treatment of organic dyes and obtained excellent degradation efficiencies in catalytic wet air oxidation system (Wang et al., 2020b). This demonstrates that MoO3 catalyst is a very effective and potential material in the field of wet air oxidation treatment of industrial wastewater. However, this research is insufficient in the following two aspects. Firstly, most of the wastewater used in studies is simulated wastewater, while the practical wastewater used in research is relatively rare. The catalytic performances of MoO3 synthetic materials in wet air oxidation of pharmaceutical production wastewater are also rarely reported. Secondly, these studies do not thoroughly analyze the degradation mechanism of catalysts for catalytic wet air oxidation of wastewater. This study has made some supplements to these two aspects. We obtained practical wastewater from pharmaceutical companies to analyze the degradation effect of MoO3 catalyst in CWAO. The organic pollutants produced in pharmaceutical wastewater are in large quantities, do great harm, and are difficult to be degraded due to their complex compositions. If MoO3 catalyst could be applied to the wet air oxidation treatment of pharmaceutical wastewater, it would not only promote the application of MoO3 catalytic material, but also provide a new choice of technique for the treatment of pharmaceutical wastewater, thus alleviate the harm of pharmaceutical wastewater to the environment. Also, with the aid of Density Functional Theory (DFT) calculations, we attempt to reveal the microscopic mechanism of MoO3 as a catalyst to degrade pharmaceutical wastewater in CWAO through analyzing the energy band structure of material. Hence, in this paper, MoO3 catalyst is prepared and synthesized by hydrothermal synthesis methods. The synthetic material properties are fully characterized by liquid specific surface area analyzer, X-ray diffractometer (XRD), scanning electron microscope (SEM), thermogravimetric analyzer, and other modern and advanced analytical techniques. The synthetic catalyst is applied to degrade pharmaceutical wastewater by catalytic wet air oxidation. The impact of catalytic dosage, reaction temperature, and reaction time on the degradation of pollutants is investigated. In addition, the reaction process of catalytic wet air oxidation on the pharmaceutical wastewater is modeled by the first-order and second-order kinetic equation. The recycling of catalyst in wet air oxidation system is also discussed. Furthermore, to illustrate the catalytic mechanism of MoO3 on the degradation of pharmaceutical wastewater, the band structure and Density of States (DOS) of catalyst were obtained through DFT calculation. Experimental equipment and methods Preparation of catalyst The hydrothermal synthesis method is performed for the preparation of catalyst. The mixture of 56 g (NH4)6Mo7O24·4H2O and KNO3 was dissolved in distilled water at 70°C and fully mixed. Among the mixed liquor, the amount of KNO3 was 5.1 g. To control the pH of solution, 0.1 mol/L HNO3 and KOH solution was used to adjust the pH (range from 4 to 7). The solution was transferred to the KH-50 type hydrothermal synthesis reactor at 125°C for 24 h. The outer shell of the hydrothermal reactor was made of stainless steel, and the inner lining was made of polytetrafluoroethylene produced by Beijing Getimes Technology Co., Ltd. Then, the resulting precipitate was filtered and washed with deionized water at room temperature until the pH value of the filtrate reached 7. The mixture was then dispersed in the 450 ml acetone and mixed for 2.5 h. After that, the mixture was filtered, and the residue was collected and dried at 85°C for 16 h in the electroApplication of MoO3 as an efficient catalyst for wet air oxidation treatment of pharmaceutical wastewater... 49 -thermostatic blast oven. The sample was ground and roasted in a muffle furnace at 430°C for 4 h. Finally, the catalytic material was obtained by naturally cooling at room temperature in the Carbolite muffle furnace. Characterization of catalyst The synthetic catalyst is characterized by liquid specific surface area analyzer, X-ray diffractometer, scanning electron microscope, Fourier transform infrared spectroscopy, thermogravimetric analyzer, X-ray photoelectron spectroscopy spectra (XPS), and so on. In this experiment, we used Xigo liquid specific surface area analyzer (XiGo Nanotools, USA) to measure the specific surface area of the catalyst, and evaluate the properties of synthetic material. The morphology and composition of the catalyst was determined by S-3400N II scanning electron microscope (Hitachi Company, Japan). The characteristic functional group structure, molecular structure and chemical composition of the catalyst are analyzed by Thermo Scientific Nicolet is 5 Fourier transform infrared spectroscopy (FTIR) using pressed KBr discs. Moreover, the image of the catalyst was analyzed by X’tra X-ray diffractometer (ARL Company of Switzerland). The thermoweight of the catalyst was measured by Pyris 1 DSC thermal analyzer (PerKinElmer Company, the United States). The X-ray photoelectron spectroscopy spectra were recorded on PHI 5000 VersaProbe XPS equipment. The UV-vis spectrum research was carried out by PerkinElmer Ultraviolet spectrophotometer. Catalytic wet air oxidation The experiment of catalytic wet air oxidation to treat the wastewater was carried out in G