Study On Preparation Of ZrO_x-ZnO Catalyst And Microwave-Assisted Photocatalytic Degradation Of Typical EDCs

Endocrine disruptors (EDCs) are a species of chemicals which can cause bioaccumulation and reproductive toxicity, and they are very harmful to human health and ecological environment. Moreover, it is difficult to degrade them by traditional wastewater treatment because of their low biodegradability, high toxicity and small environmental capacity. Therefore, it is eager to develop effective treatments for EDCs wastewater. In this paper, a novel mix-valenced ZrO_x-ZnO catalyst with simultaneous fine response to both microwave and UV light irradiation was developed, and a microwave-assisted photocatalytic oxidation process (MW/PC) has been set up using the EDCs substances dimethyl phthalate (DMP) and herbicide Atrazine as the target pollutants. The synergetic effect among microwave, UV light and catalyst in the MW/PC process was also discussed.A CTAB-assisted precipitation-hydrothermal method was applied to prepare a novel ZrO_x microwave-assisted photocatalyst. The preparation conditions of the ZrO_x were optimized, and composited with ZnO enhanced the catalytic activity and broaded the UV light response range of the mix-valenced ZrO_x. Furthermore, a ZrO_x-ZnO/γ-Al₂O₃ catalyst was produced by loading the ZrO_x-ZnO on aγ-Al₂O₃ carrier to meet the requirements of circulating degradation. XPS, XRD, BET, SEM and XRF techniques were applied to characterize the prepared catalysts. It was found that the Zr on the surfaces of all the three catalysts exist as mix-valences. The catalysts were all well crystallized and contained abundant macroporous and mesoporous structures. The band gap of the ZrO_x was much wider than that of P25 TiO₂, but the absorption band edge of the ZrO_x-ZnO has obviously moved to longer wavelength after composited with ZnO, and the UV light response range of the ZrO_x-ZnO was greatly broadened. The ZrO_x-ZnO had a mixcrystal structure, and the composition of ZrO_x and ZnO did not lead to a solid solution, and. Loaded on theγ-Al₂O₃ clearly enhanced the surface area and macropore/mesopore amount of the ZrO_x-ZnO.A sequencing batch type integrated microwave and UV light reaction equipment was designed using a microwave discharged electrodeless lamp as the UV light source and a microwave experimental oven with continuously adjustable power as the microwave source. Based on the designed equipment, a sequencing batch type MW/PC process was set up using the ZrO_x-ZnO as catalyst. The efficiency was investigated using DMP simulated wastewater as target pollutant, and the reaction conditions were optimized. Besides, the intermediates formed during the degradation of DMP were detected, and the MW/PC degradation mechanism of DMP was proposed. Results revealed that the MW/PC process had fast degradation rate, high degradation efficiency and complete mineralization degree. The removal of 100 mL 50 mg/L DMP could reach 90% and the TOC removal of DMP could achieve 71% on the conditions of microwave power 400 W, UV light intensity 9.79 mW/cm² (254 nm), catalyst dosage 1.0 g/L and reaction time 20 min.On the basis of sequencing batch type reaction equipment, a circulating type integrated microwave and UV light reaction equipment was developed and a circulating type MW/PC process was subsequently built up using the ZrO_x-ZnO/γ-Al₂O₃ as catalyst. The degradation efficiency was studied using Atrazine as target pollutant, and the reaction conditions were optimized. The removal of 3 L 10 mg/L Atrazine could reach 94% and the TOC removal of Atrazine could achieve 78% on the conditions of microwave power 400 W, UV light intensity 9.79×3 mW/cm² (254 nm), catalyst dosage 50 g/L, circulation flow rate 109 L/h, flow temperature 40±5℃and reaction time 60 min. Besides, the stability and lifetime of the ZrO_x-ZnO/γ-Al₂O₃ catalyst were investigated. Results showed that no obvious decrease on the catalytic activity and change on of the catalyst property were observed after continuously used for 8 times, which indicated that the ZrO_x-ZnO/γ-Al₂O₃ catalyst was an efficient and stable microwave-assisted photocatalyst and was applicable for practical wastewater treatment.Furthermore, the reaction mechanism of MW/PC process was studied, and it has been found that significant synergistic effect existed between microwave and photocatalysis, and the introduction of microwave effectively enhanced the photocatalytic efficiency and shortened the reaction time. Moreover, non-thermal effect of microwave played a significant role in the improvement of photocatalytic efficiency. On the other hand, in the MW/PC process, the catalytic activity of the mix-valenced ZrO_x was much higher than the P25 TiO₂ and ZrO₂. The high activity of the ZrO_x could be explained from two aspects. On one hand, the co-existence of different valence Zr ions offered the ZrO_x better microwave response ability, and the ZrO_x was easier to be activated under simultaneous microwave and UV light irradiation. On the other hand, the wide bandgap structure of the ZrO_x catalyst made the photo-generated holes on it have stronger oxidative ability. The composite ZrO_x-ZnO catalyst not only kept the good microwave response ability of the mix-valenced ZrO_x, but also the photocatalytic ability of the composite catalyst was further improved because the composition of different semiconductors could effectively separate the photo-generated electrons and holes.The·OH formation rules in different processes were also studied using a molecular fluorescence spectrum technique. Microwave alone and catalyst alone could barely produce·OH, and singular UV light irradiation had small contribution on the formation of·OH. However, synergetic effect exited between microwave and photocatalysis on·OH formation, and microwave irradiation could greatly enhance the formation of·OH in the photocatalytic system. Meanwhile, adding ZrO_x-ZnO/γ-Al₂O₃ catalyst could obviously improve the formation of·OH.In this paper, microwave irradiation and photocatalytic process were combined to provide a highly efficient wastewater treatment for EDCs in water. Moreover, novel ZrO_x-based catalysts with simultaneous good responses to both microwave and UV light were developed, and offered some new insights into designing highly efficient MW/PC catalysts. Modified ZrO_x might introduce a promising new generation of catalysts for MW/PC environmental remediation.