Study On Efficiency And Mechanism Of Magnesium-containing Metal Oxides Catalytic Ozonation Of Organic Pollutants In Water

Ozone is widely used as an effective oxidizing agent in drinking water and wastewater treatment.Selective oxidation of ozone with organic compounds leads to a low oxidative efficiency.Hence,ozone based advanced oxidation processes?AOP-O₃?have been developed,such as catalytic ozonation,H₂O₂/O₃,UV/O₃,in which ozone is effectively transformed into reactive oxygen species,thus improving the oxidation efficiency.Heterogeneous catalytic ozonation has been received increasing attention owing to its high efficiency and easy operation.The key factor for this process is to prepare a good efficient and stable catalyst.A serious of solid base catalysts?magnesium-containing metal oxides?were prepared in this dissertation,and their catalytic activities were investigated in ozonation of simulated wastewater.Specially,some works have been done to explore the mechanism for ozone decomposition in magnesium-containing metal oxides catalytic ozonation and provide an idea for preparing a more stable catalyst.The main contents and results are shown below.First,the meachanism of solid base catalytic ozonation was explored.Mg O/ceramic honeycomb?Mg O/CH?composite catalyst was prepared by the excessive impregnation method,and its catalytic activity was investigated in ozonation of acetic acid?Ac OH??100 mg/L?at a neutral p H for 30 min.Mg O was coated on the surface of CH according to the analyses of XRD,TEM and S_BET.Ac OH removal efficiencies in ozonation alone,CH catalytic ozonation and Mg O/CH catalytic ozonation were 20.7%,18.7%,81.6%,respectively.Absorption effect of Mg O/CH for Ac OH was negligible.Mg O/CH catalytic activity in ozonation of Ac OH was contributed by Mg O coated on the surface of CH.Besides the increase in the solution p H caused by Mg O/CH,Mg O/CH could provide the surface basic sites to accelerate ozone decomposition into hydroxyl radicals.Additionally,Mg O/CH exhibited good stability after continuous use in ozonation.Then,the meachanism of solid base catalytic ozonation and the stability of solid base catalyst were improved.Magnesium-doped Zn O?x-Mg Zn O,x represents the molar ratio of Mg to the sum of Mg and Zn?powders synthesized by the traditional thermal decomposition were used as catalysts for ozonation of isoniazid?INH??20mg/L?at a neutral p H for 9 min.Magnesium substituted zinc in wurtzite structure and the Zn—O—Mg bond was formed in Mg-doped Zn O on the basis of the results of X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?analyses.INH removal efficiencies were enhanced in Mg-doped Zn O catalytic ozonation processes?57.7%by 0.05-Mg Zn O and 76.3%by 0.10-Mg Zn O?,compared with those in ozonation alone?50.5%?and Zn O catalytic ozonation?49.5%?.Absorption effects of the catalysts for INH were negligible.Mg-doped Zn O exhibited good catalytic activites in ozonation of INH owing to Mg doping.When the initial p H of 7.2 was lower than the p H_PZC?point of zero charge?of Mg-doped Zn O,surface hydroxyl groups of the catalysts were protonated and the solution p H gradually increased during Mg-doped Zn O catalytic ozonation.The increase in the solution p H value mainly induced ozone decomposition into superoxide radical?·O₂^-?.Furthermore,protonated surface hydroxyl groups?S—OH₂⁺?on Mg-doped Zn O contributed a little to ozone decomposition.Additionally,the hydration of 0.10-Mg Zn O was mitigated according to the result of ICP-MS,compared with that of Mg O.0.10-Mg Zn O powder showed high stability after continuous use in ozonation.Finally,the catalytic activity and the stability of solid base catalyst were enhanced by adding of acid sites.Mg M_xO_y?M=Mn,Fe?powders prepared by the coprecipitation method were used as the catalysts for ozonation of p-hydroxybenzoic acid?p-HBA?and acetic acid?Ac OH??20 mg/L?at a neutral p H for 9 min.84.8%,88.2%,86.7%p-HBA were removed in ozonation catalyzed by Mg Fe_xO_y,Mg Mn_xO_y and Mg O,respectively.49.3%,65.8%,64.3%Ac OH were removed in ozonation catalyzed by the three catalysts,respectively.Absorption effects of the catalysts for p-HBA and Ac OH were negligible.Mg Mn_xO_y exhibited the highest catalytic activity in ozonation of p-HBA and Ac OH owing to the action of acid sites of Mg Mn_xO_y for additional ozone decomposition with the assistance of basic sites.The increase of solution p H,the protonation of surface hydroxyl groups and the electron transfer on acid sites induce ozone decomposition into reactive oxygen species in Mg M_xO_ycatalytic ozonation.Additionally,hydration of Mg Mn_xO_y and Mg Fe_xO_y was evidently mitigated according to the analyses of FTIR and ICP-MS,compared with that of Mg O.In the stability test,catalytic activity of Mg Fe_xO_ysurpassed that of Mg O,and Mg Mn_xO_y still exhibited the highest catalytic activity.