Efficiency And Mechanism Of Low- Dimensional MnO₂ Catalyzed Ozonation For Degradation Of Phenolic Wastewater

The treatment technology of phenolic wastewater has been widely studied due to its characteristics of wide source and high biological toxicity.If the pollution is not controlled thoroughly,the harm to ecological environment so immeasurable.Heterogeneous catalytic ozonation technology has become the first choice of refractory wastewater treatment technology due to its characteristics of low selectivity,complete degradation,fast reaction rate and no secondary pollution.Preparation of high-efficiency catalysts and mechanism of this technology are hot topic.Mn has many valence states to provide electrons through ion pair circulation,so it has high catalytic activity,especially low dimensional MnO₂.However,there are different opinions on which crystal type has stronger catalytic activity of MnO₂,and it is rare to study the relationship between the microstructure of MnO₂ and its catalytic activity.Meanwhile,there are few studies on the improvement of catalyst performance by oriented metal doping based on microstructure.Low-dimensional MnO₂ with different crystal types??-MnO₂,?-MnO₂,?-MnO₂,?-MnO₂?,prepared by controlling hydrothermal precursor,temperature and time,were used as a solid phase catalyst for catalytic ozonation of phenol.Microstructure of the catalyst,analyzed by characterization methods of XRD,SEM,BET,XPS,FTIR,combined with the degradation efficiency of phenol to analyze the effect of catalyst structure on catalytic activity.Afterwards,Co,whose ion radius closed to Mn,was doped by liquid phase synthesis in the preparation of MnO₂,and used to catalyze ozonation phenol.The microstructure and catalytic activity of the catalysts before and after cobalt doping were compared to analyze the effect of Co-doping on catalytic ozonation activity of MnO₂.Finally,the adsorption of calatysts,hydroxylation of catalyst surface and active oxygen free radicals?ROS?that may exist in the catalytic ozonation system were analyzed to explore the reaction mechanism of MnO₂ catalytic ozonation phenol,as well as the possible mechanism of Co doping changing the catalytic ozonation activity.The following are the conclusions of this study:?1?The MnO₂ of four different crystal types??-MnO₂,?-MnO₂,?-MnO₂,?-MnO₂?prepared by the experiment is consistent with the expectation.Mn elements in MnO₂ of four different crystal types all existed as+3 and+4 valences,thus forming Mn³⁺and Mn⁴⁺redox cycle.O elements exist in the form of lattice oxygen(O _latt)and adsorption oxygen(O _ads).hydroxyl bonds?O-H?were found on the surface of the catalysts.Therefore,catalysts have strong catalytic effect on degradation phenol.?-MnO₂ has the strongest catalytic ozonation activity because of its large specific surface area,pore volume,Mn³⁺,O _ads and surface hydroxyl relative content.?2?The catalytic ozonation of?-MnO₂,?-MnO₂,?-MnO₂ were promoted by Co doping and the optimal doping ratios were 2:10,1:10 and 2:10,respectively.The catalytic activity of?-MnO₂ decreased after Co added,and the catalytic activity was the weakest when the molar ratio of Co/Mn was 3:10.Co doping increased the specific surface area,pore volume,Mn³⁺,adsorbed oxygen(O _ads)and surface hydroxyl relative contents of?-MnO₂,?-MnO₂,?-MnO₂,but of?-MnO₂ were decreased.It is explained that the catalytic ozonation activity of?-MnO₂,?-MnO₂,?-MnO₂ was increased but?-MnO₂ was decreased after Co doped from the the perspective of structural change with Co-doping.?3?The Co ions in Co-?-MnO₂,Co-?-MnO₂,Co-?-MnO₂ mainly enter the catalyst in the way of replacing Mn of MnO₂ skeleton structure with Co³⁺,and in Co-?-MnO₂enter the catalyst structure in the form of embedded layer spacing.This leads to a change in the intensity of the characteristic peak of MnO₂ X diffraction,but the Angle2 of the X diffraction peak is almost unchanged,and there is no characteristic peak of any Co oxide in catalysts.?4?It is speculated that the adsorption of organic matter by catalyst belongs to physical adsorption.On the surface of MnO₂ catalyst,Mn³⁺metal ions and adsorbed oxygen combine with water molecules to form surface hydroxylation,which was provided as the active site to absorb ozone and decompose to generate reactive oxygen species?ROS?,such as·OH and·O^2-,to improve the utilization rate of ozone and promote the removal of phenol.Specific surface area,pore volume,Mn³⁺and adsorbed oxygen(O _ads)relative contents were changed by Co doping,thus,the surface hydroxylation of catalyst,the utilization rate of ozone and the amount of reactive oxygen species?ROS?,such as·OH and·O^2-,generated by the decomposition of O₃,so that,the catalytic ozonation of phenol was influenced.