Photocatalysis Of Organophosphorus Pesticides And Pb²⁺ And CFD Simulation Optimization Of The Reactor

With the development of industry and agriculture,the coexistence of organophosphorus pesticides and Pb²⁺has appeared in many major river basins and water bodies in China,and will show greater toxicity than when they exist alone.In recent years,the photocatalytic treatment of organophosphorus pesticides and Pb²⁺in water by using semiconductor photocatalyst TiO₂ has been widely reported,but the research on the photocatalytic treatment of organophosphorus pesticides and Pb²⁺and the recovery of inorganic phosphorus products at the same time is very rare.In addition,powdered TiO₂ is difficult to separate and recover from water,and has a high photo electron hole recombination rate,resulting in reduced photocatalytic efficiency.In this study,MgFe2O4 was compounded with TiO2 to prepare a composite photocatalysis material MgFe₂O₄@TiO₂.Dichlorvos（DDVP）was used as the target pollutant of pesticide organophosphorus,and lead nitrate was used as the target pollutant of Pb2+.The composite material not only has higher photocatalytic performance and synchronous adsorption ability of inorganic phosphorus,but also has good magnetism,which makes it easy to be separated from water magnetically.The materials were characterized by X-ray powder diffraction（XRD）,scanning electron microscope（SEM）,UV Vis diffuse reflectance spectrometer（UV VIS）and vibrating sample magnetometer（VSM）.The influencing factors and reaction mechanism of MgFe₂O₄@TiO₂ photocatalytic treatment of DDVP and Pb²⁺and their simultaneous removal were investigated.Computational fluid dynamics（CFD）software FLUENT 16.0 was used to simulate and optimize the flow field of photocatalytic reactor,which improved the removal efficiency of pollutants and saved energy,The results are as follows:Firstly,TiO₂ composites with different MgFe₂O₄ doping ratios were prepared by sol-gel method,and the optimum doping ratio between MgFe₂O₄ and TiO₂ was determined.When the MgFe₂O₄ doping ratio was 1%,the best photocatalytic efficiency was achieved,and the composite material was recorded as MgFe₂O₄@TiO₂.When the initial concentration is 20 mg/L,MgFe₂O₄@TiO₂ dosage is 1 g/L,UV irradiation,the photocatalytic removal rate of Pb²⁺reaches 62%,the photocatalytic degradation rate of organic phosphorus reaches95%,and the simultaneous adsorption capacity of inorganic phosphorus reaches 15%.At the same time,XRD,SEM,UV-Vis,VSM characterization results show that MgFe₂O₄@TiO₂ is a micron scale material composed of MgFe₂O₄ and anatase TiO₂.Compared with pure TiO₂,the optical band gap is narrower,and the saturation magnetization is 13.68 emu/g.it is easy to separate MgFe₂O₄@TiO₂ from solution by external magnetic field.Secondly,the factors affecting the photocatalytic activity of DDVP and Pb²⁺by MgFe₂O₄@TiO₂were investigated,including the initial pH value,material dosage and initial concentration of pollutants.The results showed that the initial pH value of the solution in the range of 2-10 had little effect on the photocatalytic activity of DDVP.When pH>5.5,Pb²⁺would precipitate.When pH≤5.5,the higher the pH value,the better the photocatalytic activity.When the initial pH value of the solution was 5.5 and the dosage of MgFe₂O₄@TiO₂was 1.5 g/L,the photocatalytic removal rate of Pb²⁺with initial concentration of 20 mg/L was up to 76%,the highest photocatalytic degradation rate was 95%with the initial concentration of DDVP was 20 mg/L,and the simultaneous adsorption rate of inorganic phosphorus was 35%.MgFe₂O₄@TiO₂ had 100%photocatalytic treatment capacity for low concentration DDVP and Pb²⁺below 10 mg/L,but with the increase of concentration,the treatment capacity would decline.At the same time,the simultaneous removal ability of DDVP and Pb²⁺was investigated,and the reaction mechanism of Pb²⁺was analyzed by XPS.The results showed that Pb²⁺was first reduced to Simple substance form by photogenerated electrons,and then further oxidized to Pb O or Pb O₂ by·OH and·O₂^-generated by photocatalysis.On the one hand,Pb²⁺removal will compete with organophosphorus for strong oxidants,and the acid generated by DDVP photolysis will inhibit the removal of Pb²⁺,which is not conducive to the simultaneous removal of both,On the other hand,the reduction of Pb²⁺will consume photogenerated electrons,thus inhibiting the recombination of photogenerated electron hole pairs,which is beneficial to the simultaneous removal of both.Therefore,when the initial pH value of the solution is 5.5,the material dosage is 1.5g/L,and the concentration ratio of DDVP and Pb²⁺is 10:20,the simultaneous removal effect is the best.Compared with the single pollutant treatment,the organic phosphorus maintains100%photodegradation effect,and the synchronous adsorption rate of inorganic phosphorus increases from 44.4%to 90%.At the same time,the photocatalytic removal rate of Pb²⁺increased from 76%to 85.3%,which realized the simultaneous photocatalytic removal of organophosphorus and Pb²⁺and the adsorption of inorganic phosphorus.The results show that MgFe₂O₄@TiO₂ has good simultaneous removal effect for DDVP and Pb²⁺in mineral processing wastewater,and it still has good photocatalytic activity after four desorption cycles..Uniform and full contact between photocatalyst and photocatalyst is very important to improve the efficiency of photocatalysis and save energy consumption.In this study,from the perspective of uniform distribution of photocatalyst,CFD software FLUENT 16.0 was used to simulate the flow field in the photocatalytic reactor,optimize the structure and reaction conditions of the reactor,and the reliability of the simulation results was verified by experiments,which laid a foundation for further scale-up experiments and guiding engineering practice.