Construction Of DBDP/MnFe₂o₄ Catalytic Process And Mechanism For 4-fluorophenol Degradation In Water

As an emerging contaminant treatment process,dielectric barrier discharge plasma（DBDP）shows advantages of high efficiency on water treatment.However,shortcomings such as incomplete utilizations of chemical and physical effects generated by discharge,as well as high energy consumption also exist.In this work,based on the design,building and optimization of DBDP reactors,the efficient removal of 4-fluorophenol（4-FP）was achieved through an affiliation of Mn Fe₂O₄catalysts.The mechanisms of DBDP and DBDP/Mn Fe₂O₄ processes for 4-FP abatement were explored,respectively.This work can provide data and theoretical references for the removal of refractory organic pollutants in water by advanced oxidation processes（AOPs）based on DBDP.Based on the circuit structure of water to be treated as ground electrode,a coaxial cylinder（coaxial cylinder type）DBDP reactor with treated water being membrane and circularly flowing through discharge area was designed and constructed on the basis of typical DBDP reactor with parallel flat plate（flat plate type）.And,performance comparison and optimation,as well as corresponding mechanisms,were conducted and analysed between the DBDP reactors with electrodes in the types of parallel plate（flat reactor）and coaxial cylinder（coaxial cylinder reactor）.Among them,parallel plate structures were defined as vertical flat plate and horizontal flat plate reactors,respectively.Compared to horizontal flat plate reactor,the existence of solution circulation at a rate of 250 m L·min^-1 in vertical flat plate reactor can promote the mass transfer effect of the generation of the active material in the gas phase to the liquid phase.Thus,it can enhance the effective contact between the active materials generated by discharge and the target pollutant 4-FP in water to boost the 4-FP degradation.And,with the increase of applied voltage from 12 k V to 20 k V,the corresponding improvement caused by solution circulation increased from 0.60%to 11.68%sustainedly.4-FP abatement achieved the maximal value in vertical flat plate reactor at 20 k V.But the maximal value was only 29.96%.The change of vertical flat plate reactor to coaxial cylinder reactor can effectively promote the degradation of 4-FP by maintaining an equal spacing between the high voltage electrode to the corresponding ground electrode,increasing the discharge intensity and the effective contact area of the dischar ge plasma and the treated water.With an applied voltage of 18 k V and a water circulation rate of 100 m L·min^-1,the removal rate of 4-FP was 94.85%in cylindrical reactor,being significantly higher than the corresponding value of 17.0%in vertical flat plate reactor.Besides,in coaxial cylinder reactor,the aeration of discharge gas into the tank can realize the secondary utilization of O₃ generated during discharge,and thus further promoted the 4-FP removal.To cope with the insufficient utilization of physical and chemical effects of the DBDP system,the difficulty of powder catalyst separation and the time-consuming process of functional material synthesis,the rapid synthesis of magnetic Mn Fe₂O₄catalyst was realized by microwave hydrothermal method.And a catalytic system combining DBDP and Mn Fe₂O₄ catalyst was constructed by mixing the synthetic Mn Fe₂O₄ catalyst powder and the water to be treated.Magnetic Mn Fe₂O₄ catalysts were simply and rapidly synthesized by microwave hydrothermal method.Compared with the conventional methods with various synthesis time in the range of120～1800 min,hydrothermal,coprecipitation and thermal decomposition methods for example,the corresponding value of 30 min for the microwave hydrothermal synthesis had an absolute advantage.Moreover,the microwave hydrothermal process was simpler without complex pretreatment and calcination process.The structures and properties of the synthesized Mn Fe₂O₄ catalysts were characterized using detection instruments such as SEM,XRD,XPS,FT-IR,UV-Vis DRS and VSM.The DBDP/Mn Fe₂O₄ process can effectively remove the 4-FP in water.In the absence of discharge gas aeration,for the DBDP process,the presence of Mn Fe₂O₄catalysts enabled the 4-FP removal at lower applied voltages achieved a comparable or higher level of that at higher applied voltages.With the increase of applied voltage from 12 to 20 k V,the enhancement of 4-FP removal rate caused by Mn Fe₂O₄catalysts increased gradually in the range of 4.68-20.25%.But,the maximum energy efficiency values of the DBDP and DBDP/Mn Fe₂O₄ processes were both achieved at16 k V,being 1891.17 mg·k Wh^-1 and 2391.80 mg·k Wh^-1,respectively.In addition,the increase in the water circulation rate,on the one hand,can reduce the hydraulic retention time in the discharge zone;on the other hand,can increase the absolute contact area between the treated water and the discharge plasma.According to the continuous decrease of 4-FP abatement accompanied by the water circulation rate rising from 100 m L·min^-1 to 300 m L·min^-1,the influence of hydraulic retention time played a leading role in DBDP/Mn Fe₂O₄ process.It’s worth noting that the enhancement of 4-FP degradation caused by Mn Fe₂O₄ catalysts showed a steady trend after the first increase,indicating that the increase of the water circulation rate was beneficial to the Mn Fe₂O₄ catalytic performance.The further introduction of discharge gas aeration boosted the improved 4-FP removal in DBDP process caused by Mn Fe₂O₄ catalysts,increasing from 14.63%to 30.76%.The mechanism of active substance generation to remove the refractory organic pollutant 4-FP was investigated for the DBDP/Mn Fe₂O₄ process established in this work.Based on the analysis of radical capture and blank experimentas of DBDP process,·OH and O₃ generated by discharge both played important roles on the degradation of 4-FP.The existence of the O₃ generated by discharge participated in4-FP removal directly or indirectly by promoting the conversion of generated H₂O₂to·OH.For DBDP/Mn Fe₂O₄ process,the presence of Mn Fe₂O₄ catalysts promoted the 4-FP abatement and also made the related degradation mechanism more complex.Based on the comparison of 7 groups under different conditions based on DBDP/Mn Fe₂O₄ process and radical capture experiments,·OH and O₃ formed by discharge both played important roles on 4-FP degradation.And the catalysis of Mn Fe₂O₄ occured through Fenton-like,ozone-catalysis,photocatalysis and photo-assisted Fenton-like processes.Moreover,a possible degradation pathway of4-FP in DBDP/Mn Fe₂O₄ process was proposed through intermediate product detections,simulation calculations and related references.