The Research On Degradation Of DEET In Aqueous Solution By Wetted-wall Dielectric Barrier Discharge Plasma Combined With Fe-coated Pumice

As a typical kind of PPCPs,DEET is a broad-spectrum insect repellent and widely contained in various insect repellent products.Due to the extensive use and improper disposal,the level of DEET in the water ecosystem is increasing,which would cause environment risk and endanger human health.As an efficient advanced oxidation technology,Non-thermal plasma which combines high energy electron radiation,active free radical action,ozone oxidation and ultraviolet photodecomposition,has been widely researched for pollution control,for instance,in tail water depth treatment,water micropollution control,and preoxidation of wastewater.In this paper,DEET was removed by wet-wall dielectric barrier discharge(wetted-wall DBD)combined with Fe-coated pumice,and the relevant experimental study was carried out.Firstly,we conducted three operation modes:(1)Model-1:Generated plasma gas was bubbled into aqueous solution alone.(2)Model-2:DEET solution flowed through the discharge area alone.(3)Model-3:DEET solution flowed through the discharge area,meanwhile the generated plasma gas was bubbled into DEET solution.Results showed that,after 27 min of reaction time,DEET removal efficiency is highest in Model-3 which could reach 76.8%,and DEET removal efficiency is poor in Model-1 which was only 5.96%under the same operating parameters(discharge voltage 15 kV,gas bubbling flowrate 200 L/h,liquid circulating flowrate 160 mL/min,initial concentration 20 mg/L,pH 6.48).Secondly,the effect of parameters on DEET degradation was investigated in Model-3.The experimental results showed that increasing discharge voltage is beneficial to DEET degradation,but is adverse for energy yield.Reducing the initial concentration would reduce probability of DEET reaction and lower energy yield.Larger gas bubbling flowrate and liquid circulation flowrate are beneficial to the improvement of the removal efficiency and energy yield of DEET.The pH increase was not conducive to the degradation of DEET,but the removal efficiency of DEET showed a slight improvement with pH increasing from pH 9.02 to 11.04 because of O3 decomposition to produce ·OH.Finally,Fe-coated pumice was prepared using impregnation method,and XRF,XRD,SEM,FT-IR and BET analysis showed that the Fe-coated pumice has good catalytic features.Study on DEET removal by combining wetted-wall DBD and Fe-coated pumice indicated the catalyst can improve DEET removal efficiency based on effectively catalyzing ozonation and decomposing O3 into ·OH.Also,the catalytic performance of Fe-coated pumice is better than that of single pumice.Moreover,increasing the dosage of catalyst can improve DEET removal efficiency.After repeated use of Fe-coated pumice,it is not obvious that the ultimate removal efficiency of DEET is reduced.Therefore,Fe-coated pumice has good catalytic performance and stability.In addition,H2O2 and O3 were determined and the experimental results manifested that the content of H2O2 in the Model-2 was the highest which could reach 131.18 ?M at 27 min,and the O3 content of Model-1 was the highest reached 7.125 ?M at 27 min.UV-Vis absorption spectra,chromatograms and TOC analysis showed that a large number of containing benzene intermediates were produced which were difficult to mineralize further.TOC removal rate in Model-3 was highest about 25.4%,catalyst could significantly improve the removal rate of TOC.Free radical capture experiments showed that both ·OH and ·H were involved in DEET degradation in Model-2 and Model-3.Based on theory analysis of Gaussian 09 software,the locations of DEET molecule which might be attacked were inferred.Combined with LC-MS detection results,the possible degradation products and pathways were proposed.