Study On Degradation Of DMF In Wastewater By Dielectric Barrier Discharge Plasma Under Catalyst Adsorption

N,N-dimethylformamide（DMF）causes a serious threat to water environmental and human health for its stable chemical properties,toxic and poor biodegradability.It is the right time to find economical and efficient methods of degrading the DMF in aqueous.Recently,plasma technology has aroused extensive attention of scholars at domestic and abroad due to its non-selectivity,fast and effective effect,and do not require any chemical agents.Dielectric barrier discharge plasma（DBDP）is one method of plasma generation.It can produce ozone,oxygen radicals,hydroxyl radicals and other active species in the process of discharge,which has been widely applied to ozone generation and removal of volatile organic pollutants.In recent years,the application of DBDP in wastewater degradation has also spreaded significantly.However,the effect of direct application of DBDP on DMF wastewater treatment is not clear yet.In the process of using DBDP technology,the efficiency of energy utilization was not ideal,and the some by-products such as ozone,hydrogen peroxide,and nitrogen oxygen was produced.Hence,it is critical to discover a method that can reduce the generation of discharge by-products when DBDP is used and further improve the removal efficiency of pollutants.At present,catalysts combined with plasma technology has been widely used in waste gas purification because it can decrease the generation of discharge by-products and elevate energy efficiency,but it is rarely used in wastewater treatment.In this study,DMF in wastewater was the target pollutant,DBDP technology was used as a technology to explore the degradation feasibility of DMF wastewater.The effects of different conditions on the degradation of DMF wastewater were investigated detailly.The degradation mechanism of DMF in wastewater treated by DBDP was studied,and the degradation pathways of DMF was analyzed.On this basis,the effect of catalyst addition on the degradation of DMF wastewater by DBDP and the inhibition of by-products were further investigated.The main conclusions were as follows:（1）The effects of discharge time,input voltage,input current,initial DMF concentration,pH of initial solution and initial conductivity of solution on the degradation of DMF wastewater treated by DBDP were systematically investigated.The results showed that DBDP could effectively degrade DMF in aqueous.When the input voltage was 90V and the input current was 0.65A,the degradation efficiency of1000mg/L DMF wastewater treated by DBDP for 40min could reach 52.2%,and the energy efficiency of the system was 13.30mg/kJ.In addition,the degradation efficiency of DMF decreased with the increasing of initial DMF concentration,but the energy efficiency of the system was higher when the concentration of DMF in wastewater was higher.Meanwhile,neutral and alkaline environments were more beneficial to the degradation of DMF.As the initial conductivity of DMF solution varied in the range of 0²0000μs/cm,the lowest degradation efficiency was 40%after 40min,and the highest could get to 62.1%.It demonstrated that DBDP could degrade DMF wastewater regardless of the initial solution conductivity.（2）The formation and the effect of active species in DBDP discharge system was defined.The results manifested that a large number of active species could be generated during DBDP discharge,including ozone,hydrogen peroxide and hydroxyl radicals.Through the comparative experiment of dimethyl sulforxide（DMSO）capturing hydroxyl radicals,it was found that the contribution rate of hydroxyl radicals in the degradation of DMF in wastewater reached more than 80%.The value of TOC in DMF solution decreased from 790.5mg/L to 507mg/L in 40min,which indicated that DBDP had the ability to mineralize pollutants in DMF wastewater directly.Moreover,Fourier Transform Infrared Spectrometer（FTIR）,High Performance Liquid Chromatography（HPLC）,Nuclear Magnetic Resonance Spectroscopy（¹H-NMR）and the small molecular organic compounds analysis before and after the DBDP degradation detected the intermediates of DMF in degradation process were N-methylformamide,methanol,formaldehyde and formic acid,which were finally mineralized into ammonia nitrogen,CO₂ and H₂O.（3）AC、H₂O₂/AC、Mn-AC、Mn-H₂O₂/AC were selected as catalysts,which combined with DBDP system to investigate the cooperation effect of the degradation of DMF wastewater.The results showed that the degradation efficiency of DMF was significantly improved with the addition of catalyst.Among them,Mn-AC exhibited the most obvious enhancement effect,which was 30.94%higher than that of DBDP treatment alone.TOC decreased from 826.2mg/L to 345.3mg/L,and the removal efficiency reached 58.2%.Also,mineralization effect was significantly improved too.At the same time,the addition of Mn-AC made the ozone,hydrogen peroxide and nitrate generated in DBDP discharge decreased obviously,which indicated that the addition of catalyst was beneficial to the degradation of DMF by DBDP technology,and effectively reduced the production of by-products in the process.（4）Analysis of degradation pathway:According to the detection results of the intermediates and the active species during the degradation processes of DMF by DBDP,Mn-AC combined with DBDP could significantly improve the first step of demethylation reaction,so that a great deal of large number of DMF could be converted into N-methylformamide and free methyl.After that,N-methylformamide could react with hydroxyl radicals to form N₂,CO₂ and H₂O.The methyl obtained by the first step would be degraded through the pathway of methanol,formaldehyde and formic acid,and eventually be carbonized into CO₂ and H₂O.