The Mechanism Study Of Emerging Contaminants Degradation By The Photochemical Reactions Of Nitrate And Nitrite

Since Emerging Contanimants(ECs)is refractory and with high potential risk,they have attracted widespread attention recently.The hazard of ECs for human beings and environment is severe in that its environmental behavior is still unknown.Advanced Oxidation Technologies(AOTs)is a promising wastewater treatment technology which have great advantages in dealing with ECs,especially all homogeneous and heterogeneous photocatalytic technologies.They can be applied in advanced treatment of pollutants using chemical energy with high energy density tranformed from solar energy whose energy density is low.However,most heterogeneous photocatalytic technologies have some problems needing to be solved.For example,strict conditions and high cost for preparation,difficult separation process after used,and low energy utilization rate of solar light.For these reasons,this study proposed to create an AOT process using the photochemical activities of nitrate and nitrite(NO2-and NO3-)along with appropriate UV light to deeply degrade all kinds of ECs,to investigate the efficiency and mechanism of this technology.It can also avoid the problem of photocatalysts recycle in the heterogeneous system since NO2-and NO3-can be generated in nitrification process during wastewater treatment process.This thesis contains three parts:(1)systematic study of the mechanism of NO2-and NO3photocatalytic technology and the conversion of NO2-and NO3-in this system;(2)study of NOM(Natural Organic Matter,NOM)effect on the NO2-/NO3-photocatalytic technology;(3)study the effect of important water quality parameters and actual wastewater on the NO2-/NO3photocatalytic system.Firstly,this paper investigated the efficiency of sole NO2-or NO3-photocatalytic technology in different conditions,using photoinert compound——dimethyl phthalate(DMP)as target organic compound.The results showed that:the photochemical activities of NO2-or NO3-is wavelength-dependent which would be more active in low wavelength conditions.There were hydroxyl radical(·OH)and superoxide anion free radical(O2-)detected in both NO2-and NO3system.However,the dominant free radical is OH in that O2-do not react with DMP.Additionally,the effect of NO2-/NO3-photocatalytic technology is related with the wavelength of light resource and the relative relationship between NO2-/NO3-and organic pollutants.254 nm UV light could decompose DMP efficiently,but photolysis of DMP was slow at λ=300 nm and noneffective at λ=350 nm,which could be catalyzed by the presence of NO2-or NO3-.The radical scavenging effect of NO2-is main reason affecting the degradation rate of DMP.However,improving the initial DMP concentration could inhibit the radical scavenging effect of NO2-.Besides,it could also promote the conversion between NO3-and NO3-.NO2-→NO3-process is much easier to happen compared with NO3-→NO2-process.At the end of this part,the analysis of DMP degradation products by LC-MS revealed that the mechanism of using NO2-/NO3photocatalytic technology to degrade DMP is the attact of the benzene ring by ·OH or the H atom in fat chain replacing by ·OH.Furthermore,this paper focused on the effect of Natural Organic Matter on the degradation of sulfadiazine(SDZ),a photoactive antibiotic,induced by the NO2-which had higher quantum yield under UVA wavelengths.The study showed that under 350 nm condition,the radical scavenging effect of NO2-disappeared because the dominant free radical is O2·-in the NO2photocatalytic system.The presence of FA slowed down the degradation process of SDZ because of FA’s "inner filter" effect,on the other hand,FA could not only scavenge free radicals but also turn the intermediates of SDZ into SDZ.The analysis of degradation intermediates of SDZ by LC-MS revealed that the main pathways of SDZ degradation is electron tranfer of sulfonyl leading to the loss of SO2 and then the formation of main intermediate.Besides,SDZ can also be degraded by the break of sulfonyl,forming small molecule products.Finally,the third part investigated the effect of actual wastewater on the NO2-and NO3photocatalytic process using Bisphenol A(BPA),a kind of endocrine disrupting compounds,as selected target pollutant.The results showed that NO3-photocatalytic system achieved better degradation rate and higher mineralization rate of BPA when initial BPA concentration is 0.01 mmol/L.Nonetheless,actual wastewater condition would weaken the performance of NO2photocatalytic system and promote the BPA degradation in NO3-photocatalytic system.It was also found that BPA was easier to be degraded in alkaline condition in that BPA is mainly ionic form in alkaline conditions.The NO3-→NO2-conversion process is also easier to happen in alkaline condition while the NO2-→NO3-process is easier to happen in acid condition.To sum up,this study provides us new insights and theory to build a new in situ wastewater treatment technology which is green and energy-saved to deeply decompose emerging contaminants.