Theoretical Study On The Degradation Mechanism Of N-Containing Organic Micropollutants In Novel Advanced Oxidation Processes

N-Containing organic micropollutants(N-OMPs)are a class of emerging contaminants widely distributed in the aqueous environment,mainly including anilines,sulfonamides,phenylureas and N-containing heterocyclics.Advanced oxidation processes(AOPs)are one of the effective means to remove N-OMPs in water.Compared with the AOPs with HO· as the main reactive species,the novel AOPs based on chlorine,peracetic acid(PAA)and persulfate(PS)have the advantages of higher quantum yields,stronger decontamination capacity and less influence by the water matrix,which have broader practical application prospects.However,the formation and transformation of important reactive radicals and reaction intermediates are difficult to be fully determined by experimental testing means alone,leading to an inadequate understanding of the processes and characteristics of the radical reactions of contaminants,which in turn is detrimental to the development and refinement of novel AOPs.This thesis combined quantum chemical calculations with QSAR model,kinetic modeling and experimental approach to study the degradation mechanisms,efficiencies and products of typical N-OMPs under three novel AOPs,to develop the linear models of the reaction rate constants of ClO·,CH3COO·,CH3COOO·,HO· and SO4·-,to investigate the inhibition mechanism in the pollutant mixed systems,and to evaluate the aquatic toxicities of the target N-OMPs and their degradation products.The main findings of this thesis are as follows:(1)Degradation studies of caffeine and aromatic compounds in chlorine-based AOPsIn chlorine-based AOPs,the concentrations of ClO· and BrO· are 3～5 orders of magnitude higher than the other reactive radicals.In this section,the reaction mechanisms and kinetics of HO·,ClO· and BrO· with caffeine were firstly studied and compared,and then the reaction characteristics of ClO· with 43 aromatic compounds were extensively investigated.The results showed that the reaction rate constants of three oxygen-centered radicals with caffeine were in the order of HO·>ClO·>BrO·.The reaction rate constants of ClO· with aromatic compounds are 102～1010M-1 s-1.ClO· has high reactivity and selectivity for aniline compounds.EHOMO and Mulliken electronegativity were used to develop the QSAR model of CIO’ initiated reaction rate constants.Addition dominates the reactions of HO·,ClO·,and BrO· with caffeine and also is the major reaction pathway of most aromatic compounds with ClO·.In the ClO·-mediated subsequent degradation of contaminants,O-Cl bond breakage and Cl transfer are the key steps in the generation of hydroxylation and chlorination products.In chlorine-based AOPs,ClO· played the most important role in caffeine degradation,and the half-lives of four simple anilines were within 5 min,indicating that N-OMPs could be effectively removed in a limited time.The results of this part demonstrate the promising application of chlorine-based AOPs for N-OMPs removal.(2)Degradation studies of acetaminophen in PAA-based AOPsPAA is an alternative to chlorine disinfectants,and PAA-based AOPs can also degrade micropollutants by generating HO’ and organic radicals.In this section,the reaction mechanisms and kinetics of acetaminophen with HO’ and organic radicals were calculated and compared,and the reaction characteristics of important organic radicals was further investigated.It was found that the reaction rate constants of HO’ and organic radicals with acetaminophen were linearly correlated with the reduction potentials of these radicals,and their order is:CH3COO·>HO·>CH3O·>CH3COOO·>>·CH3>CH3C·O>CH3OO·.In the UV/PAA process,HO·,CH3COO· and CH3COOO· together dominated the degradation of acetaminophen,and the roles of other organic radicals could be negligible.Although acetaminophen was effectively removed under UV/PAA,its half-life was predicted as 15.63 s一 2.30 min,which was longer than that under UV/chlorine(2.4 s).CH3COO· had relatively high reactivity towards aromatic compounds with the reaction rate constants of 1.40×106～6.25 ×1010 M-1 s-1,while CH3COOO· was highly selective and had high reactivity only for aromatic compounds with strong electron-donating groups such as aniline.The oxidation potentials of aromatic pollutants affect their reaction rate constants with organic radicals,degradation efficiency and dominant factors under UV/PAA.(3)Degradation studies of aromatic amine compounds in PS-based AOPsPS-based AOPs mainly remove N-OMPs in water by generating HO’ and SO4·-.In this section,fourteen N-OMPs and twelve phenols were selected as the models to investigate the reaction mechanisms and kinetics of two ternary systems,SO4·--phenol-aniline and HO·SO4·--N-containing heterocyclic compounds.The results showed that the reaction rate constants of the N-OMPs with SO4·-were 108／1010 M-1 s-1,and their order are:simple anilines>N-containing heterocyclics>sulfonamide antibiotics≈phenylurea herbicides.The main reaction mechanisms of N-OMPs with SO4·-are the H-atom abstraction(HAA)on N atoms and single electron transfer(SET)reactions,and their main reaction intermediates are nitrogen-centered radicals and radical cations,while with HO’ is the addition reaction,and the primary products are their hydroxylated derivatives.In the SO4·--phenol-aniline ternary system,the roles of phenols are:(Ⅰ)consuming the reactive radical SO4·-;(Ⅱ)repairing nitrogen-centered radicals and aniline radical cations by HAA and SET reactions,respectively;(Ⅲ)generating phenoxy radicals to oxidize anilines.Similar reaction mechanisms occur in the ternary system of HO·-SO4·--nitrogenous bases,leading to the self-inhibited degradation of nitrogenous bases that the hydroxylated derivatives of nitrogenous bases can repair their nitrogen-centered radicals and radical cations through HAA and SET reactions,respectively.SET and HAA reactions are important reaction mechanisms for the degradation intermediates of N-OMPs.The reaction rate constants of SET are related to the oxidation potential of the reactants,while those of HAA reactions are also influenced by the steric hindrance.PS-based AOPs are an effective method for removing N-OMPs,but the inhibition mechanism affects the degradation efficiencies of the mixtures of N-OMPs.The reaction mechanisms and kinetics of reactive radicals such as HO·,ClO·,CH3COO·,CH3COOO· and SO4·-with emerging contaminants were investigated and compared.Both EHOMO and oxidation potentials are important descriptors of the ability of a compound to lose electrons,and reduction potential is an important descriptor of the ability of a radical to oxidize.Degradation efficiency of N-OMPs was significantly higher under UV/chlorine than under UV/H2O2,but chlorinated disinfection byproducts(DBPs)were produced;under UV/PAA,N-OMPs were effectively removed and the production of harmful DBPs was reduced,but the half-lives of N-OMPs were longer than those under UV/chlorine.In the PSbased AOPs,back-reduction caused the combined degradation of N-OMPs to be inhibited.