Study On Advanced Oxidative Degradation Mechanism And Its Ecotoxicity Prediction Of Typical Phenolic Pollutants In Water

Phenolic organic contaminates(POCs),as important raw materials and intermediates,are widely used in a variety of industrial productions.POCs are toxic and difficult to biodegrade.POCs can exist in the environment for a long time,leading to potential health risks to organisms.The discharge of a large amount of wastewater containing POCs makes the residual POCs enter the human living environment,causing harm to the human body.The effective treatment of phenol-containing wastewater can avoid threats to the water environment and biological health.At present,POCs have been detected in surface water and freshwater environments because of the wide distribution of phenol-containing wastewater.Therefore,the research of removing POCs has attracted the attention and exploration of many researchers.Advanced oxidation processes(AOPs)with good application prospects in the treatment of organic wastewater can mineralize most organic pollutants until they are completely decomposed.AOPs mainly use highly active substances to complete the oxidative degradation of organic compounds,in order to achieve the removal of organic compounds in wastewater.The common reactive species in the process of AOPs for organic wastewater treatment are sulfate radicals(SO4·-)and hydroxyl radicals(HO·).In addition,some other reactive oxygen species were also found in the experiments,such as peroxyhydroxyl radicals(HO2·),superoxide radicals(O2·-),chlorine oxide radicals(ClO·),bromine oxide radicals(BrO·),carbonate radicals(CO3·),nitrate radicals(NO3·),dihydrogen phosphate radical(H2PO4·),hydrogen phosphate radical(HPO4·-)and phosphate radicals(PO42-·),however,the reactions relying on these free radicals to scavenging POCs in water need to be further investigated.At present,reports on phenolic pollutants in experimental studies are mainly concentrated on a few phenolic compounds.There are certain limitations the application of experimental methods for the in-depth study of the complex oxidative degradation behavior in the water environment.Therefore,a variety of phenolic organic pollutants involved in industrial wastewater were selected as the research object in this paper,specifically including isomers of aminophenol(AP),dihydroxybenzene(DP),methylphenol(MP),chlorophenol(CP),cyanophenol(YP),nitrophenol(NP),and 2,4-dinitrophenol(DNP)and dinitrodiazophenol(DDNP).The oxidative degradation processes of phenolic compounds were demonstrated at the micro level using quantum chemical calculation.The structural parameters,reaction mechanisms,kinetic properties and aquatic ecotoxicities of the corresponding phenolic compounds were clarified.The research results obtained as follows:(1)Degradation reaction mechanisms and kinetics of phenolic pollutants initiated by SO4·-and HO' in water and aquatic ecotoxicity assessment of hydroxylated addition productsThe initial reactions of 19 kinds of POCs by SO4·-and HO·were calculated using quantum chemistry methods.The reaction laws between POCs and two free radicals in water were explored and compared.The rate constants of the above initial oxidation reactions are obtained based on the transition state theory.The oxidation reactions initiated by SO4·-and HO·have been confirmed that POCs can follow three reaction mechanisms:radical adduct formation(RAF),hydrogen atom abstraction(HAA),and single electron transfer(SET).The results turn out that the stronger the electron donating effects of the substituent on POCs,the better the reactivity of POCs with two free radicals.SET mechanism is main reaction pathway for SO4·--initiated oxidation reaction,while RAF mechanism is the favorable way for HO·-initiated oxidation reaction.Furthermore,the ecotoxicity assessment shows that the order of acute toxicity of the 19 POCs is p-DP>o-AP>m-AP(p-AP)>MP>CP>NP>m-DP(o-DP)>phenol>YP,while the chronic toxicity is p-DP>o-AP>m-AP(p-AP)>MP>CP>NP>YP>phenol>m-DP(o-DP).Since most HO' adducts have higher toxicities on aquatic organisms than their parent reactants,the subsequent reactions of POCs oxidation degradation by free radicals should be futher investigated.(2)Reaction mechanisms,kinetics and ecotoxicity evaluation of oxidative degradation of DNP by different oxygen-containing free radicals in aqueous solutionThe thermodynamic feasibilities,optimal reaction sites and reaction characteristics of DNP with eleven kinds of oxygen-containing free radicals(HO',SO4·-,H2PO4·,NO3·,HPO4·-,CO3·-,BrO·,ClO·,HO2·,PO42-· and O2·-)were systematically and comprehensively analyzed using density functional theory.The degradation reactions of DNP initiated by H2PO4·,NO3·,and HO' are relatively easy to occur.In particular,the degradation reaction paths of HO' and DNP were proposed.The generation channels of the major intermediates/products such as hydroquinone,maleic acid and fumaric acid detected in the experiment were determined.The total reaction rate constant of HO· and DNP is 1.23 × 108 M-1s-1.The ecotoxicity evaluation suggests that the ecotoxicity of most of the products is lower than their parent compounds in the degradation process,but a small part of by-products still are toxic and require more attention.(3)Mechanism and kinetics of Oxidative degradation reaction of DDNP by SO4·-and HO· in aqueous solutlion and ecotoxicity evaluation of degradation productsThe oxidation degradation reaction of DDNP by SO4·-and HO' in aqueous solution was discussed in detail under the level of M06-2X/6-311++G(3df,2p)//M06-2X/6-31+G(d,p).The reaction mechanisms include nine initial reaction routes based on three mechanisms of RAF,HAA and SET for SO4·-and HO·,and subsequent reaction paths of main hydroxyl intermediates.O2,HO' and H2O participate in the subsequent reactions to obtain unsaturated aldehyde/ketone/alcohol products.The reactions of DDNP and two free radicals are controlled by the activation reaction process.The reaction rate constant of the hydroxylation of DDNP is 4.26 × 106 M-1s-1,and the reaction rate constant of DDNP and SO4·-is 1.49 × 105 M-1s-1.The aquatic toxicity of most degradation products has been reduced.The impact of a few products on the environment needs to be noticed,because they have potential developmental toxicity and mutagenicity.