Theoretical Study On Degradation Mechanism And Ecotoxicity Of Three Organic Drug Molecules In Water

Pharmaceuticals and personal care products（PPCPs）,as a kind of emerging environmental pollutants,are often found in water environment.Due to their high toxicity,persistence,bioaccumulation and long-distance migration,PPCPs pose great risks to the ecological environment.The effective degradation methods have become the research focus of scientists around the world.Advanced oxidation processes（AOPs）are high-performanceand and high-benifit technologies,which would degrade organic pollutants through the generation of highly reactive radicals such as hydroxyl radicals（·OH）,sulphate radicals(SO₄^·-)and carbonate radicals(CO₃^·-)for the oxidative degradation of organic pollutants.AOPs can be widely used in wastewater and general water remediation treatment.Therein,·OH is formed by photolytic cleavage of H₂O₂and has a high reactivity towards PPCPs.In addition,SO₄^·-,a primary radical,is generated by UV/persulfate.Another reactive species,CO₃^·-,is of increasing interest due to its presence in the aqueous environment and wastewater.However,the rapid and complex reactions of free radicals to initiate a range of organic pollutants make it very difficult to explore experimental studies.Thus,experimental studies are limited.Therefore,This paper uses a quantum chemical approach to investigate the mechanism,kinetics and toxicity of the reactions of three representative organic pollutants in the environment with·OH,SO₄^·-and CO₃^·-,respectively.The research contents are as follows:1.Theoretical investigation on the removal of sulfadiazine in water environments:Oxidation of·OH,SO₄^·-and CO₃^·-and captured by（Ti O₂）_nclusters（n=1–6）.This study reported the degradation of sulfadiazine through reactive radicals like·OH,SO₄^·-and CO₃^·-.Sulfadiazine（SDZ）is widely used antibiotic and emerging water pollutant.The degradation of sulfadiazine by·OH,SO₄^·-and CO₃^·-was investigated through quantum chemistry methods.The degradation of SDZ by·OH radicals depended radical adduct formation（RAF）reactions while degradation of SDZ by SO₄^·-radical was caused by RAF,hydrogen atom transfer（HAT）and single electron transfer（SET）.The degradation of SDZ by CO₃^·-radical caused by RAF,HAT and SET.The initial degradation rates of SDZ by·OH,SO₄^·-and CO₃^·-radicals were calculated separately:k _SDZ-SO4·-(5.29×10¹⁰M^-1s^-1)>k _SDZ-·OH(1.38×10¹⁰M^-1s^-1)>k _SDZ-CO3·-(1.28×10⁸M^-1s^-1).The degradation mechanism of SDZ by（Ti O₂）_n（n=1–6）cluster was investigated.The conformation between benzene ring and the aza ring of SDZ has a lower adsorption energy and the（Ti O₂）_ncluster was the most stable adsorption conformation with SDZ when n=5.The toxicity assessment showed that most of the degradation products are less toxic and eco-friendly.However,few degradation products exhibited more severe aquatic toxicity.2.N,N-diethyl-m-toluamide（DEET）degradation by·OH and SO₄^·--assisted AOPs in wastewater treatment:Theoretical studies into mechanisms,kinetics and toxicity.As an emerging pollutant,insect repellent N,N-diethyl-m-toluamide has a certain impact on the water environment.Therefore,mechanism,kinetics and toxicity were studied by quantum chemical calculations,which could help us better understand degradation of DEET initiated by·OH and SO₄^·-.The calculated rate constants for the·OH and SO₄^·--initiated reactions were 1.18×10⁹M^-1s^-1and 2.95×10⁹M^-1s^-1at298 K and 1 atm,respectively,which were consistent with the corresponding experimental data.It is indicated that the·OH and SO₄^·-generated from advanced oxidation processes present the same good performance for degradation of DEET in water treatment.The results demonstrated that·OH-initiated DEET degradation was mostly aroused by hydrogen abstraction and addition.Moreover,SO₄^·--initiated DEET degradation mainly proceeded single electron transfer and addition.The important and initial intermediates could further react with O₂,NO and·OH leading to the end products in which had been detected by experimental methods.DEET parallel to the graphene surface performed good adsorption properties.The toxic results showed that DEET degradation was still at harmful level.DEET and its degradation products were also potentially developmental toxicants,which should cause concern in degradation process of DEET.3.Theoretical calculation on the reaction of acetaminophen degradation initiated by hydroxyl and sulfate radicals in the aqueous phase.The·OH and SO₄^·-play a vital role on degrading pharmaceutical contaminants in water.In this paper,theoretical calculations have been used to discuss the degradation mechanisms,kinetics and ecotoxicity of acetaminophen（AAP）initiated by·OH and SO₄^·-.Two significant reaction mechanisms of radical adduct formation（RAF）and hydrogen atom transfer（HAT）were investigated deeply.The results showed that the RAF takes precedence over HAT in both·OH and SO₄^·-with AAP reactions.The whole and branched rate constants were calculated in a suitable temperature range of 198-338 K and 1 atm by using the Ki SThel P program.At 298 K and 1 atm,the total rate constants of·OH and SO₄^·-with AAP were 3.23×10⁹M^-1s^-1and 4.60×10¹⁰M^-1s^-1,respectively,considering the diffusion-limited effect.The chronic toxicity showed that the main degradation intermediates were harmless to three aquatic organism,namely,fish,daphnia,and green algae.From point of view of the acute toxicity,some degradation intermediates were still at harmful or toxic level.These results provide theoretical guidance on the practical degradation of AAP in the water.