Theoretical Study On The Degradation Of Three Types Of Organic Pollutants In The Environment

Organophosphorus pesticides,benzene and long-chain cyclic alkanes are three types of pollutants that are widely found in the environment.Organophosphorus pesti-cides are widely used in agricultural production due to their effective treatment of pests and diseases,but they have volatile and soluble chemical properties and thus enter the environment;different organophosphorus pesticides have different toxicity and enter the organism by inhibiting the activity of cholinesterase and other enzymes,thus causing toxicity to the organism.Benzene is widely present in petroleum materials and can be volatilized into the atmosphere and water environment with industrial production and combustion.Because of their high bulk calorific density,long-chain cyclic alkanes are widely used in industry as an additive component in aviation fuels and high-quality diesel fuel,and enter the atmosphere with exhaust emissions.In this paper,three organophosphorus pesticides（acephate,mevinphos and mono-crotophos）,1,2,3-trimethylbenzene and 1,2,4,5-tetramethylbenzene,and C12～C14long-chain cyclic alkanes were selected as the objects of study.The reaction mechanism was investigated based on the transition state theory;the reaction rate constant variation with temperature was studied;and the oral toxicity of the three pollutants to aquatic or-ganisms（Daphnia,green algae and fathead minnows）and rats were predicted by ECOSAR and T.E.S.T software based on the quantitative structure-activity relationship（QSAR）theory.The results are as follows:1、The mechanism of oxidative degradation,reaction rate and toxicity change of typical benzene compounds in the environmentAt the level of M06-2X/6-311++G（3df,2pd）//M06-2X/6-311+G（d,p）,the degradation reactions of 1,2,3-trimethylbenzene as well as 1,2,4,5-tetramethylbenzene with OH rad-icals have similar mechanisms,including OH addition to the benzene ring,H abstrac-tion and CH₃ abstraction,and substitution reactions.For 1,2,3-trimethylbenzene,OH radical addition to the benzene ring C with an intermediate methyl group is the primary pathway with energy barriers of 24.5 and 20.5 k J/mol in the gas and liquid phases,re-spectively;OH radical abstraction of the H atom on the intermediate methyl group is the secondary pathway.For 1,2,4,5-tetramethylbenzene,the OH radical abstraction of the H atom on the benzene ring is the dominant pathway with reaction energy barriers of 6.3and 10.8 k J/mol in the gas and liquid phases,respectively,while the OH radical addition to the benzene ring C without the methyl group is the minor pathway with reaction en-ergy barriers of 12.2 and 5.6 k J/mol in the gas and liquid phases.The products of the H abstraction reaction pathway react with atmospheric O₂,NO,NO₂,H₂O,and OH in a subsequent reaction resulting in more stable products.The atmospheric lifetime of1,2,3-trimethylbenzene at 298 K and 1 atm is 5 h.The degradation time in natural water is up to 7.8 h.The degradation time of the advanced oxidation water treatment method is 2.83×10^-2～2.83×10^-3 s.The atmospheric lifetime of 1,2,4,5-tetramethylbenzene is1.18 h.The degradation time for the advanced oxidation method was 2.28 h.Therefore,the reaction time was significantly reduced under the advanced oxidation water treat-ment method.Ecotoxicity prediction showed that the toxicity of the products of1,2,3-trimethylbenzene degradation reaction to aquatic organisms increased compared to the reactants,while the toxicity of 1,2,4,5-tetramethylbenzene decreased during the degradation process.2、The mechanism of oxidative degradation of typical organophosphorus pesticides in the environment,reaction rate and toxicity changesThe degradation mechanisms,reaction rates and toxicity changes of acephate,mevinphos and monocrotophos in the environment were studied at the level of w B97XD/6-311++G（3df,2pd）//w B97XD/6-311++G（d,p）.The results showed that all three organophosphorus pesticides can react addition,abstraction and substitution reac-tion mechanisms.Acephate has a different structure of P=O and P=S double bonds from those of mevinphos and monocrotophos,resulting in a different degradation mechanism from those of mevinphos and monocrotophos.The H-abstraction pathway of acephate is the dominant pathway,while the remaining substitution and addition pathways are not easy to react.The dominant pathway for trichlorfon is OH radical addition to the C=C double bond,and H abstraction is the secondary pathway.Monocrotophos is similar to fast-acting phosphorus,with H abstraction as the major pathway and addition to C=C double bond as the minor pathway.In terms of reaction rates,the degradation of the three organophosphorus pesticides in the environment was consistent with temperature,showing a pattern of decreasing and then increasing with temperature in the gas phase,and a pattern of gradual decrease in the liquid phase.The atmospheric lifetimes of mevinphos and monocrotophos in the atmosphere are 36.46～364.60 s（2.53-25.31 s）,and the half-lives during advanced oxidation water treatment are 1.41×10^-2～1.41×10^-1 s(8.92×10^-4-8.92×10^-3 s).Ecotoxicity assessment showed that degradation of acephate in the presence of OH radicals contributed little to the acute and chronic tox-icity to fish,daphnia and green algae;and all degradation products showed develop-mental toxicity and were not mutagenic to rats.The toxicity of mevinphos and mono-crotophos to three aquatic organisms and rats was significantly reduced during the deg-radation process.3、The reaction mechanism and reaction rate change of long-chain cyclohexane in the atmosphereThe chemical reactions of C₁₂～C₁₄ long-chain cyclohexane with OH radicals in the atmosphere and the subsequent reaction mechanism of H abstraction products in the atmosphere were calculated using the DLPNO-CCSD（T）/cc-pv TZ//M06-2X/6-311++G（d,p）method.The H abstraction reactions of C₁₂～C₁₄ with OH radicals occurred soon after their release to the atmosphere.The energy barrier of H abstraction from CH₃ is the highest for OH and the energy barrier of H abstraction from CH is the lowest.Since the energy barriers of each H extraction channel are not high（-5.0～32.1 k J/mol）,C₁₂～C₁₄ can easily undergo atmospheric chemical reactions with OH radicals.The reaction rate constants of 1.55,3.09 and 3.75×10^-11 cm³ molecule^-1·s^-1 for the three long-chain cyclohexanes in the atmosphere at room temperature are approximate to the experi-mental rates.From 198 to 800 K,the reaction rate constants of C₁₂～C₁₄n-alkylcyclohexane with OH radicals first decreased and then increased from 300 K.Combined with the analysis of the products measured in the laboratory,the H abstrac-tion products will react rapidly with atmospheric O₂ and NO to produce intermediate products of C₁₂H₂₅O,C₁₃H₂₇O and C₁₄H₂₉O,processes that theoretical calculations find to be barrier-free.A single water molecule has a detrimental effect on the reaction,which would slow down the reaction at room temperature.