Comparison Of Photochemical Degradation Characteristics Of Malathion And Malaoxon Via UV Assisted Advanced Oxidation Processesin In Aqueous Solution

Malathion,a widely used organophosphorus pesticide,can be easily converted to malaoxon,an oxidative analogue,during preoxidation process of conventional water treatment.Compared with malathion,malaoxon is more toxic,hydrophilic,and harmful greater to water quality.Since the traditional water treatment process are not effective for removing the micro-organic contaminants,it is essential to research degrade OPPs and their oxidation metabolites oxons using UV-based advanced oxidation processes?UV-AOPs?.Many relevant literatures previously focused on the degradation kinetic,identification of intermediate products,and proposing of the possible degradation routes of target OPPs in UV-AOPs.However,few study have been conducted to elucidate the degradation of oxons.Since the degradation behavior of OPPs and corresponding oxon analogue by UV-AOPs may be affected by the different molecular structure between them.It is necessary to investigate the degradation of malathion and malaoxon by UV-AOPs.The following research was conducted in this paper.?1?To demonstrate the effect of organic solvents on the reaction rate constant of malathion,the direct UV and UV/H₂O₂ process have been investigated at the organic solvents concentration levels that would likely be derived from stock solution.several organic solvents commonly used for stock-solution preparation were selected,including acetonitrile?ACN?,acetone?Ac?,methanol?MeOH?,ethanol?EtOH?,1-propanol?PrOH?,1-butanol?BuOH?and 1-pentanol?PeOH?.The results show that the reaction rate of malathion in the UV/H₂O₂ process could be affected by the presence of these organic solvents,even at a concentration well below that possibly introduced during the preparation of working solutions from the organic solvent stock solutions?e.g.0.00005%,v/v?.The suppressive effect on the degradation rate constant depends on the type and concentration of organic solvents.With the increase of organic solvent concentration,the reaction rate constant of UV/H₂O₂ gradually decreases to the value for photolysis alone.A stronger reaction activity with·OH of organic solvents tends to impose a greater effect on the reaction rate constant.This is most likely attributable to the competitive reactions of the organic solvents with·OH,which reduce the·OH available for oxidizing malathion.?2?The degradation kinetics constants,degradation products,and degradation pathways of malathion and malaoxon were compared in UV,UV/H₂O₂,UV/TiO₂,and UV/Fenton process.It was found that the degradation reactions of malathion and malaoxon both followed the pseudo-first-order reaction kinetics model in all four processes.The degradation kinetics constants?k?of malaoxon were significantly slower than malathion's among all studied UV-AOPs process,and the ratio of k value of malathion to malaoxon arrange from 4.3 to 5.5.UV/H₂O₂,UV/TiO₂,and UV/Fenton can further oxidative degrade the oxidation products of malathion and malaoxon into smaller molecules rapidly in comparison to direct UV process did.Malathion and malaoxon have different degradation pathways in different UV-AOPs processes.Malathion shows seven products in four processes,but malaoxon has only three of them?ethyl 2-hydroxysuccinate,diethy malate,Diethyl-2-sulfanylbutanedioate?.Moreover,malathion would partially convert to malaoxon and further degrade in each treatment process.