Study On Formation Of Hydrogen Peroxide In Photodegradation Of Chloroacetic Acids In Water By UV₂₅₄and Influences Of Selected Compounds

The disinfection by-product is a class of highly toxic organic pollutants with carcinogenic,teratogenic and mutagenic properties.Haloacetic acid is the most representative one.In order to ensure water quality during water supply,a certain amount of disinfectant is added to inhibit microbial activity.Consequently,the occurrence of HAA cannot be effectively inhibited.Therefore,it is necessary to carry out research on the effective removal of HAA.Previous studies have shown that effective removal of HAA can be achieved by irradiation with UV light at 254nm(UV₂₅₄).While the molar absorption coefficient of chloroacetic acids are weak,and the reaction mechanism of UV degradation is still unclear and controversial.Therefore,further research and analysis are needed in this part.The product analysis of three chloroacetic acids including monochloroacetic acid,dichloroacetic acid,and trichloroacetic acid was carried out using UV₂₅₄method.The results showed that hydrogen peroxide?H₂O₂?,formic acid and chloride ions were formed during the photolysis of chloroacetic acid.At the same time,monochloro,dichloro and trichloroacetic acid also formed three characteristic intermediates of glycolic acid,glyoxylic acid and acetic acid.When the irradiation was prolonged,the concentration of chloride ion was gradually increased,while the other products were degraded.Under the same initial mass concentration,the maximum yield of formic acid and H₂O₂ is dichloroacetic acid>monochloroacetic acid>trichloroacetic acid.In order to further explain the characteristics of the formation and variation of the above products,the photolysis products were analyzed for three characteristic intermediates of the same initial mass concentration.The results showed that formic acid and H₂O₂ were formed in the intermediates,and the maximum yield of them was glyoxylic acid>glycolic acid>acetic acid.Therefore,the difference in the amount of chloroacetic acid final product formation may be due to the difference in the conversion efficiency of the respective intermediates.In order to further reveal the effect of products on the production of H₂O₂during the process of photolysis of chloroacetic acid,different concentrations of products were added to the photoreaction system.The results showed that the low concentration of the product had no significant effect on the photolysis rate of chloroacetic acid and the formation of H₂O₂.The high concentration of formic acid and intermediate products would significantly increase the formation of H₂O₂ during photolysis.However,the competitive absorption would inhibit the photodegradation rate of chloroacetic acid.The chloride ion concentration has little effect on the photolysis rate of chloroacetic acid,but it will significantly reduce the formation of H₂O₂.In addition,the mechanism of photodegradation of chloroacetic acid was tentatively explored by using oxygen-controlled and tert-butanol.The results showed that both oxygen-control and tert-butanol had significantly inhibitory effects on the degradation rate of chloroacetic acid.Furthermore,the inhibitory effect was consistent during the photolysis process,indicating that the indirect photolysis may be caused by dissolved oxygen in water.During photolysis,UV₂₅₄ excites chloroacetic acid to form an excited state chloroacetic acid radical,which combines with oxygen to produce superoxide anion radicals,and then forms H₂O₂ in the water.H₂O₂ photolysis produces hydroxyl radicals and finally reacts with chloroacetic acid to achieve indirect photolysis.