The Photodegradation Of Haloacetic Acids: A Study Of Mechanisms And Influencing Factors

Haloacetic acids are a kind of halogen containing organic matter which ha ve a lot of applications in industry.The good chemi cal stability and water affinity make them widely existing in the water systems that may easily spread to other environmental systems.It is worth noting that haloacetic acids also exist in people's daily drinking water,and their content are only second to halogenated methane of all drinking water disinfection by-products.Therefore,it is necessary to exlore the degradation mechanism and influencing factors of haloacetic acids in water.In this work,the degradation mechanism of seven haloacetic acids,including chloroacetic acid(monochloroacetic acid,dichloroacetic acid,trichloroacetic acid),monobromoacetic acid,bromochloroacetic acid,monoiodioacetic acid and dibromoacetic acid,was studied by ultraviolet radiation,and the effects of various environmental factors were investigated on the removal efficiency of haloacetic acids.The results showed that the process of dehalogenation and mineralization could effectively degrade haloacetic acid in water.After 10 ~ 360 min photolysis,the removal rate of haloacetic acids could reach above 80%,and the photolysis of haloacetic acids followed the pseudo-first-order reaction kinetics.The photolysis rate constants of haloacetic acids increased as the degree of halogenation increased and the molecular weight of halogen element increased.The common products of haloacetic acids photolysis were halogen ions,formic acid,formaldehyde and halogenated methane,in which chlorinated haloacetic acids produced carbon dioxide mainly through the mineralization pathway,and brominated and iodinated haloacetic acids produced halogen-free intermediates mainly through stepwise dehalogenation and decarboxylation.By establishing the quantitative structure activity relationship model between the logarithm of photolysis rate constant and chemical descriptors,it was found that the logarithm of photolysis rate constant was well simulated by the space effect constants and electric induced effect constants,indicating that the molecular structure of haloacetic acid in the photolysis process was an important factor affecting their rate constants.The factors influencing photolysis of haloacetic acid s in this work included water samples,the initial concentration of haloacetic acids,chloride and bromide and iodide,nitrate,carbonate,bicarbonate,humic acid and residual chlorine.The results showed that the ultraviolet photolysis rate of haloacetic acids in real water was slow than that in ultrapure water.The chloride,carbonate and bicarbonate lead to lower photolysis rate of chloroacetic acid.The bromide and nitrate inhibited the photolysis of haloacetic acids other than monoiodioacetic acid.The photodegradation rates of haloacetic acids were significantly inhibited by humic acid.While iodide could effectively promote the photolysis of dichloroacetic acid,monobromoacetic acid and dibromoacetic acid.Under the condition of ultraviolet light irradiation,residual chlorine in the water accelerated the photolysis of haloacetic acids.In ultrapure water,the initial concentration had no obvious effect on the photolysis rate constants of haloacetic acids.In summary,the removal of haloacetic acid in water was effective by ultraviolet radiation,which could significantly reduce content of haloacetic acid from the water.Ultraviolet-absorbing substances in water could affect the photolysis of haloacetic acids.