Formation Mechanism Of Chlorinated Ketones/Aldehydes Disinfection By-Products And Evaluation Of The Formation Potential In Conventional Water Treatment Process

Disinfection by-products（DBPs）are a kind of substances with teratogenic,carcinogenic and mutagenic effects,which pose a great threat to the safety of drinking water.Halogenated ketones（HKs）and halogenated aldehydes（HAs）,as new emerging disinfection by-products,usually detected simultaneously at level ofμg·L^-1 in the disinfection process of drinking water,are only second to trihalomethanes（THMs）,haloacetic acids（HAAs）and haloacetonitriles（HANs）in drinking water.Therefore,they have attracted wide attention in recent years.The formation and control of halogenated ketones and aldehydes disinfection by-products are of great significance to the safety of drinking water.In this paper,two typical new emerging disinfection by-products,chlorinated ketones（CKs）（dichloroacetone,1,1-DCP;trichloroacetone,1,1,1-TCP）and chloral hydrate（CH）,were studied to explore their formation mechanism in chlorine disinfection system and evaluate their formation potential in conventional water treatment process.The formation potential of chlorinated ketones and aldehydes in chlorination disinfection process was studied with 14 kinds of natural organic substances in 6categories.High halogenated ketones and aldehydes formation potential was observed when fulvic acid,citric acid,malic acid,L-threonine and L-asparagine were served as precursors in chlorination disinfection system.And the formation pathway of chlorinated ketones and aldehydes from organic compounds mainly included two stages:the precursors firstly generated acetaldehyde and acetone through oxidation and decarboxylation,and then acetaldehyde and acetone were gradually chlorinated to produce chloroacetaldehydes and chloroacetones.The disinfection experiments in actual water showed that the formation potential of chlorinated ketones and aldehydes was not necessarily related to the concentration of organic compounds in water samples,but positively related to the content of alcohol hydroxyl and carbonyl functional groups in water samples（R²=0.69⁰.98）.The chlorine dosage,pH,bromide ions and metal ions in the reaction system all had effects on the formation potential of chlorinated ketones and aldehydes.There was a good linear relationship between chlorine dosage and formation potential of 1,1-DCP,1,1,1-TCP and CH（R²=0.87⁰.99）,and the formation potential increased with the increasing of chlorine dosage.The formation potential of 1,1-DCP,1,1,1-TCP and CH was significantly affected by pH conditions.When the pH increased from 5 to 9,the formation potential of 1,1-DCP,1,1,1-TCP and CH increased firstly and then decreased.The maximum production was obtained in neutral conditions,while in alkaline conditions,the formation potential was significantly lower than that in neutral and acidic conditions.The presence of bromide ion inhibited the formation of 1,1,1-TCP and CH,and the degree of inhibition was positively correlated with the concentration of bromide ion,but when citric acid was used as precursor,bromide ion promoted the formation of CH;low concentration of bromide ion can promote the formation of1,1-DCP,but when the concentration of bromide ion was higher than 0.25 mmol·L^-1,the production of 1,1-DCP was also inhibited.Fe³⁺and Cu²⁺promoted the formation potential of 1,1-DCP,1,1,1-TCP and CH in different degrees,and the higher the concentration of metal ions,the more obvious the promotion effect.When the concentration of metal ions was 5 mg·L^-1,the promotion effect on 1,1-DCP and1,1,1-TCP was extremely significant（p<0.01）,and the promotion of CH was significant（p<0.05）.The effects of pre-oxidation,coagulation and precipitation processes and advanced treatment processes on the formation potential of 1,1-DCP,1,1,1-TCP and CH were different.For potassium permanganate（KMnO₄）or ozone（O₃）pre-oxidation processes,when fulvic acid was used as precursor,the production of 1,1-DCP,1,1,1-TCP and CH was greatly promoted（p<0.05）;for small molecular organic acids,when citric acid was used as precursor,the formation of CKs and CH was promoted by KMnO₄pre-oxidation technology,and for L-threonine and L-asparagine,KMnO₄ pre-oxidation presented the same effect on the formation of CKs,but it had a significant inhibition on CH.After O₃ pre-oxidation,the generation potential of 1,1-DCP,1,1,1-TCP and CH were inhibited from small molecular organic acids.Coagulation and sedimentation processes inhibited the formation of 1,1-DCP,1,1,1-TCP and CH,and the inhibiting effect was less than 20%.The removal efficiency of 1,1-DCP,1,1,1-TCP and CH in advanced treatment process was very obvious and reached to 52.63%⁶6.89%,69.26%⁸3.17%and 99.49%⁹9.76%in activated carbon adsorption,NF and RO membrane,respectively.