The Chlorination Disinfection By-products Formation Potential And Pathway Of Amino Acids

Drinking water risk issue was always widely concerned by people and has become international research hotspot over the recent years. Biologic safety of drinking water has been always ensured by disinfection. Chlorination disinfection, as the most common disinfection technology in the water treatment, has been applied in most water plants.But in the process of chlorination disinfection, there will be disinfection by-products(DBPs) produced, which are harmful to person body. Hence, it is very importment to control the formation of DBPs. Amino acids are the representative nitrogenous organic compounds and important precursors of DBPs, the influencing factors and formation mechanisms of DBPs formation via amino acids have been studied.In order to find out the DBPs formation potentials of amino acids, experiments of chlorination disinfection used 20 amino acids as precursors have been carried out. The yields of DBPs are analyzed by Gas Chromatogram. As shown in the experiments results,tryptophan(Trp), has the highest THMs formation potential, and tyrosine(Tyr) takes the second place, histidine(His), asparagines(Asn), phenylalanine (Phe), threonine (Thr) and aspartic acid(Asp) also have higher THMs formation potential than other amino acids. Asp has the highest HAAs formation potential, and His takes the second place, Trp, Asn, Tyr, proline (Pro), Phe and Thr also have higher HAAs formation potential than others. Asn has the highest HANs formation potential, and Tyr takes the second place, glutamine (Gln), glutamic acid (Glu), His, Trp, Phe and Pro also have higher HANs formation potential than others. The results of chlorination disinfection also showed that chlorine demands of amino acids are different. Amino acids with activated aroma rings have higher chlorine demands,and amino acids with hydroxide group, sulfydryl group and amido group also have high chlorine demands.These function groups can lose electron.In view of the differences of structure among the 20 amino acids,in order to confirm the effects of different structure in the course of DBPs formation,the chlorination of six representative amino acids have been studied,which are alanine containing alkyl group,Tyr containing hydroxide group,Asn containing acyl group,Asp containing carboxyl group, Trp containing indole function group and His containing imidazole function group. The effects of influence factors of DBPs, such as reaction time, chlorine concentration, pH, temperature and bromine, have been reviewd. Results shown that the yields of THMs and HAAs both increase with the increase of reaction time, and with the increase of reaction time, the yield of HANs increase at first and then decreased. Chlorine concentration, pH and temperature also influence the formation of DBPs greatly, and the yields of DBPs take on different results as the different structure of amino acids.When there was bromine ion in water, bromine-substituted productions appeared, and the total yields of DBPs increased with the increase of bromine concentration.Some results have been gained by chlorination disinfection experiments and analysis in chemistry theory as follows: Ala react with chlorine to bring N-chloroalanine, which decomposed to acetaldehyde, and then acetaldehyde react with chlorine to produce dichloroacetaldehyde and chloral, via oxidation to form DCAA and TCAA. Tyr react with chlorine can bring trichlorophenol, while trichlorophenol can produce TCM, so via this way, TCM formed from Tyr. While, Tyr also can produce Ala, so TCM formation from Tyr also can via Ala. Asn react with chlorine can bring to chloral, which via nucleophilic reaction can make TCM. Otherwise, DCAN hydrolysis also can induce the formation of TCM.Asn chlorination can produce imine intermediate products, which further react with chlorine to bring cyanogen chloramines, then further to form DCAN. Otherwise, Asn can also produce dichloroacetaldehyde and chloramines, which can react with each other to DCAN.DCAA formation from Asn can via the hydrolysis of DCAN, and also can via the intermediate productsβ-carboxyl. Asp can bring to chloral by reaction with chlorine, chloral via nucleophilic reaction can make TCM. The reaction between Trp with chlorine can open the ring of pyrrole, then further chlorination to form TCM. On indole group of Trp the reaction ofαsubstitute carried at first, then ring opening to bring imine intermediate products, which can further chlorination to DCAN. His react with chlorine to form chloroimidazole, viaαsubstitute, chlorination, ring opening to produce TCM.