Formation And Control Of Nitrogen-containing Disinfection By-products From Amino Acid Precursors

Nitrogen-containing disinfection by-products?N-DBPs?are a new type of disinfection by-products with strong cytotoxicity and genotoxicity.Amino acids are typical nitrogen-containing natural organic compounds?NOM?,which widely exist in surface water.It is very important to study the formation process and control technology of N-DBPs in the chlorination process of amino acid precursors.The formation potential of typical amino acids to produce N-DBPs during chlorination disinfection was studied using seven common amino acids as precursors.The results showed that Dichloroacetamide?DCAcAm?,trichloroacetamide?TCAcAm?and dichloroacetonitrile?DCAN?were detected in the chlorination systems of histidine,aspartic acid,asparagine and tryptophan.The order of specific molar formation potential?SFP?of N-DBPs from amino acids in 0 to 56 h was asparagine>aspartic acid>histidine>tryptophan.And the SPF of N-DBPs produced by asparagine was significantly higher than that of other amino acids?p<0.01?.While in asparagine chlorination disinfection system,the sequence of SPF for each N-DBPs was DCAcAm>DCAN>TCAcAm.And the SPF of DCAN was significantly higher than that of DCAcAm and TCAcAm?p<0.01?.Taking asparagine as precursor,the influencing factors and formation mechanism of N-DBPs from amino acid chlorination were studied.The experimental factors,such as chlorination amount,pH value,anion and cation,all affected the process of N-DBPs formation from asparagine chlorination.The decarboxylation and substitution of asparagine increased with the increase of chlorine addition,while the SPF of DCAcAm and DCAN increased,but the formation of TCAcAm was inhibited.The formation potential of HAcAms and HANs under neutral and alkaline conditions was significantly higher than that under acidic conditions?p<0.05?.Bromine ions in disinfection system inhibitd the formation of DCAcAm and TCAcAm,but promoted the formation of brominated disinfection by-products.Cu²⁺promoted the formation of HAcAms.The formation of DCAcAm,TCAcAm and DCAN from asparagine involves a series of chlorine substitution reactions,Hofmann degradation reactions,decarboxylation reactions and hydrolysis reactions.The removal of N-DBPs and their amino acid precursors by coagulation and precipitation process,potassium permanganate pre-oxidation-chlorine disinfection process and activated carbon-membrane separation process were studied.The types of coagulants,dosage of reagents and rotational speed all had certain effects on the removal of N-DBPs precursors.And the relationships of these factors are:type of coagulants>rotational speed>dosage of reagents.The removal efficiency of amino acid precursors was the best when the type of reagent was FeCl₃,the dosage was 30mg·L^-1,the rotational speed was 200 r·min^-1?2 min?+10 r·min^-1?8 min?.Potassium permanganate oxidation can effectively destroy the molecular structure of amino acids,thus significantly reducing the concentration of N-DBPs produced in the subsequent chlorination disinfection process.Activated carbon and activated carbon-membrane separation technology can effectively remove N-DBPs from water.The average removal rates of three N-DBPs?DCAN,DCAcAm and TCAcAm?by activated carbon adsorption,activated Carbon-Nanofiltration and activated carbon-reverse osmosis were81.49%,85.91%and 99.6%,respectively.