Study On The Mechanism Of Disinfection By-products Of Algal Neurotoxin 2,4-diaminobutyric Acid

In recent years,the frequent eutrophication not only leads to the decline of water quality,but also causes cyanobacteria blooms.Besides,the toxic and harmful algal toxins released by cyanobacteria,such as neurotoxin 2,4-diaminobutyric acid（DAB）,obviously have the risk of affecting human health.However,it is difficult to remove DAB effectively for the conventional process of water treatment.As a result,the residual DAB in water may not only produce various disinfection by-products（DBPs）through the reaction with disinfectants in disinfection process,but also enter human body directly through drinking water.It is a serious threat to human health and the safety of drinking water quality.Therefore,finding a reliable and effective water treatment process to control and remove DAB and the DBPs of DAB is of great practical significance.The mechanism and formation of DBPs from chlor（am）ination of DAB were examined and compared in this study.It’s the conclusion that dichloroacetonitrile（DCAN）,dichloroacetic acid（DCAA）,trichloroacetic acid（TCAA）,and trichloromethane（TCM）were mainly formed from chlor（am）ination of DAB.Among them,the concentration of DCAA was highest,and the amount of chloraminated DBPs was much lower than that of chlorinated DBPs,while the concentration of DBPs from chlorination of DAB was at a high level,which posed a high threat to the safety of drinking water.Except DCAN,the amount of DBPs increased with the increase of disinfectant dosage.Alkaline condition promoted TCM from chlorination,inhibited TCM from chloramination,and inhibited DCAN during both chlorination and chloramination.Adding iodide（I^-）and bromine（Br^-）to water maked DAB chlor（am）inated to produce bromine-trihalomethanes（Br-THMs）and iodo-trihalomethanes（I-THMs）with higher toxicity.After chloramination,DAB showed higher conversion rates of Br-THMs and I-THMs at the same concentration of Br^-and I^-.DAB can produce trichloroacetaldehyde through chlor（am）ination,decarboxylation and hydrolysis,and then TCM can be produced by further reaction during chlor（am）ination.Both theα-andγ-amino groups of DAB can form DCAN through a series of chlorination and chloramination reactions,but the nitrogen（N）of some DCAN formed by chloramination may come from chloramine（NH₂Cl）.DAB was chlor（am）inated to form an intermediate product with cyano group,then reacted to form amide structure,hydrolyzed to form carboxylic acid,and finally DCAA was formed.However,the chlorinated DCAA was partially hydrolyzed by DCAN.Besides,TCAA was further formed by chlor（am）ination of DCAA.Then,the oxidation of DAB by chlorine dioxide（ClO₂）was studied,and the subsequent treatment was combined with chlor（am）ination to investigate the effect of ClO₂preoxidation on the DBPs from DAB and amino acids.It’s the conclusion that the degradation efficiency of DAB by ClO₂was higher than that by sodium hypochlorite（Na ClO）,NH₂Cl and potassium permanganate（KMn O₄）,and ClO₂can rapidly degrade DAB in a short time,and the removal rate increased with the increase of ClO₂dosage.Based on the luminol model,it was speculated that DAB-ClO₂adduct was formed as soon as ClO₂solution is added,which completed the single electron transfer of electron rich nitrogen atoms to ClO₂,and then further reacted to form subsequent oxidation products,resulting in the rapid decrease of DAB concentration.Besides,ClO₂preoxidation can significantly reduce DBPs from post-chlorination of DAB,and only slightly increase TCM and DCAN from post-chloramination.Furthermore,adding ClO₂preoxidation effectively reduced the levels of Br-THMs in subsequent chlorination at the same Br^-level,and the higher the Br^-concentration,the higher the reduction rate.However,the conversion rate of Br-THMs increased after chloramination.When I^-existed in water,ClO₂preoxidation treatment can effectively control the subsequent chlor（am）inated I-THMs.In addition,the DBPs ofβ-N-methylamino-L-alanine（BMAA）,alanine and leucine during chlor（am）ination were less than those of DAB,and ClO₂pretreatment could effectively reduce the DBPs of the three amino acids.In this study,the results showed that the use of ClO₂preoxidation can effectively remove DAB in water and significantly reduce the formation of DBPs,which has important significance to provided a theoretical basis for practically dealing with the outbreak of water bloom and ensuring the safety of drinking water.