Study On The Mechanism Of Conversion Of Inorganic Bromine And Iodine Into Organic Bromine And Iodine Disinfection Byproducts During Chlorination

The disinfection by-products（DBPs）produced by chlorination disinfection process in water purification plants are a threat to human health.Natural organic matter（NOM）in water is the main precursor of DBPs.In the coagulation and precipitation process before disinfection process,the removal efficiency of small molecular weight（<10KDa）natural organic matter in existing water purification plants is low,which affects the formation of DBPs.In addition,in recent years,due to seawater intrusion,salt tide backfilling and other reasons,there are certain concentrations of bromine and iodine ions in the source water.Their presence will interact with NOM and disinfectants to produce brominated and iodine disinfection by-products which are more carcinogenic than chlorinated disinfection by-products.This poses a new challenge to the safety of drinking water.In this paper,water from a water source reservoir in northern China was used as NOM matrix.Firstly,the variation of total organic halides（AOX,X=Cl,Br,I）in water was investigated after the concentration of bromine and iodine ions increased,and the utilization rate of inorganic bromine and iodine ions by-products of bromine and iodine disinfection in this process was calculated.Secondly,the molecular size of the source water was graded by ultrafiltration membrane method.The variation of Br-THMs and I-THMs produced by different NOM components in the disinfection process with the influencing factors（chlorine dosage,pH and temperature）was discussed respectively in the presence of bromine and iodine ions.Finally,the influence of chlorination disinfection on Br-THMs and I-THMs formation potential in different NOM components with different concentration ratios of bromine and iodine ions was explored,and the formation pathway of by-products of bromine and iodine disinfection was speculated,from which the main control factors or inhibition conditions were found.The results showed that in the presence of Br^-chlorine（NaClO）produced more AOBr than chloramine（NH₂Cl）disinfectant,and the amount of AOBr increased with the increase of Br^-concentration.In the process of increasing Br^-concentration from 50 to 1000μg/L,the utilization of inorganic Br^-in water by AOBr is between 27.15%and 60.38%.In the presence of I^-,chloramine（NH₂Cl）disinfectant produces more AOI than chlorine（NaClO）disinfectant.When the concentration of I^-increases from 50 to 400 ug/L,the utilization rate of inorganic I^-by AOI is 39.2%⁶9.95%。Chlorine disinfection of NOM with different sizes containing 500μg/L bromide ions showed that NOM with MW<1K Da was the main precursor of Br-THMs.With the increase of chlorine dosage from 2 to 16 mg/L,the formation of CHBr₃ in each NOM component changed to the initial 1/6,1/4,1/8 and 1/4.BUF decreased by 3.6%,2.0%,3.8%and 1.2%respectively.Chlorobromide mixed trihalomethane was the dominant species of Br-THMs.The total amount of Br-THMs and its components increased with the increase of pH and temperature.At pH 8,the BUF of each NOM component was above 7%,and the temperature was 35℃.At the same time,the BUF of each NOM component is more than 4%.Chloramine disinfection was carried out on NOMs with different sizes containing 200μg/L iodine ions.The results showed that CHI₃ content was the largest in the generated I-THMs and was the dominant species.With the increase of chlorine input,the production of I-THMs of each NOM component increased first and then decreased.At 4mg/L（Cl₂/DOC=1）,the maximum value of I-THMs was 25.42,23.37,17.42 and 25.70μg/mg-C,respectively.The IUF of each NOM component was slightly higher than that of the1K<MW<3K Da component,about 8%under the condition of DBPFP determination,while the utilization rate of large molecular weight organic compounds（MW>10K Da）was higher than that of 20%under the condition of alkalinity（pH=8）and high temperature（T>30℃）.This may be due to the consumption of more chlorine in the conversion of macromolecular organic matter to small molecular organic matter.Chlorine and chloramine were used to disinfect NOM components containing different ratios of bromine and iodine ions.The results showed that the THMs produced by NaClO disinfection were only Br-THMs.when the ratio of Br^-/I^-concentration was increased from 100/200（μg/μg）to 1000/200（μg/μg）,the total amount of Br-THMs produced by each NOM component increased by 1.73 times,1.74 times,1.95 times and 1.60 times,respectively.In this process,I-THMs were difficult to produce,which was the result of strong oxidation of HOBr.However,when NH₂Cl was disinfected,the THMs produced included both Br-THMs and I-THMs,in which I-THMs was the dominant species.The total amount of Br/I-THMs produced in each NOM component decreased with the increase of Br^-/I^-concentration ratio,and the generation potential decreased by 4.41,6.35,14.17 and2.36μg/mg-C,respectively.The causes of the above phenomena are related to the types of HOX（X=Cl,Br,I）in water.According to the characterization of NOM by SUVA and 3D-EEM,small-component organic compounds（MW<3KDa）with weak hydrophilicity and aromaticity are the main precursors of Br-THMs and I-THMs.