Effects Of O₃ Or UV/O₃ Pre-Oxidation On The Formation Of Iodo-trihalomethanes Duiring Drinking Water Disinfection

People pay more and more attention to the harmful disinfection by-product(DBPs),produced by the reaction between disinfectants and natural organic matter(NOM)during drinking water disinfection.Among them,iodinated disinfection by-products(I-DBPs)has higher genotoxicity and cytotoxicity than other DBPs,although its concentration is low.The pre-oxidation process has gradually sprung up,for which is more effective than the conventional disinfection process to control the formation of I-DBPs.Among them,O3 is widely used in the pre-oxidation disinfection process of waterworks because of its remarkable germicidal efficacy and strong oxidation ability.However,there is a lack of in-depth understanding of how O3 pre-oxidation affects the formation of I-THMs.In this paper,the effects and mechanisms of O3 and UV/O3 pre-oxidation on the formation of iodo-trihalomethanes(I-THMs)in subsequent chlorination and chloramination was investigated.The main research contents and conclusions are as follows:1.Effect of O3 pre-oxidation on the formation of I-THMs during chlorination and chloraminaion.Humic acid(HA)was adopted as representative NOM,the formation of I-THMs in chlorination and chloramination after pre-oxidation with different concentrations of O3 was explored,and the effects of O3 concentration on the concentration of HOI and the variation of NOM were analyzed.Finally,the total toxicity changes of O3-Cl2(ozone and subsequent chlorination)and O3-NH2Cl(ozone and subsequent chloramination)processes were evaluated with Chlorella vulgaris as model organisms.The results showed that the concentration of I-THMs increased at first and then decreased with the increase of O3 concentration during chlorination and chloramination,and reached the maximum values of 168.6±4.1 and 498.9±57.0 nM when the O3 concentration was 20 and 10 ?M,respectively.When the concentration of O3 is low,with the increase of the concentration of O3,on the one hand,the concentration of HOI increases,on the other hand,the molecular weight of HA decreases,and the fluorescent components with complex structure and large molecular weight transfer to the components with simple structure and low molecular weight to form a kind of I-THMs precursor,so the concentration of I-THMs increases.When the O3 concentration increased further,O3 affected the formation of I-THMs mainly by affecting the HOI concentration,so the decrease of HOI concentration led to the decrease of I-THMs concentration.The disinfection process of O3-Cl2 and O3-NH2Cl had a certain effect on the growth inhibition of Chlorella.With the increase of O3 concentration,the inhibition of Chlorella increased at first and then decreased2.The effect of O3 pre-oxidation of different NOM on the formation of I-THMs during chlorinaton and chloramination.Taking representative NOM such as humic acid.fulvic acid and bovine serum albumin,as well as raw water and filtered water of waterworks as research objects,the variation of different NOM caused by O3 were analyzed,and the effect of O3 pre-oxidation on I-THMs production of different NOM was discussed.The results show that the molecular weight of the five kinds of NOM decreases,from fluorescent components with complex structure and high molecular weight to those with simple structure and small molecular weight with the increase of O3 concentration.Compared with HA and FA,RW,FW and BSA are more easily oxidized by O3,the aromatic chromophore is degraded,and the aromatic structure is transformed into small molecular organic compounds with non-ultraviolet absorption,because of the low SUVA value.During chlorination and chloramination of five kinds of DOM,the total concentration of I-THMs increased at first and then decreased with the increase of O3 concentration.During chlorination,the I-THMs formed in HA and FA water was significantly higher than that in BSA,RW and FW water,while the I-THMs formed by five kinds of NOM had little difference during chloramination.3.Effect of UV/O3 pre-oxidation on the formation of I-THMs during chlorination and chloramination.HA was adopted as representative NOM,the effect of UV/O3 pre-oxidation on the formation of I-THMs during chlorination and chloramination compared with O3 pre-oxidation was discussed.The changes of HOI concentration during UV/O3-Cl2(UV/ozone and subsequent chlorination)and UV/O3-NH2Cl(UV/ozone and subsequent chloramination)and the effects of UV/O3 pre-oxidation on the structure and composition of HA were explored.Finally,the effects of UV radiation dose and Br-on the formation of I-THMs were investigated.The results show that when the concentration of O3 is 0,the I-THMs formed by UV/O3 pre-oxidation is higher than that formed by O3 pre-oxidation,while when the O3 concentration increases from 10 to 80 ?M,the concentration of I-THMs increases at first and then decreases with the increase of O3 concentration,and the I-THMs formed by O3 pre-oxidation is lower.When the O3 concentration is 0,the direct photolysis of UV induces the formation of more active aromatic structures,which leads to more precursors of I-THMs,so the concentration of I-THMs is higher.While when the concentration of O3 is not zero,the HO· formed by UV/O3 can partially oxidize NOM,in which the aromatic structure and conjugated double bonds are preferentially degraded,on the other hand,it promotes the conversion of I-to IO3-,so the concentration of I-THMs is lower.With the increase of UV dose,the concentration of I-THMs during UV/O3-Cl2 decreased,while the concentration of I-THMs increased during UV/O3-NH2Cl.The addition of Br-can reduce the I-THMs concentration during chlorination and chloraminaction.