I-THMs Formation Mechanisms During Disinfection And The Prediction Model

Disinfection is of vital importance during the drinking water treatment process to prevent the spread of diseases caused by waterborne pathogens.Chemical disinfectants are widely used in drinking water treatments,including ozone(O3),chlorine dioxide(ClO2),chlorine(Cl2),and chloramine(NH2CI).However,the reaction between disinfectants and nature organic matter(NOM)leads to the formation of hazardous disinfection byproducts(DBPs),which pose risks to human health.Iodinated DBPs(I-DBPs)have been reported to be more cytotoxic and genotoxic than the corresponding regulated chlorinated and brominated DBPs,which is an issue that has attracted public attention.In this paper,the effect of NOM molecular size on the formation of iodo-trihalomethane(I-THMs)was investigated at first.Then variation of the DOM structure during Ultraviolet/hydrogen peroxide(UV/H2O2)pre-oxidation was evaluated and the effect of UV/H2O2 preoxidation with following chlorination or chloramination on I-THMs formation was investigated.The synergistic effect of UV and disinfectants(including chlorine and chloramine)on the structural transformation of DOM and formation of I-THMs in water were investigated.Finally,three kinds of formation models of I-THMs were developed.The main research contents and experimental results are listed below:1.This paper evaluate the function of the NOM molecular size on the formation of I-THMs during chlorination and chloramination.Humic acid was adopted as the NOM matrix and fractionated into four molecular weight(MW)groups.Various parameters,including iodide,bromide,NOM concentrations,pH,and pre-chlorination time,were investigated for each MW fraction.During chlorination,high MW fractions(i.e.,MW>100 K Da and 50 K<MW<K00 K Da)produced more I-THMs compared with small MW fractions(i.e.,MW<3 K Da and 3 K<MW<50 K Da).With the increase in the I-or NOM concentration,the formation of I-THMs increased for small MW fractions,while a slight reduction occurred for high MW fractions during chlorination.Higher pH resulted in more I-THM formation for small MW fractions,while the opposite was true for high MW fractions during chlorination.Compared to small MW fractions,bromide was relatively more reactive with high MW fractions in the formation of I-THMs during chlorination.During chloramination,the I-THM yields decreased with the increasing NOM concentration for high MW fractions.The concentration of bromine-containing I-THMs decreased with increasing pH for all MW fractions during chloramination.Additionally,with the prolongation of pre-chlorination time,the total amount of I-THMs decreased remarkably for MWs higher than 3 K Da,while a slight change for MW lower than 3 K Da occurred during chloramination.The results from this study suggest that the molecular weight of the NOM plays an important role in the formation of I-THMs during chlorination and chloramination.2.This study investigated the influence of UV/H2O2 pretreatment on the formation of I-THMs during disinfection with chlorine or chloramine.The changes of precursors,I-and Br-,after UV/H2O2 pretreatment were investigated,and then,the formation and speciation of I-THMs during chlorination or chloramination after pre-oxidation were explored.Additionally,the effects of UV doses and H2O2 concentrations on the formation and speciation of I-THMs were studied.It was found that UV/H2O2 pretreatment could change larger MW DOM to smaller MW species,which had less aromatic organic compounds and fluorescence.Additionally,insignificant transformations of I" and Br-were observed after UV/H2O2 treatment.Compared to direct disinfection,UV/H2O2 pretreatment resulted in 23.0 ± 3.5%reduction in I-THMs formation during post-chlorination at a UV dose of 460 mJ/cm2 and 20 mg/L H2O2,while an enhancement was observed during post-chloramination.Moreover,total I-THM concentration increased from 43.7 ±2.4 to 97.6 ± 14.9 nM with the increase of UV doses from 0 to 1400 mJ/cm2 during the post-chlorination process,while reduced when the UV fluence was greater than 460 mJ/cm2 during the post-chloramination.Additionally,the generation of I-THMs during both post-chlorination and post-chloramination was positively related to the H2O2 levels from 0 to 20 mg/L in the UV/H2O2 pretreatment.3.The synergistic effect of ultraviolet light and disinfectants on the structural transformation of HA and formation of I-THMs in water were investigated.The impact of different treatments on the transformation of HA was investigated by FTIR,EEM and two dimensional correlation spectroscopy(2D-COS)of synchronous fluorescence spectra.The transformation of HA was enhanced upon co-exposure to UV and chlorine while insignificant changes was observed during UV/NH2Cl.UV treatment accelerated the consumption rate of Cl2 and NH2Cl and HCl would be generated to reduce the pH of the solution.Compared with direct disinfection,UV/Cl2 can increase the production of I-THMs and UV/NH2Cl can reduce the formation of I-THMs.The addition of PBS increased the quantum yield and I-THMs formation during UV/Cl2 while decreased for UV/NH2Cl treatment.4.The results of an experimental investigation into the factors forming I-THMs in the presence of iodide ions were presented and three kinds of formation models of I-THMs were developed.The experiments were conducted using synthetic water samples with different characteristics(including I-,DOC,Cl2 concentration,pH,temperature and reaction time).Different combinations of these characteristics were considered in the experimental program.The results showed that the increase of I-concentration led to an increase in the yields of I-THMs while DOC and HOCl concentration show negative correlation with I-THMs formation.As pH increased from 6 to 8.5 or temperature increased from 4 0C to 190C,the yields of I-THMs was greatly increased in different reaction time periods.Three kinds of formation models for I-THMs were developed and statistical adequacy was assessed using appropriate diagnostics,including residual plots and interaction plot for all factors.The predictive performance of the nonlinear regression model was found to be more excellent.