Formation Mechanism And Control Of Chloral Hydrate During Chlorination Of Natural Organic Matter In Source Water

The Northeast Plain in Northeast China is one of the four black soil regions in the world,which is rich in natural organic matter（NOM）.A reservoir in the plain is used as an important water source for a city in northeast China.As the snow and ice melting and surface runoff scouring,NOM from the soil will enter the reservoir,resulting in a high natural organic matter content in the reservoir throughout the year.Most NOM will enter the clear water through the conventional drinking water treatment process and consume chlorine,producing strong carcinogenic and teratogenic chloral hydrate（CH）and other disinfection byproducts（DBPs）.To effectively control the formation of CH,this study first identified the source and molecular features of NOM,screening out the possible molecular structural composition of CH precursors,and representative model compounds were selected to explore the formation mechanism of CH during chlorination.On this basis,the effective control method of CH precursor was studied and the precursor evaluation index of CH was put forward.Variations of NOM in the source water with different years,different molecular weight,and different water treatment processes,were discussed.For example,the concentration of NOM in the source water reached 7.32 mg/L in December 2016 during the icebound period.After the precipitation process and entering March of the following year,the concentration of NOM decreased to 6.44 mg/L.As the spring flood ended in June,the NOM concentration dropped to 4.08 mg/L.In different hydrological years,terrestrially hydrophobic humic acid organics dominated the source water,and those humic acid organics were mainly distributed in the range of molecular weight less than5 k Da.Along the conventional water treatment process,the proportion of humic and fulvic acids in the effluent of each process decreased from 73.49%in the raw water to69.25%in the settled water and further changed to 71.72%in the filtered water,which shows that the NOM in each water treatment unit is mainly consisted of humic and fulvic acids.The molecular features of NOM were analyzed by Fourier ion cyclotron resonance mass spectrometry（FTICR-MS）.The correlation coefficient between the relative peak intensity of FTICR-MS and the relative fluorescence intensity of fluorescent organic compounds was determined by Spearman correlation analysis,and 840 molecular characteristics were assigned to different fluorescence components at the confidence level of 95%.758 of them were assigned to humic and fullic acids,including the strongest ion peak in raw water corresponding to the molecular formula C₁₇H₁₈O₁₁.Of the 758 formulae assigned to humic and fulvic acids,97 were strongly correlated with the components of humic and fulvic acids（r=1.000）.Ninety-seven molecular features strongly associated with humic acids showed higher levels of unsaturation and aromaticity than the other molecular features assigned to humic acids.The rule of CH generation and the main factors affecting CH generation during chlorination of the water source were disscussed.During chlorination,CH production increased gradually with the prolongation of reaction time.The yield of CH was significantly correlated with the yield of other DBPs within 12 h chlorination.With the increase of chlorine dose,the CH concentration increased firstly and then remained unchanged.Inorganic Ca and Mg ions,as well as Fe and Cu ions could affect the CH production.The former had different effects at different reaction time,while the latter significantly promoted the CH production at all reaction time.The yields of CH from chlorination of NOM in source water of different years,NOM of different molecular weight,and NOM in different drinking water treatment units,were discussed.The CH yield per unit of organic carbon does not depend on the molecular weight of NOM.Pearson correlation analysis shows that from different hydrological year section,different molecular weight profile or different drinking water treatment unit profile,there existed good correlation between the fluorescence intensity of humic acid and CH yield,implying carboneous humic acids are main precursors of CH.Four possible structures of C₁₇H₁₈O₁₁,the strongest molecular ion peak of humic acids,were identified by searching Chemispider website,and they were lignin and tannic acid derivatives.The chlorine consumption per mole of organic carbon,the CH concentration and the percentage of the chlorine consumed converted to CH were further analyzed for 12tannic acid and lignin derivatives after chlorination.Among all the model compounds,benzoic acid containing carboxyl group had the lowest consumption of Cl₂,while phenol containing hydroxyl group had the highest consumption of Cl₂.At the same time,the yield of CH of phenol is much higher than that of benzoic acid,indicating that the potential of CH generation is related to the electron donating capacity of benzene ring.Spearman correlation with r=0.75,p<0.05 was found between the yield of CH and chloroform（TCM）in molar organic carbon of the 12 model compounds,suggesting that the process of CH generation was accompanied by the generation of TCM.In addition to resorcinol,the recognized DBP precursor,the p-coumaric acid and ferulic acid lignin derivatives with acrylic group on the aromatic ring had the highest potential for CH formation in turn.Among them,0.9%of chlorine consumed in the chlorination process of coumaric acid was transferred to CH,and the conversion percentage of chlorine ranked the highest among all model compounds.This paper further discusses the chlorination kinetics of coumaric acid,ferulic acid and erucic acid,the influencing factors of CH formation and the chlorination reaction paths.Electrophilic substitution,HOCl addition,oxidative demethyl and oxidative decarboxylation are the main reactions involved in CH formation.High concentration of hydroxylated products was found in the chlorinated systems of ferulic acid and p-coumaric acid,which existed for a long time in the reaction solution,possibly accounting for the high potential of CH generation in both systems.The control technology for CH is discussed and a real-time assessing method for CH precursors is proposed.Preoxidation technology increased the CH concentration.Advanced treatment unit targeted to remove humic acids with medium and low molecular weight and rich anionic groups exhibits a significant effect on the removal of CH precursors and aromatic organic compounds.Considering that the aromaticity of the organics in the source water fluctuates with seasons and water treatment conditions,this study developed an assessing technology for CH precursors,which is suitable for NOM with different aromaticity,hydrophilicity/hydrophobicity,and humification degree.Both the UV absorption value at 340 nm and the fluorescence intensity at the fluorescence peak（Ex/Em,310/415 nm）could effectively evaluate the concentrations of precursors of CH and other DBPs in humified and microbially derived NOM.Compared with UV₃₄₀,the fluorescence peak（Ex/Em,310/415 nm）has a higher sensitivity and can predict the concentration changes of CH and other DBP precursors in a wider range.In addition,the fluorescence intensity at the fluorescence peak（Ex/Em,310/415 nm）can evaluate the removal of CH precursors in the advanced treatment unit within the prediction error range of 10%,and can effectively evaluate the variation of CH and other DBP precursors in different drinking water treatment units in different drinking water treatment plants.