Degradation Of Halogenated Phenols During Chlorination And UV/chlorination: Kinetics And DBPs Formation

Halogenated phenols are a typical group of harmful disinfection by-products（DBPs）in chlorination.They are odor pollutants with low odor thresholds and also the precursors of low-molecular-weight DBPs.However,the information on their degradation and formation potential of low-molecular-weight DBPs was very limited.Thus,in this study,2,4-dichlorophenol（2,4-DCP）and 2,4-dibromophenol（2,4-DBP）were chosen as target pollutants.Firstly,the extraction-analysis method using liquid-liquid extraction and ultra-high performance liquid chromatography was optimized.Then the degradation characteristics of 2,4-DCP and 2,4-DBP and their formation potential of low-molecular-weight DBPs during chlorination and UV/chlorination under different chlorine dosages,pH values and bromide concentrations were studied,which had important significance for the protection of drinking water safety.The optimized extraction conditions were as the following:the dose of sodium chloride was 100 g·L^-1;the volume ratio of the extractant to the solution was 1/10,and pH value of the solution was in the neutral range.The recoveries of 2,4-DCP and 2,4-DBP were 89.9%～100.1%.The minimum detection limits were 14μg·L^-1 and 11μg·L^-1,respectively.The method is easy and convenient for implement with high accuracy.In chlorination,increasing chlorine dosage,pH values or bromide levels,accelerated the consumption of free chlorine.At 0.2-0.8 mg·L^-1 as Cl₂and pH of 6.0-8.0,the half-life values of 2,4-DCP and 2,4-DBP ranged from 135.9 min to 16.7 min and from 28.9 min to 4.2 min,respectively.Increasing the chlorine dosage,pH value and bromide concentration accelerated the degradation of 2,4-DCP and 2,4-DBP.The reaction rate constants of 2,4-DBP were higher than that of 2,4-DCP.With the absence of bromide,2,4-DCP generated chloroform（TCM）and 1,1-dichloro-2-propanone（1,1-DCA）;2,4-DBP generated TCM,bromo-THMs（Br-THMs）,1,1-DCA and 1,1,1-trichloro-2-propanone（1,1,1-TCA）.With the presence of bromide,both 2,4-DCP and2,4-DBP generated TCM,Br-THMs and 1,1-DCA.Increasing the chlorine dosage promoted the production of trihalomethanes（THMs）and halogenated acetones（HKs）.Alkaline conditions favored the production of THMs and inhibited the production of HKs.The yields of Br-THMs increased with the concentration of bromide.The conversion ratio of 2,4-DBP to Br-THMs was higher than that of 2,4-DCP.The main products of 2,4-DCP were bromodichloromethane（BDCM）and dibromochloromethane（DBCM）while the main product of 2,4-DBP was bromoform（TBM）.In UV/chlorination,at 2.0～4.0 mg·L^-1 as Cl₂and the pH of 6.0～8.0,the half-life values of 2,4-DCP and 2,4-DBP ranged from 124.7 s to 40.8 s and from 176.8 s to 31.7s,respectively.With the presence of bromide,the half-life values of 2,4-DCP and 2,4-DBP ranged from 84.9 s to 67.2 s and from 84.6 s to 64.6 s,respectively.The reaction rate constants of 2,4-DCP and 2,4-DBP increased with the increment of chlorine dosages,pH values and concentrations of bromide.With the absence of bromide,2,4-DCP generated TCM and 1,1-DCA;with the presence of bromide,TCM,1,1-DCA and Br-THMs were generated.2,4-DBP generated TCM,Br-THMs,1,1-DCA and 1,1,1-TCA.In UV/chlorination,the increment of chlorine dosages and pH values increased the total production of low-molecular-weight DBPs from 2,4-DCP and 2,4-DBP.The presence of bromide promoted the conversion from halogenated phenols to Br-THMs and the major brominated product was BDCM.