Study On The Formation And Coagulation Control Of Halobenzoquinone Precursors In Micro-polluted Water

Halobenzoquinones（HBQs）is a new class of uncontrolled disinfection by-products（DBPs）in drinking water.It has become a hot research topic because of its strong teratogenicity,reproductive and developmental toxicity and cytotoxicity.The low content of HBQs in drinking water is ng/L level,but the toxicity is much higher than that of conventional DBPs,which has a huge potential risk to drinking water safety.Therefore,the research on the formation of HBQs and the removal of its precursors by coagulation is of important theoretical and practical value to ensure the safety of drinking water.Firstly,humic acid was selected as the precursor model to investigate the effects of reaction time,chlorine content,pH value,temperature,bromide ion concentration and organic properties on the formation of dichloro-p-benzoquinone（DCBQ）,dibromo-p-benzoquinone（DBBQ）,2,3-dibromo-5,6-dimethyl-p-benzoquinone（DBDMBQ）andtetrabromo-p-benzoquinone（TBBQ）.Then,three different types of coagulants were used to carry out the corresponding coagulation tests for the water mixed with standard humic acid in the diversion yellow reservoir to study the differences in the removal of HBQs precursors in the coagulation process of aluminum,iron and titanium coagulants.Finally,according to the changes of turbidity,DOC,UV₂₅₄,Zeta potential and floc particle size in the coagulation process,the control effect of coagulation on HBQs was further analyzed by means of three-dimensional fluorescence spectrometry,liquid chroma to graphie-organic carbon detection（LC-OCD）and molecular weight distribution of organics.The relevant research results are as follows:（1）The results showed that the yield of DCBQ and DBBQ was higher than that of DBDMBQ and TBBQ.During the experiment,the effects of changes in chlorination conditions and precursor concentration on the formation of HBQs are as follows:With the increase of reaction time,the concentration of the four HBQs showed a tendency of increasing rapidly at first,then remaining stable and slightly decreasing,and the yield reached the maximum after the chlorination reaction for 48 hours.Increasing temperature increases the amount of HBQs.With the increase of bromide ion concentration,the amount of chloro-halobenzoquinone production gradually decreases,and the content of bromo-halobenzoquinone gradually increases.The weak acid condition is favorable for the formation of HBQs.In the range of 3-30 mg/L,the influence of the amount of four HBQs is different.The production of DCBQ and DBBQ increased with the increase of the chlorine dosage,while the production of TBBQ and DBDMBQ increased first and then gradually decreased with the increase of the chlorine dosage.With the increase of DOC,the production of HBQs increased gradually,and the unit organic HBQs with MW<1 KDa component had the highest potential.（2）The results show that based on the removal of DOM,the optimal dosages of PAC,FeCl₃and Ti（SO₄）₂ are 10 mg/L,30 mg/L,30 mg/L,respectively,and the optimal initial pH is pH=6,pH=7,pH=7,respectively.The removal effect of Ti（SO₄）₂ on DOM is better than PAC and FeCl₃.According to the changes of Zeta potential and floc particle size in the coagulation process,the PAC coagulation mechanism is mainly based on electrical neutralization,while FeCl₃ and Ti（SO₄）₂ have strong adsorption bridging and net capture capabilities,especially titanium salts.（3）Comparing the three coagulants,the removal efficiency of Ti（SO₄）₂ to HBQs precursors is about 1.2 times of FeCl₃ removal rate and 1.4 times of PAC removal rate.Before and after coagulation,the proportion of DCBQ,DBBQ,TBBQ and DBDMBQ in total HBQs in water was almost unchanged,with an average proportion of 47.5%,45.2%,1.5%and 5.8%,respectively.The removal efficiency of four HBQs precursors by coagulation was similar,and the removal effect of Ti（SO₄）₂ on HBQs was significantly better than that of PAC and FeCl₃.The removal effect of Ti（SO₄）₂ on disinfection by-product precursors was significantly better than that of PAC and FeCl₃.（4）According to three-dimensional fluorescence spectrum analysis,the average removal rates of PAC,FeCl₃ and Ti（SO₄）₂ for each fluorescent organic compound were 85.05%,87.36%,and 90.22%,respectively.Coagulation has the best removal effect on humic acid and tyrosine components,with average removal rates as high as 90.77%and 90.60%.From the liquid chromatography-organic carbon detection spectrum,it can be seen that the removal efficiency of FeCl₃ and PAC for total DOC is similar,53.0%and 51.6%,respectively,while the removal efficiency of Ti（SO₄）₂ is 66.9%,and Ti（SO₄）₂ has a good removal effect for all organic components,especially for low molecular weight organics.The results of molecular weight distribution of organics before and after coagulation show that the removal efficiency of high molecular weight organics is better than that of low molecular weight organics.For the removal of low molecular weight organics,the removal rate of Ti（SO₄）₂ was significantly higher than that of PAC and FeCl₃.（5）The SUVA value of raw water is 3.48,and the SUVA values of effluent from FeCl₃,PAC,and Ti（SO₄）₂ are 1.26,1.36,and 1.22,respectively,which have a certain inhibitory effect on the production of disinfection by-products.Coagulation is effective and feasible for the removal of HBQs precursors in drinking water,but the effect of coagulation on the removal of HBQs precursors is not ideal.Under the optimal coagulation conditions,three coagulants for HBQs precursors removal rate is only between 31%and 43%.