Transformation Of Phenolic Endocrine Disruptors By Aqueous Chlorination And Exploration Of Main Reaction Mechanisms

Phenolic endocrine disrupting chemicals（EDCs）such as 2,4-dibromophenol（2,4-DBP）,bisphenol A（BPA）,4-chlorophenol（4-CP）and nonylphenol（NP）are the most commonly detected pollutants in natural environments.2,4-DBP can be processed into polymer intermediates,wood preservatives and flame retardant intermediates,and thus are usually released into natural waters through waste streams.Besides artificial synthesis,a large amount of bromophenolic pollutants（Br Ps）can be naturally formed in the marine environment.BPA is mainly incorporated into polycarbonate,epoxy resin,polysulfone resin and many other polymer materials.4-CP is commonly found in industrial wastewater effluents from the production of pesticides,paper and pulp,and petrochemical products.NP is widely used as a raw material for the manufacture of nonionic surfactants and lubricant additives.It has reported that phenolic EDCs can perform endocrine disrupting effects and cytotoxicity,which has received extensive attention.Chlorine is the most commonly used reagent in drinking water disinfection and municipal wastewater treatment.Since production and domestic wastewater are the main sources of phenolic EDCs discharged into the environment directly,it is important to study the transformation mechanism during chlorination process.Therefore,this paper systematically studied the degradation kinetics,conversion mechanism and degradation products of phenolic contaminants in the chlorination system in water environments.Specifically,the following three aspects are launched:（1）The transformation of 2,4-dibromophenol（2,4-DBP）during chlorination process was fully explored in this research.It was found that active chlorine can react with 2,4-DBP effectively in a wide p H range of 5.0-11.0,with an apparent second-order rate constant(k_app)varying from 0.8 M^-1·s^-1 to 110.3 M^-1·s^-1.The addition of 5 m M ammonium ions almost completely suppressed the reaction via competitive consumption of free chlorine.By contrast,bromide ions at a concentration of 5 m M accelerated the process by about 4 times,due to the formation of hypobromous acid.On the basis of the eleven products（with eight nominal masses）identified by LC-TOF-MS,electrophilic substitution reactions and single-electron transfer reactions were mainly involved in the chlorination process.The concentration of primary chlorine-substituted products was about 4 times that of the dimer products,demonstrating that electrophilic substitution reaction was predominant during chlorination of 2,4-DBP.Density functional theory（DFT）based calculations revealed that HOCl is the dominant active oxidizing species for elimination of 2,4-DBP and coupling reaction occurs more easily at para and ortho position of hydroxyl group in the phenolic moiety.（2）The chlorination process of sing4-CP,BPA and NP during water treatment was investigated.After 20 min of reaction,the removal efficiency of 4-CP（1.0μM）reaches34.2%,79.8%,95.5%and 100%at 10.0,15.0,30.0 and 60.0μM chlorine,respectively.The experiment found that chlorination reaction is a typical second-order reaction.In the subsequent kinetic experiments,the residual chlorine needs to be measured simultaneously to calculate the apparent second-order rate constant k_app.A maximum k_app value was observed at p H～8.5 for the three phenolic compounds.On the basis of LC-Q-TOF-MS analysis,the main chlorination reactions are also divided into two categories:（1）electrophilic substitution reactions leading to the chlorine-substituted products,and（2）single-electron transfer reactions leading to the polymers via radical coupling.Finally,the reaction mechanisms were elucidated by quantum chemical computations.The positive Gibbs free energy changes further demonstrate that HOCl is the major reactive species in the oxidation process of sodium hypochlorite.（3）The transformation of mixtures of the three phenolic pollutants during chlorination process was studied for the first time in this work.Single phenol can be effectively removed by free chlorine via electrophilic substitution and electron transfer reactions,with the generation of chlorinated phenols and self-coupling products.In mixture solution,the apparent second-order rate constants(k_app)of 4-CP,BPA and NP at p H 8.0 was increased by 8.6%,25%and 16%,respectively,as compared to the degradation of single-compound.It is interesting that the removal also followed the order of BPA>NP>4-CP in the mixture solution.According to the products identification and density functional theory calculations,the cross-coupling process can occur more easily than the self-coupling reaction,which might accelerate the removal of phenolic mixtures.In addition,humic acid（HA）exerts some inhibitory effect on the degradation of phenolic compounds,due to the competition for chlorine consumption and the reduction of phenoxy radicals to parent phenols.The correlation between the calculated k_app values and the HA concentrations was described by a two-channel kinetic model.These findings shed light on the environmental fate of phenolic EDCs during aqueous chlorination process,which provide new information for the application of chlorination in water and wastewater treatment.