Study On The Formation Mechanism Of Trihalomethanes During Chlorination Disinfection And The Effect Of Metal Ions On It

Due to the substandard discharge of industrial wastewater and seawater intrusion,the concentration of anions such as Br^-,I^-and cations such as Pb²⁺,Mn²⁺,Ca²⁺and Mg²⁺in natural water increased.These inorganic ions affect the formation of disinfection by-products（DBPs）in the process of disinfection.Then,in this study,fulvic acid（FA）and model compounds（resorcinol,2,6-dihydroxybenzoic acid and citric acid）were used as the model precursors.the effects of Pb²⁺,Mn²⁺,Ca²⁺,Mg²⁺)and anions（Br^-,I^-）on trihalomethanes(THMs,including bromochlorinated trihalomethanes（THM₄）and trihalomethanes iodides（I-THMs）were investigated.The main contents and results of this paper are as follows:1.The effects of different factors on THMs,n（Br）and n（I）in the chlorination process of FA were studied.It was found that higher chlorine dosage,higher Br^-concentration,lower I^-concentration and higher Br/I ratio improved the formation of THM₄ and inhibited the formation of I-THMs.At different chlorine dosages,the higher pH resulted in the formation of THM₄ and I-THMs.Compared with chlorine disinfection,the production of THM₄ decreased,but I-THMs increased during chloramine disinfection.During the chlorination of FA,when the Br/I ratio changeed in a specific concentration range,the increase of Br/I ratio led to the increase of n（Br）and the decrease of n（I）.Under different chlorine dosage,n（Br）increased at first and then decreased with the higher pH.The trend of n（I）with pH was changed with different chlorine dosage.2.The study investigated the formation of THMs during the chlorine/chloramination reaction of model compounds（resorcinol,2-dihydroxybenzoic acid and citric acid）.When the carboxyl substituent located in the m-position of phenolic hydroxyl group,it was beneficial to the decarboxylation and hydrolysis reaction during chlorination,which led to the formation of a large amount of THMs.During chloramine disinfection,m-phenolic hydroxyl groups were more likely to be replaced by halogens,and promoted the generation of THMs.According to the change of model compound THMs with Cl₂/N,it was considered that the carboxyl structure on the aliphatic chain was responsible for the effect of Cl₂/N on the THMs’formation during chlorination.Combined with the THMs changes and UV absorption spectra of resorcinol and 2,6-dihydroxybenzoic acid,it was found that the formation of THMs through the way of destroying the unsaturated structures in organic compounds during chlorination and chloramination.3.The effects of metal ions on THMs,n（Br）and n（I）in the chlorination process of FA under different pH conditions were investigated.The presents of Pb²⁺and Mn²⁺promoted the formation of THMs under acidic condition,while inhibited the formation of THMs under alkaline condition.Ca²⁺could inhibit the formation of THMs in acidic and alkaline conditions,and the concentration of THMs in neutral condition increased at first and then decreased with the increase of Ca²⁺concentration.Under the above three pH conditions,the effects of Mg²⁺on THM₄ were promoted at low concentration and inhibited at high concentration.Under acidic conditions,the effect of Mg²⁺on I-THMs was also promoted at low concentration and inhibited at high concentration,while I-THMs was inhibited under neutral and alkaline conditions.Under acidic and neutral conditions,the presence of Pb²⁺increased the value of n（Br）,while decreased n（Br）under alkaline conditions.The effect of Mn²⁺on n（Br）was opposite to that of Pb²⁺.The effect of Ca²⁺and Mg²⁺on n（Br）varied with ion concentration.In addition,under the three pH conditions,the value of n（I）was directly proportional to the concentration of Pb²⁺,and inversely proportional to the concentration of Mn²⁺,Ca²⁺and Mg²⁺.