Study On Mechanism Of Formation Of NDMA From Impurities In PDMDAAC

The surface water,i.e.source or raw water in China is generally polluted,among which tiny polluted raw water accounts for a high proportion.Chloramine as a disinfectant is not generally used in the raw water treatment in China,however,chlorine is usually used as a pre-chlorination agent and disinfectant.Nevertheless,chloramine might be produced,when adding chlorine in the tiny polluted raw water which contains ammonia nitrogen as one of pollution components.The produced chloramine might interact with dimethylamine group-contained compounds existing in raw water or from added water treatment agents to form disinfection by-product NDMA.In the inorganic salt/PDMDAAC composite coagulant used for the enhanced coagulation treatment of tiny-polluted raw water,PDMDAAC is a polymer containing the dimethylamine groups,and furthermore some impurities contained in PDMDAAC like dimethylamine,dimethylallyl amine and dimethyl diallyl ammonium chloride from its monomer production.Therefore,it is necessary to study the possibility and regularity of the reaction between these impurities and chloramine to form NDMA.In this paper,the preliminary exploration experiments were carried on at first,and then the detection method of NDMA was verified,after that the lowest dosages of reactants to form detectable NDMA was measured within the detection limitation of NDMA.Finally,the kinetics and activation energy of the reaction of these impurities with chloramine to form NDMA were studied,respectively.It provided basic data to predicate the amount of NDMA generated.The main work accomplished in this paper was shown as follows.Firstly,the regulations of the reaction of impurities with a concentration of 3 mg/L with chloramine to form NDMA were studied,respectively.The results showed that different reaction conditions such as pH value and reactant concentration had certain effects on the formation amount of NDMA.In order to study the effects of the impurities on the NDMA detection method from literatures,the effects of the impurities on both HPLC and solid phase extraction detection were investigated,respectively.The results showed that there was no detected effect of the impurities on the solid phase extraction and HPLC detection processes of NDMA.since the detected recovery rate of NDMA was within 98.7%～99.2%.Secondly,in order to study the possibility of producing NDMA via the impurities in the near actual application dosage of PDMDAAC,the exploration experiment of lowest dosages of reactants,three impurities to form detectable NDMA within the detection limitation of NDMA were carried on,based on the actual process of treatment of tiny-polluted raw water by using the enhanced coagulation of composite coagulants in China.The results showed that the lowest dosage of adding the three impurities,i.e.dimethylamine hydrochloride,dimethyldiallylamine hydrochloride and dimethyldiallylammonium chloride to form NDMA with the detectable magnitude of 10 ng/L were about 0.25,0.50 and 1.0 mg/L,respectively.According to the situation of pre-chlorination in the enhanced coagulation of tiny-polluted raw water in China,the actualy used PDMDAAC dosage was around 2 mg/L,then the estimated amount of NDMA produced under the maximum adding dosages of the three impurities were much lower than the detectable magnitude of 10 ng/L.Finally,in order to more deeply understand the process of the impurities contained in PDMDAAC which reacted with chloramine to form NDMA,the reaction kinetics of the impurities with chloramine were studied,respectively.The dependence of reactant concentration on time were determined by using an isolation method,and then the reaction orders,rate constants,rate equations and activation energys of three kinds of impurities reacted with chloramine were obtained by using a tentative method,respectively.The results showed that the reaction orders of the impurities with chloramine were all first order reaction,and that of chloramine were also first order,too.The rate constants of dimethylamine hydrochloride,dimethylallylamine hydrochloride and dimethyl dially ammonium chloride with chloramines were 7.4012×10-3,1.9811×10-3 and 1.5601×10-3 mol-1·min-1.respectively at 25 ℃,5.3023×10-3;2.8213×10-3 and 1.1434×10-3 mol.1·min-1,respectively at 15 ℃,1.3385×10-3,5.1480×10-4 and 2.1879×10-4 mol-1·min-1 at 5 ℃,respectively.The activation energy of the reaction of dimethylamine hydrochloride and dimethylallylamine hydrochloride with chloramine were 51.27 and 53.57kJ/mol,respectively.All above accomplished work could be the powerful experimental foundation for application of the composite coagulation containing PDMDAAC in enhanced coagulation of the tiny polluted raw water.