Effects And Mechanism Research Of Ammonium Concentration On Breakpoint Chlorination Process

At present,excess ammonium,accompanied by emerging organic pollutants(EOCs),still exists in Chinese aquatic environment and seriously restricts the safe supply of drinking water.In recent years,many studies have found that breakpoint chlorination process can realize effective removal of ammonium,and at the same time,produce a variety of highly reactive oxidants,including HO’,reactive chlorine species(e.g.,Cl·,Cl2·-and CIO·)and reactive nitrogen species(RNS)to degrade EOCs,including those pollutants that are refractory to chlorine).However,as a common co-existing substance in water,ammonium varies greatly in concentration in aqueous.Previous studies have also found that the change of initial ammonium concentration affected the chlorine/nitrogen mass ratio(Cl/N)at breakpoint and pollutant removal in the breakpoint chlorination process.Therefore,the influential mechanism of ammonium concentration on the chlorine-nitrogen ratio at the breakpoint and the distribution of reactive species in the breakpoint chlorination process are still unclear and deserve further study.In addition,previous studies that investigated the pollutant degradation in breakpoint chlorination process usually focused on a single pollutant,and the concentration was significantly higher than the actual concentration in drinking water treatment(ng/L-μg/L),which provided limited reference for the practical application of breakpoint chlorination process.Therefore,it is of more practical significance to explore the degradation efficiency of various EOCs in breakpoint chlorination process under different ammonium concentrations.This study measured the Cl/N ratio reached to breakpoint in breakpoint chlorination process under different ammonium concentration.Results showed that the CI/N ratio of achieving breakpoint decreased from 11.0 to 9.7 with elevating ammonium concentration from 0.5 to 2.0 mg-N/L,Multi-adding mode of chlorine experiments suggested that the scavenging effect of chloramines on reactive species was more significant than that of chlorine,and the degradation of pollutants was related to the transformation of chloramines.Radical scavenging experiments showed that RNS and Cl2·-affected most on breakpoint formation.Consequently,the mechanism of the shift of Cl/N ratio at breakpoint under the higher ammonium concentration could be summarized as:with the initial ammonium concentration increasing,the chloramine formation was accelerated and the scavenging of reactive species,especially RNS and Cl2·-,by chloramine was dramatically enhanced.Hence.less free chlorine was consumed by the reactive species and majority of chlorine was prone to transform ammonium/chloramine.leading to the decrease of Cl/N ratio of reaching breakpoint.In breakpoint chlorination process,the removal of pollutants enhanced with the increasing ammonium concentration.At a certain concentration of ammonium,the degradation efficiency increased a lot with increasing the Cl/N ratio reached to breakpoint.The distribution of reactive species in breakpoint chlorination process showed that the concentrations of HO’,RCS,and RNS were significantly enhanced as the Cl/N ratio increased to breakpoint,which was ascribed to the decrease of scavenging effect on reactive species by chloramines.As the ammonium concentration increased by three times,the concentration of Cl2·-and RNS at breakpoint increased around ten folds,which was attributed to the enhanced transformation effect on reactive species by chloride and in situ formed chloramines under higher ammonium concentration.This study developed the quantitative structure-activity relationships to reveal that ClO·is reactive to compounds substituted with electron-rich groups(e.g.,methoxy-).To better evaluate the application potential of breakpoint chlorination process under actual conditions,this study selected 12 EOCs of diversity structures to explore the degradation efficiency of broad-spectrum EOCs at actual concentration in the breakpoint chlorination process.Results displayed a satisfactory removal of selected EOCs in breakpoint chlorination process.The removal rate of EOCs could be predicted effectively through kinetics model of pollutant degradation,and the error was mainly due to the choice of reaction rate constant of pollutant with radical.Besides,this study emphasized that the contribution of RNS to the degradation of pollutants in breakpoint chlorination process should not be ignored and should be further explored.