Efficiency And Mechanism Of VUV/sulfite System Towards Disinfection By-products In Water

Disinfection is often accompanied by the generation of disinfection by-products（DBPs）.It is necessary to remove DBPs produced in the process of disinfection for their wide detection and the toxicity.Monochloroacetic acid（MCAA）is often generated in the process of chlorine disinfection,while bromate is generated in the ozone disinfection.The removal rate of MCAA is low and other problems exist in the conventional treatment process and bromate cannot be removed by oxidation basically.Advanced reduction processes have great advantages,such as low toxicity,high efficiency,to generate e^-_aq,H·and other reducing radicals,and could efficiently reduce target pollutants.This paper presents a VUV/sulfite system,which can efficiently degrade a variety of DBPs in water.In this thesis,the efficiency and mechanism of degradation of DBPs by the VUV/sulfite system were studied with MCAA and bromate as target pollutants.The degradation rate of MCAA and bromate are up to 13.08μM/min and 6.36μM/min under the conditions of p H=9.0,25℃and[sulfite]₀=1 m M in the VUV/sulfite system.The effects of solution p H,sulfite concentration,initial concentration of target pollutants,and environmental condition on the degradation efficiency of MCAA and bromate by the VUV/sulfite system were investigated.Increasing the p H,sulfite concentration,temperature,and circulating flow rate in the reaction system can increase the degradation effect of target pollutants in the system.The increased initial concentration of target pollutants and environmental conditions,such as Cl^-,HCO₃^-,NO₃^-and humic acid,inhibit the degradation of MCAA and bromate by VUV/sulfite system.Two batch experiments were conducted in tapwater to investigate the degradation efficiency of MCAA and bromate by the VUV/sulfite system.The degradation efficiency of MCAA and bromate in tapwater was lower than that in pure water.The degradation of MCAA and bromate by the VUV/sulfite system is mainly via the reduction pathways with e^-_aq as the dominant reactive species.The reaction rates of MCAA with e^-_aq are 1.9×10⁹ M^-1 s^-1.Depending on the properties of the target pollutants,·OH in water may also contribute to the formation of advanced reduction-oxidation processes.The presence of·OH also provides the possibility of further degradation and mineralization of intermediate substances in the system,which makes up for the shortcoming of the partial reduction systems.the reaction rates of MCAA.The reaction rates of bromate with e^-_aq are 3.7×10⁹ M^-1 s^-1,and reduction reaction was dominant while·OH showed some inhibition.We explored the degradation and dehalogenation efficiency of seven haloacetic acid（HAAs）in the VUV/sulfite system.The degradation and dehalogenation rates of seven HAAs follow the order of Br>Cl>>F;From the point of halogenation degree,both degradation and dehalogenation rates follow the order of Tri>Di>Mono,except chloro-substituted halogenated acetic acid,whose reaction rate is Tri<Di<Mono.The generation and transformation pathways of free radicals in the VUV/sulfite system were proposed,and the degradation pathways of MCAA and bromate were explored.In the degradation of MCAA,most of the MCAA was converted to acetic acid,and formic acid in sequence,and finally to CO₂.Bromate is reduced to Br O₂^-,Br O^-,and Br^-successively.In a word,the VUV/sulfite system displayed satisfying performance on the removal of HAAs,and bromate,indicating it is a promising technique in real application.