Mechanistic study on chlorine/nitrogen transformation and disinfection by-product generation in a UV-activated mixed chlorine/chloramines system

Abstract

The conversion mechanisms of chlorine species (including free chlorine, monochloramine (NH2Cl), dichloramine, and total chlorine), nitrogen species (including ammonium (NH4+), nitrate (NO3−), and nitrite (NO2−)) as well as the formation of disinfection by-products (DBPs) in a UV-activated mixed chlorine/chloramines system in water were investigated in this work. The consumption rates of free chlorine and NH2Cl were significantly promoted in a HOCl/NH2Cl coexisting system, especially in the presence of UV irradiation. Moreover, the transformation forms of nitrogen in both ultrapure and HA-containing waters were considerably affected by UV irradiation and the mass ratio of free chlorine to NH2Cl. NO3− and NO2− can be easily produced under UV irradiation, and the removal efficiency of total nitrogen with UV was obvious higher than that without UV when the initial ratio of HOCl/NH2Cl was less than 1. The roles of different radicals in the degradation of free chlorine, NH2Cl and NH4+ were also considered in such a UV-activated mixed chlorine/chloramines system. The results indicated that OH• was important to the consumption of free chlorine and NH2Cl, and showed negligible influence on the consumption of NH4+. Besides, the changes of DOC and UV254 in HA-containing water in UV-activated mixed chlorine/chloramines system indicated that the removal efficiency of DOC (24%) was much lower than that of UV254 (94%). The formation of DBPs in a mixed chlorine/chloramines system was also evaluated. The yields of DBPs decreased significantly as the mass ratio of HOCl/NH2Cl varied from 1 : 0 to 0 : 1. Moreover, compared to the conditions without UV irradiation, higher DBPs yields and DBP-associated calculated toxicity were observed during the UV-activated mixed chlorine/chloramine process.