New Insights into Micropollutant Abatement in Ammonia-Containing Water by the UV/Breakpoint Chlorination Process
Ehsan Aghdam, Yingying Xiang, Li Ling, Chii Shang
Abstract
Ultraviolet (UV)/chlorine processes are emerging advanced oxidation processes (AOPs) for micropollutant abatement. When excessive chlorine (Cl/N mass ratio of >7.6) is applied to ammonia-containing water to yield free chlorine residual for the UV/free chlorine AOP, the AOP becomes UV/breakpoint chlorination. This study investigated micropollutant degradation by and radical yields from UV/breakpoint chlorination under various conditions using benzoic acid (BA) and nitrobenzene (NB) as representative micropollutants and radical probes. Two stages of degradation were observed. During stage I, BA and NB were rapidly degraded; this was attributed to the reactive species generated from breakpoint chlorination (Rbrk) and UV photolysis of chlor(am)ines and breakpoint chlorination intermediates/products (Rpho). During stage II, the degradation followed pseudo-first-order kinetics, and UV photolysis of unknown irreducible residuals produced from breakpoint chlorination greatly contributed to generating reactive species, in addition to UV photolysis of free chlorine. At pH 7 and an ammonia concentration of 1 mg/L as N, an optimum chlorine dosage for micropollutant degradation existed at 9.6 mg/L as Cl2 due to the strong scavenging from excessive free chlorine. The findings demonstrate that UV/breakpoint chlorination is a promising approach for degrading refractory micropollutants in real-world applications.