Elimination of chlorine-refractory carbamazepine by breakpoint chlorination: Reactive species and oxidation byproducts

Breakpoint chlorination can be commonly observed in the chlorination of water treatments when ammonia is present. In this study, it was found that breakpoint chlorination can remarkably eliminate a ubiquitous and chlorine-refractory micropollutant, carbamazepine (CBZ), with the removal of 72% at neutral condition. At neutral pH, low CBZ elimination was observed at a chlorine/ammonia molar ratio (Cl/N) of 1.0 and higher CBZ elimination was observed as Cl/N ratio increased from 1.0 to 1.6 (breakpoint), indicating that CBZ elimination was closely related to the generation and decomposition of chloramines. The chloramines generation and decomposition rates were affected by the pH, so that the CBZ elimination rate was highest at pH 7.0 and lower in acidic and basic solutions (pH 5.5 and pH 9.5, respectively). The CBZ elimination at pH 7.0 was 72.4% after 10 min of breakpoint chlorination, while reaction times about 30 min and 60 min were required to achieve the same elimination at pH 5.5 and pH 9.5, respectively. Breakpoint chlorination of CBZ was strongly suppressed by radical scavenger tBuOH and moderately suppressed by N2 purging, the inhibiting ratios being 87.7% and 27.8% at breakpoint, respectively. Electron spin resonance experiments suggested that unidentified radicals were generated by breakpoint chlorination. The OH and unidentified radical species contributions to CBZ elimination were <23.7% and >76.3%, respectively, when a pseudo steady state breakpoint chlorination was performed in a microinjection system with nitrobenzene as OH probe. Although CBZ were efficiently eliminated, breakpoint chlorination of CBZ generated adsorbable organic chlorine. The cytotoxicity of the CBZ solution was therefore increased by breakpoint chlorination, suggesting that biological risk caused by the breakpoint chlorination of micropollutants should be taken into consideration.