Enhanced co-removal of thallium and ammonium nitrogen via breakpoint chlorination

Hypochlorite (ClO − )-based chlorination has shown effectiveness in oxidizing and purifying ammonium nitrogen (NH 4 + -N) and thallium (Tl) individually. However, the co-removal of these contaminants remains understudied. Addressing this gap, our study investigated the behaviors and mechanisms of NaClO-based breakpoint chlorination in oxidizing and removing both Tl and NH 4 + -N. The interaction among thallous ions (Tl + ), NH 4 + -N and NaClO in wastewater created a synergistic oxidation effect, which led to high removal efficiencies, exceeding 97 % for both Tl and NH 4 + -N, achieved with a NaClO concentration of 7 mmol/L, across a pH range of 6 to 10 in less than 5 min. Notably, the presence of Tl + , rather than Tl 3+ or Tl 2 O 3 , significantly enhanced the conversion of hypochlorite alongside NH 4 + -N. This coupled process generated chloramines and nitroxyl, along with oxygen-independent hydroxyl radicals formation, which further transformed into chloride radicals, dichlorine radical anions, and chlorine oxide radicals. Introducing these highly reactive species into the binary-pollutant system marked a substantial improvement over single-pollutant systems, yielding significantly higher removal efficiencies. These findings shed new light on the role of Tl in breakpoint chlorination for wastewater containing both Tl and NH 4 + -N.