Mechanistic insight into the environmental fate of highly concerned transformation products of aqueous micropollutants during the solar/chlorine treatment

Transformation products (TPs) arising from the degradation of micropollutants have been frequently detected in various water bodies and may exhibit higher toxicity than their parent compounds. However, the current understanding of their chemical reactivity remains limited, and the mechanisms underlying the solar-driven oxidation processes (e.g., solar/chlorine system) of TPs have not been well investigated. This study explored the elimination of six typical TPs derived from carbamazepine (CBZ) and atrazine (ATZ) by solar/oxidant systems. It was observed that these TPs could be effectively degraded in the solar/oxidant systems, except for the solar/hydrogen peroxide system. The reactivity evaluation and quantitative contribution analysis revealed that hydroxyl radicals (^•OH) and ozone played pivotal roles in the removal of all six typical TPs by the solar/chlorine system, whereas the reactive chlorine species contributed minimally. The transformation mechanisms of carbamazepine 10, 11-epoxide (CBZ-EP) involved hydroxyl addition and electron transfer, while the TPs of ATZ underwent dealkylation only. The computational study indicated that ^•OH primarily reacted with CBZ-EP via radical addition reaction. Furthermore, the TPs of CBZ-EP and hydroxyatrazine showed no obvious change in environmental persistence but enhanced mobility and toxicity compared to the parent compounds, implying treatment-driven secondary risks. Overall, this investigation provided an in-depth mechanistic exploration of the transformation behaviors, fate, and secondary environmental risks of highly concerned TPs under the solar/oxidant treatments.