Evolution of reactive species and their contribution to the removal of ketamine and amine-containing pharmaceuticals during the sunlight/chlorine process

Abstract

Sunlight-induced photoirradiation of chlorine (sunlight/chlorine) can be observed in outdoor swimming pools and open-channel disinfection units for wastewater treatment. In this study, the degradation of ketamine, an environmentally persistent pharmaceutical, under sunlight irradiation in the presence of a low concentration of chlorine (1 mg/L as Cl2) was investigated to elucidate the evolution of reactive species and their contribution to ketamine removal. •OH dominates the initial stage of sunlight/chlorine; however, after chlorine depletion, reactions still progress with an observed rate constant (kobs = 7.6 ± 0.50 × 10−3 min−1) an order of magnitude higher than photolysis alone (kobs = 2.9 ± 0.15 × 10−4 min−1). When chlorine is depleted, O3 becomes the major reactant that degrades ketamine. High O3 yields were found in both sunlight/HOCl (12.5 ± 0.5% at pH 5) and sunlight/ClO− (10 ± 1% at pH 10) systems. At sub-µM levels, O3 resulted in substantial removal of ketamine, and even faster rates were observed in the presence of sunlight. A kinetic model was also established, and evaluate time-dependent concentration levels during sunlight/chlorine. The model simulation showed that the cumulative O3 concentration could reach 0.91 μM, and O3 contributed 31% ketamine removal during the sunlight/chlorine process. Primary and secondary amine functional groups were demonstrated to be the reaction sites of O3; other pharmaceuticals, such as atenolol and metoprolol, underwent similar phenomena. In addition, the experimental and model results further indicated that sunlight/ClO2− or ClO2 also participates in the degradation of ketamine with a minor role; trace amounts (below nM level) of ClO2− and ClO2 were estimated by the simulation.