Degradation of methadone by the sunlight/FC process: Kinetics, radical species participation and influence of the water matrix

Free chlorine sunlight photolysis (sunlight/FC) markedly enhances the degradation rate of methadone, a synthetic opioid used medically, over that obtained using sunlight alone. The pseudo-first-order rate constants of methadone degradation under acidic conditions ([methadone] = 0.2 mu M, [free chlorine] = 4 mu M, and pH =4) for sunlight/FC and sunlight photolysis are 7.0 + 1.1 x 10(-2) min(-1) and 1.4 +/- 0.2 x 10(-2) min(-1), respectively. The improved methadone degradation can be attributed to the production of HO center dot and reactive chlorine species (RCS) during sunlight/FC photolysis. HO center dot and RCS predominantly accounted for degradation during sunlight/FC photolysis under acidic and neutral conditions, while direct photolysis was the major contributor towards methadone degradation in alkaline conditions. The initial pH (pH 4-11) and free chlorine concentration (1-6 mu M) significantly influenced the overall degradation efficiency of methadone. The presence of HCO3-, Cl- and dissolved organic matters, which may competitively react with HO center dot and RCS, retard the degradation of methadone in synthetic wastewater. Consequently, a 50% lower methadone degradation rate was observed when deionized (DI) water was replaced with tap water. These results emphasize the need to consider different water matrices when applying sunlight/FC photolysis for water treatment. (C) 2018 Elsevier Ltd. All rights reserved.