ABSTRACT
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]â¯=â¯0.2⯵M, [free chlorine]â¯=â¯4⯵M, and pHâ¯=â¯4) for sunlight/FC and sunlight photolysis are 7.0⯱â¯1.1â¯×â¯10-2â¯min-1 and 1.4⯱â¯0.2â¯×â¯10-2â¯min-1, respectively. The improved methadone degradation can be attributed to the production of HO and reactive chlorine species (RCS) during sunlight/FC photolysis. HO 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⯵M) significantly influenced the overall degradation efficiency of methadone. The presence of HCO3-, Cl- and dissolved organic matters, which may competitively react with HO 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.