美沙酮於氯化消毒下之太陽光解 – 降解動力學及其對N-亞硝基二甲胺(NDMA)形成影響

Abstract

N-nitrosodimethylamine (NDMA) is a potent carcinogenic emerging disinfection byproduct (DBP) for humans at ng/L level. The presence of NDMA in drinking water is most commonly associated with chloramination of amine-based precursors. However, previous literature indicated that the high concentration of NDMA formed during the chlorination of swimming pool water (considered pools filled with drinking water). The formation of NDMA in swimming pool water mainly occurs through the reaction between free chlorine and natural organic matter, which can be derived from the pool water and swimmers. Swimmer activities in swimming pools release various substances (such pharmaceutical), the unintentional exposure to NDMA in swimming pool water is a particularly important area of concern. The aim of this study was the investigation and evaluation of the removal efficiency of pharmaceutical and associated NDMA formation during sunlight photolysis of free chlorine (sunlight/FC) and sunlight photolysis of chloramine (sunlight/chloramine) process. Additionally, the degradation kinetics, relative contributions of the reactive species, byproducts formation, change in toxicity, and effect of water quality parameters during the sunlight/FC or sunlight/chloramine process were comprehensively explored. This study selected methadone, a potential NDMA precursor, as a model compound and found that sunlight/FC markedly enhances the degradation rate of methadone, over that obtained using sunlight alone. The pseudo-first-order rate constants of methadone degradation under acidic conditions ([Methadone] = 0.2 μM, [Free chlorine] = 4 μM, and pH = 4) for sunlight/FC and sunlight photolysis are 7.0±1.1 × 10–2 min−1 and 1.4±0.2 × 10–2 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 μM) 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. Sunlight/chloramine enhances the degradation of methadone and reduces NDMA formation compared to those under sunlight or chloramination processes alone. The degradation of methadone under all investigated sunlight/chloramine conditions followed pseudo-first-order kinetics, and the degradation rate constant increased with the dosage of dichloramine (NHCl2). The highest conversion of NDMA from methadone occurred at a chlorine-to-ammonia (Cl2/N) of 1.5, and the NDMA formation increased with increasing pH (pH = 4−9.5). The presence of nitrate further decreased NDMA formation since the production of nitrite consumes monochloramine (NH2Cl) under sunlight irradiation. These results emphasize the need to consider different water matrices influencing methadone degradation and the formation of NDMA in chlorinated or chloraminated waters under natural sunlight, as is found in oxidation ponds post-chlorination or in swimming pools.