Degradation of metoprolol by UV/sulfite as an advanced oxidation or reduction process: The significant role of oxygen

•Metoprolol degradation was comparatively studied in the UV/sulfite process without and with oxygen.•Both eaq− and H• participated in metoprolol degradation by the UV/sulfite process without oxygen, while SO4•− prevailed in the case with oxygen.•eaq−/H• and SO4•− might share similar degradation mechanisms.•The toxicity of solution treated by the two UV/sulfite processes was estimated.