Photodegradation kinetics, products and mechanism of timolol under simulated sunlight

► The indirect degradation of timolol is first investigated in fulvic acid solution. ► 3FA* and 1O2 accounted for the degradation of timolol in the aerated FA solutions. ► The presence of halides inhibited the degradation in the order of Cl− < Br− < I−. ► The role of I− in the degradation was first found to be concentration-dependent. ► The photoproducts of timolol were identified by LC-DAD/ESI-MS/MS analysis.