Insights into the mechanisms of natural organic matter on the photodegradation of indomethacin under natural sunlight and simulated light irradiation

•Degradation of INDO was studied under natural and simulated irradiation.•Local DOM increases the photolysis of INDO by 30% than commercial DOM.•The contribution of 3DOM* in INDO photodegradation exceeds 70% in DOM solution.•DFT calculation further confirms the degradation pathway of INDO.•Indirect photolysis products in DOM are more toxic to aquatic organisms.