Sunlight induced photocatalytic degradation of herbicide diquat in water in presence of ZnO

•ZnO as a solar photocatalyst for efficient removal of diquat from water.•H2O2 inhibits the photocatalytic degradation of diquat over ZnO slightly.•O2 is essential for the photocatalytic degradation of diquat over ZnO.•Photocatalytic degradation of diquat over ZnO follows variable kinetics.•Identified major intermediates in the photocatalytic mineralization of diquat.