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.

Solar photocatalytic degradation of very small amounts of the pollutant herbicide diquat in water in presence of ZnO catalyst is investigated. ZnO is found to be an effective photocatalyst for the decontamination of water in presence of sunlight thus indicating its potential for achieving the twin objectives of solar energy harvesting and wastewater treatment. The influence of various parameters such as catalyst dosage, initial concentration of diquat, pH, O2 and H2O2 on the rate of degradation is investigated and optimised. The degradation follows pseudo first order kinetics. Externally added H2O2 inhibits the degradation initially though the effect becomes less significant as the reaction progresses. The degradation is slow in deaerated reaction systems and extra O2 introduced into it externally enhances the degradation. The study clearly shows that solar photocatalysis using ZnO as the catalyst can be used as an effective tool for the tertiary treatment of diquat contaminated water and the pollutant gets completely mineralised in the process.