Riboflavin-sensitized photooxidation of phenylurea herbicide monuron in aqueous peroxide solution

A natural photosensitizer riboflavin (Rf), known as vitamin B₂, and an environmental friendly strong oxidant hydrogen peroxide (H₂O₂) were employed for the riboflavin-sensitized photooxidation process (UV/Rf/H₂O₂) in this study. This approach could be considered as a hybrid process of UV/Rf (photosensitization) and UV/H₂O₂ (advanced oxidation). The photodecay of a phenylurea herbicide monuron was investigated by varying the doses of Rf and H₂O₂ in this work. The use of UV/Rf/H₂O₂ showed a promising performance for degrading monuron compared with the direct photolysis (UV), UV/Rf, and UV/H₂O₂ processes. It was found that more than 99% of the monuron was removed in the UV/Rf/H₂O₂ process in less than 70 min depending on the dosages of Rf and H₂O₂. In general, the overall improvement of UV/Rf/H₂O₂ process would achieve as high as ∼450% comparing to sole-UV process. In addition, a kinetic model for the UV/Rf/H₂O₂ process was developed successfully in this study to facilitate the prediction of the additional performance on the degradation of monuron comparing with the daughter processes (UV, UV/Rf, and UV/H₂O₂).