Toward better understanding vacuum ultraviolet—iodide induced photolysis via hydrogen peroxide formation, iodine species change, and difluoroacetic acid degradation

Recently, a photochemical process induced by ultraviolet (UV), vacuum UV (VUV), and iodide (I−) has gained attention for its robust potential for contaminant degradation. However, the mechanisms behind this process remain unclear because both oxidizing and reducing reactants are likely generated. To better understand this process, this study examined the evolutions of hydrogen peroxide (H2O2) and iodine species (i.e., iodide, iodate, and triiodide) during the UV/VUV/I− process under varying pH and dissolved oxygen (DO) conditions. Results show that increasing DO in water enhanced H2O2 and iodate production, suggesting that high DO favors the formation of oxidizing species. In contrast, increasing pH (from 6.0 to 11.0) resulted in lower H2O2 and iodate formation, indicating that there was a decrease of oxidative capacity for the UV/VUV/I− process. In addition, difluoroacetic acid (DFAA) was used as an exemplar contaminant to verify above observations. Although its degradation kinetics did not follow a constant trend as pH increases, the relative importance of mineralization appeared declining, suggesting that there was a redox transition from an oxidizing environment to a reducing environment as pH rises. The treatability of the UV/VUV/I− process was stronger than UV/VUV under pH of 11.0, while UV/VUV process presented a better performance at pH lower than 11.0.