Efficacy of vacuum ultraviolet photolysis for bromate and chlorate removal

We introduce vacuum ultraviolet (VUV) photolysis at 172 nm as a more efficient process for bromate and chlorate removal than conventional ultraviolet (UV) photolysis at 254 nm. We discuss the effects of pH and coexisting salts on VUV photolysis. In experiments at various pH levels, the VUV photolysis performance decreased in the alkaline region for pH above 7. Furthermore, nitrate and inorganic carbon compromised the performance, probably owing to the generation of active oxidant species (such as hydroxyl radicals and carbonate radicals) during the photolysis. However, photochemical experiments conducted in pure water revealed that VUV photolysis is 4.1-fold more energy efficient than UV photolysis using a low-pressure mercury lamp. Although the performance of VUV photolysis in tap water was about 40% less efficient than that in pure water, mainly owing to the presence of bicarbonate ions, the performance was still higher than that of UV photolysis in pure water. However, VUV photolysis may not be feasible in tap water with a high concentration of nitrate ions, because nitrate produces a stronger inhibition effect than other salts.