Abatement of algal extracellular organic matter by vacuum ultraviolet: Effect of dissolved oxygen and role of in-situ formed reactive oxygen species

Abstract

Algal extracellular organic matter (EOM) in water produced during the outbreak of algal blooms is difficult to be effectively removed by conventional water treatment processes. Compared with ultraviolet (UV), vacuum ultraviolet (VUV) can additionally radiate 185 nm photons, and VUV can split water to produce reactive oxygen species (ROS) in-situ. Herein, abatement and characteristic variation of EOM under VUV irradiation and the mechanism were investigated. Results indicated that EOM abatement by VUV irradiation were significantly higher than that by UV irradiation. The mineralization rate constant of EOM under VUV was 2.25 times of that under UV. Besides, under VUV irradiation, the abatement efficiency of EOM was promoted with the increased dissolved oxygen (DO), and EOM with a lower initial concentration had a higher abatement efficiency. Intensities of characteristic fluorescence peaks of EOM decreased obviously after VUV irradiation, and removal rates of specific fluorescent organic components were ≥ 74.4%. Under VUV irradiation, organic matter in EOM with higher molecular weight was transformed into that with lower molecular weight. Moreover, hydroxyl radical (•OH) generated in-situ by VUV dissociating water with different DO was directly identified by fluorescence molecular probe technology, and the higher DO caused the higher production of •OH. Furthermore, the mechanism of EOM abatement by VUV was proposed. DO and the produced ROS enhanced the abatement efficiency of EOM, and they played important roles in EOM abatement. This study disclosed abatement characteristics of EOM under VUV irradiation and provided great value for expanding the utilization of VUV in water treatment.