Mineralization, characteristics variation, and removal mechanism of algal extracellular organic matter during vacuum ultraviolet/ozone process

Abstract

Extracellular organic matter (EOM) produced by algal blooms in source water is detrimental to drinking water treatment processes and supplied water quality. Ozonation has been used to treat algal EOM, but it could not mineralize EOM effectively. In this study, mineralization and characteristics variation of EOM by vacuum ultraviolet/ozone (VUV/O3) and its sub-processes were comprehensively investigated. Results showed that EOM removal in different processes followed the order of VUV/O3 > UV/O3 > O3 > VUV > UV. For VUV/O3 process, removal efficiencies of dissolved organic carbon (DOC), UV254, protein, and polysaccharide at 50 min were 75.6%, 80.8%, 80.1%, and 78.0%, respectively, and fluorescence components received very high removal rates (≥92.8%, at 10 min). The yield of trichloromethane dropped from 102.0 to 30.1 μg/L after treating for 50 min by VUV/O3. Besides, effects of O3 dosage, initial pH, and water matrices on EOM removal in VUV/O3 process were investigated. Moreover, fluorescent molecular probe experiments confirmed that hydroxyl radical and superoxide radical were the main reactive oxygen species (ROS) in VUV/O3 process, and the transformation of ROS was proposed. The mechanism of EOM removal by VUV/O3 included VUV photolysis, direct O3 oxidation, and ROS oxidation. Furthermore, the removal of EOM in filtered water by VUV/O3 was satisfactory. All results indicated that VUV/O3 process had great application potential in treating EOM-rich filtered water.