Vacuum UV-based processes for water and wastewater purification: From unitary to multicomponent systems.

Vacuum ultraviolet (VUV) is profitable to strengthen the efficiencies of UV and reduce chemicals use, attracting more attention to water and wastewater purification. Herein, VUV-based water treatment processes from unitary VUV to multicomponent systems were reviewed for the first time to promote VUV applications. The rate of pollutant removal by unitary VUV was 1.3-57 times that of UV, in which hydroxyl radical oxidation was dominant. And the reducibility of hydrated electron and hydrogen atom radical in unitary VUV dehalogenated organics and reduced metal ions. Besides, VUV-based binary systems mainly included processes of VUV/HO, VUV/persulfate, VUV/ozone, VUV/chlorine, VUV/sulfite, VUV/iron ion, and VUV-based heterogeneous oxidation. VUV-based ternary systems basically contained three types: VUV-based Fenton-like, VUV coupling dual oxidants, and VUV combined with other technologies activating oxidants. Performance, characteristics, reactive species, and mechanisms of VUV-based binary and ternary systems were summarized. Moreover, the characterization, contribution, and role of reactive species in VUV-based processes were analyzed, and the combination of multiple methods was conducive to accurately identifying the mechanism of reactive species. Furthermore, the combination of VUV and other technologies expanded the application potential of VUV. Compared to UV-based processes, VUV-based processes significantly reduced energy consumption and were more promising in removing contaminants in actual waters. Finally, hot spots and directions (develop new techniques, reduce by-products, combine simulation and experiment, broaden removal objects, enhance pilot studies) of VUV-based water treatment technologies in future were prospected. Overall, VUV-based advanced oxidation processes are expected to be used in water treatment to improve process efficiency.