Roles of radical species in vacuum-UV/UV/peroxydisulfate advanced oxidation processes and contributions of the species to contaminant degradation at different water depths

Abstract

Vacuum-UV (VUV) (wavelength 185 nm)/ UV (wavelength 254 nm) are applied to improve performances of UV-based advanced oxidation processes. However, the improvements were strongly affected by water depth because of poor VUV transmittance in water. In this study, VUV/UV and peroxydisulfate (PDS) were used to degrade carbamazepine. More SO4•− oxidation occurred in VUV/UV/PDS than VUV/UV with similar •OH oxidation occurring. The additional SO4•− oxidation could be caused by VUV/PDS in superficial water or UV/PDS in deeper water. The synergistic factor for VUV/UV/PDS processes relative to VUV/UV and UV/PDS processes was 1.32. VUV/UV/PDS performances were affected by competition for photon absorption by dissolved organic matter (32–58 % inhibition), radical quenching by CO32−/HCO3− and NO3−, and conversion of •OH and SO4•− into reactive chlorine species by Cl−. Radical probe experiments and steady-state kinetic modeling simulations indicated that 34 %, 25 %, and 40 % of carbamazepine degradation occurring in 2-cm-deep bulk solution was due to •OH oxidation through VUV/H2O, SO4•− oxidation through VUV/PDS, and SO4•− oxidation through UV/PDS, respectively. Contribution of VUV-driven processes decreased with increasing water depth and became equivalent to contribution of 3.5-cm-deep UV-driven processes, which indicated the importance of optimizing water depth in VUV/UV-advanced oxidation process reactors.