Efficient permanganate activation under UV 222 nm irradiation for enhanced pollutant abatement

As an emerging advanced oxidation process (AOP), the ultraviolet (UV)-driven permanganate [Mn(VII)] activation process has drawn increasing attention in water treatment. Hydroxyl radical (HO^•) and various reactive manganese species (RMnS) were simultaneously generated, resulting in enhanced pollutant abatement. However, Mn(VII) activation efficiency by the current UV lights was quite limited, with unsatisfactory apparent quantum yields of Mn(VII) (Φ_{app, Mn(VII)} < 0.25 mol/Einstein). Recently, the krypton chloride (KrCl*) excimer lamp, which emits UV light mainly at 222 nm (UV₂₂₂), has received growing interest due to its superior photon energy, safety, environmental friendliness, and lifespan compared with currently applied UV lamps. Herein, the KrCl* excimer lamp was applied to activate Mn(VII) for the first time. Φ_{app, Mn(VII)} was determined to be as high as 0.65 mol/Einstein in UV₂₂₂/Mn(VII). The fluence-based pseudo-first-order reaction rate constants of target pollutants (flumequine and 4-hydroxybenzoic acid) in UV₂₂₂/Mn(VII) were several to thousands of times higher than those in other UV/Mn(VII) AOPs reported in literature, suggesting significantly higher efficiency of UV₂₂₂ in Mn(VII) activation. Compared with HO^•, RMnS played more significant roles in pollutant abatement. The total contributions of RMnS to pollutant abatement were approximately 60-70% under pH 4.0-9.0, while the contributions of HO^• were around 10-25%. Moreover, hypomanganate [Mn(V)] was important RMnS responsible for the abatement of two pollutants, while the role of trivalent manganese [Mn(III)] was limited based on experimental and computational results. Pollutant abatement was inhibited with the increase of pH, while was promoted with the increase of light intensity and Mn(VII) concentration. Furthermore, water matrix components (chloride, bicarbonate, and humic acid) showed negligible or slight influences on pollutant abatement due to the selective oxidation features of RMnS. This study demonstrates the superior potential of the UV₂₂₂/Mn(VII) AOP in water treatment.