Revisiting the UV222/permanganate process: HO• and overlooked triplet-state intermediates drive benzoate derivative degradation.

The UV222 KrCl* excimer lamp outperforms traditional UV254 lamp in advanced oxidation processes (AOPs) due to higher photon energy, improved oxidant activation, and environmental safety. While previous studies primarily ascribed contaminant degradation in UV222/permanganate to HO• and reactive manganese species (RMnS), the role of substrate excited states remains largely underexplored. Here, we revisited benzoate derivative degradation mechanism in the UV222/permanganate process by elucidating the origins of HO• and the contribution of triplet-state benzoates. The degradation kinetics of terephthalic acid (TA) and 2-hydroxyterephthalic acid (2HTA) in UV222/permanganate were 2.1 -7.3 times higher than UV254/permanganate. Both HO• and triplet benzoate derivatives contributed to their degradation. HO• originated primarily from H2O (67.9%) and permanganate (22.5%). Meanwhile, triplet-state TA (3TA*) and 2HTA (32HTA*) reacted rapidly with permanganate at second-order rate constants (k) of 2.2 × 109 and 5.5 × 109 M⁻1s⁻1, respectively, followed by reactions of secondary intermediates (2HTA•+ and eaq-). Triplet-state-related pathways contributed 36.3% to 2HTA removal, and their roles were promoted under N2 purging. Even treating simulated drinking water, the synergistic effect of UV222 and permanganate was still significant. This study clarifies a novel degradation mechanism for benzoate derivative degradation in the UV222/permanganate process, emphasizing the overlooked roles of triplet benzoate derivatives.