Reactive\nOxygen Species Generated in Situ During Carbamazepine\nPhotodegradation at 222 nm Far-UVC: Unexpected Role of H<sub>2</sub>O Molecules

When\n222 nm far-UVC is used to drive AOPs, photolysis emerges as\na critical pathway for the degradation of numerous organic micropollutants\n(OMPs). However, the photodegradation mechanisms of the asymmetrically\npolarized OMPs at 222 nm remain unclear, potentially posing a knowledge\nbarrier to the applications of far-UVC. This study selected carbamazepine\n(CBZ), a prevalent aquatic antiepileptic drug that degrades negligibly\nat 254 nm, to investigate its photodegradation mechanisms at 222 nm.\nAccelerated CBZ treatment by 222 nm far-UVC was mainly attributed\nto in situ ROS generation via self-sensitized photodegradation of\nCBZ. By quenching experiments and EPR tests, •OH radicals were\nidentified as the major contributor to the CBZ photodegradation, whereas\nO<sub>2</sub><sup>•–</sup> played a minor role. By deoxygenation\nand solvent exchange experiments, the H<sub>2</sub>O molecules were\ndemonstrated to play a crucial role in deactivating the excited singlet\nstate of CBZ (<sup>1</sup>CBZ*) at 222 nm: generating •OH radicals\nvia electron transfer interactions with <sup>1</sup>CBZ*. In addition, <sup>1</sup>CBZ* could also undergo a photoionization process. The transformation\nproducts and pathways of CBZ at 222 nm were proposed, and the toxicities\nof CBZ’s products were predicted. These findings provide valuable\ninsights into OMPs’ photolysis with 222 nm far-UVC, revealing\nmore mechanistic details for far-UVC-driven systems.