Reactive Oxygen Species Generated in Situ During Carbamazepine Photodegradation at 222 nm Far-UVC: Unexpected Role of HO Molecules.

When 222 nm far-UVC is used to drive AOPs, photolysis emerges as a critical pathway for the degradation of numerous organic micropollutants (OMPs). However, the photodegradation mechanisms of the asymmetrically polarized OMPs at 222 nm remain unclear, potentially posing a knowledge barrier to the applications of far-UVC. This study selected carbamazepine (CBZ), a prevalent aquatic antiepileptic drug that degrades negligibly at 254 nm, to investigate its photodegradation mechanisms at 222 nm. Accelerated CBZ treatment by 222 nm far-UVC was mainly attributed to in situ ROS generation via self-sensitized photodegradation of CBZ. By quenching experiments and EPR tests, •OH radicals were identified as the major contributor to the CBZ photodegradation, whereas O played a minor role. By deoxygenation and solvent exchange experiments, the HO molecules were demonstrated to play a crucial role in deactivating the excited singlet state of CBZ (CBZ*) at 222 nm: generating •OH radicals via electron transfer interactions with CBZ*. In addition, CBZ* could also undergo a photoionization process. The transformation products and pathways of CBZ at 222 nm were proposed, and the toxicities of CBZ's products were predicted. These findings provide valuable insights into OMPs' photolysis with 222 nm far-UVC, revealing more mechanistic details for far-UVC-driven systems.