Dual Pathways in 222 nm Far-UVC Photodegradation of Sulfonamides: Direct Photolysis and Radical-Mediated Self-Sensitized Oxidation

222 nm Far-UVC irradiation offers a promising mercury-free approach for removing sulfonamides (SAs) from water, but its photolytic mechanisms remain unclear. Here, the photodegradation of six representative SAs under 222 nm irradiation was investigated, emphasizing the roles of dissociated forms (neutral and deprotonated) and substituent structures (five-membered and six-membered heterocyclic). Results demonstrated that SAs' photodegradation proceeded through both direct photolysis and radical-mediated self-sensitized pathways. Dissociated forms primarily influenced the kinetics and the relative contributions of dual pathways: deprotonated forms (SAs^0,-) generally degraded faster than their neutral forms (SAs^+,-), and they are also more prone to attack by selectively photogenerated radicals. Substituent structures dictated bond-cleavage modes in direct photolysis: five-membered ring (5R) SAs underwent O-N or S-N cleavage, while six-membered ring (6R) SAs favored SO₂ extrusion. In the self-sensitized pathway, hydroxyl radicals (^•OH) and superoxide radicals (O₂^•-) were identified as the major reactive oxygen species (ROS), with ^•OH exhibiting a unique dual origin from both dissolved oxygen (DO) and H₂O molecules. These findings provide mechanistic insights into 222 nm far-UVC-driven SAs degradation and inform the design of efficient, mercury-free UV-based water treatment systems.