Microheterogeneous photogeneration of hydroxyl radical in sunlit dissolved black carbon solution

Dissolved black carbon (DBC), a potent electron acceptor and sorbent, plays a pivotal role in numerous environmental geochemical processes. Despite its significance, the mechanistic details underlying the photogeneration of hydroxyl radical (HO^•) in DBC solution remain poorly understood. This study investigates the photodegradation of four HO^•-labile pharmaceuticals in DBC solution under simulated solar irradiation. By analyzing photodegradation kinetics, a mathematical model elucidating the microheterogeneous phototransformation mechanism of pharmaceuticals is established. This model highlights the dual degradation pathways: within the DBC microphase via DOM-HO^• and in the aqueous phase via free HO^•. Our findings reveal that charge interactions are critical in the photodegradation of these pharmaceuticals in DBC solutions. Notably, the concentration of DOM-HO^• produced in the DBC microphase was calculated to be two orders of magnitude (∼ 85-fold) that of the concentration of free HO^• generated in the aqueous phase. Quenching experiments further demonstrated that DOM-HO^• formation follows an O₂-independent pathway, whereas free HO^• is produced through an O₂-dependent pathway. This newly identified HO^• formation pathway offers profound insights into the photochemistry of DBC in aquatic systems, enhancing our understanding of its role in environmental processes.