Accelerated Photolysis of Pharmaceutical and Personal Care Products at the Air-Water Interface

The widespread use and environmental release of pharmaceutical and personal care products (PPCPs) pose a global threat to ecosystems and human health. Photolysis plays a critical role in determining the transformation and fate of PPCPs in natural environments. While the photolysis of PPCPs in bulk aqueous systems has been well-documented, their photochemical behavior at environmental interfaces, such as the air-water interface, remains largely unexplored. Here, we demonstrate significantly altered photolysis kinetics and pathways of PPCPs in microdroplets compared to bulk water. The photolysis rates of multiple recalcitrant PPCPs in microdroplets were universally accelerated by an astonishing (3.0 ± 0.1) × 10¹- to (4.4 ± 0.6) × 10⁴-fold relative to bulk water. This enhancement was primarily attributed to the accumulation of hydrophobic PPCP molecules at the air-water interface. Interfacial accumulation facilitates enhanced direct PPCP photolysis due to localized light amplification and promoted photon absorbance. Additionally, indirect photolysis of PPCPs was also intensified in microdroplets, driven by an increased production of interfacial hydroxyl radicals (^•OH) and their subsequent reactions. Therefore, the photolysis products of PPCPs in microdroplets differed markedly from those in bulk water, with significantly higher levels of hydroxylated transformation products detected in microdroplets. These findings unveil a previously unrecognized yet rapid pathway for the photolysis of PPCPs at the air-water interface, offering novel insights into their natural attenuation and detoxification processes.