Photo-initiated Degradation Kinetics of the Organic UV filter Oxybenzone in Solutions and Aerosols: Impact of Salt, Photosensitizers, and the Medium
ADAM COOPER, Alexis Shenkiryk, Maya Morris, Tessa Tafuri, Jonathan Slade, University of California San Diego
Abstract Number: 553
Working Group: Chemicals of Emerging Concern in Aerosol: Sources, Transformations, and Impacts
Abstract
Organic UV filters in sunscreens, such as oxybenzone (BP3), are concentrated and persistent pollutants in seawater because of their broad use and resistance to photolysis. BP3 is hydrophobic and suspected to undergo ocean-to-air transfer via sea spray aerosol (SSA), potentially leading to different reactivity in the aerosol phase compared to bulk seawater. This study contrasts the photo-initiated degradation reactions and kinetics of BP3 in artificial seawater mixes and real seawater with those in SSA mimics comprised of NaCl and a model photosensitizer, 4-benzoyl benzoic acid (4-BBA). Pure, binary, and ternary mixtures of BP3, NaCl, and 4-BBA were studied in the dark and irradiated to isolate the impacts of salt and the photosensitizer on BP3 degradation. Experiments were conducted in a quartz cuvette exposed to solar-simulated light and analyzed via UV-VIS spectroscopy for bulk solutions and an oxidation flow tube reactor equipped with broadband UV lamps for aerosols, analyzed via extractive electrospray ionization time-of-flight mass spectrometry (EESI-TOF). Results indicate significant differences between the solution and aerosol phase effective photolysis rates (Jeff) scaled to equivalent solar exposure times, with much faster degradation observed in the aerosol phase (Jeff ≈10-3-10-2 s-1 or t1/2<10 min) than in bulk solutions (Jeff ≈10-6 s-1 or t1/2>1 day) in all mixtures. The photosensitizer enhanced BP3 photodegradation in both environmental phases but more so in solution compared to mixtures with salt or all three components, whereas BP3 photodegradation was enhanced the most in the presence of salt in the aerosol phase. The faster kinetics in the aerosol phase are consistent with previous studies demonstrating greater photodegradation in aerosols due to morphology-dependent resonances and a decreased solvent-cage effect compared to bulk solutions. High-resolution molecular identification of transformation products via Orbitrap liquid chromatography tandem mass spectrometry (LC-MS/MS) showed an enhanced generation of reduced forms of BP3, notably benzophenone, alongside smaller fragmentation products, such as benzoic acid and benzaldehyde, in the aerosol phase compared to the bulk, which enhances the aerosol’s toxicity (i.e., predicted LD50 for rats using EPISTAR). These results demonstrate the stark differences in photodegradation between the bulk and aerosol phases of a prevalent and toxic UV filter and indicate that photo-induced reactions of BP3 in SSA may be relatively more important than in seawater concerning its effects on particle toxicity.