Ice vs. aqueous photochemistry of selected hydroxy-naphthalenes: Distinct reactive oxygen species, pH-dependent degradation, and divergent pathways

Hydroxy-naphthalenes (OH-Naps) are of growing concern as ubiquitous emerging micropollutants in cold aquatic systems, urging novel insights into their differences between ice and aqueous photochemistry. This study aimed to compare the glacial and aqueous photodegradation kinetics, pathways, and photo-induced toxicity of six OH-Naps. Under the same simulated sunlight, most OH-Naps exhibited faster apparent photolysis in ice than in liquid water. Based on reactive-oxygen-species scavenging experiments, OH-Naps suffered from self-sensitized photolysis mediated by •OH and ¹O₂ during the apparent photo-process, with •OH playing a more significant role in ice, accounting for 9.17%-53.56%. Competition kinetic experiments further indicated that the bimolecular reactivity of most OH-Naps towards •OH/¹O₂ in liquid water was higher than in ice. Interestingly, the apparent photolysis of OH-Naps exhibited a pronounced dependence on pH conditions and dissociated species. The majority of OH-Naps underwent more efficient photolysis in their molecular forms than in their anionic forms in both media, although 2-NO₂-1-OHN was an exception to this. By combining DFT calculations with HPLC-MS/MS analysis, major transformation pathways were identified involving photo-oxidation, photo-hydroxylation, multiple hydroxylation, denitrification, and photo-isomerization. Many products differed significantly between ice and liquid water for most OH-Naps, exemplified by 1,4-DHN and 2,3-DHN. V. fischeri bioassays and ECOSAR predictions confirmed that most photoproducts exhibited equivalent or higher toxicity compared to the parent OH-Naps. Furthermore, the environmental photochemical transformation half-lives of OH-Naps were extrapolated in natural sunlit ice (27.79-3417.18 min) and surface waters (22.12-6163.01 min). These results clarified the glacial and aqueous photochemical differences of OH-Naps, facilitating the assessment of the persistence and risks of these newly concerned pollutants in seasonally frozen water systems.