New insights into the environmental photochemistry of hydroxynaphthalene congeners in water and in ice: A distinct comparative study

Abstract

Hydroxynaphthalene congeners (OH-Naps) are newly recognized contaminants, urging new insights into their photodegradation in water and in ice. In the study, the important differences between the aqueous and ice photochemistry of four OH-Naps were found. Under simulated sunlight irradiation (λ > 290 nm), they photolyzed faster in ice than in equivalent water in most cases, indicating that their photodegradation was related to whether they resided in water or ice. Meanwhile, the photolytic kinetics were influenced greatly by the substituent groups (–OH, –Cl, and –NO₂) and positions, resulting in the fastest photolysis of 2-hydroxynaphthalene (2-OHN) or 4-chloro-1-hydroxynaphthalene (4-Cl-1-OHN), and the slowest photodegradation of 4-nitro-1-hydroxynaphthalene (4-NO₂-1-OHN) in the two phases. Furthermore, their apparent photolysis was found to be faster at alkaline pH, attributing to the stronger photo-absorption, electron density and higher reactivities of the anionic forms. The •OH photooxidation kinetics also depended on the specific OH-Nap and the matrix type. Through the key photoproduct identification, the phototransformation of 4-Cl-1-OHN and 4-NO₂-1-OHN involved different pathways in the two phases. Only in ice, the two OH-Naps underwent multi-hydroxylation, and 4-NO₂-1-OHN suffered from photoisomerization as well. The bioassay to Vibrio fischeri indicated the higher photo-modified toxicity of most OH-Naps in ice than in water, attributing to the generation of more toxic multiple-hydroxyl adducts in ice. Based on extrapolating the lab-derived data to the real environment, the photochemical fate of OH-Naps highly depended on latitudes and solar intensities. These results are significant for evaluating the environmental persistence, fate and risk of the newly recognized contaminants.