Photolysis of hexa(methoxymethyl)melamine in aquatic systems: Critical roles of UV-shielding effect, hydrated electron and 3DOM* induced by dissolved organic matter.

Organic contaminants and dissolved organic matter (DOM) inevitably coexist and are exposed to sunlight in the aquatic environments, underscoring their interrelated roles in degradation processes. Hexa(methoxymethyl)melamine (HMMM), a widely used tire additive, has been frequently detected in aquatic environmental media, and exhibits a chemical structure that suggests potential ecological risks. DOM significantly influences the contaminant fate by acting as a photosensitizer and by generating reactive species. Photolysis is an essential environmental transformation pathway affecting both HMMM and DOM. In this study, HMMM and DOM were studied as an integrated system to investigate their UV-induced photolysis behavior, identify key degradation factors, and assess the toxicity of transformation products. Results demonstrate that both DOM and HMMM undergo transformation during photolysis. Notably, DOM generally inhibits the photodegradation of HMMM, while simultaneously facilitates a reductive degradation pathway initiated by reactive reducing species (RRS, including 3DOM* and eaq-). Our findings comprehensively provide evidence for RRS generation, supported by direct detection via electron capture signals and indirect indicators such as photolysis kinetics within HMMM-DOM system. These results refine the current understanding of photolysis pathways for tire-related contaminants in aquatic environments and emphasize the importance of evaluating contaminants and DOM as an integrated system in environmental photochemistry.