Molecular structure controls sunlight-driven photodegradation and persistence of trace organic contaminants in surface waters.

Sunlight-induced phototransformation plays a key role in the attenuation of trace organic contaminants (TrOCs) in surface waters, yet rapid and mechanistically grounded approaches for assessing their persistence remain limited. Here, we developed a tiered computational framework to evaluate the sunlight-driven photodegradation potential and persistence of 150 TrOCs spanning 11 chemical categories. The framework integrates quantum-chemical calculations, thermodynamic evaluation of 10 representative phototransformation pathways, and structure-reactivity analysis to identify molecular features associated with photochemical fate. The lowest triplet excitation energies of the selected TrOCs varied widely among chemical classes and strongly influence their sunlight reactivity. Energy transfer pathways with dissolved organic matter and oxygen were broadly favorable, whereas most electron transfer pathways were thermodynamically unfavorable under environmentally relevant conditions. Across the 150 TrOCs, the lowest triplet excitation energy correlated positively with the energy gap between the highest occupied and lowest unoccupied molecular orbitals and negatively with the extent of π-conjugation. Based on these relationships, simple structural thresholds were identified that efficiently distinguished photoreactive from photostable contaminants. Overall, this study provides a mechanistically informed framework for contaminant prioritization, persistence screening, and fate assessment in sunlit surface waters, offering practical guidance for environmental risk assessment and supporting the identification of persistent TrOCs warranting priority monitoring and regulation.