A critical review on environmental-risk-oriented Fenton-like oxidation of N-containing heterocyclic contaminants in water: From structure-activity relationships to QSAR modeling.

N-containing heterocyclic contaminants (NHCs) have emerged as structurally diverse and recalcitrant aquatic pollutants because of their stable heterocyclic frameworks, nitrogen-driven electronic effects, poor biodegradability, and non-negligible toxic potential. They have been widely detected in wastewater, surface water, groundwater, and sludge, often showing environmental persistence and mixture contamination. Because conventional biological treatment does not consistently remove many NHCs, Fenton-like oxidation has been extensively investigated as a promising advanced oxidation approach. However, parent-compound removal alone does not demonstrate potential risk reduction, as oxidation may generate transformation products (TPs) with uncertain persistence and toxicity. In this review, a comprehensive overview of environmental-risk-oriented Fenton-like oxidation of NHCs in water is introduced. Specifically, the structural classification, environmental occurrence, contamination profiles, degradation performance, operational features, and mechanistic basis of representative NHCs and different Fenton-like systems are summarized. The review further elucidates how hydroxyl radicals, sulfate radicals, and non-radical pathways contribute to NHC oxidation, while also discussing transformation-product formation and toxicity evolution during Fenton-like treatment. Structure-activity relationships and recent advances in QSAR modeling are further discussed to connect molecular features with degradation behavior and toxicity prediction. Finally, key challenges and future research priorities are identified, including reliable reactive-species identification, prioritization of potentially hazardous TPs, and improved model transferability, which together could support environmental risk assessment and treatment prioritization.