Open the door to the new world of singlet oxygen in the peracetic acid system: Generation and identification.

Peracetic acid (PAA) is rapidly emerging as a versatile oxidant for advanced oxidation processes (AOPs). Among the reactive oxygen species generated, singlet oxygen (1O2) is notable for its high selectivity toward electron-rich contaminants, broad pH tolerance, and resilience to inorganic anions. However, a comprehensive understanding of its formation pathways and reliable identification methodologies remains elusive, often leading to ambiguous and even contradictory conclusions in the literature. This review consolidates recent progress on PAA-derived 1O2, delineating two principal mechanistic avenues, namely, peroxide-derived pathways and superoxide-mediated sequences, and clarifying the conditions that steer their predominance. The analytical toolbox is critically assessed, covering electron paramagnetic resonance, quencher assays, probe-based approaches, and solvent isotope effects (H2O/D2O), with emphasis on working principles, applicability limits, and frequent artifacts such as cross-reactivity and false positives. Building on these insights, a dynamic identification framework is advanced to couple orthogonal diagnostics with kinetic analysis for quantifying 1O2 yields and apportioning its contribution to pollutant decay. The synthesis further highlights catalyst and process levers, including oxygen vacancies, heteroatom doping, and metal redox cycling, as well as pH, temperature, and matrix constituents, that can bias pathways toward selective, non-radical oxidation. By integrating the mechanism with the method, the review provides practical guidance for standardizing 1O2 reporting and for rationally designing high-efficiency in PAA-based AOPs (PAA-AOPs).