Concentration-Dependent Kinetics of Micropollutant Oxidation: Revealing the Hidden Role of Long-Lived Reactive...

Micropollutants (MPs) are ubiquitously present in aquatic environments at nanomolar (nM) concentrations or lower, yet they pose substantial risks to both ecosystems and human health. However, laboratory studies often examine MP transformation at micromolar (μM) levels due to analytical constraints, potentially overlooking certain MP behaviors under environmentally relevant concentrations. This study investigated the oxidation kinetics (k_obs) of model MPs across a wide range of initial concentration (C₀), with particular emphasis on the distinct roles of short- and long-lived reactive species (SLRS and LLRS) formed in irradiated dissolved organic matter (DOM) solutions. A generalized kinetic framework was developed and revealed that SLRS, such as triplet-state DOM, dominated MP oxidation at higher C₀ (≥μM), while LLRS, primarily DOM radicals, drove MP degradation at sub-μM levels. Laser flash photolysis confirmed that LLRS exhibits much longer lifetimes (∼ms) than SLRS (∼μs), enabling elevated LLRS concentrations under trace MP conditions and significantly enhancing MP oxidation. The observed k_obs-C₀ relationship features a characteristic "three-platform" behavior governed by RS formation rates, reactivities, and lifetimes. These findings advance our understanding of MP fate under real-world conditions and underscore the critical yet often overlooked role of LLRS in aquatic systems.