Concentration-dependent micropollutant oxidation by DOM-derived long-lived reactive species: roles of pollutant...

Dissolved organic matter (DOM) can generate a suite of organic reactive intermediates during oxidation, known as long-lived reactive species (LLRS), which are increasingly recognized as important contributors to micropollutant (MP) transformation, especially at sub-micromolar (sub-µM) levels. However, it remains unclear how this LLRS-mediated oxidation is jointly governed by the molecular structure of MPs and the compositional features of DOM. Here, we examined the dependence of oxidation kinetics (k_obs) on MP concentration (0.1-10 μM) in UV/DOM systems across diverse MPs and DOM matrices. Enhanced oxidation at sub-µM concentrations was broadly observed, with observed rate constants increasing by up to 13.9-fold as the initial MP concentration decreased from 10 to 0.1 µM. Probe, quenching, and electron paramagnetic resonance (EPR) experiments supported DOM-derived LLRS as major contributors to this enhancement. A concentration-dependent kinetic framework yielded two apparent LLRS parameters: an apparent, MP-specific LLRS availability (R_LLRS) and a loss-to-reactivity ratio (k_d,LLRS/k_LLRS,MP), with values of 1.08 × 10^-9 M s^-1 and 6.77 × 10^-8 M, respectively, for 2,4,6-trimethylphenol in Suwannee River natural organic matter (2 mg_C L^-1, pH 7.0). Exploratory descriptor correlations suggested that higher R_LLRS was associated with lower MP ionization potential (r = -0.81) and greater DOM unsaturation (r = 0.74), whereas higher k_d,LLRS/k_LLRS,MP was associated with greater MP electronegativity (r = 0.79) and DOM aliphaticity (r = 0.75). These findings establish a tentative structure-reactivity framework linking pollutant electronic properties and DOM molecular composition to the concentration-dependent importance of LLRS-mediated oxidation.