Determination of the Hydroxyl Radical Reaction Rate Constant of Amines in the Aqueous Phase

Amines are volatile, alkaline, water-soluble organic compounds that have been studied because of their significant contribution to atmospheric pollution in terms of new particle formation, ultrafine particle growth, and the neutralization of aerosol acidity. Although numerous studies have investigated the gas-phase kinetics of amine oxidation by the hydroxyl (OH) radicals, aqueous-phase reactions have not been extensively explored. Herein, we investigated the bulk aqueous-phase kinetics of the oxidation of alkylamines and alkanol amines by OH radicals using relative rate kinetics. The experimentally calculated rate constants at pH 5 for monoethanolamine (MEA), diethanolamine, triethanolamine, and diethylamine were 4.6 ± 0.27 × 10⁸, 9.3 × 10⁸, 4.9 × 10⁸, and 3.7 × 10⁸ M^-1 s^-1, respectively. The oxidation rate of MEA gradually increased by 10-20% across a pH range of 2.0-6.0. Amine oxidation kinetics was also assessed in relation to the structural positioning of the hydroxyl and alkyl groups, which induce steric hindrance. In addition, under rural, urban, and marine scenarios, the photo-Fenton reaction resulted in 100, 60, and 40% degradation of MEA, whereas the Fenton reaction led to only 40, 15, and 8% degradation, respectively, after 20 min, illustrating the importance of the catalytic effect of iron in amine photodegradation and its kinetics. The atmospheric lifetimes of amines were calculated from the observed rate constants to predict the fate of amines in the atmosphere. Product analysis and model fitting of experimental data were used to confirm that hydrogen abstraction at α-carbon or β-carbon rather than the terminal methyl group is the dominant pathway under relevant atmospheric conditions. The obtained results strengthen the understanding of the photochemical oxidation kinetics of amines and can help predict the lifetime of amines in the aqueous phase and, hence, their partitioning into the gas and particle phases.