Reactivity of poly-alcohols towards OH, NO3 and SO4− in aqueous solution

Abstract

Radicals and radical anions such as OH, NO₃ and SO₄⁻ can act as important oxidants in aqueous solutions, e.g. as a part of the tropospheric multiphase system. In the present study the aqueous phase kinetics of OH, NO₃ and SO₄⁻ radical reactions with the following ten aliphatic poly-alcohols 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, 1,2,3-propanetriol, 1,2-butanediol, 1,4-butanediol, 1,5-pentanediol, (2R,3S)-butane-1,2,3,4-tetraol (meso-erythritol), (2R,4R)-pentane-1,2,3,4,5-pentol (arabitol) and (2R,3R,4R,5R)-hexane-1,2,3,4,5,6-hexol (mannitol) were investigated. These alcohols are both of anthropogenic and of biogenic origin and all are highly water soluble. All rate constants throughout this study were determined as a function of the temperature between 278 ≤T [K] ≤ 328. OH radical reactions with poly-alcohols (k_2nd = 1.6–4.7 × 10⁹ L mol⁻¹ s⁻¹) are the fastest among the three investigated radicals. The corresponding NO₃ radical (k_2nd = 0.66–1.5 × 10⁷ L mol⁻¹ s⁻¹) and SO₄⁻ radical (k_2nd = 2.7–5.3 × 10⁷ L mol⁻¹ s⁻¹) reactions are on average two orders of magnitude slower. All three radicals react by an H-abstraction mechanism preferably at the α-carbon atom of the investigated alcohols. Kinetic measurements were performed using laser flash photolysis-long path absorption (LFP-LPA). Rate constants were measured either directly following the NO₃ and SO₄⁻ concentration–time profile or by a competition kinetics method using a well characterized reference reaction (SCN⁻) for OH radicals. Furthermore, obtained kinetic data are used to extend existing reactivity correlations for H-abstraction reactions with bond dissociation energies in aqueous solution and to calculate the tropospheric lifetimes of the alcohols.