Production of Hydroxyl Radical via the Activation of Hydrogen Peroxide by Hydroxylamine

The production of the hydroxyl radical (HO·) is important in environmental chemistry. This study reports a new source of HO· generated solely from hydrogen peroxide (H₂O₂) activated by hydroxylamine (HA). Electron paramagnetic resonance analysis and the oxidation of a HO· probe, benzoic acid, were used to confirm the production of HO·. The production of HO· increased with increasing concentrations of either HA or H₂O₂ as well as decreasing pH. The second-order rate constant for the reaction was (2.2 ± 0.2) × 10^-4 M^-1 s^-1. HO· was probably produced in two steps: the activation of H₂O₂ by protonated HA and then reaction between the H₂O₂ and the intermediate protonated aminoxyl radical generated in the first step. Such a two-step oxidation can possibly be ascribed to the ionizable hydroxyl moiety in the molecular structure of HA, as is suggested by comparing the reactivity of a series of HA derivatives in HO· production. The results shed light on a previously unknown source of HO· formation, which broadens the understanding of its role in environmental processes.