Oxidation of formaldehyde by the hydroxyl radical in aqueous solution

Abstract

The results of a detailed kinetic and mechanistic study of the aqueous oxidation of formaldehyde by the OH radical at 293 K are presented. In the presence of hydrogen peroxide and absence of O₂, the reaction proceeds by a chain mechanism. Propagation occurs by the reaction of the hydrated formyl radical, HC(OH)₂, with H₂O₂, which produces formic acid and regenerates the OH radical: k_a[HC(OH)₂+ H₂O₂]=(3.5 ± 1.2)× 10⁶ dm³ mol^–1 s^–1. A second channel for this reaction has also been identified, in which the products are the hydroperoxy radical, HO₂, and hydrated formaldehyde: k_b[HC(OH)₂+ H₂O₂]=(7.4 ± 1.7)× 10⁵ dm³ mol^–1 s^–1. In O₂-saturated solutions, the chain reaction is suppressed initially by the reaction of HC(OH)₂ with O₂, which produces formic acid and the hydroperoxy radical, HO₂: k[HC(OH)₂+ O₂]=(3.5 ± 1.6)× 10⁹ dm³ mol^–1 s^–1. The self-reaction of the hydrated formyl radical also produces formic acid: k[HC(OH)₂+ HC(OH)₂]=(1.4 ± 0.1)× 10⁹ dm³ mol^–1 s^–1. Product analysis provides no evidence for the dehydration of HC(OH)₂ to form HCO. Formic acid is the only significant product of the aqueous oxidation of formaldehyde by the OH radical. The yield of CO associated with this reaction is negligible. The implications of these results for atmospheric chemistry are discussed.