OXIDATION OF FORMALDEHYDE BY THE HYDROXYL RADICAL IN AQUEOUS-SOLUTION

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) x 10(6) 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) x 10(5) 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) x 10(9) 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) x 10(9) 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.