Photochemistry of iron(III)/iron(II) complexes in atmospheric liquid phases and its environmental significance

Abstract

The generation of hydrogen peroxide (H2O2) and depletion of oxalic and other carboxylic acids by photochemical/chemical cycling of Fe(nr)/Fe(H) complexes in sunlight has been studied under conditions typical of acidified atmospheric water. H2O2 is produced through the reduction of oxygen by intermediates formed from photo-reactions of Fe(m)-oxalato complexes. The rate of H2O2 formation increases with sunlight intensity, and with both oxalate and Fe(TJI) concentration within the concentration range used. This rate is 3.7 nM s-1 when a solution of 1 \iM Fe(IH) and 5 \iM oxalate at pH 4 is exposed to September-noon-sunlight, and oxalic acid is photolyzed with a half-life of a few minutes. The dissolved iron is present as a photochemical catalyst. Fe(III) is reduced by photons to Fe(II). In the presence of O2, the Fe(II) formed is reoxidized, leading to the reformation of Fe(DI) complexes. At solar noon in September, the cycling time of Fe(III)-Fe(II)-Fe(III) is on the order of minutes. Speciation calculations based on the concentration range of Fe(m) and oxalic acid present in atmospheric water indicate that Fe(III)-oxalato complexes are often the predominant species of dissolved Fe(III). The concentrations of Fe(IQ)-oxalato complexes are sufficiently large to make their photolysis a dominant source of in-cloud HO2/O2 radicals and H2O2, and a major sink for atmospheric oxalic acid. Irradiation of authentic fog waters has shown a good correlation between the formation of H2O2 and depletion of oxalic acid. Mechanisms and kinetics for the photochemical/chemical cycling of Fe(lH)/Fe(II) complexes, and for the formation of H2O2 and the depletion of oxalic acid are discussed in detail.