Superoxide-mediated dissolution of amorphous ferric oxyhydroxide in seawater

We have investigated the kinetics of superoxide-mediated dissolution of amorphous ferric oxyhydroxide (AFO) in seawater by spectrophotometrically examining the rate of formation of a ferrous-ferrozine complex (Fe(II)(FZ) ₃) with particular attention given to the effect of aging and iron concentration on the rate of superoxide-mediated dissolution of AFO. The production rates of Fe(II)(FZ)₃ decreased with aging of AFO for iron concentrations from 50 to 500 nM, indicating that changes to the chemical and physical properties of AFO affected the reactivity of inorganic ferric iron species with superoxide. A kinetic model developed by assuming that Fe(II)′ formation is preceded by the thermal dissolution of AFO provided a good description of the Fe(II)(FZ)₃ production rates over time. First-order rate constants for Fe(II)′ formation were found to depend on the total iron concentration, suggesting that superoxide-mediated Fe(III) reduction is affected not only by the rate of thermal dissolution of AFO but also by the rate of AFO precipitation. The reported high rates of superoxide production by both photochemical and biotic pathways in aquatic systems coupled with the ability of superoxide to dissolve freshly formed ferric oxides suggest that this process may have a significant impact on the biogeochemical cycling of iron, especially if organisms have an affinity for ferrous iron.