A unified model for Fe(II) speciation and oxidation by molecular oxygen is described. This model combines Pitzer's concept of specific interaction with classic ion pair formation theory to describe ferrous iron speciation under conditions typicalof natural waters. Using this model, it was determined that ferrous carbonate complexes Fe(CO{sub}3), Fe(CO{sub}3){sub}2{sup}2-, and Fe(CO{sub}3)(OH){sup}-) dominate the speciation of Fe(II) in natural waters containing greater than 1 mM carbonatealkalinity. The speciation data were then utilized to evaluate the species-specific rates of Fe(II) oxidation by molecular oxygen for a range of media compositions and ionic strength. At pH values below 6.0, the oxidation rate of Fe(II) is well describedin terms of the Fe{sup}2- and FeOH{sup}- species. However, for pH values above 6, the Fe(CO{sub}3){sub}2{sup}2- complex is the most kinetically active species. The combined speciation/oxidation model for Fe(II) was successful at predicting observedoxidation rates in lake water, well-defined salt solutions, and seawater.
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