Role of Carbonate Speciation on the Oxidation Rate of Fe(II) in Aquatic Systems

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 typical of natural waters. Using this model, it was determined that ferrous carbonate complexes [Fe(CO3), Fe(CO3)22-, and Fe(CO3)(OH)-] dominate the speciation of Fe(II) in natural waters containing greater than 1 mM carbonate alkalinity. 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 described in terms of the Fe2+ and FeOH+ species. However, for pH values above 6, the Fe(CO3)22- complex is the most kinetically active species. The combined speciation/oxidation model for Fe(II) was successful at predicting observed oxidation rates in lake water, well-defined salt solutions, and seawater.