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)2 2-, 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)2 2- 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.

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We report the extracellular production of superoxide in cultures of the marine diatoms Thalassiosira weissflogii and Thalassiosira pseudonana. In EDTA-buffered media with ϳ45 pmol L Ϫ1 iron (Fe)(III)Ј, over half the Fe(III) reduction is mediated by extracellular O production. Surprisingly, even though we saw a consistent inhibition ofFe reduction by the addition of superoxide dismutase (SOD) and enhancement of Fe reduction due to superoxide production using the xanthine-xanthine oxidase system (X-XO), we observed no effect of SOD or X-XO on Fe uptake in these cultures. We also observed no effect of SOD on the uptake of Fe from either ferrihydrite, an Felabeled porphyrin, or as regenerated by metazoan grazers. Our data reveal that Fe(II)Ј is formed by O via the

The rate of oxidation of ferrous iron was measured in samples from Lake Greifen, a eutrophic lake in Switzerland. Fe(II) concentrations were followed using an automated flow injection analysis system employing luminol-based chemiluminescence detection of Fe(II). For kinetic studies at pH > 7.8, the system was modified to allow a time resolution of less than 1 s. Oxidation rates were measured in unfiltered samples at 2−2000 nM Fe(II). The pH was varied between 6.8 and 8.3 by bubbling with CO2 and synthetic air. Above pH 7.4, rates were consistent with the rate law determined in pure carbonate systems. Between pH 6.8 and pH 7.3, however, the apparent rate was independent of pH. This surprising finding may be explained by some naturally occurring (organic, colloidal, or surface) ligand(s) that accelerate the oxidation of Fe(II). The relative importance and pH dependence of the direct reaction with O2 in comparison to that with H2O2 was determined, and the enhancement of the overall rate was attributed to the reaction of Fe(II) with oxygen.