The rate of iron(II) oxidation in the North Sea water of pH 5.5-10 in the range 10-25°C has been studied. The oxygenation rate depends linearly on the iron(II) and dissolved oxygen concentrations. The second-order dependence on (OH-), found by several investigators for synthetic solutions, was confirmed in seawater, but only for pOH>6.9. For pOH<5.9 the rate appeared to be independent of the pOH. In the intermediate range, pOH<5.9-6.9, corresponding to the natural pH of seawater, a first-order dependence on the pOH is obeyed. The important discrepancy in the literature between the second-order rate constant for NaCHO3 solutions and for seawater can be attributed predominantly to the incorrect assumption of a second-order pOH dependence in natural seawater. The results can be useful, for example, in predicting the effect of dumping acidic iron waste from the titanium-dioxide industry into the ocean.
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