The role of Fe(II) species on the oxidation of Fe(II) in natural waters in the presence of O2 and H2O2

The oxidation of Fe(II) at nanomolar levels with O2, H2O2 and their mixtures has been studied and a kinetic model has been applied to describe the experimental results. A first order pH dependence is obtained when the H2O2 controls the oxidation, due to the large contributions of FeOH+ and FeCO3 to the overall rate constant. The second order pH dependence for the oxidation of Fe(II) with O2 is due to the contributions of Fe(OH)2 and Fe(CO3)22− species. Trace amounts of H2O2 or O2 in the solutions radically affect the observed oxidation rates, and the presence of these two oxidants at the same time can explain some deviations obtained from the expected behavior when one of the oxidants is in excess. The experimental data and model results indicate that the oxidation of Fe(II) with H2O2 plays a relatively major role in most natural waters. At the pH of seawater, O2 is the most important oxidant when [H2O2] is below 200 nM and [Fe(II)] is at nanomolar levels.