Oxidation of iron (II) nanomolar with H2O2 in seawater

The oxidation of Fe(II) with H₂O₂ at nanomolar levels in seawater have been studied using an UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5 to 8.2), H₂O₂ (7.2 × 10⁻⁸ M to 5.2 × 10⁻⁷ M), HCO₃⁻ (2.05 mM to 4.05 mM) and Fe(II) (5 nM to 500 nM) as a function of temperature (3 to 35 °C) on the oxidation of Fe(II) are presented. The oxidation rate is linearly related to the pH with a slope of 0.89 ± 0.01 independent of the concentration of HCO₃⁻. A kinetic model for the reaction has been developed to consider the interactions of Fe(II) with the major ions in seawater. The model has been used to examine the effect of pH, concentrations of Fe(II), H₂O₂ and HCO₃⁻ as a function of temperature. FeOH⁺ is the most important contributing species to the overall rate of oxidation from pH 6 to pH 8. At a pH higher than 8, the Fe(OH)₂ and Fe(CO₃)₂²⁻ species contribute over 20% to the rates. Model results show that when the concentration of O₂ is two orders of magnitude higher than the concentration of H₂O₂, the oxidation with O₂ also needs to be considered. The rate constants for the five most kinetically active species (Fe²⁺, FeOH⁺, Fe(OH)₂, FeCO₃, Fe(CO₃)₂²⁻) in seawater as a function of temperature have been determined. The kinetic model is also valid in pure water with different concentrations of HCO₃⁻ and the conditions found in fresh waters.