Oxidation of iron (II) nanomolar with H 2O 2 in seawater

Abstract

The oxidation of Fe(II) with H 2O 2 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), H 2O 2 (7.2 × 10 −8 M to 5.2 × 10 −7 M), HCO 3 − (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 HCO 3 −. 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), H 2O 2 and HCO 3 − 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(CO 3) 2 2− species contribute over 20% to the rates. Model results show that when the concentration of O 2 is two orders of magnitude higher than the concentration of H 2O 2, the oxidation with O 2 also needs to be considered. The rate constants for the five most kinetically active species (Fe 2+, FeOH +, Fe(OH) 2, FeCO 3, Fe(CO 3) 2 2−) in seawater as a function of temperature have been determined. The kinetic model is also valid in pure water with different concentrations of HCO 3 − and the conditions found in fresh waters.