Modeling the kinetics of Fe(II) oxidation in the presence of citrate and salicylate in aqueous solutions at pH 6.0-8.0 and 25 degrees C

Abstract

The kinetics of Fe(II) oxidation in the presence of low concentrations of citrate and salicylate have been investigated in aqueous solutions over the pH range 6.0-8.0 using colorimetry. A kinetic model has been developed to describe the oxidation of Fe(II) with specific attention given to the oxidation of inorganic Fe(II), formation and dissociation of Fe(II) complexes and the oxidation of these complexes. At low concentrations of salicylate, both experimental data and model show that the common approach to modeling Fe(II) oxidation that assumes pre-equilibrium between metal and ligand prior to their oxidation is not valid. Complexation of Fe(II) by salicylate is found to be relatively slow, and oxidation of the complex formed occurs rapidly. Citrate, on the other hand was found to be in rapid equilibrium with Fe(II) but the complex formed was oxidized slowly. Both citrate and salicylate complexes are found to dissociate at a rate much faster than previously thought. A model of the oxidation kinetics of Fe(II) species that incorporates the formation and dissociation kinetics of Fe(II) and Fe(III) complexes of citrate and salicylate as well as the reactions of these species with oxygen and reduced oxygen species including superoxide and hydrogen peroxide provides an excellent description of data obtained over a wide range of concentration and pH conditions.