Oxygenation of Fe(II) in the presence of citrate in aqueous solutions at pH 6.0-8.0 and 25 °C:: Interpretation from an Fe(II)/citrate speciation perspective

The kinetics of Fe(II) oxidation in the presence of various citrate concentrations have been investigated in aqueous solutions over the pH range 6.0-8.0 using colorimetry and speciation modeling. Oxidation of Fe(II) was interpreted and quantitatively modeled in terms of oxidation of various Fe(II)-citrate species. Using the model, it is possible to predict whether the presence of citrate would dominate the Fe(II) oxidation and thus enhance/retard the oxidation rate of Fe(II) and vice versa. The study also supports the presence of other Fe(II)-citrate species rather than just the monomeric species at circumneutral pH. At low pH and in a system where complexation of Fe(II) by citrate is dominant, oxidation of Fe(II) is controlled by the oxidation of both Fecit(-) and Fecit(2)(4-). As the pH increases, the oxidation of Fe(OH)cit(2)(5-) becomes increasingly important and dominates the oxidation of Fe(II) at pH 8.0. Rate constants for the oxidation of all five suggested Fe(II)citrate species have been estimated and may be used to predict the rate of Fe(II) oxidation at any combination of pH and citrate concentration.