Oxygenation of Fe(II) in natural waters revisited: Kinetic modeling approaches, rate constant estimation and the importance of various reaction pathways

Abstract

The kinetics of Fe(II) oxygenation in aqueous solutions over the pH range 6.0–8.0 have been revisited in terms of kinetic modeling approach, rate constant estimation and the importance of various oxidation pathways. Despite the experimental agreement with earlier studies, previous oxidation models (which describe the oxidation of micromolar Fe(II)) have failed to provide an adequate description of Fe(II) oxidation at nanomolar concentrations. This failure could be due to the difficulties in reliably estimating values for the large number of kinetic and stability constants involved but is more likely associated with the fact that several reaction pathways that could become important at low Fe(II) concentrations have not been included in the previous models. A condition-specific model has been developed which uses a single Fe(II) entity as a representative of all Fe(II) species to overcome the difficulty of dealing with the large number of unknown kinetic constants. By incorporation of various reaction pathways, the model is shown to be capable of adequately describing the oxidation of Fe(II) over a range of pH and initial Fe(II) concentrations. While the oxidation of Fe(II) by oxygen and superoxide is critically important at any pH and initial Fe(II) concentration considered, oxidation of Fe(II) by hydrogen peroxide only becomes critical at high pH and high Fe(II) concentrations or in the later stage of the oxidation process. Back reduction of Fe(III) by superoxide is important at low initial Fe(II) concentrations and high pH. Precipitation of Fe(III) on the other hand exerts a marked effect in the overall oxidation of Fe(II), particularly at high pH. Disproportionation of superoxide has minimal effect on the overall oxidation of Fe(II), despite the rapidity of this process at low pH.