Role of heterogeneous precipitation in determining the nature of products formed on oxidation of Fe(II) in seawater containing natural organic matter.

A detailed kinetic model has been developed to describe the formation of the oxidation products, organically complexed Fe(III) and amorphous ferric oxide (AFO), on oxidation of Fe(II) in seawater containing Suwannee River fulvic acid (SRFA). Experimental data were collected using spectrophotometric detection of the Fe(III)-SRFA complex for a range of initial concentrations of Fe(II) and SRFA. Initial sensitivity analysis identified rate constants to which the model was most sensitive including those for heterogeneous precipitation of AFO and Fe(II)-SRFA formation and dissociation which to date have only been determined with a high degree of uncertainty. Using these rate constants as fitting parameters, an accurate fit to the experimental data could be obtained using a kinetic model describing key processes. However, reasonable fits could only be achieved with the inclusion of the heterogeneous precipitation reaction suggesting the importance of this reaction in determining the outcome of oxidation in the presence of organic ligands. The rate constants for Fe(II)-SRFA formation and dissociation were highly correlated and could not be determined uniquely, however their ratio revealed a stability constant of approximately 10(5), 3 orders of magnitude higher than previously reported. The fitted model also suggested that a complex interaction between Fe(II) and SRFA in the initial stages of the oxidation process determines the pathway of Fe(III)-SRFA formation.