The chemical speciation of Fe(III) in freshwaters

Dialysis and chemical speciation modelling have been used to calculate activities of Fe3+ for a range of UK surface waters of varying chemistry (pH 4.3–8.0; dissolved organic carbon 1.7–40.3 mg l-1) at 283K. The resulting activities were regressed against pH to give the empirical model: log aFe3+ = 2.76(±0.53) – 2.63(±0.08)•pH Predicted Fe3+ activities are consistent with a solid–solution equilibrium with hydrous ferric oxide. However, the apparent solubility of the solid phase decreases as pH decreases, consistent with some previous studies on Fe(III) solubility in the laboratory. The empirical model was used to predict concentrations of Fe in dialysates and ultrafiltrates of globally distributed surface and soil/ground waters. The predictions were improved greatly by the incorporation of a temperature correction for aFe3+, consistent with the temperature–dependence of previously reported hydrous ferric oxide solubility. The empirical model, incorporating temperature effects, may be used to make generic predictions of the ratio of free and complexed Fe(III) to dissolved organic matter in freshwaters. Comparison of such ratios with observed Fe : dissolved organic matter ratios allows an assessment to be made of the amounts of Fe present as Fe(II) or colloidal Fe(III), where no separate measurements have been made