The Influence of Iron (II) and (III) on the Kinetics of Goethite Dissolution by EDTA

The dissolution of goethite in aqueous media containing EDTA, Fe(II), and Fe(III) has been studied. The main dissolution pathways involve the inner or outer sphere electron transfer in mixed-valence EDTA-bridged surface complexes and in surface “ion pairs.” A synergistic effect is observed between ferrous EDTA and free EDTA. The rate dependencies on [EDTA] and [FeII] are interpreted in terms of Fe(II) phase transfer from three different surface complexes: ≡FeIII–Y–FeII(Y is the EDTA anion), ≡FeIII–Y···FeIIY, and the dimeric species with adjacent sites ≡FeIII–Y and ≡FeIII–Y–FeII. Addition of iron(III) produces a rate decrease that is explained on the basis of the adsorption behavior of FeIIIY−, for which adsorption data are presented. The redox equilibration of the surface is discussed.