The Influence of Oxyanions on the EDTA-Promoted Dissolution of Goethite
Organic ligands, such as EDTA, accelerate the dissolution of silicate and oxide minerals. In natural systems oxyanions can compete with organic ligands for mineral surface sites thereby affecting ligand-promoted dissolution rates. Oxyanions can either enhance or inhibit dissolution depending upon pH. It has been suggested that the type of surface complex formed by the oxyanion is responsible for differences in ligand-promoted dissolution rates with mononuclear being dissolution enhancing and bi- or multinuclear being dissolution inhibiting. However, recent research has demonstrated that the type of surface complex formed by the oxyanion is not responsible for differences in ligand-promoted dissolution rates of oxide minerals. Accordingly, this manuscript examines the influence of selenite, molybdate, and phosphate on the EDTA-promoted dissolution of goethite and proposes a mechanism for the observed differences in dissolution rates over a pH range of 4 - 8. We propose that the surface complex formed by EDTA is the controlling factor for the observed dissolution rate, with mononuclear EDTA surface complexes accelerating goethite dissolution compared to bi- or multinuclear complexes. These experimental results suggest that EDTA forms multinuclear complexes with goethite surfaces at pH values greater than or equal to 6 and mononuclear complexes with goethite surfaces at pH values < 6. Dissolution results show that when the oxyanion and the EDTA are present in the system at concentrations nearly equaling the surface sites available for adsorption, the oxyanion reduces the adsorption of EDTA and inhibits dissolution over the pH range of 4 - 8.
However, a different mechanism occurs at pH values greater than or equal to 6 when the oxyanion is present at 0.1 mM. EDTA adsorbs but the number of carboxylic groups that can sorb to the surface is reduced causing the formation of mononuclear complexes. This shift from multi- to mononuclear surface complexes enhances the EDTA-promoted dissolution of goethite in the presence of the oxyanions compared to EDTA-promoted dissolution in their absence.