DISSOLUTION KINETICS OF MINERALS IN THE PRESENCE OF SORBING AND COMPLEXING LIGANDS

This study investigated and modeled the important reactions controlling oxide dissolution in a system containing strongly complexing and strongly sorbing ligands. Polyphosphate and ferrihydrite were the model ligand and oxide studied, respectively. The dissolution of ferrihydrite in these systems is via a process called ligand-promoted dissolution, which is controlled by a surface reaction and is initiated by sorption of tripolyphosphates. Fe and tripolyphosphate leave the surface as one entity. If empty surface sites are available, Fe–tripolyphosphate complexes that are released to solution can quickly readsorb to the surface, forming a species different from the precursor to the dissolution reaction. The strong adsorption of complexes counteracts the iron dissolution reaction and leads to nonlinear net dissolution kinetics. An adsorption/ dissolution kinetic model is developed that successfully simulates the experimental observations here as well as previous work reported in the literature for systems where adsorption of free ligands and complexes is much weaker. © 1990, American Chemical Society. All rights reserved.