Adsorption of metals and dissolution of ferrihydrite in the presence of polyphosphates

In many natural environments, the concentrations of metal ions are regulated by precipitation and adsorption/dissolution reactions occurring at water/oxide interfaces. This study investigated and modelled the important reactions dominating the oxide dissolution process and controlling the partitioning of dissolved metal ions in a system containing strongly complexing and strongly sorbing ligands. Polyphosphates and ferrihydrite were used to represent such a system.;The partitioning of metal ions was directly related to that of polyphosphates, which adsorb strongly to the surface. Metal removal usually increases as polyphosphate is added at acidic pH (where free metals sorb weakly), as a result of complexation reactions with polyphosphates in solution and/or at the surface. When total polyphosphate added is greater than that necessary to saturate the surface, the dissolved polyphosphate controls the metal speciation in solution and can cause desorption of metals which are bound either to surface sites or to sorbed polyphosphates. At alkaline pH where free metals sorb strongly, the dissolved polyphosphates compete with the surface for dissolved metal ions and decrease metal sorption. In general, the metal-polyphosphate complexes behave in a ligand-like fashion.;The dissolution of ferrihydrite is controlled by a surface reaction and is enhanced by sorption of tripolyphosphates via a process called ligand-promoted dissolution. As a result, Fe and tripolyphosphate leave the surface as one entity. If all the tripolyphosphate in a system can be adsorbed by the available surface sites, Fe-tripolyphosphate complexes which are released to solution can quickly re-adsorb to the surface, forming a different species from the precursor to the dissolution reaction. The rapid dissolution and strong adsorption of complexes counteract the iron dissolution reaction and lead to non-linear net dissolution kinetics.;An adsorption/dissolution kinetic model was developed which successfully simulates the experimental observations here as well as previous work reported in the literature for systems where adsorption is weaker than dissolution. The model verified the important role of the adsorption of complexes, which accounts for the non-linear relationship between rate of dissolution and surface ligand concentration.;This work characterized the significant role of strongly sorbing and strongly complexing ligands in controlling metal partitioning and oxide dissolution. The results provide useful information for understanding the behavior of heavy metals in natural systems and in waste treatment operations, such as when metal oxides are applied to treat metal-bearing wastes containing strongly sorbing and strongly complexing ligands. The model developed based on such a system provides a useful tool for studies of dissolution kinetics of minerals in the presence of various ligands such as humic acids.