The effects of strongly complexing ligands on the adsorptive partitioning of metal ions

Abstract

This work investigated the effects of polyphosphate, a model ligand which forms strong complexes with metals, on metal sorption onto ferrihydrite. Adsorption experiments were conducted to study the partitioning of metal ions in systems containing a range of metal and ligand concentrations. The sorption of ligands, the associated sorption of charge and complexation of metals by ligands are important in controlling metal partitioning. In metal-free systems, the apparent maximum adsorption density is about 0.1 mol of polyphosphate per mol of Fe. The sorption of metals and/or metal-ligand complexes increases the maximum sorption density of ligands compared to systems where metal ions are not present. At total polyphosphate concentrations less than that needed to saturate the oxide surface, the presence of polyphosphate enhances the sorption of metals. When the concentration of polyphosphate exceeds that needed to saturate the oxide surface, the sorption of metals diminishes due to both the competition between dissolved and surface polyphosphate for metal ions and the dissolution of iron oxides.