THE REACTIVITY OF FE(III) (HYDR)OXIDES - EFFECTS OF LIGANDS IN INHIBITING THE DISSOLUTION

The rates of dissolution of Fe(III) (hydr)oxides are critically dependent on the coordinative interactions taking place on their surfaces, above all of the reactions of the surface functional > FeOH groups with H+, OH-, metal ions and suitable ligands. Surface processes rather than transport processes are typically the rate-controlling steps. Thus the dissolution rates depend on the type of surface species present. In this work, the inhibition of the dissolution of gamma-FeOOH by various organic and inorganic solutes, benzoate, arsenate, phosphate, selenite, sulfate and the inhibition of the acid dissolution of alpha-FeOOH by Cr(III) and chromate was investigated. Enhancement and inhibition of dissolution are relative terms and have to be measured operationally in relation to a reference condition. As a reference ligand in ligand-promoted and ligand-inhibited dissolution, ethylene diamine tetraacetate (EDTA) was chosen because it is dissolution-active over a relatively large pH range. The experimental results show pronounced inhibition of the dissolution by phosphate, arsenate and selenite at near neutral pH values. At pH < 5, phosphate, arsenate and selenite accelerate the dissolution. It is postulated that binuclear surface complexes inhibit the dissolution, because much more energy is needed to remove simultaneously two center atoms from the crystalline lattice than to remove solely one center. However, mononuclear complexes, especially if they are nucleophilic and bidentate, accelerate the dissolution.