The dissolution of magnetite by nitrilotriacetatoferrate(II)

Abstract

The dissolution of magnetite particles in solutions containing nitrilotriacetatoferrate(II) has been studied as a function of total nitrilotriacetic acid (NTA) and iron(II) concentrations, pH and temperature. Experimental results are interpreted in terms of adsorption equilibria involving the free ligand and its metal complexes, and inner- and outer-sphere interfacial electron transfer from the adsorbed electroactive Fe^II species to surface—Fe^III sites, either free or complexed by NTA. Ion transfer-controlled dissolution was ruled out by the experimental evidence. FeY^–ions [Y = N(CH₂CO₂)^3–₃] have been identified as the electroactive species, electron transfer being an outer-sphere process for the present experimental conditions: the precursor complex can be represented by—Fe^III—NTA…FeY^–. Dissolution rate deviates from first order in [FeY^–]; the order decreases with increasing [FeY^–] as a consequence of the relatively high affinity of the complex for —Fe^III—NTA surface sites. Reaction order on proton concentration is 0.67, reflecting the requirement of H⁺ ions adjacent to the site where the reductant is adsorbed. The apparent activation energy is 73 kJ mol^–1, which is a composite of equilibrium and kinetic parameters.