Influence of organic ligands on the stoichiometry of magnetite nanoparticles

Abstract

Magnetite, a ubiquitous mineral in natural systems, is of high interest for a variety of applications including environmental remediation, medicine, and catalysis. If the transformation of magnetite to maghemite through the oxidation of Fe²⁺ has been well documented, mechanisms involving dissolution processes of Fe²⁺ in aqueous solutions have been overlooked. Here, the effect of dissolved organic ligands (EDTA (ethylenediaminetetraacetic acid), acetic, lactic and citric acids) on Fe²⁺ solubility and on the stoichiometry (Fe(II)/Fe(III)) of magnetite–maghemite nanoparticles (∼10 nm) was investigated. These ligands were chosen because of their environmental relevance and because they are widely used as coating agents for nanotechnology applications. Results show an insignificant effect of 2 organic ligands (acetate and lactate) on the dissolution of Fe. By contrast, citrate and EDTA enhanced Fe solubility because of the formation of dissolved Fe(II)– and Fe(III)–ligand complexes. Both ligands selectively bound Fe(II) over Fe(III), but EDTA was much more selective than citrate. The combined effects of oxidation and H⁺- and ligand-promoted dissolution of Fe from magnetite were predicted using a magnetite–maghemite solid solution model, accounting for the formation of dissolved Fe(II)– and Fe(III)–ligand complexes. Therefore, these results show that citrate and EDTA (i) enhance Fe solubility in the presence of magnetite nanoparticles and (ii) modify magnetite stoichiometry, which affects its environmental behavior and its properties for nanotechnology applications.