Portal de Periódicos da CAPES

The reductive dissolution of iron oxides by ascorbate

1990; Elsevier BV; Volume: 138; Issue: 1 Linguagem: Inglês

10.1016/0021-9797(90)90181-m

1095-7103

Marı́a dos Santos Afonso, Pedro J. Morando, Miguel A. Blesa, Steven A. Banwart, Werner Stumm,

Characterization and Applications of Magnetic Nanoparticles

There are four general pathways of dissolution of reducible metal oxides in acidic aqueous solution: proton-assisted (acid), ligand-promoted acid, reductive, and ligand-promoted reductive dissolution. The presence and reactivity toward the surface of protons, chelating ligands, and reductants dictate the mechanism(s) controlling the dissolution. For the massive reductive dissolution of magnetic by ascorbic acid, the experimental rate law R = k[HA−]12[H+] suggests the involvement of surface ≡FeIII A− complexes. Adsorption isotherms of ascorbic acid onto hematite at pH 3 and 25°C yield a Langmuir-type surface complexation constant Ks = (9.57 × 108 M−1). Slow dissolution follows with an empirical rate law R = kobs(≡FeIIIA). It is concluded that the formation and kinetic reactivity of surface complexes determine the rate of dissolution. Dehydroascorbic acid also dissolves magnetite, but at slower rates. Oxalate accelerates the reductive dissolution of hematite by ascorbate even though it competes with ascorbate for surface sites; enhanced detachment of ≡FeII surface species by oxalate complexation may be involved. Autoacceleration of the reductive dissolution by dissolved FeII-carboxylate complexes is observed in EDTA/ascorbic acid mixtures; the rate reaches a maximum at intermediate [EDTA] values, where synergistic effects between EDTA and FeII-EDTA complexes are important. Autoacceleration may also operate in oxalate solutions.