Regularities of Formation of Iron(III) Nanocrystalline Oxides and Oxyhydroxides during Oxidation of Iron(II) Compounds in an Alkaline Medium

摘要

Regularities of formation of nanocrystalline iron(III) oxides and oxyhydroxides via oxidation of iron(II) compounds in an alkaline pH range (pH ≥ 12) were studied using pH and Eh measurements, chem- ical analysis, electron microscopy, and X-ray diffraction. When the molar ratio [OH–]/[FeII] ~ 2 (pH ~ 12–12.5), the oxidation process yields cube-shaped magnetite Fe3O4 particles. An excess of an alkaline agent with an overstoichiometric concentration equal to or higher than 0.5 mol/L (pH ≥ 13.5) induces the formation of anisotropic particles of nanocrystalline goethite α-FeOOH over the entire range of the synthesis parameters studied. Reaction products (Fe_3O_4 and/or αFeOOH) are formed immediately as the initial Fe(OH)_2 starts oxidizing by the dissolution–oxidation–precipitation mechanism near the surface of Fe(OH)_2 precursor par- ticles. Carbonate ions considerably change the structure and shape of newly formed α-FeOOH particles.