Influence of ruthenium ions on the precipitation of α-FeOOH, α-Fe2O3 and Fe3O4 in highly alkaline media

The influence of ruthenium ions on the precipitation of goethite (α-FeOOH), α-Fe2O3 and Fe3O4 in highly alkaline media was investigated by 57Fe Mössbauer and FT-IR spectroscopies, thermal field emission scanning electron microscope (FE SEM) and EDS. The presence of Ru-dopant strongly affected the precipitation of α-FeOOH at highly alkaline pH, i.e. the formation of α-Fe2O3 was also noticed. A decrease of hyperfine magnetic field (HMF) at RT from 35.1 T (undoped α-FeOOH) to 31.3 T for sample with [Ru]/([Ru] + [Fe]) = 0.0196 was assigned to the incorporation of ruthenium ions into the α-FeOOH structure. Mössbauer spectroscopy showed the formation of stoichiometric Fe3O4 for [Ru]/([Ru] + [Fe]) = 0.0291–0.0909. α-Fe2O3 and Fe3O4 did not show a tendency to the formation of solid solutions with ruthenium ions. FE SEM observations of the samples showed that reference α-FeOOH sample contained acicular particles of good uniformity, which increased the length up to ∼5 times with increase of concentration of ruthenium ions. On the other hand, large octahedral Fe3O4 crystals (particles) were associated with small particles of ruthenium (hydrous) oxide with a size in the range ∼100 nm or less. A possible catalytic action of ruthenium that created reduction conditions for Fe3+ ions and formation of Fe2+ ions for precipitation of Fe3O4 was discussed.