The substitution of Fe²⁺ ions by Ni²⁺ ions in green rust one compounds

Abstract

The oxidation of Fe(OH)₂ in the presence of Cl⁻ or CO ²⁻₃ ions leads, in the first stage of the reaction, to chloride-containing green rust one (GR1), 3Fe(OH)₂· Fe(OH)₂Cl·nH₂O, or carbonate-containing GR1, 4Fe(OH)₂·Fe₂(OH)₄CO₃·nH₂O, respectively. These GR1 compounds give the ferric oxyhydroxides by further oxidation. If a hydroxide Ni _x Fe_1−x (OH)₂ is initially precipitated, the reaction leads to a nickelous-ferric compound isomorphous to the ferrous-ferric GR1, but stable with respect to the oxidizing action of air. Similarly, the oxidation of a nickelous-ferrous hydroxide, in the presence of excess OH⁻ ions, leads to a nickelous-ferric GR1, a layered hydroxide with anionic interlayers made of OH⁻ ions and water molecules. The Mössbauer spectra of these nickelousferric GR1 display two ferric doublets, D₀ withIS=0.34 mm/s andQS=0.45 mm/s and D₁ withIS=0.36 mm/s andQS=0.86 mm/s. The existence of a ferrous-ferric GR1 incorporating OH⁻ ions, a compound never observed so far, is strongly suspected.