Green rusts synthesis by coprecipitation of Fe-Fe ions and mass-balance diagram

A basic solution is progressively added to various mixed FeII-FeIII solutions. The nature and the relative quantities of the compounds that form can be visualised in a mass-balance diagram. The formation of hydroxysulphate green rust {GR()} is preceded by the precipitation of a sulphated ferric basic salt that transforms in a badly ordered ferric oxyhydroxide. Then octahedrally coordinated FeII species and anions are adsorbed on the FeOOH surface and GR() is formed at the solid/solution interface. By using the same method of preparation, other types of green rust were synthesised, e.g. hydroxycarbonate green rust {GR()}. Like other layered double hydroxides, green rusts obey the general chemical formula with x1/3. Al-substituted hydroxysulphate green rust consists of small hexagonal crystals with a lateral size 50 nm, which is significantly smaller than the size of the GR() crystals (500 nm).