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

A basic solution is progressively added to various mixed Fe-II-Fe-III 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(SO42-)} is preceded by the precipitation of a sulphated ferric basic salt that transforms in a badly ordered ferric oxyhydroxide. Then octahedrally coordinated Fe-II species and SO42- anions are adsorbed on the FeOOH surface and GR(SO42-) 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(CO32-)}. Like other layered double hydroxides, green rusts obey the general chemical formula [(M(1-x)MxIII)-M-II(OH)(2)](x-.)[(x/n) A(n-) (.) mH(2)O](x+) with x <= 1/3. Al-substituted hydroxysulphate green rust consists of small hexagonal crystals with a lateral size similar to 50 nm, which is significantly smaller than the size of the GR(SO42-) crystals (similar to 500 nm).