Chemical stability of hydroxysulphate green rust synthetised in the presence of foreign anions: carbonate, phosphate and silicate

Hydroxysulphate green rust {GR(SO42-)} species were precipitated in the presence of various anions. GR(SO42-) is stable at similar to pH 7 and is transformed into a mixture of magnetite and ferrous hydroxide when the pH raised at similar to 12. In the presence of carbonate species, GR(SO42-) is partially transformed into a mixture of magnetite and siderite at similar to pH 8.5. This transformation is stopped when silicate anions are present in the solution. As already observed for phosphate anions, the adsorption of silicate anions on the lateral faces of the GR(SO42-) crystals may explain this stabilization effect. Sulphate anions are easily exchanged by carbonate species at similar to pH 10.5. In contrast, anionic exchange between sulphate and phosphate anions was not observed.