Dehydration of synthetic and natural vivianite

Thermal analyses of synthetic and natural vivianite (Fe2+)3(PO4)2·8H2O) were determined using a high-resolution thermal analyser coupled to a mass spectrometer. Five dehydration weight loss steps were observed for the natural vivianite at 105, 138, 203, 272 and 437 °C. The first weight loss step involves the reaction (Fe2+)3(PO4)2·8H2O→(Fe2+)3(PO4)2·3H2O+5H2O. The TGA/MS for the synthetic vivianite gave similar results to that of the natural sample. Mass spectrometry shows that water is lost up to 450 °C and after this temperature oxygen is lost. Changes in the structure of vivianite were followed using infrared emission spectroscopy. A model is proposed for the dehydration of vivianite.