The role of iron(II) excess and phosphate to synthesize hydroxychloride green rust

Green rust (GR) is a chemically reactive mineral of interest for environmental remediation. However, the lack of stability of this compound is a major drawback for practical applications. In this study, the precipitation of hydroxychloride green rust type 1 (GR1Cl) from soluble Fe II and Fe III species was studied by analyzing pH titration curves and employing XRD, Raman and Mössbauer spectroscopies to find optimal conditions for synthesizing GR1Cl. Using the stoichiometric condition for Fe II -Fe III corresponding to the [Fe II 3 Fe III (OH) 8 ]Cl compound, Fe III O(OH) 2.9 Cl 0.1 was formed in a first step during the addition of NaOH and then transformed into a mixture of GR1Cl and Fe 3 O 4 . The addition of phosphate anions (PO 4 ) at a molar ratio PO 4 : Fe = 1: 10 in the initial solution was shown to avoid the formation of undesired magnetite. Interestingly, GR1Cl was also the only product without adding PO 4 using an excess of Fe II species in solution, i.e. for an initial ferric molar fraction x = Fe III : Fe tot of 0.2. First experimental evidences of a hydroxyphosphate green rust type 2 GR2PO 4 were also provided by XRD and Raman spectroscopy. Therefore, this work may attract interest for a better understanding of the geochemical cycle of iron and phosphate in natural environments such as hydromorphic soils or hydrothermal chimneys where fougerite, the mineral homologue of synthetic GR, is supposed to play a significant role. • Fe3O4 is formed during green rust (GRCl) synthesis in stoichiometric conditions. • Adding the appropriate amount of PO4 to the FeII-FeIII solution stabilizes GRCl. • Excess of FeII in solution leads to single GRCl phase even without adding PO4. • A specific XRD pattern reflects PO4 and Cl exchange in green rust.