Stability of vivianite and ion-pair formation in the system fe3(PO4)2-H3PO4H3PO4-H2o

A portion of the solubility isotherm for synthetic vivianite (Fe3(PO4)2·8H2O) in the three component system Fe3(PO4)2-H3PO4-H2O has been determined at 25°C. Interpretation of the data allowing for the formation of the ion-pairs FeH2PO4+ and FeHPO4° resulted in the solubility product constant for vivianite of 10−36.0±0.1. The association constants obtained were 102.7±0.2 and 103.6±0.3 respectively for FeH2PO4+ and FeHPO4°. The solubility data show that vivianite may be of significance in controlling the phosphate concentration in anaerobic sedimentary environments and that Fe2+ can be an effective precipitant for removing orthophosphates from anaerobic digester supernatant and related process streams. The ΔGf° for vivianite of −1037 kcal/mole along with the other data from the literature were used to construct diagrams depicting the stability fields of vivianite and strengite (FePO4·2H2O) in different chemical environments.