Nanophase REE phosphate crystallization induced by vivianite oxidation: mechanistic insights and mineralogical implications

Our study investigates the interaction between multi-component rare earth element (REE; La, Ce, Pr, Nd, Dy)-bearing aqueous solutions and vivianite (Fe₃ ²⁺(PO₄)₂·8H₂O) grains under hydrothermal conditions (50-165 °C). The results revealed the solution-mediated, progressive oxidation and dissolution of vivianite. This resulted in the formation of iron phosphates, metavivianite [Fe²⁺Fe₂ ³⁺(PO₄)₂(OH)₂·6H₂O], and giniite [Fe²⁺Fe₄ ³⁺(PO₄)₄ (OH)₂·2H₂O], iron oxide hematite [Fe₂O₃], and rare earth phosphates, rhabdophane [REE(PO₄)·H₂O] and monazite [(LREE)PO₄]. The extent of the reactions was found to be dependent on temperature, pH, and the concentration and ionic radii of the rare earths in solution. The rate of vivianite oxidation and dissolution increased with increased temperature, with 50% of vivianite transformed after 32 days at 50 °C, and 100% transformed after 28 days and 4 hours at 90 and 165 °C respectively. The pH of the solutions at all three temperatures maintained the stability of rhabdophane, and only at the highest temperature of 165 °C it began to transform to monazite. Understanding the stability of iron phosphates, their transformation products, and their capacity to incorporate REEs is crucial for resource recovery, especially in the extraction of REEs from waste materials.