Impact of Pb2+ presence on precipitation of REE phosphates (analogs of rhabdophane) from aqueous solutions.

Research on rare earth elements (REE) is currently of high importance due to these elements being considered as critical raw materials. Particularly REE phosphates are the subject of laboratory experiments since they have various applications resulting from their structural diversity. In crystalline rocks REE phosphates occur as monazites and xenotimes (REEPO₄, monoclinic and tetragonal, respectively). Secondary phosphates precipitate out of aqueous solution usually in the form of rhabdophane or churchite (REEPO₄·nH2O, trigonal and monoclinic). Their formation results in immobilization of REE which plays a significant role in controlling the solubility of REE in nature, in beneficiation processes, and in technological applications.

The precipitation of rhabdophanes is usually studied experimentally in pure systems. However, it is recognized that the presence of other solution components can significantly affect the processes and the final product. Since both Pb and REE readily form phosphates that precipitate out of aqueous solutions, it has been hypothesized that the precipitation of REE-phosphates in the presence of Pb would result in the formation of mixed phosphate phases (containing both Pb and REE) or a mixture of two phases: phosphoschultenite PbHPO_4­ and rhabdophane REEPO₄·nH₂O. Despite the difference in ionic charge between Pb²⁺ and REE³⁺, formation of either phosphoschultenite PbHPO_4­ partially substituted with REE, or rhabdophanes partially substituted with Pb was also considered.

Synthesis of La-, Ce-, and Sm-rhabdophanes was attempted in the absence (control) and in the presence of Pb²⁺ ions in the solution (at ambient conditions, pH between 2 and 4). The final solutions were analyzed with inductively coupled plasma optical emission spectroscopy (ICP-OES) for Pb and REE concentrations, while solids were filtered, dried, and analyzed with powder X-ray diffraction (PXRD), scanning electron microscopy (SEM/EDS), Raman spectroscopy, and differential thermal analysis (DTA/TG).

As expected, monoclinic analogs of rhabdophanes precipitated in the absence of Pb: LaPO₄·0.67H₂O, CePO₄·0.67H₂O, and SmPO₄·0.67H₂O. However, at the presence of Pb²⁺, distinct new phases were formed. This precipitation removes REE elements from the solution very efficiently. The product forms extremely fine (<1 mm) crystalline precipitate in the form of globular aggregates. XRPD patterns of each of them are nearly identical, shifted towards higher angles as the ionic radius decreases in the order La – Ce – Sm. At this stage of research, the structure of these phases could not be identified conclusively. Chemical composition was approximated using SEM/EDS microanalysis (the precipitate is too fine for a microprobe). Raman spectrum indicates that the phases are hydrated and DTA analysis allowed to estimate the water content. At this stage of research, the chemical formula has been determined as La₂Pb₃(PO₄)₄ · nH₂O, Ce₂Pb₃(PO₄)₄ · nH₂O, and Sm₂Pb₃(PO₄)₄ · nH₂O, where n is between 3.3 and 3.5. Such unexpected results provide better insight into the new recovery pathways currently being explored for these critical raw materials. The results of these preliminary experiments open up new avenues for exploring as yet unknown crystalline phases composed of Pb-REE phosphates with potentially interesting practical applications.

This research was partly funded by NCN research grants no. 2021/43/O/ST10/01282 and 2019/35/B/ST10/03379.