The aqueous geochemistry of the rare earth elements. Part XIV. The solubility of rare earth element phosphates from 23 to 150 °C

Abstract

Rare earth element (REE) phosphates such as monazite and xenotime are important as ore minerals, potential hosts of radioactive waste, and target phases for isotopic dating. However, there are still insufficient thermodynamic data with which to model dissolution and precipitation of these phases in crustal fluids quantitatively. Therefore, the solubilities of end-member La(III)-, Nd(III)-, Sm(III)-, and Y(III)- phosphates were determined at 23 and 50 °C in NaCl–HCl and NaClO 4–HClO 4 solutions with pH m from 0 to 2 and ionic strengths of 0.1, 0.5, 1.0, and 5.0 m. The solubility of Nd(III)-phosphate was also determined in chloride solutions at 150 °C. The La(III)- and Nd(III)-phosphates had the monazite structure, and Sm(III)-phosphate and Y(III)-phosphate had the rhabdophane and xenotime structures, respectively. The dependence of solubility on pH and chloride concentration, together with data from the literature, indicated that H 3PO 4 0 and Ln 3+ (where Ln 3+ represents any free, hydrated trivalent REE ion) were the predominant species in our experimental solutions. At each ionic strength and temperature investigated, conditional equilibrium constants ( Q s3) were determined for reactions of the following type: LnPO 4+3H +↔Ln 3++H 3PO 4 0 The conditional equilibrium constants determined at various ionic strengths were extrapolated empirically to obtain the equilibrium constants at infinite dilution ( K s3). These constants were then converted to solubility products ( K s0) for the following reaction: LnPO 4(s)↔Ln 3++PO 4 3− using acid dissociation constants for H 3PO 4 0 available in the literature. The values of log K s0 so obtained are: 23°C 50 °C 150 °C La −25.7 −25.4 – Nd −25.8 −26.6 −30.8 Sm −24.6 −24.8 – Y −27.9 −27.8 – These values are in reasonable agreement with the majority of those in the literature, with the exception of the values for Y(III)-phosphate, which are substantially lower. Our results, combined with data in the literature, suggest that the solubility products of REE phosphates are retrograde (i.e., decrease with increasing temperature) up to at least 300 °C. Moreover, the solubility of REE phosphate is quite low up to 300 °C, even at low pH and high chloride concentrations, confirming the robustness of these phases as hosts for radionuclides.