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


Cetiner Z., Wood S., Gammons C.

CHEMICAL GEOLOGY, cilt.217, ss.147-169, 2005 (SCI İndekslerine Giren Dergi) identifier identifier

Özet

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 degrees C in NaCl-HCl and NaClO4-HClO4 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 degrees C. The La(III)- and Nd(III)-phosphates had the monazite structure, and Sm(Ill)-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 H3PO40 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: