Experimental determination of synthetic NdPO 4 monazite end-member solubility in water from 21°C to 300°C: implications for rare earth element mobility in crustal fluids

Abstract

The solubility of synthetic NdPO 4 monazite end-member has been determined experimentally from 21 to 300°C in aqueous solutions at pH = 2, and at 21°C and pH = 2 for GdPO 4. Measurements were performed in batch reactors, with regular solution sampling for pH measurement, rare earths and phosphorous analysis by inductively coupled plasma mass spectrometry (ICP-MS) coupled with a desolvation system. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to check that no reprecipitation of secondary phases occurred and that the mineral surfaces remained those of a monazite. Coupled with speciation calculations, measured solution compositions permitted the determination of NdPO 4 and GdPO 4 solubility products which are in general agreement with previous experimental determination on rhabdophane at 25°C, but showing that monazite is more than two orders of magnitude less soluble than inferred on the basis of previous thermodynamic estimates. The temperature evolution from 21 to 300°C of the equilibrium constant (K) of the NdPO 4 monazite end-member dissolution reaction given by: NdPO 4( s) ⇆ Nd 3++ PO 4 3− can be described by the equation: −log K= 7.621+ 0.04163T+ 1785/T where T is in Kelvins. Integration of this expression permitted the determination of the enthalpy, free energy and entropy of dissolution and formation of the NdPO 4 monazite end-member. Solubility-speciation calculations show that the presence of aqueous ligands, notably fluoride, carbonate or hydroxide in water strongly affect monazite solubility, depending on pH and temperature. These calculations also show that monazite will exhibit retrograde solubility only under acidic conditions from 70°C to 300°C and to a lesser extent in neutral aqueous solutions from 150°C to 300°C. Solubility-speciation calculations performed on natural seafloor vent hydrothermal fluids and on thermal springwaters from granitic areas at aquifer temperature show that these fluids are equilibrated with respect to monazite. Thus, our study suggests that monazite may play an active role on the control of REE concentrations in crustal waters. Finally, results of this experimental study suggest that a monazite-like nuclear waste form stored underground will show extremely low solubility when in contact with water, especially with granite-equilibrated groundwater. These results therefore confirm the excellent suitability of monazite as a nuclear ceramic.