Abstract
Thermochemical characterization of Ca4La6(SiO4)6(OH)2a synthetic La- and OH-analogous of britholite: implication for monazite and LREE apatites stabilityIn this contribution, monazite (LREEPO4) solubility is addressed in a chemical system involving REE-bearing hydroxylapatite, (Ca, LREE)10(PO4,SiO4)6(OH)2. For this purpose, a synthetic (La)- and (OH)-analogous of britholite, Ca4La6(SiO4)6(OH)2, was synthesised and its thermodynamic properties were measured. Formation enthalpy of -14,618.4±31.0 kJ·mol-1was obtained by high-temperature drop-solution calorimetry using a Tian-calvet twin calorimeter (Bochum, Germany) at 975 K using lead borate as solvent. Heat capacities (Cp) were measured in the 143-323 K and 341-623 K ranges with an automated Perkin-Elmer DSC 7. For calculations of solubility diagrams at 298 K, the GEMS program was used because it takes into account solid solutions. In conditions representative of those expected in nuclear waste disposal, calculations show that La-monazite is stable from pH = 4 to 9 with a minimum of solubility at pH = 7. La-bearing hydroxylapatite precipitates at pH > 7 with a nearly constant composition of 99% hydroxylapatite and 1% La-britholite. Each mineral buffers solution at extremely low lanthanum concentrations (log{La} = 10-10-10-15mol·kg-1for pH = 4 to 13). In terms of chemical durability, both La-monazite and La-rich apatite present low solubility, a requisite property for nuclear-waste forms.
Highlights
Its ubiquity, high chemical durability and resistance to metamictisation confer to monazite qualities of a robust U-Th-Pb chronometer (e.g. Spear, Pyle 2002) as well as a potential candidate as nuclear waste form (Boatner 1988; Ewing, Wang 2002)
Formation enthalpy of –14,618.4 ± 31.0 kJ·mol–1 was obtained by high-temperature drop-solution calorimetry using a Tian-calvet twin calorimeter (Bochum, Germany) at 975 K using lead borate as solvent
In conditions representative of those expected in nuclear waste disposal, calculations show that La-monazite is stable from pH = 4 to 9 with a minimum of solubility at pH = 7
Summary
High chemical durability and resistance to metamictisation confer to monazite qualities of a robust U-Th-Pb chronometer (e.g. Spear, Pyle 2002) as well as a potential candidate as nuclear waste form (Boatner 1988; Ewing, Wang 2002). A thermochemical study compiling measured formation enthalpy, entropy and heat capacity was given for synthetic monazite-(La) (Janots et al 2007) With those thermochemical properties, solubility of monazite end-members and corresponding REE speciation can be nowadays calculated for different conditions of temperature (T), pressure (P), pH and aqueous speciation. Thermochemical data have been measured and calculations attempted to investigate the conditions under which REE-bearing apatite is less soluble than monazite and controlled the REE concentrations in solution. For this purpose, calorimetric data have been collected on a synthetic (La)- and (OH)- equivalent of britholite, Ca4La6(SiO4)6(OH), which forms a solid solution with the apatite, Ca10(PO4)6(OH,F) (Ito 1968). Solubility diagrams were calculated for geochemical conditions representative of high level deposit in clay
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