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Synthesis and dissolution of phosphate matrices having the monazite structure type

Abstract : In the context of the French research law dedicated to the radioactive waste management, several ceramics were proposed for the specific conditioning of actinides. Among them, monazites and monazite/cheralite solid solutions were particularly considered in this work. Thus, monazites LnPO4 (Ln = La → Gd) were prepared by thermal conversion of low-temperature rhabdophane precursors, LnPO4·0.667H2O. From synchrotron experiments, the crystal structure of rhabdophane was solved. It was found to be monoclinic (space group C2) as monazite (space group P21/n). The multiparametric study of the kinetics of dissolution of monazites LnPO4 (Ln = La → Gd) was performed in nitric acid solutions and using dynamic conditions. The normalized dissolution rates remained very low whatever the acidity, the temperature, and the lanthanide element considered. The partial order of the reaction related to the protons activity varied from 0.7 ± 0.2 to 1.5 ± 0.3. Moreover, the apparent activation energy of the dissolution mechanism was found to vary with temperature, suggesting a change in the rate-limiting step. The decrease of the apparent activation energy for T  313 K was assigned to the impact of saturation processes with respect to the rhabdophane. Thus, the solubility products as well as thermodynamic data associated to the formation of rhabdophanes were evaluated from over- and under-saturation conditions. The similar values obtained by both approaches confirmed the reversibility of the equilibrium associated to the rhabdophane precipitation. Solubility products as well as thermodynamic data only slightly varied along the lanthanide elements series : –2151 ± 13 ≤ ∆f Ho (298K) ≤ –2130 ± 12 kJ.mol-1, –2004 ± 2 ≤ f Go (298K) ≤ –1984 ± 2 kJ.mol-1 and –504 ± 11 ≤ ∆f So (298K) ≤ – 473 ± 12 J.mol-1.K-1, excepted for europium that presented the highest values of free energy (‒1896 ± 2 kJ.mol-1), enthalpy (‒2057 ± 9 kJ.mol-1) and entropy (–538 ± 11 J.mol-1.K-1) of formation. The comparison of the data obtained for rhabdophanes and monazites showed that the values associated to rhabdophane can be deduced from those of monazite by adding the contribution of 0.667 water molecule. It confirms either the monoclinic structure of the rhabdophane and the number of water molecules present in the structure. For the first time, structural and microstructural evolution of monazite pellets was monitored during dissolution thanks to several complementary surface analysis techniques (ESEM, AFM, GI-XRR and GI-XRD). This study confirmed the very good chemical durability of these ceramics (only 0.04 wt.% and 0.3 wt.% of dissolved ceramic in 0.1 M HNO3 and 0.25 M HNO3, respectively, at 363 K after 300 days of dissolution). These different techniques also highlighted that pre-existing defects (pores, cracks, polishing marks) acted as preferential dissolution zones. However, no secondary phase was clearly evidenced onto the surface of the samples even if the solutions were found to be close to equilibrium with rhabdophane. Finally, an original protocol of precipitation of Th-doped rhabdophanes, Nd1-2xCaxThxPO4·0.667H2O, was developed during this work. The first estimation of the apparent solubility constants, performed in under-saturated conditions in 0.25 M HCl solution, did not evidence any significant variation in the temperature range investigated.
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Contributor : Clémence Gausse Connect in order to contact the contributor
Submitted on : Wednesday, November 29, 2017 - 11:14:30 AM
Last modification on : Friday, August 5, 2022 - 10:37:43 AM


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  • HAL Id : tel-01611137, version 1


Clémence Gausse. Synthesis and dissolution of phosphate matrices having the monazite structure type. Chemical Sciences. Université de Montpellier, 2016. English. ⟨tel-01611137⟩



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