Phosphate-hydrogenphosphate Hydrate Research Articles

Solubility product of the thorium phosphate hydrogen-phosphate hydrate (Th2(PO4)2(HPO4)·H2O, TPHPH)

The standard solubility constant of TPHPH, Th2(PO4)2(HPO4)·H2O was evaluated at 298K by performing dissolution experiments of a synthetic sample in different nitric acid solutions at a constant ionic strength of 1molL−1. The average value of the solubility constant and the associated expanded uncertainty U(log10Ks,0∘ (298K), 0.95 level of confidence) were evaluated to log10Ks,0∘ (TPHPH, 298K)=−(71.2±1.3), while the ΔfG∘ (TPHPH, 298K) value reached −(5200±23)kJ·mol−1. From the experiments performed at different temperatures, the standard molar enthalpy as well as the standard molar entropy of the dissolution reaction was determined. The solubility of TPHPH was found to be retrograde over the investigated range of temperatures (298–363)K. The values of the standard enthalpy of formation and the standard molar entropy of TPHPH are equal to: ΔfH∘(TPHPH)=-(5640.5±16.0)kJ·mol-1 and Sm=(298±68)JK−1mol−1, respectively.

Thermodynamic studies of thorium phosphate diphosphate and phase investigations of Th-P-O and Th-P-H2O systems

Abstract The standard molar enthalpy of formation of thorium phosphate diphosphate, β-Th4(PO4)4P2O7(cr) has been determined using an isoperibol solution calorimeter, a differential scanning calorimeter and phase transition data of high temperature calorimeter. The enthalpy of precipitation of thorium phosphate-hydrogenphosphate hydrate, Th2(PO4)2(HPO4)·H2O(cr), was measured at 298.15 K using an isoperibol solution calorimeter. Heat capacities of α-Th4(PO4)4P2O7(cr) and β-Th4(PO4)4P2O7(cr) were measured using a Differential Scanning Calorimeter. Combining these experimental data, and other auxiliary data from the literature, thermo-chemical reaction scheme was devised to calculate the standard molar enthalpy of formation of β-Th4(PO4)4P2O7(cr) to be {−10565.5 ± 13.6} kJ mol−1. The thermodynamic functions for β-Th4(PO4)4P2O7(cr) have been computed using Δ f H m o ( 298.15 K ) , C p , m o ( T ) and literature data. The Gibbs phase diagram and predominant area diagrams for Th-P-O system have been computed using the thermodynamic information of the various phase present in Th-P, P-O and ThO2-P2O5 systems. EH–pH diagram of Th-P-H2O system has been computed to determine stability of Th4P6O23(cr) in ground water.

From Thorium Phosphate Hydrogenphosphate Hydrate to β‐Thorium Phosphate Diphosphate: Structural Evolution to a Radwaste Storage Ceramic.

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From thorium phosphate hydrogenphosphate hydrate to β-thorium phosphate diphosphate: Structural evolution to a radwaste storage ceramic

β-Thorium phosphate diphosphate ( β-TPD), considered as a very promising radwaste storage material, was obtained from thorium phosphate hydrogenphosphate hydrate (TPHPH) precursor through dehydration and hydrogen phosphate condensation. The structures of TPHPH, intermediate α-thorium phosphate diphosphate ( α-TPD) and its hydrate ( α-TPDH) have been resolved ab initio by Rietveld analysis of their synchrotron diffraction patterns. All were found orthorhombic (space group Cmcm) and similarly composed of [ThPO 4] 4 4+ slabs alternating with disordered layers hosting either [HPO 4·H 2O] 2 4− (TPHPH), [P 2O 7·2H 2O] 4− ( α-TPDH), or [P 2O 7] 4− ( α-TPD), unlike the 3D structure of β-TPD. The diphosphate groups of α-TPD and α-TPDH are strongly bent. The irreversible transition to the final β-TPD consists in a shearing of the slabs and a reduction of the interslabs cavities that explains the stability of this high-temperature form.

Improvement of the preparation of sintered pellets of thorium phosphate-diphosphate and associated solid solutions from crystallized precursors

Several compositions of thorium–uranium (IV) phosphate–hydrogenphosphate hydrate (Ux/2Th2−x/2(PO4)2(HPO4)·H2O, TUPHPH) were prepared starting from actinides chloride solutions and concentrated phosphoric acid. The experimental synthesis parameters were optimized in order to get the quantitative precipitation of the cations and a good crystallization state. The extensive characterization of the solids demonstrated the existence of a complete solid solution between Th and U end-members and evidenced the good homogeneity of the powders. Their behaviors during heating treatment were then checked and confirm the formation of anhydrous thorium–uranium (IV) phosphate–hydrogenphosphate (TUPHP) and α-UxTh4−x(PO4)4(P2O7) (α-TUPD) acting as intermediates. Finally, the low-temperature crystallized precursors were used in original sintering processes in order to improve the efficiency of the former cold-pressing sintering procedure.

Behavior of thorium–uranium (IV) phosphate–diphosphate sintered samples during leaching tests. Part II. Saturation processes

Sintered pellets of thorium–uranium (IV) phosphate–diphosphate solid solutions (β-Th 4− x U x (PO 4) 4P 2O 7, β-TUPD) were altered in several acidic media. All the results reported in the first part of this paper confirmed the good chemical durability of the samples. The evolution of the normalized weight loss showed that, in several media, thorium quickly precipitates in a neoformed phosphate-based phase while uranium (IV) is released in the leachate due to its oxidation into the uranyl form. The characterization of neoformed phases was carried out through several techniques involving grazing XRD, infrared and μ-Raman spectroscopies, EPMA, SEM and TEM. SEM micrographies showed that the dissolution mainly occurs at the grain boundaries, leading to the break away of the grains: only the first 15 μm are altered for 2 months in 10 −1 M HNO 3. From EPMA and BET measurements, neither the chemical composition nor the specific surface area are significantly modified. Near equilibrium, two neoformed phases were observed and identified by grazing XRD and/or μ-Raman spectroscopy at the surface of the leached pellets: one is found to be amorphous and progressively turns into the crystallized thorium phosphate–hydrogenphosphate hydrate (TPHPH). From the results obtained, a chemical scheme of the dissolution of β-TUPD sintered samples is proposed. The behavior of the actinides in the gelatinous phase appears mainly driven by their oxidation state: thorium remains in the tetrapositive state and is quickly and quantitatively precipitated while uranium (IV) is oxidized into uranyl then released in the leachate. The Th-precipitation as TPHPH first appears scattered then covers the entire surface of the pellet, inducing a delay of the actinides release in the leachate. Both phases act as protective layers and should induce the significant delay of the release of actinides (Th, U) to the biosphere.

Characterization of the thorium phosphate-hydrogenphosphate hydrate (TPHPH) and study of its transformation into the thorium phosphate-diphosphate (β-TPD)

The preparation of thorium phosphate-diphosphate (Th 4(PO 4) 4P 2O 7, TPD) was developed through the precipitation of thorium phosphate-hydrogenphosphate hydrate (Th 2(PO 4) 2(HPO 4)·H 2O, TPHPH) at 150–160 °C in closed PTFE container or in autoclaves. From EPMA analyses and SEM observations, the initial precipitate was single phase and multilayered. The behaviour of TPHPH (orthorhombic system with a = 21.368(2) Å, b = 6.695(1) Å and c = 7.023(1) Å) was followed when heating up to 1250 °C. It was first dehydrated leading to the anhydrous thorium phosphate-hydrogenphosphate (TPHP, orthorhombic system with a = 21.229(2) Å, b = 6.661(1) Å and c = 7.031(1) Å at 220 °C) after heating between 180 and 200 °C. This one turned progressively into the new low-temperature variety of TPD (called α-TPD, orthorhombic system with a = 21.206(2) Å, b = 6.657(1) Å and c = 7.057(1) Å at 300 °C) correlatively to the condensation of hydrogenphosphate groups into diphosphate entities. These three phases (TPHPH, TPHP and α-TPD) exhibit closely related 2D layered structures, therefore different from the 3D structure of the thorium phosphate-diphosphate (high-temperature variety). This latter compound, now called β-TPD, was obtained by heating α-TPD above 950 °C. All the techniques involved in this study (XRD, Raman and IR spectroscopy, 1H and 31P NMR) confirmed the successive chemical reactions proposed.

Synthesis and characterization of uranium (IV) phosphate-hydrogenphosphate hydrate and cerium (IV) phosphate-hydrogenphosphate hydrate

A new uranium (IV) phosphate of proposed formula U 2(PO 4) 2HPO 4·H 2O, i.e. uranium phosphate-hydrogenphosphate hydrate (UPHPH), was synthesized in autoclave and/or in polytetrafluoroethylene closed containers at 150 °C by three ways: from uranium (IV) hydrochloric solution and phosphoric acid, from uranium dioxide and phosphoric acid and by transformation of the uranium hydrogenphosphate hydrate U(HPO 4) 2· nH 2O. The new product appears similar to the previously published thorium phosphate-hydrogenphosphate hydrate Th 2(PO 4) 2HPO 4·H 2O (TPHPH). From preliminary studies, it was found that UPHPH crystallizes in monoclinic system ( a = 2.1148 ( 7 ) nm , b = 0.6611 ( 2 ) nm , c = 0.6990 ( 3 ) nm , β = 91.67 ( 3 ) ° and V = 0.9768 ( 10 ) nm 3 ). Heated under inert atmosphere, this compound is decomposed above 400 °C into uranium phosphate-triphosphate U 2(PO 4)P 3O 10, uranium diphosphate α -UP 2O 7 and diuranium oxide phosphate U 2O(PO 4) 2. Crystallized cerium (IV) phosphate-hydrogenphosphate hydrate Ce 2(PO 4) 2HPO 4·H 2O (CePHPH) was also synthesized from (NH 4) 2Ce(NO 3) 6 and phosphoric acid solutions by the same method (monoclinic system: a = 2.1045 ( 5 ) nm , b = 0.6561 ( 2 ) nm , c = 0.6949 ( 2 ) nm , β = 91.98 ( 1 ) ° and V = 0.9588 ( 9 ) nm 3 ). When heating above 600 °C, cerium (IV) is reduced into Ce (III) and forms a mixture of CePO 4 (monazite structure) and CeP 3O 9.

Synthesis and characterization of low-temperature precursors of thorium–uranium (IV) phosphate–diphosphate solid solutions

Several compositions of new precursor of thorium–uranium (IV) phosphate–diphosphate solid solutions (Th4−xUx(PO4)4P2O7, called β-TUPD) were synthesized in closed PTFE containers either in autoclave (160°C) or on sand bath (90–160°C). All the samples appeared to be single phase. From XRD data and TEM observations, the diffraction lines matched well with that of pure thorium phosphate–hydrogenphosphate hydrate (TPHPH), Th2(PO4)2(HPO4)·H2O, which confirmed the preparation of a complete solid solution between pure thorium and uranium (IV) compounds. TGA/DTA experiments showed that samples of thorium–uranium (IV) phosphate–hydrogenphosphate hydrate (TUPHPH) prepared at 150–160°C were monohydrated leading to the proposed formula Th2−x/2Ux/2(PO4)2(HPO4)·H2O. The variation of the XRD diagrams versus the heating temperature showed that TUPHPH remained crystallized and single phase from room temperature to 200°C. After heating between 200°C and 800°C, the presence of diphosphate groups in the solid was evidenced. In this range of temperature, the solid was transformed into the low-temperature monoclinic form of thorium–uranium (IV) phosphate–diphosphate (α-TUPD). This latter compound finally turned into well-crystallized, homogeneous and single-phase β-TUPD (orthorhombic form) above 930–950°C for x values lower than 2.80. For higher x values, a mixture of β-TUPD, α-Th1−zUzP2O7 and U2−wThwO(PO4)2 was obtained. By this new chemical route of preparation of β-TUPD solid solutions, the homogeneity of the samples is significantly improved, especially considering the distribution of thorium and uranium.

Synthesis, Characterization, Sintering and Leaching of Thorium – Uranium (IV) Phosphate Diphosphate Solid Solutions from Low-Temperature Precursors

ABSTRACTIn the field of the immobilization of high level activity and long life radwaste for a deep underground repository, several phosphate matrices were already proposed to delay the release of actinides to the biosphere. Among them, Thorium Phosphate-Diphosphate (TPD) and associated solid solutions were considered. In order to improve the homogeneity of the samples prepared, a new way of preparation based on the precipitation in closed containers, at 150°C, of the Thorium-Phosphate HydrogenPhosphate hydrate (TPHP) or associated Thorium – Uranium (IV) Phosphate HydrogenPhosphate hydrate (TUPHP) was developed. Well crystallized and single phase TUPD solid solutions of general formula Th4−xUxP6O23 were prepared after heating above 950°C. Dense TUPD pellets were also obtained through a two-steps procedure involving a uniaxial pressing of the powder at room temperature then a heating treatment at 1250°C for 10 hours. Several properties of the pellets such as their chemical durability during leaching tests were considered carefully. All the normalized dissolution rates of powdered and sintered TUPD solid solutions remained low by comparison to that reported for several other ceramics studied in the same objective.