Coprecipitation of gadolinium with calcium sulphate dihydrate

Abstract

The present study reports the coprecipitation of gadolinium with calcium sulphate dihydrate, in aqueous solutions of sulphuric acid, at constant ionic strength using a radiotracer technique.The experimental results obtained at ambient temperature, show that the quantity of gadolinium ions coprecipitated in gypsum identified by X-ray diffraction (XRD) and elemental analysis is very small, lower than 4 at. %. The mechanism of coprecipitation of Gd(III) has been discussed, although the elementary processes of the mechanism are not clear.The distribution of gadolinium between precipitate of gypsum and saturated aqueous solutions of H 2 SO 4 has been measured between 30 and 70 • C. Experiments of coprecipitation indicate that the coefficient of distribution of Gd(III) depends on the solubility and nature of solid calcium sulphate phase.

The effect of fluoride and silicate ions on the coprecipitation of gadolinium with calcium in phosphoric and sulpho-phosphoric media

“…This acid in the concentration range used (C H 2 SO 4 ≥ 0.1 M) inhibits the almost total coprecipitation of Gd(III) recorded in NH 4 H 2 PO 4 medium. The distribution coefficient averaging 0.09 ± 0.02, obtained in the studied sulpho-phosphoric media, is comparable to the value determined by Bouhlassa and Salhamen [15], for sulphuric medium C H 2 SO 4 ≤ 0.1 M, where the Gd(III) is coprecipitated with gypsum. This finding suggests that the Gd(III) is coprecipitated with calcium sulphate dihydrate and the presence of NH 4 H 2 PO 4 in the medium does not influence this coprecipitation.…”

“…The incorporation of the gadolinium as a micro-component by the sulphate dihydrate as a macrocomponent is independent of both C H 2 SO 4 and C NH 4 H 2 PO 4 . In a previous work carried out by Bouhlassa and Salhamen [15], it was established that this coprecipitation controlled by the concentration of the macro-component, does not result from homogeneous crystallization or isomorphous substitution of calcium by gadolinium. In the media considered, the process of coprecipitation would be described by a heterovalent solid-solution or heterovalent substitution of gadolinium in gypsum as suggested for the incorporation of REE in other minerals [24][25][26] and supported by the results of the coprecipitation of eu- ropium with barite [27].…”

Substitution of Calcium with Ce, Nd, Er, and Tb in the Structure of Microcrystals of Calcium Sulfates with Controlled Hydration Water: A Proposed Mechanism

“…In addition, the difference between the bond types and energies of Ca 2+ –SO 4 2– ions in dihydrate crystal lattice in the absence and presence of any other ions or additives in the solution that leads to the formation of new bonds between ions can modify the surface energies of specific crystal planes and change the nucleation and growth kinetics depending on the type of bonds and surface energies. It is also shown by other investigators that the acidity of the system affects the cocrystallization of REE with calcium by sulfate ions; therefore, the surface mechanism is more probable, and the isomorphous substitution is not a good mechanism to explain the coprecipitation of REE with CSD.…”

Americium

“…The only recorded aliphatic compounds of americium appear to be the citrates Am(C 6 H 5 O 7 ) · xH 2 O and [Co(NH 3 ) 6 ][Am(C 6 H 5 O 7 ) 2 ] · xH 2 O (Bouhlassa, 1983), and the salicylate Am(C 7 H 5 O 3 ) 3 · H 2 O (Burns and Baldwin, 1976); its structure is described in Section 8.9.2.…”