The distribution behaviour of the trivalent europium ion at a micro-component scale, between barium sulphate and aqueous solution, was studied at ambient temperature. Experiments were carried out using radioactive tracers. Results indicate an enrichment of the micro component in the solid phase relative to the solution. The effects of the concentrations of the micro and macro-elements on the coprecipitation have been examined. Europium distribution coefficient D Eu increases from 1.1 ± 0.2 to 3.2 ± 0.4 when initial europium concentration decreases from more than 17 × 10 −5 to 1.4 × 10 −5 M, in sulphuric media with SO 4 2− in excess or C Ba 2+ /C SO 4 2− < 1. The excess of barium in opposite, results in a decreasing D Eu . The coprecipitation of europium with barium sulphate as a heterovalent solid-solution is described by heterogeneous model obeying the Doerner and Hoskins logarithmic partition law. The weaker partition coefficients lower than unity (λ = 0.25 when C Eu(III) ≈ 1.4 × 10 −5 M and λ = 0.13 when C Ba 2+ /C SO 4 2− < 1 and 17.8 × 10 −5 ≤ C Eu(III) ≤ 153.5 × 10 −5 M) lead to crystals increasingly enriched in the trace element.

Gadolinium / Coprecipitation / Phosphoric and sulpho-phosphoric media / Fluoride / SilicateSummary. This work was carried out with the aim to establish the effect of some impurities on the coprecipitation of gadolinium with calcium phosphate and gypsum. The tests were performed using the radioactive tracer technique to monitor the fate of gadolinium in various phosphoric and sulpho-phosphoric media containing fluoride and silicate ions as impurities. In 10 −2 to 10 −1 M NH 4 H 2 PO 4 solution, the Gd(III) at a concentration of 10 −4 M is almost entirely precipitated as amorphous phosphate. However, the presence of H 2 SO 4 at C H 2 SO 4 ≥ 0.1 M, reduces the coprecipitation to less than 8 at. %, on average. The fluoride ions in solution, even at C HF ≤ 0.1 M, induce a reduction of coprecipitation of 10 to 30% according to HF concentration. In the media containing 10 −2 to 10 −1 M NH 4 H 2 PO 4 and 10 −1 M H 2 SO 4 , the effect of HF is almost negligible in the absence of Si(IV); nevertheless, the presence of fluorosilicate in solution may contribute to the enhancement of the coprecipitation of the REE. The sulphuric acid (0.1 M) in phosphoric media (0.74 ≤ C H 3 PO 4 ≤ 4.44 M), leads to a significant coprecipitation of the REE (1.3 ± 0.2 ≤ D Gd ≤ 3.1 ± 0.5), whilst the addition of HF (0.1 M) to these media enhances the solubility of the REE ( D Gd = 0.06 ± 0.01).XRD, IR spectroscopy and elemental analyses of the solid phases in conjunction with the variation of the distribution coefficient D indicate that the coprecipitation of the REE is likely controlled by heterovalent substitution of REE in gypsum and its precipitation as phosphate or fluorosilicate.

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