Abstract
Plutonium is released in the environment as consequence of the nuclear accidents and nuclear weapons testing. The migration of the plutonium depends of several factors such pH, ionic strength, oxidation state and complexation with organic and inorganic ligands, e.g; phosphate. In order to improve models on Pu behavior with phosphate species it is necessary to collect fundamental thermodynamic and structural data on Pu-phosphate interaction. However, to bypass the difficulties to manipulate plutonium (redox properties, radiotoxicity and limit amount authorized in academic laboratory), experimental protocols have been developed using chemical analogs chemical analogs that display stable oxidation state: Eu(III), Nd(III) have been used to simulate Pu(III) and U(VI) as analog of Pu(VI). In this context, the study of the speciation of europium (III) with phosphate ligands was based on two approaches. The first one is the determination of thermodynamic data by liquid-liquid extraction in and Eu(III) /HDEHP-Toluene systems, using ¹⁵²Eu at ultra-trace scale (10‾¹⁰M). The experiments were performed at constant ionic strength and different temperatures. The composition of the complexes and their associated stability constants have been deduced from the variations of the distribution ratio of ¹⁵²Eu as a function of total phosphate concentration. The second part of the Eu(III)-phosphate study was focused on the development of a protocol that can be applied with low amount of Pu(III)-phosphate compounds. For that purpose, a stable rhabdophane Nd₀.₉₉Eu₀.₀₁PO₄·0.667H₂O and one doped with ¹⁵²Eu have been synthetized. The stable solid was characterized by XRD, FTIR, TGA and SEM. Its dissolution has been proved to be congruent. Then, dissolution experiments have been conducted with Nd₀.₉₉Eu₀.₀₁PO₄·0.667H₂O doped with ¹⁵²Eu using amount of solid from 50 to less than 2 mg. The evolution of the dissolution was monitored by γ-spectrometry. Whatever the initial mass of rhabdophane, the equilibrium was reached at the same rate and the values of solubility products were found similar. This study clearly demonstrates that a low amount of solid can be used to determine a solubility product. Thus, it has been applied to PuPO₄ monazite, a compound synthesized and characterized at CEA Marcoule. The evolution of the dissolutions was monitored by UV-Vis spectrophotometry, PERALS and α-spectrosmetry. The study of the complexation of uranium (VI) with phosphate was performed using UV-Vis spectrophotometry. The stability constants were determined at I=1 M in HClO₄ and (Na,H)ClO₄ media and different temperatures. The study of uranium-phosphate speciation has been complemented by a structural study using X-Ray absorption spectroscopyin order to determine the coordination chemistry and the interatomic distances of the metal-ion.