Effect of the synthesis method on the properties of a Pb-bearing (Y–Gd–Ce) rare-earth phosphate used for the confinement of high-level radioactive waste

Abstract In this study, a Pb-containing (Y–Gd–Ce) rare-earth phosphate with the general chemical formula of (Y0.1Pb0.1Ce0.4Gd0.4)PO4 was synthesized by two methods, namely the sol–gel and the metallurgical method. The sol–gel route consists of an external gel precipitation method, followed by two calcinations at 873 and 1473 K; and the dry route was a natural sintering at 1473 K of a mixture of micropowders activated at 873 K. The sol–gel route of synthesis gives a stronger and harder monazite mineral than the one obtained by the dry route of synthesis, both of them have a very low porosity. The sintering densities are 4.70 and 4.55 g/cm3 for both the sol–gel and dry-route made monazites. The X-ray diffraction (XRD) analysis shows a main monoclinic crystalline structure for the two ways of synthesis. However, a secondary anorthic phosphate phase appears for the dry-route made monazite. Three leaching tests simulating several radiological events were performed: an acidic static test at different pH, a static water leach test in an argilous media and a dynamic microwave leach test. For the whole of leaching processes, the kinetic of dissolution is fast. The acidic tests at pH 1, 4 and 7 gave few amounts of dissolved Ce in the leachates, about 5.668, 0.189 and 0.346 × 10−2 g/m2 day at the steady-state, respectively. The Pb was totally dissolved at pH 1 and 3. The sol–gel made monazite has a weak chemical durability in acidic media. In neutral pH, both the sol–gel and the dry-route made monazites give comparable values of Ce normalized dissolution rates (0.346 × 10−2 and 0.389 × 10−2 g/m2 day, respectively). The leaching in kaolin media decreases with a ten factor the amounts of leached Ce. However, for the whole of the leaching tests performed in neutral pH conditions, the monazite materials have a good chemical durability. The dissolution of the minerals under the microwave leaching is partially achieved, with only 3.5% and 4.0% of solubilised minerals, for both the sol–gel and dry-route made monazites, respectively, showing the good chemical durability of the studied monazite mineral.