Distribution and leaching characteristics of La element in oxyapatite glass-ceramic derived from uranium tailings

Due to their high waste loading capacity and excellent chemical durability, rare earth (RE) - oxyapatites are a potential material for the immobilization of nuclear waste. In this work, NaLa ₉ (SiO ₄ ) ₆ O ₂ glass-ceramics (GCs) and La ₂ Si ₂ O ₇ GCs derived from different contents of uranium tailings were synthesized by the solid-state method to immobilize lanthanides (La). The effect of uranium tailings content and sintering temperature on the phase evolution and chemical durability of the solidified samples was systematically investigated. Rietveld refinement analysis was used to obtain the ratios of crystalline-glass phases in the GCs, and the La element distribution between different phases before and after the leaching test was characterized by TEM-EDS. Results demonstrated that U20 (20 wt% uranium tailing addition) samples were an ideal waste form to immobilize La. The U20 sample had a 63.80 wt% proportion of crystalline phase, and the proportion of La element on the crystalline phase was 69.95 wt%, which was obviously larger than the La content of 3.53–5.81 wt% on the vitreous part. ASTM Product Consistency Test (PCT) results indicated that U20 samples had the lowest La leaching rate over the 28-day leaching period, U40, U50, and U60 samples with La ₂ Si ₂ O ₇ as the main crystalline phase had similar leaching values with U70 samples, which were pure glass phase. After the leaching test, no lattice distortion or phase change happened on the NaLa ₉ (SiO ₄ ) ₆ O ₂ crystalline phase.