Phase evolution and chemical durability of Ca2Nd8(SiO4)6O2 glass-ceramics derived from CFA via microwave heating designed to immobilize trivalent actinides

Abstract

Oxyapatite (Ca2Nd8(SiO4)6O2) glass-ceramics (GCs) derived from coal fly ash (CFA) via microwave heating were fabricated to immobilize trivalent actinides (An3+). The effects of the Nd content and sintering temperature on the phase evolution of Ca2Nd8(SiO4)6O2 GCs were systematically investigated. The chemical durability of the Ca2Nd8(SiO4)6O2 GCs waste forms was examined under different leaching conditions (pH 4.0, 6.7, and 10.0). Results demonstrated the transformation of phase from glassy to Ca2Nd8(SiO4)6O2 GCs, when the doping content of Nd2O3 was 10, 15, and 20 wt.% at 1100, 1200, and 1300 °C, respectively, and the maximum solid solution was more than 30 wt.%. Prior to this solid solubility limit, all the Nd(Ⅲ) was incorporated to the glassy phase, whereas when the limit was exceeded, the Nd(Ⅲ) was immobilized into Ca2Nd8(SiO4)6O2 GCs. Moreover, the bulk density of the solidified sample increased with increasing of Nd-doped amount in the temperature range of 1100-1300 °C. The sample that was doped with 30 wt.% Nd2O3 and sintered at 1300 °C, exhibited the maximum bulk density (3.24 g·cm-3). Importantly, the GC waste forms exhibited superior leaching resistance, and the leaching rate of Nd element was in the order of 10-7 - 10-6 g·m-2·d-1. The NLNd values of the samples doped with 30 wt.% Nd2O3 and sintered at 1100 and 1300 °C were 8.57×10-8 g·m-2·d-1 and 4.29×10-8 g·m-2·d-1, respectively, after 28 d. In particular, the leachability of Nd element under different leaching conditions followed the order of acidic ＞ alkaline ＞ neutral. Therefore, Ca2Nd8(SiO4)6O2 GCs derived from CFA via microwave heating are promising immobilization substrates for the immobilization An3+ nuclear waste.