A novel layered zirconium molybdate as a precursor to a ceramic zirconomolybdate host for lanthanide bearing radioactive waste

Abstract

Novel layered zirconium molybdates (Mo/Zr = 2) in Na⁺- and NH₄⁺-exchanged forms with a defective crystalline structure and a specific surface area of up to 100 m² g⁻¹ were synthesized under mild hydrothermal treatment conditions (150 °C, 3 days) without structure-directing reagents. The NH₄⁺- zirconomolybdate material was tested for its ability to immobilise aqueous radioactive waste containing rare earth elements by a sorption/impregnation–crystallization process using simulant Nd³⁺/Ln³⁺ bearing solutions. The motivation for tailoring (Ln,Zr,Mo)-ceramics was the utilization of sediment-forming components of spent nuclear fuel processing solutions, such as Mo and Zr, for immobilization of transuranium radionuclides or an actinide–lanthanide (An–Ln) fraction of high-level effluents. The target (Ln,Zr,Mo)-phase acceptable for incorporation of actinides and lanthanides, Ln₂Zr₃(MoO₄)₉ (Ln = La–Tb), was selected on the basis of studying the phase formation in triple oxide systems Ln₂O₃–ZrO₂–MoO₃ (Ln = La–Lu, Y, Sc) and analysis of the known promising host phases being developed in the world for the actinide immobilization. X-ray diffraction and thermal analysis methods were used in the study of the thermochemical conversion of the Nd³⁺/Ln³⁺-zirconomolybdates resulting from loading the layered NH₄⁺-zirconomolybdate precursor with different quantities of Nd³⁺/Ln³⁺. The results on Nd³⁺ immobilization by the sorption–crystallization route reflected the influence of the acidity of the simulant solutions on the content of the target phase in the solidified (Nd,Zr,Mo)-ceramics. The impregnation/sorption–crystallization procedure provided poly-phase composites including Nd₂Zr₃(MoO₄)₉, Zr_xNd_y(MoO₄)₂ and ZrO₂, the quantity of the target phase depending on the Nd³⁺ loading. The solidification of the Ln bearing simulant solution at 16% Meⁿ⁺ loading resulted in nearly mono-phase ceramics of the Nd₂Zr₃(MoO₄)₉ structural type.