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

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 [degree]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, ZrxNdy(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.