水合稀土磷酸盐的热行为及其相变产物的化学稳定性

Thermal Behavior of Rhabdophanes(LnPO_(4)·n H_(2)O)and Chemical Stability of Their Phase Transition Products

水合稀土磷酸盐(LnPO_(4)·n H_(2)O)对锕系核素选择性高、载荷量大,易在酸性溶液中制备,是沉淀-富集高放废液中锕系核素的理想材料之一。本工作利用沉淀法制备水合稀土磷酸盐(LnPO_(4)·0_(6)67H_(2)O,Ln=La、Eu、Gd),研究了LnPO_(4)·0_(6)67H_(2)O的热行为及其相变产物的化学稳定性。结果表明,LnPO_(4)·0_(6)67H_(2)O结构稳定性和脱水吸附可逆性良好,其晶型转变过程为LnPO_(4)·0_(6)67H_(2)O 25-80℃LnPO_(4)·0.5H_(2)O 80-240℃LnPO_(4)661-935℃独居石;LnPO_(4)·0_(6)67H_(2)O及其相变产物具有优异的化学稳定性,在溶液(pH=3/7、T=90/150℃)浸出28 d后归一化浸出率LR Ln为10^(-7)~10^(-4)g/(m^(2)·d);热处理可提高水合稀土磷酸盐的化学稳定性,GdPO_(4)·0_(6)67H_(2)O及其相变产物的化学稳定性顺序为GdPO_(4)·0_(6)67H_(2)O<脱水相(GdPO_(4)·n H_(2)O)<钆独居石,pH值对稀土磷酸盐化学稳定性的影响大于温度对其的影响。

LnPO_(4)·n H_(2)O rhabdophane is considered as one of potential material for precipitation-enrichment actinides in high-level radioactive waste liquid owing to its outstanding properties relating to the high chemical stability and the high selectivity and strong incorporating capacity of actinides.In this work,the phase composition,microstructure and thermal behaviors of LnPO_(4)·0_(6)67H_(2)O(Ln=La,Eu,Gd)were systematically investigated by combining with XRD,SEM,TG-DSC,BET and ICP-MS.Based on the PCT leaching test methods,the differences of chemical stability between LnPO_(4)·0_(6)67H_(2)O and its thermal products were first discussed in detail.The results reveal that the single phase of LnPO_(4)·0_(6)67H_(2)O(Ln=La,Eu,Gd)with the microstructure of hexagonal shaped nanorodcrystals can be synthesized by chemical precipitation reaction in 90℃aqueous solution.LnPO_(4)·0_(6)67H_(2)O has the excellent structural stability and the reversibility behaviors associated with dehydration and adsorption of water molecules.The process of phase transition can be described as:LnPO_(4)·0_(6)67H_(2)O 25-80℃LnPO_(4)·0.5H_(2)O 80-240℃LnPO_(4)(rhabdophane)661-935℃LnPO_(4)(monazite).Moreover,the excellent chemical stability has been observed in LnPO_(4)·0_(6)67H_(2)O rhabdophane and its thermal products in PCT leaching test.For instance,the values of LR Ln are ranged from 10^(-7)g/(m^(2)·d)to 10^(-4)g/(m^(2)·d)after leaching 28 d in different leaching solutions(pH=3/7,T=90/150℃),where the acidic solution is more likely to corrode sample surface,further resulting in a relative high LR Ln value.Importantly,the phase transition can improve the chemical stability of initial rhabdophane.As a typical example of Gd-rhabdophane,after leaching 28 d in 90℃/pH=3 solutions,the evolution of chemical stability is found as:GdPO_(4)·0_(6)67H_(2)O(1.8×10^(-4)g/(m^(2)·d))<GdPO_(4)·n H_(2)O(0<n<0_(6)67,6.4×10^(-5)g/(m^(2)·d))<Gd-monazite(1.2×10^(-5)g/(m^(2)·d)).Furthermore,a relative smaller influence on stability of changing temperature has been observed comparing to pH impaction.