钾/钠离子对水热合成铯榴石结构的影响

水热合成铯榴石能低温、晶格固化放射性废物¹³⁷Cs，但在放射性废物中Cs⁺通常与同族的K⁺/Na⁺共存，两者可能会对水热合成铯榴石组成和结构产生影响。基于此，参考铯榴石化学计量组成CsAlSi₂O₆，首先利用偏高岭土、纳米二氧化硅和CsOH?H₂O为原料水热合成了铯榴石。然后分别以不同摩尔质量的KOH或NaOH取代CsOH?H₂O，研究了K⁺/Na⁺对水热合成产物组成和结构的影响。结果表明，铯榴石水热晶化温度相对较低，产物微观下呈多晶微球态；K⁺与Cs⁺共存时，体系水热产物主要为铯榴石微球，微球尺寸随K⁺掺入比例的提高而增加，但K⁺主要存在于晶化程度较低的钾铝硅酸盐水合物中，并未参与铯榴石形成；Na⁺与Cs⁺共存时，体系水热产物主要由铯榴石和方沸石(NaAlSi₂O₆?H₂O)组成，Cs⁺和Na⁺各自存在于铯榴石或方沸石结构中。铯榴石相对较低的水热晶化温度及其结构中Cs⁺不能迁移和进行离子交换的特性，使得水热条件下K⁺/Na⁺不能进入先形成的铯榴石结构中的碱金属离子位点。尽管如此，K⁺或Na⁺的存在可降低铯榴石的水热晶化温度、促进铯榴石晶粒生长，且当K⁺或Na⁺与Cs⁺共存时，Cs⁺仍优先进入并稳定固化于铯榴石晶体结构中。

Eeffect of K+/Na+ on Structure of Hydrothermal Synthesized Pollucite

¹³⁷Cs could be solidificated in lattice of hydrothermal synthesized pollucite at relatively low temperature. The structures of pollucite might be greatly influenced by K⁺/Na⁺ coexisting with Cs⁺ in radioactive wastes because of the similar nature and properties of alkali metal ion. Based on above, pollucite micro-spheres were firstly hydrothermally synthesized by using metakaolin, nano-SiO₂ and CsOH?H₂O according to the stoichiometric composition of pollucite (CsAlSi₂O₆). Then, the effect of K⁺/Na⁺ on formation and structures of pollucite was researched through the substitution of CsOH?H₂O by NaOH or KOH. Results indicate that relatively low hydrothermal crystallization temperature of pollucite with polycrystalline nature is observed. Pollucite micro spheres would be the main products when Cs⁺ coexists with K⁺, and diameters of pollucite spheres increase with K⁺. However, K⁺ would exist in potassium aluminosilicate hydrates with low crystallinity instead of involving in the pollucite formation. Pollucite and analcite are the main products when Cs⁺ coexists with Na⁺. Na⁺ or Cs⁺ might be in structure of analcite and pollucite, respectively. It is found that the K⁺/Na⁺ could not enter and occupy the alkali ion sites in pollucite structure under hydrothermal conditions because of the non-migration and non-exchange of Cs⁺ in pollucite structure. However, hydrothermal crystallization temperature of pollucite could be further reduced and the crystal size of pollucite would increase in the presence of K⁺/Na⁺. Cs⁺ would be still stablely solidificated in the lattice of as-formed pollucite when K⁺/Na⁺ coexists with Cs⁺.