A novel composite of layered double hydroxide/geopolymer for co-immobilization of Cs⁺ and SeO₄ ²⁻ from aqueous solution

Geopolymers are always considered as promising materials for the treatment of radioactive wastes. In order to extend the application of geopolymer to the immobilization of anionic species, a novel composite of layered double hydroxide/geopolymer (LDH/GEO) was synthesized and applied for cosorption of Cs⁺ and SeO₄ ²⁻. The ability of LDH/GEO to sorb Cs⁺ was maintained as that of pure GEO, even though the surface of geopolymer was homogeneously covered by the LDH platelets. The sorption of Cs⁺ onto LDH/GEO composite occurred via ion exchange, which was controlled by particle diffusion. It is different with Cs⁺ sorption onto pure GEO governed by film diffusion. Therefore, “Pocket diffusion” was proposed for the particle diffusion as the case of LDH/GEO because this kind of diffusion would be restricted in a certain distance around the ring entrance gate due to the amorphous essence of GEO. For SeO₄ ²⁻ sorption by LDH/GEO, ion-exchange with the interlayer NO₃ ⁻ and surface sorption could be the main mechanisms. Importantly, the sorption speed of SeO₄ ²⁻ achieved by LDH/GEO composite was much faster than that by pure LDH. In the binary system (Cs⁺+ SeO₄ ²⁻), the sorption of Cs⁺ was slightly suppressed compared to the single system, which might be due to the formation of ion-pair complex of [CsSeO₄]⁻. However, it did not have negative effect on the SeO₄ ²⁻ sorption. In the presence of other cations or anions, the cosorption performances of Cs⁺ and SeO₄ ²⁻ were satisfactorily obtained. Furthermore, the Cs⁺ and SeO₄ ^2- sorption densities were superior to the previously reported values. The combined MgAl-LDH/geopolymer composite could be a promising material for the immobilization of Cs⁺ and SeO₄ ²⁻, and this work would provide guidance for the development of geopolymer-based materials for environmental applications.