A novel composite of layered double hydroxide/geopolymer for co-immobilization of Cs+ and SeO42- 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 SeO42-. 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 SeO42- sorption by LDH/GEO, ion-exchange with the interlayer NO3- and surface sorption could be the main mechanisms. Importantly, the sorption speed of SeO42- achieved by LDH/GEO composite was much faster than that by pure LDH. In the binary system (Cs++ SeO42-), 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 SeO42- sorption. In the presence of other cations or anions, the cosorption performances of Cs+ and SeO42- were satisfactorily obtained. Furthermore, the Cs+ and SeO42- 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 SeO42-, and this work would provide guidance for the development of geopolymer-based materials for environmental applications.