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.
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