Structural characterizations of fly ash-based geopolymer after adsorption of various metal ions

Geopolymer, an amorphous substance, has been viewed as good adsorbent or catalyst and attracted much attentions from all over the world. In order to achieve the better applications of geopolymer in these fields, a deep understanding of the microstructure of geopolymer would be strongly required. In the present study, geopolymer was synthesized from coal fly ash, and the structural analysis of geopolymer after adsorption of various metal ions (Li⁺, Cs⁺, Sr²⁺ and Co²⁺) was studied using XRD, SEM-EDX, FTIR, UV-VIS DRS, TG-DTA as well as surface area and pore distribution analysis. Pair distribution function preferably illustrated that geopolymer was successfully prepared from calcined fly ash. Geopolymer possesses different affinities towards various metal ions. After exchanging with other metal ions, the main structure of geopolymer was maintained. Metal ions with a large radius would have greater effect on the existing state and amount of water molecules in geopolymer. Moreover, the specific surface area of geopolymer after exchanging with metal ions decreased as a function of the radius of them. The spectra corresponding to d-d transitions indicated that the Co²⁺ could be incorporated into the deformed six-member rings or eight-membered rings. It could be deduced that the sites for ion exchange could be different rings or even cavities distributed on the surface layer of geopolymer. Furthermore, the rings distributed in the geopolymer structure were predominant in the 6-, 8-, 10- or even 12-member rings to maintain the structure stability and charge balance with the cations.