Application of fly ash-based geopolymer for removal of cesium, strontium and arsenate from aqueous solutions: kinetic, equilibrium and mechanism analysis.

Geopolymerization is a developing reaction process for the utilization of solid wastes. In the present study, fly ash-based geopolymer and its derivative (Fe(II)-modified geopolymer) were synthesized and characterized using XRD, SEM, FTIR, BET, UV-Vis DRS as well as TG-DTA, and adopted as adsorbents for removal of Cs and Sr, and AsO from solutions. Each sorption kinetic was well fitted to the pseudo-second-order model. The sorption of Cs and Sr onto original geopolymer were better fitted to the Langmuir model. However, the Freundlich model is more befitting for sorption of AsO onto Fe(II)-modified geopolymer. The free energies calculated from the D-R isotherm indicated that the sorption for Cs and Sr were dominantly ion exchanges. Ring size plays a decisive role in ion exchanges for both Cs and Sr. Furthermore, the arrangement of SiO and AlO tetrahedrons has significant impacts on the ion exchange of Sr. XPS results indicated that a part of Fe in Fe (II)-modified geopolymer had been oxidized to Fe after sorption. Precipitation of FeAsO could partially contribute to the arsenate removal from solution. AsO sorption has also occurred through the formation of inner-sphere complexes via ion exchange reaction, which could be predominantly attached by bidentate linkages.