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 Sr2+, and AsO3- 4 from solutions. Each sorption kinetic was well fitted to the pseudo-second-order model. The sorption of Cs+ and Sr2+ onto original geopolymer were better fitted to the Langmuir model. However, the Freundlich model is more befitting for sorption of AsO3- 4 onto Fe(II)-modified geopolymer. The free energies calculated from the D-R isotherm indicated that the sorption for Cs+ and Sr2+ were dominantly ion exchanges. Ring size plays a decisive role in ion exchanges for both Cs+ and Sr2+. Furthermore, the arrangement of SiO4 and AlO4 tetrahedrons has significant impacts on the ion exchange of Sr2+. XPS results indicated that a part of Fe2+ in Fe (II)-modified geopolymer had been oxidized to Fe3+ after sorption. Precipitation of FeAsO4 could partially contribute to the arsenate removal from solution. AsO3- 4sorption has also occurred through the formation of inner-sphere complexes via ion exchange reaction, which could be predominantly attached by bidentate linkages.