Preparation of a fly ash-based geopolymer for removal of a cationic dye: Isothermal, kinetic and thermodynamic studies

A class F fly ash-based geopolymer was prepared for the removal of Basic Yellow 2 (BY2) from aqueous solutions. The geopolymerization process transformed the spherical fly ash particles into a porous and amorphous polymeric structure, sodium-alumina-silicate hydrate gel. The removal efficiency increased with the geopolymer dosage while it decreased with BY2 concentration. Specifically, the efficiency of 94.47% was achieved with the geopolymer dosage of 2.0 g L^-1. However, further increases resulted in a substantial reduction, down to 57.76% for 3.0 g L^-1. At 293 K for 300 min, the efficiency decreased from 84.69 to 64.19% for BY2 concentrations of 10 and 50 mg L^-1, respectively. Based on the isothermal, kinetic and thermodynamic investigations of the adsorption process, the Langmuir and Temkin models well correlated with the obtained results. The adsorption occurs spontaneously and shows endothermic character. The pseudo-second order and the intra-particle diffusion models are valid for 293 K while the adsorption results well suited with the pseudo-second order for 313 and 323 K. In conclusion, the prepared fly ash-based geopolymer provided favorable results for the removal of Basic Yellow 2 from aqueous solutions.