Study of the adsorption of Cd (II) from aqueous solution using zeolite-based geopolymer, synthesized from coal fly ash; kinetic, isotherm and thermodynamic studies

A specific type of zeolite, synthesized from coal fly ash, was used in our batch adsorption experiments in order to adsorb Cd (II) ions from aqueous solution. Solid-state conversion of fly ash to an amorphous aluminosilicate adsorbent (geopolymer) was investigated under specific conditions. The adsorbent ZFA was characterized using XRD, XRF, FT-IR, FE-SEM, LPS and BET surface area. The optimum conditions of sorption were found to be: a ZFA dose of 0.08 g in 25 mL of Cd (II) with contact time of 7 h and pH 5. Four equations, namely Morris–Weber, Lagergren, Pseudo-second order and Elovich have been used in order to determine the kinetics of removal process. The collected kinetic data showed that pseudo-second order equations controlled the adsorption process. According to adsorption isotherm studies, the Langmuir isotherm was proved to be the best fit for our experimental data, in comparison to Freundlich, D–R and Tempkin models. The thermodynamic parameters ΔH, ΔS and ΔG are evaluated. Thermodynamic parameters showed that the adsorption of Cd (II) onto ZFA was feasible, spontaneous and endothermic under studied conditions. To conduct desorption experiments, several solvents (including alkaline, bases and water) have been employed. 84% of desorption efficiency was achieved using NaOH.