Compositional and synthesis time effect on the removal of Cu(II) and Zn(II) ion through adsorption using fly ash-based geopolymer: isotherms, kinetics, and thermodynamics

Geopolymer is one of the materials used as an environmentally friendly adsorbent and is produced from wastes such as fly ash at temperatures below 100 °C. Therefore, this study aims to investigate the adsorption process of Cu²⁺ and Zn²⁺ on the geopolymer structure by determining the isotherms, kinetics, and adsorption thermodynamics. The synthesis of geopolymer was conducted by varied Na₂O/SiO₂ ratios and aging time. The adsorption of Cu²⁺ and Zn²⁺ on geopolymer followed the Langmuir isotherm with the highest adsorption capacity achieved using R0.3T5 with q_m 94.33 mg/g and 87.72 mg/g, respectively. Furthermore, the kinetic studies of the adsorption showed the adsorption process followed pseudo-second-order. The estimated thermodynamic parameters presented a positive value of enthalpy (ΔH°) and a negative value of Gibbs energy change (ΔG°), approving that the adsorption process on Cu²⁺ and Zn²⁺ occurs endothermically and spontaneously. More importantly, we found that the adsorption of Cu²⁺ and Zn²⁺ was influenced by the Na₂O/SiO₂ ratio and the pore volume of the geopolymer, which could be the basis for the further design of a more effective adsorbent.