Removal of copper and cobalt ions by fly ash-based geopolymer from solutions-equilibrium study

Fly ash-based geopolymer was used for removal of copper and cobalt ions from aqueous solutions by a batch technique. The experimental data were obtained at constant geopolymer particles size (0.071–0.090 mm) for the different initial concentrations (in the range from 3.881 to 39.390 mmol L−1) of copper and cobalt solutions, respectively, and at various temperatures (T = 298, 308 and 318 K). For characterization of synthesized geopolymer, before and after sorption of heavy metals, FTIR, XRD, and SEM/EDS analyses were used to confirm the dissolution of geopolymer structure. SEM/EDS analysis also confirmed that sorption process occurred, as well as the fact that Na ions are outgoing ions. Various types of isotherm models, two-parameter isotherms: Langmuir, Freundlich, Jovanovic, and Dubinin–Radushkevich and three-parameter isotherms: Hill, Redlich–Peterson, and Sips were fitted to the experimental data obtained. In order to calculate parameters of the isotherms a nonlinear regression method was used. According to the results obtained, the most appropriate models for describing equilibrium data were Langmuir, Jovanovic, Hill, and Redlich–Peterson isotherm models. Thermodynamic parameters indicated endothermic (positive values of ΔH°) and spontaneous nature (negative values of ΔG°) of sorption process.