This study explored the cobalt ions removal efficiency from aqueous solutions by the sorption process on geopolymer and zeolite NaX. The influence of concentration and temperature on the sorption process was examined. FTIR and SEM analyses were conducted to elucidate the structure of sorbents. An additional goal was to test the experimental kinetic data using several kinetic models. A kinetic study has shown that the best fit is achieved when the Blanchard model was applied, suggesting that the sorption of cobalt ions on geopolymer and zeolite NaX is a second-order reaction.

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.

The sorption of Cu ions on pure fly ash-based geopolymer, fly ash-based geopolymer with Pb ions addition (modified) and zeolite NaX was studied. Taguchi's approach was applied to determine the impact of solute concentration (c), temperature (T), mixing time (t) and sorbent type (S) on the sorption of Cu ions onto different sorbents under batch conditions. Optimum experimental conditions and influence of controllable factors were determined using the larger-the-better approach. The influence in descending order is c > S > t > T, for both removal and loading. Also, the impact of sorbent type and solute concentration on the process equilibrium was examined. The equilibrium amount of Cu retained on the sorbents in equilibrium (qe) was as follows: pure geopolymer – 1.169 mmol g−1, modified geopolymer – 1.186 mmol g−1, and zeolite NaX – 1.695 mmol g−1. The experimental data were modelled using Jovanovic, Khan, Baudu, and Fritz-Schlünder isotherm models and their goodness of fit were compared. The Baudu isotherm model was the most useful in predicting the equilibrium of Cu sorption on pure and modified geopolymer. Goodness of fit of the selected isotherm models for the sorption of Cu ions on zeolite NaX was in the order: Fritz-Schlünder > Khan > Jovanovic > Baudu.