Sorption capacity and thermodynamic properties of natural Turkish (Reşadiye) bentonite for the removal of ammonium ions from aqueous solution

Natural clays and materials such as bentonite, zeolite and sepiolite have been widely used for environmental and agricultural purposes because of their easy application and relative low cost. The aim of the present study was to investigate the sorption capacity of the natural Turkish (Reşadiye) bentonite (NTB) for the removal of ammonium (NH₄⁺) ions from aqueous solution from the viewpoint of both the sorption isotherms and the associated thermodynamics. The influence of pH, adsorbent dosage and temperature on the sorption of NH₄⁺ ions onto NTB was also studied. It was found that the non-linear Langmuir model was more suitable for describing the sorption of NH₄⁺ ions onto NTB since the application of this model exhibited a higher coefficient of determination (R² = 0.99) than that for the non-linear Freundlich model (R² = 0.98). The Dubinin-Radushkevich (D-R) model was also applied to the equilibrium data (R² = 0.99). The maximum sorption capacity towards NH₄⁺ ions was determined via the Langmiur equation and found to be 30.12 mg/g. A comparison of the sorption capacity of NTB with those for various adsorbents revealed that this adsorbent had a high sorption capacity for the removal of NH₊₄ ions. The results obtained from an application of the D-R isotherm showed that the sorption of NH₊₄ ions onto NTB was a physical process. Thermodynamic parameters such as the free energy change (ΔG⁰), enthalpy change (ΔH⁰) and entropy change (ΔS⁰) were also calculated to establish the thermodynamic adsorption behaviour of NH₄⁺ ions onto NTB. These parameters indicated that the sorption of NH₄⁺ ions by NTB was spontaneous, exothermic in nature and involved weak interactions between the NH₄⁺ ions and negative charges on the NTB surface. On the basis of these results, it was concluded that NTB has a significant potential for reducing the concentration of NH₄⁺ ions in wastewater and agricultural residues by sorption without destroying the physical and ecological properties of the environment.