Study of Ammonium Adsorption Mechanism in Hydrothermalized Pahae Natural Zeolites: Kinetic and Isotherm Adsorption, and Thermodynamics

This paper focuses on the effectiveness of hydrothermalized natural zeolite from Pahae, North Sumatera to remove ammonium ion and analysis the kinetics models of the zeolite adsorption ability, such as isothermic and kinetics adsorption; thermodynamics as well as desorption-regeneration studies. Our findings have demonstrated that natural zeolite shows good performance in terms of for ammonium removal up to 97 % depending on contact time, zeolite loading, initial ammonium concentration and pH. The adsorption kinetics is best estimated by the pseudo-second-order model, whereas the adsorption isotherm results indicated that Freundlich model provides the best fit for the equilibrium data. Furthermore, with regard to thermodynamic parameters, Gibbs free energy change (ΔG°), −19.52 kJ/mol at 25 °C, −20.45 kJ/mol at 35 °C and −22.91 kJ/mol at 45 °C, is negative due to the spontaneous nature of the adsorption process, whereas the enthalpy change (ΔH°), the energy of 30.96 kJ/mol is positive, suggesting endothermic adsorption process. The entropy change (ΔS°), 0.169 kJ/(mol/K) at 25 °C is also positive, indicating an increase of randomness at the solid-solution interface during adsorption. In addition, the desorption-regeneration studies demonstrated that desorption of ammonium on zeolite is sufficiently high using NaCl solutions. HIGHLIGHTS Natural zeolite and hydrothermal-activated zeolite were used to remove ammonium. The hydrothermal-activated zeolite shows good performance for ammonium removal with up to 79 %. The performance depends on the contact time, zeolite loading, initial ammonium concentration and pH. The adsorption kinetics is best approximated by the pseudo-second-order model. Freundlich model provides the best fit for the adsorption isotherm result. GRAPHICAL ABSTRACT