This paper focuses on the effectiveness of removing ammonium ion and the theoretical aspects of adsorption including adsorption isotherm, kinetics and thermodynamics as well as desorption-regeneration studies. Results have demonstrated that natural zeolite shows good performance with up to 97% for ammonium removal depending on contact time, zeolite loading, initial ammonium concentration and pH. The adsorption kinetics is best approximated 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. it was found that Gibbs free energy change or adsorption energy (Delta G degrees), -19.52 kJ/mol at 25 degrees C, -20.45 kJ/mol at 35 degrees C and -22.91 kJ/mol at 45 degrees C is negative indicating the spontaneous nature of the adsorption process, whereas the enthalpy change (Delta H degrees), 30.96 kJ/mol is positive indicating endothermic adsorption process. The entropy change (Delta S degrees), 0.169 kJ/(mol K) at 25 degrees C is also positive indicating increasing randomness at the solid-solution interface during adsorption. In addition, the desorption-regeneration studies demonstrated that desorption of ammonium on the zeolite is sufficiently high using NaCl solutions. (C) 2011 Elsevier B.V. All rights reserved.