The purposes of this study were to investigate the removal efficiency of ammonium (NH4+) ion from aqueous solution using the natural Turkish (Yildlzeli) zeolite and to characterize equilibrium isotherms. Experiments were carried out using batch method as a function of the solution pH, shaking time, dosage of adsorbent, and temperature. All these factors affected NH4+ ion removal from aqueous solution. Equilibrium modelling data were fitted to linear Langmuir and Freundlich models. Dubinin-Redushckevich (D-R) isotherm was applied to describe the nature of ion exchange of NH4+ and found that it occurred physically. Thermodynamics parameters such as change in free energy (Delta G degrees), enthalpy (Delta H degrees) and entropy (Delta S degrees) were also calculated. These parameters confirmed that ion exchange of NH4+ by the zeolite was feasible, spontaneous and exothermic in nature. Based on the results, it can be concluded that the natural Turkish (Yildlzeli) zeolite is suitable for the removal of NH4+ ions in wastewater treatments and agricultural purposes to in terms of sustainability of environmental quality. (c) 2006 Elsevier B.V. All rights reserved.