The paper concerns the removal of ammonium ions from aqueous solution using a modified clinoptilolite-Ca2+-formed clinoptilolite (CaY) prepared from natural clinoptilolite. The batch study results show that the pH has an effect on ammonium adsorption capacity as it can influence both the character of the exchanging ions and the clinoptilolite itself; the CaY has a high selectivity to NH4+ and the exchange decreases with increasing temperature; ammonium ion uptake onto CaY was suitably described by the Langmuir model. The column results indicated that the effluent of simulated wastewater treated with CaY could meet the integrated wastewater discharge standard of China, and CaY can be circulated through regenerating by Ca(OH)(2). (c) 2006 Elsevier B.V. All rights reserved.