This paper presents a study of the removal of ammonium ion from aqueous solutions using natural Chinese (Chende) zeolite. A series of experiments was conducted to examine the effects of solution pH, particle size, contact time, adsorbent dosage, and the presence of other cation- and anion species on ammonium removal. The findings indicated that these parameters named had a significant effect on the removal Of ammonium by the zeolite. The effect of other cations on the removal of ammonium followed the order of preference Na+ > K+ > Ca2+ > Mg2+ at identical mass concentrations, and the effect of the presence of individual anions followed the order of preference carbonate > chloride > sulfate > phosphate at identical mass concentrations of ammonium ions. Kinetic analysis showed that the adsorption of ammonium on zeolite at different ranges of particle size well followed the pseudo-second-order model and followed the intra-particle diffusion model only during the initial 60 min of the adsorption process. Equilibrium isotherm data was fitted to the linear Langmuir- and Freundlich models with the latter model providing the better description of the process (R-2 = 0.991-0.997) compared to the former (R-2 = 0.902-0.989). (C) 2009 Elsevier B.V. All rights reserved.