The natural zeolite tuff (clinoptilolite) from a Serbian deposit has been studied as adsorbent for Ni(II) ions from aqueous solutions. Its sorption capacity at 298K varies from 1.9mg Nig(-1) (for the initial solution concentration of 100mg Nidm(-3)) to 3.8mg Nig(-1) (for C-0 = 600mgNidm(-3)) and it increases 3 times at 338 K. The sorption is best described by the Sips isotherm model. The sorption kinetics follows the pseudo-second-order model, the activation energies being 7.44, 5.86, 6.62 and 6.63 kJ mol(-1) for C-0 = 100, 200, 300 and 400mg Nidm(-3), respectively. The sorption involves a film diffusion, an intra-particle diffusion, and a chemical cation-exchange between the Na+ ions of clinoptilolite and the Ni2+ ions. The sorption is endothermic (Delta H degrees being 37.9, 33.4, 30.0, 27.7 and 24.3 kJ mol(-1) for C-0 = 100, 200, 300, 400 and 600mg Nidm(-3), respectively) and spontaneous in the 298-338K temperature range. Thermal treatment of the Ni(II)-loaded clinoptilolite results in the formation of spherical nano-NiO particles of approx. 5 nm in diameter which are randomly dispersed in the clinoptilolite lattice. (c) 2010 Elsevier B.V. All rights reserved.