The compound N-[3-(trimethoxysilyl)propyl]diethylenetriamine (MPDET) was anchored onto Amazon kaolinite surface (KLT) by heterogeneous route. The modified and natural kaolinite clay samples were characterized by transmission electron microscopy (TEM), scanning electron microscopic (SEM). N-2 adsorption, powder X-ray diffraction, thermal analysis, ion exchange capacities, and nuclear magnetic nuclei of Si-29 and C-13. The well-defined peaks obtained in the C-13 NMR spectrum in the 5.0-62.1 ppm region confirmed the attachment of organic functional groups as pendant chains bonded into the porous clay. The adsorption of uranyl on natural (KLT) and modified (KLTMPDET) kaolinite clays was investigated as a function of the solution pH, metal concentration, temperature, and ionic strength. The ability of these materials to remove U(VI) from aqueous solution was followed by a series of adsorption isotherms adjusted to a Sips equation at room temperature and pH 4.0. The maximum number of moles adsorbed was determined to be 8.37 x 10(-3) and 13.87 x 10(-3) mmol g(-1) for KLT and KLTMPDET at 298 K, respectively. The energetic effects (Delta H-int, Delta(int)G, and Delta S-int) caused by metal cations adsorption were determined through calorimetric titrations. (C) 2010 Published by Elsevier B.V.