Application of modified attapulgites as adsorbents for uranyl uptake from aqueous solution—Thermodynamic approach

An attapulgite (ATPG) clay sample has been chemically modified with silylating agents N-propyldiethylenetrimethoxysilane and bis[3-(triethoxysilyl)propyl]tetrasulfide. The resulting matrices (ATPG3TPT and ATPGNPTM) have been characterized by elemental analysis, X-ray diffractometry, carbon nuclear magnetic resonance in the solid-state, and scanning electron microscopy. The natural and modified clay samples have been tested as potential adsorbents for the removal of uranyl(II) from aqueous solution at pH 2.0 and 298 ± 1 K. The maximum number of moles adsorbed was determined to be 5.55, 14.86, and 18.99 × 10−2 mmol g−1 for ATPG, ATPG3TPT, and ATPGNPTM, respectively. From calorimetric determinations the quantitative thermal effects for UO22+/center interactions gave exothermic enthalpy (−6.90 to −7.88 kJ mol−1), negative Gibbs free energy (−22.34 to −24.56 kJ mol−1), and positive entropy (51.80–56.00 J K−1 mol−1). These thermodynamic data confirmed the energetically favorable condition of such interaction at the solid/liquid for all systems.