Anchored thiol smectite clay—kinetic and thermodynamic studies of divalent copper and cobalt adsorption

A natural smectite clay sample from Serra de Maicuru, Pará State, Brazil, had aluminum and zirconium polyoxycations inserted within the interlayer space. The precursor and pillarized smectites were organofunctionalized with the silyating agent 3-mercaptopropyltrimethoxysilane. The basal spacing of 1.47 nm for natural clay increased to 2.58 and 2.63 nm, for pillared aluminum, SAl/SH, and zirconium, SZr/SH, and increases in the surface area from 44 to 583 and 585 m2 g−1, respectively. These chemically immobilized clay samples adsorb divalent copper and cobalt cations from aqueous solutions of pH 5.0 at 298±1 K. The Langmuir, Redlich–Peterson and Toth adsorption isotherm models have been applied to fit the experimental data with a nonlinear approach. From the cation/basic center interactions for each smectite at the solid–liquid interface, by using van’t Hoff methodology, the equilibrium constant and exothermic thermal effects were calculated. By considering the net interactive number of moles for each cation and the equilibrium constant, the enthalpy, ΔintH0 (−9.2±0.2 to −10.2±0.2 kJ mol−1) and negative Gibbs free energy, ΔintG0 (−23.9±0.1 to −28.7±0.1 kJ mol−1) were calculated. These values enabled the positive entropy, ΔintS0 (51.3±0.3 to 55.0±0.3 JK−1 mol−1) determination. The cation-sulfur interactive process is spontaneous in nature, reflecting the favorable enthalpic and entropic results. The kinetics of adsorption demonstrated that the fit is in agreement with a second-order model reaction with rate constant k2, varying from 4.8×10−2 to 15.0×10−2 and 3.9×10−2 to 12.2×10−2 mmol−1 min−1 for copper and cobalt, respectively.