A modeling study by response surface methodology (RSM) on Th(IV) adsorption optimization using a sulfated β-cyclodextrin inclusion complex

The inclusion complex (sulfated-beta-CD@NAA) of sulfated beta-cyclodextrin and alpha-naphthylacetic acid was synthesized for adsorption of low concentrations of thorium from aqueous solutions. The structure and property of sulfated-beta-CD@NAA were characterized by Fourier transform infrared spectroscopy, thermogravimetric, X-ray diffraction, proton nuclear magnetic resonance, specific surface area measurement and scanning electron microscopy. The characterization analysis confirmed the formation of new solid phase. The results indicated that alpha-naphthylacetic acid embedded into the cavity of sulfated beta-cyclodextrin partly from the secondary face and the thermal stability of inclusion complex was significantly increased. The experiments were designed by Box-Behnken design combined with response surface methodology. The adsorption parameters of pH value, contact time, initial concentration were used as the independent variables and their effects were investigated on the Th(IV) adsorption capacity. Second-order polynomial regression models were derived and analysis of variance was utilized to judge the adequacy of the chosen models. The maximum Th(IV) adsorption capacity (12.75 mg g(-1)) was achieved under the following conditions: pH value 2.5, contact time 35 min, initial concentration 30 mg L-1. Prediction of models was in good agreement with experimental results. The experimental adsorption kinetic data followed a pseudo-second-order equation with the correlation of 0.9996, indicating the chemical adsorption. The obtained equilibrium isotherm data were fitted well with Langmuir model in the concentration range considered. The thermodynamic parameters (a dagger H (0) < 0, a dagger S (0) < 0, a dagger G (0) < 0) indicated that the adsorption process was exothermic and spontaneous. Regeneration of Th(IV)-loaded sulfated-beta-CD@NAA with 0.1 M HNO3 solution, and the regenerated sulfated-beta-CD@NAA can be reused for adsorption of Th(IV).