Influence of exchangeable cation of smectite on HDTMA adsorption: Equilibrium, kinetic and thermodynamic studies

摘要

Organically modified clays ('organo-clays') have attracted a great deal of interest because of their wide applications in industry and environmental protection. The objective of this study was to synthesize organo-clays using Tunisian smectite saturated with Na~+, Ca~(2+) and Zn(2+) ions and the cationic surfactant hexadecyltri-methylammonium bromide (HDTMAB). The effects of initial concentration (0.1-3.0 CEC), contact time (0.5-48 h) and temperature (293-333 K) were studied. The changes in the surfaces and structures of the smectites modified with HDTMA surfactant were characterized using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR).For all clays, the adsorbed HDTMA increased with increasing initial concentration; at high surfactant concentrations, Zn-exchanged clay displayed a high affinity for HDTMA surfactant suggesting that adsorption could be described by a different process. The kinetic adsorption data were modeled using pseudo-first-order and pseudo-second-order equations. It was shown that pseudo-second-order kinetic equation could best describe the adsorption kinetics. The enthalpy (A°H) and entropy (Δ°S) of adsorption were calculated from the slope and intercept of the linear plot of Log K_d against 1/T. Calculations on the thermodynamics of adsorption showed that principal contribution to the negative value of Δ°G (-14 to 14kJ·mol~(-1)), is the large positive value of Δ°S (0.06-0.07 kj·mol~(-1)·K~(-1)), whereas Δ°H is positive ranging between 4 and 7 kj·mol~(-1). It follows that adsorption of HDTMA onto smectite was feasible, spontaneous and endothermic process. Different configurations of surfactants within smectite interlayer were proposed based on the d_((001)) basal spacings which increased with surfactant loading. The presence of symmetrical and asymmetrical vibration bands of the -CH_2 group in the IR spectrum of the modified clays and the variation of their frequencies and their intensities confirmed the results obtained by XRD.