Equilibrium, kinetics, and thermodynamics studies of Cr(Ⅵ) adsorption from aqueous solutions on organoclay using ionic liquid-type imidazolium surfactants

摘要

The organoclays C-12 mim Br-Mt and C-16 mim Br-Mt were prepared using the ionic liquid-type imidazolium surfactants 1-dodecyl-3-methylimidazolium bromide (C-12 mim Br) and 1-hexadecyl-3-methyl-imidazolium bromide (C-16 mim Br) with sodium montmorillonite (Na-Mt). Then, C-12 mim Br-Mt and C-16 mim Br-Mt were characterized using X-ray diffraction (XRD), fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TG), and scanning electron microscopy (SEM). The results showed that C-12 mim Br and C-16 mim Br were successfully inserted into the lamella of Na-Mt, and the interlayer spacing was related to the alkyl chain length and the loading level of the surfactants. The adsorption of Cr (VI) on C-12 mim Br-Mt and C-16 mim Br-Mt was studied using the operational variables, including the loading level of the surfactants, contact time, and pH. The optimal conditions were as follows: the loading level of surfactants up to two times of cation exchange capacity (CEC) of Na-Mt, contact time of 2.0 h, and pH of 2.0. At the same experimental conditions, C-16 mim Br-Mt had stronger affinity for negatively charged Cr(VI) because of electrostatic interactions. The equilibrium adsorption data showed that the adsorption of Cr(VI) on organoclay correlated well with the Langmuir isotherm model. Using this model, the maximum adsorption capacities were 20.97 mg.g(-1) and 24.56 mg.g(-1) for C-12 mim Br-Mt and C-16 mim Br-Mt, respectively. The kinetics study indicated that the adsorption of Cr(VI) onto C-12 mim Br-Mt and C-16 mim Br-Mt followed a pseudo-second order model. The thermodynamics parameters, Delta G(theta), Delta H-theta, and Delta S-theta, were calculated, and the results indicated that the adsorption of Cr (VI) onto organoclay was an exothermic spontaneous physisorption process.