Adsorption behavior and mechanism of chlorophenols onto organoclays in aqueous solution

Organoclays were prepared by replacing exchangeable Na+ ions in Na-montmorillonite (Na-Mt) with dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB). Organic modification is important in order to obtain good affinity between organoclays and organic pollutants. Hydrophobic DTAB-montmorillonite (DTAB-Mt) and CTAB-montmorillonite (CTAB-Mt) were studied as adsorbents for 4-chlorophenol and 2,4-dichlorophenol. The morphology, structure, and surface properties of Na-Mt and organoclays were characterized by scanning electron microscopy, X-ray diffraction, Fourier infrared spectroscopy, specific surface area, and zeta potential measurements. Adsorption was determined as a function of adsorbent dosage, pH, contact time, and temperature. It was found that pH and temperature had very important effects on the adsorption of chlorophenols. The Langmuir isotherm was the best choice to describe the adsorption behavior. The maximum adsorption capacities of CTAB-Mt for 4-chlorophenol and 2,4-dichlorophenol were 395.0 and 585.8 mg/g, respectively, whereas the maximum adsorption capacities of DTAB-Mt for 4-chlorophenol and 2,4-dichlorophenol were 331.1 and 458.2 mg/g, respectively. The kinetic data fitted the pseudo-second-order kinetic model. Thermodynamic parameters suggested that the adsorption process of chlorophenols onto organoclays was physisorption and exothermic. The adsorption mechanism is a complex process that involves a combination of partitioning, electrostatic attraction, and van der Waals forces. (C) 2015 Elsevier B.V. All rights reserved.