Removal of malachite green dye by sodium dodecyl sulfate modified bentonite clay: Kinetics, thermodynamics and isotherm modeling

Adsorption of malachite green (MG) onto sodium dodecyl sulfate (SDS) modified bentonite clay (MBC) was investigated. MBC was synthesized and characterized by X-ray diffraction (XRD), thermos-gravimetric analysis (TGA), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) Sorptometer. According to XRD analysis, the interlayer distance d-spacing increases from 1.2496 nm to 1.4000 nm after modification of bentonite clay (BC). TGA analysis suggests that the residual mass at 1198.4 °C is 92.74% which indicates that the MBC adsorbent is thermodynamically stable. BET analysis confirms that surface area decreases from 22.87 m²/g to 20.13 m²/g after modification. TEM, EDS and XPS analysis confirm the adsorption of MG onto MBC. The efficiency of this adsorbent for the uptake of MG was investigated using batch adsorption technique at various contact times, pHs, dye concentration, temperature, particle size, concentration of competitive ion and adsorbent dosage. Adsorption capacity rises as contact time, starting dye concentration and temperature increase, while decreasing when the particle size and competing ion concentration increase. The adsorption process follows the pseudo second order kinetics model over the pseudo first order kinetics model and the Elovich kinetics model. Compared to the Freundlich, Temkin and Dubinin–Radushkevich isotherm models, the Langmuir isotherm model provides greater insight into the adsorption process. The maximum adsorption capacity, q_m was 1988.387 mg/g at optimum conditions (45 °C, pH 10, contact time 180 min, adsorbent dosage 200 mg/L) which was calculated from the Langmuir isotherm model. Negative values of free energy change, ΔG (−2.8464 kJ/mol at 30 °C) indicated spontaneity of the adsorption of MG onto MBC and The process is endothermic, as suggested by positive values of the heat of adsorption, ΔH (5.6514 kJ/mol). The value of activation energy (E_a = 9.7956 KJ/mol) and the estimated average free energy of adsorption (E < 8) supported that the adsorption process is physisorption.