Removal of direct red 81 from aqueous solution by adsorption onto magnesium oxide-coated kaolinite: Isotherm, dynamics and thermodynamic studies

Natural clays are considered to be excellent low cost adsorbents for the removal of dyes from aqueous solutions due to high surface area and porosity, layered structure, and high cation exchange capacity. The adsorption efficiency of the clays is considerably enhanced after chemical modifications. In this work, the efficiency of kaolinite (Kaol) and magnesium oxide-coated kaolinite (Mg-Kaol) for the removal of azo-dye (direct red 81) from aqueous solution was studied. The objective of the work was to evaluate the adsorption capacity of Mg-Kaol, and optimum operation conditions such as contact time, initial DR81 concentration, adsorbent dose, pH, and temperature for uptake of direct red 81 (DR81) from aqueous solution. The experimental data were analyzed using Langmuir, Freundlich, Dubinin-Radushkevich (D-R), Temkin, and Redlich-Peterson isotherm models using nonlinear regression analysis. Freundlich isotherm best fitted the equilibrium data for adsorption of DR81 onto Kaol and Mg-Kaol. The Langmuir monolayer adsorption capacity (Q(m)) of Mg-Kaol (55.70 mg/g) showed considerable improvement compared with Kaol (26.55 mg/g). The R-L values below unity supported favorable adsorption of the dye. The nonlinear pseudo-second order kinetic model provided best correlation suggesting adsorption of one DR81 molecule onto two active sites. The mechanism of removal of DR81 was controlled by both liquid-film and intra-particle diffusion. Thermodynamic parameters (G(o) and H-o) suggested the adsorption process to be spontaneous and endothermic. The study revealed that Mg-Kaol can be an efficient low-cost adsorbent for decolorization of wastewaters containing DR81. (c) 2016 American Institute of Chemical Engineers Environ Prog, 36: 45-58, 2017