Removal of astrazon blue dye from aqueous media by a low-cost adsorbent from coal mining

The aim of this study was to evaluate the removal of astrazon blue (AB) dye from aqueous solution by adsorption on an otherwise despoiling and useless mineral waste obtained from coal mining (MWCM) and how this was affected by calcination. The solid waste, as characterized by elemental and thermogravimetric analyses, X-ray diffraction, infrared spectroscopy, specific surface area measurements and Boehm titrations, was found to be composed primarily of calcite, dolomite, and gypsum. Experiments investigated how adsorption was affected particularly by the conditions of pre-calcination of the waste, and also by the contact time, initial AB dye concentration, pH, and temperature of the aqueous medium. Optimum performance was achieved with waste that had been pre-calcinated at 400°C, MWCM400. The kinetics, equilibrium, and thermodynamics of adsorption for MWCM400 were investigated in detail with results that could be satisfactorily rationalized by a pseudo-second-order kinetic model together with a Langmuir isotherm. On the basis of this model, the maximum adsorption capacities were 74.24, 71.63, and 97.18 mg g−1 at 25, 45, and 65°C, respectively. The thermodynamic parameters indicated an endothermic process, ΔHads∘ = +64 kJ mol−1, driven by an increasing degree of freedom accompanying the adsorption process, ΔSads∘ = +186 J K−1 mol−1. The activation energy, Ea = +9.5 kJ mol−1, was consistent with adsorbate–adsorbent interactions of a predominantly physical type. The results revealed that MWCM400 is a potential low-cost adsorbent for cationic dye removal from residual waters.