Removal of dyes from aqueous solution using fly ash and red mud

Fly ash and red mud have been employed as adsorbents for the removal of a typical basic dye, methylene blue, from aqueous solution. Heat treatment and chemical treatment have also been applied to the as-received fly ash and red mud samples. It is found that fly ash generally shows higher adsorption capacity than red mud. The raw fly ash and red mud show adsorption capacity at 1.4×10−5 and 7.8×10−6 mol/g, respectively. Heat treatment reduces the adsorption capacity for both fly ash and red mud but acid treatment by HNO3 induces a different effect on fly ash and red mud. Nitric acid treatment results in an increase in adsorption capacity of fly ash (2.4×10−5 mol/g) while it decreases the adsorption capacity for red mud (3.2×10−6 mol/g). The adsorption data have been analysed using Langmuir, Freundlich and Redlich-Peterson isotherms. The results indicate that the Redlich-Peterson model provides the best correlation of the experimental data. Isotherms have also been used to obtain the thermodynamic parameters such as free energy, enthalpy and entropy of adsorption. For fly ash and red mud, adsorption of methylene blue is endothermic reaction with ΔH0 at 76.1 and 10.8 kJ/mol, respectively.