Selective removal of dual dyes from aqueous solutions using a metal organic framework (MIL-53(Al))

Abstract

The adsorption process of the cationic dye Malachite Green (MG) and the anionic dye Methyl Orange (MO) onto an MOF, namely MIL-53(Al), was studied using several experimental parameters. Batch experiments were performed to investigate the feasibility of this MOF as a possible adsorbent for the removal of MG and MO from both single and binary-dye aqueous solutions. The experimental results revealed that MIL-53(Al) can quickly bind to both types of dyes with high removal efficiencies of more than 95%. The effects of different parameters, such as adsorbent dosage, initial concentration of the dye solution, pH of the solution and temperature for the adsorption process, were examined. Adsorption kinetic data were then fitted using pseudo first-order, pseudo second-order, and Eelovich models, with the results showing that the pseudo second-order model had the best fit for both dyes amongst all the models used in both single and binary-dye systems(R2 > 0.996). Moreover, adsorption isotherm models of Langmuir and Freundlich were studied and the isotherm data was found to be in good agreement with the Freundlich isotherm for both dyes. Thermodynamic studies on the change in Gibbs free energy, enthalpy and entropy were carried out and showed that the adsorption of both dyes was endothermic and spontaneous. The selectivity test for an equimolar binary-dye solution containing both MG and MO suggested the higher affinity of MIL-53(Al) towards MO over MG, as it reached a value of 13.58 under the optimum conditions. Finally, excellent reusability of MIL-53(Al) was shown by utilizing it for 4 cycles of adsorption–desorption with almost no reduction in the adsorption capacity, bringing about an additional examination of its applicability.