Scavenging malachite green dye from aqueous solutions using pomelo (Citrus grandis) peels: kinetic, equilibrium and thermodynamic studies

Abstract

Activated carbon produced from pomelo peels (PPAC) was tested for its effectiveness in the removal of malachite green (MG) dye from aqueous solution. The PPAC prepared was characterized using TGA, BET, FTIR, SEM, pHpzc, Elemental analysis and Boehm titration, respectively. The extent of dye adsorption was investigated as a function of pH, contact time, adsorbate concentration and solution temperature. Dye removal was pH dependent, resulting in 95.06% removal at pH 8.0. Quantum chemical studies suggested that the cationic MG dye possessed minimal molecular size at planar geometry coupled with high-electrostatic interaction thereby, enhancing the adsorption at high pH. Langmuir, Freundlich and Dubinin–Radushkevich (D–R) equilibrium isotherms were used to fit the adsorption data. Langmuir isotherm fit the adsorption data most with maximum monolayer adsorption capacity of 178.43 mg/g. The kinetic data fitted the pseudo-second-order model with correlation coefficient greater than 0.99. The mean free energy E (kJ/mol) got from the D–R isotherm indicated physisorption mechanism Thermodynamic parameters; Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) changes were also calculated, and the values indicated that the adsorption process was spontaneous and endothermic in nature. Regeneration efficiency of spent PPAC was studied using 0.2 M HCl, the efficiency was found to be in the range of 92.71–96.35% after four cycles. The study showed that PPAC can be used as an effective, low cost and environmentally friendly adsorbent for the removal of MG dye from aqueous solution.