Studies on batch adsorptive removal of malachite green from synthetic wastewater using acid treated coffee husk: Equilibrium, kinetics and thermodynamic studies

Coffee husk treated with sulfuric acid (ACH) is used as a low-cost adsorbent for batch adsorptive removal of malachite green (MG) from synthetic dye solution. Structural and morphological characteristics of the prepared ACH adsorbent were analyzed by powder X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The effects of pH (2-9), adsorbent dose (0.05-1 g/L), initial adsorbate concentration (25-150 mg/L), contact time (0-120 min) and temperature (30-60 degrees C) on adsorption were studied. The adsorption of MG on ACH is confirmed by Fourier transform infrared spectroscopy. Further, modelling of isotherms, kinetics, and thermodynamics was carried out using various mathematical equations. The Sips isotherm model and pseudo second-order kinetic model fitted best with experimental data with R-2 > 0.98. The diffusion model analysis indicates the adsorption process is governed by both film and intra-particle diffusion. The maximum adsorption capacity of acid treated coffee husk was found to be 263 mg/g. Thermodynamic studies revealed that the adsorption of MG on ACH is endothermic (Delta H-0 = 7.83 kJ/mol), feasible (Delta G(0) = -20.10 to -22.87 kJ/mol) and spontaneous (Delta S-0 = 0.092 kJ/mol K). Energies of adsorption calculated from various equations showed that adsorption is a physical process. This study shows that ACH can be a promising and efficient low-cost adsorbent for the removal of MG from industrial wastewater.