An in-depth investigation into adsorption equilibrium, kinetics, and thermodynamics of spent coffee grounds for methylene blue removal

Abstract

The adsorption performance, thermodynamics, and kinetics of spent coffee grounds (SCG) in the elimination of Methylene Blue (MB) from an aqueous solution were extensively investigated in this study. Unmodified SCG were characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, BET specific surface area analyzer, and the point of zero charge (pHpzc) determination to identify surface chemistry, morphology, and textural properties. The batch adsorption experiment, and the nonlinear optimization method, combined with the chi-squared test, were utilized in modeling adsorption equilibrium and kinetics. The results demonstrated that the nonlinear Langmuir adsorption isotherm well fitted to adsorption equilibrium data, which indicated a maximum monolayer adsorption capacity of 73.38 mg/g. Adsorption kinetics exhibited a better description of the nonlinear pseudo-second-order kinetic model. The values obtained from the thermodynamics study showed the adsorption process is spontaneous at increased temperatures (ΔG°<0, ΔS°>0), and endothermic (ΔH°>0). Additionally, the magnitude of MB adsorption enthalpy change (ΔH°=4.18 kJ/mol) can assist in predicting the MB adsorption of SCG is physisorption and probably driven by hydrogen bondings combined with π-π interactions. This study demonstrates the potential of adsorbents derived from spent coffee grounds in textile wastewater treatment and reveals the nature of the interaction between MB and SCG.