Sorption mechanisms of selected heavy metals on spent coffee grounds

Abstract

This study investigated the use of spent coffee grounds (SCG) for the sorption of cadmium and nickel ions from synthetic aqueous solutions. Batch kinetic and equilibrium experiments were conducted to study the effects of pH, contact time, sorbent dosage, initial concentration and temperature on the sorption of Cd and Ni. The spent coffee was stripped of its remaining oils using supercritical fluid extraction (SFE) before sorption. Maximum sorption occurred at pH 6 for both metals. Equilibrium data for Ni sorption were best modelled by the Langmuir isotherm, with linear regression coefficient R2 = 0.9916, whereas Cd sorption was best modelled by the Freundlich isotherm (R2 = 0.9888). The theoretical sorption capacities of Cd and Ni were 2.5500 mg⋅g-1 and 2.0590 mg⋅g-1, respectively. Thermodynamic studies suggested sorption of both metals was exothermic (ΔH°Cd = -31.54 kJ∙mol-1; ΔH°Ni = -23.84 kJ.mol-1), and spontaneous (ΔG° ˂ 0) over the temperature range 10 °C to 40 °C. The ΔG° decreased with increasing temperature, suggesting sorption was more spontaneous at higher temperatures. Disorder of magnitudes ΔS°= 107.51 J/mol-1∙K-1 (Cd) and 76.44 J/mol-1∙K-1 (Ni), suggested sorption was random. Equilibrium was reached in less than 60 minutes for both Cd and Ni. Kinetic data for both metals best fitted a pseudo-second order model, indicating that chemisorption was the predominant mechanism.