Adsorption of cadmium (II) from aqueous solutions by coconut dregs residue: Kinetic and thermodynamic studies

Abstract

In this study, the potential of coconut dregs residue (CDR) as a low-cost biosorbent for the removal of cadmium (II) from aqueous solutions was investigated. The removal efficiency of Cd (II) by CDR was tested through batch adsorption experiments under different conditions including contact time, initial pH, initial Cd (II) concentration, adsorbent dose, and temperature. The optimal pH for removal was found to be 7.0 where an 87.6% removal was achieved with a metal concentration of 25 ppm, a 0.5 g/100.00 cm3 adsorbent dosage, and a 250–500 µm particle size within 120 minutes at 303 K. The Freundlich isotherm model was employed to explain the adsorption process, while the pseudo-second-order model proved to be the best-fit kinetic model. Results showed that the Gibbs free energy change in Cd (II) adsorption was −5.4 kJ mol−1 indicating a spontaneous process. The raw CDR exhibited an adsorption capacity of 4.31 mg/g. Scanning electron microscopic (SEM) analysis revealed the presence of tubular voids which contributed to the adsorption process. Additionally, activated carbon prepared from CDR showed a higher removal efficiency of 95.6% for Cd (II) as compared to the raw CDR biosorbent.