Biosorption of cadmium ions from aqueous solution onto alkaline-treated coconut shell powder: kinetics, isotherm, and thermodynamics studies

The need for low-cost and efficient biosorbent materials for the decontamination of heavy metal polluted water is important to abate the toxic effects on humans and the environment. Therefore, the present research reports the adsorption of cadmium ions from an aqueous solution onto alkaline-treated coconut shell (Alk-CCS) powder as a low-cost potential biosorbent. The biosorbent was characterized by SEM, XRD, FTIR, and EDX. Batch biosorption experiments were used to determine the effect of pH, time, temperature, dosage, and concentration on biosorption. The isotherm, kinetic, and thermodynamic modelling of the process was performed. The optimum condition for the adsorption of Cd2+ onto Alk-CCS was a pH of 6.4, adsorbent dosage of 0.2 g, and a contact time of 30 min. Moreover, a maximum monolayer adsorption capacity of 14.22 mg/g was obtained from the Langmuir model for Cd2+ adsorption. From the nonlinear isotherm and kinetics analysis, cadmium ions adsorption on Alk-CCS followed the pseudo-second-order model involving chemical interactions, while the Sips model best fitted the isotherm evaluation. According to the satisfactory fitting to the Sips isotherm model, the adsorption could not be best fitted to the classical adsorption isotherm models such as Langmuir and Freundlich. Thermodynamic parameters indicated that the adsorption of Cd ions on Alk-CCS was feasible, spontaneous (negative value of increment G degrees), and exothermic (negative value of increment H degrees). Moreover, the nonlinear modelling showed that alkaline modified coconut shell powder is a viable and efficient adsorbent for the removal of cadmium ions from polluted water.