Biosorption of cadmium metal ions on raw and chemically modified walnut shells

The abundantly available agricultural waste product walnut shells were treated with alkali for enhancement in adsorptive capacity for Cd(II) ions. The raw walnut shells (RWS) and alkali treated walnut shells (AWS) were characterized for functional groups and morphological changes using FTIR and SEM analysis. The adsorption increased drastically between pH 2 and 6. The Langmuir isotherm model provided a better fit with the experimental data than Freundlich by giving high correlation coefficients. The biosorption capacity of RWS and AWS for Cd(II) ions was found to be 4.20 and 14.29 mg g−1, respectively. Dubinin–Radushkevich (D‐R) isotherm model was used to find out the mean adsorption energy. The adsorption followed pseudo‐second order kinetics, suggesting monolayer coverage and a chemisorption process. The thermodynamic parameters including change in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were derived. Negative value of ΔG confirmed spontaneity of biosorption. Desorption efficiency showed no significant decrease till third cycle.