Characterization of cadmium adsorption on two cost-effective biochars for water treatment

The objective of this study was to characterize the cadmium (Cd) adsorption behavior on biochar derived from grape and apple pruning residues (GPR and APR, respectively). Consequently, batch experiments were carried out with increasing levels of initial Cd concentration (0 to 200 mg L−1) under different shaking times (0 to 240 min) and temperatures (10, 20, 30, and 40 °C). The adsorption of Cd on the biochars was well fitted to the Elovich kinetics and Freundlich sorption model. The maximum Cd sorption capacities of 57 mg g−1 and 49 mg g−1 were calculated for GPR and APR biochars, respectively, indicating the higher sorption capacity of GPR in comparison with the APR biochar. The sorption energy parameter (E) of Dubinin–Radushkevich isotherm (4.37 and 4.05 kJ mol−1) and negative Gibbs free energy (∆G) values (− 15 to − 19 kJ mol−1) revealed the physical adsorption and spontaneous of Cd adsorption on the biochars, respectively. The entropy (ΔS) and change in enthalpy (ΔH) were found to be 1.57 J mol−1 K−1 and 0.22 kJ mol−1 for GPR biochar and 1.50 J mol−1 K−1 and 0.21 kJ mol−1 for APR biochar, reflecting an affinity of Cd on the biochars and endothermic nature of Cd adsorption reaction. This study demonstrated the feasibility of the biochars derived from grape and apple pruning residues to be as a potential low-cost adsorbent for Cd removal from aquatic systems. Furthermore, the adsorption procedure involving kinetic time and temperature effects were optimized to achieve the maximum Cd sorption on the adsorbent. Adsorption mostly took place within 20 and 40 min for GPR and APR biochar, respectively, while the optimum temperature was 40 °C.