Removal of Cu(II) from aqueous solution by agricultural by-product: Peanut hull

Peanut hull, an agricultural by-product abundant in China, was used as adsorbent for the removal of Cu(II) from aqueous solutions. The extent of adsorption was investigated as a function of pH, contact time, adsorbate concentration and reaction temperature. The Cu(II) removal was pH-dependent, reaching a maximum at pH 5.5. The biosorption process followed pseudo-second-order kinetics and equilibrium was attained at 2 h. The rate constant increased with the increase of temperature indicates endothermic nature of biosorption. The activation energy ( E a) of Cu(II) biosorption was determined at 17.02 kJ/mol according to Arrhenius equation which shows that biosorption may be an activated chemical biosorption. Other activation parameters such as Δ H #, Δ S #, and Δ G # were also determined from Eyring equation. The equilibrium data were analyzed using the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models depending on temperature. The equilibrium biosorption capacity of Cu(II) determined from the Langmuir equation was 21.25 mg/g at 30 °C. The mean free energy E (kJ/mol) got from the D-R isotherm also indicated a chemical ion-exchange mechanism. The thermodynamic parameters such as changes in Gibbs free energy (Δ G 0), enthalpy (Δ H 0) and entropy (Δ S 0) were used to predict the nature of biosorption process. The negative Δ G 0 values at various temperatures confirm the biosorption processes are spontaneous.