Kinetics and Thermodynamics Studies of the Biosorption of Pb(II), Cd(II) and Zn(II) Ions from Aqueous Solution by Sweet Orange (Citrus sinensis) Seeds

Abstract

Kinetics and thermodynamics studies of the biosorption of Pb(II), Cd(II) and Zn(II) ions from aqueous solution by sweet orange (Citrus sinensis) seeds was investigated using batch adsorption experiment at 32 oC. The effect of pH, contact time, metal ion concentration and temperature were evaluated. The residual concentrations of the metal ions were determined by atomic absorption spectrophotometer. The results showed that maximum removal of Pb(II) and Zn(II) ions occurred at pH 6 with 98.69% and 99.82% respectively and Cd(II) ion was found to be 98.61% at pH 7. The equilibrium data fitted to Freundlich and Temkin adsorption isotherms. The order of fitness of the equilibrium data was in Temkin < Freundlich. Thermodynamics investigation showed that standard Gibb’s free energy change (ΔGo) was negative indicating that the adsorption of Pb(II), Cd(II) and Zn(II) ions by orange seeds (Citrus sinensis) were feasible and spontaneous. The negative value of standard enthalpy change (ΔHo) implies that the reaction was exothermic and physical in nature while positive value of standard entropy change (ΔSo) implies an irregular increase in the randomness at the solid/solution interface of the adsorbent during the adsorption process. The kinetics study revealed that the sorption of Pb(II), Cd(II) and Zn(II) ions could be best described by pseudo-second order kinetics models. This study demonstrated that orange seeds (Citrus sinensis) could be efficiently and effectively used as an alternative adsorbent to commercial activated carbon in the removal of Pb(II), Cd(II) and Zn(II) ions from aqueous solution.