ADSORPTION OF LEAD (II) AND CADMIUM (II) IONS FROM AQUEOUS SOLUTION ONTO MODIFIED AND UNMODIFIED RED ONION SKIN BIOMASS

Abstract

The biosorption performance of unmodified and modified onion skins as alternative low cost biosorbent for the removal of lead (II) and cadmium (II) was investigated. The effects of varying the pH, contact time, initial metal concentration, temperature as well as adsorbent dosage on the adsorption capacity of the modified and unmodified onion shells were determined. The performance of the biosorbents was evaluated in terms of percentage biosorption for each heavy metal and the extent of the maximum sorption capacity was estimated by Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models. The percent sorption decreased with increasing initial concentration of metal ions for the two metal ions studied, with higher affinity for Pb(II). At the optimum conditions for the biosorption of Pb(II) and Cd(II) ions, the highest percentage of the metal ions adsorbed were Pb(II) at pH 4 (96.1 % and 88.5 %) and Pb(II) at pH 5 (81.5 % and 73.95 %) for modified and unmodified onion shell respectively. The results showed that the chemical modification with thioglycolic acid improved the adsorption performance of onion shells. The adsorption isotherm showed that Freundlich isotherm best represents the equilibrium data for Pb(II) and Cd(II) ions than other isotherms. The adsorption performance of metal ions on the unmodified and modified onion shells were described well by the pseudo-second-order kinetic model. Thermodynamic parameters such as free energy (ΔG°), standard enthalpy (ΔH°) and the standard entropy (ΔS°) were determined. Among the two different desorbing agents tested, 67.58 % Pb(II) and 38.75 % Pb(II) recovery was achieved with 0.3 M HCl. On the whole, this experiment showed that onion shells can effectively be utilized for Pb(II) and Cd(II) removal from aqueous solutions.