Adsorption of Lead and Cadmium from Aqueous Solution by Using Almond Shells

The potential of almond shells was assessed for adsorption of heavy metal ions such as Pb2+ and Cd2+ from aqueous solution. Almond shells were pretreated separately with 0.4 mol/L NaOH, 0.4 mol/L HNO3, and distilled water and their adsorption abilities were compared. Batch adsorption experiments were carried out as a function of the initial ion concentration, pH, and adsorbent dosage. Adsorption isotherms of metal ions on adsorbents were determined and correlated with common isotherm equations such as Langmuir, Freundlich, and BET models. The alkali-modified almond shells had adsorption capacities for Pb2+ from 2 to 9 mg/g and for Cd2+ from 2 to 7 mg/g, which was much higher than acid- and water-pretreated adsorbents. Experimental results showed that the best pH for adsorption was 5-6 and the adsorption values decreased with lowering pH. Isotherm models indicated the best fit for Langmuir model for alkali-modified almond shells. In comparing the parameters of the models, it was observed that the affinity of almond shells for adsorption of lead is stronger than affinity for adsorption of cadmium.