Theoretical and experimental study of M2+ adsorption on biopolymers: III. Comparative kinetic pattern of Pb, Hg and Cd

Abstract

Chitosan, pectin and chitosan-pectin pellets were studied as adsorbents for Cd2+ from acidic aqueous solutions at selected conditions. Several adsorption sites were considered and adsorption mechanisms for the biopolymers-metal interactions are proposed taking into account theoretical and experimental results. Langmuir adsorption isotherms for Hg, Pb and Cd on pellets gave KL values of 796, 1551 and 7265Lmequiv.-1; their Memax (mequiv./g) being 0.027 (2.71 mg/g), 0.1075 (11.2 mg/g) and 0.022 (1.23 mg/g), respectively. Chitosan adsorption of Hg achieved 85-100% and 70-75% for Pb and Cd, respectively. Ci (initial concentration) was 47,045.4 μg/g chitosan for Pb, 6863.6 μg/g chitosan for Hg and 5.4436 μg/g chitosan for Cd. When pectin was the adsorbent 15% of Hg and not more than 4% Pb and Cd were adsorbed, Ci Pb being 9409.1 μg/g pectin Ci Hg=1372.7 μg/g pectin and Ci Cd=1064.5 μg/g pectin. In the case of pellets, 100% initial Hg, Cd and Pb present initially in solution were adsorbed, Ci Pb being=103.5 μg/g, Ci Hg=15 μg/g and Ci Cd=12 μg/g. Molecular mechanics (MM2) and PM3 calculations were done, trying to identify evidence of the proposed kinetic model and an explanation of the equilibrium uptake of the metal adsorption on chitosan-pectin pellets.