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

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 7265 L mequiv.−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. lar 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.