Theoretical and Experimental Study of Pb2+ and Hg2+ Adsorption on Biopolymers, 1. Theoretical Study

Abstract

Chitosan and pectic acid have been modeled as disaccharides or oligosaccharides for Hg2+ and Pb2+ adsorption. Reasonable models of both biopolymers were used. Several adsorption sites of both polysaccharides were considered, mainly NH2 in chitosan and CO2– in pectic acid. Hg2+ has several points of anchorage on chitosan. The most important one is NH2 . The Molecular Mechanic modeling permit us to compare in relative terms the different conformations of models of pectic acid and chitosan and their effect in heavy metal coordination. Using the Parameterized Model version 3 (PM3), we report the formation enthalpy of inter- and intramolecular compounds with Hg and Pb. The Extended Huckel method (EHM) results seem to indicate that electrostatic interaction (leading to adsorbed cation on NH2 and on sites different to NH2) could be the reason for the high uptake found for Hg2+ using chitosan. Besides NH2, the OH near the amine group is the preferred site for Hg2+ adsorption, especially if it is ionized. In the case of Pb2+ adsorption, several sites of chitosan present no interaction with this cation. Only the NH2 group and the ionized OH group mentioned above seem to be the preferred sites, following the EH modified (EHMO) results. The Hg- and Pb-adsorption modeling on pectic acid permit us to conclude that the best site is the same for both metals: the bridge oxygen between monomers of galacturonic acid. the carbonyl group from carboxylate is the best second site for Hg2+, whereas the internal oxygen bridge is the best second site for Pb2+. Considering the Hg2+ chemistry in aqueous solution, we evaluated the HgOH+ and HgCl+ or HgCl3– adsorption on both copolymers, using EHMO. The energetic of adsorption changed on both biopolymers for these species, comparing them with Hg2+.