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

FERREIRA, M. L.; GSCHAIDER, M. E.

MACROMOLECULAR BIOSCIENCE

WILEY-V C H VERLAG GMBH

Año: 2001 vol. 1 p. 233 - 248

1616-5187

Chitosan and pectic acid have been modeledas disaccharides or oligosaccharides for Hg2+ and Pb2+adsorption. Reasonable models of both biopolymers wereused. Several adsorption sites of both polysaccharideswere considered, mainly NH2 in chitosan and CO2± in pecticacid. Hg2+ has several points of anchorage on chitosan.The most important one is NH2 . The Molecular Mechanicmodeling permit us to compare in relative terms the differentconformations of models of pectic acid and chitosanand their effect in heavy metal coordination. Usingthe Parameterized Model version 3 (PM3), we report theformation enthalpy of inter- and intramolecular compoundswith Hg and Pb. The Extended Huckel method(EHM) results seem to indicate that electrostatic interaction(leading to adsorbed cation on NH2 and on sites differentto NH2) could be the reason for the high uptakefound for Hg2+ using chitosan. Besides NH2, the OH nearthe 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 thiscation. Only the NH2 group and the ionized OH groupmentioned above seem to be the preferred sites, followingthe EH modified (EHMO) results. The Hg- and Pbadsorptionmodeling on pectic acid permit us to concludethat the best site is the same for both metals: the bridgeoxygen between monomers of galacturonic acid. the carbonylgroup from carboxylate is the best second site forHg2+, whereas the internal oxygen bridge is the best secondsite for Pb2+. Considering the Hg2+ chemistry in aqueoussolution, we evaluated the HgOH+ and HgCl+ orHgCl3± adsorption on both copolymers, using EHMO. Theenergetic of adsorption changed on both biopolymers forthese species, comparing them with Hg2+.