Nature as Resource. Thermodynamic characterization of natural and synthetic polymers and their sequestering ability towards some bivalent metal cations

The aim of this paper is to enhance the knowledge on the behaviour and thermodynamic properties of natural and synthetic biopolymers in aqueous solutions. The acid-base properties of Carrageenan (Car), Xanthan gum (XG), Hydroxyethyl cellulose (HEC), Carboxymethyl cellulose (CMC), Acusol 445™ (Sol), Monopropylene glycol (MPG) and Polyethylene glycol (PEG) have been studied by potentiometric technique in NaNO3(aq), at I = 0.15 mol dm−3 and T = 298.15 K. In some cases, the protonation constants have been determined at different ionic strengths, and the dependence on ionic strength has been modelled by a Debye-Hückel type equation. The binding ability of the different polymers has been investigated towards Ca2+, Zn2+ and Sn2+ and, different speciation models have been obtained. For Acusol 445™, the weak interactions with Na+ and K+ have been determined. The effect on the complexing ability of a secondary inorganic ligand, the fluoride, has been evaluated on Carboxymethyl cellulose and Acusol systems; the formation of ternary mixed complexes with a significant increasing stability with respect the binary species has been obtained. The sequestering ability of the polymers towards the metal cations has been evaluated by means the empirical parameter pL0.5.