Use of Gantrez Copolymers as Potential Chelating Agent for the Selective Sequestration of Metal Ions. Studies of the Interactions in Aqueous Solution at Different Ionic Strengths and Temperatures

In this paper, the thermodynamic properties of the complex species of the GantrezTM (Trade Mark product) copolymers of different molecular weights (AN169, S97 and S95) with different divalent (Ca2+, Mg2+, Sn2+ and Zn2+) metal cations in NaCl aqueous solutions at different ionic strengths and temperatures were studied by potentiometric technique. Investigations were carried out in wide ranges of metal to ligand molar concentration ratios up to basic pH values. For the simple metal-Gantrez systems, fairly similar speciation models were obtained, independent of the experimental conditions; the main differences are due to the formation of a ternary MLOH species (M = metal ion, L = Gantrez ligand) or to the final pH value of the titrations, determined by the formation in some cases (i.e. for Sn2+ investigations) of a sparingly soluble species. The stability trend of the species was: Sn2+ >> Zn2+ > Ca2+  Mg2+. For Gantrez S95, the interaction with Zn2+ and Sn2+ were also investigated in a solution containing different amounts of fluoride, in order to investigate the formation of mixed metal-ligand’-ligand’’ species. The dependence of the stability constants on ionic strength and temperature was modelled by means of modified Debye-Hückel equations. From the gradient of stability constants with respect to temperature, rough enthalpy and entropy change values for the formation of the species were calculated, and results that the entropic contribution is the driving force of reactions. The sequestering ability of the three Gantrez ligands, evaluated by means of the pL0.5 parameter, allowed us to highlight a net higher ability of gantrez ligands to sequester Sn2+ with respect to the other metal ions (about 3-4 orders of magnitude).