Speciation of Al3+ in the presence of ligands of industrial, biological and environmental interest

This contribution is the result of an investigation on the binding and sequestering ability of three ligands different in terms of chemical point of view and of fields of application. The ligands under study are: • Gantrez®AN169 (H4GTZ), a synthetic copolymer of methyl vinyl ether and maleic anhydride, having several applications in industrial, cosmetics, pharmaceutical and oral care product fields1; • citric acid (H3Cit), a low molecular weight organic acid, with a well-known biological role2; • orthosilicic acid (H4SiO4), an inorganic compound and the main form of bioavailable silica for humans and animals3. Their interactions and binding ability towards Al3+ was investigated by performing potentiometric (ISE-H+) measurements at different conditions of metal-ligand concentration ratio (cM/cL), temperature (283.15  T/K  318.15) and ionic strength (0.10  I/mol L-1  1.00) in NaCl(aq), the main inorganic component of mostly natural and biological fluids. Different speciation models were determined taking into account the acid-base properties4-7 of the ligands and of the metal cation. For all the systems the formation of a common species, namely the protonated AlLH, was observed. The investigation on the Al3+/GTZ4- and Al3+/Cit3- interactions showed also the formation of other species with different stoichiometry (AlpLqHr), such as simple metal-ligand, polynuclear and mixed hydroxo species. The dependence of the stability constants on ionic strength and temperature was modelled by means of a Debye-Hückel type equation. From the formation enthalpy changes values, it was evidenced that all the reactions are endothermic in nature and the entropic contribution can be considered the driving force for the stability of the complexes. Furthermore, the sequestering ability of the ligands towards Al3+ was investigated by the determination of an empirical parameter, the pL0.5, already proposed by the research group8. It represents the total concentration of ligand necessary to sequester the 50% of a metal cation present in trace in solution and can be described by a sigmoid type Boltzmann equation. The analysis of the pL0.5 values determined at different experimental conditions showed that in all the cases the sequestering ability increases increasing the pH and the temperature, whilst it decreases increasing the ionic strength in NaCl(aq).