Sequestration of Aluminium(III) by different natural and synthetic organic and inorganic ligands in aqueous solution.

The speciation of Al in aqueous solutions containing organic and inorganic ligands important from a biological (citrate (Cit), gluconate (Gluc), lactate (Lac), silicate (HSiO), carbonate (CO), fluoride (F)) and industrial (Gantrez; polymethyl-vinyl-ether-co-maleic acids; GTZ S95 and GTZ AN169) point of view is reported. The stability constants of Al/L complexes (L = ligand with z charge) were determined by potentiometry at T = 298.15 K and 0.10 ≤ I/M ≤ 1.00 in NaCl (in NaNO only for Al/GTZ S95 and Al/Gluc acid systems). For Al/Cit, Al/Lac and Al/GTZ AN169 systems, the investigations were also carried out at 283.15 ≤ T/K ≤ 318.15. The dependence of the thermodynamic parameters on ionic strength and temperature was modelled with a Debye-Hückel type equation. Different speciation schemes of Al/L systems were obtained, including protonated, simple metal-ligand, polynuclear and hydrolytic mixed species. At I → 0 M and T = 298.15 K the stability trend for the AlL species is: 14.28 ± 0.02, 13.99 ± 0.03, 10.16 ± 0.03, 3.16 ± 0.08, 2.84 ± 0.10 for GTZ S95, GTZ AN169, Cit, Gluc and Lac, respectively. From the investigations at different temperatures, it results that the entropic contribution is the driving force of the reactions. The sequestering ability of the ligands towards Al was investigated determining the pL parameter at different experimental conditions, finding the following trend: Cit » Gluc > GTZ S95 > GTZ AN169 > Lac for the organic ligands, and pL: F » CO > HSiO for the inorganic ones.