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

The speciation of Al3+ in aqueous solutions containing organic and inorganic ligands important from a biological (citrate (Cit3−), gluconate (Gluc-), lactate (Lac-), silicate (H2SiO42−), carbonate (CO32−), 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 Al3+/Lz− complexes (Lz− = ligand with z− charge) were determined by potentiometry at T = 298.15 K and 0.10 ≤ I/M ≤ 1.00 in NaCl(aq) (in NaNO3(aq) only for Al3+/GTZ S95 and Al3+/Gluc− acid systems). For Al3+/Cit3−, Al3+/Lac- and Al3+/GTZ AN1694− 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 Al3+/Lz- 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(3−z) 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, Cit3−, 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 Al3+ was investigated determining the pL0.5 parameter at different experimental conditions, finding the following trend: Cit3− » Gluc- > GTZ S954− > GTZ AN1694− > Lac- for the organic ligands, and pL0.5: F− » CO32− > H2SiO42− for the inorganic ones.