Exploring various ligand classes for the efficient sequestration of stannous cations in the environment

The metal pollution has become one of the most serious environmental problems. The presence of heavy metals is due to both natural and anthropogenic sources. Tin is not generally considered to be among the most important “pollutants”, although some compounds are of great importance from an environmental point of view. Beside the reputation of organotin(IV) compounds, inorganic tin species are important from a geochemical and hydrological perspective. In fact, tin is the 24th most abundant element in the Earth’s crust and has ten isotopes, the largest number in the periodic table. Among them, 126Sn is a radionuclide produced by nuclear fission, and it is often present in radioactive wastes, with a half-life of 105 years [1]. In this contribution, the sequestering ability of several ligands will be evaluated towards the Sn2+ cation in the conditions of several natural fluids, for example sea water (I = 0.72 mol kg-1, pH = 8.0), fresh water (I = 0.0034 mol kg-1, pH = 7.0), and an acidic waste water (e.g., I = 0.05 mol kg-1, pH = 4.0). Several organic and inorganic ligands will be explored, on the basis of the functional group(s) in the molecule (carboxylic acids, amines, amino acids, phosphonates etc…), molecular weight (low and high), number of sites available for binding (monodentate, polydentate, chelants), environmental persistence (biodegradable or not). According to this classification, some ligands such as carbonate, mellitic acid, glycine, EDTA, polyacrilates, spermine, thiomalic acid will be tested. The evaluation of the sequestering ability of the ligands is done in terms of evaluation of different parameters, such as the pM (residual concentration of free metal cation), and the pL0.5 (concentration of ligand necessary to bind the 50% of metal in solution). It is fundamental to mention the difference between pM and pL0.5; in fact, while in the computing of pM, the contribution of all the species that may form complexes with Mn+ is considered (e.g., OH-, other ligands), this is not in the evaluation of pL0.5, which on the contrary takes into account only the contribution of the Mn+/L species, free from other side reactions, and from our point of view it is more objective than pM. This research group has introduced a new empirical parameter, called pL0.5, for the evaluation of the sequestering ability of a ligand towards a metal cation. Details about the calculation procedure and the features can be found elsewhere [2]. Briefly, a Boltzmann type equation is built calculating the molar fraction of the metal cation, present in trace concentration, complexed to the ligand vs. the ligand concentration (expressed as –log cL). x "= " ["1" /("1+" 〖"10" 〗^(("pL-" 〖"pL" 〗_"0.5" ) ) )] The value of the pL when 0.5 molar fraction is complexed to the ligand is called pL0.5. This simple parameter is useful for the comparison of the sequestering ability of different ligands towards the same cation and vice versa, because it takes into account all the side reactions (ligand protonation, metal cation hydrolysis, presence of a second ligand) and does not depend on the speciation scheme, but only on the experimental conditions (pH, ionic strength and temperature) in which it is determined.