Sequestration of Pd2+ by polyamino-polycarboxylic ligands

The increase of the worldwide demand of “Platinum group elements” (PGE) for application in several fields such as industry, medicine, jewellery and, especially, in catalyst converter production, caused a noticeable increasing of PGE concentration in the environment. Though palladium, among the anthropogenic PGE, is not the most abundant one, it is the most hazardous since it undergoes easily and quickly oxidation to palladium(II) when in contact with soils, with a consequent increase of its mobility in the environment. The presence of complexing agents, which form soluble complex species with palladium(II), favours the mobility of the ion with an increase of its availability to plants, animals and humans. Among anthropogenic complexing molecules, an important role is played by synthetic aminopolycarboxylic chelating agents (usually called with the acronym APC) whose concentration in the environment is progressively increasing owing to their considerable use in several fields (agriculture, industry, medicine) and a low biodegradability of most of them. The interaction of these ligands with palladium(II) ion leads to the formation of soluble complex species whose stability influences strongly the availability of palladium(II) in the environment. With the aim to assess the strength of interaction of Pd2+ with aminopolycarboxylic ligands, here we report the results of a systematic study, , on the formation of palladium(II) complex species with five APCs [ethylenediamine-N,N,N’,N’-tetraacetate (EDTA), (S,S)-Ethylenediamine-N,N′-disuccinic acid (S,S-EDDS), Nitrilotriacetate (NTA) and diethylenetriamine-N,N,N’,N’’,N’’-pentaacetate (DTPA) and triethylenetetraamine-N,N,N’,N’’,N’’’,N’’’-hexaacetate (TTHA)]. Owing to the high stability of the Pd2+ - APC complex species, the calculation of their stability constants was very difficult and was possible only by combining the results obtained from two series of ISE-H+ potentiometric titration (in NaNO3 and in mixed NaNO3 /NaI ionic medium) and from ISE-H+ potentiometric /spectrophotometric titrations (in NaClO4). As expected, the stability of Pd-APC complex species is function of the number of carboxylic and amino groups present in the ligand molecules (e.g., logKPd(APC) = 37.00, 36.31, 23.60, 23.07 and 17.82 for TTHA, DTPA, EDTA, S,S-EDDS and NTA in Na+ ionic media, at I = 0.1 mol L-1 and T = 25°C). Results obtained on the stability of species in the Pd- S,S-EDDS show that this ligand, which is the most biodegradable APC ligand, can be used successfully as environmental friendly chelating agent in substitution of the other less degradable APCs in all their application fields. From the stability data of the Pd2+ - APCs the sequestration capacity [expressed as pL50, i.e. the –log (APC concentration] necessary to bind the 50% of the metal ion) of the ligands under investigation towards palladium(II) ion was determined in the pH range considered. The pL50 is easily correlated to important physico-chemical parameters (pH, ionic strength, temperature, etc.) as shown in the Figure, were the dependence on pH of pL50 of the APCs towards Pd2+ ion is reported. As can be seen, TTHA and DTPA show almost the same sequestering ability in the pH range 2 – 10, clearly higher than that of EDTA, S,S-EDDS and NTA where a minor number of amino and carboxylic groups is present in the molecule.