Palladium(II) sequestration by natural and synthetic chelating agents

Palladium belongs to the so called “Platinum group elements” (PGE), and it is normally present at very low concentration in the environment. In the last twenty years, there has been an increasing PGE demand in different fields such as medicine, electronics, jewellery and car industry. Moreover, palladium (and other PGE) is broadly used as catalyst in a wide number of reactions. The high catalytic properties of PGE favoured their use in the production of catalytic converters. Unfortunately, this caused a noticeable increasing of PGE concentration in the environment, mainly constituted by platinum, with a particular accumulation in urban areas. However, since 1993 platinum has been increasingly replaced by palladium as the predominant substance in catalysts. Now the release of palladium in automotive catalysts is becoming just a critical problem as that of platinum. Furthermore, whilst the emission, the environmental distribution and the possible health effects of platinum have been widely investigated, there is a clear lack of information concerning palladium. Palladium also represents an environmental risk higher than platinum, because it easily and quickly undergoes oxidation processes when put in contact with soils. If the oxidation of palladium may represent a serious environmental problem, at the same time it could represent an advantage for soil and water remediation, if an appropriate chelating agent for the metal ion removal is found. Since the sequestering ability of a chelating agent is strictly related to the stability of the complex species formed with the metal ion to be removed, the speciation studies are of great importance, and represent a fundamental step for the understanding and the optimization of the whole sequestration process. To this end, we focused our attention on the sequestering ability of a natural polyphosphate chelating agent, the so called phytic acid [myo-Inositol hexakis(dihydrogen phosphate)] and of three synthetic chelating agents: the ethylenediamine-N,N,N’,N’-tetraacetate (EDTA), the diethylenetriamine-N,N,N’,N’’,N’’-pentaacetate (DTPA) and the triethylenetetraamine-N,N,N’,N’’,N’’’,N’’’-hexaacetate (TTHA). In this contribution, a series of ISE-H+ potentiometric titrations were carried out in aqueous solutions containing different amounts of the ligands used and of palladium(II) ion, in NaNO3aq at I = 0.1 mol L-1 and t = 25°C. In some cases, some measurements were also performed in NaClaq. The use of two different supporting electrolytes was useful for the evaluation of the effect of the ionic medium on the speciation of Pd2+, with particular reference to the influence of chloride ion. In fact, palladium(II) ion forms stable complex species with this anion, whilst nitrate is weakly interacting with Pd2+. Due to the great tendency of palladium(II) to hydrolyze even at low pH, some ISE-H+ titrations were also carried out to study the acid-base behaviour of Pd2+ ion in the same experimental conditions of Pd2+-ligand systems investigated. Though the literature on the hydrolysis of palladium(II) ion in different ionic media and ionic strengths is abundant, this check was necessary owing to the evident discrepancies noticed in terms of both the number of hydrolytic species and the hydrolysis constant values reported by the different authors. The sequestering ability of the investigated ligands toward Pd2+ was then evaluated by the calculation of various pL50 values in different conditions. Finally, the dependence of pL50 on pH was modelled by simple empirical relationships.