Palladium in environmental matrices: A review

Abstract

Palladium is one of the Platinum Group Elements (PGEs) and it is present on the Earth Crust (EC) at very low concentrations. Background concentrations of these noble elements in the range 0.06-0.40 mg/kg were reported by Taylor (1985), Wedepohl (1995) and Rauch et al., (2000) while a Pd average concentration of 0.015 mg/kg in the Upper Continental Crust (UCC) was proposed by Greenwood and Earnshaw (1989) and Hartley (1991). In the last two decades the use of cars equipped with VECs (Vehicle Exhaust Catalyst) was strongly implemented in order to reduce the gaseous pollutants from vehicle exhausts. PGEs are present in catalyst owing to their chemical and physical properties. In fact, at the high operational temperatures of these systems (450-550 C), they facilitate the reactions that permit the transformation of about 90% of NOx, CO and HCs in the less dangerous products as N2, water and CO2. In modern three-way catalytic converters (TWCs), PGEs are dispersed in the honeycomb structure of the catalyst. However, notwithstanding the evident benefit, the continuous release of PGEs in the environment, following the deterioration of TWCs, produces some concerns. In fact in some urban areas the PGEs concentration in soils, sediments and dusts is slightly but continuously increasing, reaching in some cases values that are one or two order of magnitude higher than the natural background ones. In addition, the small particles of PGEs mainly are in the PM10 fraction where the 3 m breathable fraction could reach 35-40% of the emissions. These concerns arise also considering recent data on PGEs demand for industrial use. In fact, since the introduction of catalytic converters a constant increase of these noble metals demand for application in auto catalysts has been observed. In Europe from 1992 to 2002 the total demand for Pt and Pd is grown of the 94.2% and 150% respectively, while their application for auto catalyst required the 75% and 84% of the total production. Worldwide data show that in the year 2000 rather 98% of the total mined Rh (16200 kg) has been used in car industry while the percentage for Pt and Pd was 31% and 61% respectively [WHO (1991, 2002), Johnson Matthey (2001), Ravindra et al., (2004)]. From when catalytic converters were first introduced, to comply with new legislation guidelines and thanks to technologic developments, catalysts have become increasingly efficient. Recently the importance of Pd has been hampered by the fact that the new three-way catalyst employ preferentially Pd and Rh instead of Pt which cost results too high for the market. So in the next future an increase on Pd emission can be reasonably expected and, cause to its highest mobility among PGEs a deeply investigation on Pd in environmental matrices should be promoted.