Changes in palladium, platinum, and rhodium concentrations, and their spatial distribution in soils along a major highway in Germany from 1994 to 2004.

Soil samples were collected along the highway A5 from the major junctions Frankfurter Kreuz to Darmstäder Kreuz from July to September, 2004 and analyzed for palladium (Pd), platinum (Pt), and rhodium (Rh). The results were compared to analyses of platinum group elements (PGE) in soils collected along this same stretch of highway in 1994. The goal of this study is to detect any changes that may have occurred in the concentration and environmental distribution patterns of these metals over this 10 year period. The concentrations of Pd in soils along the highway were found to be about 15 times higher on average than those measured in 1994. Pt and Rh concentrations increased 2 and 1.6 times, respectively, during this time period. The significant rise in Pd concentrations in soils observed for the time period of analysis is likely due to its use in automobile catalytic converters in Germany since 1993. The results also show a strong positive relationship between PGE emissions and traffic density and speed. The results indicate that increases in the concentrations of Pd, Pt, and Rh in soils along the highway are not limited to the soil surface. Pt was measured as deep as 12 cm. Pd was determined at even greater depths of 12-16 cm. The presence of Pd at lower depths compared to Pt suggests that this element has a higher solubility. Pd, Pt, and Rh concentrations display a strong inverse relationship with distance from the road, with decreasing levels with increasing distance from the highway. Nonetheless, Pd and Pt were detected in a meadow as far as 50 m from the highway, a much greater distance compared to that measured for these metals in 1994. Pt concentrations were also found to significantly correlate with levels of Rh and Pd. The ratios between the PGE analyzed (Pt/Rh and Pd/Rh) display a shift toward Pt and Pd. The results clearly show that PGE concentrations have increased over time. Observed increases in Pd concentrations are particularly a cause of concern.