Environ. Sci. Technol. 2007, 41, 451-456
Changes in Palladium, Platinum, and Rhodium Concentrations, and Their Spatial Distribution in Soils Along a Major Highway in Germany from 1994 to 2004 FATHI ZEREINI,* CLARE WISEMAN, AND WILHELM PU ¨ TTMANN Institute of Atmospheric and Environmental Sciences, Department of Analytical Environmental Chemistry, J.W. Goethe-University Frankfurt, Georg-Voigt-Strasse 16, D-60054 Frankfurt am Main, Germany
Soil samples were collected along the highway A5 from the major junctions Frankfurter Kreuz to Darmsta¨ der 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.
Introduction The platinum group elements (PGE) platinum (Pt), rhodium (Rh), and palladium (Pd) are used as catalysts in automobile * Corresponding author phone: (049) 069-798-22144; fax (049) 069-798-23502; e-mail:
[email protected]. 10.1021/es061453s CCC: $37.00 Published on Web 12/07/2006
2007 American Chemical Society
catalytic converters to reduce pollutant emissions. Elevated temperatures and vibration of the catalyst material in the converter lead to the release of small amounts of PGE in exhaust fumes (1-4). Emission rates are related to car operation conditions and the age of the catalytic converter (1-2). Platinum metals appear to be primarily emitted as particles in metallic form (5-6). Most of the emitted PGE-containing particles appear to be deposited directly on the road. The particles are washed out from the atmosphere when it rains, and they can end up in the drainage system (5). PGE can also be deposited along the roadside with the water that is sprayed from car tires under wet conditions or be transported with rainwater to roadside gullies and flood retention basins (6-8). There is also evidence which suggests that PGE can be transported via wind, with the distance of travel being dependent on the morphology and particle size of the emitted particles and climatic conditions (9-14). Studies conducted in different countries have shown that the active materials from catalytic converters (Pt, Pd, and Rh) occur in high concentrations in soils in the vicinity of roads (15-23, 8). Characteristically, PGE concentrations decrease with increasing distance from the roadside. Soils along heavily frequented roads typically display the greatest levels of these metals, indicating that PGE concentrations are related to traffic volume (24). Observed Pt and Rh concentrations, and ratios thereof, suggest that these elements have a low solubility in soils (25, 6). Experimental work confirms this (26). Emitted PGE have been reported though to be taken up by various plants and terrestrial and aquatic organisms (27-33). Pd appears to have a higher solubility compared to Pt and Rh (19). Since 1993, Pd has been increasingly used in place of platinum in Pt/Rh catalysts in automotive catalytic converters in Europe. This is also reflected in observed increases in the environmental concentrations of this element. Globally, the use of Pd in exhaust catalysts has greatly risen since the early 1990s. While 22 t of Pd were used worldwide by the catalytic converter industry in 1993, a total of 115 t were consumed in 2005 (34, 35). This represents an approximate 5-fold increase in the use of Pd over a period of 12 years. Knowledge regarding the environmental behavior and potential impact of PGE, particularly Pd, is limited. The goal of this study is to increase our knowledge regarding PGE concentrations and their distribution patterns in soils over time. This will help us to improve our ability to assess the potential environmental and human health impacts of these elements. For this, soils along the highway A5 between the major junctions Frankfurter Kreuz and Darmsta¨dter Kreuz in Germany were collected in 2004 and analyzed in terms of their Pt, Pd, and Rh concentrations. The results were compared to PGE concentrations in samples which had been collected along this same stretch of highway in 1994. This allowed for the detection of possible changes in PGE concentrations, as well as the spatial distribution of these elements, over the last 10 years. Samples in this study were collected in a similar fashion at the same sites, and analyzed using the same methods as those applied in 1994, to increase the validity and reliability of comparing sample results obtained from the two time periods.
Experimental Section Sampling. Samples were collected along the highway A5 from Frankfurter Kreuz (km 498) to Darmstadt (km 520), a stretch that we already studied in 1994. This part of the highway has one of highest traffic volumes in Germany, with a traffic load VOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of 110 000 to 131 000 per day (data from 2004, HSVV-Hessen). The A5 along the sampling stretch has eight lanes, is about 23 km in length, and runs primarily in a north-south direction. The vegetation is diverse and includes meadows, cultivated fields, and forest. A total of 54 soil samples were collected at various sites and distances from the highway from July to September 2004 and analyzed for Pt, Pd, and Rh. The concentrations of these elements were also analyzed according to depth at one site. The different soil sample types taken can be split into three categories: soils directly along the highway, soil cross sections, and soil depth profile. These are described in detail below. Soils Directly Along the Highway. Soils located directly along the highway were sampled. Such soils are not naturally formed and normally contain elevated levels of heavy metals due to traffic and street erosion. The east side of the highway was sampled as it was expected to be enriched with PGE due to the prevailing westerly wind that blows here. For each designated km of the sampling stretch, about 30-60 g of sample were collected from the top 2 cm of soil every 25-30 m and composited (N ) 23). This yielded about 1-1.5 kg of soil per sample for analysis. Samples were taken directly below the guardrail, 0.3 m from the edge of the highway. Soil Cross Sections. At three locations with different prevailing vegetation, samples were collected at distances of 0, 3, 6, 10, 15, 20, 30, 50, and 100 m from the highway, respectively. For each distance, 20 samples were taken over a breadth of up to 500 m parallel to the highway. For each sample, 30-60 g were taken from the top 2 cm of soil. A total of 27 soil samples were collected. The sampling sites were forest (at km 524-525), meadow (at km 508-509) and cultivated field (at km 514-515). The meadow and the cultivated field are about 150 cm lower than the surface of the highway, while the forest is located at approximately the same height. Soil Depth Profile. At km 519, 30 cm from the shoulder of the Highway, a quadratic piece of soil about 10 cm long and 16 cm deep was removed and sectioned into four lengthwise pieces with a thickness of 4 cm (i.e., a total of four samples). The location for this sample was chosen to ensure that neither soil morphology nor drainage grooves or conduits would serve to reduce PGE levels deposited on the soils. Soil Sample Preparation and PGE Analyses. After collection, the samples were air-dried and passed through a 2 mm sieve to isolate the
