Isotopic Signature and Impact of Car Catalysts on the Anthropogenic Osmium Budget ANDRE Ä POIRIER* AND CLE Ä MENT GARIE Ä PY † GEOTOP-UQAM-McGILL, Geochemistry and Geodynamics Research Centre, C.P. 8888, Succ. Centre-ville, Montre ´al, Quebec, Canada, H3C 3P8 Higher osmium concentrations and lower 187 Os/ 188 Os ratios in sediments from urban areas have been linked to anthropogenic osmium sources. Automobile catalytic converters that use platinum group metals (PGM) are a potential source for this Os pollution. We present the first direct Os concentrations and isotopic measurements of catalytic converters for major automobile brands to test the assumption that car catalysts release Os with a distinct signature in the environment. The analysis of four new catalytic converters yields similar low 187 Os/ 188 Os ratios (0.1-0.2), suggesting a similar source for the PGM. The Os concentrations measured are in the ppt range (6-228 ppt). From our results, the osmium contribution of the car catalysts to the environment through attrition (wearing and grinding down of the catalyst by friction) is predicted to be low, <0.2 pg Os/m 2 /year in highly urbanized environment. We show that Os loss from catalysts as volatile OsO 4 is important at car catalyst operating temperatures. Moreover, we estimate that car catalysts may be responsible for up to ∼120 pg Os/m 2 deposited per year in urban areas and that part of it may be exported to sedimentary sinks. Car catalytic converters are thus an important anthropogenic osmium source in densely populated areas. The NIST car catalyst standard (SRM- 2557, made from recycled used catalysts) yields higher concentrations (up to 721 ppt Os) and a more radiogenic isotopic composition (∼0.38), perhaps indicative of Os contamination during its preparation. Introduction The accumulation of heavy metals in the sedimentary record was greatly enhanced by the onset of the industrial era. In most urban environments, recent sediments contain metal concentrations orders of magnitude higher than pre- industrial levels. Metals with naturally variable isotopic compositions (e.g., Pb) allow a better understanding of the impact of pollution in sedimentary environments, by enabling tracing of sources, either natural or man-made. Osmium, a member of the platinum group metals (along with Pt, Pd, Rh, Ru, and Ir), is also enriched in urban shallow sedimentary records despite its rather limited industrial use (1-5). Osmium also has a wide range of isotopic compositions in nature because the 187 Os isotope varies in relative abundance in different lithologies of diverse geological ages. This is attributable to the fact that part of the 187 Os comes from the radioactive decay of 187 Re (T1/2 ≈ 42 billion years). The present- day 187 Os/ 188 Os of a rock is controlled by its Re/Os ratio and its geological age. The variability in Re/Os is due to the different geochemical behavior of the two elements; Os is more enriched in ultramafic rocks, whereas Re is preferen- tially enriched in more differentiated lithologies. Some of the most important sources for platinum group metals (PGM) are ore bodies from magmatic ultramafic intrusions of mantle origin, which were contaminated with minor amounts of (relatively radiogenic) continental osmium during emplacement. Consequently, commercial PGM from these deposits have 187 Os/ 188 Os isotopic signatures of ca. 0.15- 0.20, for example, Bushveld Complex (6). Sediments derived from continental crust, although heterogeneous in nature, have a higher average 187 Os/ 188 Os of ∼1.0 due to long-term Re enrichment of the crust (7). The large differences in Os isotopic signatures between these two sources allow the anthropogenic and natural components in a given sedi- mentary basin to be distinguished and quantified. Urban sewage sludge contains high concentrations of osmium and is suspected to be a major contributor of anthropogenic Os to shallow urban sediments (2, 3, 5). Sources of Os in this sludge, and ultimately in sediments, were attributed to the biomedical and industrial uses of this PGM; OsO4 is a lipid stain used to enhance cell structures for optical and electron microscopy (1, 4, 5). Automobile catalytic converters have been mentioned as another possible con- tributor of anthropogenic Os to recent sediments (2, 5). Esser and Turekian (1) mentioned catalytic converters as a Pd source but discarded it for Os because it is not directly employed in the automobile emission control industry. Automobile catalytic converters (hereafter termed catalysts) use Pd, Pt, and Rh to decrease the conversion temperature of carbon monoxide, nitrous oxides, and unburned hydro- carbons to less noxious exhaust fumes. In a converter, these metals are coated (using a washcoat) on to a honeycomb- structure that provides a high surface/volume ratio that is typically composed of cordierite (due to its excellent thermal properties). These devices are known to lose a fair amount of their noble metals by attrition during normal use (8). Although osmium is not directly employed in car catalysts, the strong association between the PGM means that osmium impurities can persist even among refined PGM. Thus, a significant level of Os impurity could be present among the other PGM in the catalysts and contribute to the anthro- pogenic input of Os to the urban environment. An estimate of how much osmium could theoretically be introduced into urban environments by attrition every year can be obtained from the equation used for platinum release (eq 1): where Pt attrition is the amount of platinum delivered through attrition by recent monolithic catalyst -20 to 136 ng Pt/km (8), ∑cars is the quantity of currently catalyst-equipped cars (ca. 500 million), and Km is the average annual mileage per car (15 000 km per years; 9). The Os/Pt ratio of catalytic converters is unknown, but estimating an impurity of a few * Corresponding author phone: (514)987-4080; fax: (514)987-3635; e-mail: andre@olympus.geotop.uqam.ca. † Prof. C. Garie ´py passed away in April 2004, during the writing of this manuscript. He was 51 years old. Dr. Garie ´py was instrumental in the development of isotopic techniques at GEOTOP Research Center, as well as in the widening of their applications for environmental research, as illustrated here and in many papers he co-authored in recent years. Os attrition ) Pt attrition × ∑cars × Km × ( Os Pt ) catalyst (1) Environ. Sci. Technol. 2005, 39, 4431-4434 10.1021/es0484552 CCC: $30.25  2005 American Chemical Society VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4431 Published on Web 05/17/2005