Changes in platinum concentrations in soils and dusts from UK cities

Abstract

Introduction Vehicle exhaust catalysts have been fitted to all new cars in the UK since January 1993, in order to comply with EU Stage I limits (EC Directive 91/441/EEC) on emissions of carbon monoxide, hydrocarbons and nitrogen oxides (NO2 nitrogen dioxide and NO nitric oxide). A typical catalyst contains 1 – 3 g of platinum group metal, housed in a stainless steel box. Catalysts are designed to operate for at least 80,000 km, and their numbers are increasing worldwide, therefore Pt losses are of general environmental interest. Validated data are needed to determine concentrations in environmental media in order that potential health risks and environmental impacts of this source of Pt may be assessed. The mechanism for the loss of platinum from catalysts is primarily thought to be from sintering due to thermal damage caused at the high operative temperatures experienced and poor adhesion of the catalyst layer to the ceramic monolith resulting from mechanical damage due to vibration (Zarkadis 1987). Chemical reactions have also been implied in the loss of platinum from catalysts (Schlögl 1996). Pt emissions from catalysts are generally thought to be in the metallic or oxide form (Schlögl 1987). Recent work has shown the proportion of oxide to be less than 25% of total Pt emissions from three way catalysts (Schlögl 1996). Artelt (1996) reported that in Pt particulate emissions originating from automobile exhaust catalysts, 1.0% soluble Pt was found. Under conditions simulating city traffic, emissions of Pt are two to three times higher than at constant speeds (Knobloch 1993) therefore ambient concentrations should reflect traffic densities and flows. Pt concentrations in the vicinity of roads have been discussed in the literature, for example: Sweden (Wei 1993; Wei and Morrison 1994); Germany (König et al 1992; Laschka et al. 1996; Helmers et al. 1994; Lahmann and Thron 1987; Zereini et al. 1993, Alt et al. 1993; Alt and Messerschmidt 1993); the USA (Hodge and Stallard 1986) and the UK (Farago et al. 1996, 1998). Platinum concentrations in motorway soils in Germany ranged from 15 - 30 ng/g (Alt and Messerschmidt 1993), which is considerably higher than background concentrations in soils (range 0.09 - 4 ng/g); alpine and forest soils were reported to contain 0.09 and 0.1 ng/g respectively. Surface soils from near the Frankfurt Wiesbaden highway were found to have a mean concentration of Pt of 10 ng/g with a maximum of 87 ng/g (Zereini et al. 1993).