Platinum and palladium variations through the urban environment: evidence from 11 sample types from Sheffield, UK.

Platinum (Pt) and Palladium (Pd) concentrations have been analysed in 194 samples from within the city of Sheffield in the UK. The samples were taken from road dust, gully pots (also known as drains), soils, a motorway drainage pipe, rivers, lakes, sewage sludge, incinerator ash, incinerator ash in landfill, street cleansers and gully cleansers. The introduction of Pt- and Pd-bearing automobile catalysts, has been cited as the cause of a rise in the concentration of urban Pt and Pd accumulations. Geochemical analyses for the different sample types are used here to show how the Pt and Pd accumulate in different urban environments as they are transported from their catalytic source. Initially Pt and Pd collect in road dust and gully pots at values of up to 450 ppb although most analyses for both elements are around 100 ppb. The four roadside soils analysed, have a great range in values, the highest with a value of over 600 ppb Pt and 1000 ppb Pd. Then the fate of the Pt and Pd is either to be removed by gully flushers and road sweepers (which contain around half the concentration of that in road dust) or to be washed from the roads, through the gully pots, into either the river or urban drainage systems. Due to the addition of terrestrial sediments, river samples contain much reduced values of Pt and Pd, at approximately an order of magnitude lower than in road dust. Similarly, sewage sludge contains Pt and Pd values which are lower than road dust. However, the Pt and Pd analyses are much higher in incinerated sewage (with many samples over 150 ppb for both metals), probably due to the loss of the mass of other material during the incineration process. Weathered incinerator ash in landfill has lower values of Pt and Pd than fresh ash from the incinerator. Although the range in values of Pt and Pd is similar for road dust and gully pot sediments their modal values tell a different story. The mode for Pt is very similar for both road dust and gully pot sediments, at around 100 ppb, whereas there is a drop of 50 ppb in the mode for Pd in the gully pots (from 80 ppb to around 40 ppb). Given that gully pot sediment is derived from road dust, it is suggested that in gully pots, Pd is preferentially mobilized over Pt. Furthermore, a comparison of the modal values of Pt and Pd in river sediments suggests that this process continues into the natural drainage system of the city.