Pt and Pd in the Urban Environment

Pt and Pd are extremely rare metals. Almost half the world's annual production is being used in catalytic converters fitted to cars to reduce the emissions of poisonous gasses from car exhaust systems. Particles of Pt, Pd and also Rh, are detached from these catalytic converters and land on the roads [Prichard in BA reports (1998); Jarvis et al. (2001); Higney et al. (2002).] A pilot study in 1996 on road dusts swept from six road junctions in Cardiff (Table 1, Fig. 6) showed that at all the localities Pt and Pd are above the normal background values of less than 1-2 ppb. Values at the six sites varied from 16-126 ppb Pt and 7-99 ppb Pd. The higher values correspond to the busiest roundabouts where traffic flow is high.

Table 1A Cardiff road dust as swept straight from the road.

Values in ppb

1 A48, Roath roundabout

2 A48, Penylan roundabout

3 M4, Granada services roundabout

4 Cyncoed roundabout

5 Culverhouse cross BMW entrance

6 Traffic lights A48

Rh Pt Pd

22 126 99

16 98 13

11 73 21

Table 1B Roath roundabout road dust.

Sieved into coarse and fine fractions and separated by heavy liquids

Values in ppb Rh Pt Pd Au % of sample

Light fraction 4 25 30 3 93

Heavy fraction 220 1679 284 122 7

Fine fraction below 0.09 mm

Light fraction 5 37 6 6 89

Heavy fraction 111 966 2945 272 11

M4 L

\\f / \ v\l / \ —

A\ / v"-~ l "Vt ''V -^u b—

A 48 —/ \ 1 N X, 1 km A |

Vr0-,;':' / Cardiff ^O^-'ky— Bay

Fig. 6 (a) A map of Cardiff showing sample locations for road dust collection at major road junctions (squares) and mud samples in the bay (circles); the M4 motorway is shown as a thick black line, and the other roads are shown as thinner black lines. Rivers are shown in dashed lines, Cardiff Bay and the Bristol Channel are filled with horizontal lines and the built up area is shaded grey.

Fig. 6 (b) A map showing the locations of sample sites (I) at the major road junctions marked on (A). The number of each sample site refers to sample numbers in Table 1.

Fig. 6 (b) A map showing the locations of sample sites (I) at the major road junctions marked on (A). The number of each sample site refers to sample numbers in Table 1.

The Cyncoed roundabout was found to have significantly high Pt and Pd values and, although it does not have the high traffic flow associated with the roundabouts on major route ways and motorways, it is in the centre of a wealthy part of Cardiff. Here new cars have catalytic converters that tend to disintegrate due to temperature fluctuation at the beginning of journeys. The sample with the highest Pt and Pd values at Roath, after density separation, yielded total precious metal values of 4294 ppb in the fine grained, heavy fraction.

From the road sides the PGE are washed down gullies into the complicated network of artificial and natural drainage (Fig. 7) [Laschka and Nachtwey (1997)]. PGE are collecting at points in the urban waste system at concentrations well above normal background levels. The fact that these accumulations of Pt and Pd are being moved through the urban environment is demonstrated by their presence in mud from Cardiff Bay. Here values of 20 ppb Pt plus Pd must have come from artificial sources as there are no nearby natural sources of Pt and Pd. Another example of the mobility of PGE in the urban environment comes from Os isotope evidence in sediments in Massachusetts and Cape Cod bays [e.g. Ravizza and Bothner (1996)]. Traces of Pt and Pd contamination have been recorded even in the Greenland ice sheet [Barbante et al. (2001)].

Fig. 7 Diagram showing possible pathways for the Pt and Pd as they pass through the urban environment.

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