Long-Term Monitoring of Palladium and Platinum Contents in Road Dust of the City of Munich, Germany

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The concentration of platinum group metals (PGM), namely palladium, platinum and rhodium, has increased in all earth spheres, which is mainly due to the worldwide use of automotive catalytic converters containing these metals as active components. Especially high concentrations are therefore found near busy roads and in urban environments where PGM concentrations now have reached a level that has let scientists and engineers think about possibilities to recycle these precious metals in the context of urban mining strategies. Especially palladium has been classified as particularly critical as it shows a comparatively high mobility in environmental compartments and elevated toxicological effects. It is therefore advisable and necessary to monitor traffic related PGM emissions with particular focus on palladium. One way to do this is long-term monitoring of PGM emissions under largely constant conditions. In the present work this has been done by monitoring palladium concentrations in tunnel dust of the outer city ring (B2R) of Munich, Germany. Dust samples were collected from 1994 until 2012 from the roofs of emergency telephone boxes installed in the tunnels Landshuter Allee, Candid and Trappentreu and analyzed for their palladium and some of them for their platinum content. Major and minor matrix components have also been analyzed to ensure comparability of the samples. PGM Analysis was performed with Graphite Furnace Atomic Absorption Spectrometry (GFAAS) after complete sample digestion and enrichment of palladium and platinum with N,N-Dialkyl-N’-benzoylthioureas acting as highly selective chelating agents. Matrix characterization was performed by elementary analysis and Total Reflection X-ray Fluorescence (TXRF). The main matrix components determined in the road dust samples from 1994 to 2012 show, apart from one noteworthy exception (sulfur) a quite constant composition. The sulfur content in the dust samples of all three tunnels decreased significantly in the years after 1998. This is most probably attributable to the legally required reduction of the sulphur content in gasoline and diesel fuels. The average palladium concentration in the dust samples increased significantly from 1994 to 2007 where it reached a maximum. From 2009 onwards there was a steady decline in the average palladium concentration, reaching a minimum in 2012. The increase of the palladium concentration in the tunnel dust from 1994 to 2007 can easily be explained by the gradual replacement of platinum by palladium in automotive catalytic converters. In 2007 traffic density monitored by the municipal administration of the city of Munich also reached a high level which roughly remained the same up to today. Platinum concentration in the dust samples also reached a maximum in 2007 and declined from 2007 to 2012. The most likely explanation for the decline of the palladium and platinum concentration in the tunnel dust after 2007 is progress in the production of automotive catalytic converters and/or progress in automobile engine construction.