Environ. Sci. Technol. 2000, 34, 3119-3123
Elemental Association and Fingerprinting of Traffic-Related Metals in Road Sediments
the autocatalyst. High-temperature sintering of the autocatalyst alters the nanocrystalline structure (8), while the occurrence of S in automobile exhaust can increase the solubility of Pt (9). The speciation of PGMs in road sediments is a key to understanding further biogeochemical mobility and transformations in the environment.
S EÄ B A S T I E N R A U C H , * , † GREGORY M. MORRISON,† M I K A E L M O T E L I C A - H E I N O , ‡,§ OLIVIER F. X. DONARD,‡ AND M Y R I A M M U R I S †,# Water Environment Transport, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden, and Laboratoire de Chimie Bio-Inorganique et Environnement, EP CNRS 132, He´lioparc Pau Pyre´ne´es, F-64053 Pau Cedex 9, France
The determination of the speciation of Pb and PGMs in road sediments leads to the assessment of potential mobility and bioavailability. Speciation of trace elements in sediments is governed by bulk components. However, comparison between trace and bulk elements through total analysis provides little correlation, even in extensive surveys (10). Relevant information on the heterogeneity of metal associations might be lost because the total metal concentration is considered rather than the speciation. Therefore, an analytical procedure based on the sequential dissolution into five solutions has been developed to study the association of trace elements with five geochemical phases in sediments (11). Sequential extraction of road sediments has shown that Pb is predominantly associated with geochemically defined Fe/Mn hydrous oxide and carbonate fractions (12), while Pt has a significant exchangeable fraction (4). However, this interpretation is fraught with difficulties. The separation of metals into geochemically defined fractions was originally adapted for the investigation of pollutant metals in river sediments (13) and later rigorously tested as an analytical procedure (11). Doubts emerged with regard to selectivity, recovery (14, 15), and readsorption (16). Cadmium and zinc partitioning in anoxic sediments studied using an electron microprobe and through thermodynamic calculations disagreed with sequential extraction (17). Nonetheless, scientific investigations still report and review results for sequential extractions of a range of environmental particulates (1821). To aid widespread use, a simplified three-step procedure (22, 23) with associated certified reference material (24) has been introduced.
Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) can be used for the study of elemental associations in solid environmental matrices through direct ablation. The elemental associations of Pb and platinum group metals (PGMs) in road sediments were investigated by ablating the sediment surface followed by multielemental analysis by ICP-MS. When the UV laser was tuned into a small spot size together with a low energy, the true geochemical associations of the elements in road sediments could be obtained. Pb showed a high degree of association for the Fe/Mn and Ca/Mg/Sr phases, while PGMs were not associated with a particular geochemically defined fraction. Automobile catalysts are the main source of PGMs in road sediments, and association with Ce demonstrated that PGMs remain bound to autocatalyst particles in road sediments.
Introduction With the phasedown of lead in petrol, the number of new automobiles using unleaded fuel and equipped with autocatalysts has steadily increased. Palladium, platinum, and rhodium act as catalysts for oxidation (carbon monoxide, hydrocarbons) and reduction (nitrogen oxides) reactions in automobile exhaust systems. These platinum group metals (PGMs) are deposited on the aluminum washcoat of a honeycomb ceramic support, together with additives which include Ce, Zr, La, and Ba (1). The car emission rate of PGMs into the highway environment is in the ng km-1 range (2), and elevated concentrations of PGMs in atmospheric particles (3), road sediments (4), roadside catchbasins (5), and roadside soil (6) have been reported. Platinum in the fresh autocatalyst is present not only in a metallic form but also as oxides, as chlorides, and bound to hydrocarbons, while rhodium is present in a metallic form and as an oxide (7). Transformations of PGMs during vehicle operation might change the form in which PGMs occur in * Corresponding author phone: +46 31 772 21 23; fax: +46 31 772 21 28; e-mail:
[email protected]. † Chalmers University of Technology. ‡ Laboratoire de Chimie Bio-Inorganique et Environnement. § Present address: IENS, Lancaster University, Lancaster LA14YQ, United Kingdom. # Present address: Ecole Nationale des Travaux Publics de l’Etat, rue Maurice Audin, 69518 Vaulx-en-Velin cedex, France. 10.1021/es000001r CCC: $19.00 Published on Web 07/06/2000
2000 American Chemical Society
The sequential extraction procedure is time-consuming and poorly poised for automation, although microwave and ultrasonic (25) assisted developments increase sample throughput. Unfortunately, the overriding criticism is more fundamental; sequential extraction procedures are operationally defined, and this precludes the strict interpretation in terms of elemental speciation (18, 26). It is quite feasible that the sequential extraction procedure provides inaccurate results due to redistribution of chemical speciation during extraction. A false interpretation of PGM and Pb speciation in road sediments might therefore be obtained through the application of a sequential extraction procedure. Direct solid sampling for multielemental analysis may prove a more suitable strategy for the speciation of sediments. Recent advances in ultraviolet laser microprobes (27, 28) coupled to ICP-MS (inductively coupled plasma-mass spectrometry) opens up the possibility for a new approach to metal speciation in sediments. A laser microprobe can be used to investigate elemental patterns (fingerprints) and ablated associations in a wide range of materials (29-31). This procedure involves the intercomparison of elements, rather than determining absolute concentrations, and therefore avoids the interpretation and calibration difficulties caused by differential ablation effects. Combined with the sensitivity and multielement capability of ICP-MS, laser ablation (LA) allows the investigation of elemental associations and the heterogeneity of trace elements in road sediments down to a 5-10 µm resolution. LA-ICP-MS analysis of PGMs and Pb with major mineral elements (e.g. Fe, Mn, VOL. 34, NO. 15, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Ca, Al, Si) can provide true geochemical associations in road sediments.
TABLE 1. PGM Concentrations in a
