Environ. Sci. Technol. 2008, 42, 9443–9448
Bioaccessibility of Platinum Group Elements in Automotive Catalytic Converter Particulates ANDREW TURNER* AND SIMON PRICE School of Earth, Ocean, and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, U.K.
Received April 30, 2008. Revised manuscript received August 7, 2008. Accepted September 3, 2008.
The bioaccessibilities of the platinum group elements (PGE): Rh, Pd, and Pt; and the catalyzator poison, Pb, have been determined in particles derived from milled automotive catalytic converters using a physiologically based extraction test (PBET) that simulates, sequentially, the chemical conditions encounteredinthehumanstomachandintestine.PGEaccessibility, relative to total metal concentration, was generally less than a few percent, but increased in the stomach with decreasing pH (from 4 to 1) and/or increasing chloride concentration, and with decreasing particle concentration. In most cases, bioaccessibility increased from the acidic stomach to the neutral, carbonate-rich intestine. Bioaccessibility of Pb displayed similar pH and particle concentration dependencies to PGE in the stomach, but this metal exhibited significantly greater mobilization (up to 80%) overall and a reduction in accessibility from the stomach to intestine. Reaction kinetics of PGE dissolution in the stomach at pH 2.5 were modeled using a combined surface reaction-diffusion controlled mechanism with rate constants of 0.068, 0.031, and 0.015 (µg L-1)-1 h-1 for Rh, Pd, and Pt, respectively. For Pb, however, mobilization proceeded via a different mechanism whose time-dependence was fitted with an empirical, logarithmic equation. Overall, PGE bioaccessibility appeared to be controlled by dissolution rates of metallic nanoparticles in the stomach, and solubility and kinetic constraints on inorganic species (chlorides, hydroxychlorides, and carbanatochlorides) and undefined organic complexes formed in the simulated gastrointestinal tract. Further studies are required to elucidate any effects engendered by the long-term oral exposure of small quantities of these species.
Introduction Platinum, palladium, and rhodium are platinum group elements (PGE) that have a variety of applications but whose principal use is in the catalytic converter of motor vehicles (1). Although the design and efficiency of catalytic converters have undergone a series of changes since their introduction about thirty years ago, the active, metallic components (Pt, Pd, and Rh) and their association with the underlying substrate have remained the same. Absolute and relative concentrations of PGE in catalytic converters vary, depending on the type and age of vehicle, but in total they rarely exceed 0.2% of the bulk monolithic mass (2). PGE are emitted with exhaust gases in particulate form, typically as nanoparticles attached to µm-sized fragments of * Corresponding author tel: +44 1752 584570; fax: +44 1752 584710; e-mail:
[email protected]. 10.1021/es801189q CCC: $40.75
Published on Web 11/18/2008
2008 American Chemical Society
the original substrate, and at rates of up to several hundred ng per km per vehicle (3). Airborne particles either accumulate in road side dust and local soil or are dispersed more widely (4, 5). The environmental mobility of PGE is subsequently enhanced through a variety of chemical and microbiological processes (e.g., desorption, oxidation-reduction, complexation, readsorption), ultimately resulting in their interaction with the terrestrial and aquatic biospheres (2, 6). Since plants and animals are able to accumulate PGE, the diet is considered to be an important exposure route of these metals for humans (7). Less understood, however, is the significance of PGE intake by the inhalation or inadvertent ingestion of contaminated solids, including automotive catalytic converter particulates themselves. Ingestion is of particular concern for young children because of their repetitive hand to mouthing of nonfood objects such as soil and housedust (8). Significant in this respect, therefore, are recent results of a standard physiologically based extraction test (PBET) that simulates, sequentially, chemical conditions in the stomach and intestine of children aged 2-3 years (9). Thus, it was shown that overall digestive bioaccessibilities of PGE in auto catalytic particles were less than one percent (and decreased in the order Pd > Rh > Pt) and that solubilization in the near-neutral conditions of the intestine was greater than that in the acidic conditions (pH 2.5) of the stomach. The authors suggested that PGE are unlikely to precipitate when transported from the stomach to intestine, and that chloro-complexes are important in the digestive environment. The latter assertion raises important health issues because of the toxic and allergenic effects of such species (10). Clearly, a better understanding of the factors and mechanisms controlling bioaccessibility of PGE in the human gastro-intestinal tract is required for an improved exposure and risk assessment of these metals. To this end, we study the release of PGE from catalytic particles using a conventional PBET, but examine the influence of different physicochemical variables, representative of the range of conditions and constraints encountered in the gastro-intestinal environment, on this process. Specifically, we examine the effects of solid to fluid ratio (or quantity ingested-digested), reaction time, and pH-chloride concentration in the stomach phase. Using available thermodynamic information, we also attempt to gain a qualitative insight into the aqueous inorganic species of PGE that are likely present in the stomach and intestine.
Experimental Section Reagents and Samples. All reagents used in this study were of analytical grade or better, and were purchased from SigmaAldrich, Fisher Scientific, or VWR International. Plasticware and glassware employed throughout were precleaned in 10% HCl for at least 16 h before being rinsed with distilled water. Sufficient quantities of auto catalytic particles that are representative of the size distribution and noble metal composition of particulates emitted to the environment from vehicles (2, 10) were obtained from the milling of used catalytic converters. Preliminary work focused on particles of
