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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Water Environment Technology, Department of Civil and Environmental
Engineering, Chalmers University of Technology
41692 Göteborg,
Sweden
Correspondence: Corresponding author: sebastien.rauch{at}chalmers.se
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONANTHROPOGENIC PGE EMISSIONSPGE ACCUMULATION AND DISPERSIONPHYSICOCHEMICAL FORMS AND...BIOAVAILABILITY AND TOXIC...CONCLUSIONSREFERENCES |
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KEYWORDS: platinum-group elements, automobile catalyst, urban environment, bioavailability, risk assessment
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONANTHROPOGENIC PGE EMISSIONSPGE ACCUMULATION AND DISPERSIONPHYSICOCHEMICAL FORMS AND...BIOAVAILABILITY AND TOXIC...CONCLUSIONSREFERENCES |
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Worldwide PGE production has steadily increased since the 1970s because of increasing use in applications such as automobile exhaust catalysts, industrial process catalysts, jewelry, dental implants, and electronics (FIG. 1). The subsequent release of PGE into the environment is causing a redistribution of PGE, and increasing concentrations of these elements have been reported in surface environments. It is now important to assess the potential impacts of this new contamination on humans and the environment. This review presents the current knowledge about PGE emissions, accumulation, dispersion, and impacts.
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ANTHROPOGENIC PGE EMISSIONS |
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TOPABSTRACTINTRODUCTIONANTHROPOGENIC PGE EMISSIONSPGE ACCUMULATION AND DISPERSIONPHYSICOCHEMICAL FORMS AND...BIOAVAILABILITY AND TOXIC...CONCLUSIONSREFERENCES |
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PGE Emissions from Automobile Catalysts
Automobile catalysts are generally believed to be the main source of PGE
emitted into the environment. These catalysts use Pd, Pt, and Rh to promote
the removal of gaseous pollutants in vehicle exhausts, and a fraction of the
PGE in catalysts is emitted into the environment during vehicle operation
(Moldovan et al. 2002). In
addition, catalysts contain Os, Ir, and Ru impurities, and these metals are
released into the environment alongside Pd, Pt, and Rh
(Fritsche and Meisel 2004;
Rauch et al. 2004a;
Poirier and Gariepy 2005).
Direct measurements of PGE emissions from automobile catalysts provide
emission estimates in the nanogram per traveled kilometer range. Emissions
from gasoline catalysts are expected to be in the low nanogram per kilometer
range, whereas 10-100-fold higher Pt emissions have been measured for diesel
catalysts (Moldovan et al.
2002). In contrast, an emission rate of 0.8 µgkm-1
has been inferred from indirect measurements based on the analysis of
environmental samples and traffic information
(Helmers 1997). This higher
emission estimate has been attributed to conditions encountered in real life,
for example, engine ignition problems, that are not taken into consideration
in bench tests used for direct measurements. Emission rates depend on factors
like engine and catalyst types, the PGE content of the catalyst, the mileage
of the catalyst, engine condition, vehicle speed, and driving conditions
(Ravindra et al. 2004). A
global catalyst emission of 0.8-6.0 metric tons of Pt per year can be
inferred, assuming that 500 million vehicles are equipped with catalysts, that
the average yearly mileage is 15,000 km per vehicle, and that the average
emission rate is 0.1-0.8 µg km-1
(Rauch et al. 2005).
The emission mechanism and the form of PGE in automobile exhaust are still unclear. It is generally believed that mechanical erosion of the catalyst surface is the major cause of PGE emissions, although thermal and chemical processes may also contribute. Pd, Pt, and Rh occur in particle sizes ranging from <1µm to >63 µm (particle size defined by sampling device) in automobile exhaust and in the urban environment, supporting the idea that emission is a combination of processes such as chemical and thermal aging. In addition, chemical transformation is suggested by the occurrence of soluble PGE in automobile exhaust. Whereas soluble Pt represents less than 10% of total Pt emissions, soluble Pd and Rh fractions might be greater than 50% of total emissions. The occurrence of fine PGE-containing particles and soluble PGE species in the environment raises concern about potential environmental and health risks.
PGE Emissions from Mining and Metal Production
Metal production in northern Europe has also been reported to result in PGE
emissions. Nickel smelters in the Kola Peninsula in northwestern Russia have
been identified as an important regional source of Pt and Pd, based on the
spatial distribution of these metals in environmental samples
(Niskavaara et al. 2004).
Chromium smelters in the Kemi district in Finland have been identified as a
source of Os (Rodushkin et al.
2007). However, emission rates have not been determined, and data
from other metal production sites are needed to assess the extent of PGE
emissions resulting from metal production activities. Furthermore, emissions
from PGE production activities in South Africa, the leading PGE producer, need
to be determined.
PGE Emissions from Medical Facilities
Platinum-containing drugs, including cisplatin
[cis-diamminedichloroplatinum(II)] and carboplatin
[diammine(1,1-cyclobutanedicarboxylato)platinum(II)], are used in the
treatment of several forms of cancer. Platinum is excreted by the patients
after administration of Pt-based drugs and is found in hospital effluents at
concentrations ranging from <10 ng l-1 (ppt) to 3.5 µg
l-1 (ppb), but Pt is diluted in the municipal wastewater system,
and concentrations are <10 ng l-1 in sewage effluents
(Kümmerer et al. 1999).
Emission is expected to be in the form of soluble compounds, including
administered drugs and their derivatives. Osmium is also believed to be
emitted from medical facilities where it is used as a stain fixative in
electron microscopy applications (Esser
and Turekian 1993).
Other Potential Anthropogenic Sources
Today PGE are used in a wide range of applications, and emissions might
occur during PGE production, manufacture of PGE-containing products, and use
and disposal of these products. Although emissions from PGE production and
manufacture are expected to be limited or relevant to specific sites, the use
and disposal of PGE-containing items are of concern because of the potential
leaching of PGE. Emissions from these sources, however, have not been
determined.
The contribution of natural sources, including erosion and volcanic emissions, and the potential impact of human activities on some natural sources also need to be investigated. Increased erosion resulting from agriculture or deforestation may, for instance, contribute to elevated concen trations at remote sites where no direct anthropogenic sources are present.
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PGE ACCUMULATION AND DISPERSION |
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TOPABSTRACTINTRODUCTIONANTHROPOGENIC PGE EMISSIONSPGE ACCUMULATION AND DISPERSIONPHYSICOCHEMICAL FORMS AND...BIOAVAILABILITY AND TOXIC...CONCLUSIONSREFERENCES |
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While the greatest occurrence is in the urban and roadside environment, a significant fraction of PGE emitted by automobile catalysts is dispersed at regional and global scales owing to their occurrence in fine particles (Rauch et al. 2005). PGE are found in particles with diameters ranging from less than 1 µm to over 63 µm (Gómez et al. 2002). Whereas relatively large particles are expected to be deposited close to their source, a significant fraction of particles containing PGE in automobile emissions has a sufficiently long atmospheric residence time to be transported over long distances (Rauch et al. 2005). Elevated PGE concentrations at remote sites support suggestions of widespread atmospheric dispersion of emitted PGE (Barbante et al. 2001; Barbante et al. 2004; Rauch et al. 2004b). Increasing PGE concentrations have been reported as far from automobile traffic as central Greenland (Barbante et al. 2001). Although recent results raise concern over the validity of reported Greenland concentrations (De Boni 2007), the possibility of widespread dispersion of PGE is also supported by increasing accumulation in Alpine glaciers (Barbante et al. 2004) and in a remote peat bog located approximately 300 m from automobile traffic (Rauch et al. 2004b). In the latter study, Pd, Pt, and Rh deposition was determined to be of almost exclusively anthropogenic origin by estimating the natural input using Os isotope composition as a proxy.
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PHYSICOCHEMICAL FORMS AND TRANSFORMATION |
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TOPABSTRACTINTRODUCTIONANTHROPOGENIC PGE EMISSIONSPGE ACCUMULATION AND DISPERSIONPHYSICOCHEMICAL FORMS AND...BIOAVAILABILITY AND TOXIC...CONCLUSIONSREFERENCES |
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PGE are present as finely dispersed nanoparticles in catalysts and are
likely to be emitted in the form of PGE nanoparticles or as PGE attached to
washcoat particles of 
-Al2O3. Sintering may also
result in the emission of PGE particles in the micrometer range. Subsequently
different types of PGE-containing particles are expected to be found in the
environment, resulting in differences in their environmental reactivity
(FIG. 3). In
addition, soluble PGE species have been found in automobile exhaust, although
there is no clear agreement on the amount in such emissions. Moldovan et al.
(2002) reported that the
soluble fraction represents approximately 10% of total Pt emissions, while as
much as 40% of Pd and Rh may be soluble in a weak acid solution. The remaining
fraction is expected to be in metallic form. Because the soluble fraction is
obtained by filtration at <0.45 µm, soluble PGE may also include PGE
nanoparticles. Hospital emissions are an additional source of soluble Pt
emitted into the aquatic environment as a result of the excretion of
platinum-based drugs and their derivatives by patients.
Particle size and especially the occurrence of PGE as nanoparticles may
also play a major role in the presence and formation of soluble PGE species.
As mentioned earlier, soluble PGE may include nanoparticles if the soluble
fraction is determined by filtration at <0.45 µm. In addition, fine
particles have relatively large surface areas and offer more possibility for
reactions with environmental substances. Larger particles are likely to be
composed of catalyst washcoat, and PGE nanoparticles may be released under
conditions that promote the dissolution of
-Al2O3.
PGE in automobile emissions are predominantly in metallic form. Metallic PGE are usually considered to be inert and environmentally unreactive, and it may be reasoned that they cannot be oxidized. Oxide and hydroxide forms also have limited solubility. Studies on the solubility of PGE provide an unclear picture of the amount of soluble PGE compounds in the environment. Differences are likely due to the form of PGE in different environmental compartments, as well as to the presence of reaction promoters and the readsorption of soluble PGE onto solid surfaces. It is clear, however, that Pd has a higher solubility than Pt and Rh (Jarvis et al. 2001; Moldovan et al. 2001). Naturally occurring complexing agents may play an important role in the solubilization of PGE and their fate in the environment. Such complexones are widely found in many soils and freshwater systems. The presence of humic acids, as well as triphosphate, pyrophosphate, and L-methionine, increases the solubility of platinum (Lustig et al. 1998). Siderophores (organic ligands secreted by microbes and plants to extract metal nutrients from soil) also have the potential to solubilize PGE metals and oxides, thereby increasing their environmental mobility and enabling their uptake by plants (Normand and Wood 2005; Dahlheimer et al. 2007). The extraction of PGE is in the order Pd > Pt > Rh (Dahlheimer et al. 2007). Experiments on the interactions of PGE with humic soils indicate that Pt is subject to complex transformations in soil. Pt is oxidized and released from the particle surface by a complexing agent, leaving the surface free for further oxidation (Lustig et al. 1996). However, the formation of organic complexes may also explain the relatively low mobility of PGE in soils (Lustig et al. 1996).
Emitted PGE particles may reach aquatic environments, where they are expected to remain largely insoluble. The soluble fraction may react with particles or form complexes with inorganic or organic ligands. The input of particulate or particulate-reactive PGE to aquatic systems has been demonstrated using sediment cores (Rauch et al. 2004a). A study of the behavior of soluble PGE in river water showed that a significant fraction of PGE binds to particulate matter (>45 µm). The speciation of Pt, Pd, and Rh is controlled by different mechanisms. Palladium is complexed by small hydrophobic organic ligands (<0.1 µm); Rh is complexed to these organic ligands, but also forms hydroxychlorides; Pt forms inorganic aqueous species, and the particle-water reactivity of Pt is controlled by electrostatic interactions (Cobelo-Garcia et al. 2008). The behavior of PGE in natural waters results in increasing solubility in estuarine mixing (Cobelo-Garcia et al. 2008).
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BIOAVAILABILITY AND TOXIC EFFECTS |
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TOPABSTRACTINTRODUCTIONANTHROPOGENIC PGE EMISSIONSPGE ACCUMULATION AND DISPERSIONPHYSICOCHEMICAL FORMS AND...BIOAVAILABILITY AND TOXIC...CONCLUSIONSREFERENCES |
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Uptake by Flora and Fauna
Transformation of PGE and the occurrence of soluble species in the
environment indicate that a fraction of PGE is in a bioavailable form. It
follows that exposure to PGE may result in uptake and eventually in toxic
effects.
Roadside vegetation is exposed to relatively high PGE concentrations owing to proximity to automobile traffic. Elevated PGE concentrations have been found in roadside grass (Zimmermann and Sures 2004). However, elevated concentrations are largely due to PGE deposition on the plant surface and actual uptake from soil is relatively limited, possibly due to exposure routes. Uptake from soil requires the remobilization of deposited PGE, whereas atmospheric deposition results in direct contact for adsorption. Accumulation of PGE on the plant surface is likely the result of such particle adsorption. PGE uptake from soil has also been demonstrated in laboratory experiments and may contribute to internal PGE accumulation (Zimmermann and Sures 2004). The uptake mechanism is believed to be linked to complexones (siderophores) used by plants for the extraction of metallic nutrients from soil. The highest concentrations are generally found in the roots, followed by the shoots and the leaves, indicating uptake but limited transport in the plants (Zimmermann and Sures 2004).
Aquatic organisms have been reported to take up and accumulate PGE under environmental conditions. Freshwater benthic organisms were found to contain 38.0 ng g-1 Pt, 155 ng g-1 Pd, and 17.9 ng g-1 Rh. These organisms feed on sediments, and the uptake route is believed to be dietary intake and dissolution of PGE in the digestive system (Moldovan et al. 2001). Laboratory exposure (typically performed in ligand-free water using soluble compounds at high concentrations) supports the idea that PGE are present in a bioavailable form in the aquatic environment and can be taken up by aquatic fauna, including invertebrate and fish species (Zimmermann and Sures 2004).
Studies on higher organisms are scarce. Exposure of rats to model Pt-containing particles resembling automobile emissions indicate that Pt can be taken up through inhalation or intratracheal intake (Artelt et al. 1999b). A substantial fraction of Pt was found to be bioavailable as a result of in vivo solubility. Increased concentrations were found in the blood, urine, and feces; in organs like the liver, spleen, kidneys, stomach, and adrenal, and in the femur. Over 90% of the bioavailable Pt is bound to proteins, and the remaining fraction possibly corresponds to low-molecular-weight ionic complexes (Artelt et al. 1999b). Binding to proteins is believed to play a major role in PGE accumulation in organisms (Zimmermann and Sures 2004).
In general, Pd is found to be more bioavailable than Pt and Rh. Experimental studies also reveal that uptake and accumulation depend on the chemical speciation of PGE in the environment (Zimmermann and Sures 2004); at present, knowledge about the chemical form of PGE in the environment is very limited. Further characterization of PGE in the environment and in the biosphere is needed to understand uptake and accumulation mechanisms.
Human Exposure and Health Effects
Human exposure to PGE is most likely to be through inhalation of fine
PGE-containing particles, skin contact, and dietary intake. Human exposure and
uptake have been investigated in studies comparing populations with different
exposures to automobile traffic. Adults from a large city with dense traffic
had greater urinary Pt and Rh concentrations than adults from a smaller town
with relatively low traffic density, but no clear trend was found for Pd
(Bocca et al. 2004). In
contrast, a significant correlation between urinary Pd and Rh concentrations
and traffic density was found in children, but no correlation was observed for
Pt (Caroli et al. 2001).
Despite some inconsistencies, these studies clearly show that human exposure,
possibly through inhalation, results in the uptake of PGE, although PGE may
not be transferred to organs.
Toxic effects at high concentrations reported in medical and occupational studies include sensitization, mutagenic effects in bacterial and mammalian cells, and increased tumor incidence. PGE effects have been observed in medical or occupational settings where exposure is high. However, effects have not been determined under environmental conditions as concentrations are generally considered too low for any effect to occur. While a no-effect limit concentration of 1.5 ng m-3 has been set for exposure to Pt salts in a catalyst-manufacturing plant (Merget and Rosner 2001), airborne Pt concentrations typically do not exceed 100 pg m-3 and only approximately 10% may be in the form of soluble salt. Available data on Pd and Rh are still insufficient to determine the likelihood of effects on human health, but effects from PGE exposure in the environment are considered unlikely (Merget and Rosner 2001). Further characterization of exposure, uptake, and effects are needed.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONANTHROPOGENIC PGE EMISSIONSPGE ACCUMULATION AND DISPERSIONPHYSICOCHEMICAL FORMS AND...BIOAVAILABILITY AND TOXIC...CONCLUSIONSREFERENCES |
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Despite increasing concentrations, current environmental PGE levels remain low, and risks for humans and the environment are therefore expected to be limited. However, it is important to stress that currently available data are not sufficient for an accurate assessment of potential risks. Studies on the effects of PGE are sparse and do not generally provide environmentally relevant information; studies on chronic effects at low-exposure concentrations are insufficient at present. The physicochemical form of PGE, including the occurrence of PGE as nanoparticles, and the higher bioavailability of Pd need to be taken into consideration.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONANTHROPOGENIC PGE EMISSIONSPGE ACCUMULATION AND DISPERSIONPHYSICOCHEMICAL FORMS AND...BIOAVAILABILITY AND TOXIC...CONCLUSIONSREFERENCES |
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