Environmental routes for platinum group elements to biological materials—a review

Science of the Total Environment 334 – 335 (2004) 21 – 38 www.elsevier.com/locate/scitotenv

Review

Environmental routes for platinum group elements to biological materials—a review Kristine H. Ek a,*, Gregory M. Morrison a, Sebastien Rauch b b

a Water Environment Transport, Chalmers University of Technology, SE 412 96 Go¨teborg, Sweden R.M. Parsons Laboratory 48-108, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Accepted 1 April 2004

Abstract The increased use of platinum group elements (PGE) in automobile catalysts has led to concern over potential environmental and biological accumulation. Platinum (Pt), palladium (Pd) and rhodium (Rh) concentrations have increased in the environment since the introduction of automobile catalysts. This review summarises current knowledge concerning the environmental mobility, speciation and bioavailability of Pt, Pd and Rh. The greater proportion of PGE emissions is from automobile catalysts, in the form of nanometer-sized catalyst particles, which deposit on roadside surfaces, as evidenced in samples of road dust, grass and soil. In soil, PGE can be transformed into more mobile species through complexation with organic matter and can be solubilised in low pH rainwater. There are indications that environmentally formed Pd species are more soluble and hence more mobile in the environment than Rh and Pt. PGE can reach waterbodies through stormwater transport and deposition in sediments. Besides external contamination of grass close to roads, internal PGE uptake has been observed for plants growing on soil contaminated with automobile catalyst PGE. Fine particles of PGE were also detected on the surface of feathers sampled from passerines and raptors in their natural habitat, and internal organs of these birds also contained PGE. Uptake has been observed in sediment-dwelling invertebrates, and laboratory studies have shown an uptake of PGE in eel and fish exposed to water containing road dust. The available evidence indicates that the PGE, especially Pd, are transported to biological materials through deposition in roots by binding to sulphur-rich low molecular weight species in plants. PGE uptake to exposed animals have uptake rates in the following order: Pd>Pt>Rh. The liver and kidney accumulate the highest levels of PGE, especially Pd. Urinary Pd and Rh, but not Pt, levels are correlated with traffic intensity. Dental alloys may lead to elevated urinary Pt levels. Platinum is a well-known allergen and Pd also shows a strong sensitisation potential. D 2004 Elsevier B.V. All rights reserved. Keywords: Platinum; Palladium; Rhodium; PGE; Biological material; Transformation; Mobility; Speciation; Bioavailability; Uptake

1. Introduction

* Corresponding author. Tel.: +46-31-772-2189; fax: +46-31772-2128. E-mail address: [email protected] (K.H. Ek). 0048-9697/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.04.027

Automobile catalysts were introduced in the U.S. in the mid-1970s, are mandatory on new cars in Sweden since 1989 and the European Union since 1993, and are now being introduced in developing

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Table 1

Pt, Pd and Rh concentrations in naturally exposed environmental and biological materials Year

Site

Average concentration Pt

Pd

Rh

Earth crust Autocatalyst emission (gasoline)

– –

– –

0.4 6–8

0.4 12 – 16

0.06 3 – 12

–

–

6–8

13 – 15

4

–

–

108 – 150

–

–

–

–

92 – 134

–

–

2002

Urban

38.1

14.4

6.6

4.3

2.6

0.4

8.5

4.4

1.8

3.8

1.3

1.1

12.9 5.5 1.8 ND 255

– – 2.3 – 82

– – 1.6 – –

2001

–

228.4

1997

170

1999

Autocatalyst emission (diesel)

Air particles < 30 Am Air particles < 10 Am Air particles < 10 Am Air particles < 2.5 Am Tree bark

1999 – 2000

NR NR

Road dust

NR

1998 Soil

Urban Rural Urban Rural Urban

Urban

Unit

Analytical technique

Detection limit Pt

Pd

Rh

ng g 1 ng km

ICP MASS Q and HR ICP-MS, DP CSV ICP-MS

– NR

– NR

NR

Q and HR ICP-MS, DP CSV ICP-MS ICP-SFMS

pg m

3

1

Reference

Note

– NR

Wedepohl (1995) Palacios et al. (2000a,b)

Aged autocatalysts

NR

NR

NR

NR

NR

Moldovan et al. (2002) Palacios et al. (2000a,b)

NR

NR

NR

0.18

0.22

0.14

0.07

0.06

0.05

Q ICP-MS

NR

NR

NR

Go´mez et al. (2001)

–

–

Ma et al. (2001) Becker et al. (2000) Kovacheva and Djingova (2002) Boch et al. (2002)

Moldovan et al. (2002) Kanitsar et al. (2003)

ng g

1

Q ICP-MS

0.8 ng l

ng g

1

Q ICP-MS

0.2

0.3

0.05

ng g

1

ICP-AES

–

–

–

–

GF-AAS

–

1.8 ng l

23

28

Q ICP-MS

0.4

0.4

0.1

Scha¨fer et al. (1999)

189

56

74

Q ICP-MS

NR

NR

NR

157

–

60.4

HR-ICP-MS

0.15

–

0.02

Q ICP-MS

0.003

0.003

0.003

Rauch et al. (2000) Rauch et al. (2000) Ely et al. (2001)

ng g

1

1

1

–

NR

Highway

58.3

21.1

5.4

2001 1994

Urban Highway

11.2 46

– 6

– 7

ICP-MS Q ICP-MS

1.0 0.4

– 0.4

– 0.1

NR

Highway

87

7.2

–

ID-ICP-MS

0.15

0.075

–

Aged autocatalysts

Cinti et al. (2002) Scha¨fer et al. (1999) Mu¨ller and Heumann (2000)

Pd value indicatory

Mean of samples from three roads Mean of the two most trafficked reads < 63 Am. Pd value indicatory < 63 Am. Pd value indicatory Mean of samples 0 – 1 m from three roads Heavy traffic roads

0.6 m from the highway

K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

Material

Sewage sludge

1994 – 1995

Urban

1997 Grass

Asellus aquaticus

Human serum Human urine

–

–

138

450

6.3

ng g

1

ng g

1

Q ICP-MS

0.1 ng l

1

–

–

0.4

0.4

0.1

CSV

0.05

–

0.03

1997

Highway

10.6

–

1.8

NR

Highway soil

175

5.7

27

ICP-MS

1.25

2.5

1.05

Highway

1.2

1.0

0.1

Q ICP-MS

0.003

0.003

0.003

Urban

38

155.4

17.9

1999 1986 – 2000

1

Q ICP-MS

NR

NR

NR

ng g

1

Q ICP-MS

–

–

–

0.5

1.4

0.3

1995 – 2001

2.7

0.8

0.6

0.2

0.5

0.2

1992 – 2000

0.4

0.5

0.3

0.2

0.3

0.07

1997 – 2001

0.2

–

–

0.1

–

–

1985 – 1995

0.2

0.7

0.5

0.1

0.3

0.1

1985 – 1995

0.2

0.3

0.3

0.1

0.3

0.1

30

–

–

NR

NR

Rural

ng g

Unexposed

129

–

–

ng l

1

Exposed Urban

246 ND

– ND

– ND

ng l

1

Unexposed

ND

50.2

ND

HR-ICP-MS

32

26

13

0.9

50.2

–

0.3

0.2

–

5.1

48

ND

1.2

7

9

ND

ND

ND

Sector field ICP-MS Sector field ICP-MS ICP-MS

100

200

100

HR-ICP-MS

0.25

0.03

0.03

Fire assayICP-MS

30

–

–

Sector field ICP-MS

30 0.24

– 0.17

– –

Urban

ng l ng g

1

1

Fire assayICP-MS ICP-MS

30 100

– 200

– 100

1996

Urban

0.9

7.5

8.5

NR

Unexposed

113

–

–

Exposed Unexposed

470 1.8

– 140.3

– –

1

9.5

11.7

HR-ICP-MS

0.6

5

0.5

–

ND

–

TRXF

–

2.5

–

ng l

1

Laschka and Nachtwey (1997) Scha¨fer et al. (1999) Helmers and Mergel (1998) Scha¨fer et al. (1998) Ely et al. (2001)

External contamination No external contamination washed leaves and shoot

Moldovan et al. (2001) Jensen et al. (2002) Ek et al. (2003a,b)

Farago et al. (1998)

Whole blood

Ba´ra´ny et al. (2002a,b) Begerow and Dunemann (1996) Begerow et al. (1997b) Rodushkin et al. (1999) Ba´ra´ny et al. (2002a,b) Caroli et al. (2001) Farago et al. (1998)

Below detection Pt and Rh below detection limit

below detection limit In ng g

1

creatinine

K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

Feathers (Falco peregrinus) Blood (Falco peregrinus) Eggs (Falco peregrinus) Faeces (Falco peregrinus) Liver (Falco peregrinus) Kidney (Falco peregrinus) Human blood

160

Begerow et al. (1997a) Krachler et al. (1998) Messerschmidt et al. (2000)

NR denotes not reported.

23

24

K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

countries (Kylander et al., 2003). The introduction of three-way catalysts has reduced the emissions of carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx) by 90% (Barefoot, 1997). Platinum group elements (PGE) are the active components of these catalysts: platinum (Pt) and palladium (Pd) oxidising CO to CO2 and HC to H2O, and rhodium (Rh) reducing NOx. A clear link has been established between the increasing use of automobile catalysts and increasing environmental PGE concentrations (Wei and Morrison, 1994; Helmers and Mergel, 1998; Scha¨fer et al., 1999; Rauch and Morrison, 2001; Boch et al., 2002; Jensen et al., 2002; Rauch and Hemond, 2003). Metallic PGE particles in the micrometer and submicrometer range are emitted at nanograms per kilometer rates from the automobile catalyst due to surface abrasion of the washcoat during vehicle operation (Alt et al., 1993; Moldovan et al., 1999, 2002; Palacios et al., 2000a,b; Go´mez et al., 2001; Zereini et al., 2001; Kanitsar et al., 2003). It was previously believed that the soluble fraction of automobile catalyst emitted PGE is only a few percent, but it has recently been demonstrated that this fraction is higher (Moldovan et al., 2002). In urban aerosols, PGE are mainly found in the 10 –30 Am aerodynamic diameter (AED) fraction, but further maximum for PGE rich particles of AED 1 – 2.15 Am was also found. The distribution of PGE particles in aerosols was similar to Cu, being abraded from brake linings; this indicates that PGE originate from a mechanical process such as abrasion of the catalyst surface (Kanitsar et al., 2003). Elevated PGE concentrations have been found in a variety of compartments in the urban environment (Alt et al., 1993; Wei and Morrison, 1994; Farago et al., 1996; Scha¨fer and Puchelt, 1998; Scha¨fer et al., 1999; Zereini et al., 1999, 2001; Rauch et al., 2000, 2001; Dongarra´ et al., 2003), alpine snow (Van de Velde et al., 2000) and even in remote Greenland snow (Barbante et al., 2001), indicating the possibility for long-range transport of PGE-containing particles. Since the mid1990s, Pd has been somewhat favoured as a substitute for Pt in automobile catalysts, although Pd demand has fallen since 2000 due to increased market price (Johnson, 1996, 2002; Palacios et al., 2000a,b). The use of Pd in automobile catalysts is a concern, since it has been demonstrated that Pd has a greater mobility in the environment than either Pt or Rh (Moldovan et

al., 2001). It was previously believed that PGE are relatively inert, but it has now been shown that these metals undergo environmental transformations into more reactive species which may be bioavailable (Vaughan and Florence, 1992; Lustig et al., 1996, 1998; Farago et al., 1998; Scha¨fer et al., 1998; Rauch and Morrison, 2000; Moldovan et al., 2001; Philippeit and Angerer, 2001; Rauch, 2001). The toxicity, and in particular human toxicity, of Pt has been investigated in detail (Lindell, 1997), while less is known about Pd and Rh (Melber et al., 2002). A recent review summarises the literature on platinum group metals in the environment and their health risk (Ravindra et al., 2004). The determination of PGE at environmentally relevant concentrations is a difficult task. Recent developments in analytical science have provided sensitive tools and have enabled a better understanding of the environmental pathways of PGE, from automobile emission to bioaccumulation. The purpose of this review is to summarise the current knowledge on environmental transformation, mobility, speciation and bioavailability of automobile catalyst-emitted PGE, as well as the effect of PGE on humans.

2. Sources of exposure PGE occur naturally in very low concentrations in the environment (Table 1); around 1 ng g 1 in the Earth crust and 1 pg g 1 in seawater (Wedepohl, 1995; Melber et al., 2002). PGE are extracted from mine deposits in mainly South Africa, Russia, Canada and the USA. Fig. 1 shows the world demand for platinum, palladium and rhodium by application from 1985 to 2002. Platinum is used in automobile catalysts, jewellery, as an anti-tumour drug in cancer therapy (cisplatin), in catalysts in the chemical industry, in electronics and dentistry as alloys (Lindell, 1997). Platinum emitted by hospitals is 3 – 12% of the estimated amount emitted from automobile catalysts in Europe (Ku¨mmerer et al., 1999). The principal uses of palladium are in electronics, industrial catalysts, circuitry, dental alloys, jewellery and in automobile catalysts (Kielhorn et al., 2002). Recently, 103Pd has been used in the treatment of cancer (Melber et al., 2002). The demand

K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

Fig. 1. World platinum, palladium and rhodium demand by application (Johnson, 2002).

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K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

and production of Pd has recently increased rapidly, although a decreasing tendency has been observed since 2000 (Fig. 1). WHO concluded that for the general population, dental alloys contribute considerably to the total Pd intake (< 1.5– 15 Ag person 1 day 1), and skin contact with Pd-containing jewellery is an important route of exposure (Melber et al., 2002). Palladium exposure through inhalation of Pd from automobile catalyst emissions is expected to be low (about 2.2 ng person 1 day 1). However, if the worst case air pollution scenario of 57 ng m 3 is considered, the average daily intake would reach 1254 ng person 1 (Melber et al., 2002). Rhodium is mainly used with platinum in automobile catalysts and in catalysts in the chemical industry (Habashi, 1997). However, sources of PGE other than automobile catalysts are not considered important for the environmental contamination of PGE. Modern automobile catalysts are honeycomb ceramic supports with PGE incorporated into an intermediate washcoat layer of aluminium oxide and other promoter elements such as Ce, Zr and Ba. Gasoline catalysts use a combination of Pt and/or Pd to oxidise CO and HC and Rh to reduce NOx. Diesel oxidation catalysts are based on Pt and have been used in the European Union since 1991. PGE are emitted in the exhaust gas from automobile catalysts with emission rates of 6 –8 ng km 1 for Pt, 12– 16 ng km 1 for Pd and 4– 12 ng km 1 for Rh for aged gasoline catalysts. Emission rates from aged diesel catalysts are 108 – 150 ng Pt km 1 (Palacios et al., 2000a). Emission is mainly in a metallic form bound to aluminium oxide particles together with Ce, and the diameter of the PGE particles range from submicrometers to several micrometers (Alt et al., 1993; Go´mez et al., 2001). However, platinum in fresh automobile catalysts is not only present in the metallic state, but also in the form of oxides, chlorides and bound to hydrocarbons. Rhodium is also present in the form of oxides. The soluble fraction of Pt represents about 10% and for Pd and Rh it is close to 50% of the total Pt, Pd and Rh, respectively, released from automobile catalysts (Moldovan et al., 2002). Sulphur in the fuel might bind to PGE, which could increase the solubility of PGE (Rauch and Morrison, 2001; Moldovan et al., 2003) and, therefore, emission might follow different mechanisms for gasoline and diesel catalysts. New catalysts emit more PGE, and PGE emissions are smaller when

driving at a constant speed compared to driving cycles (Moldovan et al., 1999; Palacios et al., 2000a).

3. Environmental transport, distribution and transformation 3.1. Transport and distribution PGE particles emitted from automobile catalysts deposit on the road surface or in the roadside environment. The soluble fraction of Pt has been reported to be less than 10% of the total amount of Pt in exhaust fumes of gasoline and diesel catalysts, but for Pd and Rh, the soluble fraction was in the same order of magnitude as the particulate fraction (Moldovan et al., 2002). This implies that the effects of Pd and Rh emissions could have a more adverse effect on the environment than Pt emissions. The concentration of PGE in soil decreases with increasing distance from the road, as well as with increasing soil depth (Scha¨fer and Puchelt, 1998; Mu¨ller and Heumann, 2000). The deposited particles can then be washed into rivers and water bodies during rain events, where they accumulate in sediments; levels in water are very low (Rauch, 2001). It was suggested that the predominant inorganic form of Pd in freshwater may be the neutral hydroxide species. In seawater, the predominant form is PdCl42 (Melber et al., 2002). Pd(II) can be complexed by amino acids and it has been shown that PdCl2 complexed with glycine is more stable than all known inorganic Pd(II) complexes (Melber et al., 2002). A strong co-variance between Pd and Ni in seawater implies similar biogeochemical pathways (Lee, 1983). In road dust samples collected at increasing distance (0– 10 m) from the road, the Pt/Rh ratio is relatively constant at f 7.1, while Pt/Pd ratios are more variable with a mean of 6.6 and a range between 2.0 and 26.6. This suggests that there is a significant difference in chemical behaviour between Pd and Rh (Jarvis et al., 2001). In addition, a temporal study over 1 year demonstrated that the Pt/Rh ratio is constant regardless of the amount of rainfall and correlates with the ratio in automobile catalysts, while the Pt/Pd ratio varies between 1 and 18, showing no correlation with rainfall. It was suggested that this might be evidence of the solubility and formation of chemical species of

K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

Pd (Jarvis et al., 2001). The Pt/Rh ratio varied between 4.6 and 5.6 in soil, pointing to automobile catalysts as the source, which indicates a relatively inert and immobile behaviour of Pt and Rh (Zereini et al., 1997). The Pt/Pd ratio in surface soil decreased with distance from the highway surface (Scha¨fer and Puchelt, 1998; Mu¨ller and Heumann, 2000), which is further evidence of a higher environmental mobility of Pd compared to Pt. Pt/Pd and Pt/Rh ratios in different environmental compartments are given in Table 1. Tree bark reflects the deposition of airborne PGE particles. Laser ablation analysis showed that Pt is associated with Pd and/or Rh and indicated a heterogeneous distribution of PGE in the bark compared to Pb. PGE were detected as inert metallic particles which are not readily transformed/solubilised by weathering, and a low bioavailability was proposed (Ma et al., 2001). 3.2. Transformation Scanning laser ablation of road and river sediments revealed a coincidence of PGE and Ce peaks, indicating direct transport of PGE containing catalyst particles into the river. Although total PGE concentrations are low in sediments, PGE containing particles are concentrated in a few sediment particles where they have relatively high concentrations (Rauch et al., 2002). In river sediments, PGE remain associated to Ce particles, but part of the particulate PGE might be released from the Ce particle through formation of soluble PGE species or breakdown of the particle (Rauch et al., 2002). The water-soluble fraction of Pt in tunnel dust was 3.9% and the fraction soluble in organic solvents was 3.1%, while no platinum was soluble in organic solvents for tested platinum compounds (Pt black, K2PtCl4, Na2PtCl6
6H2O). This indicates that the soluble species of Pt in tunnel dust are of an organic character. The solubility of Pt in tunnel dust was 52% and 25% in EDTA and thiourea, respectively; much higher than the solubility for tested platinum compounds. This means that 52% of the original metallic Pt in tunnel dust is mobile and oxidised in the soil. The oxidation is most probably caused by the presence of humic soil, which contains S-attached complexones that oxidise platinum, forming insoluble compounds. The reason why Pt in tunnel dust is

27

oxidised so efficiently may be related to its nanocrystalline particle size (0.1 –20 Am) and its ultrafine dispersion (Lustig et al., 1996). Pt-black and Pt-containing tunnel dust can be solubilised in water by natural complexing agents, which can result in potentially bioavailable platinum species. The ligand that had the greatest effect on the dissolution of metallic platinum was L-methionine. Tunnel dust platinum was generally dissolved to a higher extent than Pt-black, which is probably because of the smaller particle size (nanometer) and the ultrafine dispersion of tunnel dust Pt compared to Ptblack. Metallic Pt was first oxidised and then a complexing agent removed the Pt oxide formed. However, in the absence of a complexing agent, the surface of metallic Pt was rapidly saturated with the oxide, inhibiting further oxidation. It was suggested that metallic Pt in tunnel dust is oxidised in the soil and that the majority of the Pt species formed in the oxidation are immobilised; these compounds could be humic acid complexes of Pt. Pt black and hydrocomplexes, on the other hand, were transformed into inorganic Pt species (Lustig et al., 1998). The amount of soluble Pt in airborne dust was 30– 43%, while it was only 2.5 –6.9% in tunnel dust (Alt et al., 1993). It was suggested that this might be due to the different origin of the platinum, traffic being the origin of road dust while airborne dust represents more environmentally occurring Pt. In Boston harbour sediments, Pt concentrations showed a high variability within the mixed layer, indicating remobilisation of Pt within these layers on a short time scale. It was suggested that Pt is associated with organic matter and then remobilised due to organic matter oxidation. For Pd, on the other hand, there was no clear evidence of remobilisation once it was fixed in the anoxic layer (Tuit et al., 2000). Pt, Pd and Rh, in the form of complexes of chloride and nitrate, were dissolved in rain water (pH 4 –5) rich in natural organic matter (NOM) under controlled laboratory conditions. Solutions were shaken in plastic containers for a week and it was shown that roughly one half of the Pt, Pd and Rh present was complexed with NOM. This shows that PGE can be efficiently mobilised by NOM, contributing to their redistribution in the environment (Menzel et al., 2001). It was demonstrated that only a relatively small fraction of automobile catalyst Pt and Rh is dissolved

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K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

under natural environmental conditions; they are both relatively inert, although there is some evidence that the solubility of Pt may increase with time (Zereini et al., 1997). The highest solubility of Pt and Rh in ground catalytic converter material in rainwater was reached at pH 1 (0.35 – 0.5% Pt and 1.0% Rh), and decreased rapidly to a constant low level between pH 3 and 9 (0.01 – 0.025% Pt and 0.05% Rh). The relative solubility is thus higher for Rh compared to Pt. In a mixture of rainwater and soil, Pt solubility is lower than in rainwater alone (0.001% Pt), indicating a retention capability of the soil due to adsorption by clay and humic substances. Increasing sulphur concentration in soil affects Pt solubility positively and it was also enhanced with time. No effect was observed for Rh (Zereini et al., 1997). Thirty-five percent of the total Pd in road dust was shown to dissolve in a solution at pH 3, simulating rain, and a solubility gradient of Pd>Rh>Pt was observed (Jarvis et al., 2001). The solubility rate was also faster for Pd than for Pt and Rh; in a 15-h experiment, over 60% of the Pd was in solution after 3.5 h. Because of the degree of solubility observed for Pd, it was suggested that it is unlikely that Pd is in the metallic form, but may be present as chloride species. The residence time for the road dust was less than 1 week; thus, the chemical conversion from a metallic state to a chloride species must be a relatively rapid process (Jarvis et al., 2001). The high solubility of Pd might imply a more effective transport in the aqueous environment. The solubility of PGE also depends on particle size. The solubility of a model substance of platinum with 5% Pt on aluminium, closely resembling the particles emitted from catalytic converters, increased with decreasing particle size; Pt particles with a diameter of 3.4 nm had a solubility of 22%, while 25 nm particles had a solubility of 2%. Solubility was 18 and 28 times higher in 0.9% NaCl and 0.9% KCN, respectively, compared to deionised water (Nachtigall et al., 1996). The in vitro solubility of a model substance of Pt(0)-particles (approximately 4 nm in diameter coated on aluminium oxide particles), was only 0.4% in pure water, while solubility was 10% in 0.9% NaCl. It was suggested that the reason for this solubility is the small diameter of the platinum particles, because the activation energy necessary for oxidation is smaller for fine particles. Therefore, they

are more easily oxidised by oxygen present in the aqueous solution. There was a negative correlation between the Pt particle diameter and the solubility; particles with a diameter of 3 nm had a solubility of 22%, while 25 nm particles had a solubility of 1.4% (Artelt et al., 1999). In organisms, mechanisms similar to those in a 0.9% NaCl solution are likely to take place, leading to an increased bioavailability of Pt. Initially, the Pt solubility was low in vivo, but at longer times the bioavailability increased, reaching up to 7% after 90 days. It was suggested that after a longer exposure the Pt particles on the surface of the aluminium oxide particles are dissolved in the organism and become bioavailable (Artelt et al., 1999). Microorganisms do not influence the dissolution and transformation of metallic catalyst-emitted Pt into bioavailable species for shorter exposure periods (up to 60 days). There was no difference in the amount of EDTA-extractable Pt between sterile and non-sterile conditions, indicating that transformation of Pt in soil is mainly of a chemical nature (Lustig et al., 1997a). On the other hand, it was suggested that Pt in urban gullypot sediments occurring in the organic fraction is a result of bacterial action (Wei and Morrison, 1994). However, in the light of the study by Lustig et al. (1997a), that can also be explained by chemical dissolution in the presence of organic ligands such as humic acids. This was demonstrated for Pt in tunnel dust (Lustig et al., 1996). Theoretically, Pt could be transformed into a methylated species in soil, although methylation has only been demonstrated under laboratory conditions. Methylation of Pt requires the presence of both the Pt(II) and Pt(IV) oxidation states. These platinum compounds can be methylated by methylcobalamin in vitro, forming a platinum chloride-methylcobalamin complex (Fanchiang et al., 1979). Salts of Pd were observed to demethylate methylcobalamin in vitro, but at much slower rates than Pt4 +. The reactivity of the salts decreased according to: K2PdCl6>PdSO4> K2PdCl4. There is no evidence for stable methyl Pd derivatives and nor that Pd can be methylated unless it is in the 4+ or 2+ valency state (Melber et al., 2002). In general, methylated organometallic derivatives are more toxic than their inorganic precursors to higher organisms, due to the possible increase in lipid solubility, volatility and persistence in biological systems

K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

resulting from the methylation (Fanchiang et al., 1979). According to a study by the International Programme on Chemical Safety, it is not possible to draw any conclusions whether microorganisms in the environment can biomethylate platinum compounds or not (Lindell, 1997). In conclusion, PGE can be transformed into soluble species in soil by complexation with NOM, such as humic acids, through oxidation by sulphur-attached complexones. Catalyst-emitted PGE is nanocrystalline-sized and ultrafine dispersed, which allows more efficient oxidation. There is now convincing evidence that Pd is more soluble and hence more mobile in the environment than Rh and Pt. No evidence for in situ methylation or bacterial dissolution of Pt and Pd has been shown, although the former has been demonstrated in vitro.

4. Bioavailability 4.1. Plants Bioavailability of catalyst emitted Pt to nutrient plants grown on tunnel dust treated soil was very low in a 6-month experiment; 0.02 – 0.6% of the total platinum present in the soil was taken up by the plants. The platinum species were oxidised and immobilised in the soil. However, it was suggested that long-term processes might remobilise the absorbed platinum species and make them available for plants (Lustig et al., 1997b). On the other hand, a measurable transfer of PGE from contaminated soils to plants was found in another study, with PGE transfer coefficients being in the range of immobile to moderately mobile elements (Scha¨fer et al., 1998). Plants were grown in greenhouses to avoid contamination of the plant surface, and they were grown both on contaminated sand from a site adjacent to a highway and on uncontaminated agricultural soil. The Pt/Rh ratio was 6:1 in the contaminated soil, reflecting the ratio in catalytic converters. The transfer coefficient was defined as the ratio of the concentration in the plant to the concentration in the soil. Cd, Cu, Pb and Zn were also measured. PGE concentrations in plants grown on uncontaminated soil were below instrumental detection limits. The transfer coefficient for PGE decreased in the order; Pd>Pt z Rh. Pd was as mobile as

29

Zn, while Pt and Rh were as mobile as Cu (Scha¨fer et al., 1998). Water hyacinths (Eichhornia crassipes) were treated with solutions of chlorocomplexes of PGE and the recovery decreased in the following order: Pt(II)>Pd(II)>Rh(III) (Farago and Parsons, 1983). Hydroponically grown cucumber plants strongly accumulated Pt in a biologically available form [Pt(NH3)4](NO3)2, especially in the roots, while rye grass grown on sandy loam soil showed only slight accumulation (Verstraete et al., 1998). In grass treated with an aqueous solution of [Pt(NH3)4](NO3)2, about 90% of the Pt was bound to a low molecular mass species (1 kDa), and the remainder was found in species with molecular masses up to 1000 kDa. Absorption occurred only at the roots (Messerschmidt et al., 1994). In native grass, however, platinum is bound to a protein with a high molecular mass (160 –200 kDa) (Messerschmidt et al., 1995). In grass grown on soil treated with a [Pt(NH3)4](NO3)2 solution, most of the platinum was deposited in the roots. Results from size exclusion chromatography coupled to ICP-MS indicated that Pt is bound to sulphur in a mechanism involving sulfhydryl groups, for example by binding to a phytochelatine, a low molecular mass peptide (Klueppel et al., 1998). Binding of Cu and Cd to phytochelatines has been observed previously (Leopold and Guenther, 1997). After treatment with a Pd(NO3) solution, endive plants (Cichorium endivia) took up considerable amounts of Pd. The plants showed clear stress symptoms after 2 days of treatment, and after this time the mean total Pd concentration was 8.7 ng g 1. 40% of the total Pd was in a soluble form, and the Pd of this fraction was distributed in equal amounts between high (160 kDa) and low molecular weight species (< 10 kDa) (Alt et al., 2002). In conclusion, plants can take up and accumulate soluble species of PGE, especially Pd, and deposition occurs in the roots by binding to sulphur in low molecular weight species. 4.2. Animals Bioaccumulation of platinum and palladium was high in species of the bacteria Pseudomonas (Peterson and Minski, 1985). The biosorptive removal of PGE

30

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by the anaerobe Desulfovibrio desulfuricans in an aqua regia leachate of spent automobile catalyst material was investigated. The effect of Rh(III) on Pd(II) uptake was negligible, while Pt(IV) decreased Pd(II) uptake by 15%. When Pd(II) was alone, uptake was 50% of the total Pd(II) present. Uptake of Rh(III) and Pt(IV) was 12% when they were alone in the solution, but no uptake was observed when together with Pd(II), indicating a much greater affinity of Pd(II) to the bacteria (Yong et al., 2002). Exposure of PGE standard solutions (soluble chloro-complex standard solutions) to the freshwater isopod Asellus aquaticus gave bioaccumulation factors of 85, 150 and 7 for Pt, Pd and Rh, respectively (Moldovan et al., 1999). Asellids live on river sediments and are directly exposed to PGE by ingestion of sediment particles. Bioaccumulation was time dependent, and there was a higher accumulation for materials with a higher PGE content. It was also shown that no interaction occurs among the metals, because accumulation results were the same both when the animals were exposed to individual and to mixed solutions. The uptake might occur via ingestion of metal-containing sediments and via absorption of metal dissolved in water. Bioaccumulation mechanisms suggested were a metal-binding protein or elevated formation rates of metal-binding granules. When A. aquaticus was exposed to catalytic converter material, the uptake rate was the same for the three PGE, since they are present in a metallic form. The uptake rate of Pt and Rh was, however, different when exposed to environmental materials containing PGE (river sediment, road dust and tunnel dust). This may be due to transformations of the PGE species in the environment. It was shown that Pt and Rh exist in a more bioavailable form in river sediments than in tunnel dust, since a much higher increase in uptake rate between 24 and 96 h of exposure to river sediment was found compared to tunnel dust (Moldovan et al., 1999). Platinum accumulation was demonstrated to be species dependent; Pt(IV) was accumulated to a higher extent than Pt(II) in A. aquaticus (Rauch and Morrison, 1999). In a recent study, mussels were maintained in water containing road dust and the soft tissue was analysed for PGE. An uptake was shown for all PGE and the highest bioavailability was found for Pd, followed by Pt and Rh. The concentration factor of Pd was five

times higher compared to Pb and only six times lower compared to Cu (Sures et al., 2002). Eels and barb fish were exposed to water containing ground catalyst converter material (PGE particles V 10 Am) and the liver and kidneys were analysed. An uptake was observed for Pd in eels and for all PGE (Pd>>Pt>Rh) in barb fish. In conclusion, Pd showed the highest accumulation of PGE in the liver and kidneys (Sures et al., 2001). Uptake of Pd in the liver of eels maintained in water containing road dust was observed in another study. It was suggested that uptake occurs through the gills, where aqueous metal ions pass across the membrane by diffusion and enter the bloodstream from where they are transported to internal organs (Sures et al., 2001). The eoacanthocephalan parasite Paratenuisentis ambiguous in European eels (Anguilla anguilla) readily took up and accumulated Pt and Rh when eels were exposed to ground catalytic converter material. Pd uptake was not investigated. No metal uptake was found for the eels. The parasites contained as much as 1600 times higher Rh and 50 times higher Pt concentrations compared to the PGE concentration in the water (Sures et al., 2003). A model substance of Pt(0)-particles, closely resembling automobile catalyst-emitted Pt, was bioavailable to rats (Artelt et al., 1999). Up to 30% of the Pt deposited in the lung after inhalation was bioavailable and it was suggested that the bioavailability of Pt is due to in vitro solubility. Platinum bioavailability was calculated by totalling the Pt content in urine and all organs except the lung. Within 24 h, the fastest clearance processes such as expiration and ciliary clearance were complete. After intratracheal instillation, Pt was mainly found in the lung, while bioavailable Pt was mainly found in the urine. The relative contribution of urine to the bioavailable fraction of Pt increased steadily with time, reaching z 87% after 90 days. In contrast to urine, bioavailable Pt in blood decreased substantially with increasing time and reached very low levels after 90 days ( V 0.03%). The remaining Pt was mainly found in the kidneys and the liver. After inhalation, the highest concentration of Pt was found in the faeces and high levels were also found in the lung. The bioavailable Pt was again found in the urine ( f 96%). In organs, the highest levels were found in the kidney. After oral application, levels were highest in faeces and in the

K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

urine and most of the Pt was excreted within 24 h. Low levels were found in organs, with a maximum in the kidneys. Only 0.1% of the Pt in the faeces was bioavailable. The bioavailable fraction of Pt in the gastrointestinal tract was investigated by applying the soluble platinum compound K2PtCl4 intravenously. About 90% of the total Pt was found in the faeces and the urine, suggesting that Pt is poorly absorbed from the digestive tract. It was concluded that a substantial fraction (20 – 30%) of the Pt originally present as metallic Pt in the form of ultrafine particles in the nanometer range was bioavailable after inhalation. In body tissues and fluids, z 90% of the bioavailable Pt was bound to proteins; the rest was in the form of ionic complexes (Artelt et al., 1999). In rats given drinking fluid with soluble Pt4 + salts, the highest concentrations occurred in the kidney, and levels were also high in the liver (Holbrook et al., 1975). The highest absorption of 103Pd and 191Pt (10%) was obtained following an intravenous dose and the lowest ( < 0.5%) after an oral dose in rats. After an intravenous dose, 103Pd were found in all tissues analysed, in the following descending order: kidney>spleen>liver>adrenal gland>lung>bone. The concentration of 191Pt after an intravenous dose was highest in the blood, kidney, liver, spleen and adrenal gland (Moore et al., 1974, 1975). Levels of Pd and Rh were higher than Pt in the liver and kidney of raptors in their natural habitat, indicating the following uptake order: Pd>Rh>>Pt (Ek et al., in press). The Pt/Pd ratio was lower in raptor feathers compared to automobile catalyst material, air particles and road dust, indicating a higher environmental mobility and uptake of Pd (Table 1) (Jensen et al., 2002). PGE concentrations were higher in feathers of birds in urban habitats and PGE contamination of feathers is predominantly externally attached PGE containing particles (Jensen et al., 2002; Ek et al., 2003a). The total Pt level in faeces of wild peregrine falcon was relatively low compared to other body tissues and fluids, indicating little excretion of Pt (Ek et al., 2003b). In conclusion, uptake of PGE in animals decrease in the following order; Pd>Pt>Rh. PGE, especially Pd, are taken up in the liver and kidney of rats and eels exposed to PGE in the laboratory, and there are indications of an uptake of Pd in raptors in the wild. The bioavailable fraction of automobile catalyst Pt to

31

rats may be as high as 20 –30% and it is bound to proteins in body tissues and fluids. Pt and Pd can bind to metallothionein in liver and kidney through the replacement of zinc and cadmium on sulfhydryl groups.

5. Levels in biological material This section focuses on levels in biological materials, with a summary of PGE levels in various environmental materials presented in Table 1. PGE levels in all biological materials that have been investigated are in the nanograms per gram and nanograms per liter range. Pt/Pd and Pt/Rh ratios in the same environmental materials are presented in Table 2. In 1975, before the introduction of automobile catalysts in the U.S., Pd was detected in honey samples from combs located near a U.S. highway, which was suggested to derive from traffic (Tong et al., 1975). PGE levels in tree bark from urban sites in the U.K. were in the order of 1 –3 ng g 1, which is of the same order of magnitude as in grass samples exposed to traffic. The Pt/Rh ratio was 1– 3 and the Pt/Pd ratio was 1– 1.7 (Table 2) (Becker et al., 2000). These ratios are similar to ratios in airborne particles, suggesting that PGE contamination of tree bark consists of externally attached PGE-containing particles, as for feathers (Jensen et al., 2002). Pt concentrations in tree bark were much higher in samples from major Japanese and US cities compared to remote sites. Pt levels in tree bark from cities were up to 38 ng g 1, while the level was below 9 ng g 1 in tree bark at remote sites (Ma et al., 2001). Platinum was taken up by grass growing adjacent to a U.S. highway. The Pt concentration was 1.2 ng g 1 in washed leaves and shoots, and Pd and Rh levels were 1.0 and 0.1 ng g 1, respectively (Table 1). The Pt/Pd ratio was 3.5 times lower in the grass than in the soil it was growing on, indicating a higher environmental mobility of Pd (Ely et al., 2001). The PGE content in the freshwater isopod A. aquaticus from an urban river was highest for Pd, followed by Pt and Rh (Moldovan et al., 2001). The Pd level was 155.4 ng g 1, approximately four times higher than the Pt level and nine times higher than the Rh level (Table 1).

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Table 2 Pt/Pd and Pt/Rh concentration ratios in naturally exposed environmental and biological materials Material

Year

Site

Ratio Pt/Pd

Earth crust Autocatalyst emissions (gasoline) Air particles < 30 Am Air particles < 10 Am Air particles < 10 Am Air particles < 2.5 Am Tree bark Road dust

– –

– –

1 6.7 ICP MASS Wedepohl (1995) 0.4 – 0.7 0.5 – 2.7 Q and HR ICP-MS, Palacios et al. DP CSV (2000a,b)

– 2002

– Urban

0.4 – 0.6 1.5 – 2 2.6 5.8

Sewage sludge Grass

Asellus aquaticus Feathers (Falco peregrinus) Blood (Falco peregrinus) Eggs (Falco peregrinus) Liver (Falco peregrinus) Kidney (Falco peregrinus) Human blood Human urine

ICP-MS ICP-SFMS

Moldovan et al. (2002) Kanitsar et al. (2003)

Q ICP-MS

Go´mez et al. (2001)

1.7

10.8

1.9

4.7

2.9

3.5

0.8 3.1

1.1 –

Q ICP-MS ICP-AES

1997

7.4

6.1

Q ICP-MS

Becker et al. (2000) Kovacheva and Djingova (2002) Scha¨fer et al. (1999)

1999

3.4

2.6

Q ICP-MS

Rauch et al. (2000)

2.6

HR-ICP-MS

Rauch et al. (2000)

1999 – 2000

NR NR

Urban Urban

1998 Soil

Analytical technique Reference

NR

Highway

2.9

12.1

Q ICP-MS

Ely et al. (2001)

1994 NR

Highway Highway

7.7 12.1

6.6 –

Q ICP-MS ID-ICP-MS

1997 1997

Urban Highway

3.3 –

21.9 5.9

CSV

NR

Highway soil 30.7

6.5

ICP-MS

Scha¨fer et al. (1999) Mu¨ller and Heumann (2000) Scha¨fer et al. (1999) Helmers and Mergel (1998) Scha¨fer et al. (1998)

Q ICP-MS Q ICP-MS Q ICP-MS

Ely et al. (2001) Moldovan et al. (2001) Jensen et al. (2002)

Highway 1999 Urban 1986 – 2000 Rural

1.2 0.2 0.4

12 2.1 1.7

1995 – 2001

1.2

1.5

1992 – 2000

0.8

1.3

1985 – 1995

0.3

0.4

1985 – 1995

0.7

0.7

0.02 0.1 0.1 0.01 0.1

– – 0.1 – 0.1

NR

Unexposed

1996 NR

Urban Unexposed

NR denotes not reported.

Note

Pt/Rh

Ek et al. (2003b)

Sector field ICP-MS Sector field ICP-MS HR ICP-MS Sector field ICP-MS HR ICP-MS

Begerow et al. (1997b) Rodushkin et al. (1999) Caroli et al. (2001) Begerow et al. (1997a) Krachler et al. (1998)

Aged autocatalysts

Pd value indicatory

Mean of the two most trafficked roads < 63 Am. Pd value indicatory < 63 Am. Pd value indicatory Mean of samples 0 – 1 m from three roads 0.6 m from the highway

External contamination No external contamination Washed leaves and shoot

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Platinum levels in feathers, blood, eggs, faeces, liver and kidney of wild Swedish peregrine falcon (Falco peregrinus) in their natural habitat were in the range of 0.1 – 3.4 ng g 1 (dry weight) (Table 1). Palladium levels in the same material varied between 0.3 and 1.4 ng g 1 (dry weight), and rhodium levels ranged between 0.3 and 0.7 ng g 1 (dry weight) (Jensen et al., 2002; Ek et al., 2003b). PGE levels were higher in blood compared to faeces and egg and Pt levels are also higher in blood compared to liver and kidney of peregrine falcons. It was suggested that all PGE are taken up in the same way to the blood, while Pd and Rh is more readily transferred to egg, liver and kidney, with Pd also being sequestered into feathers. It was further suggested that PGE contamination of feathers is predominantly in the form of externally attached PGE-containing particles. No evidence was found for bioaccumulation of PGE in naturally exposed raptors (Ek et al., 2003b). The Pt level in various foodstuffs ranged from 0.13 ng g 1 (fresh weight) in cream to 8.1 ng g 1 in liver. The results showed that a hypothetical average diet of an adult living in Sydney, Australia, contained 1.4 Ag Pt day 1 (Vaughan and Florence, 1992). However, the high levels reported have caused some doubts about their value. The PGE concentration in human urine and blood has been investigated in several studies (Table 1). In road construction workers, the urinary Pt level was 0.9 ng l 1 and the Pd level was 52.2 ng l 1 (Begerow et al., 1998/1999). The urinary Pd level was below 10 ng l 1 in unexposed individuals (Philippeit and Angerer, 2001). The Pd level was 50 times higher than the Pt level in blood of unexposed individuals: 50.2 and 0.9 ng l 1, respectively (Begerow et al., 1997b). The concentration of PGE decreased in the order Pd>Pt>Rh in blood samples from unexposed individuals in Sweden; levels were 48, 5.1 and below 9 ng l 1, respectively, for Pd, Pt and Rh (Rodushkin et al., 1999). Pt levels were significantly higher in blood and urine of exposed individuals (precious metal workers) compared to unexposed individuals (Farago et al., 1998). In exposed individuals the Pt level was 246 ng l 1 in blood and 470 ng g 1 creatinine in urine, and for unexposed individuals the Pt level was 129 ng l 1 in blood and 113 ng g 1 creatinine in urine. Pt levels in blood and urine were significantly correlated in exposed individuals (Farago et al., 1998). Pd and

33

Rh as well as Pt and Rh were strongly correlated in blood and serum of unexposed individuals, indicating a common source of exposure of the metals, presumably automobile catalysts (Ba´ra´ny et al., 2002a). In whole blood and serum of unexposed individuals, PGE levels were below the detection limit of 100 ng l 1 for Pt and Rh and 200 ng l 1 for Pd (Ba´ra´ny et al., 2002b). The palladium level in urine of unexposed and occupationally exposed individuals was determined by total reflection X-ray fluorescence. The urinary Pd level in occupationally exposed individuals was 200 –1000 ng l 1, while it was under the detection limit of 2.5 ng l 1 in unexposed individuals (Messerschmidt et al., 2000). Pt/Pd and Pt/Rh concentration ratios indicate transformation from the original catalytic material. As shown in Table 2, ratios decrease further up the food chain; in environmental material such as airborne particles, road dust, soil and grass, the Pt/Pd ratio is approximately three, while it decreases to approximately 0.5 in material such as invertebrates and birds, and reaches 0.1 or lower in human urine and blood. The Pt/Rh ratio follows the same pattern; decreasing from approximately six in airborne particles, road dust, soil and grass (close to the ratio in automobile catalysts), to 1.3 in invertebrates and birds, and 0.1 in human urine and blood. This indicates a higher environmental mobility and thus a probable higher biological uptake of Pd and Rh compared to Pt. 5.1. Temporal trends Platinum and rhodium concentrations in externally contaminated grass growing 20 cm from a highway increased 3.5 and 9 times, respectively, from 1992 to 1997. During the same time, the percentage of cars equipped with automobile catalysts increased from 30% to 75% (Helmers and Mergel, 1998). In urban soil, Pt concentrations were six times higher in 2001 compared to 1992, correlating with the increased use of automobile catalysts (Cinti et al., 2002). Platinum and rhodium levels were close to four times higher in the upper soil profile close to a highway in 1996 compared to 1994 (Scha¨fer et al., 1999). Platinum levels were approximately three times higher in road dust sampled in 1991 compared to 1984, also correlating well with the increased use of automobile catalysts during that time (Wei and Morrison, 1994).

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Platinum and rhodium levels in urban road dust < 63 Am were approximately 3 and 1.5 times higher, respectively, in 1998 compared to 1991 (Wei and Morrison, 1994; Rauch et al., 1999; Rauch, 2001). The Rh concentration, in the form of externally attached Rh containing particles, was significantly higher in feathers of raptors living after the introduction of automobile catalysts in 1986 compared to those living before (Jensen et al., 2002), correlating with the increased Rh level in the environment since automobile catalysts were introduced.

6. Uptake, excretion and effects in humans Studies of human tissue have shown that only Pt(II) binds to proteins in human blood, but Pt(IV) can be reduced to Pt(II) before binding (Einha¨user et al., 1996; Nachtigall et al., 1997; Zhong et al., 1997). Platinum is transported with proteins to the liver and kidneys, where it accumulates before being excreted. In the liver and kidneys, Pt(II) binds to metallothionein (MT), a low molecular weight cysteine-rich protein. The association constant of Pt(II) to rabbit liver MT was approximately 30 and 107 times higher than that of Cd and Zn, respectively (Zhang et al., 1997). Binding of Pt(II) to MT is associated with the replacement of Zn or Cd atoms bound to MT, and the binding depends on the availability of sulfhydryl groups in MT (Zelazowski et al., 1984). Palladium in the form of Pd(II) ions can also bind to MT in rabbit kidneys (Zhang et al., 1998). Urinary Pt and Pd excretion was investigated in dental technicians, road construction workers, and school-leavers (Begerow et al., 1998/1999). The mean excretion of the metals was significantly higher in dental technicians compared to the other groups, indicating that occupational exposure to dental alloys leads to an internal exposure that is higher than the exposure from automobile catalysts. Road construction workers did not have higher Pt and Pd excretion compared to school-leavers (Begerow et al., 1998/ 1999). Concentrations of Pd and Rh in human urine were strongly associated with traffic density, but not Pt (Caroli et al., 2001). Pd levels were 75 times higher than Pt levels in urine of unexposed persons (Begerow et al., 1997a), and Pd and Rh levels were approximately 10 times higher than Pt levels in urine of

children from Rome, Italy (Krachler et al., 1998). Considering that environmental concentrations of Pd and Rh are lower than Pt, this might be explained by a different environmental mobility as well as different uptake and metabolism of Pd and Rh. It has been suggested that Pt/Au alloys could be the main source of urinary Pt excretion in non-occupationally exposed persons (Begerow et al., 1999; Schierl, 2001). No evidence was found for a significantly increased urinary Pd excretion after insertion of dental alloys containing Pd (Begerow et al., 1999). Another study found no evidence that Pd in dental alloys increases the urinary Pd level of non-occupationally exposed individuals, while occupational exposure lead to increased urinary Pd levels (Philippeit and Angerer, 2001). The toxicity of Pt has been investigated in detail (Lindell, 1997), while less is known about Pd and Rh. Allergy to platinum salts is well known; the condition platinosis is defined as the effects of soluble platinum salts on occupationally exposed individuals. Symptoms include irritation of the nose and the upper respiratory tract, with sneezing and coughing, and sometimes even asthmatic symptoms such as tightness of the chest, wheezing and shortness of breath. A few cases of allergic contact dermatitis from salts of palladium and occupational contact dermatitis from salts of rhodium have been reported (Bedello et al., 1987; de la Cuadra and Grau-Massane´s, 1991; Wataha and Hanks, 1996; Jappe et al., 1999). Palladium allergy almost always occurs together with nickel sensitivity (Wataha and Hanks, 1996; Jappe et al., 1999). A recent study reviewed the exposure and effects of palladium to human health (Kielhorn et al., 2002). It was concluded that the sensitisation risk of Pd is a major health concern because very low doses are sufficient to cause allergenic reactions in susceptible individuals (Kielhorn et al., 2002). In Environmental Health Criteria for palladium, WHO concluded that Pd ions are among the most frequent senzitisers within metals and that several palladium salts may cause severe primary skin and eye irritations (Melber et al., 2002). Chlorinated salts of Pt, Pd and Rh did not cause positive patch or prick test reactions in non-occupationally exposed persons with dermatitis and/or urticaria living in urban areas, while allergic reactions were positive in a number of exposed workers in a

K.H. Ek et al. / Science of the Total Environment 334 – 335 (2004) 21–38

35

PGE refinery (Santucci et al., 2000). The study concluded that present environmental levels of PGE do not increase the incidence of reactions to platinum salts in patients with dermatitis and/or urticaria, but if levels reach those in industrial settings, more frequent health effects can be expected.

urinary Pd and Rh levels reflect exposure from automobile catalyst PGE, while urinary Pt levels reflect release from dental alloys. Platinum allergy is well known and occurs in occupationally exposed persons, while Pd and Rh allergies are rare, although Pd is a potent sensitiser.

7. Conclusions

Acknowledgements

Automobile catalyst emitted particles deposit on the roadside and can be transported to waterbodies through stormwater runoff. The soluble fraction of PGE in automobile catalyst emissions is at least 10%. The solubility of metallic Pt and Rh increases with decreasing pH and the solubility of metallic Pt increases with decreasing particle size and increasing salinity. A solubility gradient and rate of Pd>Rh>Pt in a solution simulating rain has been suggested. Metallic Pt is oxidised in soil, probably due to its nanocrystalline particle size, and because the majority of the Pt species formed in the oxidation are immobile. Platinum and palladium might form complexes with humic substances in soil and water. Methylation of Pt and Pd has only been observed in laboratories, and it is not yet possible to draw conclusions on whether PGE are methylated in the environment. For automobile catalyst PGE, the uptake rate in plants and animals is highest for Pd, followed by Rh and Pt, and the transfer coefficient of PGE from soil to plants is in the same order as Zn and Cu. Several studies suggest an environmental mobility and uptake gradient of Pd>Rh>Pt. In plants, Pt and Pd are bound to sulphur in low molecular species. The bioavailability of automobile catalyst emitted Pt is up to 30% in rats, probably due to the small particle size. The greater proportion of Pt and Pd remaining after excretion are in the liver and kidney. Pt is transported with proteins to the liver and kidneys, where Pt and Pd can bind to metallothionein. The highest retention of Pt and Pd in rats occurs after an intravenous dose and the lowest after an oral dose. Absorbed Pd can be found in almost all organs, tissues and body fluids, with the highest concentrations in the liver, kidneys and spleen in experimentally dosed animals. Pd and Rh levels are higher than Pt levels in urine, suggesting a different environmental mobility as well as different uptake and metabolism of Pd and Rh. It has been suggested that

The authors would like to acknowledge MISTRA, the Foundation for Strategic Environmental Research, for financial support.

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