Platinum, palladium and rhodium release from powder particles
exposed to simulated lung fluids
Claudia Colombo*, A. John Monhemius, Jane A. Plant
Department of Earth Science and Engineering, Imperial College London, London
SW7 2AZ, UK
*
Corresponding author. Tel.: +44 (0)207 759 41362; Fax: +44 (0)207 594 7444 ; E-
mail address: claudia.colombo@ic.ac.uk
Abstract
Increases in platinum-group element (PGE) concentrations in ambient air and dust
since the introduction of vehicle exhaust catalysts (VECs) is a cause of concerns. To
assess the risks associated with the inhalation of VEC-emitted PGEs, extraction
experiments on PGE-containing materials (i.e. road dust, auto catalyst and hydroxide
samples) exposed to simulated lung fluids were performed. The aim of this study was
to measure the release of PGEs in an inhalation scenario. Two synthetic lung fluids
were used as surrogates for different areas of potential exposure in the human lungs:
Gamble’s solution is representative of the interstitial fluid of the deep lung and
artificial lysosomal fluid (ALF) is representative of the more acidic environment
following phagocytosis by alveolar and interstitial macrophages within the lung. The
results showed a higher PGE release for samples exposed to the acidic ALF compared
to the neutral Gamble’s solution, implying that the inhaled particles would have to be
phagocytized before a considerable solubilization of PGEs took place. The greatest
percentage of released PGEs (up to 88%) was associated with road dust samples, most
likely because of the transformations undertaken by these metals once deposited from
VECs in the environment (i.e. formation of soluble and mobile species through
complexation by inorganic and organic ligands common in the roadside environment).
Pd and Rh showed higher % of metal release compared with Pt, probably due to
differences in their mobilities and tendencies to form soluble complexes. The highest
absolute bioavailability is observed for Pt however, because of its greater
concentration in environmental samples. The elevated solubilization of Pt, Pd and Rh
observed in the respiratory tract could involve the formation of PGE-chloride
complexes. This observation merits particular attention considering the powerful toxic
and allergenic effects of PGE-chloride salts on human beings and living organisms
and the ever-increasing levels of PGEs in the environment due to VEC emissions.
Keywords: Platinum; palladium; rhodium; environment; lung fluids; health risk.
1
1. Introduction
Human exposure to particulate matter originating from traffic is finding
increasing interest and the widespread use of platinum, palladium and rhodium in
vehicle exhaust catalysts (VECs) has stimulated various scientists to study the
concentration of these metals in airborne samples at different urban sites and working
places {Gomez, 2001 #62}{Alt, 1993 #100}{Zereini, 2001 #113}{Bocca, 2004 #60}.
Platinum group elements (PGEs) are emitted in the exhaust gas from VECs with
emission rates in the ng/km range and their concentrations in urban air have increased
by more than two orders of magnitude in the last 20 years (up to approximately 110
pg/m3 {Merget, 2001 #72}). PGE emissions are thought to be mainly as fine
particulate material that originates from the abrasion and deterioration of the surface
of the catalyst. The annual Pt emission from VECs has been estimated at 0.5–1.4 ton
per year {Barbante, 2001 #102}. Pt, Pd and Rh are deposited along roadways, on
adjacent vegetation and soil, may be dispersed in silt, water and plants. Although
PGEs are mostly released in metallic and oxide form {Konig, 1992 #56;Artelt, 2000
#170;Schlogl, 1987 #188}, there is evidence that, following their deposition in the
environment, they can be transformed into soluble species by complexation with
ligands commonly found in nature, providing compounds able to enter the food chain
{Alt, 1993 #100}{Freiesleben, 1993 #133}{Lustig, 1996 #68}{Lustig, 1998
#131}{Zimmermann, 2003 #85}{colombo, #246}{Wood, 2004 #138}.
Because of the toxic, cytotoxic and allergenic effects of some PGE species
(especially the chloride ones) and the increasing concentrations of VEC-emitted Pt,
Pd and Rh in the environment, there is growing concern over human health and
environmental risks resulting from PGE emissions. One of the major concerns for
exposure to VEC-emitted PGEs is sensitization of the airways caused by soluble PGE
2
compounds {Merget, 2000 #212}{Cristaudo, 2005 #162}. Many PGE compounds are
known as potent respiratory allergens leading to rhinitis, conjunctivitis, asthma and
urticaria {WHO, 1991 #189}{WHO, 2002 #190}. Signs of acute toxicity of several
PGE salts (such as H2[PtCl6], RhCl3, and PdCl2) in rats or rabbits included deaths,
decrease in feed and water uptake, emaciation, cardiovascular effects, peritonitis and
biochemical changes. There were also hemorrhages of lungs and the small intestine.
Moreover interactions with DNA were observed in vitro and an inhibition of DNA
synthesis were demonstrated both in vitro and in vivo {WHO, 2002 #190}. A study
on cell viability and oxidative stress in human bronchial epithelial cells {schmid, 2007
#292} showed that Pt and Pd had considerable toxic effects that are comparable if not
even exceeding the damage induced by heavy metal species such as Cd and Cr. The
mode of action of PGE ions and of elemental PGEs as dispersed dust in biological
systems is not fully clear. Similar to other transition metals, PGE ions follow a few
basic principles in their mode of action. Due to their ability to form strong complexes
with both organic and inorganic ligands, they have the potential not only to disturb
cellular equilibria or replace other essential ions but also to interact with functional
groups of macromolecules, such as proteins, enzymes and DNA/RNA, thereby
disrupting a variety of cellular processes. From occupational studies conducted in
VEC production, a no-effect level “critical range” of 15-150 ng/m3 has been
suggested {Merget, 2001 #72}. Higher concentrations could potentially induce
hypersensitivity reactions if halogenated PGE salts are present in VEC emissions or
are formed through transformation processes in the environment.
Very little information is available on the bioavailability of VEC-emitted PGEs
after inhalation. A study by Artelt et al. {Artelt, 1999 #54} provided bioavailability
measurements of a model platinum substance, similar to that emitted by VECs. This
3
substance was applied to Lewis rats during a 90 day inhalation study. The study
showed that the percentage of bioavailable platinum was up to 30%. The authors
concluded that the elevated in vivo solubility was most likely due to the ultrafine
structure of the platinum particles.
Data on PGE release into lung fluids relevant for human exposure are nonexistent in the literature. The inhaled particles may interact with cells and tissues in
the lung compartment in many different fashions. The mechanisms of these
interactions include the release of PGE species that may have a particular effect on the
human body. Reliable information on PGE-release from the inhaled particles in the
respiratory tract is crucial when assessing the potential risk for adverse effects arising
from exposure to these substances. The solubility of metal compounds in simulated
body fluids is commonly used for determining human health risk from exposure to
specific substances of concerns {midander, 2007 #277}{herting, 2006 #275}{herting,
2007 #276}{shi, 2006 #274}{stopford, 2003 #273}. Therefore, in this study, PGEcontaining materials, such as road dust and milled recycled catalyst have been
exposed to various simulated lung fluids to provide quantitative estimates of Pt, Pd
and Rh release in an inhalation scenario representative of a human being. Synthetic
hydroxide species have also been included serving as reference materials for
comparison of data. Artificial lysosomal fluid (ALF) and Gamble’s solution have
been used to simulate different interstitial conditions in the lung. ALF is analogous to
the fluid with which inhaled particles would come into contact after phagocytosis by
alveolar and interstitial macrophages within the lung. Gamble’s solution represents
the interstitial fluid deep within the lung. These fluids have previously been used to
investigate the bioaccessibility of cobalt compounds {stopford, 2003 #273}, the
4
copper release from powder particles {midander, 2007 #277} and metal release from
stainless steel {herting, 2007 #276}.
2. Experimental procedure
2.1. Test materials
Substrates evaluated for PGE release in the lung fluid experiments included two
-Certified Reference Materials (CRM) representative of (i) road dust with a
maximum particle size of 90 µm, from the Institute of Reference Materials and
Measurements, and (ii) powdered auto catalyst with a maximum particle size of 74
µm, from the National Institute of Standards and Technology (Table 1),
- Synthetic PGE-hydroxide species Pt(OH)2, Pd(OH)2 and Rh(OH)3 prepared
from the appropriate
nitrate complexes (Johnson-Matthey) by precipitation with
NaOH, were also tested.
Test material
Pt
Pd
Rh
Road dust (BCR-723)
81.3 µg/kg
6.0 µg/kg
12.8 µg/kg
Recycled catalyst (SRM-2557)
1131 mg/kg
233.2 mg/kg
135.1 mg/kg
Table 1: PGE content of Certified Reference Materials employed in the lung fluid
experiments.
1g road dust, 1g catalyst and 0.1g hydroxide sample (composed of 0.033g
Pt(OH)2, 0.033g Pd(OH)2 and 0.033g Rh(OH)3) were employed in the lung fluid
experiments.
2.2. Composition of simulated lung fluids
5
The chemical composition of each ALF and Gamble’s solution is presented in
Table 2.
Chemicals (g/L)
ALF
Gamble’s solution
pH = 4.5
pH = 7.4
MgCl2
0.050
0.095
NaCl
3.21
6.019
—
0.298
Na2HPO4
0.071
0.126
Na2SO4
0.039
0.063
CaCl2·2H2O
0.128
0.368
C2H3O2Na
—
0.574
NaHCO3
—
2.604
C6H5Na3O7·2H2O
0.077
0.097
NaOH
6.00
KCl
C6H8O7
20.8
—
—
H2NCH2COOH
0.059
—
C4H4O6Na2·2H2O
0.090
—
C3H5NaO3
0.085
—
C3H3O3Na
0.086
—
Table 2: Composition (g/L) of extraction fluids {midander, 2007 #277}{stopford,
2003 #273}.
The components of Gamble’s solution were added following the order in Table
2 to avoid precipitation of salts. Citrate was used in place of proteins and acetate to
6
replace organic acids. All chemicals were of analytical grades and both solutions were
prepared using ultra-pure water. Except for differences in salt composition, the main
difference between Gamble’s solution and ALF is the acidity: the former has pH = 7.4
whereas the latter has pH = 4.5. The ALF acidity may explain the higher level of
solubility for metals that are phagocytized compared to those that remain extracellular
{Costa, 1995 #278}.
2.3. Extraction method
The extraction experiments were performed by mixing the test material (1.0 g
for road dust and recycled catalyst or 0.1 g for the hydroxide species) with 100mL of
simulated lung fluid. The experiments were carried out in sealed containers to
minimize interactions between the reaction fluid and atmospheric oxygen and the
potential for cross contamination. Each experiment was conducted in duplicate and
blanks (containing only simulated lung fluids) were used. Each sample was placed
under dark conditions in a shaking incubator keeping the temperature at 37°C.
Extracts were filtered through a 0.2 µm filter to avoid any solid carry over. Different
extraction periods (up to 30 days) were investigated. The pH of each sample was
controlled at every extraction time and no variation superior to 0.2 units was
observed.
2.4. PGE analysis
All the extracts were analyzed for PGE content by inductively coupled plasma
mass spectrometry (ICP-MS), using an ICP-MS Varian model 810. The operating
conditions are given in Table 3.
7
Plasma
Plasma flow:
18.00 L/min
Auxiliary flow:
1.80 L/min
Sheath gas flow:
0.30 L/min
Nebulizer flow:
1.07 L/min
Sampling depth:
5.00 mm
Power:
1.40 kW
Pump rate:
5 rpm
Stabilization delay:
10 sec
Analysis Modes
Analysis type:
Quantitative
Acquisition mode:
Steady state
Scan mode:
Peak hopping
Dwell time:
10000 µsec
Replicates/sample:
3
Scans/replicates:
30
Analytes:
63
Cu, 65Cu, 87Rb, 87Sr, 88Sr, 89Y,
103
Rh, 105Pd, 179Hf, 195Pt, 206Pb
Table 3: ICP-MS operating conditions.
ICP-MS is often used for the determination of PGEs in environmental samples
{Barefoot, 1997 #26;Ek, 2004 #33;Ravindra, 2004 #57} because of its excellent
detection limits that allow accurate determination of PGEs in the ppt range. The
determination of PGEs by ICP-MS is, however, hampered by spectral interferences
8
from monatomic and polyatomic ions produced in the argon plasma by the matrix
constituents: elements such as Cu, Y, Hf, Sr, Rb and Pb from road dust and catalyst
samples cause interferences with the PGE signals (Table 4).
Analyte
195
Pt
179
Pd
40
Ar65Cu+
89
Y16O+
88
Sr17O+
87
Rh
40
Ar63Cu+
87
Sr16O+
87
Rb16O+
206
105
103
Possible interfering species
Hf16O+
Rb18O+
Pb2+
Table 4: Possible interferences in the ICP-MS determination of 195Pt, 105Pd and 103Rh.
All the experimental solutions were prepared daily in 2% HNO3 and matrixmatched using simulated lung solutions. PGE calibration standards were prepared by
dilution of stock solutions of 1000 mg/L of Pt, Pd and Rh (all from Alfa Aesar,
Johnson Matthey). The linearity of the calibration plots (correlation coefficient >
0.99) was checked for Pt, Pd and Rh in the concentration range 0.1 – 100 ng/L and 0.1
– 100 µg/L. To compensate for instrumental drifts, 10 µg/L of internal standards (ISs)
were added to all samples. 115In for 105Pd and 103Rh and 191Ir for 195Pt were selected as
ISs because of their low abundance in the sample matrix, similar atomic mass and
chemical and physical properties to the PGEs. For the interference studies, single
element stock solutions (1000 mg/L) of Cu, Rb, Sr, Y, Hf and Pb (all from Alfa
Aesar, Johnson Matthey) were used.
3. Results
3.1. Interference correction
The isotopes
195
Pt,
105
Pd and
104
Rh were used to determine the concentration of
Pt, Pd and Rh, respectively, in the samples because of their high natural abundance
9
and because they are subject to fewer interferences; possible interferences are reported
in Table 4.
Interferences were corrected mathematically by estimating the contribution of
interfering species to the PGE signal through the analysis of single element standard
solutions of interferents {Rauch, 2001 #47;Moldovan, 1999 #25;Gomez, 2000 #63}.
The concentrations of the interfering elements in the standard solutions were at
similar levels as those in the sample solutions. Corrections were calculated using the
following equation:
SA = SA,S – (SI,S RIO)
where SA is the corrected analyte (A) signal, SA,S is the interfered analyte signal
measured for the sample solution, SI,S is the signal for the interfering element (I) in the
sample solution, and RIO is the signal ratio IO+/I+ (ratio between the concentration of
the ions produced in the plasma that interfere on the PGE signal and the concentration
of the interfering element itself, i.e.
40
Ar63Cu+/63Cu+). The signal ratio RIO for all the
interfering species needed in the correction equations were measured daily. This
mathematical correction method requires a linear dependence of the IO+ signal on the
I+ concentration. In this study, a linear dependence was observed for all the interfering
elements.
Considering all the interferences, the corrections for Pt, Pd and Rh were:
SPt = SA,S – (SHf,S RHfO)
SPd = SPd,S – (CCu,S RArCu + CY,S RYO + CSr,S RSrO + CRb,S RRbO)
SRh = SRh,S – (CCu,S RArCu + CSr,S RSrO + CRb,S RRbO + CPb,S RPb)
Unfortunately, the extremely low PGE concentrations in the extracted samples and the
high content of interfering elements in the matrixes hampered the determination of Pd
released by road dust exposed to ALF.
Even lower PGE concentrations were
10
observed in road dust samples exposed to Gamble’s solution, preventing the
determination of all the three PGEs (i.e. Pt, Rd and Rh).
3.2. PGE release
Reproducible results were observed for every duplicate experiment and the
blank analysis showed that neither of the simulated lung fluids interfered with the
PGE signal. The results on Pt, Pd and Rh release from the various test materials in
ALF and Gamble’s solution are illustrated in Figures 2 and 4 respectively. In Figures
3 and 5 results are represented in terms of % of PGE released to allow comparison
between compounds and between extraction fluids.
0.3
Pt
Rh
Total release (ppb)
0.25
0.2
0.15
0.1
0.05
0
0
a
100
200
300
400
500
600
700
Time of exposure (hours)
11
250
Pt
Pd
Rh
Total release (ppb)
200
150
100
50
0
0
100
200
300
400
500
600
700
600
700
Time of exposure (hours)
b
25
Pt
Pd
Rh
Total release (ppm)
20
15
10
5
0
0
c
100
200
300
400
500
Time of exposure (hours)
Figure 2: Total Pt, Pd and Rh release from road dust (a), auto catalyst (b) and
hydroxide samples (c) exposed to ALF.
12
100
90
80
% released
70
60
50
40
Pt (road dust)
Rh (road dust)
Pt(auto catalyst)
Pd(auto catalyst)
Rh(auto catalyst)
Pt(hydroxide)
Pd(hydroxide)
Rh(hydroxide)
30
20
10
0
0
100
200
300
400
500
600
700
Time of exposure (hours)
Figure 3: % of Pt, Pd and Rh released from road dust, auto catalyst and hydroxide
samples exposed to ALF.
16
Pt
Pd
Rh
14
Total release (ppb)
12
10
8
6
4
2
0
0
a
100
200
300
400
500
600
700
Time of exposure (hours)
13
400
Pt
Pd
Rh
350
Total release (ppb)
300
250
200
150
100
50
0
0
100
200
300
400
500
600
700
Time of exposure (hours)
b
Figure 4: Pt, Pd and Rh release from auto catalyst (a) and hydroxide samples (b)
exposed to Gamble’s solution.
0.5
0.45
0.4
% released
0.35
0.3
0.25
0.2
0.15
Pt(auto catalyst)
Pd(auto catalyst)
Rh(auto catalyst)
Pt(hydroxide)
Pd(hydroxide)
Rh(hydroxide)
0.1
0.05
0
0
100
200
300
400
500
600
700
Time of exposure (hours)
Figure 5: % of Pt, Pd and Rh released from auto catalyst and hydroxide samples
exposed to Gamble’s solution.
14
The results of the extraction experiments for the three different test materials
(road dust, auto catalyst and hydroxide) exposed to ALF showed that metal releases
changed with substrates, with the road dust samples showing much higher percentages
of released PGEs than either the catalyst or the hydroxide samples (Fig. 3). For road
dust samples, the percentage of released Pt was about 36% and released Rh about
88%, whereas for the auto catalyst and hydroxide samples, the percentage of released
PGEs was less than 8% and 10%, respectively (Fig. 3). The extraction experiments
conducted employing Gamble’s solution showed a low percentage of release for
PGEs: up to 0.45% for auto catalyst and 0.15% for hydroxide samples (Fig. 5).
The results also showed considerably lower metal release from samples exposed
to Gamble’s solution (pH 7.4) compared to ALF (pH 4.5) (Fig. 3 and 5).
For road dust samples exposed to ALF, the percentage of released Rh was
higher than the one for Pt in road dust samples exposed to ALF (Fig. 3). The auto
catalyst and hydroxide samples exposed to ALF and Gamble’s solution, showed
percentages of released PGEs in the following order: Pd > Rh > Pt (Fig. 3 and 5).
The results for the absolute PGE releases showed that for road dust samples the
amount of released Pt was greater than the amount of released Rh (Fig. 2a) and for
auto catalyst samples, the absolute release order was Pt > Pd > Rh (Fig. 2b and 4a).
Because the hydroxide sample contained the same amount of Pt, Pd and Rh (see
paragraph 2.1), the extraction experiments employing this material showed an
absolute release order identical to the percentage of release order (Fig. 2c and 4b).
4. Discussion
The elevate percentages (36% for Pt and 88% for Rh) of released PGEs
observed for road dust samples (Fig. 3 and 5) corroborate previous assumptions that
15
PGEs in road dust have been transformed from a metallic form to much more soluble
species, probably through complexation by inorganic ions {colombo, #246} and
humic substances commonly found in the environment {Wood, 1996 #222} {Lustig,
1998 #132;Lustig, 1996 #68;Lustig, 1998 #131}. Besides, the % of released Pt from
road dust observed in this study (36%) was in accordance with the results of Artelt et
al. {Artelt, 1999 #54} that exposed Lewis rats to a material resembling VECemissions during a 90-day inhalation study and found that about 30% of the Pt
deposited in the lung was bioavailable.
The higher metal release observed from samples exposed to ALF compared to
Gamble’s solution (Fig. 3 and 5) is probably due to the acidity of the former
compared to the neutrality of the latter (ALF pH = 4.5 and Gamble pH = 7.4). This
suggestion is supported by various studies reporting increased PGE solubilities in
acidic environments {Nachtigall, 1996 #129}{Fuchs, 1974 #130}{Zereini, 1997
#22}. The higher PGE release observed in ALF implies that inhaled PGE particles
would have to be phagocytized before a significant solubilization occurred.
The different % of release for Rh and Pt from environmental samples (i.e. road
dust) (Fig. 3) can be attributed to different reactions the two metals undergo once
emitted by VECs and deposited in the environment. The more mobile nature of Rh
{Rauch, 2000 #106} and its higher tendency to form complexes with humic
substances than Pt {Wood, 1996 #222} support this assumption. The percentage of
released Pd in road dust samples (which could not be determined because of
interference issues, as discussed in paragraph 3.1) would probably be the greatest due
to the highly soluble and mobile nature of this element {Ravindra, 2004 #57;Ek, 2004
#33}. The percentage of release order observed in the extraction experiments
involving auto catalyst and hydroxide samples exposed to ALF and Gamble’s solution
16
(Fig. 3 and 5) was in agreement with the findings of several solubility studies {Ek,
2004 #33;Ravindra, 2004 #57} that reported the highest solubility for Pd followed by
Rh and eventually Pt. Pt/Pd and Pt/Rh concentration ratios in environmental and
biological samples indicate different transformation for the three metals once
deposited in the environment by VECs {Ek, 2004 #33}. Concentration ratios in VECs
are about 5 for Pt/Pd and 8 for Pt/Rh (Table 1) and they decrease further up the food
chain, indicating a higher mobility and biological uptake for Pd and Rh compared to
Pt: in airborne particles, road dust and soil the Pt/Pd ratio is about 3 and Pt/Rh about
6, in invertebrates and birds the ratios decrease to 0.5 for Pt/Pd and 1.3 for Pt/Rh and
both ratios are lower than 0.1 in human urine and blood.
The results for the absolute PGE release (Fig. 2a, 2b and 4a) reflected the
relative PGE concentrations in the environmental samples (i.e. road dust and catalyst)
employed in the extraction experiments (Table 1). It can therefore be concluded that,
despite Pd and Rh being more soluble and mobile than Pt (% of release is in the order
Pd > Rh > Pt), the higher amount of Pt present in the samples accounts for its highest
absolute release in the respiratory tract.
A crucial parameter to consider when assessing the risk associated with
inhalation of dusts is the particle size of the inhaled material. Research about the size
distribution of the PGEs emitted from modern three-way VECs {Artelt, 1999
#52}{Gomez, 2002 #41}{Merget, 2001 #72} showed that the size fraction of particles
with a diameter > 10.2 µm ranged from ca. 43 to 71% and the small sized fraction of
particles (particle diameter < 3.1 µm) was between 11 and 36%. Importantly for this
study, the particle-size distribution of the reference material road dust (BCR-723) was
measured by laser diffraction spectroscopy {Zischka, 2002 #64}: the results showed a
particle top size of less than 90 μm, a median particle size of 13.2 μm, about 36% of
17
the particle (volume %) smaller than 10 μm and about 16% of the particle (volume %)
smaller than 4 μm. Particle size affects where inhaled particles may be deposited in
the respiratory tract. Small particles (Ø < 4 µm) and aggregates of nano-particles that
easily fragment are capable to penetrate the alveolar region of the respiratory system,
causing an inflammatory response {murr, 2004 #279}. Particles with a diameter > 10
µm and up to 100 µm are inhalable, but are mainly deposited in the upper respiratory
tract, whereas particles with a diameter < 10 µm have a likelihood of ≥ 50% to reach
the tracheobronchial region {Merget, 2001 #72}.
From the toxicological perspective, it is important to consider the possible
formation of PGE-chloride complexes in the respiratory environment due to the
presence of HCl. It has been shown that, in acidic conditions (ALF pH = 4.5) and in
presence of chloride ions, PGE-chloride species can be formed {colombo,
#246}{Fuchs, 1974 #130}{Zereini, 1997 #22}. Potent allergenic effects of PGEchloride salts on human beings, as well as acute toxicity in laboratory mammals,
including death, cardiovascular effects, reproductive toxicity and DNA mutagenicity
{WHO, 1991 #189}{WHO, 2002 #190} suggest an increased health risk due to the
presence of these species. Topical exposure of mice to respiratory sensitizing Pt salts
caused a similar quality of immune response to that induced by epicutaneous
application of organic allergens, such as toluene diisocyanate or cyanuric chloride
{dearman, 1998 #301}.
In a previous study, the bioavailabilities of PGEs from road dust, auto catalyst
and hydroxide samples were estimated using a physiologically based extraction test
(PBET) {colombo, #300}. In the PBET assay, digestive juices based on human
physiology are used to extract soil contaminants, in conditions that mimic those in the
human digestive tract. The PBET results supported the findings of this study: much
18
higher percentages of bioavailable PGEs were observed in road dust samples
compared to auto catalyst and hydroxide samples, Pd and Rh showed higher
bioavailabilities compared with Pt and the highest absolute release in environmental
samples was observed for Pt, because of its greater concentration. In order to evaluate
the potential pathways (inhalation and ingestion) of VEC-emitted PGEs into humans,
it is useful to compare the PGE release from road dust samples in the respiratory and
digestive environment: about 17% of Pt and 68% of Rh were solubilized during the
PBET experiments, whereas about 36% of Pt and 88% of Rh were solubilized during
extraction experiments with lung fluids. A higher PGE uptake was therefore observed
for the respiratory tract compared to the digestive tract.
5. Conclusion
VEC-emission of PGEs in populated urban areas has resulted in an increasing
concern about the hazardous effect of these new pollutants for people and other living
organisms. Very little information is available on the behaviour of PGEs after
exposure via inhalation. In order to address this information gap, a study of the release
of PGEs from road dust, auto catalyst and hydroxide samples was carried out.
Detailed data on Pt, Pd and Rh release from the various PGE-containing
materials were generated. The in-vitro extraction experiments were performed in ALF
(pH 4.5) and Gamble’s solution (pH 6.4), where the former represents conditions
occurring in conjunction with phagocytosis by alveolar and interstitial macrophages
and the latter represents the interstitial fluid deep within the lung. The acidity of ALF
is probably responsible for the higher PGE release observed in samples exposed to
this fluid compared to Gamble’s solution. This result implies that the inhaled particles
19
would have to be phagocytized before a considerable solubilization of PGEs took
place.
Road dust provided the greatest percentage of released PGEs: percentages of
release up to 88% were detected. This is most likely due to transformations that PGEs
undertook once deposited from VECs in the environment. These transformations
include complexation by inorganic and organic ligands common in the roadside
environment leading to the formation of more soluble and mobile species.
The different % of release of Pt, Pd and Rh observed in this study can be
attributed to differences in their mobilities, tendencies to form soluble complexes and
transformations in the environment. The results of the extraction experiments support
the assertion that Pd and Rh have higher mobilities and biological uptakes compared
with Pt: the % of metals release followed the order Pd > Rh > Pt. The highest absolute
release is observed for Pt, however, because of its greater content in the road dust and
auto catalyst samples.
A comparison of the results from the present study with a previous one
{colombo, #300} investigating the bioavailabilities of PGEs from road dust, auto
catalyst and hydroxide samples in fluids mimicking the human gastrointestinal tract
showed a higher PGE uptake by the respiratory tract compared to the digestive tract.
This study demonstrated that human beings can uptake up to 88% of VECemitted PGEs via inhalation exposure. Owing to the presence of chloride in the lung
fluids, the solubilization of Pt, Pd and Rh in the respiratory tract could be in part due
to, or involve, the formation of PGE-chloride complexes. This information needs
particular attention in view of the powerful toxic and allergenic effects of PGEchloride salts on human beings and living organisms and of the ever-increasing levels
of PGEs in the environment due to VEC emissions.
20
Acknowledgments
The research presented in this paper was partially supported by Anglo American
plc. We are grateful to Dr. Raquel Garcia-Sanchez and Dr. Eva Valsami-Jones at the
Natural History Museum for ICP/MS measurements. We would also like to thank
anonymous reviewers for critically reading the manuscript and suggesting
improvements to the text.
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