Implications of Platinum-Group

Element Accumulation along U.S.

Roads from Catalytic-Converter

Attrition

JAMES C. ELY,* , † CLIVE R. NEAL, †

CHARLES F. KULPA, ‡

MARK A. SCHNEEGURT, ‡,§

JAMES A. SEIDLER, † AND

JINESH C. JAIN †

Department of Civil Engineering and Geological Sciences,

156 Fitzpatrick Hall, University of Notre Dame,

Notre Dame, Indiana 46556, Center for Environmental

Science and Technology, 152A Fitzpatrick Hall,

University of Notre Dame, Notre Dame, Indiana 46556, and

Department of Biological Sciences, 403 Hubbard Hall, Box 26,

Wichita State University, Wichita, Kansas 67260

Automobile catalytic converters are dispersing platinumgroup elements (PGEs) Rh, Pt, and Pd into the environment

(1-3). This paper represents the first detailed study to

assess the PGE content of soils and grasses from U.S.

roadsides. These soils were analyzed using cation exchange

pretreatment and ultrasonic nebulizer-ICP-MS (4). Highway

and several urban sites showed Pt abundances of

64-73 ng/g immediately adjacent to the roadside, with

corresponding Pd and Rh abundances of 18-31 ng/g and

3-7 ng/g, respectively. All Pt and most Pd and Rh

abundances are statistically above local background soil

values. Platinum, Rd, and Rh show positive correlations with

traffic-related elements (Ni, Cu, Zn, and Pb) but no

correlations with nontraffic-related elements (Y, Ga).

Iridium and Ru show no correlations with any of these

trace elements. These PGE abundances are comparable

to European studies (5-7) and are approaching concentrations

that would be economically viable to recover. This study

also demonstrates transport of Pt statistically above

background more than 50 m from the roadside. Further

study is necessary to see how mobile the PGEs are in

roadside environments, but these initial data indicate only

Pt is taken up by plants.

Introduction

Catalytic converters have been in use on U.S. automobiles

since 1975 removing gaseous pollutants, such as CO,

hydrocarbons, and NOx, from exhaust by promoting chemical

reactions that change these pollutants into less toxic substances (i.e., CO2, N2, and H2O). Catalytic converters use an

alumina or cordierite monolith treated with an Al2O3washcoat

containing rare earth oxides and 0.10-0.15% (w/w) Pt, Pd,

and/or Rh to catalyze the removal of pollutants (8). The Clean

Air Act of 1990 required catalytic converters to be attached

to small gasoline engines such as those on lawn mowers,

chain saws, etc. However, it has become evident that attrition

of platinum group elements (PGEs) from the catalyst and its

subsequent release into the environment and deposition on

roadsides (e.g., ref 1) could be creating an unintended

problem (9).

The amount and rate of PGE release from catalytic converters is affected by the speed of the automobile, type of

engine, type and age of catalyst, and type of fuel additives.

A 1977 study (10) found that under highway driving conditions, Pt is lost at a rate of 0.19 μg/km, whereas a 1992 study

(11) measured levels that were 10-100 times lower. Both

studies concluded that higher speeds and catalyst temperatures enhanced attrition and emissions of PGEs. Most PGE

emissions from catalytic converters are in particulate form

(Pt > 95%, Pd > 85%, and Rh > 90%) and are dispersed into

the environment at rates of ng of PGE per kilometer per car

(11, 12). Examination of the particulates released from

catalytic converter materials has shown that ∼99% of Pt is

in the metallic state, with ∼1% present as oxidized Pt,

presumably Pt4+ (13, 14). The particulate Pt exists as a surface

oxidized metal nanoparticle that is attached to a larger

alumina (substrate) particle. However, the assumption that

PGEs are mostly released in metallic form has been challenged

by a recent study based on solubility experiments with

roadside PGEs (7), which concluded that the PGEs may not

be in a metallic form or that they have been rapidly altered

once they are deposited in the environment. Studies in

Germany demonstrated that Pt is accumulating in the upper

(0-4 cm) layers of the soil and is being immobilized,

presumably by humic acids or other organics, because natural

waters in the study area have low Pt concentrations (15).

This is consistent with laboratory studies of interactions of

Pt with natural organic matter analogues (16) as well as

environmental studies of Pt and Pd concentrations near ore

deposits (17) and roadsides (7), confirming that Pd is more

easily solubilized and transported than Pt.

Several studies have documented Pt uptake in plants, but

these were conducted using high and environmentally

unrealistic doses of Pt (18-22). Other studies have documented Pt contamination of roadside vegetation (23, 24),

and Pt has been shown to accumulate in Douglas fir twigs

(25). The roadside vegetation studies have suggested some

uptake of Pt into roots, leaves, and stems but not into the

fruit (24), although at least 50% of the Pt concentration on

roadside grass was adhering to the surface (23). Another study

(26) looked at transformations of Pt compounds in a humic

soil using tunnel dust and artificial Pt compounds at high

concentrations. They concluded that Pt binds to humic acid

and some of the tunnel dust Pt may be oxidized but reported

no Pt abundance data or associated error analysis for

comparison. A different study (27) looked at plant uptake

grown on soils contaminated with PGEs from catalytic

converters. However, detection limits range from 0.21 to 2.5

ng/g for Rh, Pd, and Pt and all Rh and Pd abundances shown

in the paper are all less than 2 ng/g. The proximity of the PGE

abundances (except Pt) in roadside soils to the detection

limits (usually low ng/g) in these studies (18-26) illustrates

the difficulties in using traditional techniques such as fire

assay or Te coprecipitation to analyze for PGEs, which is an

inherent problem in most of these published works (e.g., ref

7). The near detection limit PGE abundances, in combination

with the lack of tabulated PGE concentration data with 2

sigma errors, makes comparisons difficult and makes in-

* Corresponding author phone: (219)631-4308; fax: (219)631-9236;

e-mail: ely.1@nd.edu. † Department of Civil Engineering and Geological Sciences,

University of Notre Dame. ‡ Center for Environmental Science and Technology, University

of Notre Dame. § Wichita State University.

Environ. Sci. Technol. 2001, 35, 3816-3822

3816 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 19, 2001 10.1021/es001989s CCC: $20.00  2001 American Chemical Society

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terpretation of such data containing inherently large errors

(even if those errors are not explicitly quantified in the paper)

circumspect. The cation exchange-ultrasonic nebulization

(USN)-ICP-MS method developed at the University of Notre

Dame allows simultaneous determination of PGEs (Ir, Ru,

Rh, Pt, and Pd) in a sample with measured detection limits

in the fg/g (10-15 g/g or part per quadrillion) range (4, 28).

In particular, it makes possible the accurate analysis of a

local background sample, which is necessary to demonstrate

the amount of enrichment of roadside PGEs.

There have been several studies of environmental PGE

accumulation in the U.K., Sweden, and Germany (1, 5), and

these studies often have governmental cooperation. Since

1988, a German Cooperative Project has been sponsored by

the Ministry of Research and Technology to assess the impact

of “Emissions of Precious Metals” (18). A 1997 study in the

U.K. (29) highlighted that further work on the transport of

the platinum group metals was critical. This project resulted

in a database for the concentrations of Pt, Pd, and Rh in road

dust and soil adjacent to a major highway in the U.K. While

there have been recent studies of PGEs in harbor and coastal

sediments (30-32), only cursory roadside studies have been

conducted in the U.S. (2, 3). These were performed before

the complete conversion of automobiles to unleaded gasoline

and modern monolith catalytic converters (as opposed to

older pelletized catalysts) and were severely limited by the

high PGE detection limits of the analytical techniques used.

In recent years, more sensitive instrumentation with lower

detection limits has renewed interest in anthropogenic PGE

contamination of the environment.

This paper represents the first stage of an in-depth study

of PGE (Ir, Ru, Rh, Pt, and Pd) pollution in soils along U.S.

highways in relation to their potential adverse health effects

and possible economic exploitation. The study includes local

roads, interstates, and urban highways and has quantified

the PGE contents of roadside soils and grasses.

Experimental Section

Soil samples were collected from interstate highways, urban

roads, and side streets in and around South Bend, IN (33).

All soils collected were of the Tyner-Oshmeto soil association (34) and were sandy loams or loamy sands collected from flat slopes (0-2%). Three soils immediately adjacent to highways were also collected from Perth, Western

Australia and are included here only for comparative purposes (33) but were mostly unconsolidated bedrock. Soils

were collected by scraping up the surficial soil (0-1 cm

depth) with a plastic (HDPE) spatula in an area approximately 10 cm2 and stored in plastic sample bags. Soils from

the Indiana Toll Road (I-80/90, mile-marker 68) were

separated using HDPE sieves with polyester-mesh screen

sizes of 10 (2 mm), 18 (1 mm), 35 (0.5 mm), and 60 (0.25 mm)

and a receiver pan. On the basis of catalytic converter attrition

studies (13), the majority of PGE particles should be in the

finest fraction (i.e., <0.25 mm). Consequently, all “pan”

samples were analyzed, along with two coarser fractions from

the soil immediately adjacent to the road. Soils from Australia

and around the University of Notre Dame campus were

processed as bulk samples. A South Bend, IN, background

soil sample of the same type and general composition as the

other South Bend area soil samples was collected approximately 1000 m from the nearest road. No background

soil for the Australian samples was collected. All soil samples

were powdered in an alumina mill and dissolved following

the method of Ely et al. (4). Only grass leaves and shoots

were collected (no roots), with half of the sample being rinsed

with ethanol to wash off surface materials. All grass samples

were weighed, dried in an oven at 100 °C, reweighed, and

then powdered in an agate mortar and pestle for dissolution.

The cleaning of the agate mortar and pestle followed the

protocol described in ref 4.

Results and Discussion

Identification of elevated PGE contents of soils through

anthropogenic contributions requires that a natural background level be established as a reference point. PGE

abundances in soils are dependent on such factors as soil

composition, mineralogy, grain size, and underlying bedrock,

and these factors can be locally highly variable. Therefore,

a local background sample of the same soil type and general

composition as the roadside soils was used as a normalization

reference (Table 1). This background sample has PGE

abundances generally similar to literature values for upper

crust (35-37), which are provided in Table 1 for comparison.

However, each upper crust sample has slight differences in

different PGEs, highlighting the variability of, and therefore

the importance of, establishing a local background reference

soil composition. The PGE data demonstrate significant

abundances of Rh, Pt, and Pd above background in each of

the soil samples, especially those closest to the road (Table

1). On the basis of Pt/Rh, Pt/Pd, and Pd/Rh ratios, we can

demonstrate that catalytic converter attrition was responsible

for the elevated abundances (Figure 1A-C). The majority of

data fell within the field defined by catalytic converter

compositions (38), although the pan samples from the

Indiana Toll Road persistently plotted toward lower Rh and

Pd values for a given Pt concentration (Figure 1A,B). When

starting at the PGE abundances of the local background soil

(black cross in Figure 1A-C), these samples form a trend of

increasing PGE abundances that is parallel or subparallel to

the field formed by the PGE ratios of three way catalytic

converters. As PGE ratios in catalytic converters are variable

(38), it is likely that studies that draw conclusions from ratio

data alone to determine differential PGE mobility are

potentially flawed (e.g., ref 7).

The soils from less traveled roads (Douglas Road and

University Drive) near the University of Notre Dame campus

also exhibited higher abundances of Rh, Pt, and Pd relative

to Ir and Ru (Table 1). However, these Rh, Pt, and Pd

abundances were lower than those from the soil at 0-1 m

from IN-933 or the 0 m soil from the IN Toll Road,

demonstrating the positive correlation of Rh, Pt, and Pd

abundances with the traffic volume data (see ref 33). The

three Australian highway soils do not exhibit the same degree

of enrichment in Rh, Pt, and Pd as seen in the U.S. roadside

soils (Table 1; Figure 1A-C).

Both the washed and unwashed grass samples contained

PGE abundances within error of each other (Figure 1A-C;

Table 1). Platinum was the most abundant PGE in the grasses

ranging from 1.2 to 1.7 ng/g, but, compared to the analyzed

soils, the grasses contained the lowest PGE abundances.

PGE abundances in soils decreased away from road

surfaces (Figure 2). Two sampling traverses were conducted,

one perpendicular to the Indiana Toll Road (I-80/90; Figure

2A) and one perpendicular to IN-933 (US-31; Figure 2B) (33).

There was little change in Rh, Pt, and Pd abundance between

0 and 2 m from I-80/90 (Figure 2A). Both traverses demonstrated that 4-5 m from the road, Rh, Pt, and Pd

abundances decreased dramatically, by a factor of 2 along

I-80/90 (Figure 2A) and by a factor of 5 along IN-933 (Figure

2B). At a distance of 10 m from I-80/90 the abundances were

further reduced, but by a factor of 3-5 along I-80/90.

However, beyond 10 m, there was no further decrease in the

PGE content of the soil, but the abundances still are above

local background (Figure 2A; Table 1). Iridium and Ru, which

supposedly are not present in catalytic converters (but may

possibly be present as impurities; Ir may also be present in

some new diesel catalysts), were detectable in the soils at

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levels less than 1 ng/g and at relatively constant abundances

(within error) across the length of each traverse (Figure 2).

Normalization of the roadside soil PGE abundances to

the background soil produces profiles that are instructive for

examining relative enrichment above background as well as

differences between the samples (Figure 3A-D). All soil

samples generally contain Pd (except the Toll Road 10 m pan

soil) and Pt abundances statistically different from (i.e.,

higher) those of South Bend background soil abundances

(Table 1) at the 95% confidence level. Rhodium abundances

for all soils except the Toll Road 10 and 24 m pan soils were

also statistically higher than background soil concentrations

at the 95% confidence level. Somewhat surprisingly, Ir

abundances for all soil samples except the Toll Road 10 and

24 m, Douglas Road and University Drive soils were statistically higher than background soil abundances at the 95%

confidence level. Enrichments of Ir were 5-7 times above

background within 4 m of both the Toll Road and IN-933.

For Ru, only the Toll Road 0 m, 18 m, and the IN-933 0-1

m soils were statistically higher than background soil

abundances at the 95% confidence level. These enrichments

are also seen in the normalization plots.

In the Indiana Toll Road (I-80/90) soils, the finest fraction

contained the greatest PGE concentrations. Enrichments

were approximately 18 times above background for Pt, 12

times above background for Pd, and 40 times above

background for Rh. The coarser fractions of the I-80/90 soils

contained almost an order of magnitude lower abundances

of Pt, Pd, and Rh (enrichments of Pt, Pd, and Rh were 3-5

times, 3-4 times, and 12-15 times, respectively, see Figure

3A). Platinum and Pd appeared to be more widely dispersed

compared to Rh, consistent with experimental observations

and laboratory studies (15-17). All soils analyzed in this study

contained Pt and Pd abundances above the natural background, whereas Rh decreased to within error of background

levels at distances approximately 10 m from the road (Table

1; Figures 1, 2A, and 3A). This may reflect lower amounts of

Rh relative to Pt and Pd in catalytic converters and the fact

that Rh has only been used in catalytic converters since the

mid-1980s, while Pt and Pd have been used since 1975. This

is particularly relevant for understanding any potentially

adverse impact such dispersion could have.

The situation is similar with the traverse perpendicular

to IN-933 (US-31) (Figure 3B), although here bulk soils were

analyzed. In comparing the data from I-80/90, IN-933 (US31), Douglas Road, and University Drive there was a direct

correlation between Rh, Pt, and Pd contents of the roadside

(0 m) soils and the amount of traffic on these roads (33),

even though the PGEs in the I-80/90 soils have been

concentrated through sieving. This clearly demonstrates the

anthropogenic input of PGEs through the attrition of

automobile catalytic converters. Enrichments were even

higher than the Toll Road 0 m soil, with Pt, Pd, and Rh being

20 times, 20 times, and 74 times above background,

respectively. The Australian roadside soils also exhibit

enrichment of Rh, Pt, and Pd but to a lesser degree than the

South Bend soils (Pt, Pd, and Rh enrichments were on average

9 times, 9 times, and 17 times, respectively, compared to

average upper crust-see Figure 3D). However, interpretations

are limited because a local background soil was not collected,

so the average upper crust-normalized profiles only show

the general degree of enrichment. These profiles do illustrate

that Pt in the Australian soils is less enriched relative to Pd

and Rh than the South Bend soils, as the upper crust average

Pt value is lower than the South Bend background Pt

abundance.

Caution should be exercised in trying to compare PGE

roadside abundances from different regions because of the

generally poor quality of data in the literature as well as the

large number of variables affecting the production, accumulation, and subsequent transformations of PGEs in the

roadside environment. The quality of PGE data in the

TABLE 1: PGE Abundances for 16 Roadside Soils, 4 Roadside Grasses, Local Background, Reference Material UMT-1, and

Literature Upper Crust Estimates with Errors Given As 2 Sigma Valuesa

sample name

Ir

(ng/g)

2

sigma

Ru

(ng/g)

2

sigma

Rh

(ng/g)

2

sigma

Pt

(ng/g)

2

sigma

Pd

(ng/g)

2

sigma

IN Toll Road 0 m soil (pan) 0.43 0.07 0.59 0.11 3.35 0.25 64.44 7.55 18.52 0.88

IN Toll Road 2 m soil (pan) 0.34 0.11 0.57 0.11 3.85 0.33 66.13 6.31 18.24 1.46

IN Toll Road 4 m soil (pan) 0.14 0.04 0.38 0.13 1.54 0.21 35.46 1.99 9.76 2.03

IN Toll Road 10 m soil (pan) 0.20 0.11 0.64 0.44 0.24 0.11 12.27 1.42 2.08 0.90

IN Toll Road 18 m soil (pan) 0.41 0.11 0.93 0.27 0.32 0.14 17.65 0.92 4.62 1.03

IN Toll Road 24 m soil (pan) 0.17 0.13 0.41 0.16 0.16 0.08 11.15 1.86 3.72 0.81

IN Toll Road 0 m soil mesh 35 (18-35) 0.21 0.06 0.44 0.09 1.11 0.06 17.54 0.74 6.34 0.35

IN Toll Road 0 m soil mesh 60 (35-60) 0.21 0.07 0.24 0.05 1.38 0.09 11.26 1.00 4.62 0.46

IN-933 0-1 m 0.30 0.10 0.71 0.13 6.68 0.81 73.25 4.85 31.48 3.10

IN-933 3-4 m 0.39 0.08 0.28 0.08 1.75 0.21 16.95 1.11 6.61 0.34

IN-933 50 m 0.27 0.05 0.18 0.05 0.34 0.06 5.17 0.59 2.51 0.32

Douglas Road 0-1 m 0.21 0.15 0.44 0.08 6.20 0.60 37.35 9.87 13.16 1.32

University Drive 0-1 m 0.09 0.06 0.27 0.02 0.84 0.07 10.61 0.84 6.51 0.81

Australian (Tonkin Highway) 0.73 0.30 0.55 0.12 3.54 0.57 22.79 3.02 14.39 1.12

Australian (Leach Highway #1) 0.69 0.21 0.63 0.12 2.73 0.22 16.24 1.06 8.40 0.36

Australian (Leach Highway #2) 0.59 0.17 0.75 0.21 2.63 0.44 13.26 1.14 10.32 1.23

IN Toll Road 0 m washed grass 0.07 0.04 0.22 0.05 0.10 0.02 1.23 0.27 1.03 0.23

IN Toll Road 18 m washed grass 0.09 0.04 0.20 0.05 0.10 0.03 1.69 0.19 0.97 0.37

IN Toll Road 0 m unwashed grass 0.07 0.05 0.16 0.02 0.11 0.04 1.73 0.44 1.29 0.25

IN Toll Road 18 m unwashed grass 0.06 0.04 0.23 0.04 0.12 0.05 1.41 0.68 1.44 0.30

UMT-1 average (n ) 7) 7.50 1.10 10.03 0.98 9.18 0.96 119.4 12.8 97.8 10.7

UMT-1 certified values 8.8 0.6 10.9 1.5 9.5 1.5 129 5 106 3

upper crust (37) 0.025 0.407 0.39 0.653

upper crust (38) 0.03 1.06 0.38 2.0

upper crust (39) 0.05 0.10 0.06 0.40

South Bend, IN background soil 0.06 0.04 0.43 0.13 0.09 0.03 3.63 0.82 1.54 0.41

average upper crust (refs 37-39 + S.B. bkgd) 0.04 0.50 0.18 2.01 1.15

a 10-200 g samples were rehomogenized to minimize the “nugget effect”. Aliquots of ∼250 mg were dissolved and analyzed. Determination

limits (10 sigma) for this technique are given in ref 32 and are in the 0.07-3 pg/g range. A VG Elemental PQII STE quadrupole ICP-MS was used

for analysis. Two sigma errors were based on standard addition regression error with details of error calculations in ref 44.

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literature has been commented upon (39, 40) and discussed

above and is most often affected by factors such as

compositional sample heterogeneities (“nugget effect”), the

lack of errors provided with the data, and the insufficient use

of direct methods of reproducibility to assess the precision

of the data (39).

While the same general group of auto manufacturers

supply Australia as well as Europe, Japan, and the U.S., there

are marked differences in roadside Pt, Pd, and Rh enrichments, as demonstrated by the European studies (1). These

differences may be due to the different timing of mandatory

catalytic converter use by individual countries in conjunction

with other factors (e.g., variable composition of each country’s

car population by automaker and car model, age of catalytic

converter, speed of car, volume of traffic, climate and rainfall,

soil type, etc.). Furthermore, necessary information about

how the PGEs transform in the roadside environment, how

mobile they are, or even the bioavailability and toxicity of

PGEs from catalytic converters are only now beginning to be

investigated. Note that the apparent Ir enrichment in the

Australian soils (Figure 3D) is a function of the different

normalization used in this diagram (see Table 1). Further

study is necessary to determine the precise causes for the

lower Pt, Pd, and Rh abundances in Australia compared to

published studies in Europe and this U.S. study.

Certain trace metals (e.g., Ni, Cu, Zn, and Pb) have been

used to evaluate the contribution of traffic to the surrounding

urban environment (41-43). As an illustration, Pt, Pd, and

Rh and Ir abundances from IN-933 were plotted against Ni,

Cu, Zn, and Pb to see if any correlation was present (Figure

4A-D). The PGEs were also plotted against the lithophile

elements Y and Ga, for which there should be no correlation,

as these elements should be constant in all the soils as there

is no documented anthropogenic input of Y and Ga. As can

be seen in Figure 4A-C, Pt, Pd, and Rh all show positive

correlations with Ni, Cu, Zn, and Pb. Note that the PGE

abundances are plotted increasing from right to left, as the

distance from the road is shown from left to right (actual

distance data is in Table 1 and Figure 2B). Also note that the

two sigma errors for the PGEs are for clarity only shown on

the Ni data points, but are the same for all trace element data

shown for each given PGE. These positive correlations of Pt,

Pd, and Rh with Ni, Cu, Zn, and Pb further demonstrate that

all of these metals are anthropogenic inputs to the roadside

from automotive traffic. Also note that the Ni, Cu, Zn, and

Pb background soil abundances are much lower than even

the IN-933 50 m soil abundances, indicating substantial

transport of these elements as well as the PGEs. There are

no correlations of Pt, Pd, and Rh with Y and Ga (i.e., they are

flat), and all Y and Ga abundances in the IN-933 soils are

within error of those in the South Bend background soil.

FIGURE 1. Element-element plots of Rh, Pd, and Pt abundances

in soil and grass samples. Errors shown are 2 sigma and if not

visible are within the size of the symbol. A: log-log plot of Rh vs

Pt abundances. Abbreviations in the legend are as follows: T.R.-

Toll Road; S.B.-South Bend, IN. IN Toll Road soils (pan) were sampled

at 0, 2, 4, 10, 18, and 24 m and IN Toll Road 0 m soils mesh sizes

shown are 35 (0.5-1 mm diameter) and 60 (0.25-0.5 mm diameter).

IN-933 soils were sampled at 0-1, 3-4, and 50 m. The gray field

represents three-way (Pt-Pd-Rh) catalytic converter (TWC) ratios

(36). B: log-log plot of Pd vs Pt abundances. Symbols and field

are as in A. C: log-log plot of Rh vs Pd abundances. Symbols and

field are the same as in A.

FIGURE 2. Perpendicular PGE profiles away from the roads. A.

Indiana Toll Road, with PGE abundances given in ng/g (vertical log

scale) and pan soil samples at 0, 2, 4, 10, 18, and 24 m. Errors on

the points are 2 sigma and are within the size of the symbol if not

visible. Note that Pt, Pd, and Rh are significantly elevated near the

Toll Road, while Ir and Ru are always below 1 ng/g. When compared

with the background soil sample, Ir abundances in all Toll Road

samples are elevated above the background, while Ru abundances

in all samples are similar to the background abundance. B. IN-933

with PGE abundances given in ng/g (vertical log scale) and bulk

soil samples at 0-1, 3-4, and 50 m. Symbols are the same as in

A. Errors on the points are 2 sigma and are within the size of the

symbol if not visible.

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Iridium was also plotted against Ni, Cu, Zn, and Pb but

showed no consistent positive correlations (Figure 4D). Note

the much smaller Ir abundance scale compared to Figures

4A-C and the fact that all of the Ir abundances shown are

within error of each other (note again that the two sigma

errors are shown only on the Ni data points for clarity, but

are the same on Ir for all trace elements shown). While it was

noted earlier that Ir abundances are elevated above local

background (Figure 3B, Table 1), the geochemical data do

not confirm traffic as the source. However, the elevated Ir

FIGURE 3. Soil and grass PGE data normalized to local background soil sample for South Bend, IN samples and average upper crust for

Australian samples. Two sigma errors for all the samples were omitted for the sake of clarity (see Table 1 for all errors). A: Indiana Toll

Road (I-80/90) soils. The dashed lines represent soil fractions other than those of the sieve pan. B: Soils taken from roads around the

Notre Dame campus. All analyses were conducted on the bulk soil. C: Indiana Toll Road (I-80/90) grass samples. “Washed” indicates

that the grass was washed with ethanol to remove the outer cuticle and any PGEs adhering to the surface. D: Soils from Australian

highways around Perth, Western Australia. Note that the Australian soils are normalized to average upper crust as a local background

(i.e., uncontaminated) soil was not collected in this area.

FIGURE 4. Soil PGE data plotted against traffic-related (Ni, Cu, Zn, and Pb) and lithophile (Y, Ga) trace element data for samples from

IN-933. Local background soil PGE and trace element abundances are also plotted as points for comparison. A: Pt. Two sigma errors

are shown for Pt on the Ni data points and are the same for all other series but were omitted for clarity. Trace element errors and errors

not visible are within the size of the symbol. B: Pd. Symbols and errors are the same as in A. C: Rh. Symbols and errors are the same

as in A. D: Ir. Symbols and errors are the same as in A. Note the lack of a positive correlation for any of the automotive-related trace

elements (Ni, Cu, Zn, and Pb) and that all Ir abundances are within error of each other.

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abundances in the soil are most likely being deposited along

with Pt, Pd, and Rh as Ir is probably present as an impurity

in automotive Pt-Pd-Rh catalysts and also is being used in

some new diesel catalysts with Pt (8). Ruthenium (not shown)

also demonstrated no correlations with any of the trace

elements, and abundances were within error of that in the

local background soil.

The results of the I-80/90 grass analyses demonstrate that

Pt was taken up by the grass (Table 1, Figures 3C). However,

the Pt abundances in all grass samples were not statistically

different from those of the local background soil (Table 1)

at the 95% confidence level. There was also no difference

between the Pt abundances of the washed and unwashed

grasses, demonstrating that Pt adhering to the surface of the

grass leaves and stems was insignificant and that the Pt was

incorporated into the grass tissue. Previous work (23, 24)

had shown that Pt was taken up by roadside vegetation and

that a positive correlation of Pt abundances and volume of

traffic was documented. Other studies contradicted these

conclusions (44, 45), stating “the bioavailability of catalyst

emitted Pt is remarkably low”. Based on our preliminary

data, there is uptake of Pt by grass. It also appears there is

no correlation between the Pt abundances in the grass and

the amount of Pt deposited in the roadside soils. This may

be a function of the amount of Pt the grass can incorporate

into its structure. Further study is required to conclude

whether there is a correlation with traffic volume on U.S.

roads, especially as the previous studies that showed such

a correlation (23, 24) may have had another source of Pt

present (i.e., industrial). Clearly future work is needed and

will clarify whether the correlation with volume of traffic can

be reproduced, what local background grass PGE abundance

is (i.e., grass grown on uncontaminated soil), and whether

this Pt uptake poses any health risks.

A positive impact of increased usage of catalytic converters

in automobiles is that roadside soils may become an

economic resource for the PGEs. For example, 100 ng/g

translates to 0.1 g per metric ton. The only operating Pt mine

in the U.S., the Stillwater Mining Company of Montana,

processed 1.6 million metric tons of ore in the year 2000,

producing 12 190 kg of Pt+Pd at a cash cost of $9300 per kg

and resulting in an economically viable ore grade of 7.47 g

per metric ton (46). If the roadside soil 0-1 m from the

roadway and 1 cm deep were collected from all the urban

and interstate roadways in Indiana (34 000 km, which is 23%

of the total road mileage in Indiana), and assuming an average

Pt concentration in the soil of 100 ng/g and Pd concentration

of 50 ng/g, then 510 000 metric tons of soil would be

processed, producing 76 kg of Pt+Pd for an ore grade of 0.15

g per metric ton. While this ore grade is lower, the cash cost

of collecting and processing the soil would be considerably

reduced. The roadside soil is easily accessible and simpler

and more environmentally friendly to process, compared to

mining PGE ore from 1.6 km depth and smelting and

processing that ore. As PGE roadside abundances will

continue to accumulate (e.g., a German study (5) found Pt

values of 1100 ng/g in roadside dust from Stuttgart and a

recent British study (7) found Pt abundances of greater than

500 ng/g in road dust and surface samples), this recycling of

PGEs from roadside soils could become economically viable

and merits further research. Finally, if the PGEs were recycled,

any potentially adverse effect to human health would be

negated.

In summary, it is evident that attrition of automobile

catalytic converters is introducing an anthropogenic contribution of Rh, Pt, and Pd into roadside soils. The data

presented here shows that Pt and Pd were being transported

relatively large distances (up to 50 m) from the road. Because

of vegetation between the IN-933 50 m soil site and IN-933

as well as the level terrain, it appears that the transport

mechanism for the PGEs to this site is airborne. This is

consistent with a recent study from Germany (47) where

increased Pt and Rh abundances were detected in airborne

samples collected at distances of 150 m from the road.

Furthermore, the data also show that at least Pt was being

incorporated into plants. With agricultural crops bordering

much of the Indiana Toll Road, it remains to be seen how

Pt is being incorporated into these foodstuffs; our study

demonstrated the potential is there, and we caution that the

toxic effects of Pt (+ Rh and Pd) should not be underestimated.

While catalytic converters are essential in reducing smogcausing automobile emissions, it is essential that we do not

replace one pollutant with another. A solution to what is a

potential problem also has an economic advantage. The

roadside soils, especially in urban areas, would be nearly

enriched enough to make them economically viable as

sources of Pt and possibly Rh and Pd. If such exploitation

were conducted, the PGEs would be removed from the

environment before they could enter the food chain.

Acknowledgments

This research was funded by a seed grant from the Center

for Environmental Science & Technology at the University

of Notre Dame. J.C.J. was supported under NSF grant EAR93-

15930, and the ICP-MS laboratory was supported by the

University of Notre Dame and NSF grant ECS92-14596 to

C.R.N. Three anonymous reviewers and especially B. PeuckerEhrenbrink are thanked for reviewing an early version of this

manuscript.

Literature Cited

(1) Anthropogenic Platinum-Group Element Emissions; Zereini, F.,

Alt, F., Eds.; Springer: Berlin, Germany, 2000.

(2) Medical and Biologic Effects of Environmental Pollutants:

Platinum-Group Metals; National Academy of Sciences: Washington, DC, 1977.

(3) Hodge, V.; Stallard, M. Environ. Sci. Technol. 1986, 20, 1058-

1060.

(4) Ely, J.; Neal, C.; O’Neill, J., Jr.; Jain, J. Chem. Geol. 1999, 157,

219-234.

(5) Scha ̈fer, J.; Eckhardt, J.; Berner, Z.; Stu ̈ben, D. Environ. Sci.

Technol. 1999, 33, 3166-3170.

(6) Farago, M., et al. Fresenius J. Anal. Chem. 1996, 354, 660-663.

(7) Jarvis, K.; Parry, S.; Piper, J. Environ. Sci. Technol. 2001, 35,

1031-1036.

(8) Palacios, M.; Moldovan, M.; Gomez, M. In Anthropogenic

Platinum-Group Element Emissions; Zereini, F., Alt, F., Eds.;

Springer: Berlin, Germany, 2000; pp 3-14.

(9) Barefoot, R. Environ. Sci. Technol. 1997, 31, 309-314.

(10) Hill, R.; Mayer, W. IEEE Trans. Nucl. Sci. 1977, NS-24, 2549-

2554.

(11) Ko ̈nig, H.; Hertel, R.; Koch, W.; Rosner, G. Atmos. Environ. 1992,

26A, 741-745.

(12) Moldovan, M.; Gomez, M.; Palacios, M. J. Anal. At. Spectrom.

1999, 14, 1163-1169.

(13) Artelt, S.; Levsen, K.; Ko ̈nig, H.; Rosner, G. In Anthropogenic

Platinum-Group Element Emissions; Zereini, F., Alt, F., Eds.;

Springer: Berlin, Germany, 2000; pp 33-44.

(14) Schlo ̈gl, R.; Indlekofer, G.; Oelhafen, P. Angew. Chem., Int. Ed.

Engl. 1987, 26, 309-319.

(15) Zereini, F.; et al. In Anthropogenic Platinum-Group Element

Emissions; Zereini, F., Alt, F., Eds.; Springer: Berlin, Germany,

2000; pp 73-83.

(16) Wood, S. Can. Mineral. 1990, 28, 665-673.

(17) Wood, S.; Vlassopoulos, D. Can. Mineral. 1990, 28, 649-663.

(18) Wegscheider, W.; Zischka, M. Fresenius J. Anal. Chem. 1993,

346, 525-529.

(19) Lee, M.; To ̈lg, G.; Beinrohr, E.; Tscho ̈pel, P. Anal. Chim. Acta

1993, 272, 193-203.

(20) Messerschmidt, J.; Alt, F.; To ̈lg, G. Anal. Chim. Acta 1994, 291,

161-167.

(21) Messerschmidt, J.; Alt, F.; To ̈lg, G. Electrophoresis 1995, 16, 800-

803.

(22) Parent, M.; Vanhoe, H.; Moens, L.; Dams, R. Fresenius J. Anal.

Chem. 1996, 354, 664-667.

VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3821

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 Publication Date (Web): August 24, 2001 | doi: 10.1021/es001989s
(23) Dietl, C.; Laschka, D.; Wa ̈ber, M.; Peichl, L. In Anthropogenic

Platinum-Group Element Emissions; Zereini, F., Alt, F., Eds.;

Springer: Berlin, Germany, 2000; pp 65-71.

(24) Eschnauer, H. In Anthropogenic Platinum-Group Element

Emissions; Zereini, F., Alt, F., Eds.; Springer: Berlin, Germany,

2000; pp 115-120.

(25) Riese, W.; Arp, G. In Mineral Exploration: Biological Systems

and Organic Matter; Carlisle, D., et al., Eds.; Prentice Hall: New

York, 1986; pp 170-182.

(26) Lustig, S.; Zang, S.; Michalke, B.; Schramel, P.; Beck, W. Sci.

Total Environ. 1996, 188, 195-204.

(27) Scha ̈fer, J.; Hannker, D.; Eckhardt, J.; Stu ̈ben, D. Sci. Total

Environ. 1998, 215, 59-67.

(28) The low detection limits of the analytical method allow low

abundance isotopes (e.g., 191Ir and 198Pt) that are free of

polyatomic interferences (such as HfO) to be quantified. This

makes mathematical correction of the polyatomic interferences

unnecessary, as this procedure has been found by the authors

to be somewhat suspect when PGE concentrations are in the

low ng/g range and Hf concentrations are 1-10 μg/g, which is

common in soil samples.

(29) Platinum Group Metals; Environ. Chem. Grp. Newslett. 6, Royal

Society of Chemistry: London, 1997.

(30) Esser, B.; Turekian, K. Environ. Sci. Technol. 1993, 27, 2719-

2724.

(31) Tuit, C.; Ravizza, G.; Bothner, M. Environ. Sci. Technol. 2000,

34, 927-932.

(32) Helz, G.; Adelson, J.; Miller, C.; Cornwell, J.; Hill, J.; Horan, M.;

Walker, R. Environ. Sci. Technol. 2000, 34, 2528-2534.

(33) Soil samples were collected at mile marker 68 along I-80/90,

the Indiana Toll Road, at 0, 2, 4, 10, 18, and 24 m from the road,

and grass samples were taken at 0 and 18 m. Only soil samples

were collected from a busy four-lane state (previously federal)

road running through South Bend (IN-933-formerly US-31 when

samples were collected) at 0, 3, and 50 m from the road. Soils

immediately adjacent to the road were taken from a mediumtraveled two-lane road (Douglas Road) bounding the University

of Notre Dame, a low-traveled two-lane road (University Drive),

and from two four-lane highways, one rural (Tonkin) and one

urban (Leach), in Perth, Western Australia. Annual average traffic

data, in vehicles/day, was available for the following roads in

this study: I-80/90 Toll Road (24 350), IN-933 (38 000), and

Douglas Road (8000). Data are from the Indiana Department

of Transportation, the Indiana Toll Road Authority, and the St.

Joseph County Department of Transportation (personal communications, 2000).

(34) Soil Survey of St. Joseph Co., IN, USDA Soil Conservation Service,

1977.

(35) Peucker-Ehrenbrink, B.; Jahn, B. abstract presented at Ninth

Annual V. Goldschmidt Conference, Abstract #7320. LPI Contribution No. 971, Lunar and Planetary Institute: Houston, 1999.

(36) Schmidt, G.; Palme, H.; Kratz, K. Geochim. Cosmochim. Acta

1997, 61, 2977-2987.

(37) Wedepohl, K. Geochim. Cosmochim. Acta 1995, 59, 1217-1232.

(38) Auto manufacturers do not publish the ratios of Rh, Pd, and Pt

in their catalytic converters and are constantly changing them

to optimize performance. In addition, ratios vary between cars

manufactured in the U.S., Europe, and Japan and even from

cars from the same production line (8). Therefore, a field

represents three-way catalytic converters (TWC) in Figure 1.

These fields are based on estimates of Pt:Pd:Rh ratios of catalytic

converters reported in refs 1, 6, 12, and 48, which are Pt:Pd

(1-2.5), Pd:Rh (4-9), and Pt:Rh (5-16).

(39) McDonald, I. Geostand. Newslett. 1998, 22, 85-91.

(40) Ely, J.; Neal, C. Geostand. Newslett. 2001, in press.

(41) DeMiguel, E.; Llamas, J.; Chacon, E.; Mazadiego, L. Sci. Total

Environ. 1999, 235, 355-357.

(42) Hares, R.; Ward, N. Sci. Total Environ. 1999, 235, 169-178.

(43) Barbosa, A.; Hvitved-Jacobsen, T. Sci. Total Environ. 1999, 235,

151-159.

(44) Lustig, S.; Schramel, P. In Anthropogenic Platinum-Group

Element Emissions; Zereini, F., Alt, F., Eds.; Springer: Berlin,

Germany, 2000; pp 95-104.

(45) Lustig, S.; Zang, S.; Michalke, B.; Schramel, P.; Beck, W. Fresenius

J. Anal. Chem. 1997, 357, 1157-1163.

(46) Stillwater Mining Company website, 2001, http://www.

stillwatermining.com/.

(47) Zereini, F.; Wiseman, C.; Alt, F.; Messerschmidt, J.; Mu ̈ller, J.;

Urban, H. Environ. Sci. Technol. 2001, 35, 1996-2000.

(48) Gaita, R.; Al-Bazi, S. Talanta 1995, 42, 249-255.

Received for review December 19, 2000. Revised manuscript

received June 28, 2001. Accepted July 5, 2001.

ES001989S

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