Spatial distribution and speciation of lead around corroding bullets in a shooting range soil studied by micro-X-ray fluorescence and absorption spectroscopy.
ABSTRACT We investigated the spatial distribution and speciation of Pb in the weathering crust and soil surrounding corroding metallic Pb bullets in a shooting range soil. The soil had a neutral pH, loamy texture, and was highly contaminated with Pb, with total Pb concentrations in the surface soil up to 68 000 mg kg(-1). Undisturbed soil samples containing corroding bullets were collected and embedded in resin, and polished sections were prepared for micro-X-ray fluorescence (micro-XRF) elemental mapping and micro-X-ray absorption near edge structure (micro-XANES) spectroscopy. Bullet weathering crust material was separated from the metallic Pb cores and analyzed by powder X-ray diffraction analysis. Our results show a steep decrease in total Pb concentrations from the bullet weathering crust into the surrounding soil matrix. The weathering crust consisted of a mixture of litharge [alpha-PbO], hydrocerussite [Pb3(CO3)2-(OH)2], and cerussite [PbCO3], with litharge dominating near the metallic Pb core and cerussite dominating in the outer crust, which is in contact with the soil matrix. On the basis of these results and thermodynamic considerations, we propose that the transition of Pb species after oxidation of Pb(O) to Pb(II) follows the sequence litharge --> hydrocerussite --> cerussite. Consequently, the solubility of cerussite limits the activity of Pb2+ in the soil solution in contact with weathering bullets to < or =1.28 x 10(-6) at pH 7, assuming that the CO2 partial pressure (PCO2) in the soil is equal or larger than in the atmosphere (PCO2 > or = 0.000 35 atm).
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ABSTRACT: This short communication documents chemical transformations caused by weathering of two Pb compounds that commonly occur in house dust. Chamber experiments were designed to simulate humid indoor environment conditions to determine whether Pb compounds undergo chemical transformations influencing bioaccessibility. Reference compounds of Pb metal (12 % bioaccessibility) and Pb sulfate (14 % bioaccessibility) were subjected to an oxygenated, humidified atmosphere in closed chambers for 4 months. X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) spectroscopy were used to characterize the main Pb species, and the change in Pb bioaccessibility was determined using a simulated gastric acid digestion. At the end of the weathering period a small amount of Pb carbonate (9 % of total Pb) appeared in the Pb sulfate sample. Weathering of the Pb metal sample resulted in the formation of two compounds, hydrocerussite (Pb hydroxyl carbonate) and Pb oxide, in significant amounts (each accounted for 26 % of total Pb). The formation of highly bioaccessible Pb carbonate (73 % bioaccessibility), hydrocerussite (76 % bioaccessibility), and Pb oxide (67 % bioaccessibility) during weathering resulted in a measurable increase in the overall Pb bioaccessibility of both samples, which was significant (p = .002) in the case of the Pb metal sample. This study demonstrates that Pb compounds commonly found in indoor dust can 'age' into more bioaccessible forms under humid indoor conditions.Environmental Geochemistry and Health 06/2013; 35:153-159. · 2.08 Impact Factor
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ABSTRACT: Heterogeneous chemistry of nitrogen dioxide with lead-containing particles is investigated to better understand lead metal mobilization in the environment. In particular, PbO particles, a model lead-containing compound due to its wide spread presence as a component of lead paint and as naturally occurring minerals, massicot and litharge, are exposed to nitrogen dioxide at different relative humidity. X-ray photoelectron spectroscopy (XPS) shows that upon exposure to nitrogen dioxide the surface of PbO particles react to form adsorbed nitrates and lead nitrate thin films with the extent of formation of nitrate relative humidity dependent. Surface adsorbed nitrate increases the amount of dissolved lead. These reacted particles are found to have an increase in the amount of lead that dissolves in aqueous suspensions at circumneutral pH compared to unreacted particles. These results point to the potential importance and impact that heterogeneous chemistry with trace atmospheric gases can have on increasing solubility and therefore the mobilization of heavy metals, such as lead, in the environment. This study also show that surface intermediates, such as adsorbed nitrates, that form can yield higher concentrations of lead in water systems. In the environment, these water systems can include drinking water, ground water, estuaries and lakes.Environmental Science & Technology 10/2012; · 5.26 Impact Factor
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ABSTRACT: Canadian military small arms firing range (SAFR) backstop berm soils contain high concentrations of Pb, Cu, Sb, and Zn. Three Teflon® cylindrical lysimeters were installed in the vadose zone of the backstop berm in order to characterize the pore water quality. Copper-Zn concentrations were below their respective Québec drinking water thresholds, and Pb-Sb concentrations were above the criteria in many cases indicating that a metal vertical migration occurs in the vadose zone. Metal concentrations in pore water were, in decreasing order: Zn>Pb>Sb>Cu. Calculated annual metal leaching rates showed that a rather small portion of the metals in the soil were mobilised in the vadose zone, and that metals were a potential source of contamination for extremely long periods of time, highlighting the importance of finding ways to remediate such soils in order to limit metal mobility in SAFR backstop berms.Soil and Sediment Contamination 01/2012; · 0.51 Impact Factor
Spatial Distribution and Speciation
of Lead around Corroding Bullets in
a Shooting Range Soil Studied by
Micro-X-ray Fluorescence and
D E L P H I N E V A N T E L O N ,† , ‡
A N T O N I O L A N Z I R O T T I ,§
A N D R E A S C . S C H E I N O S T ,† , |A N D
R U B E N K R E T Z S C H M A R *, †
Institute of Terrestrial Ecology, Swiss Federal Institute of
Technology (ETH) Zurich, CH-8952 Schlieren, Switzerland,
and The University of Chicago, Consortium for Advanced
Radiation Sources, Chicago, Illinois
We investigated the spatial distribution and speciation of
Pb in the weathering crust and soil surrounding corroding
metallic Pb bullets in a shooting range soil. The soil had
a neutral pH, loamy texture, and was highly contaminated
with Pb, with total Pb concentrations in the surface soil
up to 68 000 mg kg-1. Undisturbed soil samples containing
corroding bullets were collected and embedded in resin,
and polished sections were prepared for micro-X-ray
fluorescence (µ-XRF) elemental mapping and micro-X-ray
absorption near edge structure (µ-XANES) spectroscopy.
Bullet weathering crust material was separated from the
metallic Pb cores and analyzed by powder X-ray diffraction
analysis. Our results show a steep decrease in total Pb
concentrations from the bullet weathering crust into the
surrounding soil matrix. The weathering crust consisted of
a mixture of litharge [R-PbO], hydrocerussite [Pb3(CO3)2-
(OH)2], and cerussite [PbCO3], with litharge dominating near
the metallic Pb core and cerussite dominating in the
outer crust, which is in contact with the soil matrix. On
we propose that the transition of Pb species after
oxidation of Pb(0) to Pb(II) follows the sequence litharge
f hydrocerussite f cerussite. Consequently, the solubility
of cerussite limits the activity of Pb2+in the soil solution
in contact with weathering bullets to e1.28 × 10-6at pH 7,
assuming that the CO2partial pressure (PCO2) in the soil
is equal or larger than in the atmosphere (PCO2g 0.000 35
greatest risk emanates from direct ingestion of bullets,
contaminated soil or plants, and Pb-rich dust (2-6). Lead
contamination of soils may stem from various sources,
of spent batteries, waste incineration, and traffic (7, 8). In
significant source of Pb pollution. Annual Pb deposition by
hunting and recreational shooting varies between 200 and
6000 t in The Netherlands, Denmark, Canada, and England
has more than 2000 community shooting ranges used for
mandatory shooting exercises of Swiss Army personnel and
for recreational shooting. An estimated 90 million bullets
are fired each year, resulting in an annual emission of 400-
500 t of Pb (12). On many shooting ranges, the stop butt
consists of a natural slope or of an artificial mound made of
soil. Consequently, soils at and behind the stop butts are
often highly contaminated, with maximum soil Pb concen-
trations ranging up to 150 000 mg kg-1(6, 9, 13-17). For
comparison, the average natural background concentration
of Pb in soils is typically on the order of 10-30 mg kg-1(1).
The environmental impact of soil contamination around
Pb bullets were regarded as rather inert. However, once a
bullet has penetrated the soil, the surface of the metallic
Pb(0) core is slowly oxidized to Pb(II). The time required for
estimated to range from 30 to 200 years, depending on soil
adsorbed by roots, soil organic matter, clays, calcite, or
manganese and iron oxides (1, 6, 8). As long as corroding
surrounding the corroding bullets and are an important
source of bioavailable Pb. It is therefore important to
understand the mineralogical composition and spatial
distribution of Pb species around corroding Pb bullets in
species formed during metallic lead corrosion in soils by
X-ray diffraction analysis. The most abundant secondary Pb
phases were found to be hydrocerussite [Pb3(CO3)2(OH)2]
and cerussite [PbCO3] (9, 15, 19-24). In addition, massicot
[?-PbO] has been reported to occur in weathering shotgun
bullets (15, 21, 22) and litharge [R-PbO] in antique artifacts
and weathered lead pipes (23, 24). Small amounts of
[Pb10(PO4)6(OH)2] may also occur, depending on the soil
chemical conditions. However, powder X-ray diffraction
analysis may not detect amorphous or microcrystalline Pb
phases and yields no direct information on the spatial
(EMPA), Lin (15) observed weathering crusts around bullets
from the bullet core to the outer part of the crust. However,
this technique does not allow a direct and spatially resolved
apply a combination of micro-X-ray fluorescence (µ-XRF)
and micro-X-ray absorption near-edge structure (µ-XANES)
spectroscopy to investigate the spatial distribution and the
speciation of Pb around corroding Pb bullets in shooting
Materials and Methods
Soil Samples. Soil samples were collected at a communal
300-m shooting range at Oberuzwil (47°25′51′′ N, 9°7′12′′ E)
in the canton of St. Gallen, Switzerland. The shooting range
* Corresponding author phone: +41 44 6336003; fax: +41 44
6331118; e-mail: email@example.com.
5232 Villigen. E-mail: firstname.lastname@example.org.
§The University of Chicago.
|Present address: The Rossendorf Beamline, European Synchro-
Environ. Sci. Technol. 2005, 39, 4808-4815
48089ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 13, 2005 10.1021/es0482740 CCC: $30.25
2005 American Chemical Society
Published on Web 05/21/2005
has been in operation for about 90 years. The shooting
practice at the site consists primarily of rifle training with
two calibers of bullets, 6.5 mm (GP11) and 7.5 mm (GW Pat
90). These bullets consist of a metallic core of Pb hardened
with 2-4% Sb, encased in a cupronickel steel jacket. The
stop butt is located within a natural slope. The immediate
impact area is covered with a thin layer (1-2 cm) of sawdust
To characterize the magnitude and distribution of Pb
the stop butt area to 23 m behind the targets. To allow a
more detailed sampling and the collection of undisturbed
soil cores, a soil pit was opened 9 m behind the target, in the
no distinct soil horizons, except a higher density of roots in
the top 15 cm. No hydromorphic soil properties were
most parts of the year. Corroding bullets were observed in
soil depths up to 90 cm. While these bullets most likely
mixing of soil and bullets during the periodic renewal of the
stop butt cannot be excluded. However, the exact history of
reconstruction of the stop butt is unknown.
From the opened profile, in addition to collecting bullets
and disturbed soil for bulk chemical and mineralogical
analyses, we collected undisturbed soil samples, at depths
of 25 cm (sample A) and 55 cm (sample B), for the
in custom-made 8 × 6 × 5 cm3aluminum boxes, dried at 40
°C, and embedded in resin (LR White, SPI Supplies, West
Chester, PA) at 60 °C under vacuum during 24 h. Each
µm thick, by use of a diamond saw; the sections were then
polished with grit 500-5000 sandpaper (Accutom-50 and
Labopol-5, Struers, Birmensdorf, Switzerland). Based on
visual inspection of all sections, one representative section
per box was selected for synchrotron µ-XRF and µ-XANES
Soil Analysis. Soil pH was measured with a combination
pH electrode (Metrohm, Herisau, Switzerland) after 10 g of
dry soil was equilibrated with with 25 mL of 0.01 M CaCl2
solution for 30 min (25). Exchangeable cations were deter-
mined by extracting 2 g of soil with 20 mL of 1 M NH4NO3
solution for 24 h on a rotary shaker (26). The suspensions
were then centrifuged for 15 min at 1500 g and the
supernatant solutions were analyzed for Mg, Ca, Na, and K
by atomic absorption spectrometry (Spectra 220FS, Varian,
Palo Alto, CA). Selective sequential extractions (SSE) of the
soil were performed following the seven-step procedure
developed by Zeien and Bru ¨mmer (26). For total elemental
analysis, the soils were ground to <60 µm in a vibratory disk
mill (Retsch, Haan, Germany). Total C and N contents were
measured on a CHNS analyzer (CHNS-932, Leco, St. Joseph,
(Na to U) were determined by X-ray fluorescence spectrom-
thoroughly mixed with a binding component (C-Wachs,
Hoechst) at a fixed soil/wax ratio of 40/9 and then analyzed
on an energy-dispersive XRF spectrometer (X-Lab 2000,
a series of five secondary targets, and an ab initio method
to improve the deconvolution of spectra, a lower detection
limit of 0.5 mg kg-1could be achieved for most elements.
Accuracy of the XRF analysis was routinely verified by use
of a certified standard sample (D133, MCACAL).
in the soil pit excavated at 9 m behind the targets were
physically deformed and strongly corroded, as indicated by
a crust of brown and gray material. To analyze the corrosion
products in this crust, six bullets were placed in a beaker,
submerged in deionized water, and ultrasonicated at 300 W
until no further crust material was removed. The resulting
The material was then analyzed by powder X-ray diffraction
(Scintag XRD-2000, Cupertino, CA) analysis with Cu KR
radiation (wavelength 1.5405 Å). The powder mounts were
scanned over a Bragg angle range 2-70° 2θ with a step size
assigned by use of the Scintag DMSNT program (ICDD
database, JCPDS card numbers 41-0677, 47-1734, 05-0490,
Elemental Mapping and µ-XANES. Elemental mapping
in the energy range 2-20 keV was performed on a bench-
scale µ-XRF instrument (µ-Eagle II, Ro ¨ntgenanalytik GmbH,
Taunusstein, Germany) equipped with a Rh X-ray tube
(emission line ∼20 000 eV) and a Si(Li) detector. The beam
was focused by a polycapillary lens to a spot size of about
30 µm in diameter. Areas of 2.5 × 2 mm2were scanned with
20 µm step size and 5 s dwell time.
Pb LIIIedge µ-XANES measurements were performed on
beamline X-26A, at the National Synchrotron Light Source
(NSLS), Brookhaven National Laboratory, Upton, NY. The
by use of two 100 mm long, dynamically bent silica mirrors
arranged in Kirkpatrick-Baez (KB) geometry (27). The soil
sections were placed on a stage at an angle of 45° relative to
the incident beam and could be observed with a CCD
microscope. The Si(111) monochromator energy was cali-
a lithium drifted silicon detector [Canberra SL30165 Si(Li)]
at room temperature. Energy resolution was 1.8 eV around
the Pb K edge. Synchrotron µ-XRF spectra were collected
an IDL MCA routine developed at the beamline.
use of the FLUO code (version 002, May 1999) developed by
html). They were normalized by use of a linear function for
the pre-edge and a second-degree polynomial function for
smoothed by Fourier filtering. They were analyzed by linear
combination fits with WinXAS 2.3 (28). The fits were
performed across the edge from 13 010 to 13 110 eV. The
energy was allowed to float during the fit within (1 eV.
Reference spectra were taken from the SSRL XAFS spectra
Results and Discussion
Soil Contamination. Table 1 gives the total concentrations
of Pb, Sb, Zn, Cu, and Ni of the soil samples collected along
the lateral transect behind the target area. In the surface soil
(0-5 cm depth), concentrations increase with increasing
distance from the targets until the area of maximum bullet
The maximum total Pb concentration is 68 g kg-1. At 23 m
behind the targets, the surface soil was still strongly
were lower owing to their lower concentrations in bullets
(12). The concentration of Pb generally decreased with soil
natural background concentration in soils. Extremely high
VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY94809
subsoil concentrations (>5000 mg kg-1) of Pb were found at
2 and 9 m behind the targets (Table 1). Bullet fragments in
the subsoils indicate that these elevated concentrations are
related either to bullet penetration into the soil or to soil
mixing by the periodic reconstruction and erosion of the
Mineralogy of Soil and Corroding Bullets. The soil
samples collected along the transect have slightly acidic to
slightly alkaline pH values (Table 1) and clayey to loamy
texture. A powder XRD analysis of the soil at 9 m behind the
target revealed the following mineral composition: quartz
(42%), muscovite (10%), dolomite (16%), calcite (14%),
plagioclase (6%), microcline (5%), chlorite (5%), and horn-
blende (2%). The effective cation exchange capacity (ECEC)
of the soil was 380 mmolckg-1, and Ca was the dominant
from 95 g kg-1at 25 cm depth to 35 g kg-1at 55 cm depth.
Figure 1 shows a powder XRD pattern of the weathering
(23%), and hydrocerussite (12%), with smaller amounts of
calcite (7%) and plagioclase (2%). Quartz, calcite, and
plagioclase are minerals derived from the parent rock, while
cerussite [PbCO3] and hydrocerussite [Pb3(CO3)2(OH)2] are
secondary Pb mineral phases formed during weathering of
the Pb bullets (9, 15, 19).
Previous reports indicate that hydrocerussite is the most
prevalent secondary Pb mineral phase (up to 80%) of bullet
weathering in different soils with pH values ranging from 3
to 8 (9, 15, 19-21). In contrast to these previous studies, we
found that cerussite was the predominant Pb mineral in the
weathering crust. This difference may be due to a high CO2
partial pressure in the Oberuzwil soil. Cerussite and hydro-
cerussite have similar solubilities, but elevated CO2partial
pressures relative to the atmosphere favor the formation of
cerussite over hydrocerussite (23, 29). Traces of additional
range soils (9, 15, 19-22). In our samples, no additional Pb
phase could be identified by XRD analysis except for traces
of lead oxide (PbO). However, the signal of PbO was too
(?-PbO) or litharge (R-PbO).
Selective Sequential Extraction. Table 2 presents the
results of a selective sequential extraction applied to bulk
soil samples collected in the vicinity of samples A and B.
Only 0.5-1.3% of the total Pb was exctractable with a 1 M
NH4NO3solution (SSE 1), suggesting that only a very small
percentage of the total Pb was bound by nonspecific cation
exchange. The largest fraction of the total Pb (55-75%) was
extractable with ammonium acetate at pH 6 (SSE 2). This
extraction step is assumed to dissolve Pb associated with
carbonates and also weakly complexed or chemisorbed Pb.
This is in agreement with the high concentration of lead
carbonate phases, cerussite and hydrocerussite, detected in
extracted mainly with extraction steps SSE 3 and SSE 4,
phase extracted by total digestion (SSE 7). This residual
fraction may include some metallic Pb(0).
elemental distribution around corroding Pb bullet cores are
shown in Figures 2 and 3. On the optical microscope images
of sample A (Figure 2a; 25 cm depth) and sample B (Figure
marked. The metallic Pb bullet cores appear as light gray
areas, labeled BC (spots 1A and 1B). The bullet core is
of approximately 100 µm (spots 2A and 2B). In some areas,
this gray material is covered with a second layer of light gray
to white material (spots 3A and 3B), which is surrounded by
the soil matrix (spots 4A and 4B). In Figure 3a, one can also
material, which appear to be similar to the weathering crust
around the bullet core. Bullet steel jackets are not visible in
these sections. Presumably, they have been separated from
the core by the bullet impact into the soil.
Typical synchrotron µ-XRF spectra of the bullet core
(Figure 4, spot 1A) are dominated by the two strongest X-ray
emission lines (Pb LR1at 10.552 keV and Pb L?1at 12.614
in Pb concentration from spot 1A (bullet core) to spot 4A
(soil matrix at 250 µm distance from weathering crust) is
evidenced by the strong decrease in the Pb fluorescence
peaks. This trend is confirmed by the elemental mapping
TABLE 1. Soil pH and Total Metal Concentrationsa
metal concn (mg kg-1)
(in 0.01 M
CaCl2)Ni CuZnSb Pb
280 1260 1025
585 1100 23 620
645605 14 330
350365 10 860
570 3020 67 860
900755 16 090
905 19 100
200630 14 450
755585 14 550
445 560 12 910
160 590 13 440
aDetermined by XRF analysis as a function of soil depth along a
transect ranging from 2 to 23 m behind the targets.
FIGURE 1. Bulk XRD pattern of the bullet crust material. H )
hydrocerussite, C ) cerussite, Q ) quartz, L ) litharge, M )
48109ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 13, 2005
(Figure 2b) and a transect concentration profile (Figure 5),
which shows high Pb concentrations in the bullet core and
the weathering crust and a sharp decrease in Pb counts in
the surrounding soil (2 orders of magnitude within 200-300
µm). This sharp decrease is in agreement with Pb concen-
trations calculated from energy-dispersive XRF spectra
collected for bulk soil (∼15 000 mg kg-1Pb) and crust
(∼180 000 mg kg-1Pb) material. Despite the large number
of corroded bullet fragments in sample B (Figure 3a), the
sharp decrease in Pb concentration within a few hundred
TABLE 2. Results of Sequential Selective Extractions of Disturbed Soil Samplesa
(% of total)c
B extracting solution (conditions)b
interpretation of Pb speciesb
1 M NH4NO3(20 °C, 24 h)
1 M NH4OAc, pH 6 (20 °C, 24 h)
0.1 M NH3OHCl + 1 M NH4OAc, pH 6 (20 °C, 0.5 h)
0.025 M NH4EDTA, pH 4.6 (20 °C, 1.5 h)
0.2 M ammonium oxalate, pH 3.25 (20 °C, 4 h)
0.1 M ascorbic acid + 0.2 M ammonium oxalate,
pH 3.25 (97 °C, 0.5 h)
total digestion in concentrated HNO3, HCl, HF
bound to carbonates, weakly complexed
bound to Mn (hydr)oxides
bound to organic matter
bound to Fe (hydr)oxides of low crystallinity
bound to crystalline Fe (hydr)oxides
bound in residual fraction
aCollected in the vicinity of sample A (25 cm depth) and sample B (55 cm depth).bAccording to Zeien and Bru ¨mmer (26).cAverage of two
replicates, total calculated as sum of all fractions.
FIGURE 2. Photomicrographs of bullet area in sample A (a) (BC )
bullet core) and X-ray fluorescence elemental maps: (b) Pb in red,
Fe in green, Si in blue; (c) Pb in red, Al in green, Ca in blue. Maps
of 30 µm. The step size is 20 µm and dwell time is 5 s. The map
size is 2.5 × 2.0 mm2.
FIGURE 3. Photomicrographs of bullet area in sample B (a) (BC )
bullet core) and X-ray elemental maps (b) Pb in red, Fe in green,
on a bench-scale µ-XRF instrument with a spot size of 30 µm. The
step size is 20 µm and dwell time is 5 s. The map size is 2.5 × 2.0
VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY94811
In addition to the fluorescence lines of Pb, lines at 3.605
keV (Sb LR1) and 3.844 keV (Sb LR1) are indicative of the
relatively small amount of Sb in the bullet cores (2-4% by
weight) (Figure 4a). Other metals were not detected by XRF
of Fe (Figure 4b, spot 2A), which may stem from the
surrounding soil or from the corroded bullet jackets. This
Mn are also clearly detectable in spots 3A and 4A (Figure 4).
However, in the soil surrounding the corroding bullets, the
the distributions of those elements.
at the Pb LIIIedge. The µ-XANES spectra and corresponding
first derivatives for the spots marked in Figures 3 and 4 are
concentrations at spots 1A and 1B (bullet core) and lowest
concentrations at spot 4A (soil matrix). For this purpose, the
Pb concentration at each spot was qualitatively estimated
from Pb LR emission peak areas in µ-XRF spectra (Figure 4).
Relative concentrations were then calculated with spot 1 as
reference for the bullet core.
from the core state to unoccupied valence states of the
in the XANES spectra (31, 32). While atoms beyond the
coordination sphere may also contribute to the XANES
spectra, their influence is much smaller as compared to the
The electronic configuration of Pb is Xe 4f145d106s26p2;
thus, 6d is the first free electronic orbital. Hence, the Pb LIII
edge corresponds to the electronic transition 2p f 6d. The
edge position indicates the Pb valence, with the inflection
point of metallic (zerovalent) Pb being assigned to 13 035
with standards, that divalent Pb prevails in the soil sur-
rounding the bullets.
The XANES spectra and first derivatives of spots 1A and
shown). The XANES spectra and first derivatives collected
on spots 2A and 2B were similar to the reference spectra for
litharge. The edge position of 13 054 eV is in line with Pb(II).
Further, a pre-edge feature is clearly visible at 13 029 eV.
Bargar et al. (31), Eiden-Assmann et al. (33), and Rao and
Wong (32) assigned it to the electronic transition 2p f 6s.
always occurs (31). When Pb is divalent, 6s is an occupied
orbital. This transition can occur only if the 6s electronic
orbital is hybridized, which is allowed in case of PbO where
the 6s2pairs are commonly described as stereochemically
uncertain. Several authors, on the basis of XANES and NMR
can exist only in certain symmetries such as the C4vsquare-
authors, on the basis of ab initio calculations in density
functional theory (DFT), have assigned it to Pb 6s-O 2p
hybridization (36, 37). Recently, Farges et al. (38) showed by
FIGURE 4. XRF spectra from sample A collected on NSLS beamline X26A. Spot locations are reported in Figure 2a. Spectra were collected
with an incident beam of 13 035 eV.
FIGURE 5. Pb concentration profile, according to the Pb Lr peak
intensity of µ-XRF measurements, along transects 1 and 2 marked
in Figure 2b.
48129ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 13, 2005
arise from multiple scattering of the photoelectron among
feature similar to that of litharge, but the edge features are
more similar to those of hydrocerussite. Thus, both litharge
and hydrocerussite may contribute to this spectrum. The
of hydrocerussite, cerussite, and possibly aqueous or ad-
pyramidal coordination, which is similar to inner-sphere
sorption complexes of Pb on surfaces (39).
that several lead compounds may coexist in the different
and quantity, we performed linear combinations fits using
combinations are plotted as blue lines in Figure 6. The
corresponding fitting parameters are reported in Table 3.
fitting represents the most likely lead species in soils (29).
Relative concentrations derived by the linear least-squares
fitting approach should, in general, be considered accurate
kept as few components as possible and removed compo-
nents with little abundance since they did not significantly
improve the fitting result. The fits are considered valid since
the least-squares fit residual values (?2) are less than 200
are below 0.95, and the sum of components ranged from
98.42% to 100.3%.
FIGURE 6. Normalized µ-XANES spectra (a) and corresponding first derivatives (b) of samples A and B (dotted lines), sorted along
decreasing Pb concentration from top to bottom. The blue lines are the linear combination fits (Table 3) based on the reference spectra,
which are shown as black solid lines. Spot locations are as in Figure 2a.
TABLE 3. Abundance of Reference Pb Species in Experimental
[Pb]bullet Pb(0)LHC Pb4(OH)44+
45 30 25
20 30 50
aAbundance is calculated by linear combination fitting and sorted
according to Pb content. Pb(0) ) metallic Pb, L ) litharge, H )
spectra partial concentrations. Residual (%) and ?2are deviations
resulting from the least-squares refinement.
VOL. 39, NO. 13, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY94813
Discussion of the Weathering Process. The results
cerussite, and cationic Pb2+, in agreement with thermody-
the first products resulting from the direct oxidation of
metallic lead by oxygen. Since formation of PbO2is unlikely
at the prevalent redox conditions in soils, only PbO occurs
(Pb +1/2O2f PbO). In previous studies, massicot (?-PbO)
was reported as the PbO phase formed in bullet corrosion
(15, 20-22) and litharge (R-PbO) as the PbO phase formed
in corroded artifacts and antic lead pipes (23, 24). In our
study, linear combination fits performed with massicot as
reference were not satisfactory. XANES spectra of litharge
and massicot have similar shapes (41), but the spectral
latter exhibits a single peak at 13 071 eV, which was also
in the massicot spectrum. Thus litharge is the PbO phase
and temperature tetragonal R-PbO (litharge) is predicted to
predominate (36, 42, 43), both massicot and litharge were
observed in natural systems. Possibly, there is an influence
was evidenced in studies where corrosion products were
in the laboratory was performed. Litharge was detected in
as in our study. Massicot may be a metastable, fast-forming
PbO phase that converts into litharge with time (23).
here. It can be easily converted to other compounds such as
low abundance of sulfur in the soil (450 mg kg-1), the
formation of anglesite [PbSO4] is not favored. At CO2partial
pressures PCO2 g 0.0003 atm and 25 °C, the formation of
cerussite is thermodynamically favored over the formation
(Table 3) and in agreement with Lin (15), hydrocerussite is
the first carbonated species formed in the weathering crust.
The competitive equilibrium reactions are (29)
By combining eqs 2 and 3 to yield eq 4, one can predict that
hydrocerussite is favored when the ratio of water activity to
bullets. If the activity of water is unity, this corresponds to
PCO2 toward the outer surface of the weathering crust,
hydrocerussite is then transformed into cerussite following
The dissolution of cerussite controls the activity of Pb2+in
the soil solution in contact with the corroding lead bullets.
and pH 7, the activity of Pb2+in solution in equilibrium with
cerussite at 25 °C is 1.28 × 10-6. Due to soil respiration, the
CO2partial pressure in soils is usually strongly elevated and
can exceed 0.02 atm even in well-aerated soils. Since the
solubility of cerussite decreases proportionally with increas-
ing PCO2, the activity of Pb2+in the soil solution in contact
with the corroding Pb bullets is expected to be even lower.
the soil matrix is strongly adsorbed to minerals and soil
organic matter, resulting in low mobility of Pb in the soil.
In summary, we have shown that the spatial distribution
and speciation of Pb in the weathering crust of Pb bullets in
soils can be studied by µ-XRF elemental mapping and
synchrotron µ-XANES spectroscopy. In contrast to previous
studies, we found litharge (R-PbO) to be the first weathering
(OH)2] and then to cerussite (PbCO3), which was the
We thank Michael Plo ¨tze (ETH Zurich) for powder XRD
analysis, Kurt Barmettler and Irene Xifra (ETH Zurich) for
their support with soil analyses, and Bill Rao (Consortium
for Advanced Radiation Sources, University of Chicago) for
Research carried out at the National Synchrotron Light
Source, Brookhaven National Laboratory, was supported by
under Contract DE-AC02-98CH10886. Use of the Beamline
X26A was supported by the Department of Energy, Basic
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Received for review November 5, 2004. Revised manuscript
received March 25, 2005. Accepted April 11, 2005.
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