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Science of the Total Environm
Determining speciation of Pb in phosphate-amended soils:
Method limitations
Kirk G. Scheckela,T, James A. Ryana, Derrick Allena, Ninnia V. Lescanob
aUS EPA, ORD, NRMRL, LRPCD, RCB, 5995 Center Hill Avenue, Cincinnati, OH 45224, United StatesbUniversity of Cincinnati, 2624 Clifton Avenue, Cincinnati, OH 45221, United States
Received 14 October 2004; accepted 12 January 2005
Available online 26 April 2005
Abstract
Determining the effectiveness of in situ immobilization for P-amended, Pb-contaminated soils has typically relied on non-
spectroscopic methods. However in recent years, these methods have come under scrutiny due to technical and unforeseen error
issues. In this study, we analyzed 18 soil samples via X-ray diffraction (XRD), selective sequential extraction (SSE), and a
physiologically based extraction test (PBET). The data were compared against each other and to previous data collected for the
soil samples employing X-ray absorption fine structure spectroscopy coupled with linear combination fitting (XAFS-LCF),
which spectroscopically speciates and quantifies the major Pb species in the samples. It was observed that XRD was incapable
of detecting pyromorphite, the hopeful endpoint of the immobilization strategy for reduced Pb bioavailability in our studies.
Further, the SSE and PBET extraction methods demonstrated an increase of recalcitrant Pb forms in comparison to the XAFS-
LCF results suggesting that SSE and PBET methods induced the precipitation of pyromorphite during the extraction
procedures. The theme of this paper illustrates the experimental concerns of several commonly employed methods to investigate
immobilization strategies of amended, metal-contaminated systems which may not be in true equilibrium. We conclude that
appropriate application of spectroscopic methods provides more conclusive and accurate results in environmental systems (i.e.,
Pb, Zn, Cd, etc.) examining P-induced immobilization.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Selective sequential extractions; Speciation; Bioavailability; Pb immobilization; Pyromorphite
1. Introduction
In situ remediation of lead (Pb)-contaminated soils
via phosphate amendments to sequester the Pb as
0048-9697/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.scitotenv.2005.01.020
T Corresponding author. Tel.: +1 513 487 2865; fax: +1 513 569
7879.
E-mail address: Scheckel.Kirk@epa.gov (K.G. Scheckel).
pyromorphite [Pb5(PO4)3X, where X=Cl, OH, or F]
has been extensively studied in the literature (Davis et
al., 1993; Cotter-Howells et al., 1994; Ruby et al.,
1994; Cotter-Howells, 1996; Laperche et al., 1997;
Traina and Laperche, 1999; Hettiarachchi et al., 2000;
Ryan et al., 2001; Stanforth and Qiu, 2001; Cao et al.,
2003; Melamed et al., 2003). The objectives of these
previous studies were to demonstrate a reduction in Pb
ent 350 (2005) 261–272
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272262
bioavailability by converting soil-Pb into biologically
inert and environmentally stable pyromorphite. How-
ever, speciating and quantifying the extent of soil-Pb
transformation to pyromorphite in phosphate-
amended soils has been difficult to ascertain. Attempts
have been made with scanning electron microscopy
coupled with energy dispersive X-ray spectroscopy
(SEM–EDX; Cotter-Howells and Thornton, 1991;
Davis et al., 1993; Laperche et al., 1997; Manecki et
al., 2000; Cao et al., 2003), X-ray diffraction (XRD;
Cotter-Howells et al., 1994; Cotter-Howells, 1996;
Laperche et al., 1997; Zhang and Ryan, 1999;
Manecki et al., 2000; Scheckel and Ryan, 2002; Cao
et al., 2003), transmission electron microscopy (TEM;
Cotter-Howells et al., 1994, 1999; Zhang et al., 1998;
Zhang and Ryan, 1999), electron microprobe analysis
(EMPA; Davis et al., 1993; Link et al., 1994), X-ray
absorption spectroscopy [XAS—encompassing X-ray
absorption near edge (XANES) and X-ray absorption
fine structure (XAFS) spectroscopies; Cotter-Howells
et al., 1994, 1999; Ryan et al., 2001; Scheckel and
Ryan, 2002, 2004], and selective sequential extrac-
tions (SSE; Ryan et al., 2001; Agbenin, 2002; Cao et
al., 2002, 2003; Melamed et al., 2003; Scheckel et al.,
2003). Despite their application in the literature, most
of these methodologies are limited by perturbation of
a nonequilibrated system, detection limits, improper
assumptions, and misinterpretation of results. For
example, when using energy dispersive X-ray spec-
troscopy with scanning electron microscopy (Davis et
al., 1993; Yang et al., 2001; Cao et al., 2003) and
transmission electron microscopy (Cotter-Howells et
al., 1994, 1999; Gulson et al., 1994) to speciate Pb in
P-amended soils it is very difficult to visually
distinguish pyromorphite from an array of hexagonal
minerals typically found in soils by microscopy and
EDX only provides a gross total element concen-
tration generally over a large beam spot size which
may indicate that the input amendment of P and
contaminated soil-Pb are both present within the beam
window, but provides inconclusive evidence that P
and Pb are chemically associated as pyromorphite.
Research efforts using XRD for P-amended, Pb-
contaminated soils are generally hampered by the
quantity and degree of crystallinity of pyromorphite
for identification purposes. Typically, a 5% weight
concentration of well-ordered crystals is necessary to
provide suitable XRD peaks for identification and
matching by a library card database. However, most
Pb-contaminated soils rarely reach the 5% contaminant
concentration and the full conversion of non-crystalline
Pb to pyromorphite at such a site would require a
tremendous amount of phosphate that would ultimately
alter the soil consistency (Porter et al., in press). EPMA
studies work well in defining homogeneous, well-
defined systems (Davis et al., 1993; Link et al., 1994);
however, examination of Pb in environmental samples
by EPMA is insensitive to chemical and physical
aspects of submicron matrix heterogeneity and cannot
distinguish effectively between metal adsorption and
(co)precipitation in a complex matrix.
X-ray absorption spectroscopy in situ studies of
soil-Pb and phosphate systems have been successful
in distinguishing pyromorphite (Cotter-Howells et al.,
1994, 1999; Ryan et al., 2001; Scheckel and Ryan,
2002, 2004) through comparison to reference spectra
and the identification of nearest neighboring atoms
relative to Pb as well as interatomic bond distances
and coordination numbers. XAS is advantageous
relative to other mentioned methods because it is
element specific with low detection limits. However,
quantifying the amount of pyromorphite in soils based
solely on XAS data cannot be accomplished since the
overall signal collected is an average of all species
present of a particular element. Recent advances for
speciating metals in heterogeneous soil and sediment
environments have involved the tandem use of X-ray
absorption fine structure (XAFS) spectroscopy and
statistical analysis via linear combination fitting
(LCF) or principle component analysis (PCA; Beau-
chemin et al., 2002; Isaure et al., 2002; Roberts et al.,
2002; Scheinost et al., 2002; Scheckel and Ryan,
2004). The technique combines the element specific,
in situ capabilities of XAFS with the comprehensive
statistical analysis of LCF or PCA by examining
unknown sample spectra relative to known reference
compounds to identify various metal species and also
quantify the multiple components. Scheckel and Ryan
(2004) employed XAFS-LCF to evaluate the trans-
formation of soil-Pb near a smelter facility in Joplin,
MO, to pyromorphite following amendment with
phosphate or combinations of phosphate with an iron
rich waste product and composted biosolids. The
XAFS-LCF results determined a maximum pyromor-
phite content of 45% relative to the total Pb
concentration which is significantly lower than SSE
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272 263
residual fraction values that can be over 80%
(Agbenin, 2002) under similar circumstances. Drasti-
cally overestimating the amount of pyromorphite in a
contaminated system via chemical alteration from
field status provides a false sense of security that Pb is
sequestered in a biologically unavailable form.
The methodology with the greatest potential for
error in bspeciationQ of Pb in P-amended, non-
equilibrated soils is selective sequential extractions
which employ multiple extraction steps of progres-
sively mordant solutions to remove metals into
operationally defined fractions or species. Selective
sequential extraction methods were initially
designed to examine the distribution of trace
concentrations of metals in sediments (Tessier et
al., 1979), not percent metal levels in soils. Many
critical concerns of SSE include sampling/preserva-
tion procedures (Rapin et al., 1986), chemical
properties of the target element and sample (Martin
et al., 1987), little specificity for the solid phase
attacked (Jouanneau et al., 1983), lack of reference
materials for quality control (Fiedler et al., 1994), lack
of standardized procedures (Qiang et al., 1994;
Quevauviller et al., 1994; Usero et al., 1998),
redistribution of elements (Qiang et al., 1994;
Raksasataya et al., 1996; La Force et al., 1999), and
transformation of elements to less soluble phases
during the extraction procedure (Ryan et al., 2001;
Scheckel et al., 2003). Selective sequential extraction
procedures have shown that a substantial portion of
the soil-Pb was transformed to pyromorphite in
phosphate-amended soils as evident by an increase
of Pb concentration in the residual fraction relative to
a control (Ma and Rao, 1997; Ryan et al., 2001;
Agbenin, 2002; Cao et al., 2002, 2003; Chen et al.,
2003); however, these studies cannot rule out nor
disprove the probability that pyromorphite formed as
a result of the extraction procedure due to the likely
nonequilibrium of the amended soil and to rapid
kinetic formation of pyromorphite from soluble Pb
and P (Nriagu, 1973, 1974; Scheckel and Ryan,
2002). This phenomenon was illustrated by Scheckel
et al. (2003) in SSE experiments with pure Pb
components within a sand matrix [Pb-acetate, cer-
ussite (PbCO3), anglesite (PbSO4), galena (PbS), and
chloropyromorphite] in the presence and absence of
solid calcium phosphate. Employing XAFS and XRD
to examine the solid residue after extraction steps, it
was evident that pyromorphite easily formed when
initially independent sources of Pb and P were reacted
during extraction steps. In fact, pyromorphite was
identified after the first (least aggressive) extraction
step via XRD and XAFS (Scheckel et al., 2003).
These results illustrate that pyromorphite forms during
the extraction steps of SSE of a pure experimental
system and maybe a speculated reason why research-
ers that employ SSE methods to P-amended, Pb-
contaminated soils achieve residual fraction values
greater than values recorded for spectroscopic studies
(Scheckel and Ryan, 2004). In fact, the same
problematic issues associated with SSE methods are
probably present when considering in vitro extraction
schemes that attempt to characterize metal bioavail-
ability in nonequilibrated systems; therefore, these
schemes must require in vivo animal feeding studies
and spectroscopic techniques to validate and correlate
the relationship (Arnich et al., 2003; Hettiarachchi et
al., 2003).
The objective of this study was to compare
previously published XAFS-LCF data (Scheckel and
Ryan, 2004) with XRD, SSE and PBET results of Pb
speciation in soil samples that have been treated with
phosphate. The underlying hypothesis is that: XAFS-
LCF provides a more accurate assessment of Pb
speciation than XRD, SSE and PBET in soils treated
with phosphate, phosphate and compost, or phosphate
and iron. Although XRD, SSE, and PBET have
become acceptable methods to evaluate Pb speciation
in P-amended soils within the literature, we feel these
methods do not provide accurate results. This is
suggested because (1) XRD is not sensitive enough
for typical Pb concentrations in these soils to identify
pyromorphite and (2) the extraction methods (SSE
and PBET) induce the rapid formation of pyromor-
phite which over predicts the amount of Pb bound in
the soils.
To test the hypothesis we examined the SSE and
PBET distribution of Pb in P-amended soils from a
previous XAFS-LCF investigation (Scheckel and
Ryan, 2004) in order to evaluate the relationship
between the concentration of Pb in the residual
fraction of SSE and extractable Pb in PBET studies
with quantified speciation results from XAFS-LCF.
Additionally, high-resolution XRD was employed to
attempt to identify pyromorphite in the amended soil
samples.
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272264
2. Experimental section
2.1. Soil samples
The field experiment (42 m�47 m) was estab-
lished in the spring of 1997 in a residential setting
adjacent to a Pb smelter which operated from the
1880s to its closing in the late 1960s at Joplin, MO.
This site has been the focus of several studies with
additional site characteristics available in Brown et al.
(2004) and Ryan et al. (2004). Smelter emission was
the primary source of Pb contamination to the site.
The soil-Pb concentration at the site is variable and
ranges from 1100 to 5300 mg Pb (kg soil)�1 with the
majority of the site being in the range of 2000 to 3000
mg Pb (kg soil)�1. Total soil-Pb concentrations were
determined by EPA Method 3051 for microwave
assisted digestion followed by inductively coupled
plasma atomic emission spectrometry (ICP-AES)
according to EPA Method 6010B and EPA Quality
Assurance guidelines. The soil had a neutral pH (6.9–
7.2), organic carbon content of 46–56 g kg�1, a cation
exchange capacity of 27.2–32.2 cmol kg�1, and a
Bray extractable P of 12–39 mg P (kg soil)�1.
Treatments (Table 1) were installed during March
1997, using a completely randomized design with 4
replicates for statistical examination. A HDPE barrier
was placed around the perimeter of each of the 2
m�4 m plots to reduce the potential of inter plot
contamination. In addition to phosphate treatments
and the influence of residence time, an iron rich (IR)
waste product and composted biosolids (CBS) were
included as treatments which have been reported to
reduce bioavailability (Ryan et al., 2004). Amend-
Table 1
List of treatments for field plots
Treatments (dry weight application)
Control Phosphate only
Control 0% H3PO4a 1.0 TSPb
3.2% TSP
1.0% PRc
0.5% H3PO4a
1.0% H3PO4a
a Phosphoric acid, residence times of 3, 18, and 32 months.b Triple super phosphate.c Phosphate rock.
ments were weighed and hand applied on a per plot
basis to the tilled soil. For the field study, triple super
phosphate (TSP) and phosphoric acid (H3PO4) were
purchased at a local fertilizer dealer in Joplin, MO.
Rock phosphate (RP) was donated by Occidental
Chemical in Florida. The iron rich (IR) paint
processing by-product was donated by the DuPont
Company, Wilmington, DE. The Compro composted
biosolids (CBS) was shipped from Montgomery
County, MD. Applications were made on a dry weight
basis with the assumption that the bulk density of 1
m3 of soil=1050 kg based on particle density, pore
space, and organic matter content. Application rates of
P treatments were calculated on the basis of total P
addition. After amendment, plots were covered with a
commercial landscape fabric to reduce erosion.
In May 1997, the fabric was removed, Ca(OH)2(71% purity, quick lime) was added and rototilled to a
10 cm depth into each plot to bring the pH to 7, and
the plots were hand-seeded with Kentucky 31 Tall
Fescue (Festuca elatior cv. K31). The amount of lime
required ranged from 39.4 kg/plot (10% com-
post+0.32% P as TSP) to 157 kg/plot (3.2% P
TSP). This corresponds to approximately 50 Mt lime
ha�1 for the 10% compost + 0.32% TSP and 200 Mt
lime ha�1 for the 3.2% TSP treatments. In the case of
the phosphoric acid treatments, the liquid fertilizer
grade (85%) phosphoric acid and fertilizer grade
(45%) KCl were surface applied rototilled, 10 days
later lime [Ca(OH)2] was applied and hand raked to
incorporate to a depth of 10 cm and 30 days later the
plots were seeded.
At set times of 0, 3, 18, and 32 months, laboratory
samples were collected from each plot for analysis.
Phosphate+iron rich
material (IR)
Phosphate+composed
biosolids (CBS)
1.0% TSP+1.0% IR 10% CBS
0.32% TSP+2.5% IR 0.32% TSP+10% CBS
1.0% TSP+2.5% IR 1.0% TSP+10% CBS
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272 265
Random samples from each plot were combined with
the random samples from the quad-replicated treat-
ments. In total, approximately 20 kg of soil for each
treatment was collected and stored at 4 8C.
2.2. Selective sequential extraction procedure
The procedure described by Tessier et al. (1979)
was employed with slight modifications. For the
organic matter bound extraction, a NaOCl solution
was utilized rather than H2O2 since a number of
researchers have demonstrated that H2O2 can destroy
or alter Mn oxides, carbonates, and phosphates
(Jackson, 1956; Anderson, 1963; Lavkulich and
Wiens, 1970). EPA Method 3051 (microwave diges-
tion in concentrated nitric acid) was utilized instead of
a HF-HClO4 mixture for assessment of the final
residual phase for safety reasons. The Tessier et al.
(1979) method has been modified numerous times;
however, the objective of this study was not to
develop a sequential extraction procedure but to
evaluate the overall validity of these tests for
perturbed systems under reasonable conditions. All
chemicals employed in the laboratory studies were
ACS certified from Fisher Scientific (Pittsburgh, PA)
unless otherwise noted. Taking 1-g soil samples, we
used the following detailed sequential extraction
procedure in triplicate for each sample.
2.2.1. Fraction 1—exchangeable
The samples were extracted at room temperature
for 1 h with 8 mL of magnesium chloride solution
(1 M MgCl2, pH 7.0) with continuous agitation.
2.2.2. Fraction 2—bound to carbonates
The residue from Fraction 1 was leached at room
temperature with 8 mL of 1 M NaOAc adjusted to pH
5.0 with acetic acid (HOAc). Continuous agitation
was maintained and the time necessary for complete
extraction was evaluated based on Pb carbonate prior
to experimental trials and was determined to be 3 h.
2.2.3. Fraction 3—bound to iron and manganese
oxides
The residue from Fraction 2 was extracted with 20
mL of 0.04 M NH2OHd HCL in 25% (v/v) HOAc.
This fraction experiment was performed at 96F3 8Cwith intermittent agitation for 6 h.
2.2.4. Fraction 4—bound to organic matter
To the residue from Fraction 3, 20 mL of 7 M
NaOCl (adjusted to pH 8.5 with HCl) was added, and
the mixture was heated to 90F2 8C for 2 h with
occasional agitation. After centrifuge separation, a
second 20-mL aliquot of NaOCl (adjusted to pH 8.5
with HCl) was then added and the sample was heated
again to 90F2 8C for 2 h with intermittent agitation.
2.2.5. Fraction 5—residual
The residue from Fraction 4 was digested with
concentrated HNO3 in a microwave digester accord-
ing to EPA method 3051 and diluted 50:1 with 18 mV
DI water.
After the prescribed time interval for each extrac-
tion (Fractions 1–4), samples were centrifuged (7000
rpm, Sorvall RC-5B Superspeed Centrifuge, New-
town, CT) and the supernatant filtered through a 0.45
Am filter. The remaining solid sample was washed
twice with Millipore DI water before continuing with
the next extraction step. The supernatants were
collected in 15 mL plastic vials, acidified with
concentrated HNO3, and stored at 4 8C. For Fraction5, the final dilution was filtered through a 0.45 Amfilter and stored at 4 8C. Once an entire sequential
extraction replication was completed, the samples
were analyzed for Pb by inductively coupled plasma
atomic emission spectrometry (ICP-AES) according
to EPA Method 6010B and EPA Quality Assurance
guidelines.
2.3. X-ray diffraction
Powder XRD patterns were obtained using a
Phillips PW3040/00 X’Pert-MPD Diffractometer
system with a Cu anode ceramic diffraction X-ray
tube operating at 50 kV and 40 mA. The sample
platform used for the XRD experiments was a
PW3064 Sample Spinner and was automated in
conjunction with a sample (changer) batch program.
The soil samples were back-filled into 32 mm
circular sample holders to obtain a flat, randomly
oriented surface for analysis. Using the X’Pert Data
Collector software, a relative scan method was
employed that used the upper diffracted beam path
and a goniometer scan axis. Data were collected in a
step scan mode from 10 to 75 28Q at a step size of
0.0158 and a rate of 1.50 s/step. The samples were
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272266
rotated on the spinning sample platform at two
revolutions per second so that each step of the scan
observed three full rotations of the sample. After the
raw XRD patterns for the soil samples were
collected, X’Pert Graphics and Identity software
was employed to conduct a peak search, smooth
the raw spectra to a factor of 1, Ka2 stripping with a
factor of 1, and a search-match of the identified
peaks relative to the ICDD library card database.
A 1% (10,000 ppm) pyromorphite standard was
prepared in respirable amorphous silica (0.05%
crystalline quartz content; J.M. Huber, Edison, NJ)
and analyzed to aid in the identification of pyromor-
phite in the amended soil samples with minimal
background interference. The background of the
chloropyromorphite standard was set equal to 4 counts
s�1 (average count value of a zero-background sample
holder for the X’Pert Data Collector) using X’Pert
Plus software from Philips Analytical (now PANaly-
tical, The Netherlands).
2.4. PBET in vitro extraction method
All soil samples were prepared for the in vitro
studies by drying (b 40 8C, 24 h) and sieving to b 250
mm. The b 250-mm size fraction was used because
this particle size is representative of that which
adheres to children’s hands with respect to the oral
exposure route for Pb. Samples were thoroughly
mixed prior to use in bioavailability studies to ensure
homogenization. The main piece of equipment
required for this procedure is an extractor motor that
has been modified to drive a flywheel connected to a
Plexiglass block situated inside a temperature-con-
trolled water bath. The Plexiglass block contains
twelve (12) 5-cm holes with stainless steel screw
clamps, each of which is designed to hold a 125-mL
wide-mouth high density polyethylene (HDPE) bottle.
The water bath was filled such that the extraction
bottles were immersed. Temperature in the water bath
was maintained at 37F2 8C using an immersion
circulator heater to simulate body temperature. The
leaching procedure for this method used an aqueous
extraction fluid at pH values of 1.5, 2.0, or 2.5. A pH
meter was used to measure the pH of the extraction
fluid prior and post experiment. The extraction fluid
consisted of a 0.4 M glycine buffered solution at 37
8C. Concentrated HCl (trace metal grade) was added
until the solution pH reached values of 1.50, 2.00, or
2.50F0.05 and the final solution volume was brought
to 2 L with DI water.
Soil samples of 1.00F0.05 g were added (b 250
mm) to a 125-mL wide-mouth HDPE bottle in
triplicate. Next, 100F0.5 mL of the extraction fluid,
using a calibrated dispenser attached to a bottle
reservoir, was measured into the 125-mL wide-mouth
HDPE bottle. Each bottle top was hand-tightened and
inverted to ensure that no leakage occurs, and that no
material was caked on the bottom of the bottle. The
bottle was placed into the extractor, making sure each
bottle was secure and the lid(s) were tightly fastened.
For each run, 10 soil samples, 1 blank, and 1 quality
control (QC) check were examined. The temperature
of the chamber was recorded with control limits of
35–39 8C. The extractor was rotated end over end at
30F2 rpm for 1 h to simulate conditions and
residence time of solid material within the stomach
of mammals. When extraction (rotation) was com-
plete, the bottles were removed from the apparatus,
wiped dry, and placed upright on the bench top.
Extract aliquots were drawn directly from reaction
vessel into a disposable 20-cc syringe to which a 0.45-
Am cellulose acetate disk filter (25 mm diameter) was
then attached, filtered into a clean 15-mL polypropy-
lene centrifuge tube, and acidified for ICP-AES
analysis. The pH of fluid remaining in the extraction
bottle was measured and recorded. If the fluid pH was
not within F 0.5 pH units of the starting pH, the test
was discarded and the sample reanalyzed; however,
this was never necessary. The samples were analyzed
for Pb by ICP-AES according to EPA Method 6010B
and EPA Quality Assurance guidelines.
3. Results and discussion
Fig. 1 shows the XRD results for all 18 soil
samples examined via XAFS-LCF (Scheckel and
Ryan, 2004), SSE, and PBET analyses along with a
1% chloropyromorphite standard. As Fig. 1 illustrates,
the soil spectra are dominated by a quartz peak at
26.64 28Q. The reference spectrum for chloropyro-
morphite (denoted by a circle-lined curve) is not
predominantly evident in the full graph of Fig. 1. The
inset of Fig. 1, which shows the section of the XRD
curve with the two most dominant peaks for chlor-
10 20 30 40 50 60 70 80
0
6
25000
20000
15000
10000
5000
0
Cou
nt In
tens
ity
29.0 29.2 29.4 29.6 29.8 30.0 30.2 30.4 30.6 30.8 31.0-100
100
200
300
400
500
600
700
Cou
nt In
tens
ity
2°Θ
2°Θ
Fig. 1. XRD patterns of unamended and P-amended soils contaminated with Pb. The curve consisting of circles represents a 1% (diluted)
chloropyromorphite standard.
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272 267
opyromorphite, demonstrates that the chloropyromor-
phite standard blends into the background noise of the
soil spectra making identification of pyromorphite in
soil samples extremely difficult, if not impossible. It is
important to note that the highest Pb concentration for
any of the soil samples did not exceed 5500 ppm and,
further, XAFS-LCF data for these samples (Scheckel
and Ryan, 2004) did not identify any sample with a
pyromorphite content greater than 45% of the total Pb.
Considering that the chloropyromorphite standard was
diluted to 10,000 ppm, identifying pyromorphite by
XRD in soil samples is practically impossible unless
extremely contaminated sites (approaching 5% Pb)
amended with substantial amounts of phosphate (N
3%) are examined.
The results of the selective sequential extraction
experiments on the P-amended, Pb-contaminated soil
samples are show in Fig. 2 (standard deviation error
0.47–3.04%, n=57). For the non-treated sample
(Control), the residual fraction was determined to be
approximately 50%, most likely influenced by the
presence of galena and, possibly, fertilizer P reacting
with soluble Pb to form pyromorphite (Scheckel et al.,
2003). While galena is a stable mineral phase, the
potential exists for oxidation and formation of more
soluble phases to occur, thus, periodic monitoring is
necessary to reassess risk for non-treated aerobic
systems. With the exception of the biosolids (BS)-
amended samples, which saw a decrease in residual
fraction Pb but a significant increase in organic
fraction associated Pb, the amount of Pb in the
residual fraction increased as a function of P concen-
tration and amendment solubility. Data regarding the
PBET extraction scheme for these samples are shown
in Fig. 3. The PBET studies were conducted at three
pH values covering the range of values that have been
Fig. 2. Selective sequential extraction results of unamended and P-amended soils contaminated with Pb. SSE fractions: F1—exchangeable, F2—
carbonate, F3—oxide, F4—organic, and F5—residual.
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272268
empirically employed in the literature (Ruby et al.,
1996; Zhang et al., 1998; Hamel et al., 1999; Basta
and Gradwohl, 2000; Hettiarachchi et al., 2000;
Casteel et al., 2001; Stanforth and Qiu, 2001; Arnich
et al., 2003). The non-treated samples show, that
regardless of pH, approximately 60% of the Pb in the
Fig. 3. Physiologically based extraction test results of unamended and P
extracted into solution.
system is extractable. The treated samples show an
overall trend of decreasing extractability as P concen-
tration, P solubility, and pH increase. For example, the
TSP only treated samples of 1% and 3.2% show a
significant reduction in Pb extractability from 50% to
22% at pH 2.5. Comparing the solubility of P, the 1%
-amended soils contaminated with Pb showing the percent of Pb
0
15
30
45
60
75
0 15 30 45 60 75
% P
b in
Res
idu
al F
ract
ion
% Pyromorphite & Pyromorphite+GalenaDetermined by XAFS-LCF
PyromorphitePyromorphite + Galena
y = 0.439x + 46.736R2 = 0.808
y = 0.631x + 39.086 R2 = 0.923
Fig. 4. Relationship of the percent of pyromorphite and the sum of
pyromorphite and galena determined by XAFS-LCF versus the
percent of Pb measured in the residual fraction of the selective
sequential extraction procedure.
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70 80
% P
b in
In V
itro
So
lid P
has
e
% Pyromorphite Determined by XAFS-LCF
pH 1.5
pH 2.0
pH 2.5
y = 0.226x + 36.398 R2 = 0.790
y = 0.424x + 39.769 R2 = 0.927
y = 0.757x + 43.492 R2 = 0.933
Fig. 5. Relationship of the percent of pyromorphite and the sum o
pyromorphite and galena determined by XAFS-LCF versus the
percent of Pb remaining in the solid phase of the soil measured in
the physiologically based extraction test.
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272 269
rock phosphate sample noted approximately 45%
extraction at pH 2.5 relative to 19% Pb extraction
for the 1% phosphoric acid sample (2.5 years) which
is obviously more soluble than rock phosphate. As pH
increased from 1.5 to 2.0 to 2.5, the amount of
extracted Pb decreased from 53% to 36% to 22% for
the 3.2% TSP-amended sample.
To substantiate and understand this research, we
compared recent research from our lab (Scheckel and
Ryan, 2004) that employed XAFS-LCF to speciate
and quantify the major Pb species in these soil
systems to the data collected in this study. XAFS-
LCF determined that pyromorphite concentration
ranged from 0% (control soil) to 45% (1% phosphoric
acid amendment, residence time of 32 months)
relative to the total Pb concentration. The Pb
speciation in the non-amended control field plots
included Pb-sulfur species (galena+angelsite=53%),
adsorbed Pb (inner-+outer-sphere+organic bound=
45%), and Pb-carbonate phases (cerussite+hydrocer-
ussite=2%). The addition of P promoted pyromor-
phite formation and the rate of formation increased
with increasing P concentration. Supplemental addi-
tion the iron rich (IR) byproduct with TSP enhanced
pyromorphite formation relative to independent TSP
amendment of like concentrations. The amendment of
biosolids and biosolids plus TSP observed little
pyromorphite formation (1–16% of total Pb), but a
significant increase of sorbed Pb was measured by
XAFS-LCF. With this baseline in addition to previous
findings indicating the induced formation of pyro-
morphite during extractions of P-amended, Pb-con-
taminated soils (Scheckel et al., 2003), Figs. 4 and 5
show the comparison of XAFS-LCF data to the results
of selective sequential extraction (residual fraction)
and in vitro PBET values, respectively, for the Joplin
soils. In comparing the results of XAFS-LCF to the
percent of Pb in the residual fraction of the SSE
experiments (Fig. 4), one observes a balance shift
away from the theoretical 1:1 line (dashed line in Fig.
4) towards higher values related to the amount of Pb
measured by SSE. Even after adding the concentration
of galena determined by XAFS-LCF, the offset in Fig.
4 could not be explained. These results imply that an
enrichment of the residual fraction occurred during the
SSE method most likely as a result of pyromorphite
formation during the extraction steps. The same
phenomenon may explain the data present in Fig. 5
for the PBET studies. As PBET extraction solution pH
increases, the amount of Pb remaining in the solid
phase increases. Comparing the quantity of pyromor-
phite measured by XAFS-LCF to the amount of Pb
remaining in the solid phase for PBET analysis, one
notices an enrichment of Pb above the 1:1 line
indicating an overestimation of Pb remaining in the
f
30
40
50
60
70
80
90
30 40 50 60 70 80 90
pH 1.5
pH 2.0
pH 2.5
% P
b in
In V
itro
So
lid P
has
e
% Pb in SSE Residual Fraction
Fig. 6. Relationship of the percent of Pb measured in the residual
fraction of the selective sequential extraction procedure versus the
percent of Pb remaining in the solid phase of the soil measured in
the physiologically based extraction test.
K.G. Scheckel et al. / Science of the Total Environment 350 (2005) 261–272270
solid phase by PBET results relative to XAFS-LCF
data (Fig. 5). However, this latter observation is not all
bad in terms of potential bioavailability. If, for
instance, a child ingests soil from a P-amended, Pb-
contaminated site and the child’s stomach and
digestive system induces further pyromorphite for-
mation, then the ultimate bioavailability of Pb in the
system is reduced. However, developing a method to
measure and prove this hypothesis would surely be
difficult particularly in the context of the multiple in
vitro extraction methods that are currently employed
in the literature. Direct animal feeding studies would
be more conclusive but expense and public criticism
often limit this type of research.
Comparing one extraction method to another
should be approached with extreme caution as
demonstrated in Fig. 6 for data collected in this study.
Fig. 6 shows no direct relationship between the
percent of Pb in the residual fraction of the SSE
procedure and the percent of Pb in the solid phase for
the in vitro PBET experiments. As experimental
options arise to make research cheaper and quicker
for commercial purposes, common sense should
dictate that a dual extraction analysis is not a viable
alternative. Nonetheless, Figs. 4 and 5 demonstrate
that extraction methods overestimate the quantity of
Pb in the recalcitrant solid phase relative to XAFS-
LCF data which may lead to erroneous assumptions
regarding the true risk of an amended site. To alleviate
the confusion of examining amended, metal-contami-
nated systems in terms of immobilization effective-
ness and bioavailability indexes, we strongly
recommend the adaptation of advanced, molecular-
level spectroscopic techniques to truly speciate and
explain ex situ extraction results and speciation-
limited instrument analyses.
Acknowledgements
The U.S. EPA has not subjected this manuscript to
internal policy review. Therefore, the research results
presented herein do not, necessarily, reflect Agency
policy. Existing data collected from the various
sources not generated by EPA employees for infor-
mational purposes were not subjected to nor verified
by EPA’s quality assurance procedures. Mention of
trade names of commercial products does not
constitute endorsement or recommendation for use.
NVL is grateful for the opportunity to participate in
the 2003 U.S. EPA-UC High School Apprenticeship
Program.
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