Determining speciation of Pb in phosphate-amended soils: Method limitations

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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: [email protected] (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.


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


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


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.


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.


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.


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.


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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Determining speciation of Pb in phosphate-amended soils: Method limitations

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