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Polyhedron 29 (2010) 2364–2372
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
The structure and solubility of carbonated hydroxyl and chloro lead apatites
Mitchell P. Sternlieb a, Jill D. Pasteris b, Benjamin R. Williams a, Katherine A. Krol a, Claude H. Yoder a,*
a
b
Department of Chemistry, Franklin and Marshall College, Lancaster, PA 17603, United States
Department of Earth and Planetary Sciences, The Center for Materials Innovation, Washington University in St. Louis, St. Louis, MO 63130-4899, United States
a r t i c l e
i n f o
Article history:
Received 23 March 2010
Accepted 6 May 2010
Available online 13 May 2010
Keywords:
Carbonated lead apatite
Pyromorphite
Solubility
XRD
Raman
Lead NMR
a b s t r a c t
The properties of carbonated hydroxyl and chloro lead apatites, Pb10(PO4)6(OH)2 and Pb10(PO4)6Cl2, serve
as models for the incorporation of carbonate into their medically important calcium analogs, and there is
likely incorporation of carbonate in an insoluble lead phosphate phase during lead remediation. We have
synthesized a series of carbonated lead hydroxyl- and lead chloro-apatites at 60–80 °C. The incorporation
of carbonate into the apatite structure was documented by X-ray powder diffraction, IR and Raman spectroscopy, 207Pb solid state NMR spectroscopy, and elemental analysis. The carbonate content was determined by combustion analysis and confirmed by Raman spectroscopic analysis. As carbonate content
increases in hydroxyl lead apatite, Raman spectra show changes in the phosphate stretching modes at
925 and 950 cm1, an increase in intensity and downshift of a new peak at 1050 cm1, and changes in
the spectral features of the O–H stretch at about 3560 cm1. The variation in unit cell parameters for
the chloro lead apatite as a function of carbonate content is similar to that documented for B-type substitution in calcium apatites. The 207Pb NMR spectra corroborate B-type substitution. For the hydroxyl
lead apatite, the changes in cell parameters suggest a combination of A- and B-type substitution. Solubilities of the carbonated lead apatites, determined by ICP-MS, increase slightly at low to moderate carbonate content, but more strongly at ca. 5.0 wt.% carbonate content. Ksp values extrapolated to zero carbonate
content reveal that the chloro lead apatite is indeed less soluble than the hydroxyl analog.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
The apatite family of materials is ubiquitous on Earth, although
typically found in low concentrations geologically. A carbonated
form of the mineral species hydroxylapatite (Ca10(PO4)6(OH)2),
designated here as CaApOH) is the principal inorganic constituent
of bone and teeth and therefore essential to human life. Perhaps
the most intriguing feature of the apatites is their extraordinary
ability to undergo substitution by a great variety of cations and anions. This property is responsible for their use as phosphors, containment agents to immobilize radioactive decay products,
catalysts, and ion exchange and stationary phase materials for
chromatography [1]. These generally hexagonal P63/m compounds
have structures that have been described as approximately closest
packed phosphate ions [2] or as closest packed cations [3,4]. The
structure of Ca10(PO4)6Cl2, given in Fig. 1, shows the presence of
two types of calcium ions: the Ca(2) ions (channel-defining ions)
that surround the chloride ions at the corners of the unit cell and
the Ca(1) cations located in the ‘‘body” of the unit cell. There are
six Ca(2) channel-defining cations and four Ca(1) body cations in
* Corresponding author. Tel.: +1 717 2913806.
E-mail addresses: [email protected] (J.D. Pasteris), [email protected] (C.H. Yoder).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.05.001
the unit cell. The phosphate ions are arranged as columns of staggered tetrahedra parallel to the c-axis.
The channels, in which the monovalent anions (in this case
chloride) reside, permit substitutional solid solution of some anions, including fluoride, chloride, and hydroxide. Although larger
anions occupy the channel positions less symmetrically, giving
monoclinic structures for pure hydroxyl- and chlor-apatite, solid
solution generally gives rise to hexagonal structures [1].
Of particular importance in the growth of bone and the use of
phosphates as agents of lead and nuclear waste remediation is
the substitution of carbonate ion. In Ca10(PO4)6(OH)2, substitution
by carbonate occurs to up to ca. 20 wt.% [5,6]. This divalent anion is
believed to substitute for either phosphate (B-type) or the monovalent anion (A-type) or both [7,8]. Under ambient conditions, Btype substitution dominates [2] as can be expressed by the formula
Ca10x(PO4)6x(CO3)x(OH)2x, where x is the number of moles of
carbonate contained in one mole of the apatite. A-type substitution, represented by the formula Ca10(PO4)6(CO3)x(OH)22x, is observed predominantly in reactions at high temperature [2].
Additional substitutions at much lower concentrations can occur,
such as of HCO3 , HPO4 2 and H2 PO4 , which require appropriate
charge-balancing. During the synthesis of carbonated apatites,
counter-ions such as sodium are present that also can be incorporated. A variety of mechanisms have been postulated for charge
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2. Experimental
2.1. Syntheses
Fig. 1. The structure of CaApCl as viewed down the c-axis. Calcium ions and
portions of tetrahedral phosphate groups outside of the unit cell (Ca10(PO4)6Cl2) are
included in order to show the monovalent anion (Cl) channels defined by
triangular planar groups of calcium ions positioned around the anion. Atoms closer
to the reader are shown darker. In the unit cell, six cations surround the channel site
containing the monovalent anions, and four cations are embedded in the body of
the unit cell. The phosphate ions extend down the c-axis in staggered columns. In
the hydroxyl homolog, the OH is located between the two planes of calcium
triangles along the c-axis.
balance in the apatite structure during substitution, but there is no
general understanding of substitution in the calcium apatites. It is
clear, however, that substitution of carbonate for phosphate in the
mineral apatite increases its solubility [9–11] as does the carbonate in natural phosphate (fluorapatite-rich) rock [12].
Stoichiometric, end-member apatite species of other cations exist that are also of considerable importance. These apatites include
the pyromorphite group of minerals, i.e., Pb10(PO4)6(OH,F,Cl)2,
which figure prominently in discussions of lead remediation because of their low solubilities over a wide range of pH [13–15].
Areas of lead contamination can be treated with a form of phosphate in order to induce precipitation of a solid pyromorphite
phase and thereby immobilize the lead [12,15–21]. As with the calcium apatites, the structures of both hydroxyl and chloro lead apatites [41] have hexagonal P63/m symmetry. In the pyromorphite
group, the chloropyromorphite member is generally thought to
be less soluble than the hydroxylpyromorphite member. There is,
however, an increasing amount of evidence that chloropyromorphite is more soluble [22,23] than indicated by a frequently quoted
Ksp [24]. Natural samples show considerable substitution of hydroxyl for chloride [25–28] as well as Ca for Pb and AsO4 for PO4 [29–
32]. Only a few authors [12,26,33] have addressed the carbonate
concentration of natural pyromorphites in spite of the carbonaterich environment of many lead deposits. Indeed, it is concern for
the high solubility of lead minerals, such as the lead carbonate
cerussite, that has triggered studies on how to re-incorporate lead
into highly insoluble minerals [12,15,19,34–40].
It is the intent of this study to explore substitution of carbonate
in hydroxyl and chloro lead apatites (Pb10(PO4)6(OH)2 and
Pb10(PO4)6Cl2) in order to determine the mode of substitution,
and, particularly, the effect of carbonate on the solubility of the
lead phosphate phase. We have therefore prepared a series of carbonated hydroxyl and chloro lead apatities (CPbApOH and
CPbApCl) with a range of carbonate contents. To verify carbonate
incorporation into the pyromorphite structure, we have employed
Raman and IR spectroscopy, which have been useful in studies of
carbonated CaApOH [42–49], and X-ray powder diffraction (XRPD).
We have used 207Pb solid state NMR spectroscopy and XRPD to
ascertain the mode of substitution of carbonate. We have also
determined the solubilities of the carbonated lead apatites using
ICP-AES and ICP-MS.
All water was Milli-Q doubly deionized water that was purged
with nitrogen gas to remove carbon dioxide. In the preparation
of CPbApOH an amount of ammonium hydrogen carbonate (Sigma–Aldrich, 99%) calculated for a specific carbonate:phosphate ratio was dissolved in 250 mL of water, heated to 60 ± 2 °C, and
stirred magnetically. The pH of the solution was raised to
9.0 ± 0.3 by adding 6 M NH3 (Pharmco, ACS reagent grade) and
maintained manually at that value throughout the reaction by reference to a pH meter. A total of 25 mL of 0.15 M Pb(NO3)2 (Sigma–
Aldrich, 99%) and 25 mL of a 0.09 M NH4H2PO4 (Acros) solution
were added simultaneously to the reaction vessel at a rate of
1 mL/min. The precipitate-solution mixture was maintained for
2 h at 60 °C and pH 9 and was then allowed to cool to room temperature. The precipitate was isolated by suction filtration, washed
three times with water, and then dried in a vacuum desiccator at a
pressure of 2 torr for 24 h.
The synthesis of CPbApCl differed from that of CPbApOH only in
the addition of 90 mg of ammonium chloride (Sigma–Aldrich) to
the ammonium hydrogen carbonate at the beginning of the
reaction.
2.2. Analyses
Weight percent carbon was determined by Schwarzkopf Microanalytical Laboratories, Woodside, NY, using a C-, H-, N-elemental
analyzer after combustion at 1000 °C in a stream of oxygen. The reported carbonate concentrations have an experimental error of
±0.2 wt.%. Lead and phosphorus analyses, including lead analyses
by ICP-MS, were obtained from Galbraith Laboratories, Knoxville,
TN and have a relative uncertainty of ca. 5%.
2.3. XRPD
Powder X-ray diffractograms were obtained with a Phillips
3520 X-ray diffractometer using Cu Ka radiation in conjunction
with a monochromator. A step size of 0.02° 2h and a dwell time
of 1 s were used over the range 2–60° 2h. The instrument was calibrated regularly using a quartz standard. Cell parameters were
determined using the program UNIT CELL (Cambridge Earth Sciences,
Holland and Redfern, 2007).
2.4. IR and Raman
Infrared spectra were obtained with a Bruker Tensor 37 spectrometer using a ZnSe ATR accessory. Analyses were based on
256 scans from 600 to 4000 cm1 with a resolution of 2 cm1.
Raman microprobe spectra were obtained using a HoloLab Series 5000 Laser Raman Microprobe (Kaiser Optical, Ann Arbor, Michigan). A 532 nm, frequency-doubled Nd-YAG laser delivered a
maximum of 11 mW to the sample surface. Spectra were recorded
from 100 to 4000 cm1 with a resolution of about 3 cm1. A typical
spectrum represents the average of 32 acquisitions of 4 s each. Raman peak positions are accurate to within 0.3 cm1, and the reproducibility is 0.1 cm1 based on repeated analysis of the same
hydroxylapatite standard during each measurement session. Analyses were made using an 80 ultra-long-working-distance objective with a numerical aperture of 0.75, which provided a spatial
resolution on the order of a couple of micrometers. The effective
penetration depth of the laser is probably less than 2 lm into these
strongly light-scattering powders.
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2.5. NMR
Solid state 207Pb NMR spectra were obtained on a Varian INOVA
500 MHz instrument operating at 104.167 MHz at 23 °C. Samples
were contained in 3.2 mm rotors spinning at rates from 8 to
20 kHz. Delay times of 10–200 s with an average number of transients of 5000 were used with and without proton decoupling.
An external substitution reference of solid lead nitrate was set to
550 ppm [50].
2.6. Solubility measurements
Finely ground synthetic pyromorphite samples were equilibrated at 25 ± 1 °C in a shaker bath for 6 days in phosphate buffers
consisting of 96.7 ppm phosphate and 18.4 ppm ammonium ion
(0.1171 g (1.018 mmol) NH4H2PO4 in 1 L H2O) in Milli-Q deionized
water. For comparison, two samples were also equilibrated for 5,
10, and 15 days in water. The supernatant was isolated by centrifugation and passage through a 0.45 lm disc filter. The lead concentrations were determined using a Spectro Ciros CCD ICP-AES
(at 168.215 nm) and a Perkin–Elmer Sciex Elan 6100 ICP Mass
Spectrometer (Galbraith Laboratories). The relative uncertainties
in the lead concentrations are estimated to be ±5%. Activities of
each species were calculated using Visual MINTEQ (Version 2.61)
from experimental values of lead and phosphate concentrations
and pH values at the conclusion of each equilibration period.
more similar to that of PbApOH. As the percent carbonate increases, the two peaks at about 30° 2h move closer until, at 4.2%
carbonate, there is only one peak in this area.
Several XRPD patterns for CPbApOH are given in Fig. 3 and show
little effect of carbonate on position and width of peaks.
3.2. Carbonate incorporation
The weight percent carbonate detected by combustion analysis
in the precipitated reaction product was generally related to the
ratio of carbonate relative to phosphate in the reaction solution.
The maximum carbonate incorporation of 5.0 weight percent was
achieved for CPbApOH at a solution mole ratio of carbonate to
phosphate of 25 to 1. The dependence of the carbonate composition of the product on the ratio of carbonate to phosphate in the
reaction mixture for CPbApCl is shown in Fig. 4. The figure suggests
approach to carbonate saturation of PbApCl at a mole ratio of 20–
25 to 1, with a maximum concentration of incorporated carbonate
of 4.2 wt.%. A similar, but less regular trend was observed for
CPbApOH.
When no carbonate was added to the reaction mixture, 1.1 and
0.75 wt.% carbonate were detected in the CPbApOH and CPbApCl
products, respectively, presumably a result of the sorption of CO2
from the air by the basic reagents.
After careful washing and drying in vacuo, the products were
examined by petrographic microscopy at 800. Less than 5% of
3. Results and discussion
3.1. Syntheses
In our preparation of both CPbApOH and CPbApCl, ammonium
salts were used to prevent the formation of insoluble sodium lead
double salts [51]. The slow addition of solutions of lead nitrate and
ammonium dihydrogenphosphate to a solution of ammonium
hydrogen carbonate provided a high initial carbonate concentration and produced yields ranging from 90% to 99%.
The X-ray powder diffraction patterns (XRPD) for the carbonated pyromorphite samples showed no evidence of the lead carbonate phases cerussite or hydrocerussite and were in good
agreement with the patterns reported for unsubstituted PbApOH
(ICDD PDF# 00-008-0259) and PbApCl (ICDD PDF# 00-019-0701).
The powder X-ray diffraction patterns of both the least and most
carbonated samples of CPbApCl are shown in Fig. 2, from which
it is apparent that the least carbonated has the general appearance
of PbApCl, while the most carbonated has a diffraction pattern
Fig. 2. Powder X-ray diffraction patterns of the least (1.1 wt.%) and most (4.2 wt.%)
carbonated CPbApCl, as well as the ICDD patterns for uncarbonated PbApCl
(bottom) and PbApOH (top), showing convergence of the peaks around 30° 2h as
carbonate concentration increases in CPbApCl.
Fig. 3. Powder X-ray diffraction patterns of the 2.5 and 5.0 wt.% carbonated
CPbApOH, as well as the ICDD pattern for uncarbonated PbApOH (bottom, PDF 00008-0259), showing little change in both position and width of peaks as carbonate
content increases.
Fig. 4. The relationship between weight percent carbonate in the product CPbApCl
and the starting mole ratio of carbonate to phosphate in the reaction mixture.
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the pyromorphite crystals (the largest of which was approximately
0.3 mm in length) contained a small number of inclusions. The significantly higher birefringence of the inclusion solids than the host
crystal strongly suggests a lead carbonate phase. However, optical
analysis suggests contamination levels below 0.05 volume%. Several other procedures for introducing the reactants were explored
in order to minimize the coprecipitation of, presumably, cerussite
or, more likely, hydrocerussite [52]. Addition of a mixture of carbonate and phosphate to the lead-bearing solution, as well as
simultaneous addition of carbonate, phosphate, and lead solutions
to water, however, produced about the same population density of
inclusions as the original procedure. The testing of PbApOH-hydrocerussite mixtures indicated that hydrocerussite can be detected
by XRPD at the 1 wt.% level; below this level, the hydrocerussite
in control mixtures could not be reproducibly identified. XRPD
analysis of all samples indicated the presence of only the carbonated apatite phase.
Raman microprobe analysis was also used to probe the possibility of a second phase in the product powders on a micrometer spatial scale. Three to five individual analyses were made on each
sample. Fig. 5 shows that cerrusite and hydrocerussite are readily
distinguishable from hydroxyl pyromorphite in the Raman spectra,
and no spectral evidence of those two lead carbonates was observed in any of the products.
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3.3. Raman analyses
Because Raman spectra are sensitive to changes in bonding configuration and bond energy, they reflect different structural aspects
of a solid than do XRPD patterns. Progressive changes occur in the
Raman spectra of our products, which are strong evidence of incorporation of the carbonate into the PbApOH structure. Fig. 6 shows
the development and increasing intensity of a peak at about
1050 cm1 and broadening of both peaks of the characteristic Pb
ApOH doublet at about 925 and 950 cm1 with increasing carbonate content.
The latter two peaks of the P–O stretch also downshift in response to carbonate substitution. In B-type substitution in calcium
apatites, replacement of phosphate by carbonate results in a deficiency of negative charge, which must be compensated for by a decrease in the number of calcium and hydroxyl ions (or
cosubstitution of other anions such as HPO4 2 ). Fig. 7 shows the
decrease in intensity of the O–H stretch at about 3561 cm1 (normalized to the intensity of the 925 cm1 peak) with increasing carbonate content. The peak remains at about the same position but
shows broadening and development of a shoulder on the lowwavenumber side of the O–H band.
The two strongest Raman bands in our CPbApOH spectra occur
at about 925 (m1 symmetric P–O stretch) and 950 cm1 (m3 asymmetric P–O stretch) [26]. As carbonate concentration in the sample
increases, the band at about 1050 cm1 increases in intensity relative to the 925 and 950 cm1 peaks. This spectroscopic development is analogous to what occurs in carbonated hydroxylapatite
(CCaApOH) produced by methods analogous to those used here;
in the case of CCaApOH, a carbonate-induced band arises at about
1070 cm1 [42–46,48,53]. As shown in Fig. 5, the lead carbonate
phases cerussite and hydrocerussite have their dominant m1 symmetric C–O stretches very close to the 1050 cm1 band of carbonated pyromorphite. In carbonated CaApOH, the 1070 cm1 band
historically has been assigned to the m1 of carbonate [26–32].
Recently, however, Pasteris et al. [55], Antonakos et al. [49], and
Mason et al. [56] have suggested that this carbonate-induced
band is an asymmetric m3 phosphate stretch. The unexpected position of the band may reflect deformation of the phosphate tetrahedra, which changes the P–O bond length and downshifts the
appropriate Raman band. The m3 mode likely loses its degeneracy
Fig. 5. Raman spectra of CPbApOH and possible carbonate impurity phases. Shaded
area is expanded on the right. Note the difference in shape of the 1050 cm1 bands
of cerrusite and hydrocerrusite compared to the nearest band in CPbApOH, as well
as differences in the O–H stretching region at 3600 cm1 for hydrocerrusite and
CPbApOH.
Fig. 6. Changes in the 925, 950, and 1050 cm1 Raman peaks of CPbApOH as a
function of carbonate content.
Fig. 7. Changes in the 3561 cm1 Raman peak (normalized to intensity of 925 cm1
peak) of CPbApOH as a function of weight percent carbonate.
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due to substitution of carbonate at several different phosphate
sites.
Downshifting of the bands associated with the P–O stretch of
phosphate as a function of carbonate concentration has been observed in the CCaApOH system by several workers [43,48] as well
as in our own studies. The downshift suggests a progressive lengthening, i.e., weakening, of the P–O bond as the amount of carbonate
increases. The mechanism of this weakening is not clear but can be
rationalized by a simple model. The replacement of phosphate by a
smaller ion of lower charge (i.e., carbonate) leaves a local increase
in positive charge, which must be compensated at a somewhat
longer range by a decrease in positive charge or an increase in negative charge. The consequence of the local increase in positive
charge is an increased electrostatic attraction between the oxygens
in the remaining phosphate tetrahedra and the local excess positive charge, which then produces a lengthening of the P–O bond(s)
and a decrease in vibrational frequency.
Likewise, extensive substitution of PO4 by CO3 groups may sufficiently alter the environment of the channel sites to cause a shift
in the vibrational frequency of some OH-groups, leading to the
development of a shoulder on the 3561 Dcm1 OH-band at high
carbonate concentrations (see Fig. 6, samples 6–8). Alternatively,
at high carbonate content, CO3 2 may begin to substitute preferentially into the channel site.
The Raman spectra of CPbApCl in the same region are shown in
Fig. 8. Although effects from incrementally increasing incorporation of carbonate clearly are seen, the spectra do not show significant shifting of the 946 cm1 peak, as might be expected in analogy
with the OH-analog. It is not clear why the Cl-member of the Pb
apatites shows much less spectral effect from carbonate incorporation than do the OH-members of the Pb and the Ca apatites. It is
intriguing that PbApCl shows a strong response to increasing carbonate concentration in its XRPD patterns but not in its Raman
spectra [54], whereas PbApOH shows just the opposite response.
This issue will be pursued further in the future.
spectra show the asymmetric carbonate stretch at about
1400 cm1 and the bending modes, m4 and m2 at 682 (693) and
838 (857) cm1 for cerussite and hydrocerussite, respectively
[57]. Analogous behavior is observed in the IR spectra of CPbApCl,
as shown in Fig. 10.
3.4.1. XRPD analyses
In contrast to the significant decrease in crystallinity of CCaApOH as a function of carbonate content [58,59] the XRD peaks
of CPbApOH and CPbApCl show little increase in width as the percent carbonate increases. The lattice parameters for a series of our
carbonated pyromorphites shown in Figs. 11 and 12 should be
compared to the cell parameters reported for PbApOH (a-axis, caxis), i.e., 9.774 and 7.291 by single-crystal analysis [41] and those
obtained by XRPD: 9.877, 7.429 [60]; 9.8828, 7.4406 [61]; 9.8355,
7.41 [61]; 9.866, 7.426 [62]; 9.877, 7.426 [63]; 9.877, 7.427 [64].
The unit cell parameters reported for single-crystals of PbApCl include 9.95, 7.31 [65]; 9.993, 7.334 [66]; 9.9981, 7.344 [67]; 9.9764,
7.3511 [68]. The data from XRPD patterns show 9.976, 7.351 (PDF
# 99-000-3062); 9.987, 7.33(PDF#00-019-0701). The averages of
the reported PbApOH cell parameters are 9.86 Å for the a- and baxis and 7.41 Å for the c-axis. For PbApCl the averages are 9.98 Å
3.4. Infrared analyses
Fig. 9 shows the infrared spectra of two samples of CPbApOH
(incorporating 0.75 and 3.6 wt.% carbonate) compared to spectra
of the 0.75 wt.% CPbApOH spiked with cerussite and with hydrocerussite, each at 3.0 wt.% carbonate. The spectrum of the singlephase pyromorphite with 3.6 wt.% carbonate (second from top) is
sufficiently different from the spectra of the spiked samples, especially the sample spiked with the more likely ‘‘contaminant” phase
hydrocerussite, to support the hypothesis that carbonate is structurally incorporated, not precipitated as a separate phase. The
Fig. 8. The Raman spectra of CPbApCl.
Fig. 9. The IR spectra of CPbApOH incorporating 0.75 wt.% carbonate and CPbApOH
incorporating 3.6 wt.% carbonate (top two traces) and CPbApOH incorporating
0.75 wt.% carbonate spiked with 3.0 wt.% cerussite or hydrocerussite.
Fig. 10. Expansion of the infrared spectra of two CPbApCl samples of 2.75 wt.%
carbonate and 4.20 wt.% carbonate. For comparison are spectra of 2.2 wt.%
carbonated CPbApCl spiked with 3 wt.% cerussite or hydrocerussite.
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Fig. 11. Variation of a- and c-axis length with carbonate content in CPbApOH.
Fig. 13. The MAS 207Pb NMR spectrum of CPbApCl containing 1.1 wt.% carbonate
with a delay time of 200 s and a spin rate of 20 k.
Fig. 12. Variation of unit cell parameters with carbonate content in CPbApCl.
for the a- and b-axis and 7.34 Å for the c-axis. The values of the aand c-axes for the least carbonated samples in this study were
9.885 and 7.438 Å for PbApOH and 9.968 and 7.335 Å for CPbApCl,
in reasonable agreement with the averages reported. The cell
parameters for CPbApOH samples of different carbonate content
are shown in Fig. 11, which indicates a small increase in both aand c-axis length with an increase in carbonate content. For carbonate substitution in the Ca-analog CCaApOH, B-type substitution
into the phosphate site results in a decrease in the a-axis and a
slight increase in the c-axis, whereas substitution of the A-type
for the channel anion produces a lengthening of the a-axis and
slight contraction of the c-axis [5,8]. The increases in both cell
dimensions observed for our lead apatites could be attributed to
a combination of A- and B-site substitution, or, it may be the case
that the cell parameters for the lead apatites do not depend in the
same way on the substitution mode as do calcium apatites (see below). It should be noted that the approximate increase of 0.02 Å
observed for the a-axis is considerably smaller than the magnitude
of the decrease of ca. 0.125 Å observed by LeGeros for carbonated
calcium apatites with up to 20% carbonate [5].
The unit cell parameters for CPbApCl obtained from the diffraction patterns are shown in Fig. 12, where it appears that the a-axis
length decreases, while the c-axis increases with increased carbonate content. The variation in the a-axis for CPbApCl is opposite to
that observed for carbonated PbApOH, but is consistent with the
trend reported for B-site substitution in CaApOH [8]. Although
Fig. 14. The MAS 207Pb NMR spectrum of CPbApCl containing 3.8 wt.% carbonate
with a delay time of 200 s and a spin rate of 20 kHz.
the trend in the c-axis appears to be determined primarily by the
point at 1.1 wt.% carbonate, this c-value is consistent with the average value reported for uncarbonated PbApCl.
3.4.2. 207Pb solid state NMR spectra
The 207Pb NMR spectrum of the least carbonated CPbApCl obtained at a spin rate of 20 kHz (Fig. 13) shows two distinct manifolds of peaks. The isotropic chemical shift is 130 ppm for the
narrow high-field manifold and 790 ppm for the broader low-field
manifold. The spectrum in Fig. 13 was obtained with a delay time
of 200 s, but does not differ appreciably from that obtained with a
delay time of 10 s. The integrated ratio of the two manifolds is
approximately 4 (low-field) to 6 (high-field). The crystal structure
of pyromorphite, like that of CaApCl (Fig. 1), contains two different
lead sites, those defining the channel and those in the interior of
the unit cell. The atomic occupancies of these sites are in the ratio
of 6 to 4 in the Pb10(PO4)6Cl2 unit cell. On the basis of this ratio, the
high-field resonance can be attributed to the channel-defining lead
ions.
As the carbonate content increases (Fig. 14), the low-field manifold becomes increasingly less distinct (more noisy) and the highfield manifold broadens. The use of delay times up to 200 s did not
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significantly improve these spectra (although a sharp peak at
790 ppm is still visible). Although the nuances of all of these
changes must be left for a future study, there seems to be little
doubt that incorporation of carbonate changes the structure of
pyromorphite and probably affects the interior lead ions to a greater extent than the channel ions. It appears therefore that carbonate
replaces phosphate, which produces nonequivalent interior lead
ions. Of course, the replacement of phosphate must also involve
rejection of lead and chloride from the structure for charge balance. The relative changes in the intensities of the manifolds make
it appear that the interior lead ions may be preferentially removed.
The breadth of the channel manifold may reflect nonequivalent
channel lead ions or dipolar coupling. These spectral changes are
noticeable even at carbonate incorporation of 4.2%.
occupied by the same set of six phosphate and two hydroxide ions
in both apatites. The difference between the anion volumes
(536469 = 62 Å3) is presumably due to structural effects that
are not taken into account by the simplistic addition of ion volumes and may also be related to greater covalent interactions for
the more Lewis acidic lead ion.
However, the modified cell volumes themselves do not speak
directly to the size of the two substitution sites, which are exceedingly difficult to quantify. It seems likely, however, that there is no
steric inhibition to substitution at either site in the lead apatites,
and perhaps the greater residual volume of the latter accounts
for the relatively smaller changes in cell parameter lengths for
CPbApX compared to the calcium analogs.
3.5. Substitution mode
3.6. Solubility
Although the lead, phosphorus, and carbon elemental analyses
are not of sufficient accuracy to yield charge-balanced empirical
formulas, they can be used to evaluate the lead to phosphorus ratio. These experimentally derived atomic ratios of 1.76, 1.79, 1.77,
and 1.90 for CPbApOH samples with wt.% carbonate values of 2.15,
2.95, 3.60, and 5.00, respectively, suggest an increase in Pb:P ratio
as the carbonate content increases. This general trend is consistent
with B-site substitution, whereas A-site substitution generates a
constant value of 1.67 for the lead to phosphorus ratio regardless
of the carbonate content. It is also interesting to note that, according to the B-type substitution formula of Pb10x(PO4)6x(CO3)x
(OH)2x, 5 wt.% carbonate corresponds to x = 1.8. Complete substitution of carbonate for hydroxide in A-site substitution produces
Pb10(PO4)6CO3, which contains only 2.2% carbonate. Thus, even
for CPbApOH, where the cell parameters could indicate a combination of A- and B-site substitution, it appears more likely from the
Pb/P ratios, NMR data, and the maximum carbonate percentage
that substitution occurs at the B-site.
In an attempt to relate the substitution mode of the lead apatites with those established for the calcium apatites, it may be
instructive to look at several ways in which the cation might affect
the substitution behavior. Perhaps the most obvious difference between CaApX and PbApX is the size of the cation. The 8-coordinate
ionic radii of Ca2+ and Pb2+ are generally given as 1.12 Å and 1.29 Å,
respectively [69,70]. Simplistically, the 10 cations in the unit cell
make a contribution (4/3pr3) of 59 and 90 Å3 for calcium and lead,
respectively. The slightly larger size and volume of lead ions is reflected in the cell volumes of 528 and 626 Å3 for CaApOH [4] and
PbApOH [63], respectively. Subtraction of the volumes of the cations from the cell volumes produces values of 469 and 536 Å3 for
CaApOH and PbApOH. These residual volumes presumably are
Solutions equilibrated with solid CPbApOH in a phosphate buffer (pH 5.7) contained 3–39 ppb of lead, as shown in Fig. 15. The
data indicate an almost constant concentration of dissolved lead
in equilibrium with the samples of low to moderate carbonate content, but a markedly higher concentration for the sample with
maximum (5.0 wt.%) carbonate. Equilibration of a 5.0 wt.% sample
prepared in a different synthesis produced the same concentration
within experimental error. The increase in concentration could be
attributed to a discontinuous effect of the carbonate on the structure of the apatite. However, there is no evidence for such a discontinuity in any of the spectroscopic and X-ray measurements. There
is also no evidence for a second, possibly more soluble, lead phase.
The lead concentrations in equilibrium with solid CPbApCl
show the same general behavior (Fig. 16).
For CPbApOH samples with 5.0 wt.% carbonate content equilibrated for 5, 10, and 15 days, the measured solubilities were constant within 2 ppb over those time periods, indicating that
equilibrium is reached within a 5-day equilibration period. In
water (without buffer), the lead concentrations after 6 days of
equilibration were 5 and 7 ppb for CPbApOH with carbonate contents of 5.0 and 3.6 wt.%, at pH values of 6.0 and 6.6, respectively.
These values can be compared to the EPA maximum contaminant
level (MCL) in drinking water for lead of 0.015 mg/L (15 ppb) [71].
In order to relate these lead concentrations for phases with different carbonate content to the actual solubility of the phase, the
Ksp values for the phase must be obtained from the experimental
values of phosphate, pH, carbonate, and lead, appropriately speciated. We have assumed B-site substitution and have used the formula obtained from the carbonate content to formulate a Ksp
expression for each of the carbonated apatites. For example, a carbonate percent of 2.5 wt.% corresponds to a value of x = 1.0 in the
Fig. 15. Dissolved lead concentrations (ppb) in equilibrium with solid CPbApOH
after 6-day equilibration in phosphate buffer.
Fig. 16. Dissolved lead concentrations (ppb) in equilibrium with solid CPbApCl
after 6-day equilibration in phosphate buffer.
Author's personal copy
M.P. Sternlieb et al. / Polyhedron 29 (2010) 2364–2372
Fig. 17. Log(Ksp) and molar solubility values for samples of CPbApOH of different
weight percent carbonate (assuming a composition of Pb10x ðPO4 Þ6x ðCO3 Þx ðOHÞ2x )
equilibrated in phosphate buffer. The value of x was determined from the percent
carbonate. Ksp values were determined from the activities of the species generated
by Visual MINTEQ.
2371
M10(PO4)6X2 are converted to values appropriate for the empirical
formulas M5(PO4)3X, values of 1079 and 1086.5 are obtained for
Pb5(PO4)3OH and Pb5(PO4)3Cl, respectively. These values are consistent with the literature values of 1076.8 for PbApOH [72] and
1084.4 for PbApCl [73,74], but not with the value of 1079.1 for
PbApCl [58].
The mobility of lead in phosphate-remediated soils is therefore
somewhat affected by the presence of low to moderate concentrations of incorporated carbonate but appears to be strongly affected
by large concentrations of carbonate. Solution pH also plays a major role [33] in the solubility of the apatite phase. For example, at a
pH of 9, using a Ksp for Pb10(PO4)6(OH)2 of 10160, the molar solubility can be calculated as approximately 1010, whereas at pH 4, a
value comparable to that obtained in acidic ground waters, the solubility increases three orders of magnitude to approximately
107 M. In terms of an actual soil environment, the most important
point is that lead apatite with expected degrees of carbonation remains less soluble than other lead minerals that may be present
and undergoing dissolution, as demonstrated by the work of Cao
and colleagues [75], who investigated lead contamination from
PbO, PbCO3, and PbSO4 in soils under a range of pH conditions.
Acknowledgements
The authors are indebted to Dr. Richard Schaeffer of Messiah
College, Dr. Cecil Dybowski of the University of Delaware, Anne
Rowand and Steve Sylvester of Franklin and Marshall College for
technical assistance and the Keystone Innovation Zone, the Hackman Program of Franklin and Marshall College, and the Center
for Materials Innovation at Washington University in St. Louis for
funding. Partial support for this work also was provided by NIH
funding through Grant 1R21AR055184-01A2.
References
Fig. 18. Log(Ksp) and molar solubility values for CPbApCl of different weight percent
carbonate (assuming a composition of Pb10x ðPO4 Þ6x ðCO3 Þx ðClÞ2x ) equilibrated in
phosphate buffer. The value of x was determined from the measured percent
carbonate. Ksp values were determined from the activities of the species generated
by Visual MINTEQ.
formula Pb10x(PO4)6x(CO3)x(OH)2x and therefore to the Ksp
expression
2þ
K sp ¼ aðPb Þ9 aðPO4 3 Þ5 aðCO3 2 ÞaðOH Þ
The Ksp values obtained using the activities derived from Visual
MINTEQ are shown in Fig. 17. Because each Ksp refers to a different
empirical formula, the Ksp values cannot be compared directly as
an indicator of relative solubilities. Therefore, we have determined
the molar solubilities from the Ksp expression and Ksp for each carbonated phase using Visual MINTEQ. These values, useful for a
comparison of the effect of carbonate on solubility, are also shown
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PbApOH and PbApCl, based on the first three (lowest-concentration carbonate) points, produces Ksp values of 10158 and 10173,
respectively, clearly indicating a lower solubility for ‘‘pure” PbApCl
than for PbApOH. When these Ksp values for the unit cell formulas
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