Anal. Chem. 2002, 74, 3819-3823

X-ray Absorption Fine Structure Combined with
Fluorescence Spectrometry for Monitoring Trace
Amounts of Lead Adsorption in the Environmental
Conditions
Yasuo Izumi,*,† Fumitaka Kiyotaki,† Taketoshi Minato,† and Yoshimi Seida*,‡Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering,Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, and Institute of Research andInnovation, 1201 Takada, Kashiwa 277-0861, JapanThe local structure of trace amounts of lead in anadsorbent matrix that contains a high concentration ofiron and magnesium (Mg6Fe2(OH)16(CO3)‚3H2O) was
successfully monitored by means of X-ray absorption finestructure spectroscopy combined with fluorescence spectrometry. A eutectic mixture of PbCO3 and Pb(OH)2
coagulated when Pb2+ was adsorbed from a 1.0 ppm
aqueous solution, and in contrast, the major species wasion-exchanged Pb2+ in the case of adsorption from a 100
ppb aqueous solution. The difference was ascribed to thebalance between the precipitation equilibrium for coagulation and the rate of the ion exchange reaction withsurface hydroxyl groups.
X-ray absorption fine structure (XAFS) spectroscopy has
become widely used to monitor the local structure of noncrystalline and heterogeneous/hybrid materials. Even with the state-ofthe-art technology of synchrotron radiation and advanced X-ray
detectors, there are still experimental difficulties measuring theabsorption edge of trace amounts of an element in samples that
contain a high concentration of heavy element(s).
We chose as an example a trace amount of lead adsorbed on
Mg6Fe2(OH)16(CO3)‚3H2O (adsorbent 1). The adsorbent is oneof the basic polyhydroxide compounds and exhibits a layered
structure.1 Several adsorbents containing aluminum are used forthe removal of heavy metal elements. However, the risk of usingaluminum in relation to the acceleration of Alzheimers diseasecannot be excluded at the moment. Using a conventional XAFS
measurement technique,2 the X-ray fluorescence derived from a
trace amount of lead is extremely weak relative to the backgroundof the dominant scattered and emitted X-rays derived from thelarge quantities of iron and magnesium contained in the samples.
This paper deals with a new analytical approach to the observed* Corresponding author: (e-mail) izumi@chemenv.titech.ac.jp.
† Tokyo Institute of Technology.
‡ Institute of Research and Innovation.
(1) Seida, Y.; Nakano, Y.; Nakamura, Y. Water Res. 2001, 35, 2341-2346.
(2) Lytle, F. W.; Greegor, R. B.; Sandstrom, D. R.; Marques, E. C.; Wong, J.;
Spiro, C. L.; Huffman, G. P.; Huggins, F. E. Nucl. Instrum. Methods Phys.
Res. 1984, 226, 542-548.
10.1021/ac025550p CCC: $22.00
Published on Web 06/17/2002© 2002 American Chemical Society

trace amount of lead and exhibits the speciation of adsorbed lead
species.3,4Lead, 1.0 wt %, was adsorbed on 1 from an aqueous Pb2+
solution of 1.0 ppm concentration. One of the most familiar
environmental conditions relating to lead is the treatment ofdrinking water to suppress lead concentrations of 100-1000 ppb
derived from soil contamination5-7 or from pipes made of lead. A
concentration of 1.0 ppm is the upper limit of Pb2+ concentrationin the environment. The lower concentration was set to 100 ppb
as the lower limit of Pb2+ in the environment. The removal from100 ppb aqueous Pb2+ solution was tested up to lead contents of
0.12 and 0.30 wt % on adsorbent 1.
It is a challenge to obtain local structural information for 0.12
wt % adsorbed lead from the 100 ppb aqueous solution. The Pb/
Mg atomic ratio is 0.000 62. The edge jump for XAFS measurements in transmission mode8 is 0.022 versus a total absorption of
4.0. The photon number ratio of Pb LR1 to Fe KR1 is 0.0058 intraditional fluorescence mode by using an ion chamber.2 To enablemeasurements with a reasonable signal/background (S/B) ratio,
a solid-state detector (SSD) may be ineffective.9 An alternative
measurement of the Pb K edge (88 005 eV) instead of the L3 edge
cannot give enough information about the bonding to oxygen
atoms due to broadening effects,10 and “double fluorescence
X-rays” methods utilizing a Mn metal plate and Cr filter cannot
selectively monitor the Pb LR1 signal.13 In this work, XAFS
combined with a high-energy-resolution fluorescence spectrometer16-18 was applied to selectively monitor trace amounts of leadand obtain reliable spectra, free from the problems associated with
(3) Perera, W. N.; Hefter, G.; Sipos, P. M. Inorg. Chem. 2001, 40, 3974-3978.
(4) Fisher, E.; van den Berg, C. M. G. Anal. Chem. Acta 2001, 432, 11-20.
(5) Mortvedt, J. J. In Fertilizers and Environment; Rodriguez-Barrueco, C., Ed.;
Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996; pp 5-11.
(6) Gimeno-Garcia, E.; Andreu, V.; Boluda, R. In Fertilizers and Environment;
Rodriguez-Barrueco, C., Ed.; Kluwer Academic Publishers: Dordrecht The
Netherlands, 1996; pp 491-493.
(7) Ubavic, M.; Bogdanovic, D.; Cuvardic, M. In Fertilizers and Environment;
Rodriguez-Barrueco, C., Ed.; Kluwer Academic Publishers: Dordrecht The
Netherlands, 1996; pp 551-553.
(8) Rao, K. J.; Wong, J. J. Chem. Phys. 1984, 81, 4832-4843.
(9) The SSD should be operated in a detection mode with a high counting ratio
to detect the weaker Pb LR1 in the presence of an excess of other X-rays.
In this situation, photon counting loss phenomena and a lowering of the
S/B ratio are inevitable.

Analytical Chemistry, Vol. 74, No. 15, August 1, 2002 3819

photon-counting losses or broadening effects at higher photon
energies.
EXPERIMENTAL SECTION
Samples. Adsorbent 1 was synthesized using the proceduredescribed in ref 1. Adsorbent 1 effectively adsorbed 100-1000
ppb Pb2+ at high flow rates (space velocity, 50-800 min-1). The
specific BET surface area of adsorbent 1 was 110 m2 g-1. The
distance between the nearest two layers was 7.79 Å based on the
X-ray diffraction.
A 1.0 ppm lead(II) nitrate solution was adsorbed at a flow rate
(space velocity) of 50 min-1 until the Pb content reached 1.0 wt% (Pb/Fe atomic ratio: 0.016), as checked by inductively coupledplasma (ICP) measurements. In a separate experiment, 100 ppb
of lead(II) nitrate solution was interacted with adsorbent 1 at thesame flow rate until the Pb contents reached 0.12 and 0.30 wt %,as checked by ICP. The Pb/Fe atomic ratio in the two specimens
corresponds to 0.0019 and 0.0047, respectively. These samples
were dried and pressed into disks (diameter, 20 mm).
Standard inorganic PbII compounds were used as received anddiluted to 5.0 wt % Pb by mixing well with boron nitride using amortar and pestle. The mixture was then pressed into disks(diameter, 20 mm). Ion-exchanged standard samples were prepared from Pb2+ aqueous solutions on three kinds of zeolites. The
exchange ratio was 85.5 (NaY, JRC-Z-Y4.8, Na58Al58Si134O384‚
240H2O), 6.3 (sodium mordenite, JRC-Z-M10, Na8Al8Si40O96‚
24H2O), and 100% (NH4-ZSM-5, PQ Corp., NaxAlxSi96-xO192‚16H2O(x < 27)). The former two samples were distributed by theCatalysis Society of Japan. The case in which all the cation sites
were exchanged with Pb2+ (substitute one Pb2+ for two Na+ orNH4+) was defined as 100%.
X-ray Emission and Absorption Measurements. The X-ray
measurements utilizing a fluorescence spectrometer were performed on the Undulator Beamline 10XU of SPring-8 (Sayo). The
storage ring energy was 8.0 GeV, and the ring current was 10072 mA. A Si(111) double crystal monochromator was used. A
rhodium-coated double mirror set was inserted in the incident
X-ray beam path in order to suppress X-ray higher harmonics.
(10) Several K edge spectra at the higher energy (Ce 40 453-Pt 78 381 eV) werestudied.11 The mean free path λ of photoelectron at higher energy isexpressed λ ) 1/(1/λ + mΓK/2pk), where p, m, k, ΓK, are the Planck’s
constant, the mass, the wavenumber of the photoelectron, and the lifetime
width of level K, respectively. The oscillation χ of EXAFS is expressed as
the contributions of neighboring atoms i to absorbent χ(k) ∝ ∑ie-2Ri/λ )∑ie-2Ri/λe-(Rim/p)(ΓK/k). Thus, the lifetime broadening effects at higher energy
should be emphasized by larger ΓK for lead and a lower k value region.11,12(11) Nishihata, Y.; Kamishima, O.; Kubozono, Y.; Maeda, H.; Emura, S. J.
Syncrotron Rad. 1998, 5, 1007-1009.
(12) Uruga, T. at SPring-8, private communication.
(13) Secondary fluorescence X-rays, Mn KR1 (5898.75), KR2 (5887.65 eV), by
the emission of Pb LR1 are monitored. The Mn K fluorescence yield is0.314.14 When a Cr filter (K absorption 5988.8 eV) is placed between the
Mn metal plate and detector, mass absorption coefficients15 are 62 and 471
for Mn KR1 and Fe KR1, respectively. The detection of Pb LR1 is not selective
compared to the case of XAFS + fluorescence spectrometer.
(14) Tertian, R.; Claisse, F. Principles of Quantitative X-ray Fluorescence Analysis;
Heyden: London, 1982.
(15) International Tables for X-ray Crystallography; MacGillavry, C. H., Rieck, G.
D., Eds.; Reidel: Dordrecht, The Netherlands, 1985.
(16) Izumi, Y.; Kiyotaki, F.; Seida, Y. J. Phys. Chem. B 2002, 106, 1518-1520.
(17) Izumi, Y.; Oyanagi, H.; Nagamori, H. Bull. Chem. Soc. Jpn. 2000, 73, 20172023.
(18) Izumi, Y.; Kiyotaki, F.; Nagamori, H.; Minato, T. J. Electron Spectrosc. Relat.
Phenom. 2001, 119 (2/3), 193-199.

3820 Analytical Chemistry, Vol. 74, No. 15, August 1, 2002

Figure 1. Configuration of the XAFS system combined with afluorescence spectrometer.

The X-ray fluorescence from the sample was analyzed by a
Rowland-type spectrometer (R ) 220 mm) equipped with a
Johansson-type cylindrically bent Ge(555) crystal (50 × 50 mm2,
Crismatec) and a scintillation counter (SC, Oken) (Figure 1).17,18
The size of slit 0 was fixed at 1.0 × 1.0 mm2 throughout themeasurements. The samples were set in a plane slightly tilted
from a horizontal plane. The Rowland circle was in the verticalplane. The top surface of sample holder was tilted toward the
direction of the incident X-ray from the horizontal plane (θ ) 6.0°,
Figure 1) and also toward the direction perpendicular to incident
X-ray and in the same horizontal plane (χ ) 7.0°, Figure 1). The
configuration in Figure 1 effectively focuses the divergenceoriginating from the incident X-ray beam (polarized horizontally).
The energy resolution was 1.1 eV at the Cu KR1 emission (8 keV),
including the contribution of the beamline.18
The Pb LR1 emission spectrum was measured for Pb2+
adsorbed on 1 with the excitation energy set at 13 064.3 eV. The
samples were at 290 K. The scan step was ∼0.9 eV, and 80 points
were scanned between 10 535 and 10 605 eV. The dwell time of apoint was 60 s. The photon counts at SC decrease as the energy
resolution is improved. The energy resolution was lowered to 10.1
eV to obtain minimal photon counts (130 counts s-1) at SC for0.12 wt % Pb on adsorbent 1 by controlling the vertical size ofslits 1 and 2 (Figure 1).19 The emission energy of the Pb metal
was calibrated to 10 551.5 eV.20 The secondary fluorescence
spectrometer was used to discriminate the minor Pb LR1 fluorescence from the scattered + emitted X-rays originating from the
major Fe and Mg in the sample (Fe KR1 6403.84, KR2 6390.84,
Kβ1 7057.98, Mg KR 1253.60 eV). Similar optics have beenreported in applications to improve the energy resolution of X-ray
absorption near-edge structure (XANES) spectra21,22 and in spin23 or site-selective(19) Georgopoulos, P.; Knapp, G. S. J. Appl. Crystallogr. 1981, 14, 3-7.
(20) Bearden, J. A. Rev. Mod. Phys. 1967, 39, 78-124.
(21) Stojanoff, V.; Hamalainen, K.; Siddons, D. P.; Hastings, J. B.; Berman, L. E.;
Cramer, S.; Smith, G. Rev. Sci. Instrum. 1992, 63, 1125-1127.
(22) Garcia, J.; Sanchez, M. C.; Subias, G.; Biasco, J.; Proietti, M. G. J. Synchrotron
Radiat. 2001, 8, 892-894.
(23) Wang, X.; Grush, M. M.; Froeschner, A. G.; Cramer, S. P. J. Synchrotron
Radiat. 1997, 4, 236-242.

Figure 2. Pb LR1 emission spectrum for Pb2+ (0.12 wt %) on 1.
The solid line is the experimental data, and the dotted line is the fit
with a pseudo-Voigt function. The intensity ratio of the Lorentzian andGaussian components was fixed to 1:1.

XAFS.24,25 In Figure 1, the Ge(555) bent crystal received a solid
angle of 0.009 sr for the X-ray fluorescence emitted from thesample.
The fluorescence spectrometer was tuned to the peak of the
Pb LR1 emission spectrum, and the Pb L3 edge XAFS spectrum
was measured. The scan steps were ∼0.3 and ∼0.6 eV for theedge and postedge regions, respectively. The Undulator gap andthe piezoelement were optimized to maximize the X-ray beam flux
at each data point. The dwell time of each data point was 40-120
s. The rising edge energy of the Pb metal was calibrated to
13 040.6 eV.20
Reference Pb L3 edge XAFS spectra for standard PbII inorganic
compounds and Pb2+ ion-exchanged zeolites were measured using
a double crystal Si(311) monochromator in transmission mode at
290 K.
Analysis. Background subtraction of the raw XANES spectrum
was performed by using a Victoreen parameter. The subtraction
was not necessary in the case of XANES utilizing the fluorescence
spectrometer. A smooth spline function was calculated in thepostedge region, and then the spectrum was normalized at
13 095-13 115 eV. Derivative spectra of XANES utilizing thefluorescence spectrometer were smoothed using the locallyweighted least-squares error method of KaleidaGraph 3.5.1(Synergy Software).
RESULTSPb Lr1 Emission Spectra. The Pb LR1 emission spectrum
is depicted in Figure 2. The sample contained 0.12 wt % Pb
adsorbed on 1 from a 100 ppb Pb2+ solution. The peak appearedat the same energy as for Pb metal (10 551.5 eV). The Pb LR1emission spectra for other adsorbed Pb samples on adsorbent 1
and standard PbII compounds (not shown) were essentially thesame with respect to peak energy position and peak width. The
chemical shift on going from Pb0 to PbII may be small or has an(24) Grush, M. M.; Christou, G.; Hamalainen, K.; Cramer, S. P. J. Am. Chem.
Soc. 1995, 117, 5895-5896.
(25) Izumi, Y.; Nagamori, H. Bull. Chem. Soc. Jpn. 2000, 73, 1581-1587.

Figure 3. Normalized Pb L3 edge XANES spectra for Pb2+ on 1
measured by utilizing a secondary fluorescence spectrometer (ac). The Pb content was 1.0 wt % adsorbed from 1.0 ppm Pb2+ solution(a) and 0.30 (b) and 0.12 wt % Pb (c) from 100 ppb solution.
Reference XANES spectra for PbY zeolite (d), PbO (e), Pb(NO3)2(f), 2PbCO3‚Pb(OH)2 (g), Pb6O4(OH)4 (h), and PbCO3 (i). (d-i) were
measured in the conventional transmission mode.

equivalent level of experimental error (<0.4 eV).17,25 Therefore,site (state) selection of PbII was difficult in the case of Pb LR1.
Furthermore, the core-hole lifetime width for Pb LR1 (M5 f L3)
is greater (the core-hole lifetime width of Pb L3 is 5.81 eV26) thanthat of the K fluorescence lines of the first row of transition metalelements (1-2 eV; Mn Kβ1, Fe Kβ1, Cu KR1). Fortunately, only
the PbII species are included in samples of this study.
The peak cannot be fit well using only Lorentzian or Gaussianfunctions. However, the peak can be fit well using a pseudo-Voigt
function (a simple sum of Lorentzian and Gaussian functions)27(Figure 2, dotted line). The obtained full width at a half-maximum(fwhm) was 11.7 eV. Taking the core-hole lifetime width of Pb L3(5.81 eV) and a smaller M5 into account, the energy resolution of
the spectrometer was evaluated to be 10.1 eV, including thecontribution of the beamline.
Pb L3 Edge XANES Spectra. Normalized Pb L3 edge XANES
spectra are depicted in Figure 3. The rising edge positions arelisted in Table 1. The rising edge for standard PbII inorganic
compounds appeared within 13 043.4-13 044.9 eV, shifted by 2.84.3 eV toward higher energy from 13 040.6 eV for Pb0. The risingedge for lead (0.12-1.0 wt %) adsorbed on 1 appeared within therange 13 042.9-13 044.2 eV, demonstrating that the adsorbed leadstate was II and excluding the possibility of a lead site structure
similar to PbIVO2 or PbII,IV3O4. Based on the comparison of both
the rising edge (Table 1) and inflection point energies (Figure3), the spectrum for 1.0 wt % Pb on 1 (13 044.2 and 13 056.3 eV,
respectively; Figure 3a) resembled only that for 2PbCO3‚Pb(OH)2(13 044.0 and 13 055.5 eV, respectively; g). Although the acetate(26) Krause, M. O.; Oliver, J. H. J. Phys. Chem. Ref. Data 1979, 8, 329-338.
(27) Izumi, Y.; Glaser, T.; Rose, K.; McMaster, J.; Basu, P.; Enemark, J. H.;
Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121,
10035-10046.

Analytical Chemistry, Vol. 74, No. 15, August 1, 2002

3821

Table 1. Rising Edge Positions of Pb L3 Edge
Absorption for Pb2+ on 1 Measured Using a
Fluorescence Spectrometer and for Standard Pb
Compounds Measured in a Transmission ModesamplesourcePb 1.0 wt % on 1
Pb 0.30 wt % on 1
Pb 0.12 wt % on 1
PbY zeolite
PbO
Pb(NO3)2
2PbCO3‚Pb(OH)2
Pb6O4(OH)4
PbCO3
Pb metalfrom 1.0 ppm solnfrom 100 ppb soln

rising edgeenergy (eV)
13 044.2
13 042.9
13 043.1
13 043.4
13 043.8
13 044.9
13 044.0
13 044.2
13 043.4
13 040.6

group does not exist in this adsorption system, Figure 3aresembled also that for Pb(CH3CO2)2‚3H2O (not shown), suggesting the similarity of lead site structure to the case of Pb(CH3CO2)2‚3H2O. The samples of 0.30 and 0.12 wt % Pb on 1 wereboth prepared from 100 ppb Pb2+ solution, and the two spectra(b) and (c) resembled each other. The rising edge position andthe inflection point shifted by 1.1-1.3 and 1.4-2.7 eV, respectively,
toward lower energy on going from the Pb 1.0 wt % sample to the0.30-0.12 wt % samples. Based on the comparison of both risingedge and the inflection point energies, these two spectra (13 043.0( 0.1 and 13 054.3 ( 0.7 eV, respectively) resembled only thatfor PbY (13 043.4 and 13 054.4 eV, respectively; d). The XANES
spectra for lead mordenite and Pb-ZSM-5 were also measured (notshown). They were very similar to the spectrum for PbY (Figure
3d).
The above comparisons, based on the rising edge and theinflection point energies, can be rationalized by the comparisonof spectrum patterns in the whole energy range of Figure 3. Theedge regions of PbO, Pb(NO3)2 and Pb6O4(OH)4 (Figure 3e, f,
and h) were entirely different from those for adsorbed lead (ac). Two postedge peaks appeared for PbCO3 (i), which wererelatively intense compared to the weak, flat postedge features in(a-c). No significant peaks in the postedge region were observedin (d), and (g), either. The very weak, broad peak at 13 097 eV of(a) corresponded to that at 13 100 eV in (g). A postedge broad,weak peak appeared at 13 094 eV for Pb 0.30 and 0.12 wt % (band c). The energy position was similar to the case of PbY (13 090
eV, d) rather than that of 2PbCO3‚Pb(OH)2 (13 081 and 13 100
eV, g).
An unresolved shoulder peak was observed at 13 049 eV in
Figure 3a-c. A similar peak was also observed for 2PbCO3‚Pb(OH)2 (g). No shoulder peak appeared in this region for PbY (d).
This difference is clearly visualized in the derivative plots of thenormalized XANES spectra (Figure 4). Two split peaks wereobserved at 13 044.0 and 13 052.2 eV for 2PbCO3‚Pb(OH)2 (Figure4e), whereas single peak at 13 043.4 eV was observed for PbY(d). The signal-to-noise (S/N) ratio is worse in (a-c), but dispite
this, a separated peak feature clearly exists at 13 052 eV in addition
to a major peak (i.e., the rising edge) at 13 042.9-13 044.2 eV in(a-c).
3822 Analytical Chemistry, Vol. 74, No. 15, August 1, 2002

Figure 4. Derivative of the normalized Pb L3 edge XANES spectrain Figure 3. The Pb content was 1.0 wt % adsorbed from a 1.0 ppm
Pb2+ solution (a) and 0.30 (b) and 0.12 wt % Pb (c) from a 100 ppb
solution. The reference derivatives are from PbY zeolite (d) and2PbCO3Pb(OH)2(e).

In summary, most of the Pb2+ coagulated as a eutectic
mixture28 of PbCO3 and Pb(OH)2 on 1 in the case of a 1.0 ppm
solution. The formations of pure PbCO3 by the reaction withintercalated/dissolved carbonate or pure Pb6O4(OH)4 by a reaction
with surface hydroxyl groups should be negligible. In contrast,in the case of the 100 ppb solution, the major lead phase was ionexchanged Pb2+ via the surface reaction OH + Pb2+ f OPb+ +H+. A shoulder peak at 13 049 eV suggests a minor contribution
from coagulated eutectic mixture of PbCO3 and Pb(OH)2 on 1.
DISCUSSION
By analyzing the X-ray fluorescence emitted from 1 containing
trace amounts (0.12-1.0 wt %) of lead, the Pb LR1 signal was
selectively monitored using an energy resolution of 10.1 eV. This
energy resolution was good enough to discriminate the weaker
Pb LR1 signal (10 551.5 eV) among dominant Fe KR1, KR2, Kβ1,
Mg KR, and scattered X-rays (incident X-ray beam energy for
excitation 13 064.3 eV).
Our methodology to selectively measure XANES utilizes a partof X-ray fluorescence directed to Johansson-type bent crystal (50× 50 mm2) among the solid angle of 2π from the sample surface,leading to a relatively low S/N ratio of XANES data. On the other
hand, X-ray fluorescence collected at SC (Figure 1) was essentially
all due to lead and background signal was negligible, leading to
an extremely high S/B ratio.
In the case of goethite, an ion exchange mechanism was
proposed for the lead removal.29 The proposed ion exchange inthe case of 1 proceeds as in eq A. If we assume the apparent
density of 1 to be 0.5 g cm-3, the concentration of Pb2+ to be100-1000 ppb in the test solution, and a flow rate (space velocity)of 50 min-1, it will take 4.5-45 days for the molar ratio of adsorbed(28) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford,
U.K., 1984; p 628.
(29) Forbes, E. A.; Posner, A. M.; Quirk, J. P. J. Soil. Sci. 1976, 27, 154-156.

[Mg3Fe(OH)8]+ + Pb2+ f [Mg3Fe(OPb)(OH)7]2+ + H+
(A)
Pb2+/[Mg3Fe(OH)8]+ to reach unity. Step A is faster;1,30 i.e. theion exchange rate is essentially determined by diffusion. The rate
can be compared to the coagulation rate in the proximity of 1 foran acidic Pb2+ solution (pH value, 3-5.5).

Pb2+ + nCO32- + 2(1 - n)OH- anPbCO3‚(1 - n)Pb(OH)2 (B)The coagulation was proposed to proceed by the buffering effectsof the basic surface of 1 (Figure 5). The pH value in the proximityof 1 becomes 7-8 due to the effect of hydroxyl groups of theadsorbent and coagulant chemicals released through a slightdissolution of the adsorbent during the buffering.31,32
For the adsorption from 1.0 ppm aqueous Pb2+ solution,nPbCO3‚(1 - n)Pb(OH)2 coagulated at a basic pH. Only the lead
species similar to 2PbCO3‚Pb(OH)2 were detected by XANES,
utilizing a fluorescence spectrometer (Figure 3a). The numberof free Pb2+ ions that were able to access to the surface hydroxyl
groups should be negligible.
In the case when the concentration in aqueous solution
decreased from 1.0 ppm to 100 ppb, the equilibrium of coagulation
shifted to the left-hand side of eq B. The ratio of the free Pb2+
ions in solution dramatically increased, and thus, the ion exchange
reaction dominantly proceeded (eq A). Hence, the major species
of ion-exchanged Pb2+ (right-hand side of step A) and the minor
phase of the coagulated eutectic mixture of Pb(CO3) and Pb(OH)2(right-hand side of step B) were detected by means of XANES byutilizing a fluorescence spectrometer (Figure 3b and c).
The lead removal by adsorbent 1 is efficient in the range offlow rates (space velocity) of 50-800 min-1, where a major partof lead ion cannot be trapped by activated carbon. In this rangeof flow rates, the concentration of Pb2+ solution was found to
determine the kind of adsorbed lead species: eutectic mixture
coagulation of Pb(CO3) and Pb(OH)2 at higher concentrations (1.0ppm) versus ion-exchanged Pb2+ with surface hydroxyl groups
at lower concentrations (100 ppb).
This method combining XAFS with fluorescence spectrometry
can be applied to perform a systematic survey of the removal
process of dilute toxic elements, e.g., Cr, Cu, Zn, As, Cd, or Hg.
Quantitative estimation should be extended in future in combination with the theory of XANES and EXAFS. The ultimate
advantage of this method is when the energies of the minor andmajor X-ray fluorescence are different by less than 100 eV. In this(30) Seida, Y.; Nakano, Y. Water Res. 2000, 34, 1487-1494.
(31) Seida, Y.; Nakano, Y. J. Chem. Eng. Jpn. 2001, 37, 906-911.
(32) Seida, Y.; Nakano, Y. Water Res. 2002, 36, 1306-1312.

Figure 5. Pb2+ adsorption mechanism on 1 from Pb2+ 1.0 ppm and100 ppb aqueous solutions.

case, no other detection systems can selectively detect the minor
X-ray fluorescence derived from trace amounts of an element. The
setup we have achieved (Figure 1) can achieve this with an energy
resolution of 0.3-10 eV.
CONCLUSIONSThe chemical state and local structure of 0.12 wt % Pb2+ werefirst determined by XAFS combined with fluorescence spectrometry. XANES is often utilized for speciation, and this paper extends
the lower limit of concentration as small as 0.12 wt % lead in thepresence of 17.35 wt % iron. The local structure was found to be
dependent on the concentration of Pb2+ in the test aqueous
solution: a eutectic mixture of PbCO3 and Pb(OH)2 and dominant
ion-exchanged Pb2+ species in the case of adsorption from 1.0
ppm and 100 ppb Pb2+, respectively. This difference can be
explained by the balance between the precipitation equilibriumof the mixture PbCO3 and Pb(OH)2 and the ion exchange ratewith the surface hydroxyl groups. The speciation of lead salts will
be helpful in the design of materials for water treatment.
ACKNOWLEDGMENT
The experiments were performed under the approval of theSPring-8 Program Review Committee (2001A0022-NX-np and2001B0004-CX-np). Y.I. expresses thanks for financial support fromthe Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology (13555230,
12740376), Yamada Science Foundation (2000), and Toray Science
Foundation (98-3901).
Received for review January 28, 2002. Accepted May 6,
2002.
AC025550P

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