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J. Phys. Chem. B 2002, 106, 1518-1520

Monitoring of Trace Amounts of Lead on an Adsorbent by X-ray Absorption Spectroscopy
Combined with a Fluorescence Spectrometer
Yasuo Izumi,*,† Fumitaka Kiyotaki,† and Yoshimi Seida*,‡Department of EnVironmental Chemistry and Engineering, Interdisciplinary Graduate School of Science andEngineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, andInstitute of Research and InnoVation, 1201 Takada, Kashiwa 277-0861, Japan
ReceiVed: September 12, 2001; In Final Form: NoVember 28, 2001

A trace amount of lead on adsorbent Mg6Fe2(OH)16(CO3)‚3H2O was selectively monitored by XAS combinedwith a fluorescence spectrometer. In the presence of dominant scattered and emitted X-rays derived fromheavy elements (Fe and Mg), large quantities of heavy elements predominantly absorb the incident photons
and the X-ray fluorescence derived from trace amounts of elements is very weak compared to scattered +emitted X-rays derived from heavy elements. Pb LR1 emission was counted during XAS measurements witha fluorescence spectrometer of energy resolution of 1.1 eV. The obtained spectrum was compared to thespectra for standard Pb2+ compounds. The ion exchange mechanism FeO(OH) + Pb2+ f FeO-OPb+ + H+
was rejected, but a eutectic mixture of PbCO3 and Pb(OH)2 may be coagulated on the basic surface of adsorbent.

Introduction
X-ray absorption spectroscopy (XAS) has become widelyused to monitor the local structure of noncrystalline andheterogeneous/hybrid materials. Recent innovations in synchrotron radiation and X-ray detectors have enabled us to study
lower-concentration systems, e.g., metalloenzymes in solution,1
impurity in materials,2 and surface sites.3 However, even withthis state-of-the-art technology, there is still a large barrier tomeasuring the absorption edge of trace amounts of an elementin samples that contain a high concentration of heavy element(s). Large quantities of heavy elements in samples will
predominantly absorb the incident photons, and consequently
the S/B ratio for the minor-element absorption edge becomes
extremely depressed. When the X-ray fluorescence associatedwith the relaxation to the core-hole as produced by photon
absorption is monitored, the X-ray fluorescence derived from atrace amount of elements must be selectively monitored in thepresence of the dominant scattered and emitted X-rays derived
from heavy elements.
In this Letter, we chose as an example a trace amount oflead on adsorbent Mg6Fe2(OH)16(CO3)‚3H2O (Figure 1). Whenthe ratio of adsorbed Pb and Mg was in the range 0.00050.005 (mol/mol), XAS measurements in transmission mode are
difficult to achieve because the Pb L3-edge jump is only 0.0150.15 versus a total absorption of 4.0. Experiments carried outin traditional fluorescence mode using a Lytle detector2 are alsodifficult to interpret because the photon number ratio of Pb LR1/Fe KR1 is only 0.006-0.056 in the concentration range of Pb.
To enable measurements of trace amounts of Pb, the selective
detection of Pb LR1 (10551.5 eV) among the scattered X-rays,
Fe KR1 (6403.8 eV), Kβ1 (7058.0 eV), and Mg KR (1253.6eV), and the X-ray fluorescence from the filter (to reduce thescattered X-rays) is necessary. A solid-state detector (SSD) may
be used. The SSD should be operated in a detection mode with* Corresponding author. E-mail: yizumi@chemenv.titech.ac.jp.
† Tokyo Institute of Technology.
‡ Institute of Research and Innovation.

Figure 1. Layered structure of pyroaurite and proposed Pb2+ adsorption
site from low-concentration Pb2+ (1.0 ppm) at a high flow rate (spacevelocity: 50 min-1).

a higher counting ratio to detect the weaker Pb LR1 in thepresence of an excess of other X-rays. In this situation, photon
counting loss phenomena and a worse S/B ratio are inevitable.
In this work, XAS combined with a high-energy-resolution
fluorescence spectrometer was applied to selectively monitor
trace amounts of Pb, free from the problems associated withphoton counting loss.
Experimental Section
Pyroaurite is one of the basic polyhydroxides (Figure 1) andis a new adsorbent used for water treatment instead of activatedcarbon. It adsorbs low concentrations of Pb ions at high flow
rates.4 The common oxidation state of Pb is II, which is achronic toxin toward humans and animals.5
A 1.0 ppm solution of PbII nitrate was interacted with
pyroaurite at a flow rate (space velocity) of 50 min-1 until thePb/Mg molar ratio reached 0.005, as checked by ICP. The XAS
was measured for a dried sample using the Undulator Beamline

10.1021/jp015539d CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/24/2002

LettersFigure 2. Pb L3-edge XANES spectra for Pb2+ on pyroaurite measured
with a secondary fluorescence spectrometer (a). Reference XANES
spectra for Pb(CH3COO)2‚3H2O (b), PbY zeolite (c), PbO (d), Pb(NO3)2(e), 2PbCO3‚Pb(OH)2 (f), Pb6O4(OH)4 (g), and PbCO3 (h). (b)-(h) were
measured at KEK-PF 10B (double-crystal Si(311) monochromator) intransmission mode.

10XU of SPring-8. The storage ring energy was 8.0 GeV, andthe current was 99-72 mA. A Si(111) double crystal monochromator was used. The X-ray fluorescence from the sample
was analyzed by a Rowland-type spectrometer (R ) 220 mm)
equipped with a Johansson-type Ge(555) crystal and scintillation
counter.6 The energy resolution was 1.1 eV, including thecontribution of the Beamline at 8 keV.7 The typical energy
resolution of an SSD is 100 eV. A secondary fluorescence
spectrometer was used to discriminate the X-ray fluorescenceoriginating from the minor element from the scattered + emitted
X-rays originating from the major elements in the sample. Asimilar experimental setup has been reported for other applications: site-selective,8,9 spin-selective,10 and high-energy-resolution XAS,9,11 but its application to the present purpose has notbeen reported previously.
The Pb LR1 emission spectrum was measured for Pb2+ on
pyroaurite with the excitation energy set at 13 064.3 eV. Next,
the Pb L3-edge X-ray absorption near-edge structure (XANES)
was measured by utilizing the fluorescence spectrometer. The
fluorescence spectrometer was tuned to the maximum peakenergy (40 cps) of the emission spectrum. The Ge(555) crystal
received a solid angle of 0.004 sr for the X-ray fluorescencefrom the sample. The scan steps were ∼0.3 and ∼0.6 eV forthe edge and postedge region, respectively. The Undulator gap
was optimized to maximize the beam flux at each data point.
The dwell time of each data point was 120 s. The rising edgeenergy of the Pb metal was calibrated to 13 040.6 eV.
Results and Discussion
The Pb LR1 emission spectrum for Pb2+ on pyroaurite
appeared at the same energy as that for PbII oxide, demonstratingthat the valence state of adsorbed lead was II. As the core-hole
lifetime width for Pb LR1 (MV f LIII) is greater (the width ofPb LIII, 5.81 eV12) than for Mn Kβ1, Fe Kβ1, or Cu KR1 (1-2eV) previously reported for site-selective XAS,6-11 site selection
was rather difficult for Pb LR1. However, the Pb2+ state wasbasically chosen by tuning the fluorescence spectrometer to thepeak of the Pb LR1 emission spectrum with a narrow band path.
Figure 2 shows the normalized Pb L3-edge XANES spectra.
In the case of Pb/pyroaurite (Figure 2a), the rising edge was at

J. Phys. Chem. B, Vol. 106, No. 7, 2002 1519
13 043.8 eV and the inflection point was at 13 056.3 eV. As isevident in the first derivative (not shown), a shoulder wasobserved at 13 049 eV. As the edge position corresponded toan oxidation state of 2+, the spectrum was compared with theXANES spectra of standard PbII compounds and Pb2+exchanged zeolites13 (Figure 2b-h). The shape around theinflection point (13 050 to 61 eV) of Pb oxide (d) and Pb nitrate(e) was entirely different from that of (a). Relatively strong peaksin the postedge region of Pb6O4(OH)4 (g) and Pb carbonate (h)appeared at 13 105 and 13 089 eV, which is different from thebroad, weak feature at 13 097 eV for (a). The XANES spectra
for the three Pb2+-exchanged zeolites were essentially the same.
The rising edge for PbY was at 13 043.4 eV and the inflection
point was at 13 054.4 eV (c), similar to the case of (a). However,no shoulder peak around 13 049 eV was detected. In the caseof goethite, Pb2+ removal by ion exchange in acidic solution
was proposed: FeO(OH) + Pb2+ f FeO-OPb+ + H+.4 In thecase of Pb2+ adsorption on the basic surface of pyroaurite, theion exchange model was rejected.
For Pb acetate, the rising edge, the inflection point, and thepostedge peak were at 13 043.6, 13 055.8, and 13 094 eV,
respectively (Figure 2b). The differences from the corresponding
values of (a) were only 0.2-3 eV. The shoulder at 13 049 eV
was also observed. The spectrum pattern of 2PbCO3‚Pb(OH)2(Figure 2f) was very similar to that of Pb acetate (b), and therising edge was at 10 344.0 eV, similar to that in Figure 2a(13 043.8 eV). In summary, Figure 2a most closely resembled(b) and (f).
On the basis of these XANES results, it was found that atrace amount of Pb2+ on pyroaurite did not form pure PbCO3
by the reaction with intercalated or dissolved carbonate, norpure Pb6O4(OH)4 by a reaction with surface hydroxyl groups.
A possible mechanism may be coagulation as a eutectic mixtureof PbCO3 and Pb(OH)2. As Figure 2a most closely resembled(b), approximately four oxygen atoms may coordinate to Pb2+
as a statistical average.
The Pb L3-edge extended X-ray absorption fine structure(EXAFS) was measured utilizing the fluorescence spectrometer
for Pb/pyroaurite up to k ) 10 Å-1 (Å ) 0.1 nm). The Pb-O
bond was found at 2.4 Å and the coordination number (N) was∼3 based on curve fitting analysis. Empirical fit parameters fromthe EXAFS of PbO2 (rPb-O ) 2.18 Å, NPb-O ) 6)14 were used.
The average Pb-O distances and the N values were determined
to be 2.30 Å (N ) 4) for PbO,14 2.69 Å (N ) 9) for PbCO3(cerussite),15 2.47 Å (N ) 4) for Pb(CH3COO)2‚3H2O, 2.338
Å (N ) 4) for Pb6O4(OH)4,16 and 2.34-2.54 Å (N ) 3) for[Pb4(OH)4]4+.17 The observed Pb-O distance of 2.4 Å for Pb/
pyroaurite was in the range associated with the oxygen atoms
in Pb oxide, hydroxyl, or acetate, but too short for oxygen atomsof Pb carbonate. Hence, it was suggested that on average, three/four oxygen atoms of both hydroxyl and carbonate werecoordinated to Pb2+. The major ligand may be the hydroxyl
group. It is not plausible that a phase of high (as much as nine)
NPb-O(carbonate) could exit in the coagulated mixture on pyroaurite.
Both 2PbCO3‚Pb(OH)2 and PbCO3‚Pb(OH)2 are known asminerals.14 At the moment, the ratio of PbCO3 and Pb(OH)2
cannot be determined in the case of a trace amount of Pb2+ on
pyroaurite.
A new analytical methodology was described to selectively
measure the local structure of trace amounts of Pb2+ in Fe- andMg-based adsorbent materials by utilizing a high energyresolution fluorescence spectrometer. This method can beapplied to the removal process of dilute toxic elements, e.g.,
Cr, Cu, Zn, As, Cd, or Hg in the environment, impurity effects

1520 J. Phys. Chem. B, Vol. 106, No. 7, 2002
in the steel industry, and the active/promoter sites of heterogeneous catalysts supported over TiO2, ZnO, ZrO2, or Nb2O5.
Acknowledgment. The experiments were performed under
the approval of the SPring-8 Program Review Committee(2001A0022-NX-np). Y.I. is thankful for grants from the Grantin-Aid for Scientific Research from the Ministry of Education,Culture, Sports, Science, and Technology (13555230, 08874066,and 12740376), the Toray Science Foundation (98-3901), andthe Yamada Science Foundation (2000).
References and Notes(1) Hodgson, K. O.; Hedman, B.; Zhang, H. J. Inorg. Biochem. 1997,67, 424.
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Methods Phys. Res. 1984, 226, 542-548.
(3) Oyanagi, H. Appl. Surf. Sci. 1992, 60/61, 522-528.
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(5) Perera, W. N.; Hefter, G.; Sipos, P. M. Inorg. Chem. 2001, 40,3974-3978.

Letters(6) Izumi, Y.; Oyanagi, H.; Nagamori, H. Bull. Chem. Soc. Jpn. 2000,73, 2017-2023.
(7) Izumi, Y.; Kiyotaki, F.; Nagamori, H.; Minato, T. J. Electron
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(13) From Pb2+ aqueous solution, the exchange ratio was 85.5 (NaY),
6.3 (Na-mordenite), and 100% (NH4-ZSM-5). The case that all the cationsites were exchanged with Pb2+ (substitute one Pb2+ for two Na+) was
defined to 100%.
(14) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon
Press: Oxford, U.K., 1984; p 628.
(15) Sahl, K. Z. Kristallogr. 1974, 139, 215-222.
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(17) Kagaku-binran (Chemistry Handbook) Basic Edition, 4th ed.;Maruzen: Tokyo, 1993.