Monitoring Trace Amounts of Lead and Arsenic Adsorbed
under Environmental Conditions
by XAFS Combined with Fluorescence Spectrometry

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The risk of trace amounts of metals in environmental
water to human health has been pointed out [1]. The
regulations of metal concentration in water are
becoming more severe: e.g., 100 ppb for lead and 10ppb for arsenic. Co-precipitation and adsorption arepromising methods for the removal of trace amountsof metals in water. Information on the removed metalsite structure and the chemical state is crucial in theassessment of the removal effectiveness for the mosthazardous valence state of metals.
X-ray absorption fine structure (XAFS) has become
widely used with the state-of-the-art technology ofsynchrotron radiation and advanced X-ray detectors tomonitor the local structure of noncrystalline andheterogeneous/hybrid materials. However, there arestill experimental difficulties in measuring at highenergy resolution the absorption edge of the trace
amount of an element in samples containing a highconcentration of heavy element(s).
In this account, 0.12 wt% Pb adsorbed from a 100
ppb aqueous solution and 0.20 wt% As adsorbed froma 200 ppb arsenite solution were monitored by theXAFS combined with a high-energy-resolution
fluorescence spectrometer [2]. The energyresolution s were 0.3 eV at 5 keV and 1.1 eV at 8 keV.
The latter value includes the contribution of thebeamline [3]. These values are smaller than the corehole lifetime widths of Pb L3 (5.81 eV) and As K (2.14eV). Thus, the advantages of the method applied inthis account are (i) the removal of lifetime broadening[4], (ii) reasonable signal/background ratio, and that(iii) the method is free from the problems associatedwith photon-counting losses.
The X-ray fluorescence from Pb adsorbed on
Mg 6Fe 2(OH) 16 (CO 3)•3H 2 O(1) was analyzed by a
R o w l a n d - t y p e s p e c t r o m e t e r ( r a d i u s 2 2 0 m m)equipped with a Johansson-type cylindrically bentGe(555) crystal and a scintillation counter at beamline
BL10XU [5]. The spectrometer was tuned at 10551.5
eV (Pb Lα1 emission), and the obtained Pb L3-edge Xray absorption near-edge structure (XANES) spectra
are shown in Figs. 1(a-c). Based on the comparisonof the rising edge and post-edge peak energies,
spectrum a for Pd adsorbed from the 1.0 ppm solution
resembled only that for 2PbCO 3•Pb(OH)2 (Fig. 1(g)).
Figures 1(b,c) for Pb adsorbed from a 100 ppb56.3

e0.8 dc0.4 ba0.01302097Value in figure= Energy (eV) – 13000
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Energy (eV)Fig. 1. Normalized Pb L3-edge XANES spectra for Pb2+ on1 measured utilizing a secondary fluorescence spectrometer(a – c). The Pb content was 1.0 wt% adsorbed from a 1.0ppm Pb 2+ solution (a) and Pb 0.30 wt% (b) and 0.12 wt%(c) adsorbed from a 100 ppb solution. XANES spectra for
PbY zeolite (d), PbO (e), Pb (NO3 )2 (f), 2PbCO3 •Pb(OH)2(g), Pb6O 4(OH)4 (h), and PbCO3 (i) were measured in theconventional transmission mode.

solution resembled each other. The rising edge
position shifted from –1.1 to –1.3 eV compared to Fig.
1(a). Figures 1(b,c) resembled only those of PbY
zeolite (Fig. 1(d)), Pb mordenite or Pb-ZSM-5. Pb 2+
ions replace the protons or Na+ sites of zeolites.
An unresolved shoulder peak was observed at13049 eV in Figs. 1(a-c). A similar peak was also
observed for 2PbCO 3 •Pb(OH) 2 (Fig. 1(g)). Noshoulder peak appeared in this region for PbY (Fig.
1(d)).
In summary, most of the Pb2+ coagulated as aeutectic mixture of PbCO3 and Pb(OH)2 on 1 in theadsorption from a 1.0 ppm solution. In contrast, in theadsorption from a 100 ppb solution, the major Pb
phase was ion-exchanged Pb2+ via surface reaction A.
– OH + Pb2+ → – OPb+ + H+(A)A shoulder peak at 13049 eV suggests a minor
contribution from the coagulated eutectic
mixture of PbCO 3 and Pb(OH)2 on 1. Theinterpretation of this difference that depend s onthe Pb concentration is illustrated in Fig. 2. The
surface of 1 has a buffering effect and the pH
value in the proximity of 1 becomes 7 – 8 due tothe effect of coagulant chemicals r eleased
through a slight dissolution of 1 [5]. In the pHregion, a forward reaction of B proceeds from a
1.0 ppm solution.

(B)

In the adsorption from a 100 ppb Pb2+solution, equilibrium shifted to the left-hand sideof equation B. The ratio of free Pb2+ in thesolution dramatically increased, and thus the,ion exchange reaction (equation A) dominantly
proceeded. Hence, the major species of ionexchanged Pb 2 + and the minor phases of thecoagulated eutectic mixture of PbCO3 and Pb(OH)2
were detected (Figs. 1(b,c)).
The As K -edge XANES spectra were alsomeasured s imilarly t o the Pb L 3 -edge spectra
described above. The fluorescence spectrometer wastuned to 10544.3 eV (As Kα 1 emission). Samplesof 0.48 – 0.20 wt% As a dsorbed from 16 ppm ofarsenate, 16 ppm of arsenite, and 200 ppb ofarsenite aqueous solutions were studied (Figs.
3 ( d - f )) on an Fe-impregnated montmorillonite(Na1.5Ca0.096Al5.1Mg1.0Fe0.33Si12 O27.6(OH)6.4 )(2) [3].
The three spectra resembled one another. The strong
peak positions in the post-edge region were very
close to that for the spectrum of KH 2 AsV O4 (Fig. 3(c) )rather than that for the spectrum of AsIII2O3 (Fig. 3(b)).
Thus, the observed oxidation of trace amounts ofarsenite upon adsorption on 2 to arsenate is importantin the preservation of the environment because
arsenate affects far less human health than arsenite.

Fig. 2. Pb2+ adsorption mechanism on 1 from1.0 ppm and 100 ppb aqueous Pb2+ solutions.

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Pb2+ + nCO32– + 2(1–n)OH–→ nPbCO3 •(1–n)Pb(OH)26fe4dc2ba011860

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Energy (eV)Fig. 3. Normalized As K-edge XANES spectra for As
metal (a), As 2 O3 (b), KH 2AsO4 (c), and As adsorbed on 2(Fe 15.3 wt%, d – f ) from solutions of 16 ppm ofKH2AsO4 (d), 16 ppm of As2O3 (e), and 200 ppb of As2O3(f). Spectra (d – f) were measured utilizing a high-energyresolution fluorescence spectrometer. The spectra (a – c)were measured in the conventional transmission mode.

References[1] http://www.who.int/water_sanitation_health/en/[2] Y. Izumi et al.: J. Electron Spectrosc. Relat.
Phenom. 119 (2001) 193.
[3] Y. Izumi, F. Kiyotaki, T. Minato, D. Masih and Y.
Seida: Phys. Scr. - in press.
[4] Y. Izumi and H. Nagamori: Bull. Chem. Soc. Jpn.
73 (2000) 1581.
[5] Y. Izumi et al.: Anal. Chem. 74 (2002) 3819.

Yasuo Izumi
Tokyo Institute of Technology

E-mail: yizumi@chemenv.titech.ac.jp

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