J. Phys. Chem. B 2005, 109, 3227-3232

3227

Characterization of Intercalated Iron(III) Nanoparticles and Oxidative Adsorption of
Arsenite on Them Monitored by X-ray Absorption Fine Structure Combined with
Fluorescence Spectrometry
Yasuo Izumi,*,† Dilshad Masih,† Ken-ichi Aika,† and Yoshimi Seida‡
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,4259-G1-16, Nagatsuta, Midori-ku, Yokohama 226-8502, and Institute of Research and InnoVation,
1201 Takada, Kashiwa 277-0861, Japan
ReceiVed: June 5, 2004; In Final Form: NoVember 9, 2004

This paper first deals with the screening and optimization of FeIII-based adsorbents for arsenic adsorption
from 0.2 to 16 ppm test solutions of arsenite/arsenate. The best adsorption capacity has been reported on
R-FeO(OH) on an adsorbent weight basis. Better results were found on intercalated Fe-montmorillonites forarsenite adsorption below the equilibrium dissolved As concentration of 310 ppb and for arsenate adsorption
in all of the concentrations studied. Next, the speciation of As adsorbed was performed by As K-edge x-ray
absorption fine structure (XAFS) combined with high-energy-resolution fluorescence spectrometry. Major
oxidative adsorption of arsenite was observed on Fe-montmorillonite from the 0.2-16 ppm test solutions.
The reasons for the higher capacity of arsenic adsorption and oxidative adsorption of arsenite on
Fe-montmorillonite are discussed.

Introduction
Regulations concerning the concentration of arsenic in public
water supplies are becoming severe, at 10 ppb, due to the highcarcinogenic risk.1,2 The removal of As from water by theadsorption method is economically advantageous compared tomethods using membrane, ion exchange, or reverse osmosis.
For the removal of trace amounts of As in drinking water, we
screened FeIII-based materials to optimize the greatest adsorption
capacity of arsenite/arsenate.3 The adsorbents included R-FeO(OH) with specific surface area (SA) of 16-54 m2 g-1 andintercalated FeO(OH) nanoparticles between montmorillonites.
In this paper, the responsible structure was studied for the
optimized adsorbent by using Fe K-edge EXAFS (extended
X-ray absorption fine structure).
The understanding of the removal mechanism from low
concentrations of metals in water is important to feed back to
the investigation of adsorbents. Recently, the switching of theremoval mechanism of Pb2+ on Mg6Fe2(OH)16(CO3)‚3H2O wasreported. By the observations using XAFS combined withfluorescence spectrometry, it was found that coagulation took
place from 1.0 ppm test solutions of Pb2+, in contrast to theobservation that the surface ion exchange occurred with surfacehydroxyl groups from 0.1 ppm test solutions of Pb2+.4,5 In
comparison to conventional XAFS, the combination of XAFS
with high-energy-resolution fluorescence spectrometry enabled
one to observe low concentrations of Pb with far better signal/
background (S/B) ratios in the presence of high concentrationsof Fe in samples.
Furthermore, better energy resolution of XANES (X-ray
absorption near-edge structure) spectra is required to quantify
each oxidation state for arsenic (AsIII and AsV). Valence* To whom correspondence should be addressed. E-mail: yizumi@
chemenv.titech.ac.jp.
† Tokyo Institute of Technology.
‡ Institute of Research and Innovation.

information is crucial to the study of arsenic removal, because
arsenite is particularly toxic.1,2 In this study, XAFS combinedwith fluorescence spectrometry was applied to As adsorbed on
the adsorbent optimized among the FeIII-based materials.
Experimental Section
Preparation of Fe-Montmorillonites. A quantity (1.1-7.2mL) of a 0.43 M FeIII nitrate solution was mixed at 290 K for
2 h with 200 mL of a 0.5 wt % Na-montmorillonite (Kunipia
F; Na1.5Ca0.096Al5.1Mg1.0Fe0.33Si12O27.6(OH)6.4) suspended in
distilled water. Na-montmorillonite was used as received. The
primary particle size of Na-montmorillonite was 0.1-1.0 µm.
A 0.75 M sodium hydroxide solution was added dropwise to
the above mixture until the molar ratio of Fe3+ added andhydroxide reached 1:2.
The mixture was stirred at 290 K for 12 h. It was filtered
and washed with distilled water until the pH of the filtrate
became higher than 6.5. The resultant powder was dried at 313
K for 72 h. Intercalated FeIII-based materials prepared by thisroute are called Fe-montmorillonites.
Samples were also prepared by simple Fe3+ ion exchangewith Na+ and Ca2+ sites of Na-montmorillonite or by theinteraction of Na-montmorillonite with the colloid in aqueous
solution formed by the reaction of Fe3+ and NaOH (molar ratioof 1:2). The sodium content in Fe-montmorillonite (Fe 14.0 wt%) and simple Fe3+ ion-exchanged montmorillonite (Fe 2.8 wt%) powder was determined to be 0.173 ((0.002) and 0.188((0.002) wt %, respectively, by inductively coupled plasma(ICP) measurements combined with optical emission spectroscopy (Prodigy, Leeman Labs) at the Center of Advanced
Materials Analysis, Tokyo Institute of Technology (Dr. T. Toya).
Hence, 94.0% of Na+ sites in Na-montmorillonite were exchanged with Fe3+ by simple ion exchange. An excess amountof Fe3+ introduced and/or the addition of NaOH to prepare Fe-

10.1021/jp047571o CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/21/2005

3228 J. Phys. Chem. B, Vol. 109, No. 8, 2005
montmorillonite had a negligible effect on Na+ site substitution.
A high level (94.5%) of Na+ sites at the interlayer space wereeffectively substituted by Fe3+ ions for Fe-montmorillonite (Fe14.0 wt %).
Arsenic Adsorption Tests. A quantity (20 mL) of 0.2-16
ppm solutions of AsIII2O3 or KH2AsVO4 was introduced to 50
mg of Fe-montmorillonites or R-FeO(OH) (Kanto Chemicals,
Goethite >95.0%) in a 30-ml polypropylene tube. The mixture
was shaken at a rate of 130× min-1 at 290 K for 12 h. The
system reached an adsorption equilibrium within 12 h. The
equilibrium dissolved As concentration was determined by ICP
measurements.
BET Surface Area and X-ray Diffraction Measurements.
The BET surface area was measured using a BELSORP 28SA
(Bell Japan) with nitrogen gas as the adsorbate. The Femontmorillonites were evacuated at 313-423 K for 1 h before
measurements. X-ray diffraction (XRD) data were obtainedusing a Multiflex-S diffractometer (Rigaku). Cu KR emission
was used with a Ni filter. The diffractions were monitored for
Bragg angles (θB) between 2θB ) 2.0° and 60.0°.
Fe K-edge XAFS Measurements. The Fe-montmorillonites(0, 2.8, and 14.0 wt % Fe added) were evacuated at 290 K andtransferred to a Pyrex glass cell. The windows were sealed with
Kapton film of 12.5 µm thickness. R-Fe2O3 was mixed wellwith boron nitride (BN) and pressed into a disk of φ ) 20 mm
to give an Fe K absorption edge jump value of 1.0. The Fe
K-edge XAFS spectra were measured at Beamline 10B of KEKPF in a transmission mode at 290 K. The storage ring energy
was 2.5 GeV, and the ring current was 440-315 mA. A Si(311) double crystal monochromator was used. Nitrogen gas
and a mixture of N2 (85%) and Ar (15%) gases were used in
the I0 and It ion chambers (Oken), respectively. The rising edgeenergy of the Fe metal foil was calibrated at 7111.20 eV.6 The
scan steps were ∼0.1, ∼0.5, and ∼2 eV for the edge, postedge (XANES), and EXAFS regions, respectively. The integration time for a data point was 1-10 s.
As K-edge XAFS Measurements. As K-edge XAFS spectra
were measured at 290 K for 0.13-0.48 wt % of As adsorbed
onto Fe-montmorillonites at Beamline 10XU of SPring-8 byutilizing a high-energy-resolution fluorescence spectrometer.7-9
All of the samples were pressed into a disk of φ ) 20 mm.
Standard As samples were diluted with BN to 2.0 wt % As.
The storage ring energy was 8.0 GeV, and the ring current was
97-50 mA. A Si(111) double crystal monochromator was used.
A rhodium-coated double mirror set was inserted in the incident
X-ray beam path at 3.0 mrad in order to suppress the higher
harmonics. The undulator gap and the ∆θ of the second crystalof the monochromator (rocking curve) were optimized tomaximize the beam flux at each data scan point. The scan steps
were ∼0.2, ∼0.5, and ∼2 eV for the edge, post-edge (XANES),
and EXAFS regions, respectively. The integration time for adata point was 10-60 s.
The homemade high-energy-resolution Rowland-type fluorescence spectrometer selectively counts the X-ray fluorescenceemitted from the sample over a small energy band (0.3-10 eV)
and thus enables higher-energy-resolution XAFS spectra7,8
and the measurement with high S/B ratio of trace amountsof elements in samples that contain high concentrations ofheavy elements.4,5 The Rowland radius was 220 mm anda Johansson-type Ge(555) crystal (Saint-Gobain) was
mounted.
XAFS Data Analysis. The EXAFS data analysis was
performed using XDAP (XAFS Services International) basedon the works of M. Vaarkamp, H. Linders, and D. Koingsberger.

Izumi et al.
TABLE 1: Basic Physicochemical Data of
Fe-Montmorillonites with the Fe Loading of 0-14.0 wt %Fe loadinga(wt %)

Fe3+/(Na+ + 2Ca2+)

d001
(Å)

specific SA(m2 g-1)

0
2.8
7.0
14.0

0
0.34
1.1
2.0

12.2
12.7
13.5
15.3

25.8
45.8
66.7
100

a

Fe included in as-received Na-montmorillonite was not counted.

The pre-edge background was approximated by the modified
Victoreen function

C2
E

2

+

C1
+ C0
E

The background was approximated by a smoothing spline
calculated by the equation
N

∑
i)1

(µxi - BGi)2exp(-0.075ki2)

e smoothing parameter

Multiple shell analyses were performed for the Fourier-filtered
EXAFS data using empirical parameters for Fe-O andFe‚‚‚Fe extracted from EXAFS for R-Fe2O3 and for As-O andAs‚‚‚Fe extracted from EXAFS for FeAsO4. The bond distancesand coordination numbers were based on crystal structure data
for R-Fe2O310,11 and FeAsO4.12 Listed σ2 values (Tables 2 and
3) are relative (∆σ2) to those of corresponding model parameters.
The goodness of fit was given as requested by the Committee
on Standards and Criteria in X-ray Absorption Spectroscopy.
Results
Screening of Adsorbents among FeIII-Based Materials.
Screening of adsorbents among FeIII-based materials was
performed with 0.2-16 ppm (starting concentrations) of arsenic
test solutions. The weight ratio of adsorbed As and adsorbent
was optimized at the equilibrated condition (12 h). Lenoble etal.13 and Dixit et al.14 reported the greatest adsorption amountsof arsenite and arsenate, respectively, on R-FeO(OH) in theequilibrium dissolved As concentration range of 0.5-11 ppm(Figure 1). Data for R-FeO(OH) with a specific SA of 16 m2
g-1 were added in this work. Greater As adsorption wasobserved as the specific SA increased, but exceptions of thecorrelation order were found (Figure 1A,B).
The screening using Fe3+ ion-exchanged montmorillonite andusing montmorillonite intereacted with colloid particles formed
in aqueous solution by the reaction of Fe3+ and NaOH was
unsuccessful.3 The weight ratio of adsorbed As and adsorbent
was always smaller than the values for R-FeO(OH) with a
specific SA of 54 m2 g-1.14 The screening included former
adsorbents with an Fe loading of 2.8 wt % and latter adsorbents
of 2.0-26.5 wt % Fe. As entirely no adsorption of AsIII/AsV
was observed on native Na-montmorillonite, Fe species were
responsible for the As uptake.
In the adsorption of arsenate, the greatest weight ratio ofadsorbed As and adsorbent was found on the Fe-montmorillonite(see the Experimental Section). The screening included samplesof 2.8-14.0 wt % Fe. The best results were shown in Figure
1B for the sample of 14.0 wt % Fe. Because of the remarkabledifference in adsorption capacity, the equilibrium dissolved As
concentrations were very small for the Fe-montmorillonite. The
data also fit well with a Langmuir-type equation. We did notOxidative Adsorption of AsIII on FeIII Nanoparticles

J. Phys. Chem. B, Vol. 109, No. 8, 2005 3229

TABLE 2: Best-Fit Results of Fe K-edge EXAFS for Na-Montmorillonite and Fe-Montmorillonites with Fe Loadings of 2.8 and14.0 wt %asample of fit

Fe-OFe‚‚‚Fe

goodness of fit

Nc

(Å)
R

(eV)
∆E0

(Å2)
∆σ2

3.3-11.0
1.1-2.1

6.0((0.4)

2.122((0.002)-4.3((0.8)

Na-Montmorillonite-0.0057((0.0006)

3.4-11.0
1.1-3.0

6.4((1.0)

2.074((0.007)-0.9((1.3)

Fe-Montmorillonites, Fe 2.8 wt %-0.00191.4
3.269((0.0009)((0.3)((0.008)-2.7((1.8)

0.0016((0.0008)

113

3.1-11.0
1.1-3.2

6.0((1.0)

2.046((0.009)-0.7((1.7)

Fe-Montmorillonites, Fe 14.0 wt %0.000312.5
3.270((0.0005)((0.6)((0.010)-3.1((2.2)

0.00029((0.0002)

256

R-Fe2O3b
R-FeO(OH)b6
6

2.03052.021

γ-FeO(OH)b

6

2.034

R-range (Å)
k-range (Å-1)

Nc

7
2
6
6

(Å)
R

(eV)
∆E0

(Å2)
∆σ2
930

3.12933.015,
3.390
3.066

a Values in parentheses are the fit errors. b Based on X-ray crystallographic data in refs 10-11.

c S 2 values were set to 1.
0

TABLE 3: Best-Fit Results of As K-edge EXAFS for As Adsorbed on Fe-Montmorillonite (Fe 14.0 wt %) from 200 ppb Test
Solution of Arsenitea
sample of fit
R-range (Å)
k-range (Å-1)
3.9-12.0
0.9-3.2
FeAsO4b

As-O

As‚‚‚Fe

Nc

(Å)
R

(eV)
∆E0

(Å2)
∆σ2

4.2
((0.8)
4

1.679((0.004)
1.68734.8((1.6)

0.0034((0.002)

goodness of fit

Nc

(Å)
R

(eV)
∆E0

(Å2)
∆σ2

2.1
((0.6)
4

3.308((0.01)
3.36

2.7((1.1)

0.0040((0.001)456

a Values in parentheses are the fit errors. b Based on X-ray crystallographic data in ref 12. c S 2 values were set to 1.
0

Figure 1. Adsorption isotherms of arsenite (A) and arsenate (B) at
290 K on Fe-montmorillonite (14.0 wt % Fe) (O) or R-FeO(OH) (2,B, and 9). Observed data were plotted as points, and the fits to firstorder Langmuir-type equatios were drawn as lines. The data for R-FeO(OH) (39 and 54 m2 g-1) are from refs 13 and 14.

add the data of adsorption tests from arsenite solution inconcentrations of 50-100 ppm, because 16 ppm of arsenite isnear the upper limit of plausible modern environmental conditions.
In the adsorption of arsenite, the greatest weight ratio ofadsorbed As and adsorbent was observed on the Fe-montmorillonite below the equilibrium dissolved As concentrationsof 310 ppb, but above the As concentration, R-FeO(OH) (39m2 g-1) was the best instead (Figure 1A). This is due to a larger
equilibrium adsorption constant for Fe-montmorillonite compared to that for R-FeO(OH) samples. In the practical point ofview, a lower As concentration range is more important, because
the application is to reduce the equilibrium dissolved As
concentrations to less than 10 ppb.
Adsorbed amounts of As were also compared to the saturated
adsorbed As amount per Fe amount using fits with theLangmuir-type equation. The values for AsIII were 57 and 26
mgAs/gFe on Fe-montmorillonite and R-FeO(OH) (39 m2 g-1),
respectively. Those for AsV were 540 and 21 mgAs/gFe on Fe-

montmorillonite and R-FeO(OH) (54 m2 g-1), respectively.
Thus, the practical superiority of Fe-montmorillonite is clear.
Preliminary adsorption tests of arsenic on Ti, Mn, Co, Ni, Zn,
Zr, Nb, or Mo intercalated between montmorillonites were also
performed. The saturated adsorbed amounts of AsIII/AsV weregreatest in the case where Fe was intercalated.
In summary, the Fe-montmorillonite (14.0 wt % Fe) was thebest adsorbent for low concentrations of both arsenite andarsenate (<310 ppb) on the basis of the screening tests among
the FeIII-based materials.
Basic Physicochemical Characterization of Fe-Montmorillonites. Basic physicochemical data for the Fe-montmorillonites were summarized in Table 1. The maximum loadingof Fe was 14 wt %. Excessive Fe greater than 14.0 wt % was
eluted by washing with distilled water during the sample
preparation. Fe (2.8 wt %) corresponds to the maximum ionexchangeable Fe3+ amount replacing Na+ and Ca2+ sites of Namontmorillonite, with each charge taken into account (Table
1).
On the basis of the strongest peak angles due to (001)diffraction (and also the relatively weaker (002) diffraction peak)in the XRD spectra, interlayer spacings of montmorillonite weremonitored. The interlayer spacings and specific SA gradually
increased starting from 12.2 Å and 25.8 m2 g-1 (Na-montmorillonite) to 15.3 Å and 100 m2 g-1 (Fe-montmorillonite, 14.0
wt % Fe), respectively (Table 1). These data demonstrate thatthe intercalated FeIII species increased the interlayer spacingsand the clearance for N2 molecules.
Fe K-edge EXAFS. The structure of intercalated FeIII species
was studied by Fe K-edge EXAFS. The EXAFS spectra of Namontmorillonite (one montmorillonite layer consists of three
sheets, and 1.55 wt % of Fe is contained in the middle sheet15)
and Fe-montmorillonites (Fe 2.8 and 14.0 wt %) were studied(Table 2).

3230 J. Phys. Chem. B, Vol. 109, No. 8, 2005

Izumi et al.

Figure 2. Fe K-edge EXAFS spectrum measured at 290 K for Na-montmorillonite (A) and Fe-montmorillonites (Fe 2.8 wt % (B) and 14.0 wt %(C)). (a) k3-weighted EXAFS χ function, (b) its associated Fourier transform (s, magnitude; ‚‚‚, imaginary part), and curve-fitting analysis to theinversely Fourier-filtered k3χ function in k-space (c) and in R-space (d) (s, experimental; ‚‚‚, calculated).

The Fourier transforms are depicted in Figure 2b. Although
the strongest peak at 1.6 Å (phase-shift uncorrected) was
common for the three samples studied (A-C), the next-nearest
peak appeared at 2.5 Å (phase-shift uncorrected) for Namontmorillonite (A-b), shorter than 2.8 Å (phase-shift uncorrected) for Fe-montmorillonites (B-b and C-b). Thus, themajor backstattering atoms are Si and Al for Na-montmorillonite15 and Fe for Fe-montmorillonites.
Best-fit results are listed in Table 2. The coordination number(N) of Fe-O was 6.0 for Na-montmorillonite, consistent withthe octahedral symmetry in the middle sheet of the montmorillonite layer.15 Obtained N values of Fe-O were 6.0-6.4
for Fe-montmorillonites. [FeO6] units were suggested. The
Fe-O distances for Fe-montmorillonites (2.8-14.0 wt % Fe)
were 2.046-2.074 Å, relaxed from 2.021 Å for R-FeO(OH)and 2.034 Å for γ-FeO(OH) (Table 2). The obtained Fe‚‚‚Fe
distance was essentially constant (3.269-3.270 Å) for
Fe-montmorillonites (Table 2). This Fe‚‚‚Fe distance is within
the range for edge-shared [FeO6] octahedral structure (2.973.28 Å).16 The N values obtained for Fe‚‚‚Fe were 1.4-2.5.
In a similar preparation route of Fe-montmorillonite but inthe absence of NaOH (the molar ratio of Fe3+/NaOH was 1:2
in our study), an intercalated Fe(OH)2-like sheet structure wasproposed.17 The alkaline addition may have the effect of growing
the Fe species to nanoparticles between layers. When our Femontmorillonite sample (Fe 14.0 wt %) was heated at 623 K ina vacuum, the XANES pattern and EXAFS data (not shown)became similar to those of R-Fe2O3. Thus, nonheated Femontmorillonites studied in Table 2 are suggested to be FeO(OH)-like nanoparticles.
In summary, intercalated FeO(OH) particles of Fe-montmorillonite (14.0 wt % Fe) consisted of edge-shared [FeO6]
octahedral units. The size was limited on the basis of N forFe‚‚‚Fe (2.5), consistent with the d001 value increase by 3.1 Åcompared to that for Na-montmorillonite (Table 1). The
octahedral structure was relaxed compared to the units in R-FeO(OH) or γ-FeO(OH) crystals.

Figure 3. As KR1 emission spectrum for As adsorbed on Femontmorillonite (14.0 wt % Fe) from 200 ppb test solution of arsenite(points). A fit to data with pseudo-Voigt function (s) and the energy
resolution of the fluorescence spectrometer (‚‚‚) were also drawn.

As K-edge XANES Combined with Fluorescence Spectrometry. The As KR1 emission peaks for the As adsorbed on
Fe-montmorillonite (14.0 wt % Fe) from 0.2 to 16 ppm ofarsenite solutions always appeared at 0.6 eV higher than thatfor As metal (Figure 3). The chemical shifts of the As KR1
peak were -1.4 and +0.6 eV for As2O3 and KH2AsO4,
respectively, with reference to As metal. The fwhm (full widthat half-maximum) of the observed peak was 3.36 eV (Figure3). Because the lifetime width of As KR1 is 3.08 eV,18 the energy
resolution of this fluorescence spectrometer was estimated tobe 1.3 eV (‚‚‚, Figure 3), including the contribution of thebeamline. Because the energy resolution value was smaller thanthe core-hole lifetime width of As K-level (2.15 eV),18 thelifetime broadening should be removed at the As K-edge.7,8,19,20
The fluorescence spectrometer was tuned to 10 544.3 eV(peak top). Then, XANES spectra were measured using highenergy-resolution X-ray fluorescence spectrometry for As adsorbed on Fe-montmorillonites (Figure 4d-f). Note that spectra
a-c were measured in transmission mode, and the peaks werebroader compared to those in spectra d-f. The white line peakOxidative Adsorption of AsIII on FeIII Nanoparticles

J. Phys. Chem. B, Vol. 109, No. 8, 2005 3231
As‚‚‚Fe bond of 3.308 Å with an N value of 2.1 (Table 3). The
As‚‚‚Fe distances have been reported as 3.6, 3.24-3.32, and2.83-2.85 Å for monodentate mononuclear, bidentate binuclear(bridging), and bidentate mononuclear AsV complexes,
respectively,11,21-24 and 3.36-3.40 Å for bidentate binuclear
AsIII species on Fe oxyhydroxides.22 The As‚‚‚Fe distanceobtained in this work corresponds to the bidentate binuclear
AsV: the arsenate being connected to the adjacent apexes ofedge-sharing [FeO6] units.
Discussion

Figure 4. (A) Normalized As K-edge XANES spectra combined withfluorescence spectrometry at 290 K for As adsorbed on Fe-montmorillonite (14.0 wt % Fe) (d-f) from aqueous test solutions of 16 ppm
of KH2AsO4 (d), 16 ppm of As2O3 (e), and 200 ppb of As2O3 (f).
Reference XANES spectra measured in transmission mode for As metal(a), As2O3 (b), and KH2AsO4 (c). (B) First derivative spectra of (A).

Figure 5. As K-edge EXAFS spectrum measured at 290 K for As on
Fe-montmorillonite from 200 ppb test solution of arsenite. (a) k3weighted EXAFS χ function, (b) its associated Fourier transform(s, magnitude; ‚‚‚, imaginary part), and curve-fitting analysis to theinversely Fourier-filtered k3χ function in k-space (c) and in R-space(d) (s, experimental; ‚‚‚, calculated).

for As adsorbed from 16 ppm test solution of arsenate appeared
at 11 872.1 eV (d), at the identical energy as KH2AsO4 (c).
The white line peak for As adsorbed from 0.2 to 16 ppm test
solutions of arsenite appeared at 11 871.7-11 871.9 eV (e andf), very near to the peak for KH2AsO4 (c) rather than As2O3 (b,11 868.6 eV). The peaks were not symmetrical. These comparisons of peak energy position were carefully checked for the
first derivative spectra (Figure 4B).
In summary, the speciation of As was successfully performedby the chemical shifts of the As KR1 peak and the energy shiftsof the white line in the As K-edge XANES. A major part ofarsenite was oxidized to arsenate upon the adsorption on the
Fe-montmorillonite from 0.2 to 16 ppm test solutions. This
oxidation was not observed in the adsorption from 50 to 100
ppm of arsenite test solutions.
As K-edge EXAFS. The adsorbed state of As from a 200
ppb test solution of arsenite was studied using As K-edge
EXAFS (Figure 5 and Table 3). The As-O distance obtained
was 1.679 Å. These data supported the oxidative adsorption ofarsenite. As-O bond distances of 1.78-1.79 and 1.69 Å havebeen reported for AsIII 21,22 and AsV species,21 respectively,
adsorbed on γ-FeO(OH) and R-FeO(OH).
A weak peak appeared at 2.8 Å (phase shift uncorrected) in
the Fourier transform (Figure 5b). The peak was fit as an

By the screening tests of FeIII-based adsorbents with 0.2-16
ppm test solutions of arsenite/arsenate, the Fe-montmorillonite(14.0 wt % Fe added) was found to be the best (Figure 1). Asa general trend, the weight ratio of adsorbed As and adsorbent
increased as the specific SA of adsorbent grew greater (Table
1 and Figure 1). The specific SA for Fe-montmorillonite (14.0
wt % Fe) was 100 m2 g-1.
The trend of adsorption isotherms between arsenite andarsenate adsorptions was in good contrast (Figure 1). The Femontmorillonite (14.0 wt % Fe) was found to be best for arsenite
adsorption capacity in the equilibrium dissolved As concentration smaller than 310 ppb, in contrast to the dramatic improvement of arsenate adsorption capacity in the entire As concentration range studied. In the As adsorption conditions of thisstudy, arsenite and arsenate are dissolved in water as AsIII(OH)3and [AsVO2(OH)2]-, respectively.25 The charge of As molecules
may affect the complex formation with the surface of FeO(OH).
The thickness of one layer of montmorillonite is ∼9.3 Å.24
Therefore, the average interlayer clearance is estimated to 6.0
Å for Fe-montmorillonite (Fe 14.0 wt %, Table 1). On the basisof the Fe K-edge EXAFS, the structural unit of edge-shared[FeO6] was suggested for the adsorbent. Because the Fe‚‚‚Fe
distance obtained (3.270 Å, Table 2) was more than the half of
6.0 Å, the interconnection of [FeO6] units should be in the
direction (nearly) parallel to the layers. The N value forFe‚‚‚Fe (2.5) suggested a limited number of interconnectionsof [FeO6] units. It is important to note that the Fe K-edge
EXAFS data for the Fe-montmorillonites exhibited negligible
change after the As adsorption tests (Figure 1) compared to dataobtained prior to the tests (Figure 2B,C). The montmorillonite
swells up in aqueous solution, and the FeO(OH) particles may
not be closely packed between layers during adsorption testsof As. Hence, the performance observed in Figure 1 is primarily
due to the unsaturated nature of the very small FeIII particle
size and not a geometric effect between packed layers.
On the basis of the As K-edge XANES combined withfluorescence spectrometry (Figure 4), oxidative adsorption ofarsenite was observed on Fe-montmorillonite (14.0 wt % Fe)from 0.2 to 16 ppm test solutions. The energy levels at which
the derivative XANES data cross the line of y ) 0 for the
standards of AsIII and AsV were 11 868.6 and 11 872.1 eV,
respectively (Figure 4B, b and c). Corresponding values foradsorbed As derived from As2O3 were 11 871.3 (e) and 11 871.6
eV (f). If a linear combination of AsIII and AsV peaks is assumed,
the population of As species that converted from arsenite toarsenate upon adsorption was estimated to be 77% and 85%,
respectively. Note that this estimation was improved whencombined with high-energy-resolution fluorescence spectrometryby the removal of lifetime broadening.7,8,19,20 The white line
peaks for d-f utilizing fluorescence spectrometry were sharper
compared to spectra a-c obtained in the transmission mode.
This finding of oxidative adsorption of arsenite was supported

3232 J. Phys. Chem. B, Vol. 109, No. 8, 2005
by the chemical shifts of As KR1 emission spectra (Figure 3)
and As-O and As‚‚‚Fe bond distances obtained by As K-edge
EXAFS (1.679 and 3.308 Å, respectively; Table 3). The surface
As complex was suggested to be bidentate binuclear AsV
species.11,21-24 The fit values may not be exact, because ∼15%
of AsIII coexisted on the basis of As K-edge XANES (Figure
4f).
Homogeneous oxidation of a 30 ppm arsenite solution wasreported at pH greater than 9.26 The pH in the arsenite adsorptionof this study (Figure 1A) was 6.0-3.9, and the possibility ofhomogeneous oxidation of arsenite is rejected. No oxidation
upon adsorption was reported for 6.0-15 ppm test solutions ofarsenite on R-FeO(OH) and γ-FeO(OH).21 Thus, the smaller
particle size of FeO(OH) on the scale of 6 Å, and accordingly
higher population of the adjacent apexes of edge-sharing [FeO6]
units, may be the primary reason of the greatest As adsorption
capacity (Figure 1) among FeIII-based materials.
As observed in Figure 1, it is a general trend that arsenate
uptake is several times greater than that of arsenite on FeIIIbased materials except for R-FeO(OH) (39 m2 g-1). In thiscontext, one of the reasons for superior arsenite uptake on Femontmorillonite at lower As concentrations is its oxidation toarsenate upon adsorption. Final adsorbed As local structuresfrom 0.2 to 16 ppm test solutions were very similar either fromarsenite and arsenate (Table 3 and Figures 4 and 5). The
responsible sites on FeO(OH) nanoparticles may work as AsIII(OH)3 + FeO(OH)(surface) f AsV(OH)2O2Fe(surface) + H2O.
Two prerequisites, this oxidation step and a higher populationof coordinatively unsaturated sites on FeO(OH), may determine
the uptake amonts of arsenite on Fe-montmorillonites. The Fe
K-edge shift was studied for Fe-montmorillonite in 16 ppm ofarsenite test solution (Figure 1). No change was observed at
the edge upon adsorption of As, because the molar ratio of
adorbed As and Fe was only 0.031.
Conclusions
Among FeIII-based materials studied and in the literature,intercalated Fe-montmorillonites prepared in this work exhibitedthe best arsenate adsorption capacity on an adsorbent weightbasis from 0.2 to 16 ppm test solutions. The adsorbents werealso best for arsenite sorption capacity below the equilibrium
dissolved As concentrations of 310 ppb.
The structure of responsible FeIII sites was studied for
optimized Fe-montmorillonite (14.0 wt % Fe). The size ofintercalated FeO(OH) particles was 6.0 Å in the direction
perpendicular to montmorillonite layers based on XRD. The
information for Fe-O at 2.046 Å and Fe‚‚‚Fe bonds at 3.270
Å by Fe K-edge EXAFS demonstrated octahedral [FeO6] unitsinterconnected in the direction (nearly) parallel to the layers.
The number of units per particle should be very limited.
The speciation of As adsorbed on the Fe-montmorillonite(14.0 wt % Fe) was performed by As K-edge XAFS combinedwith high-energy-resolution fluorescence spectrometry. The
technique removed lifetime broadening and thus enabled one
to quantitatively monitor the oxidative adsorption of a majorpart (77-85%) of arsenite to arsenate from 0.2 to 16 ppm test

Izumi et al.
solutions. This finding was supported by the chemical shifts of
As KR1 emission peaks and the information for As-O at 1.679
Å and As‚‚‚Fe bonds at 3.308 Å by As K-edge EXAFS.
The greatest As sorption capacity per adsorbent weight for
Fe-montmorillonite may be due to the metric size (6 Å) ofintercalated FeO(OH) particles and accordingly higher populations of the adjacent apexes of edge-shareing [FeO6] units.
Acknowledgment. The XAFS experiments were performed
under the approval of the SPring-8 Program Review Committee(2002B0738-NX-np, 2003A0145-NX-np) at As K-edge andunder the approval of the Photon Factory Proposal Review
Committee (2001G092, 2003G074) at Fe K-edge. The authors
are thankful for a grant from the Grant-in-Aid for Encouragement of Young Scientists from the Ministry of Education,
Culture, Sports, Science, and Technology (Y.I., 1474 0401).
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