Microporous and Mesoporous Materials 94 (2006) 243–253
www.elsevier.com/locate/micromeso

Creation of micro and mesoporous FeIII materials utilizing organic
template followed by carboxylates exchange for the
low concentrations of arsenite removal
Yasuo Izumi a,*, Dilshad Masih a, Ken-ichi Aika a, Yoshimi Seida baInterdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-16, Midori-ku,
Yokohama 226-8502, Japanb
Institute of Research and Innovation, 1201 Takada, Kashiwa 277-0861, Japan
Received 27 October 2005; received in revised form 24 March 2006; accepted 4 April 2006
Available online 22 May 2006

Abstract
The instability of inorganic framework has been the greatest barrier to synthesize micro and mesoporous FeIII/II materials. This paper
uses dodecylsulfate to organize lamellar-structure FeOx(OH)y composite followed by smaller carboxylates exchange (formate, acetate, or
propionate) to create micropores. XRD, nitrogen sorption/desorption, HR-TEM, FT-IR, ICP, EPMA, TG–DTA, and Fe K-edge
EXAFS measurements were used for the characterizations. The lamellar-structure reorganized to wormhole-like framework stabilizedwith carboxylate anions adsorbed inside micropores. Upon heating at 423 K, a half of acetates/propionates diminished and the speciﬁc
surface area increased to as much as 230 m2 g1. Based on the Fe K-edge EXAFS for FeOx(OH)y composite and derivative porous FeIII
materials, Fe–O bonds were observed at 2.04–2.09 Å with the coordination number 5–6. Farther Fe  Fe bonds also appeared at 3.21–
3.25 Å. The coordination number was obtained to 2–3, reﬂecting higher dispersion and higher surface area for these porous FeIII
materials. The acetate-exchanged FeOx(OH)y heated at 423 K exhibited greatest saturated sorption amount ð21 mgAs g1
adsorbent Þ andIII
equilibrium sorption constant ð1:0  107 ml g1
materials and FeIIIAs Þ in 0.2–32 ppm of arsenite test solutions among other relevant Fe
nanoparticles intercalated between clay layers.
Ó 2006 Elsevier Inc. All rights reserved.
Keywords: Iron oxyhydroxide; Carboxylate exchange; Arsenite; Lamellar; Wormhole-like

1. Introduction
Recent regulations concerning the concentration ofarsenic in public water supplies are at 10 ppb due to thehigh carcinogenic risk reported in Bangladesh, West Bengal, Taiwan, Argentina, Vietnam, etc. [1]. Especially, arsenite has fatal eﬀects on human health. For the economicand eﬀective removal of trace amounts of As in drinking
water, we recently reported the optimization of FeIII materials for the sorption of low concentrations of As [2]. The
screening included a-FeO(OH) with speciﬁc surface area*

Corresponding author. Tel./fax: +81 45 924 5569.
E-mail address: yizumi@chemenv.titech.ac.jp (Y. Izumi).

1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2006.04.002(SA) of 16–54 m2 g1 [3,4], c-FeO(OH) with speciﬁc SA
of 27–176 m2 g1, Fe3+-exchanged montmorillonite withspeciﬁc SA of 46 m2 g1, and dispersed FeOx(OH)y nanoparticles (average particle size 6 Å) intercalated between
montmorillonites with speciﬁc SA of 25.8–100 m2 g1, in
50 ppb–16 ppm of test As solutions. The uptake capacity
on intercalated FeOx(OH)y as much as 14 wt.% Fe was
the best from both arsenite and arsenate in the equilibrium
dissolved As concentration range smaller than 310 ppb [2].
In order to apply them to practical As sorption, further
increase of Fe content in sorbents is essential. The superiority of FeIII nanoparticles stabilized between montmorillonite layers was owing to greater equilibrium sorption
constant [2]. The As sorption on unsaturated [FeO6] sites

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Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

was stronger and stable. In this paper, highly dispersed FeIII
porous materials were synthesized using anionic organic
template followed by anion exchange with smaller carboxylates to enable high dispersion and high concentration of Fe
in sample simultaneously. Due to the molecular size diﬀerence of template and carboxylates anions, micropores weresuccessfully formed.
FeIII inorganic/organic composites were synthesized in
the forms of lamellar [5–7], 2-dimensional hexagonal [8],and wormhole-like (holes ﬁlled with templates) [9–12].
However, reports are limited that removed the templates
by thermal [10–12] or solvent treatment [7–10,12] to eﬀectively form meso or micropores. When the templates were
removed, assembled Fe framework decomposed to larger
porous structure [12] or fragmented to nanoparticles[9,11]. We discuss the role of carboxylates exchanged andthe micro/mesostructure transformation in the heating process. Thus-prepared microporous FeIII materials wereapplied to the low concentrations of arsenite removal.
2. Experimental section
2.1. Preparation of FeIII porous materials
Fifty milliliters of ferrous chloride (0.10 M) was mixed
with 10 ml of sodium dodecylsulfate (0.070 M) at 290 K
followed by addition of 10 ml of H2O2 (0.25 M) [5–7,13].
The pH value of solution changed from 6 to 3. The FeII
oxidation route was chosen to control moderate rate ofsolid formation. The solution was stirred for 1 h andﬁltered. Obtained yellow powder is called FeOx(OH)y composite. When the molar ratio of FeII and Na dodecylsulfate
was in the range 2:1–1:2 [7], two independent sets of peaks
appeared in the X-ray diﬀraction (XRD) data during treatments. When the ratio was 7:1 as above, all the XRD peakintensity behaved as a single set. Therefore, the ratio was
ﬁxed to 7:1.
The FeOx(OH)y composite was mixed with 75 ml of
sodium formate, acetate, or propionate (0.050 M for each)at 290 K in ethanol [14]. Obtained brown powders arecalled anion-exchanged FeOx(OH)y. As a control, theFeOx(OH)y composite was washed with ethanol [7–10,12]
at 290 K. This sample is called ethanol-washed FeOx(OH)y.
Each anion-exchanged or ethanol-washed FeOx(OH)y wasset in furnace in the nitrogen ﬂow with the rate of
40 ml min1. The temperature was linearly raised from
290 to 523 K with an elevation rate of 2 K min1.
2.2. XRD, sorption/desorption isotherms, TEM, FT-IR,
ICP, EPMA, and TG–DTA measurements
XRD data were obtained using a Multiﬂex-S diﬀractometer (Rigaku). Cu Ka emission was used with a Ni ﬁlter.
The diﬀractions were monitored in the 2hB range of 2–
70° (hB: Bragg angle). The speciﬁc SA was measured at
77 K using a BELSORP Mini (Bell Japan) with N2 as theadsorbate. The samples were evacuated at 393–423 K for

1 h before measurements. Transmission electron microscope (TEM) observations were performed using ﬁeldemission-type JEM-2010 F (JOEL) at the accelerating
voltage of 200 kV at the Center of Advanced Materials
Analysis (CAMA), Tokyo Institute of Technology (Dr.
A. Genseki). The powder was dispersed in carbon tetrachloride using ultrasonic and mounted on carbon-coated
Cu grid.
FeIII materials (0.15 g) were thoroughly mixed with KBr
powder (IR-grade, Wako) to be 1.0 wt.% Fe and pressed
into a disk of 20 mm in diameter. The Fourier-transformed
infrared (FT-IR) spectra were measured in transmission
mode using Valor III (JASCO) equipped with a MCT
detector. The energy resolution was set to 1 cm1. Nitrogengas was ﬂowed in sample room to avoid moisture during
measurements.
The amounts of FeOx(OH)y, dodecylsulfate, and formate were based on OH stretching (3400 cm1), CH
stretching (2921 and 2852 cm1), and COO stretching (as,s) (1602 and 1350 cm1) peaks, respectively (Fig. 4). The
amounts of acetate and propionate were based on COO
stretching (as, s) peaks (1535 and 1425 cm1, respectively).
The amount of dodecylsulfate was also evaluated based on
the stretching peak of sulfate (1226 cm1). The evaluatedamount was essentially identical based on either mCH or
mSO4 peak. No peaks due to CCO stretching of ethoxylgroup were detected at 1050 cm1 (as) and 970–
870 cm1(s) [15] for ethanol-washed samples, demonstrating negligible ethanol adsorption on FeIII materials in thispaper. These peak areas were compared to corresponding
peak areas obtained for diluted a-FeO(OH) disks to
0.1–2.0 wt.% Fe and powder disks to 0.1–2.0 wt.% of dodecylsulfate and Na formate, acetate, or propionate diluted
using KBr.
The elemental compositions of Fe, C, O, S, Na, and Cl
in acetate-exchanged FeOx(OH)y samples were analyzed
using inductively coupled plasma (ICP) measurements
combined with optical emission spectroscopy (Prodigy,
Leeman Labs) and electron probe microanalysis (EPMA)
measurements (JXA-8100, Jeol) at CAMA (Dr. T. Toya).
LaB6/W ﬁlament was used and the accelerating voltage
of incident electrons was 15 kV.
Thermogravimetric diﬀerential thermal analysis (TG–
DTA) was performed using DTG-60 (Shimadzu) for
a-FeO(OH) and acetate-exchanged FeOx(OH)y. The ﬂow
rate of air was 120 ml min1 and the ramping rate of sample was 4 K min1.
2.3. Fe K-edge XAFS measurements and analysis
The powders of FeIII materials were evacuated at 290 K
and transferred to the Pyrex glass cell. The windows weresealed with Kapton ﬁlm of 12.5 lm in thickness. Sample
thickness was controlled to give a Fe K absorption edge
jump of 1.0. The Fe K-edge X-ray absorption ﬁne structure(XAFS) spectra were measured in the beamlines 10B and
12 C at KEK-PF in a transmission mode at 20–290 K.

Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

245

The storage ring energy was 2.5 GeV and the ring current
was 437–320 mA. A Si(311) double crystal monochromator
was used. N2 gas and a mixture of N2 (85%) and Ar (15%)
were used in the I0 and It ion chambers, respectively. The
rising edge energy of Fe metal was calibrated at
7111.20 eV [16]. The scan steps were 0.1, 0.5, and2 eV for the pre-edge + edge, post-edge (X-ray absorption near-edge structure: XANES), and EXAFS (extended
X-ray absorption ﬁne structure) regions, respectively. The
accumulation time for a data point was 1–5 s.
The EXAFS data analysis was performed using XDAP(XAFS Services International) based on works of M.
Vaarkamp, H. Linders, and D. Koningsberger. The details
were described elsewhere [2]. Multiple shell analyses were
performed for the Fourier-ﬁltered k3-weighted EXAFS
data in the k- and R-space using empirical parameters
extracted from EXAFS for a-Fe2O3 [2]. The bond distancesand coordination numbers of Fe–O and Fe  Fe werebased on crystal structure data for a-Fe2O3 [17,18]. The
r2 values are relative to those for a-Fe2O3. The many-body
reduction factor S 20 was assumed to be equal for sample andreference. The goodness of ﬁt was given as requested by theCommittee on Standards and Criteria in X-ray Absorption
Spectroscopy.
2.4. Arsenite sorption tests
Twenty milliliters of 0.2–16 (or 0.2–32) ppm solution of
III
AsIII
material in a
2 O3 was introduced to 50 mg of each Fe
30-ml polypropylene tube. The mixture was shaken at a rateof 130 times min1 at 290 K for 12 h. The system reached anequilibrium within 12 h. The equilibrium dissolved As
concentration was determined by ICP measurements.
3. Results
3.1. XRDThe XRD spectra measured for FeOx(OH)y composite
and the derivative samples were depicted in Fig. 1.
Obtained d spacing values based on the diﬀraction angle
were summarized in Table 1. In the XRD spectrum for
FeOx(OH)y composite (Fig. 1A(a)), strong, medium, andweak peaks were observed centered at 2hB = 2.44°, 4.9°,
and 7.6°, respectively. These were assigned to (0 0 1),(0 0 2), and (0 0 3) diﬀractions, respectively, of lamellarstructure [5–7,19] with d spacing of 36.2 Å.
The spectrum A(a) changed by carboxylate anions
exchange. When formate was used, all peaks became too
weak (Fig. 1A(b)). When acetate and propionate wereused, a broad peak appeared centered at 2hB = 4.5° (A(c,d); corresponding d spacing 20 ± 2 Å). These peaks were
not due to the (0 0 2) diﬀraction for lamellar-structure
because corresponding (0 0 1) diﬀraction peak anticipated
to appear at 2hB = 2.3° was not observed. For the ethanol-washed FeOx(OH)y, a medium peak appeared at
2hB = 3.2° (A(e); d spacing 28 ± 1 Å).

Fig. 1. XRD spectra. (A) Lower angle region measured for FeOx(OH)ycomposite (a), formate, acetate, and propionate-exchanged FeOx(OH)y(b–d, respectively), ethanol-washed FeOx(OH)y (e), and b–e heated at
423 K (f–i, respectively). (B) Higher angle region measured for FeOx(OH)ycomposite (a), acetate-exchanged FeOx(OH)y (b), b heated at 423 K (c),formate, acetate, and propionate-exchanged FeOx(OH)y heated at 533 K(d–f, respectively), and ethanol-washed FeOx(OH)y heated at 533 K (g).
(*) Peaks derived from FeO(OH) (PDF No.03-0440); (h) peaks derived
from Fe3O4 (ICSD No. 36314, PDF No. 75-0033).

When these samples were heated at 423 K, the broad
peaks for acetate/propionate-exchanged samples shifted
downward to 4.4° (Fig. 1A(g)) and 3.9° (h), respectively.
These angles corresponded to the d spacings of 20 ± 2
and 23 ± 2 Å (Table 1g and h). The peak at 2hB = 3.2°
for ethanol-washed sample remained at the same angle(Fig. 1A(i)). The peak intensity deceased to one third.
When these samples were heated at 533 K, all the peaks
disappeared in the 2hB range of 2–10°.

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Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

Table 1
Basic physicochemical data of the FeOx(OH)y composite, the derivatives, and reference materials
Entry

Treatment

Reactant

Speciﬁc SA (m2 g1)aAverage pore size (Å)

Pore volume (ml g1)d001 or d100 (Å)ba

FeIII Composite

As-prepared

2.8

–

–

36.2

b
c
d
e

Anion exchanged

FormateAcetatePropionate
Ethanol washed

–
–
–
–

–
–
–
–

–
–
–
–

–
20 ± 220 ± 228 ± 1fghi

423 K heatedc

FormateAcetatePropionate
Ethanol washed95
230 (208)e128
170

Micropore
Micropore
Micropore
Micropore

0.054d0.10d (0.09)d,e
0.062d
0.085d–20 ± 2 (22 ± 2)e23 ± 228 ± 1jklm

533 K heatedc

FormateAcetatePropionate
Ethanol washed

81
145
132
68

37f
37f
50f
Micropore

0.10f
0.14f
0.18f
0.008d

–
–
–
–

100
16

–
–

0.067d<0.001

15.3–

FeIII-montmorillonite [2]a-FeO(OH) (Kanto) [2]aMeasured after treatment in vacuum at 423 K.
On the basis of XRD.
c
Heated in nitrogen gas ﬂow.
d
On the basis of t-plot.
eMeasured after the sorption test in 1.0 ppm of arsenite for 12 h.
f
On the basis of DH-plot.
b

3.2. Sorption/desorption isotherms

3.3. HR-TEM

The sorption and desorption isotherms were depicted inFig. 2A(a–c) measured for anion-exchanged FeOx(OH)ypreheated at 423 K. These isotherms belong to Type I of
IUPAC classiﬁcation [20]. Average pore size was in micropore range based on the t-plot (Fig. 2B). The isotherms forethanol-washed sample heated at 423 K belonged to a pattern as the transition from Type I to IVb (Fig. 2A(d)) [20].
Similar H2-type hysteresis was reported for ethanolwashed FeIII material prepared from FeIII ethoxide andcetyltrimethylammonium bromide [11]. When the heating
temperature was varied between 373 and 573 K, the speciﬁc
SA reached a maximum (230 m2 g1) at 423 K for acetateexchanged sample. After 423 K-heating, the speciﬁc SA for
ethanol-washed samples reached the maximum 170 m2 g1.
The former speciﬁc SA value changed to 208 m2 g1 after
the sorption test in 1.0 ppm of arsenite for 12 h (Table
1g). The limited decrease (9%) may be partial damage of
microporous framework in water. The changes of pore volume and d-spacing after sorption test were very small: from
0.10 to 0.09 ml g1 and from 20 to 22 Å, respectively (Table
1g).
Next, FeIII samples were heated at 533 K. Obtained isotherms belong to Type IVb for anion-exchanged samples(Fig. 2C(a–c)). The average pore size was expanded tomeso-range: 37–50 Å based on the DH-plot (Fig. 2D).
For ethanol-washed sample, the dV/dRp peak was negligibly weak (Fig. 2D(d)). Therefore, the mesopore population
was negligible when carboxylate was not used. The speciﬁc
SA decreased when heated at 533 K compared to corresponding samples heated at 423 K except for the propionate-exchanged one (128 ! 132 m2 g1; Table 1).

Representative TEM images were depicted in Fig. 3
measured for acetate-exchanged FeOx(OH)y and the423 K-heated one. Irregular distribution of thin contrast
spots was observed in both images with the size of a few
nanometers. The image (a) suggested totally amorphousnature. In image (b), the surface section began to form
crystalline phase, and major bulk section seemed to remain
as amorphous.
3.4. FT-IR
FT-IR spectra measured for FeOx(OH)y composite andthe derivative samples were depicted in Fig. 4. Obtained
ratios of FeIII phases and anions were summarized in Table
2. The molar ratio of dodecylsulfate versus Fe was evaluated to 0.14 for the FeOx(OH)y composite in consistent
with the ratio of introduced amount during the synthesis(1:7).
After carboxylates those are ﬁve molar times of dodecylsulfate in composite sample were introduced to the FeOx
(OH)y composite, the amount of dodecylsulfate was significantly decreased (Table 2b–d). The molar ratio of carboxylates incorporated in sample versus Fe was 0.19–0.42,
excessive than the molar ratio of dodecylsulfate in the
FeOx(OH)y composite. When the FeOx(OH)y composite
was washed with ethanol, 25% of dodecylsulfate remained(e). In XRD measurements in the higher angle region, abroad peak centered at 35° invariably appeared for FeOx(OH)y composite and carboxylates-exchanged samples(Fig. 1B(a, b)). The broad peak feature implied amorphousnature of the FeOx(OH)y. In summary, the exchange with

Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

247

Fig. 2. (A) The sorption and desorption isotherms for exchanged FeOx(OH)y with formate (a), acetate (b), and propionate (c) or ethanol-washed
FeOx(OH)y (d) all heated at 423 K. (B) Calculated pore size distribution based on t-plot method. (C) Corresponding samples to (A) heated at 533 K. (D)Calculated pore size distribution based on DH-plot method.

Fig. 3. TEM images for acetate-exchanged FeOx(OH)y (a) and the one heated at 423 K in nitrogen gas ﬂow (b).

smaller carboxylates removed most of dodecylsulfate
molecules to aﬀord micropores inside the amorphous
FeOx(OH)y framework.

When these samples were heated at 423 K, 16% of formate and 51% of acetate and propionate diminished (Table
2f–h). The molar amount of formates remained was still by248

Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

formed into Fe2O3 by dehydration. The 40% of FeOx(OH)ytransformed into Fe2O3 for ethanol-washed one (Table 2i).
These samples were further heated at 533 K. Most oforganics diminished in Table 2j–l. FeOx(OH)y (80–50%)
was lost for carboxylates-exchanged samples (Fig. 4j–l).
In the XRD spectra, a set of peaks due to Fe3O4 crystalline
was observed (Fig. 1B(d, e, f)) in contrast to the appearance of crystalline FeO(OH) phase for ethanol-washedsample (g). Remained dodecylsulfate may have decomposed until 533 K and hydrocarbon section desorbed asgas because sulfate peak at 1226 cm1 remained whereas
no peak remained in mCH region (Fig. 4m) [13]. Note thatthe values for dodecylsulfate and carboxylates in Table 2
were exclusively for intact forms of these anions. Quantitative elemental analysis should be based on ICP or EPMA
measurements.
3.5. ICP and EPMA analyses for elemental compositions
Fig. 4. (a) FT-IR spectra for FeOx(OH)y composite (a), exchanged
FeOx(OH)y with formate (b), acetate (c), and propionate (d), ethanolwashed FeOx(OH)y (e), and b–e heated at 423 K (f–i, respectively) or at
533 K (j–m, respectively).

The results of ICP and EPMA analyses for acetateexchanged FeOx(OH)y samples (as exchanged and 423 K
heated) were listed in Table 3. Sulfur (0.773–0.731 wt.%)
were detected in the two samples. Compared to the molar
ratio dodecylsulfate/FeIII of 0.14 in initial composite before
acetate exchange, 18.1% and 17.7% of dodecylsulfate (orsulfur species derived from it) remained, respectively. Small
amounts of Na and Cl (0.5–3.1 wt.%) were not completely
washed out from the acetate-as-exchanged sample.
Assuming the exchanged molar amount of acetate/FeIIIof 0.19 based on FT-IR (Table 2c) and molar ratio of
remained dodecylsulfate/FeIII of 0.026 based on ICP, theelemental composition of acetate-exchanged FeOx(OH)y(as exchanged) was calculated (Table 3). The calculated

2.5 times greater than that of dodecylsulfate in FeOx(OH)ycomposite. The inorganic phase remained as FeOx(OH)ybecause the dehydration was diﬃcult to proceed on theinternal surface of FeOx(OH)y densely-covered with formates. In contrast, 40–60% of FeOx(OH)y was lost for acetate/propionate-exchanged FeOx(OH)y by heating at 423 K
based on the mOH peak decrease (Fig. 4g and h). Corresponding broad XRD peak intensity at 35° became a half
(Fig. 1B(b, c)). Thus, half of amorphous FeOx(OH)y trans-

Table 2
Evaluated chemical composition of FeIII materials on the basis of FT-IRa
Entry

Treatment

Reactant

Weight (mg)FeOx(OH)yb

Fe2O3 or Fe3O4c

Dodecylsulfate(as intact anion)

Carboxylate(as intact anion)

Molar ratio ofPPanion/ FeaComposite

As prepared

114

0

46

0

0.14

b
c
d
e

Anion exchanged

FormateAcetatePropionate
Ethanol washed

132
142
134
147

0
0
0
0

<5
<5
<5
15

28
18
26
<5

0.42
0.19
0.24
0.034

f
g
h
i

423 K heated

FormateAcetatePropionate
Ethanol washed

134
85
60
87

<10
(68)
(91)
(63)

<5
<5
<5
10

24
7
9
<5

0.35
0.094
0.12
0

j
k
l
m

533 K heated

FormateAcetatePropionate
Ethanol washed

29
80
54
–d

(131)
(80)
(106)
–

<5
<5
<5
9

<5
<5d
<5d
<5

0
0
0
0

a

Sample weight was kept constant, 160 mg.
The composition was simpliﬁed as FeO(OH) for calculations.
cValues indirectly obtained as the rest of Fe phases.
dUnknown peak exists.
b

Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

249

Table 3Elemental compositions of acetate-exchanged FeIII materials on the basis of ICP and EPMA
SampleMethodAs exchanged

ICPa
EPMAa
Calculatedb
ICPa

423 K-heated

Weight ratio (%)

Total

Fe

C

O

S

Na

Cl

H

51.88
51.4
52.2
49.99

–
6.7
7.7
–

–
37.0
37.2
–

0.773
0.80
0.77
0.731

–
3.1
0
–

–
0.5
0
–

–
–
2.1
–

100
100

The contribution of sorbed water (19.0 wt.% in sample) was subtracted based on the sample weight change before/after vacuum (106 Pa) treatment at
290 K for these samples.
b
Assuming acetate/Fe molar ratio of 0.19 based on FT-IR and dodecylsulfate/Fe molar ratio of 0.026 based on ICP. Simpliﬁed composition FeO(OH)
was also assumed.
avalues were well in accord with the analyzed values using
EPMA.
3.6. TG–DTA
TG–DTA data was measured for acetate-exchanged
FeOx(OH)y sample (Fig. 5). Gradual weight decrease wasobserved between 290 and 500 K due to the desorption/
decomposition of acetate and dehydration of FeOx(OH)y.
Then, abrupt weight decrease between 500 and 580 K was
associated with exothermic peak. Within this temperature
range (533 K), the transition of amorphous FeOx(OH)yto Fe3O4 was observed in the XRD spectra (Fig. 1B).
3.7. Fe K-edge EXAFS

100

60

90

40

80

20

70

400

500
600
Temperature (K)700

DTA (μV)

Weight loss (%)

The EXAFS spectra were measured for FeOx(OH)ycomposite, acetate-exchanged FeOx(OH)y, the one heatedat 423 K, ethanol-washed FeOx(OH)y, and the one heatedat 473 K. In all the Fourier-transform spectra (Fig. 6b),strong peak appeared at 1.5–1.6 Å (phase shift uncorrected) due to the Fe–O bond. A medium peak at 2.7–
2.8 Å (phase shift uncorrected) was clearly observed due
the Fe  Fe interaction.

0

Fig. 5. TG–DTA data measured for acetate-exchanged FeOx(OH)y in air.
The ramping rate 4 K min1. TG (solid line) and DTA (dotted line)
curves.

For crystallines a-Fe2O3, a-FeO(OH), and c-FeO(OH),the Fe–O bond distance was between 2.0305 and 2.034 Å
and the coordination number NFe–O was 6 for the octahedral [FeO6] coordination [2,21]. Obtained Fe–O bonddistance FeOx(OH)y composite and the derivative microporous FeIII materials were between 2.043 and 2.088 Å,slightly relaxed from those for standard compounds (Table
4). Associated NFe–O values fell within 4.6–6.0. Relatively
smaller value (4.6) given for acetate-exchanged FeOx(OH)ymay be because of less complete array of [FeO6] units due
to highly dispersed microporous nature and/or surface
reduction by acetate or ethanol. The RFe  Fe values also
elongated (3.213–3.245 Å) compared to the value for cFeO(OH) (3.066 Å). These values were within the rangeof Fe  Fe distance for edge-shared [FeO6] units (2.97–
3.28 Å) [21]. Obtained NFe  Fe values (1.8–3.1), suggestinghighly dispersed, unsaturated nature of microporous FeIII
materials.
In summary, Fe K-edge analyses demonstrated that
FeIII materials studied consisted of edge-shared [FeO6]
octahedral units and the unsaturated nature.
3.8. Arsenite sorption tests
Two porous FeIII samples of highest speciﬁc SA werechosen. Sorption tests using our FeIII porous materials ingroundwater collected in Kalalanwala (Pakistan), wherein
mysterious disease due to arsenic in drinking water is serious [22], are reported elsewhere. The superiority of sorption
capacity varied dependent on the equilibrium dissolved
arsenite concentration range (Fig. 7). When it was lower
than 1.3 ppm, the superiority order was acetate-exchanged
FeOx(OH)y  ethanol-washed FeOx(OH)y > Fe-montmorillonite [2] > a-FeO(OH) [4]. The ﬁrst two samples werepreheated at 423 K. This is plausible condition for application because the environmental regulation of As concentration is 10 ppb [1].
When the concentration was higher than 2.6 ppm, theorder was acetate-exchanged FeOx(OH)y  a-FeO(OH) >ethanol-washed FeOx(OH)y  Fe-montmorillonite. The
diﬀerent trend was due to the diﬀerence of equilibrium sorption constant: 1:0  107 ðacetate-exchanged
FeOx ðOHÞy Þ  2:0  106 ðethanol-washed FeOx ðOHÞy Þ >1:4  106 ðFe-montmorilloniteÞ > 6:4  105 ml g1
As ða-FeO

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Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

(a)

(b)

(d)

(c)

4

4

2

2
2

(A)

2
0

0

0

0

-2

-2

-2

-2

6

4

4

0

Fourier transform

0

-2

4

3

-4

k χ (CF)

3

kχ

0

-2

-2

4
2

2

(C)

Fourier transform (CF)

2

2

(B)

2

2

2

-2

2

0

0

0

0

0

-2

-2

-2

-2
-4

4
4

2

2
2

2

(D)

0

0
-2
-4

-2
4

6

8

10

12

0

0

0

-2

-2
1

k(Å-1)

2

3

4

5

4

6

8

10

12

0

1

k(Å-1)

R(Å)

2

3

4

5

R(Å)

Fig. 6. Fe K-edge EXAFS spectra measured at 290 K for FeOx(OH)y composite (A), acetate-exchanged FeOx(OH)y (B), B heated at 423 K (C), andethanol-washed FeOx(OH)y (D). (a) k3-weighted v function, (b) its associated Fourier transform (solid line: magnitude, dotted line: imaginary part), andcurve-ﬁt analysis to the inversely Fourier-ﬁltered k3v function in k- (c) and R-space (d) (solid line: experimental, dotted line: calculated).

Table 4
Best ﬁt results of Fe K-edge EXAFS for FeOx(OH)y composite and the derivative porous materialsa
SampleFeOx(OH)ycomposite
Acetate-exchanged
FeOx(OH)yAcetate-exchanged
FeOx(OH)yheated at 423 K
Ethanol-washed
FeOx(OH)ya-Fe2O3ba-FeO(OH)b

R-range forﬁt (Å)Fe–O
N

R (Å)DE0 (eV)

Dr (Å )N

R (Å)DE0 (eV)

Dr (Å )3.7–12.0

1.11–3.13

3.7–12.0

1.14–3.08

3.7–11.6

1.09–3.15

6.0(±0.8)
4.6(±0.6)
5.6(±0.5)

2.070(±0.013)2.065(±0.005)
2.043(±0.005)1.0(±1.0)
0.0(±1.1)
1.9(±0.9)

0.0002(±0.0007)
0.0009(±0.0004)
0.0010(±0.0002)

1.8(±0.2)
2.5(±0.4)
3.1(±0.2)

3.245(±0.003)
3.213(±0.020)3.241(±0.013)

1.0(±0.5)
0.4(±3.4)
2.1(±2.0)

0.0009(±0.0003)
0.0022(±0.0007)
0.0049(±0.0003)

3.7–11.9

1.11–3.10

5.1(±1.6)
6
6

2.088(±0.027)2.03052.021

7.0(±3.5)

0.0027(±0.0031)

0.0047(±0.0005)

2.034

3.233
(±0.011)3.1293
3.015
3.390
3.066

0.3
(±1.4)

6

2.6(±0.1)
7
2
6
6

c-FeO(OH)babFe  Fe

k-range forﬁt (Å1)

Values in parenthesis are estimated ﬁt errors.
From Refs. [17,18].

2

2

2

2

Goodness
of ﬁt
198
141
264

136

Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

14

(a)

251

FeO(OH)As ads / adsorbent (10-3 g/g)

36.2Å
12

dodecylsulfate
10

Na acetate

Ethanol

8(b)

FeO(OH)6(c)disordered
FeO(OH)layers

4

20±2Å

2

(XRD)
423 K

423 K
(A)

0

0

1

2

3

4

5

6

7

(d)

FeO(OH)+ Fe2O3

14

FeO(OH)+ Fe2O3

Microporous
230 m2g—1
20±2Å (XRD)

12
As ads / adsorbent (10-3 g/g)

(e)

10

Microporous
170 m2g—1
28±1Å (XRD)

533 K

8

(f)

6

Fe3O4 +FeO(OH)

533 K(g)

FeO(OH)+ Fe2O3

Mesoporous
145 m2g—1
37Å (DH-plot)

4
2

Fig. 8. Proposed structure transformation from lamellar-type FeOx(OH)ycomposite to acetate-exchanged/ethanol-washed micro/mesoporous FeIII/II
materials.

0
0
(B)

0.1

0.2

0.3

0.4

0.5

Equilibrium dissolved As (mg/l)

Fig. 7. Sorption isotherms of arsenite at 290 K on acetate-exchanged
FeOx(OH)y heated at 423 K (speciﬁc SA 230 m2 g1; ), ethanol-washed
FeOx(OH)y heated at 423 K (speciﬁc SA 170 m2 g1; ), FeIII nanoparticles intercalated between montmorillonites (14 wt.% Fe, speciﬁc SA
100 m2 g1; ) [2], and a-FeO(OH) (speciﬁc SA 54 m2 g1; m) [4]. The
data were plotted as points and the ﬁts to ﬁrst-order Langmuir-type
equation were drawn as lines.

ðOHÞÞ evaluated assuming the ﬁrst-order Langmuir-type
dependence. Thus, practical superiority of acetateexchanged FeOx(OH)y was evident.
4. Discussion
4.1. Lamellar and reorganized microporous FeOx(OH)yThree peaks appeared in the lower angle range of XRD
for FeOx(OH)y composite (Fig. 1A(a)) as a typical patternof lamellar-structure [5–7]. A broad peak at 35° suggested
amorphous nature of the FeOx(OH)y (Fig. 1B(a)). The d
spacing was calculated to 36.2 Å (Table 1a) similar toreported values for lamellar-structure formed using dodecylsulfate (35.2 Å [5] and 36.6 Å [6]). The length of a dodecylsulfate anion is 17.4 Å [6]. Thus, about half of thelength for dodecylsulfate may interdigitate to form double
layer of surfactant [6] (Fig. 8a).

Dodecylsulfate (82%) was exchanged with formate, acetate, or propionate in ethanol. In addition to exchange
reactions, extra amount of carboxylate anions (210–41%,
Table 2a–d) may be exchanging with the hydroxyl groupsof FeOx(OH)y. The XRD peak for acetate- and propionate-exchanged FeOx(OH)y corresponded to d spacingsof 20 ± 2 Å (Fig. 1A). Thin spots of a few nanometers wereobserved in TEM (Fig. 3a). Thus, microporous structure[9–12] adsorbed with carboxylate anions was proposed(Fig. 8b). The FeOx(OH)y was amorphous based on broadness of the XRD peak at 35° (Fig. 1B(b)).
When the FeOx(OH)y composite was washed with ethanol, 24% of dodecylsulfate remained. Due to the lack oforganized surfactant layer, the piling regularity of inorganic layers was seriously lost judging from the broadnessof (0 0 1) diﬀraction peak (Fig. 1A(e)). The centers were difﬁcult to determine corresponding to the (0 0 2) and (0 0 3)
peaks.
4.2. Heating to create micro and mesopores
When samples were heated at 423 K, 51% of acetate andpropionate diminished (Table 2g and h). Based on the mOH
peak intensity, 40–60% of FeOx(OH)y transformed into
Fe2O3. The molar amount of transformed Fe corresponds

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Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

to ﬁve times of acetate or propionate diminished. Thus, thedehydration of FeOx(OH)y took place spontaneously. The
inorganic phases were still amorphous based on the broadness of diﬀraction peaks at 35° (Fig. 1B(c)). The irregularthin contrast spots still remained after heating at 423 K
in TEM (Fig. 3b). The speciﬁc SA (230 m2 g1) decrease
was less than 10% after the 1.0 ppm of arsenite sorption
test for 12 h. The pore volume and d-spacing changes after
the sorption test was also small (Table 1g). The adsorbed
amount of arsenic was only 0.04 wt.% of sorbent in one
run of sorption test and the sorption capacity negligiblychanged in ﬁve consecutive sorption tests in 1.0 ppm ofarsenite. Thus, the stability of this microporous structure
was demonstrated for the application of As removal andrepeated use.
In summary, by utilizing dodecylsulfate and subsequentacetate/propionate exchange, lamellar-structure eﬀectively
reorganized into microporous structure (Fig. 8d). A halfof the carboxylates diminished by the heating at 423 K
and micropores were created.
Mesostructured FeIII materials were directly synthesizedfrom FeIII and palmitic or lauric acids. The mesostructurewith d spacings of 30.8 and 24.5 Å, respectively, collapsed
at 423 K in air or oxygen [10]. The FeIII framework using
directly carboxylates may be less stable or the heating inoxidative atmosphere (heating in nitrogen in our work)
destabilized the framework.
When the ethanol-washed FeOx(OH)y was heated at
423 K, microporous framework was formed and thespeciﬁc SA was 170 m2 g1 (Table 1). Via the synthesis
from FeIIICl3 and hexadecylsulfonate followed by washingwith methanol and propanol, speciﬁc SA of 262 and236 m2 g1 were reported, respectively [7].
When these samples were heated at 533 K, the isotherms
pattern changed from Type I to Type IVb (Fig. 2A and C).
When most of the organics were removed, inorganic framework was lost. The average pore size was 37–50 Å for carboxylate-exchanged ones (Fig. 8f). In XRD, the peak at 35°
switched into Fe3O4 crystalline peaks (Fig. 1B(d–f)). 80%,
50%, and 70% of FeOx(OH)y seemed to transform into
Fe3O4 for formate-, acetate-, propionate-exchanged samples, respectively, via the following exothermic reaction(Fig. 5)
RCOO ðadsorbedÞ þ 3FeOðOHÞ ! ROH þ CO2
þ H2 O þ Fe3 O4 ðR ¼ H; Me; EtÞhydrophobicOHhydrophobicAs
CH3 HO OH CH3
O Cδ— OOH Cδ— — H O
2
O
O
δ+ δ+
Fe
Fe
Fe
O
O

hydrophobichydrophobic

O

O
CH3
As
C
O
O Cδ—
δ— O
O
O
δ+ δ+
Fe
Fe
Fe
O
O

CH3

Fig. 9. Proposed promoted sorption mechanism of low concentrations ofarsenite on acetate-exchanged FeOx(OH)y heated at 423 K.

loss by heating up to 723 K was 27.5% for acetateexchanged FeOx(OH)y by TG analysis (Fig. 5). Because
the acetate contents and the weight loss due to the transformation from FeO(OH) into Fe3O4 are 11.3% and 13.1%,
the rest of 3.1% may be due to adsorbed water.
4.3. Responsible sites for optimized arsenite sorption
The arsenic uptake on Fe oxyhydroxides was correlated
to the adjacent apexes of edge-sharing [FeO6] units[2,24,25]. The Fe  Fe coordination number (1.8–3.1) was
smaller for FeOx(OH)y composite and the derivative FeIII
materials compared to those for standard FeIII compounds(Table 4). The Fe site information deduced from EXAFS
was basically common for all the FeIII materials studiedin contrast that the acetate-exchanged FeOx(OH)y preheated at 423 K exhibited exceptionally excellent sorption
performance for arsenite (Fig. 7). Both the saturated sorption amount ð21 mgAs g1
adsorbent Þ and the equilibrium sorption constant ð1:0  107 ml g1
As Þ were by far greater thanethanol-washed FeOx(OH)y, Fe-montmorillonites, andconventional bulk crystalline FeO(OH) [2,4].
Thus, the reason of this superiority seems not the formation of speciﬁc adsorption FeOx sites for arsenite uptake.
Plausible explanations may be (1) the high speciﬁc SA,
microporous framework separated by 20 ± 2 Å in average(Fig. 8d, pore size 10 Å based on semi-quantitative t-plot
analysis) was advantageous for the diﬀusion of As(OH)3(maximum molecular size 6.8 Å [26] taken van der Waals
radius into account) in water, (2) adsorbed acetate inside
the micropores (a half of exchanged acetate left after heating at 423 K) (Table 2) eﬀectively exchanged with As(OH)3,and (3) adsorbed acetate molecules promoted the adsorption of As(OH)3 (surface site segregation by hydrophobic
eﬀect of alkyl groups, electrostatic eﬀect, etc.) (Fig. 9).

ð1Þ
5. Conclusions

The amount of carboxylate anions (0.42, 0.19, and 0.24
versus the amount of Fe) was suﬃcient to form Fe3O4
via Eq. (1) (Table 2). The reduction of FeIII to FeII by acetate was reported for FeIII hydroxoacetate heated in N2 gas
above 523 K [23].
Only very weak peak was observed in hydroxyl stretching region for formate-exchanged sample heated at 533 K(Fig. 4j). Therefore, in general for the FT-IR measurementsin this work, the contribution of adsorbed water should beminor compared to that of iron oxyhydroxide. The weight

The lamellar-structure of FeOx(OH)y coupled withdodecylsulfate double layers reorganized to wormhole-likemicroporous structure stabilized with adsorbed carboxylate anions. Upon heating at 423 K, a half of acetate/propionate anions diminished and the speciﬁc surface area grewto 230–128 m2 g1. The inorganic framework consisted ofamorphous FeOx(OH)y and dehydrated Fe2O3. This preparation method to exchange with smaller carboxylates wasfound to be eﬀective to prepare microporous FeIII materials

Y. Izumi et al. / Microporous and Mesoporous Materials 94 (2006) 243–253

of high speciﬁc SA. The acetate-exchanged FeOx(OH)y preheated at 423 K (230 m2 g1) exhibited the best arsenite
sorption capacity among all adsorbents that we havescreened. The microporous FeIII materials consisted of lesscomplete array of [FeO6] units based on smaller Fe  Fe
coordination number (1.8–3.1). The reason that acetatedexchanged FeOx(OH)y exhibited exceptionally excellent
performance for the arsenite sorption seems not the creation of speciﬁc FeOx surface sites. The eﬀects of microporous framework and the presence of adsorbed acetate
may be kinetically advantageous.
Acknowledgements
Fe K-edge XAFS experiments were performed under theapprovals of Photon Factory Proposal Review Committee(2001G092, 2003G074, 2004G278). We are thankful for the
Grant-in-Aid for Encouragement of Young Scientists from
the Ministry of Education, Culture, Sports, Science, andTechnology (Y.I., 1474 0401).
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