Water Air Soil Pollut: Focus (2009) 9:203–211
DOI 10.1007/s11267-009-9206-1

Arsenic Removal from Dilute Solutions by High Surface
Area Mesoporous Iron Oxyhydroxide
Dilshad Masih & Yoshimi Seida & Yasuo Izumi

Received: 28 November 2008 / Revised: 7 July 2009 / Accepted: 20 July 2009 / Published online: 30 September 2009
# Springer Science + Business Media B.V. 2009

Abstract Mesostructured iron oxyhydroxide (FeOx)
and iron oxyhydroxide–phosphate (FeOxP) composites
were organized using dodecylsulfate surfactant as atemplate. X-ray diffraction studies depicted a lamellar
structure of the product. Ion exchange and solvent
extraction methods were employed for the removal of
the surfactant. Carboxylate ions exchanged lamellar
type mesostructured material reorganized to awormhole-like mesoporous material when heated underN2 atmosphere. Surfactant was completely removed bycarboxylate ions as observed by the Fourier transform
infrared spectra. High surface area acetate-exchanged
FeOx (230 m2 g−1) was obtained after the surfactant
removal from the composite (2.8 m2 g−1). Surface areaof acetate-exchanged FeO x P was the highest
D. Masih (*)
Global Edge Institute, Tokyo Institute of Technology,4259-G1-33 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japane-mail: masih.d.aa@m.titech.ac.jp
Y. Seida
Department of Environmental Chemistry and Engineering,Tokyo Institute of Technology,4259-G1-33 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japane-mail: seida.y.aa@m.titech.ac.jp
Y. Izumi
Graduate School of Science, Chiba University,
Yayoi 1-33, Inage-ku,
Chiba 263-8522, Japane-mail: yizumi@faculty.chiba-u.jp(240 m2g−1) after the removal of the surfactant. Local
structure of iron species of FeOx was investigatedby X-ray absorption fine structure spectroscopy.
Further, Fe···Fe bond appeared at 3.21–3.25 Åwith coordination number 2–3, showing a high degreeof un-saturation of Fe···Fe bonds. As comparedwith bulk iron oxyhydroxide and iron-intercalated
montmorillonite, the mesoporous iron materials
were highly effective for arsenic removal from low
concentrations of aqueous solutions. Furthermore,mesoporous iron materials were stable in aqueous
phase.
Keywords Arsenic removal . Mesoporous .
Iron oxyhydroxide

1 Introduction
Arsenic contamination of drinking water sources is amajor environmental problem around the globe affecting
millions of people (Smedley and Kinniburgh 2002;
Charlet and Polya 2006). Due to high health hazards,
especially the carcinogenic risks from the intake ofarsenic-contaminated water, its permissible limit inpublic water supplies has been reduced from 50 to10 μg/L (WHO 2001). Arsenic species are prevalent as
inorganic forms in the contaminated aquifers. Depending
upon environmental conditions, inorganic arsenic exists
in water as arsenite and arsenate oxyanions or neutral
species. Mobility and bioavailability of arsenic depend

204on the particular type of its species, arsenite being themost toxic one. Hydrochemistry of arsenic-enrichedaquifers in the calamity-hit Bengal area indicate reducing
conditions under which arsenite species should be
prevalent in the groundwater (Charlet and Polya 2006;
Nath et al. 2008). Besides, considerable amount of
inorganic arsenite species has been reported even in thesurface waters (Ren et al. 2007).
Iron-based materials have high affinity for arsenic,
but removal of the most toxic arsenite species isdifficult (Farquhar et al. 2002; Dixit and Hering2003). Most of the studies on arsenic removal fromwater deal with arsenate only. Due to high affinity ofiron for arsenic, iron compounds have been used evenfor the cost-effective chemical immobilization ofarsenic in contaminated soils (Gemeinhardt et al.
2006). Adsorption is one of the most effective andcomparatively simple and economical methods forarsenic removal from aqueous solutions. However, tocope with the new regulations, effective adsorbent
materials are required, especially for the removal ofarsenite species from the solutions of low concentrations. Iron species intercalated in montmorillonite wereeffective for oxidative adsorption of arsenic (Izumi et al.
2005; Masih et al. 2005). High affinity and oxidative
power of iron-intercalated montmorillonite for thearsenite was observed because of the high extent of
un-saturation of Fe∙∙∙Fe coordination (N=2.5) and their
homogeneous distribution (Masih et al. 2007).
Mesostructured iron oxyhydroxides (FeOx) are
reported to have a high degree of un-saturation ofFe∙∙∙Fe coordination number (Wirnsberger et al.
2001). Synthesis of mesostructured iron materials
from iron precursor and surfactant has been reported
earlier (Tolbert et al. 1997). But after the removal of
the surfactant, it was difficult to maintain theorganized framework structure. By employing soft
methods for surfactant removal, the degree of Fe∙∙∙Fe
coordinative un-saturation of mesostructured FeOx
was retained and iron contents were also not muchdifferent than that of the bulk iron oxyhydroxides(Izumi et al. 2006). As-prepared lamellar materialtransformed into a wormhole-like mesoporous structure when template was substituted with carboxylateions followed by heating under N2 atmosphere.
Bruner–Emmett–Teller (BET) surface area and porosityof the materials were controlled by these modifications
and mild heat treatments. Amorphous iron (III) phosphate is reported to have good adsorption capacity for

Water Air Soil Pollut: Focus (2009) 9:203–211

heavy metals (Yin et al. 2007). Guo et al. reported thesynthesis of high surface area mesoporous iron
phosphate with the surfactant sodium dodecylsulfate(Guo et al. 2001). In this study, mesoporous iron
oxyhydroxide–phosphate (FeOxP) analog of FeOx
was prepared to see the effect of framework
phosphate in strengthening the framework structureand surface acetate on arsenic removal. The materials
were used for the arsenic removal from the aqueous
solutions in the concentration range of environmental
conditions.

2 Experimental
2.1 Preparation of Adsorbent Materials
Syntheses of mesostructured iron oxyhydroxide(FeOx) and iron oxyhydroxide–phosphate (FeOxP)
composites were carried out under ambient conditions. For the synthesis of mesostructured FeOx withiron to surfactant molar ratio of 7:1, 50 mL of 0.10 M
ferrous chloride (Wako) was mixed with 10 mL of
0.07 M sodium dodecylsulfate (Wako) followed bydropwise addition of 10 mL of 0.25 M H2O2 (Wako)
under vigorous stirring. The final reaction mixture
was kept on stirring for 1 h and then filtered andwashed with deionized water. The obtained yellow
powder of mesostructured material is called FeOx
composite. For the synthesis of mesostructured FeOxP,
2.5 mL of 0.05 M Na3PO4 (Wako) was mixed with
50 mL of 0.10 M ferrous chloride followed by thesame procedure as described above for the FeOx
composite. The surfactant was removed by solvent
extraction with ethanol and ion exchange methods. The
composite was mixed with 75 mL of 0.05 M sodium
formate, sodium acetate, or sodium propionate solutionin ethanol and stirred for 30 min at 290 K. The
obtained brown powders are called carboxylateexchanged FeOx and FeOxP. Surfactant was also
extracted with ethanol at 290 and 333 K for 10 h.
Each of the carboxylate-exchanged or ethanol-washed
FeOx and FeOxP samples was heated under N2 flow.
The color of the obtained powder was reddish brown.
2.2 Characterization of Adsorbent Materials
X-ray diffraction (XRD) patterns of powder samples
were recorded on Multiflex-S diffractometer (Rigaku)Water Air Soil Pollut: Focus (2009) 9:203–211

using Cu Kα radiation. N2 adsorption–desorption
isotherms were measured at 77 K on Belsorp-mini(Bel Japan) after pre-treatment at 423 K for 2 h underN2 flow. Specific surface area was calculated by the
BET method. For each type of the International Union
of Pure and Applied Chemistry (IUPAC)-classifiedadsorption–desorption isotherm and hysteresis loop, a
specific method is available for the calculation ofdifferent parameters. Pore size distribution and porevolume were determined by appropriate t-plot (microporous material) and DH-plot (mesoporous materials,
synonymous BJH method) methods. Thermogravimetric(TG) and differential thermal analyses (DTA) were
performed on DTG-60 (Shimadzu). For the measurement of vibrational spectra, 1.0 wt.% of as-preparedand modified samples and reference materials weremixed very well with KBr (Wako) and pressed into
20-mm-diameter disks. Each disk was prepared from
0.16 g homogeneous mixture. The measurements werecarried out under ambient conditions on VALOR-III
FTIR (Jasco) equipped with a mercury cadmium
telluride detector.
2.3 X-Ray Absorption Fine Structure Studies
For the measurements of X-ray absorption finestructure (XAFS) spectra, FeOx samples and referenceiron materials were mixed well with boron nitride
while keeping the Fe K-edge absorption jump valueof 1.0. XAFS spectra were measured at beamline
10B of Photon Factory in transmission mode using a
Si (311) double crystal monochromator. The rising
K-edge energy of the 4-μm Fe metal foil was calibrated
at 7,111.20 eV (Bearden 1967). Fe K-edge XAFS data
analyses were performed using program XDAP. The
Fe–O and Fe···Fe bond distances and coordination
numbers were based on crystal structure data for
α-Fe2O3.

205

concentration of the liquid phase was quantified byinductively coupled plasma-optical emission spectroscopy measurements.

3 Results and Discussion
3.1 Ordered Framework Structure
XRD patterns measured for the FeOx composite andits derivative compounds are depicted in Fig. 1. In thesmall-angle XRD pattern of FeOx composite, main
diffraction peak (001) of ordered framework structure
was centered at 2.44° (d-spacing of 36.3 Å) followedby the second order diffraction peak (002) at 4.92°
depicting a typical lamellar structure. As shown in
Fig. 2, the main diffraction peak (001) for FeOxP
composite (2.74°) appeared at higher diffraction angle
as compared to FeOx composite (2.44°) indicating adecrease in the d-spacing. Calcination was noteffective for the removal of the surfactant, andcrystallization to α-Fe2O3 started at 573 K before
reaching the temperature (>673 K) required for thecomplete removal of the surfactant. After the surfactant removal and ion exchange with carboxylates, thed-spacing decreased to 20.2 Å (acetate) along with adecrease in the regularity of the framework structureof FeOx. The d-spacing of ethanol-washed FeOx

2.4 Arsenic Adsorption Test
Arsenic adsorption tests were performed under batch
setup at 290 K in polypropylene tubes. Adsorbent
materials (50 mg) were immersed in 20 mL of
0.2–32 mg/L test solutions of arsenite (pH 6.6–7.1)
and arsenate (pH 6.3–6.7). The mixtures were shaken
at a rate of 130 times per minute on a reciprocating
shaker for 12 h. The solid and liquid phase wasseparated by 0.45 μm cellulose membrane. Arsenic

Fig. 1 XRD patterns of FeOx composite (a), ethanol-washed
FeOx (b), and acetate-exchanged FeOx (c)206Fig. 2 XRD patterns of FeOxP composite (a), ethanol-washed
FeOxP (b), and acetate-exchanged FeOxP (c)sample was 28.0 Å. A similar decrease in d-spacing
and regularity of the framework structure wasobserved for the FeOxP material after the removalof the surfactant. Wide-angle XRD patterns in therange of 15–70° showed that the amorphous natureof the as-prepared composites was maintained after
the surfactant extraction with ethanol and exchangewith carboxylate ions. Small-angle XRD pattern ofarsenic adsorbed mesoporous FeOx showed stabilityof the framework structure after interaction withaqueous solution. High-resolution transmission electron micrograph of acetate-exchanged FeOx sample
heated at 423 K depicted its prevailing amorphousnature and thin contrast spots of a few nanometers
showed a wormhole-like porous nature of thematerial (Izumi et al. 2006).
3.2 N2 Adsorption–Desorption Isotherms andPorosity
Type-IVb isotherms of IUPAC classification withcharacteristic H2-type hysteresis loop were observed
for the carboxylate-exchanged FeOx samples heated at
533 K as shown in Fig. 3 (Rouquerol et al. 1999).
Type-IVb isotherms are typical for the wormhole-likemesoporous materials. DH-plot method is appropriate
for the calculation of pore size and pore volume fromtype-IVb isotherms. With DH-plot method, an aver-Water Air Soil Pollut: Focus (2009) 9:203–211

Fig. 3 N2 adsorption (solid symbols)–desorption (hollow
symbols) isotherms for acetate-exchanged FeOx (a) andpropionate-exchanged FeOx (b) materials heated at 533 K(DH-plot is shown in the inset)

age pore diameter of 37 Å was determined fromdesorption branch for the mesoporous material
obtained from acetate-exchanged precursor as shownin the inset of Fig. 3. Pore size distribution curve offormate-exchanged FeOx was also centered at 37 Å.
Homogeneous pores of 50 Å was observed forpropionate-exchanged FeO x heated at 533 K(Fig. 3). With the change of the carbon chain lengthof the carboxylate ions, it was possible to calibrate thepore size for homogeneous mesoporous materials.
N2 adsorption–desorption isotherms measured forthree kinds of carboxylate-exchanged FeOx materials
heated at 423 K were of type I. For the type-I
isotherms, t-plot method is suitable to calculate poresize and pore volume of the materials. From thedesorption branch, an average pore size of 10 Å was
calculated with t-plot method. The isotherm forethanol-washed sample heated at 423 K belonged totype IV. Figure 4 shows N2 adsorption–desorption
isotherm measured for acetate-exchanged FeO xPheated at 423 K. The isotherm is typical ofwormhole-like mesoporous materials, and pore size
was 37 Å as calculated by the DH-plot method, as
shown in the inset in Fig. 4. Pore size distribution of
acetate-exchanged FeOxP was also homogeneous.
Other researchers have also reported wormholelike morphologies of mesoporous iron materials.
Srivastava et al. reported the synthesis of mesoporous

Water Air Soil Pollut: Focus (2009) 9:203–211

207

FeOx, the pore volumes were 0.10 and 0.14 cm3 g−1
respectively. BET surface area of acetate-exchanged
FeOxP was the highest (240 m2 g−1) among all the
mesoporous materials under study. Similar to thehighest SBET value, the pore volume of acetateexchanged FeOxP was also maximum (0.24 cm3 g−1)among all the mesoporous materials.
3.3 Thermal Behavior and FT-IR Studies

Fig. 4 N2 adsorption (solid symbols)–desorption (hollow
symbols) isotherms for acetate-exchanged FeOxP heated at
423 K (DH-plot is shown in the inset)

Fe2O3 by sonochemical technique, but the producthad inhomogeneous pores of 39 and 75 Å (Srivastava
et al. 2002). Jiao et al. reported two- and threedimensional mesoporous Fe2O3 with inhomogeneous
pore size of 54 and 70 Å (Jiao and Bruce 2004).
Mesoporous materials synthesized in this study havehomogeneous pores, which is important for the
specific application of the materials as adsorbents.
An ideal adsorbent material for the removal of arsenic
was predicted to have uniform and interlinked pores(Kim et al. 2004). Variations of carboxylate ions wereeffective to synthesize mesoporous materials withuniform-sized calibrated pores. For the ethanol-washedsample, the pores were also lying in the range ofmesopore size (41 Å). However, dV/dRp peak intensity
was by far weaker than the mesoporous materials
obtained from the carboxylate-exchanged FeOx.
As-prepared FeOx composite was essentially nonporous with BET surface area (SBET) of 2.8 m2 g−1.
When surfactant was extracted with ethanol, specificsurface area increased to 170 m2 g−1. Among FeOx
materials, the SBET value was the highest for
the acetate-exchanged FeOx (230 m2 g−1) material
heated at 423 K. It is important to note that, forthe acetate-exchanged FeOx, the SBET value remained
high (208 m2 g−1) even after the interaction with
1.0 mg/L arsenic solution. Pore volumes of mesoporous materials synthesized from propionate-exchanged
FeOx was 0.18 cm3 g−1. For the mesoporous materials
synthesized from formate- and acetate-exchanged

For the acetate-exchanged FeOx, a gradual loss ofweight was observed between 290 and 500 K as
shown by the TG curve in Fig. 5. An early weight loss
starting at 330 K was observed because of thedesorption of water and acetate species and dehydrationof FeOx. An abrupt weight loss occurred from 500
to 580 K because of the exothermic decompositionof the acetate species as shown by the DTA pattern(Fig. 5). Theoretical weight loss was precisely inagreement with the one observed from the TG
curve.
The surfactant, dodecylsulfate, was completely
exchanged with carboxylate ions as monitored by
Fourier transform infrared (FT-IR) spectroscopy. The
spectra measured for FeOx composite and its derivatives are depicted in Fig. 6. Estimation of thesurfactant in the FT-IR spectrum showed that themolar ratio of surfactant and iron in the FeOx
composite was consistent with the starting ratio.
When the surfactant was extracted with ethanol, about
25% of the surfactant remained intact as shown by
the peak derived from the sulfate (1,227 cm−1) in theFig. 5 TG–DTA curves of acetate-exchanged FeOx

208

Water Air Soil Pollut: Focus (2009) 9:203–211

Fig. 6 FT-IR spectra of FeOx composite (a), ethanol-washed
FeOx (b), acetate-exchanged FeOx (c), and acetate-exchanged
FeOx heated at 423 K (d)Fig. 7 Fourier transforms of iron K-edge EXAFS spectra
measured at 290 K for 14%Fe-montmorillonite (dotted line),
and acetate-exchanged FeOx (solid line)

FT-IR spectrum (Fig. 6). After surfactant exchangewith carboxylates, essentially no dodecylsulfate was
detected as depicted by total disappearance of thestretching at 1,227 cm−1 due to sulfate. About 51% of
the acetate (peaks at 1,423 and 1,540 cm−1) was
decomposed on heating at 423 K (Fig. 6).

similar to that for the 14% Fe-montmorillonite(Fig. 7). As compared to bulk iron oxides, thesesmall values of coordination number indicated alimited number of Fe···Fe attachments and consequently a lot of free sites available to capture arsenic
species.

3.4 Local Structure of Framework Iron Species

3.5 Arsenic Removal

Iron K-edge XAFS studies showed a high degree of
coordinative un-saturation of Fe···Fe bond in the FeOx
composite and its derivatives. Figure 7 depicts Fouriertransforms of extended X-ray absorption fine structure (EXAFS) spectra for FeOx composite and 14%
Fe-montmorillonite. A high degree of coordinative
un-saturation has been reported in the 14%Fe-montmorillonite and the material is used as a reference inthis study (Izumi et al. 2005). In the Fe K-edge
EXAFS Fourier transform, the strongest peak at 1.5–
1.6 Å was from Fe–O bond. Another prominent peakat 2.7–2.8 Å was also clearly observed due to Fe···Feinteractions. The Fe–O coordination number negligibly varied around six indicating octahedral [FeO6]
symmetry of iron species. The Fe–O bond distancewas 2.05–2.09 Å, relaxed from 2.021 Å for α-FeO(OH) and 2.034 Å for γ-FeO(OH). Accordingly,
Fe···Fe bonds were slightly expanded (3.23–3.25 Å)compared to those for the crystalline iron oxides. The
Fe···Fe coordination number was in the range of 2–3,

Arsenic adsorption studies were performed for theremoval of arsenite and arsenate species from dilute
aqueous solutions in the range of 0.2–32 mg/L. This
concentration range is a plausible condition for
application to environmental samples. Mesoporous
FeOx, FeOxP, and reference materials 14%Fe-montmorillonite and α-FeO(OH) were the adsorbent
materials used for arsenic removal. Surface area of
acetate-exchanged iron oxides was the highest among
all the carboxylate-exchanged materials. And preliminary arsenic adsorption tests showed that acetateexchanged iron oxides were superior in performance.
Therefore, results of arsenic removal by acetateexchanged adsorbents are described in detail. In theapplied concentration range, arsenate species wascompletely removed by the acetate-exchanged FeOx
showing a very high efficiency of the material. Hence,the discussion is focused on the removal of most toxic
and difficult-to-remove arsenite species. In Fig. 8,
adsorption isotherms are shown for arsenite removal

Water Air Soil Pollut: Focus (2009) 9:203–211

Fig. 8 Adsorption isotherms of arsenite at 290 K on the acetateexchanged FeOx (ball), ethanol-washed FeOx (cube), acetateexchanged FeOxP (inverted triangle), 14% Fe-montmorillonite(diamond), and ethanol-washed FeOxP (triangle)

from 0.2–32 mg/L solutions. The order of adsorption
capacity was mainly depending upon the arsenic
concentration at equilibrium. When it was lower than
760 μg/L, the weight ratio of adsorbed arsenic andadsorbent was greatest for the acetate-exchanged
FeOx. Adsorption capacity of bulk material (α-FeO(OH)) employed in this study was far below than thatof the acetate-exchanged FeO x . The acetateexchanged FeOx showed much higher adsorption
capacity even against the bulk iron oxyhydroxidewith the best performance as reported by otherresearchers (Dixit and Hering 2003). Next, when theconcentration was higher than 1.27 mg/L, again theacetate-exchanged FeOx was matchless in performance. Performance of ethanol-washed FeOx and
14%Fe-montmorillonite was comparable for arsenite
removal from test solution of >1.27 mg/L. Arsenite
removal behavior of modified FeOxP materials was
similar to that of modified FeOx materials. Acetateexchanged FeOxP material showed better performance
than the ethanol-washed FeOxP sample.
The difference of arsenic removal efficiencies was
mainly due to the order of equilibrium adsorption
constant (ml gAs−1): 1.0 ×107 (acetate-exchangedFeOx)≫1.4×106 (14%Fe-montmorillonite)>6.2×105(ethanol-washed FeO x ) ≈ 5.4 × 10 5 (α-FeO(OH)),

209

based on the fits with Langmuir-type equation.
Thus, the practical superiority of the acetate-exchanged
FeOx is evident. Arsenic uptake of iron oxyhydroxides
has been correlated to the adjacent apexes of edgesharing [FeO6] units. In the arsenite concentration
range studied here, sorption capacity of acetateexchanged FeOx heated at 423 K was the greatest.
Both the saturated adsorption amount (21×10−3 gAs
gadsorbent−1) and adsorption equilibrium constantvalues were by far greater than ethanol-washed
FeO x , modified FeO x P, 14%Fe-montmorillonite,and bulk α-FeO(OH) (Izumi et al. 2005; Dixit andHering 2003).
Arsenic to iron molar ratio for arsenite adsorbed on
the acetate-exchanged FeOx was 15 times higher thanthat of the bulk α-FeO(OH) employed under the same
experimental conditions. High arsenic to iron molar
ratio depicts that a large number of adsorption active
sites are available in the mesoporous material. Arsenic
to iron molar ratio of acetate-exchanged FeOx was
two times greater in comparison to the performance ofa high surface area bulk α-FeO(OH) used in anotherstudy (Dixit and Hering 2003), even though ironcontents of acetate-exchanged FeOx was 12% lessthan that of the bulk α-FeO(OH). Furthermore, theAs/M (Fe, Al) molar ratio for the arsenite adsorbed on
the acetate-exchanged FeOx was similar to the value
reported for mesoporous aluminum oxide (Kim et al.
2004). Iwamoto et al. reported effective removal ofarsenate (pH 7.5) and arsenite (pH 10.0) throughanion exchange on zirconium sulfate–surfactant
micelle mesostructure (Iwamoto et al. 2002). From
5.0 mg/L test solution, almost complete removal ofarsenate was observed with the ion exchange time of
10 h. But arsenite removal efficiency of the material
was always less than 80% even after 40 h equilibrium
time. For the test solutions of similar concentration,acetate-exchanged FeOx showed complete removal ofarsenate and importantly the arsenite removal efficiency was also >99%. High removal efficiency alongwith high capacity of the adsorbent material isimportant regarding the modern regulations of arsenic
contaminant level, and acetate-exchanged FeOx was
fulfilling the criteria.
The extent of coordinative un-saturation of Fe···Febond in the acetate-exchanged FeOx (N=2.5) andethanol-washed FeOx (N=2.6) was similar. The BET
specific surface area of the former was 1.35 times
larger. These differences were not so remarkable to210justify the extremely good performance of acetateexchanged FeOx. Results of Yoshitake et al. showed
the importance of creating particular adsorption sitesof amino-functionalized mesoporous silica for thearsenate removal but not the surface area (Yoshitake
et al. 2002). Also in this study, acetate-exchanged
FeOxP had the highest surface area not the efficiency
for arsenite removal. Arsenite exchange against thesurface carboxylate species was one of the plausible
explanations for the greatest performance of acetateexchanged FeOx. However, this cannot explain
completely the exceptional performance, as the arsenite
species were essentially neutral in the applied pH rangeof the test solutions. Oxidative adsorption of arsenite
on the acetate-exchanged FeOx could be a possible
reason of its extremely high efficiency. Performance of
acetate-exchanged FeOx and FeOxP was better than therespective ethanol-washed materials. So the porous
framework with the presence of adsorbed acetate may
be kinetically advantageous over the bare surface
mesoporous material.
XRD and BET studies gave valuable information
regarding the repeated use of the mesoporous adsorbents. Small-angle XRD pattern showed similard-spacing and extent of the ordered structure before
and after arsenic adsorption test. Similar d-spacing of
mesoporous adsorbent before and after arsenic adsorption is no surprise as the smaller arsenic species
fit in the larger vacant area without propping up theiron oxide sheets. Furthermore, the SBET value of
acetate-exchanged FeOx (230 m2 g−1) decreased to
208 m2 g−1 after interaction with 1.0 mg/L arsenic
solution. These investigations show stability of themesoporous iron materials for repeated arsenic removalfrom aqueous solutions.

4 Conclusions
Surfactant extraction and ion exchange methods weresuccessfully applied to modify mesostructured FeOx
and FeOxP composites for the efficient arsenic andparticularly arsenite removal from low concentrationsof test solutions. The lamellar structure of as-prepared
composites reorganized to a wormhole-like mesoporous
structure. Amorphous nature of the composites
was retained after modifications with ethanol andcarboxylate solutions of ethanol. Maximum surface areaof 230 and 240 m2 g−1 were obtained respectively for

Water Air Soil Pollut: Focus (2009) 9:203–211

the acetate-exchanged FeOx and FeOxP. Carboxylateexchanged FeOx heated at 533 K showed wormholelike homogeneous mesopores of 37 Å (formate/acetate)
and 50 Å (propionate). Acetate-exchanged FeOxP also
showed wormhole-like homogeneous-sized pores of37 Å and maximum pore volume of 0.24 cm3 g−1.
From XAFS studies, Fe···Fe coordination number
obtained in the range of 2–3 showed a high degree of
coordinative un-saturation of iron species. Acetateexchanged FeOx pre-heated at 423 K exhibitedthe best arsenite sorption capacity and complete
removal of arsenate. Adsorbed acetate species in thehomogeneous-sized mesoporous network of highsurface area acetate-exchanged FeOx and FeOxPmaterials were influencing the high arsenic removal
efficiency of the materials.
Acknowledgements XAFS experiments were carried out
under the approval of the Proposal Review Committee of
Photon Factory. Thanks are due to the Japanese Ministry of
Education, Culture, Sports, Science and Technology for thefinancial assistance from MEXT’s program “Promotion ofEnvironmental Improvement for Independence of Young
Researchers” under the Special Coordination Funds for Promoting
Science and Technology.

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