Eng. Life Sci. 2007, 7, No. 1, 52–60

52

D. Masih1
Y. Izumi1

Research Article

K. Aika1
Y. Seida2

Optimization of an Iron Intercalated
Montmorillonite Preparation for the Removalof Arsenic at Low Concentrations*12Interdisciplinary Graduate
School of Science andEngineering, Tokyo Institute ofTechnology, Yokohama, Japan.
Institute of Research andInnovation, Takada, Japan.

A series of iron intercalated montmorillonites (Fe-Monts) were prepared using(i) ion exchange of native sodium and calcium ions with iron ions, (ii) basehydrolysis of inserted iron ions in montmorillonite suspension, and (iii) insertionof pre-hydrolyzed iron colloid in montmorillonite. The materials were characterized by X-ray diffraction and gas adsorption-desorption techniques. The basald(001)-spacing and BET specific surface area increased after the intercalation ofiron species in montmorillonite. Local iron structure studied by X-ray absorption
fine structure (XAFS) spectroscopy showed an unsaturation of the Fe···Fe coordination number (N 2.5) of the intercalated iron species as compared to the bulk
iron oxyhydroxides (N 6). The Fe-Monts were employed for arsenic removal from
aqueous solutions at low concentration (0.2–16 mg/L). Among the Fe-Monts, theone prepared by the hydrolysis of inserted iron ions, was the best in performance.
The saturation adsorption amount of the optimized iron-montmorillonite was
4 and 28 times higher for the removal of arsenite and arsenate, respectively, as
compared to bulk iron oxyhydroxide (goethite). Compared with bulk iron oxyhydroxide, the Fe-Monts were superior for arsenate uptake and comparable forarsenite. In addition, arsenite adsorbed on the Fe-Monts was found to be oxidized
to arsenate based on XAFS spectroscopy.
Keywords: Arsenic, Bioremediation, Contaminated water, Environmental protection
Received: August 15, 2006; revised: November 16, 2006; accepted: November 24, 2006
DOI: 10.1002/elsc.200620171

1

Introduction

Arsenic contamination of drinking water resources is a major
environmental problem in many countries of the world [1, 2].
Both natural and anthropogenic sources have increased arseniclevels of water. Depending upon environmental conditions,arsenic exists in water as arsenite (AsIII) and arsenate (AsV)
oxyanions or neutral species. The mobility and bioavailabilityof arsenic highly depends upon the particular species. In particular, arsenite is the most toxic and mobile among all the–
Correspondence: Y. Izumi (e-mail: izumi.y.ac@m.titech.ac.jp), Interdisciplinary Graduate School of Science and Engineering, Tokyo
Institute of Technology, Nagatsuta 4259-G1-16, Midori-ku, Yokohama
226-8502, Japan.
[*]

Based on a lecture presented at the First International Conference on
Environmental Science and Technology (ICEST), New Orleans, LA, USA,
January 23–26, 2005.

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

common species of arsenic [3]. Due to high health hazards,
especially carcinogenic risks from the consumption of arsenic
contaminated water, the maximum contaminant level has been
reduced from 50 lg/L to 10 lg/L [1, 2, 4]. Adsorption is a simple and economical method for the removal of arsenic from
aqueous solutions. However, to cope with new regulations andprovide safe drinking water, effective adsorbent materials are
required, especially for the removal of arsenite from low concentration solutions.
The affinity of arsenic for iron-based materials is well known[5–9]. However, by using bulk iron oxyhydroxides (e.g.,goethite), the removal of arsenite is difficult without prior oxidation [10]. Iron dissolution from soil and its co-precipitation
with arsenic is important to control its fate in the environment[11]. In this study, an iron-based adsorbent was chosen and its
affinity for arsenic was investigated by preparing different
kinds of iron species. Sodium-montmorillonite (Na-Mont), a
clay mineral of the smectite group having a three-layered sheet
structure, was used as a substrate material to host iron species.
Intercalation behavior of montmorillonite is well known[12–15].

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X-ray absorption fine structure (XAFS) spectroscopy is astate-of-the-art technique for the local structure studies [16].
In this paper, XAFS was chosen as a probe for the studies ofthe molecular level of adsorbent and adsorbate species to gain
an understating of the arsenic removal mechanism and to
make modifications in the adsorbent material, in order tosatisfy modern environmental regulations. Redox behavior ofadsorbed arsenic species was monitored by XAFS combinedwith high-energy-resolution fluorescence spectrometry. Speciation of arsenic at low concentrations (0.13–0.48 wt %)adsorbed on iron species was performed successfully. The highsignal/background ratio enabled by this combined technology
has been used to monitor low concentrations of environmental
contaminants in the presence of other heavy metals [17, 18].
This study describes the preparation of different kinds ofiron species intercalated montmorillonites, and the behaviorof their physicochemical properties with different iron loadings. The arsenic removal capacity and efficiency of Fe-Monts
prepared by different routes are discussed in detail. The arsenic
adsorption activity of intercalated iron species is comparedwith that of bulk iron oxyhydroxides. The importance of intercalated iron species for the oxidative adsorption of most toxic
and highly mobile arsenite species is explained.

2

Materials and Methods

Preparation of Iron-Montmorillonites
Iron-montmorillonites (Fe-Monts) were prepared by threeroutes outlined in Fig. 1. Na-montmorillonite (Kunipia F; 99 %purity) was used as received from Kunimine Industries. The
particle size of Na-Mont was in the range of 0.1–1.0 lm andthe cation exchange capacity was 115 meq/100 g. The substratematerial also had a 2.21 wt % of Fe2O3 from the substitutionsof some aluminum sites in the central layer of the montmorillonite sheet. The iron present in the pristine montmorillonite
was essentially buried between the silicon layers. Aqueous solutions of 0.43 M Fe(NO3)3 (Wako) and 0.75 M NaOH (Wako)
were used in all the preparations. All the Fe-Monts wereprepared at 290 K using 0.5 wt % aqueous suspension of
Na-Mont.
Ion Exchange (IE)
For the ion exchange reaction, 0.5 mmol of Fe(NO3)3 wasadded dropwise into a 0.5 wt % aqueous suspension of NaMont (1.0 g) and kept stirring for 12 h. The maximum
amount of iron exchanged with the sodium and calcium sitesof montmorillonite corresponded to 2.8 wt % on a dry mass
basis. If more than 0.5 mmol of Fe(NO3)3 was introduced to
1.0 g of Na-Mont suspension, the excessive iron was eluted
out.
Hydrolysis of Inserted Positive Ions (HIPI)
An excessive amount of iron ions (> ion exchange capacity)
was introduced to a Na-Mont suspension. The inserted positive ions were stabilized by base hydrolysis with a 0.75 M© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 1. Schematic diagram for the preparation of iron-montmorillonites.

NaOH aqueous solution. The OH:Fe molar ratio was a key
factor for the successful intercalation of iron species, andthe optimized ratio was 2. A series of iron-montmorillonites
were prepared using the post-hydrolysis of different amounts
of inserted positive ions. For the maximum iron loaded
14.0 % Fe-HIPI-Mont, 3.0 mmol of Fe(NO3)3 was mixed with
0.5 wt % aqueous suspension of Na-Mont (1.0 g). The amountof iron ions added into montmorillonite suspension was 6times higher than the exchangeable amount required against
the native ions. The mixture was stirred at 290 K for 2 h, andthen 6.0 mmol of NaOH was added dropwise. The final reaction mixture was kept on stirring for 12 h.

Insertion of Pre-Hydrolyzed Iron (IPI)This method is similar to a previously reported method [14].
In this case, instead of growing iron species in the montmorillonite suspension, pre-hydrolyzed iron species were inserted in
the montmorillonite. At first, a precursor colloid of 0.20 M[Fe(OH)2]+ was prepared by mixing Fe(NO3)3 and NaOH.
The molar ratio of base to metal was kept at 2, and the mixing
was carried out at 290 K. The colloidal [Fe(OH)2]+ was added
to a 0.5 wt % aqueous suspension of the Na-Mont, and stirred
for 12 h. A series of Fe-IPI-Monts were prepared with ironloadings ranging from 2.0–27 wt %. By this route, the maxi-

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D. Masih et al.

mum amount of iron loaded in the montmorillonite was
27 wt %.
For all the preparations, the solid and liquid phases wereseparated by filtration. The obtained mass was washed with
Millipore water until the pH of the filtrate approached 7. The
final product was dried at 313 K for 72 h.

X-ray Diffraction and N2 Adsorption-Desorption Measurements
X-ray diffraction (XRD) patterns of powdered samples wererecorded on a Multiflex-S diffractometer (Rigaku) using Cu
Ka radiation. The patterns were measured in the 2-theta range
of 2–60 degrees. The gas adsorption-desorption isotherms
were measured on BELSORP 28SA and Belsorp-mini (BellJapan). The isotherms were measured at 77 K using N2 as anadsorbate. Before recording the isotherms, the samples were
outgassed by evacuation at 423 K or by heating at the same
temperature for 2 h under N2 flow. The specific surface area ofadsorbents was calculated using the Brunauer-Emmett-Teller(BET) method. Pore size distribution and pore volume were
determined from the desorption branch of isotherms using the
Dollimore-Heal (DH)-plot method.

Arsenic Adsorption Tests
The arsenic adsorption tests were performed in a batch set-up
and at 290 K. In a 30 mL capacity polypropylene tube, 50 mg
of adsorbent was immersed in 20 mL of 0.2–16 mg/L test solution of arsenite (pH 5.5–6.5) and arsenate (pH 5.7–6.7). The
mixture was shaken on a reciprocating shaker at 130 rpm for
12 h. For the adsorption test, an arsenite stock solution(1000 mg/L) was prepared by dissolving As2O3 (Wako) in aminimum amount of basic aqueous solution and the final pH
was brought to 7 by the addition of HCl solution [5]. Arsenate
stock solution was prepared from water soluble KH2AsO4(Aldrich). The aqueous phase arsenic concentration was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES), using Vista-Pro of Seiko-Varian
Instruments.

Iron K-edge XAFS Measurements
For XAFS measurements, the Fe-Monts were transferred into a
Pyrex glass XAFS cell and evacuated at 290 K. Iron K-edge
XAFS spectra were measured at the beamline 10B of KEK-PF.
A channel-cut Si(311) double crystal monochromator set wasused. The measurements were carried out in a transmission
mode at 290 K. N2 and a mixture of N2/Ar (85 %/15 %) wasused, respectively, in the ionization chambers measuring theI0 (incident) and It (transmitted) signal counts. The absorption-edge energy of 4 lm iron metal foil was calibrated to7111.2 eV [19].

Arsenic a Emission Spectra and K-edge XAFS Measurements
20 mm diameter disks were prepared for the arsenic adsorbedon the Fe-Monts, and the arsenic reference compounds© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim(Arsenic metal powder, AsIII2O3 and KH2AsVO4) setting thearsenic K-edge absorption jump value to 1.0. The arsenic
K-edge XAFS spectra were measured at an undulator beamline
10XU of SPring-8 (Japan) by utilizing a Rowland-type highenergy-resolution fluorescence spectrometer. A Si(111) double
crystal monochromator set was used. In order to suppress
higher harmonics, a rhodium-coated double crystal mirror set
was inserted at 3.0 mrad into the path of the incident X-ray
beam. The secondary spectrometer selectively counts the fluorescence X-rays emitted from the sample over a small energy
band (0.3–1.3 eV) and thus enables the measurement of the
XAFS spectra with a high-energy resolution [18].
The arsenic Ka1 emission peak was measured for eacharsenic adsorbed Fe-Mont sample. Then the fluorescence spectrometer was tuned to the peak top of the emission line, and
the XAFS spectra were measured by means of this high-energy-resolution detector. All the measurements were carried out
at 290 K. A mixture of 15 % Ar and N2 was flown into the I0
and It signal counting ionization chambers. The arsenic
absorption K-edge energy was calibrated to 11865.0 eV [19].
The XAFS data were analyzed by using the XDAP (XAFS
Services International) programme. For the iron EXAFS analysis, empirical parameters for Fe-O and Fe···Fe bond distanceswere extracted from the EXAFS spectrum of a-Fe2O3. The
bond distances and coordination numbers were based on the
crystal structure data for a-Fe2O3 [20, 21]. The values of r2 arepresented as relative (Dr2) to those of the corresponding
model parameters. The goodness of the fit was given as
required by the Committee for Standards and Criteria in X-ray
Absorption Spectroscopy.

3

Results and Discussion

3.1

Basal Spacing with Iron Loading ofMontmorillonite

Basal spacing along the (001) direction (d-spacing) for thesubstrate material Na-Mont was 12.2 Å as shown by the lowangle XRD pattern (see Fig. 2). After interaction of montmorillonite particles with iron species >94 % of sodium ions were
replaced. The sodium contents of the solutions of the FeMonts were determined by ICP-OES. The loss of sodium ionsfrom montmorillonite confirmed a successful intercalation of
the iron species.
The low-angle XRD patterns showed that the ordered structure of montmorillonite was retained after iron intercalations(see Fig. 2). For a 2.8 % Fe-IE-Mont, prepared by the ion exchange route, the d-spacing increased from 12.2 Å to 14.5 Å(see Tab. 1), which indicated the replacement of native ionswith comparatively larger and more hydrated iron species. In
this study, sodium and calcium ions of substrate material werecompletely exchanged with iron ions. One previous study
reported a decrease in the d-spacing when the introduced iron
ions were less than the exchangeable ions in the pristine material [22].
Maximum basal spacing of 15.3 Å was observed for the
14.0 % Fe-HIPI-Mont (see Tab. 1). In the HIPI route, a base

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55

The d-spacing for the 15.3 % Fe-IPI-Mont (15.3 Å) preparedby insertion of pre-hydrolyzed iron colloid was similar to thatof the 14.0 % Fe-HIPI-Mont. When the intercalated ironamount was 24.6 wt %, a decrease in basal spacing was shownby the XRD pattern (see Tab. 1 and Fig. 2). This behavior wasin accordance with other studies reporting the relation of metal loading and d-spacing of smectite clay [23]. When excessiveiron colloid was introduced into the montmorillonite suspension, the maximum amount of iron retained in the material
was 27 wt %, however, the ordered structure of iron-montmorillonite was severely damaged, as the (001) peak almost disappeared in the XRD pattern (data not shown). In the case of the
Fe-IPI-Monts with different iron loadings, the d-spacing wasincreasing with iron loading up to 15.3 wt %. Any further
increase in iron gave the same d-spacing (15.3 Å) or a shift
towards a smaller value, as shown in Fig. 2 for the 24.6 % FeIPI-Mont (15.0 Å). This behavior was similar to the Zr-exchanged pillared clay [23].
For the iron intercalated montmorillonites prepared by the
HIPI and IPI routes, the maximum iron loading was 14.0 and
27 wt %, respectively. The d-spacing values of the 14.0 % FeHIPI-Mont and the 15.3 % Fe-IPI-Mont were the same. Basedon the thickness of the montmorillonite sheet, i.e. 9.3 Å [24],the size of the intercalated iron species should be around 6 Åalong the (001) direction. For the 14.0 % Fe-HIPI-Mont, someof the introduced iron ions were occupying the point charges
on the silicon layer and other partially hydrolyzed species werebalancing the overall negative charge on the montmorillonite
sheets. Therefore, the intercalated iron species prepared via the
HIPI route should be relatively small. Furthermore, the dispersion of iron species intercalated in the montmorillonite, andthe Fe···Fe coordinative unsaturation should exceed that of theiron species intercalated by the IPI route.

Figure 2. Low-angle XRD pattern of the substrate material Namontmorillonite (a); 2.8 % Fe-IE-Mont (b); 14.0 % Fe-HIPI-Mont(c); 15.3 % Fe-IPI-Mont (d); and 24.6 % Fe-IPI-Mont (e).

hydrolysis of excessive iron ions was performed in the montmorillonite suspension. A successful intercalation was observed for the partially hydrolyzed (base to metal ratio of 2)iron species. On the other hand, for the base to metal ratio of
3, the iron species were mostly precipitated outside of montmorillonite. This argument was supported by the fact that theincrease in d-spacing was small (12.7 Å) when the base to metal ratios were 3. In the series of Fe-HIPI-Monts, the basal spacing was increasing with the iron loading. In comparison to the
Fe-Mont prepared by ion exchange, for the same iron(2.8 wt %) amount intercalated by the HIPI route, the d-spacing was 1.8 Å smaller. The reason for the larger basal spacing
obtained for the 2.8 % Fe-IE-Mont as compared to the 2.8 %Fe-HIPI-Mont was because of the high amount of water ofhydration required for iron but not for the partially hydrolyzed
iron species.

Table 1. Basic physicochemical data of Na-montmorillonite, iron-montmorillonites and bulk iron oxyhydroxides.
AdsorbentFe[wt %]

d-spacing[Å]SBET[m2/g]

Pore size[Å]Pore volume[mm3/g]Intercalated

Total

Na-montmorillonite

n.a.

1.5

12.2

26

39

33

2.8 % Fe-IE-Mont

2.8

4.3

14.5

43

41

44

14.0 % Fe-HIPI-Mont

14.0

15.3

15.3

100

41

67

15.3 % Fe-IPI-Mont

15.3

16.5

15.3

138

41

93

24.6 % Fe-IPI-Mont

24.6

25.7

15.0

166

41

92

a-FeO(OH)

n.a.

62.8

n.a.

16

n.a.

n.a.

a-FeO(OH) [6]

n.a.

62.8

n.a.

39

n.a.

n.a.

a-FeO(OH) [14]

n.a.

62.8

n.a.

54

n.a.

n.a.

n.a.: not applicable© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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3.2

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D. Masih et al.

BET Specific Surface Area and Porosity ofAdsorbent Materials

The BET specific surface area (SBET) of the Na-Mont (26 m2/g)
was in good agreement with the given value (25 m2/g). The
adsorption-desorption isotherms shown in Fig. 3 A were similar to Type-IIb of IUPAC classification [25]. With the intercalation of the ion exchanged iron (2.8 wt %) species, SBET
increased to 43 m2/g. Furthermore, for the 14.0 % Fe-HIPIMont an about four times rise in SBET (100 m2/g) was
observed (see Tab. 1). For the Fe-HIPI-Monts, a linear increasein SBET was observed with the increase of an intercalated ironamount. Similarly, the Fe-IPI-Monts also showed a linear
increase in SBET with the iron loading. A maximum SBET of
211 m2/g was found for the 27 wt % iron intercalated montmorillonite prepared via the IPI route.

iron species (see Tab. 1). Pores larger than 41 Å were not observed in any of the Fe-Monts. Basically, the pores in themontmorillonites were slit shaped openings as depicted by thetype of the adsorption-desorption isotherms and the characteristic hysteresis loops. The Na-Mont and the 2.8 % Fe-IEMont had essentially no micropores (< 20 Å), however, the FeHIPI and IPI-Monts showed a peak feature extending below
an Rp (pore radius) of 1 nm as demonstrated in Fig. 3 B. This
intrusion gave an indication for the presence of micropores,
and it was increasing with iron loading. For the 15.3 % Fe-IPIMont, SBET was lower than that of the 24.6 % Fe-IPI-Mont,but the pore volume was higher, i.e. 93 mm3/g (see Tab. 1).
The substantial increase of micropores with iron loading was
responsible for the maximum SBET value of the 27 % Fe-IPIMont.

3.3

Arsenic Adsorption Capacity and the Efficiency ofIron-Montmorillonites

The arsenic adsorption isotherms for iron-montmorillonites
and bulk iron oxyhydroxide are presented in Fig. 4. The con-Figure 3. (A) N2 adsorption (solid points)-desorption (hollow
points) isotherms for the Na-montmorillonite (a); 2.8 % Fe-IEMont (b); 14.0 % Fe-HIPI-Mont (c); 15.3 % Fe-IPI-Mont (d), and24.6%Fe-IPI-Mont (e).
(B) The corresponding DH-plots from the desorption branchshow the pore size distribution.

The pore size distribution and pore volume were calculatedby the DH-plot method. A homogeneous distribution withmeso-sized (20–500 Å) pores was observed for the adsorbent
materials (see Fig. 3 B). Na-Mont had an average pore diameter of 39 Å, which increased to 41 Å after the intercalation of© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 4. Adsorbent weight basis (A) arsenite and (B) arsenate
adsorption isotherms for the 2.8 % Fe-IE-Mont (filled diamonds), 14.0 % Fe-HIPI-Mont (filled squares), 15.3 % Fe-IPIMont (filled inverted-triangles), 24.6 % Fe-IPI-Mont (filled triangles), and bulk iron oxyhydroxide (goethites) with an SBET of
16 m2/g (filled circles).

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capacities of the 14.0 % Fe-HIPI-Mont were maximal among
all the adsorbents employed in this study (see Tab. 2). The avalues of arsenite and arsenate adsorption were found to be 4
and 28 times of that of the a-FeO(OH) applied under the same
experimental conditions. A large value for the arsenite adsorption equilibrium constant showed its superiority for low concentration solutions (< 310 lg/L). In comparison to the FeIPI-Mont with a comparable iron content (15.3 wt %) andbasal spacing, the adsorption capacities were 1.8 and 10.2
times higher for arsenite and arsenate, respectively (see Tab. 2
and Fig. 4).
The Fe-IPI-Monts were prepared with iron loadings in therange of 2.0–27 wt %. The Fe-Monts with 15.3 and 24.6 wt %iron contents were selected because of the optimum d-spacing
and comparably maximum iron loadings. A saturated arsenic
adsorption amount was reached at a maximum for iron/arsenite and arsenate adsorptions of 24.6 wt % and 15.3 wt %,respectively (see Tab. 2 and Fig. 4). Therefore, the arsenic adsorption was not increasing with the increase of the ironamount loaded in the montmorillonite. On the other side, it
was a function of the number of the active iron sites present in
the adsorbent material. Among all the Fe-Monts, the K valuesof the Fe-IPI-Monts were high for the arsenate adsorption, but
the saturated arsenic adsorption capacities were significantly
lower than those of the 14.0 % Fe-HIPI-Mont (see Tab. 2).
Iron-montmorillonites were superior to bulk iron oxyhydroxides, even though the Fe-Monts have had low iron contents. For the Fe-Monts, the As/Fe molar ratios for both thearsenite and arsenate cases were higher than that of the bulk
iron oxyhydroxides (see Tab. 2). These high As/Fe values
showed a high activity of the partially hydrolyzed iron intercalated in montmorillonites. Among the Fe-Monts, the highest
As/Fe molar ratios were obtained for the adsorbents preparedby ion exchange, and the base hydrolysis of excessive iron ionsin the montmorillonite mixture (see Tab. 2). From the overallarsenic adsorption results, the 14.0 % Fe-HIPI-Mont was
found the best in performance. The isotherms plotted on aniron weight basis also described the high arsenic adsorption

centration range of the arsenic test solutions was chosen as aplausible limit under environmental conditions. The adsorption data fit well with the Langmuir-type equation. Under theexperimental conditions, the adsorption of arsenic on the untreated Na-Mont was negligible (see Tab. 2). However, under
different conditions, some adsorption of arsenate was observed
on montmorillonite [26]. In this study, after the replacementof inactive native sodium ions of montmorillonite with iron,the modified materials became highly active for arsenic
removal.
The arsenite saturated adsorption capacity (a) of the 2.8 %Fe-IE-Mont was comparable with that of the bulk iron oxyhydroxide (a-FeO(OH); goethite), and for arsenate, it was higher(see Tab. 2) even though the iron content of a-FeO(OH) was>14 times of that of the 2.8 % Fe-IE-Mont. In comparison to
a-FeO(OH), the adsorption equilibrium constant (K) valuesof the 2.8 % Fe-IE-Mont were similar for arsenite adsorption,
but low in the case of arsenate (see Tab. 2). The arsenic adsorption isotherms plotted on an iron weight basis showed that the
2.8 % Fe-IE-Mont was not only superior to that of the
a-FeO(OH) used in this study, but also to the high SBET
a-FeO(OH) (39 and 54 m2/g) used in other studies [6, 14] (seeTab. 2). Furthermore, the arsenite K value of the 2.8 % Fe-IEMont was higher as compared to the Fe-IPI-Monts. The overall adsorption results showed the presence of a large numberof active iron sites in the 2.8 % Fe-IE-Mont, however, theintercalated iron amount could not be increased by more than
2.8 wt % (maximum exchangeable limit) through the ion
exchange route.
In addition, the base hydrolysis of the inserted positive ionsin the montmorillonite suspension was carried out to stabilize
the higher (> 2.8 wt %) iron amount and to increase the number of active sites for arsenic removal. This post-mixing basehydrolysis of excessive iron ions in the montmorillonite
allowed for a retention of a 5 times higher iron amount asobtained in the 14.0 % Fe-HIPI-Mont. From the series of the
Fe-HIPI-Monts, the adsorbent with a maximum iron loading(14.0 wt %), d-spacing and SBET was chosen for the arsenic
adsorption studies. Both the arsenite and arsenate adsorption

Table 2. Saturated arsenic adsorption capacity (a), adsorption equilibrium constant (K) and the As/Fe molar ratio
for the adsorbents used in this study, and comparison with iron oxyhydroxides used in other studies.
Adsorbent

Na-montmorillonite
2.8 % Fe-IE-Mont

Arsenite

Arsenatea[mgAs/gads]

K[mL/gAs]

As/Fe[molar ratio]a[mgAs/gads]

K[mL/gAs]

As/Fe[molar ratio]

0

0

0

0

0

0

1.4

6

1.4 ×10

6

0.037

4.6

5

4.3 × 100.122

514.0 % Fe-HIPI-Mont

8.0

1.4 × 10

0.043

75.2

4.2 × 100.401

15.3 % Fe-IPI-Mont

4.4

5.1 × 105

0.021

7.4

1.4 × 106

0.036

5.3

5

5.0 × 106.1

6

2.4 × 10

0.018

1.9

6

2.7

6

3.2 × 100.003

55.4 × 10

0.015

5

0.002

24.6 % Fe-IPI-Mont
a-FeO(OH)a-FeO(OH) [6]a-FeO(OH) [14]1.4 × 105

13.0

5.4 × 1016.6

5

5.5 × 10© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

0.016
0.002
0.015
0.019

13.0
2.0

1.5 × 10

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capacities of the Fe-Monts compared to the bulk iron oxyhydroxides (data not shown).

3.4

Local Structure of Intercalated Iron and Adsorbed
Arsenic Species

The iron species in the Fe-Monts were lacking any long-rangeorder as no characteristic peak of iron oxides appeared in the
wide-angle XRD patterns (data not shown). Thus, the local
structure of the iron species was studied by XAFS spectroscopy. Fig. 5 presents the iron K-edge k3 weighted extended X-ray
absorption fine structure (EXAFS) function, its associatedFourier transform, and the curve fitting analysis for the 14.0 %Fe-HIPI-Mont. The strongest peak at 1.6 Å in the Fouriertransform corresponded to Fe-O. For the next-nearest neighbor, the peak corresponding to Fe···Fe shell appeared at 2.8 Å.
The curve fitting results gave a Fe-O bond distance (R) of
2.046 Å with a coordination number (N) of 6, indicating anoctahedral coordination of the intercalated iron species. The
results of the curve fitting analysis are given in Tab. 3 and theobtained structural parameters typical of edge-shared octahedral are described in the model structure shown in Fig. 8. The
curve fitting results of Fe···Fe shell showed a coordinative unsaturation (N 2.5) of the edge-sharing (R 3.27 Å) iron species ascompared to the bulk iron oxyhydroxides (N 6). This unsaturation of the Fe···Fe coordination number in the intercalated
iron species was the major reason of their high activity forarsenic removal.
Chemical shifts in the arsenic Ka1 emission spectra wereobserved for the reference compounds of AsIII2O3 andKH2AsVO4. In comparison to the arsenic metal Ka1 peak, thechemical shifts of the AsIII2O3 and KH2AsVO4 Ka1 peaks were–1.4 and +0.6, respectively. The full width at half maximum(fwhm) for the most symmetrical Ka1 peaks (AsIII adsorbed on
Fe-montmorillonite) was 3.36 eV (see Fig. 6), whereas thenatural core-hole lifetime width of Ka1 was 3.08 eV. Thus, theenergy-resolution of this homemade fluorescence spectrometer
was estimated to be 1.3 eV, including the contribution of thebeamline.
The emission spectra were compared with the referencecompounds in order to find out the oxidation state of the arsenic adsorbed on the 14.0 % Fe-HPI-Mont. The Ka1 emission
peak of arsenic adsorbed from the 0.2 to 16 mg/L test solutionsof arsenite always appeared at +0.6 eV with reference to arsenic
metal. This chemical shift was the same as seen for the arsenic
Ka1 peak of KH2AsVO4. The emission spectra are displayed inFig. 6. The peak position of the Ka1 emission spectrum for thearsenic adsorbed from 16 mg/L of arsenite was observed at
10554.3 eV, similar to that of the arsenate reference compound(10554.9 eV). The starting test solution was of arsenite, but thearsenic Ka1 emission peak showed that the finally adsorbed
state was arsenate. This chemical shift indicated an environmentally important phenomenon: the oxidation of the arsenite
species to arsenate.
In addition, the comparison of the arsenic K-edge XANES
spectra showed that the peak energy positions for thearsenite adsorbed on the intercalated iron species from the© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 5. (A) Iron K-edge k3 weighted EXAFS function, (B) its associated Fourier transform and (C) the curve fitting analysis forthe 14.0 % Fe-HIPI-Mont. The XAFS spectrum was measured at
290 K. The observed data are shown as a solid line and the fit asa dotted line.

0.2–16 mg/L test solutions were (11871.7–11871.9 eV) close to
the value of arsenate (11872.1 eV) rather than the arsenite(11868.6 eV) reference compound (see Fig. 7). No oxidation
was reported for the arsenite adsorption from the 6–15 mg/L
test solutions on the bulk iron oxyhydroxides [5].
Furthermore, the arsenic K-edge EXAFS curve fitting results
for the arsenite adsorbed on iron also confirmed its oxidation(data not shown). For the arsenic adsorbed from the arsenite
test solution, the As-O bond distance of 1.68 Å (N 4.2) was

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Eng. Life Sci. 2007, 7, No. 1, 52–60

Fe-Montmorillonites for the Removal of Arsenic

59

Table 3. Iron K-edge EXAFS curve fitting results for the 14.0 % Fe-HIPI-Mont and bulk iron oxyhydroxide.
Sample name

Shell

N

R
[Å]

DE
[eV]

Dr2
[Å2]

Goodnessof the fit

Fe-montmorillonite

Fe-O

6.0 (±1.0)

2.046

–0.7

0.0003

256

Fe...Fe

2.5 (±0.6)

3.270

–3.1

0.0003

Fe-O

6.0

2.021

n.a.

n.a.

Fe...Fe

2.0

3.015

n.a.

n.a.

Fe...Fe

6.0

3.390

n.a.

n.a.

a-FeO(OH)*n.a.

N: Coordination number, R: Bond distance, DE: Change in the inner potential, Dr2: Debye-Waller factor.
* The results are based on the crystallographic data [21].
n.a.: not applicable

close to that of arsenate (1.69 Å) rather than arsenite (1.78–
1.79 Å) [5, 27]. The As···Fe N value of 2.1 (R 3.31 Å) indicateda bidentate binuclear complex formation of oxidized arsenate
species with the adjacent apexes of edge-sharing octahedral
iron species. The model structure in Fig. 8 describes the modeof the adsorbed arsenic species. XAFS studies also showed thatthe arsenate species adsorbed on the iron-montmorillonite
remained unchanged.

Figure 6. Arsenic Ka1 emission spectra for (open squares)
As2O3, (open circles) KH2AsO4 and arsenic (filled circles) adsorbed on the 14.0 % Fe-HIPI-Mont from the arsenite test solution of 16 mg/L.

Figure 8. Model structure deduced from the X-ray absorption
fine structure studies for the edge-sharing octahedral iron species intercalated in montmorillonite and arsenic adsorbed on
them.

4

Figure 7. The normalized arsenic Ka1-selecting arsenic K-edge
XANES spectra for (a) arsenic metal; (b) As2O3, (c) KH2AsO4
and arsenic adsorbed on the 14.0 % Fe-HIPI-Mont from (d) an
arsenate test solution of 16 mg/L, (e) an arsenite test solutionof 16 mg/L, and (f) an arsenite test solution of 0.2 mg/L.

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Conclusions

Iron-montmorillonites were highly active in arsenic removal.
Even though the iron contents of Fe-Monts were low as compared to the bulk iron oxyhydroxides, they were more efficientin arsenic removal. The most active iron species for arsenic adsorption was prepared by ion exchange, and the base hydrolysis of excessive iron ions in the montmorillonite suspension. A
high unsaturation of the Fe···Fe coordination number in theintercalated iron species was an important factor for their highhttp://www.els-journal.com

60

D. Masih et al.

activity in arsenic adsorption. Along with an efficient arsenic
adsorption, the intercalated iron species were effective in theoxidation of arsenite into arsenate.

Acknowledgements
The authors are highly grateful to the Ministry of Education,
Culture, Sports, Science and Technology of the government ofJapan for the financial support. The XAFS experiments forarsenic (SPring-8) and iron (KEK-PF) were performed underapprovals.

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