PROCEEDINGS OF ENVIRONMENTAL SCIENCE AND TECHNOLOGY 2005(II)

ARSENIC SPECIATION AND IDENTIFICATION ON ACTIVE IRONADSORBENT SITES BY XAFS TECHNOLOGY
Dilshad Masih, Ken-ichi Aika and Yasuo Izumi(Tokyo Institute of Technology, Midori-ku, Yokohama 226-8502, Japan)
Yoshimi Seida(Institute of Research and Innovation, Kashiwa 227-0861, Japan)
ABSTRACT: Nano-sized Fe species were formed as layered FeO(OH) and as
Fe-montmorillonites, which were identified by X-ray absorption fine structure(XAFS) spectroscopy, XRD and BET techniques, and applied for As removalfrom test solutions of 0.20 – 16 mgAs/L. Adsorption efficiency was better forthese nano-materials compared to traditional bulk iron oxyhydroxides. The
performance of layered FeO(OH) was best when it was washed with ethanol prior
to its use for arsenic sorption. Layered FeO(OH) and Fe-montmorillonites
exhibited nearly equivalent performance for arsenic uptake on an adsorbent
weight basis. On an iron weight basis, the latter was 4 times better than theformer. To detect very low concentrations of As (0.13 – 0.48 wt%) in the highconcentrations of heavy elements (i.e. Fe), As K-edge XAFS was combined withhigh-energy-resolution fluorescence spectrometry. The inflection point energy ofadsorbed arsenite shifted close to the arsenate reference compound, indicatingoxidation of AsIII upon adsorption from test solutions of 0.20 – 16 mgAs/L.
INTRODUCTIONAs Contamination and Novel Materials for its Removal. High risks to humanhealth due to the consumption of As in drinking water have prompted reduction of
the maximum contaminant level (MCL) of As from 50 g/L to 10 g/L (Smith etal., 2002). Depending upon solution pH, As exists in water as AsIII and AsVoxyanions (Munoz et al., 2002). AsIII is more toxic and more mobile.
Novel materials and methods are required to cope with modern regulations,especially for the binding and control of AsIII species. As affinity for Fe-basedmaterials is well known (Raven et al., 1998, Farquhar et al., 2002); however, itsremoval from dilute solutions of less than 1 mg/L (especially AsIII) is difficult.
Adsorbent materials where the active adsorption sites are dispersed on a nanolevel are promising for As removal. Nano-sized FeIII species were prepared fromsurfactant-assisted, layered FeO(OH) and also intercalated between
montmorillonite layers. As prepared, layered FeO(OH) was hydrophobic. A soft
removal method (washing in ethanol at 333 K) was used to eliminate surfactant
while maintaining structure and avoiding aggregation of particles.
Namontmorillonite (Na-Mont) was utilized to stabilize the FeIII nano particles. For
both the adsorbents, no characteristic peaks of crystalline Fe oxides appeared intheir XRD patterns.
XAFS Combined with Fluorescence Spectrometry. XAFS spectroscopy is astate-of-the-art technique for short-range order bonding studies.
For
environmental applications, the local structure studies of adsorbent-adsorbate
ISBN 0-9768853-5-2© 2005 American Science Press

PROCEEDINGS OF ENVIRONMENTAL SCIENCE AND TECHNOLOGY 2005(II)systems will help to optimize adsorbent materials. Molecular level studies ofenvironmental contaminants using XAFS are attracting much attention as a meansof understanding their fate, but it still has limitations for dilute contaminants (~0.1wt%), especially in the presence of other heavy elements. XAFS in combination
with a fluorescence spectrometer was able to detect the speciation of trace-levelsof Pb adsorbed under environmental conditions by utilizing the advantage ofextremely high signal/background ratio for XAFS data acquisition (Izumi et al.,2002). The fluorescence spectrometer has a high energy-resolution (0.3 eV at 6keV) and has allowed monitoring As K-edge XANES for low concentrations(0.13 – 0.48 wt%) of adsorbed As.
MATERIALS AND METHODS
Layered FeO(OH) and Fe-Montmorillonite Synthesis. Layered FeO(OH) wassynthesized from 0.1 M FeCl2 (100 mL) and 0.07 M sodium dodecyl sulfate (20mL). First, a colloidal suspension was formed to which 3 wt% H2O2 (20 mL) wasadded quickly with vigorous stirring. The reaction mixture was stirred for 30 min,and the solid and liquid phases were separated by filtration (Wirnsberger et al.,2001). The surfactant was extracted by re-suspension of the precipitates inethanol, swirling the mixture at 333 K for 2.5 h, and following with filtration.
The extraction process was repeated five times to completely eliminate thesurfactant from the solid. The material obtained was air-dried under ambientconditions, and then calcined at 423 K for 2 h in air.
Fe-montmorillonite (Fe-Mont) was prepared by two step interaction; (1)interaction of 3 mmol Fe3+ with Na+ and Ca2+ of montmorillonite (1 g; 0.5 wt%
aqueous suspension), (2) subsequent reaction with NaOH of double the molar
amount (6 mmol) of Fe3+ introduced. The reaction mixture was aged at 290 K for
12 h, filtered and washed several times with Millipore water until the pH of thefiltrate was more than 6.8. The solid obtained was dried on a glass plate underambient conditions and ground to powder. The maximum amount of Feintercalated was 14.0 wt% of the dried mass.
Characterization. XRD patterns of powdered samples were measured byMultiflex-S diffractometer (Rigaku). N2 adsorption-desorption isotherms weremeasured at 77 K using BELSORP mini (Bell, Japan).
Fe and As K-edge XAFS Studies. Reference compounds, -Fe2O3 and FeO(OH), and the samples were mixed with boron nitride (BN) and pressed intodisks (diameter 20 mm). Fe K-edge XAFS spectra were measured at 290 K intransmission mode at beamline 10B (KEK-PF). An Si(311) monochromator wasused. N2 and a mixture of N2/Ar (85/15) were used, respectively, in ionizationchambers measuring I0 and I signal counts. The rising K-edge energy of Fe metal
foil was calibrated to 7111.2 eV.
As metal (As0), As2O3 (AsIII) and KH2AsO4 (AsV) were mixed with BN tocontain 2 wt% of As and pressed into disks (diameter 20 mm). As adsorbed, airdried samples were also pressed into the same sized disks. As K-edge XANES
spectra were measured at 290 K at Undulator beamline 10XU (SPring-8). AnSi(111) monochromator was used. Fluorescence X-rays emitted from the sample

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PROCEEDINGS OF ENVIRONMENTAL SCIENCE AND TECHNOLOGY 2005(II)were analyzed by a homemade Rowland-type spectrometer. A cylindrically-bent
Johansson-type Ge(555) crystal and a scintillation counter were positioned on theRowland circle (R = 220 mm) and controlled by computer. The sample was set
on a flat plane, nearly horizontal, tilted 6o towards the incident X-rays and 7otowards the center of Ge(555) crystal (Izumi et al., 2002). With the excitationenergy of 11885.0 eV, As K 1 emission spectrum was measured. Then, thespectrometer was tuned to the center of the As K 1 peak, and XANES spectra
were measured. The rising K-edge energy of As0 was calibrated at 11865.0 eV.
As Adsorption Test. 1.0 g/L AsIII and AsV stock solutions were prepared from
As2O3 and KH2AsO4, respectively. 1, 2, 4, 8 and 16 mg/L dilutions were used forthe adsorption tests. The pH of starting solutions was in the range of 5.7 – 6.0(AsIII) and 5.7 – 6.4 (AsV). Layered FeO(OH) washed with ethanol, Fe-Mont, FeO(OH) (SBET = 16 m2/g), and Na-Mont were used for adsorption. For As batchadsorption tests, 50 mg of adsorbent was immersed in 20 mL of As solution. The
mixture was shaken for 12 h at 298 K, followed by separation using 0.4 m
cellulose membranes. The aqueous phase As concentrations were measured byICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy; Vista-Pro,Seiko-Varian Instruments).
RESULTS AND DISCUSSIONHigh Surface Area Layered Materials. For the as-prepared, layered FeO(OH),the characteristic XRD peak appeared at 2.6 degree, demonstrating a regularity of34 Å (Figure 1-A-a). After template extraction with ethanol, the peak shifted
toward 3.1 degree, indicating a d(001)-spacing of 28 Å (Figure 1-A-b). High
specific surface area (166 m2/g) and pore volume (154 mm3/g) was achieved forthe ethanol washed sample compared to as-prepared, layered FeO(OH) (Table 1).

TABLE 1. XRD and BET Results of Layered FeO(OH) and Montmorillonites
Layered FeO(OH)Na-Mont Fe-Montas-prepared
Ethanol washedd(001)-spacing (Å)
3428
12.2
15.7
SBET (m2/g)
2.8
166
25118
3Pore Vol. (mm /g)
8.3
154
3367
For Fe-Mont, the layered structure was retained after Fe intercalation asshown by XRD patterns (Figure 1-B-b), and with the intercalation of Fe species
the d(001)-spacing increased from 12.2 Å to 15.7 Å (Table 1). Specific surface
area (25 m2/g) and pore volume (33 mm3/g) also increased to 118 m2/g and 67
mm3/g, respectively, after intercalation of Fe species (Table 1).

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PROCEEDINGS OF ENVIRONMENTAL SCIENCE AND TECHNOLOGY 2005(II)(A)(B)

FIGURE 1. XRD patterns of layered FeO(OH) (A); as prepared (a), ethanolwashed (b), and montmorillonites (B); Na-Mont (a), and Fe-Mont (b).
Nano-sized Fe Species. Fe species in both the materials (layered FeO(OH) andFe-Mont) were lacking any long-range order, as no iron oxide characteristic peaks
appeared in XRD patterns. The local structure of Fe was studied by extended Xray absorption fine structure (EXAFS). In the Fourier transforms of as preparedlayered FeO(OH) and Fe-Mont the magnitudes of Fe-Fe peaks were20%compared to goethite (bulk -iron oxyhydroxide) indicating nano-size of Fespecies.
As Speciation. As low as 0.13 wt% As adsorbed from 200 g/L AsIII solution
under flow setup on the adsorbent containing high amount of Fe 15.3 wt% (total),
Si 21 wt% and Al 8 wt% was measured successfully by XAFS combined withfluorescence spectrometry. For the AsV adsorbed sample the inflection point
energy was similar to AsV reference showing no change in oxidation state. But,the inflection point energies of AsIII adsorbed samples (11871.9 eV; 0.13 wt% Asadsorbed from 200 g/L AsIII solution, and 11872.0 eV; 0.48 wt% As adsorbed
from 16 mg/L AsIII solution) were close to AsV reference (11872.1 eV) rather thanthe AsIII reference (11868.5 eV), revealing the oxidation of adsorbed As species.
This oxidation phenomenon is environmentally important regarding the binding
and control of the more toxic AsIII species.
As Removal. As adsorption isotherms followed by the adsorbents were of theLangmuir type (Figure 2-A). Saturated As adsorption (a) and equilibriumconstant (K) values obtained by fitting the adsorption isotherms to first order
Langmuir equation are reported in Table 2. As adsorption from AsV solution washigher as compared to adsorption from AsIII solution (Figure 2-A, Table 2).
Adsorption of AsIII, V by Na-Mont was negligible (isotherms not shown). So, in438

PROCEEDINGS OF ENVIRONMENTAL SCIENCE AND TECHNOLOGY 2005(II)the case of Fe-Mont, the intercalated nano-sized Fe species were totally
responsible for As adsorption.
(A)(B)FIGURE 2. As adsorption isotherms; adsorbent weight basis (A), Fe weight basis(B) for ethanol washed layered FeO(OH); AsIII (&), AsV ( ), Fe-Mont; AsIII ('),
AsV ( ), and goethite; AsIII ( ), and AsV ( ).
TABLE 2. Saturated As Adsorption (a) and Equilibrium Constant (K) Values
AsIII
AsV
K(mL/gAsa(mgAs/gadsorbent) K(mL/gAs) a(mgAs/gadsorbent))
Ethanol washed5
9.5
6.2 x 1012.8
1.3 x 108
layered FeO(OH)
Fe-Mont
8.0
1.4 x 10675.0
4.3 x 105
6Goethite
1.9
1.4 x 102.7
3.2 x 106AsIII, V adsorption efficiencies of nano-size iron species in ethanol-washed,layered FeO(OH) and Fe-Mont were comparable on adsorbent weight basis, andwere much better than bulk iron oxyhydroxide (Figure 2-A). Next, for the FeMont (Fe 14.0 wt%) the amount of As adsorbed per g Fe was 4 times higher
than the ethanol-washed, layered FeO(OH) (Fe 62.9 wt%), and 10 times higher
than the bulk iron oxyhydroxide (Fe 62.9 wt%) (Figure 2-B). Highly dispersed
nano-sized Fe species in the hybrid material had more active adsorption sites.
Especially for the removal of more toxic and highly mobile AsIII species, the FeMont even with low Fe contents (14.0 wt%), but with more active adsorption sites
was the best adsorbent material (Figure 2, Table 2).
CONCLUSIONS
Adsorption active nano-sized Fe species were identified by XAFS and theenvironmentally important phenomenon of AsIII oxidation was monitored anddemonstrated successfully by the XAFS combined fluorescence spectrometry.

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PROCEEDINGS OF ENVIRONMENTAL SCIENCE AND TECHNOLOGY 2005(II)ACKNOWLEDGMENTSThe XAFS experiments for As and Fe were performed under the approvalof SPring-8 and KEK-PF. The author is highly grateful to the Ministry ofEducation, Culture, Sports, Science and Technology government of Japan for thefinancial support.
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