J. Am. Ceram. Soc., 94 [2] 584–591 (2011)
DOI: 10.1111/j.1551-2916.2010.04098.x
r 2010 The American Ceramic Society
Journal
Enhanced Arsenite Adsorption onto Litchi-Like Al-Doped Iron Oxides
Ronghui Li,z Qi Li,z Shian Gao,z and Jian Ku Shangw,z,y
z
Materials Center for Water Purification, Institute of Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
y
Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
have strong affinities toward inorganic arsenic species and high
selectivity to arsenic in the adsorption process. In addition, they
are easily available and inexpensive to produce. However, their
adsorption kinetics is usually slow and their adsorption capacity
is usually low, especially for arsenite.
To take the advantages of iron (hydr)oxides on arsenic
adsorption, various modifications had been developed to enhance their arsenic adsorption performance. For example, Liang
et al.26 reported that iron–aluminum hydroxide complexes (with
a Fe:Al molar ratio of 7:3) exhibited a significantly higher arsenate adsorption capacity than either the iron hydroxide or
aluminum hydroxide prepared by the same procedure. Recently,
a novel Fe–Mn binary oxide adsorbent (with a Fe:Mn molar
ratio of 3:1) had been developed by Zhang et al.,27 which was
more effective for As(V) removal than pure amorphous
FeOOH. The higher adsorption capacity of the Fe–Mn binary
oxide/hydroxide was attributed to its higher specific surface area
(SSA) and pore volume.
It should be noted that high molar ratios of the second metal
element to iron were adopted in most of these modification
studies. For example, 42.9% Al/Fe ratio was used in the report
by Liang et al.,26 33.3% Mn/Fe was used in the report by Zhang
et al.,27 and 26.7% Ce/Fe ratio was used in the report by Zhang
et al.28 At these high molar ratios, a second metal oxide/
hydroxide is generally formed so that the adsorption characteristics represent those of mixed oxides rather than those of a
modified iron oxide. As a result, it is not possible to determine
potential changes in the adsorption characteristics resulting
from chemical and physical modifications of iron oxide in the
mixture. It is conceivable that the second metal element might
get into the structure of the iron oxide/hydroxides to form the
doped iron oxides/hydroxides during the synthesis of these ironbased binary oxide/hydroxides and the presence of the second
oxide could alter the structural morphology of the iron oxide,
both of which may influence the arsenic performance of these
iron-based adsorbents.22–25 However, till now no work has been
reported that elucidates how these potential chemical and physical modifications of iron oxides may directly influence the
arsenite adsorption onto iron oxide.
Current arsenic removal techniques could remove As(V) from
water effectively. However, the removal of As(III) remains a
challenge because As(III) exists mainly as nonionic H3AsO3 in
natural water with near neutral pH environment, which does not
have high affinity to the surface of various adsorbents compared
with As(V). Thus, a preoxidation of As(III) to As(V) and/or
adjusting the pH value of water before the adsorption process is
necessary for its effective removal. Hence, it is desirable to
develop an effective adsorbent on As(III) without the oxidation/pH adjustment, which could largely simplify the treatment
process and lower the treatment cost.
In this study, iron–aluminum binary hydroxide/oxides were
synthesized by a hydrothermal process, and their As(III)
removal performance was examined without the oxidation/pH
adjustment. To study the effects of Al doping on the crystal
structure, surface properties, and As(III) removal performance
Iron–aluminum complex hydroxide/oxides were synthesized
from a hydrothermal process, and litchi-like Al-doped iron oxide nanoparticles were then obtained by removing extra boehmite nanoplates with NaOH washing. While the hydrothermal
synthesis induced doping of iron oxide by Al, with the Al dopant
concentration increasing with the Al–Fe molar ratio, removal of
boehmite greatly increased the specific surface area (SSA) of
these Al-doped iron oxide nanoparticles. The resulting litchi-like
Al-doped iron oxide nanoparticles were found to strongly adsorb
As(III) from aqueous environment at neutral pH without preoxidation., and the As(III) (arsenite) adsorption capacity of Aldoped iron oxides increased with the increase of the initial ratio
of Al/Fe in the hydrothermal process, reaching as high as 2.5
times the adsorption capacity of pure iron oxide. The enhanced
arsenite adsorption was shown to result from the increases in the
amount of the surface hydroxyl group as well as in the SSA of
Al-doped iron oxide nanoparticles.
I. Introduction
A
RSENIC is a highly toxic element, and its existence in natural
water bodies is a worldwide problem. Numerous documentations regarding the health threat of arsenic contamination in
groundwater and drinking water are available.1–8 Arsenic is
usually found in the form of inorganic oxyanions in natural
water bodies. Arsenate (i.e., HAsO2
4 ) is the primary anion in
aerobic surface water, and arsenite (i.e., H3AsO3 or H2AsO
3 ) is
the primary species in groundwater. Arsenite is considerably
more mobile and toxic than arsenate.6,9 Long-term drinking
water exposure to arsenic causes skin, lung, bladder, and kidney
cancers as well as pigmentation changes, skin thickening
(hyperkeratosis), neurological disorders, muscular weakness,
loss of appetite, and nausea.9,10 To reduce the risk of arseniasis and the subsequent serious problems to human health, the
World Health Organization suggested in 1993 that the maximum contaminant level for arsenic in drinking water should not
be over 0.01 mg/L.11
Various arsenic removal techniques from drinking water had
been developed, including precipitation/coprecipitation, coagulation, sorption/adsorption, ion-exchange, and reverse osmosis.10,12–14 Among them, the adsorption process is considered as
a simple and cost-effective process. After being transferred to
the solid surface from the aqueous phase, the toxic species could
be bound to the adsorbent either by physical or chemical forces.
Many studies15–25 had demonstrated that iron (hydr)oxides are
promising adsorbent materials for arsenic removal because they
R. Moreno—contributing editor
Manuscript No. 27885. Received April 20, 2010; approved July 28, 2010.
This study was supported by the National Basic Research Program of China, Grant No.
2006CB601201, and the Knowledge Innovation Program of Institute of Metal Research,
Grant No. Y0N5A111A1.
w
Author to whom correspondence should be addressed. e-mail: [email protected]
584
February 2011
Enhanced Arsenite Adsorption onto Al-Doped Iron Oxides
of iron oxides, aluminum hydroxide phase in this binary complex was removed by NaOH washing to eliminate any possible
contributions from it. Clear evidence of Al doping was obtained
by X-ray photoelectron spectroscopy (XPS) and diffraction analyses, and its effect on the surface properties of ion oxides was
examined. Our work demonstrated that the Al doping increased
the amount of the hydroxyl groups on the iron oxide surface,
and the removal of boehmite greatly increased the SSA. Both of
these two effects are beneficial to the enhancement of the As(III)
removal performance.
II. Experimental Procedure
(1) Material System
All the chemicals used in the hydrothermal synthesis were of
analytical reagent grade and were purchased from Sinopharm
Chemical Reagent Co. (Shenyang, China). Sodium metaarsenite
(NaAsO2, Shanghai Tian Ji Chemical Institute, Shanghai,
China) was used to prepare As(III) stock solution, and concentrated hydrochloride acid (HCl, 32%–38%, Tianda Chemical
Reagents Factory, Tianjin, China) was used to stabilize the arsenic species after treatment. As(III) solutions used in the batch
experiments were obtained by diluting the As(III) stock solution
to the desired concentration with deionized (DI) water.
(2) Synthesis of Al-Doped Iron Oxides
The Al-doped iron oxides were synthesized according to the
following procedure: 0.02M Fe(NO3)3 9H2O and a certain
amount of Al(NO3)3 9H2O were dissolved in DI water to
form the precursor mixture solution. The molar ratios of
Al31/Fe31 in the mixture solution were 0% (pure Fe31), 10%,
20%, 30%, 40%, and 50%, respectively. Under vigorous magnetic-stirring, 0.04M urea was added into the mixture solution
acting as a homogeneous coprecipitation agent. The formed
transparent solution was continuously stirred for half an hour at
room temperature. The solution was then transferred into a
PTFE-lined stainless steel autoclave and heated at 1801C for 5 h.
After naturally cooling to room temperature, solid precipitates
were obtained, and they were washed several times by DI water
2
to remove NH1
4 , NO3 , and CO3 . The precipitate was then put
into 5M NaOH solution, and kept in a thermostatic bath at
601C for 5 h under continuous stirring to remove aluminum
hydroxide from the precipitate. Finally, Al-doped iron oxides
were collected by the centrifugation, washed with DI water and
absolute ethanol for several times, and finally dried in air at
801C for overnight. The samples obtained were named as 0%
Al/Fe2O3, 10% Al/Fe2O3, 20% Al/Fe2O3, 30% Al/Fe2O3, 40%
Al/Fe2O3, and 50% Al/Fe2O3, respectively, according to their
initial Al/Fe ratios.
(3) Characterization of Al-Doped Iron Oxides
The semiquantitative chemical composition and surface chemical states of the samples were examined by XPS measurements
using an ESCALAB250 X-ray photoelectron spectrometer
(Thermo Fisher Scientific Inc., Waltham, MA.) with an Al K
anode (1486.6 eV photon energy, 0.05 eV photon energy resolution, 300 W). The crystal structure of the samples was analyzed using a D/MAX-2004 X-ray powder diffractometer
(Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu
(0.15418 nm) radiation at 56 kV and 182 mA. The FESEM
image was obtained with a Supra35 field-emission scanning
electron microscope (Zeiss, Oberkoch, Germany). SEM sample
was made by attaching the samples to a copper billet with a
conductive carbon tape. Before imaging, the sample was sputtered with gold for 80 s (Emitech K575 Sputter Coater, Emitech
Ltd., Ashford Kent, U.K.). BET surface area was measured by
N2 adsorption–desorption isotherm with Autosorb-1 series surface area and pore size analyzers (Quantachrome Instruments,
Boynton Beach, FL).
585
(4) Batch Experiments For As(III) Sorption
The kinetic study of As(III) adsorption were conducted at
different time intervals with initial As(III) concentrations of
2.222 mg/L, which is in the high range of arsenic concentrations
found in natural water around the world.29 The pH value of
2.222 mg/L As(III) solution is B7.5, near the neutral state. A
series of Al-doped Fe2O3 nanoparticle samples with the same
material loadings (1 g/L) were added into the As(III) solutions
respectively and the suspensions were stirred magnetically to
disperse the samples to ensure good contact with the As(III)
contaminations. After appropriate time intervals, the sample
was recovered by centrifugation at 25 000 rpm for 10 min. One
drop of concentrated HCl was added into the clear solution
to preserve its arsenic species. Adsorption isotherm experiments
were carried out by adding 0.05 g of adsorbent into 100 mL of
As(III) solution with various initial concentrations (from 1 to
150 mg/L) in a series of reagent flasks at ambient temperature,
and stirring the suspensions for 24 h before recovering the
adsorbent by centrifugation. The adsorption capacity at different equilibrium As(III) concentrations could then be calculated
from the difference between the initial and the equilibrium
As(III) concentrations. The As(III) concentration was analyzed
using an atomic fluorescence spectrophotometer (AFS-9800,
Beijing KeChuangHaiGuang Instrument Inc., Beijing, China).
III. Results and Discussion
(1) Chemical Composition of Al-Doped Iron Oxides
The semiquantitative chemical composition of samples obtained
was analyzed by XPS. Figures 1(a)–(c) show the high-resolution
XPS scan spectra of Fe 2p, Al 2p, and O 1s peaks in the 50% Al/
Fe2O3 sample, respectively. After the hydrothermal synthesis, the
sample obtained was washed with NaOH solution to remove
AlOOH produced during coprecipitation. In Fig. 1(a), two binding energy (BE) peaks of 709.8 and 723.6 eV could be observed,
which correspond to the Fe 2p3/2 and Fe 2p1/2 peaks for Fe31 in
Fe2O3, respectively. Figure 1(b) shows a BE peak at B73.4 eV,
which corresponds to the Al 2p peak for Al31. Figure 1(c) shows
the XPS scan spectrum of O 1s. Thus, these XPS spectra confirmed that the sample obtained was composed of Fe, Al, and O.
To examine the existence of Al element within the sample, Ar1
ion sputtering was adopted to remove the surface layers of the
sample to explore the chemical composition inside the sample. In
Fig. 1(d), the atomic percentages of O, Fe, and Al at different
sputtering times were summarized. With the increase of the sputtering time (from 0 to 50 s), the atomic percentage of Al slightly
decreased from B7.18% to B5.66%. This observation shows
that Al could be readily found inside the sample, they are subsequently referred to as Al/Fe2O3. The slightly higher value at the
surface may be due to the Al residue after the removal of AlOOH
by NaOH washing. With the increase of sputtering time, the
atomic percentage of Fe increased and that of O decreased,
before they gradually reached the stable state. This observation
may be due to the presence of adsorbed H2O and –OH groups at
the external surface of the sample, which could largely increase
the atomic percentage of O at the surface. After being sputtered
for 50 s, the atomic percentage of O reached B60%, and the sum
of the atomic percentages of Al and Fe reached B40%. Thus,
the chemical formula of the samples could be expressed as
Fe2xAlxO3. With the increase of the initial Al/Fe ratio in the
precursor solution, we observed an increase of x from 0.03 (10%
Al/Fe2O3 sample) to 0.28 (50% Al/Fe2O3 sample), indicating the
increase of Al dopants in the samples obtained.
During the hydrothermal process, urea acts as a homogeneous coprecipitation agent. The hydrolysis of urea could be
described by the following Eq. (1):
COðNH2 Þ2 þ 3H2 O ¼ 2NHþ
4 þ 2OH þ CO2
(1)
Thus, the pH value of the solution will increase gradually
with the hydrolysis of urea, and Fe31/Al31 ions could react with
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Vol. 94, No. 2
Fig. 1. High-resolution XPS spectra of (a) Fe 2p, (b) Al 2p, and (c) O 1s in the 50% Al/Fe2O3 sample. (d) The atomic percentages of O, Fe, and Al at
different sputtering times of the 50% Al/Fe2O3 sample.
OH ions to produce Fe(OH)3 and Al(OH)3. The solubility
constants of Fe(OH)3 and Al(OH)3 are 4.0 1038 and
1.3 1033, respectively. Hence, Fe(OH)3 should precipitate
first because of its much smaller solubility. In the basic aqueous environment, these Fe(OH)3 colloidal particles often carry
negative charges and could attract cations around them. Besides
forming Al(OH)3 precipitates, part of Al31 ions in the solution
could be adsorbed onto Fe(OH)3 and occupy some Fe31 lattice
positions in the formation process of Fe(OH)3. After the further
phase transition from hydroxides to oxides during the
hydrothermal process, Al-doped iron oxides (Fe2xAlxO3) was
produced.
(2) Al-Doping Effect on the Crystal Structure of Al-Doped
Iron Oxides
Figure 2(a) shows the XRD patterns of the as-prepared samples
from the hydrothermal process without NaOH washing, which
demonstrates that the major phases in these Al/Fe binary complex are hematite (a-Fe2O3) and boehmite (g-AlOOH). When
the initial Al/Fe ratio was o30%, only hematite and boehmite
were obtained under the current hydrothermal treatment, and
no other phase was detected. When the initial Al/Fe ratio was
30% and over, a new peak with 2y at 21.2231 could be observed
in the XRD pattern that corresponds to the goethite (a-FeOOH)
phase. It is well known that hematite is more stable than goethite,30,31 and goethite could transform to hematite by the following dehydration reaction:
2a-FeOOH ¼ a-Fe2 O3 þ H2 O
(2)
This observation indicates that Al doping could retard the
phase transition of goethite to hematite in this hydrothermal
process. Similar result was reported by Ruan and Gilkes32 on
the dehydroxylation of aluminous goethite. With the increase of
Al doping concentration, they observed that the transition temperature increased from 2361 to 2731C. Owing to the higher
ionic potential of Al31, Al31 could retain coordinated –OH
more strongly than Fe31. Thus, –OH groups could be better
kept within the structure with Al doping during the hydrothermal process. Consequently, excess –OH groups could be preserved in the Al-doped goethite phase compared with pure
goethite, which restrains the transition from the goethite phase
into the hematite phase.
Figure 2(b) shows the XRD patterns of the samples after
NaOH washing. Compared with the XRD patterns of their
counterparts in Fig. 2(a), the XRD peaks of boehmite disappeared, indicting that the boehmite phase was completely
removed by the treatment of NaOH because of the amphoteric character of aluminum hydroxide/oxide. The XRD peaks of
hematite and goethite were kept in the same position as those in
Fig. 2(a), suggesting that the alkali solution had little or no effect
on the structure of hematite and goethite.
Detailed analysis on the XRD peak positions of hematite
phase indicated that a positive shift was present for almost all
the XRD peaks with the increase of Al-doping concentration.
For example, Fig. 2(c) clearly shows the shift to the more positive direction of peak positions of (214) and (300) of the hematite phase from samples after the NaOH treatment. According
to the Bragg equation:
2d sin y ¼ nl
(3)
where d is the lattice spacing, l is the average wavelength of the
X-ray radiation, and y is the diffracting angle, the increase
of 2y value indicates the decrease of the d-spacing value. The
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Enhanced Arsenite Adsorption onto Al-Doped Iron Oxides
587
Fig. 2. (a) and (b) XRD patterns of Al/Fe2O3 samples before and after NaOH washing, respectively (note: goethite peaks (m), boehmite peaks ( & ),
hematite peaks (other peaks); (0): 0% Al/Fe2O3, (1): 10% Al/Fe2O3, (2): 20% Al/Fe2O3, (3): 30% Al/Fe2O3, (4): 40% Al/Fe2O3, and (5): 50%Al/Fe2O3).
(c) The peak position shifts of (214) and (300) of the hematite phase after NaOH treatment. (d)–(f) The unit cell parameters of a, c, and V of the hematite
phase from samples after NaOH treatment versus initial ratios of Al/Fe.
six-coordinate ionic radii of Al31 is 67.5 pm, which is smaller
than the six-coordinate ionic radii of Fe31 of 69 pm.33 Thus, the
substitution of Fe31 by Al31 should result in a decrease of the
lattice spacing, and the decrease should become deeper with the
increase of Al-doping concentration. Our observation is in good
accordance with this theoretical analysis. The d-spacing values
of several lattice planes are summarized in Fig. S1 in the supporting information, which clearly demonstrates the decrease of
the d-spacing value with the increase of the initial Al/Fe ratio
(also
Al-doping
concentration).
Interestingly,
the
d-spacing value could be fitted very well with the initial Al/Fe
ratio to a linear relationship.
The unit cell parameters of a, c, and V of the hematite phase
from samples after the NaOH treatment are presented in Figs.
2(d), (e), and (f), respectively. It is clear that these parameters
show a linear decrease roughly with the increase of initial Al/Fe
ratio (also Al-doping concentration). For the unit cell length a, it
could be fitted very well with the linear relationship. Similar
observation had been reported and had been taken as the evi-
dence of the substitution of Al for Fe in this iron–aluminum
oxide complex material system.34 For the unit cell length c, it
shows a deviation from the linear relationship, which could be
explained by the fact that c is more affected by structure –OH
than a.34
(3) Morphology and Surface Properties of Al-Doped Iron
Oxides
The morphology of Al-doped iron oxides was examined with the
SEM observation. Figure 3(a) shows the morphology of pure
Fe2O3 without the Al doping, which exhibits irregular spherical
morphology with an average particle diameter of about 80 nm.
The high-magnification SEM image of the same sample (inset in
Fig. 3(a)) shows a litchi-like structure formed by the aggregation
of uniform nanoprotuberances with an average diameter of
about 10 nm. Figure 3(b) shows the SEM image of 50% Al/
Fe2O3 sample before the NaOH washing treatment, which is
composed of both beohmite and Al-doped iron oxides with very
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Vol. 94, No. 2
Fig. 3. (a)–(c) High-resolution SEM images of pure Fe2O3 sample, 50% Al/Fe2O3 sample before NaOH treatment, and 50% Al/Fe2O3 sample after
NaOH treatment, respectively. (d) The BET specific surface area values of Al-doped iron oxide samples versus initial ratios of Al/Fe.
different morphologies. Boehmite and Al-doped iron oxides
were formed by the coprecipitation in the hydrothermal process, and hence they subsequently became interleaved with each
other. The Al-doped iron oxides kept the spherical litchi-like
morphology, while their particle size was a little smaller than
that of pure hematite. Boehmite assumed a nanoplate morphology, which was also formed with the aggregation of boehmite
nanoparticles. After the NaOH treatment (see Fig. 3(c)), boehmite was completely removed from the binary complex. Iron
oxides kept the spherical litchi-like morphology, while some of
the ‘‘spherical litchis’’ became fragmentary particles with nonuniform size distribution. The whole structure became looser
compared with that of the pure Fe2O3, which could be attributed mainly to the removal of boehmite by the NaOH treatment
and should contribute to the increase of its surface area. Figure
3(d) summarizes the BET SSA values of Al-doped iron oxide
samples. With the increase of the initial Al/Fe ratio, their BET
SSA increases. For example, the BET SSA of 50% Al/Fe2O3 is
52.47 m2/g, while that of pure hematite is only 23.32 m2/g. Thus,
the Al doping is beneficial to the enhancement of the surface
area of iron oxides.
The surface O 1s spectra of Al-doped Fe2O3 samples were
analyzed before and after As(III) adsorption by high-resolution
XPS scan. Figures 4(a) and (b) show the surface O 1s spectra of
pure Fe2O3 and 50% Al/Fe2O3 samples, respectively. Large
differences could be observed between them, indicating that
they have different chemical states of oxygen. For both of them,
their O 1s spectra could be best fitted with three overlapped O 1s
peaks of oxide oxygen (O2), hydroxyl group –OH, and adsorbed water (H2O). The data were fitted using Lorentzian peak
shape, and the fitting parameters could be found in Table S1 in
the supporting information. For pure Fe2O3 without Al doping,
the percentage of –OH in the total surface oxygen is just about
35%, while that of 50% Al/Fe2O3 has a much higher value of
about 64%. Thus, it suggests that Al doping into Fe2O3 is beneficial to the formation of abundant M–OH, which is in accordance with the XRD analysis result. The presence of surface
hydroxyl groups is critical to the adsorption of As(III) from
water.17,25,26 After As(III) adsorption, the amounts of hydroxyl
group in both pure Fe2O3 and Al-doped Fe2O3 decreased, which
resulted in the decrease of its percentage in the total surface
oxygen. For pure Fe2O3, its percentage decreased from B35%
to B21%, while its percentage decreased from B64% to
B15.6% for 50% Al/Fe2O3. Thus, the Al doping is beneficial
to obtaining the enhanced surface area and much higher –OH
percentage in the total surface oxygen, both of which are desired
for obtaining better performance to remove As(III) from aqueous environment by adsorption.
(4) Kinetic Studies on As(III) Adsorption by Al-Doped Iron
Oxides
The adsorption kinetics of As(III) adsorbed onto Al-doped
Fe2O3 nanoparticles was studied at initial As(III) concentrations
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Enhanced Arsenite Adsorption onto Al-Doped Iron Oxides
589
Fig. 4. The surface O 1s spectra of Al-doped Fe2O3 samples. (a) pure Fe2O3 sample, (b) 50% Al/Fe2O3 sample, (c) pure Fe2O3 sample after As(III)
adsorption, and (d) 50% Al/Fe2O3 sample after As(III) adsorption.
of 2.222 mg/L. The experimental results could be best fitted into
a pseudo-second-order rate kinetic model.19,35 The pseudosecond-order adsorption kinetic rate equation could be
expressed as
t
1
1
¼
þ t
qt k2 q2e qe
(4)
where k2 is the rate constant of adsorption (g/(mg min)), qe is
the equilibrium adsorption capacity (mg/g), and qt is the amount
of As(III) adsorbed at time t, respectively. The applicability of
the pseudo-second-order rate model was quantified by the
square of the correlation coefficient R (R2), and the closeness
of R2 to 1 indicates that the model fitted the experimental data
accurately. The initial adsorption rate h (mg/g min) could be
defined as
h ¼ k2 q2e
(5)
which could be used an indicator on the adsorption rate, especially at the beginning of the adsorption process. Figure 5(a)
shows the As(III) adsorption kinetic data of the series of Aldoped Fe2O3 nanoparticle samples with the initial Al/Fe ratio of
0%, 10%, 20%, 30%, 40%, and 50%, respectively, for water
samples with the initial As(III) concentration at 2.222 mg/L. For
all these samples, the As(III) adsorption rate was rapid in the
first 30 min, and then it gradually decreased as the adsorption
approached their respective equilibrium. Their corresponding
pseudo-second-order rate model fittings are demonstrated in
Fig. 5(b), and the fitting parameters are summarized in Table
S2 in the supporting information. As demonstrated by the
square of correlation coefficient of over 0.99, these kinetic data
of As(III) were successfully fitted with the pseudo-second-order
rate equation. With the increase of the initial Al/Fe ratio (the
increase of Al-doping concentration) and the same material
loading (1 g/L), both the initial adsorption rate (h) and the equilibrium adsorption capacity (qe) increased, indicating that a better As(III) adsorption performance was achieved with Al doping
into iron oxides. The kinetic study here shows clearly that a better As(III) adsorption performance could be achieved by Al
doping of iron oxides.
(5) Equilibrium Adsorption Isotherm Study on As(III)
Adsorption by Al-Doped Iron Oxides
The adsorption capacity of Al-doped Fe2O3 nanoparticles on
As(III) at near neutral pH condition was investigated by the
equilibrium adsorption isotherm study. The adsorption data
could be best fitted with the Freundlich isotherm (with
R240.98), as demonstrated in Fig. 6(a) for pure Fe2O3, 30%
Al/Fe2O3, and 50% Al/Fe2O3 samples, respectively. The
Freundlich isotherm describes the adsorption process
where the adsorbent has a heterogeneous surface with adsorption sites that have different energies of adsorption.27 This
observation is in accordance with the heterogeneity of Al-doped
Fe2O3 nanoparticle surface. The Freundlich isotherm is given in
Eq. (6):
Qe ¼ KC1=n
e
(6)
where Qe is the amount (mg/g) of As(III) adsorbed at equilibrium, Ce is the equilibrium As(III) concentration (mg/L) in
water samples, and K and n are the constants of adsorption.
The parameters obtained in fitting the experimental data are
summarized in Table S3 in the supporting information. It is
clear that the Al doping largely enhanced the adsorption capacity of iron oxides, which is largely in accordance with the
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Journal of the American Ceramic Society—Li et al.
Fig. 5. (a) The As(III) adsorption kinetic data of the series of Al-doped
Fe2O3 nanoparticle samples with the initial Al/Fe ratio of 0%, 10%,
20%, 30%, 40%, and 50%, respectively, for water samples with the initial As(III) concentration at 2.222 mg/L. (b) The pseudo-second-order
rate model fittings of the kinetic data in Fig. 5(a).
Vol. 94, No. 2
Fig. 6. (a) The equilibrium adsorption isotherm of As(III) on pure
Fe2O3, 30% Al/Fe2O3, and 50% Al/Fe2O3 samples, respectively. (b) The
amount of As(III) adsorbed per unit surface area at the equilibrium (Qe/
SSA, mg/m2) for 50% Al/Fe2O3 sample, compared with that of pure
Fe2O3 sample. SSA, specific surface area.
IV. Conclusions
enhanced SSA of this material system by Al doping as demonstrated in Fig. 3(d).
The As(III) adsorption onto iron oxides/hydroxides is a specific adsorption process.25 Both the surface area and the
adsorption active sites play important roles in this process. To
examine other possible effects from the Al doping other than the
enhancement of the SSA, the amount of As(III) adsorbed per
unit surface area at the equilibrium (Qe/SSA, mg/m2) was calculated for 50% Al/Fe2O3 sample, compared with that of pure
Fe2O3 sample. As demonstrated in Fig. 6(b), the adsorption
capability of 50% Al/Fe2O3 with the unit surface area is generally higher than that of pure Fe2O3, especially when Ce is high.
This observation indicates that besides the increase of the surface area, Al doping could also increase the amount of adsorption active sites in iron oxides, which will also contribute to the
enhancement of the As(III) adsorption performance. According
to the surface complex model theory,36,37 hydroxyl groups on
the surface of metal oxides are the active adsorption sites for
arsenite. Thus, larger amounts of surface hydroxyl groups could
lead to higher As(III) adsorption capacity. From the highresolution XPS study on O 1s spectra of these samples
(Fig. 4), –OH percentage in the total surface oxygen of
Al-doped iron oxides is much higher than that of pure Fe2O3.
The larger amount of As(III) adsorbed per unit surface area on
Al-doped iron oxides observed here is in good accordance with
the XPS analysis result.
In this study, aluminum–iron hydroxide/oxides with different
initial ratios of Al/Fe were synthesized by a hydrothermal process. To distinguish the Al-doping effect from that of boehmite,
boehmite was removed by NaOH washing and Al-doped iron
oxide was obtained. Al-doped iron oxides showed a much better
performance on As(III) removal from water and the As(III)
adsorption capacity of 50% Al/Fe2O3 sample reached over
40 mg/g, 2.5 times that of pure Fe2O3 sample under the same
experimental conditions. The removal of boehmite resulted in a
large increase in the SSA of Al-doped iron oxides. Al doping
promoted the formation of surface hydroxyl groups, leading to an
increased amount of As(III) adsorption active sites on iron oxides. Both of these two effects are beneficial to the enhancement
of the As(III) removal performance of Al-doped iron oxides.
Supporting Information
Additional Supporting Information may be found in the online
version of this article:
Table S1. showing the Lorentzian peak shape fitting parameters for O 1s XPS peak of pure Fe2O3and 50% Al/Fe2O3 samples before and after As(III) adsorption
Table S2. showing kinetics parameters for As(III) adsorption
onto Al/Fe2O3 samples with various initial Al/Fe ratios.
February 2011
Enhanced Arsenite Adsorption onto Al-Doped Iron Oxides
Table S3. showing the Freundlich equilibrium adsorption
isotherm fitting parameters for As(III) onto Al/Fe2O3 samples
with various initial Al/Fe ratios.
Fig. S1. Showing the d-spacing values of several lattice planes
of hematite phase versus initial ratio of Al/Fe.
Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting materials supplied by the
authors. Any queries (other than missing material) should be
directed to the corresponding author for the article.
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