superparamagnetic magnesium ferrite
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Superparamagnetic magnesium ferritenanoadsorbent for effective arsenic (III, V)removal and easy magnetic separation
Wenshu Tang a, Yu Su a, Qi Li a,*, Shian Gao a, Jian Ku Shang a,b
aEnvironment Functional Materials Division, Shenyang National Laboratory for Materials Science, Institute of
Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, Liaoning Province 110016, PR ChinabDepartment of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana,
IL 61801, USA
a r t i c l e i n f o
Article history:
Received 1 February 2013
Received in revised form
14 April 2013
Accepted 15 April 2013
Available online 24 April 2013
Keywords:
Arsenite/arsenate adsorption
Magnesium ferrite nanoadsorbent
Superparamagnetic
Magnetic separation
* Corresponding author. Tel.: þ86 24 8397802E-mail addresses: [email protected], qiliuiuc
0043-1354/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.watres.2013.04.023
a b s t r a c t
By doping a proper amount of Mg2þ (w10%) into a-Fe2O3 during a solvent thermal process,
ultrafine magnesium ferrite (Mg0.27Fe2.50O4) nanocrystallites were successfully synthesized
with the assistance of in situ self-formed NaCl “cage” to confine their crystal growth. Their
ultrafine size (average size of w3.7 nm) and relatively low Mg-content conferred on them a
superparamagnetic behavior with a high saturation magnetization (32.9 emu/g). The ul-
trafine Mg0.27Fe2.50O4 nanoadsorbent had a high specific surface area of w438.2 m2/g, and
demonstrated a superior arsenic removal performance on both As(III) and As(V) at near
neutral pH condition. Its adsorption capacities on As(III) and As(V) were found to be no less
than 127.4 mg/g and 83.2 mg/g, respectively. Its arsenic adsorption mechanism was found
to follow the inner-sphere complex mechanism, and abundant hydroxyl groups on its
surface played the major role in its superior arsenic adsorption performance. It could be
easily separated from treated water bodies with magnetic separation, and could be easily
regenerated and reused while maintaining a high arsenic removal efficiency. This novel
superparamagnetic magnesium ferrite nanoadsorbent may offer a simple single step
adsorption treatment option to remove arsenic contamination from water without the
pre-/post-treatment requirement for current industrial practice.
ª 2013 Elsevier Ltd. All rights reserved.
1. Introduction and Suzuki, 2002; Brown and Ross, 2002). In order to mini-
Arsenic, a relatively scarce but ubiquitous element, is of
serious health concern due to its toxicity and carcinoge-
nicity. Long-term exposure to arsenic contaminated water
could cause skin, lung, bladder, and kidney cancers as well
as pigmentation changes, skin thickening (hyperkeratosis),
neurological disorders, muscular weakness, loss of appetite,
nausea, cardiovascular and cerebrovascular disease, dia-
betes mellitus, and adverse reproductive outcomes (Mandal
8; fax: þ86 24 [email protected] (Q. Li).ier Ltd. All rights reserve
mize its health risk, the World Health Organization (WHO)
set a new guideline limit of 0.01 mg/L in drinking water to
replace the previous 0.05 mg/L limit in 1996 for arsenic
(WHO, 1996), and this new guideline limit has been adopted
by many countries. For example, the European Union
required the compliance with this new guideline limit by
2007 (De Zuane, 2007), and this new guideline limit has also
been effective in the United States since January 2006
(USEPA, 2002).
d.
wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 6 2 4e3 6 3 4 3625
Because of its simplicity, potential for regeneration, and
sludge free operation, adsorption is considered to be one of
the most promising technologies for arsenic removal (Jang
et al., 2006). In recent years, synthesized metal oxide nano-
adsorbents, such as aluminum, iron, titanium, zirconium and
cupric based oxides, have been extensively studied for arsenic
adsorption, which demonstrated superior performance
because of their large surface areas from their nano-size and
preferred surface properties (Cao et al., 2007; Cui et al., 2012;
Dutta et al., 2004; Kim et al., 2004; Liu et al., 2010; Mohan and
Pittman, 2007; Pena et al., 2005, 2006). To implement the
stricter guideline limit of 0.01 mg/L arsenic in drinking
water, the development of more effective and cost-friendly
nanoadsorbents for arsenic removal from drinking water is
urgently needed. Furthermore, the separation of current
nanoadsorbents from treated water bodies still remains as a
challenge to their potential application in real water treat-
ment practice. The inefficient separation could cause their
dispersion into the aqueous environment, resulting in the
operation cost increase and potential damages to both natural
organisms and the environment.
Iron oxide-basedmaterials had been extensively studied as
the arsenic adsorbents because of their low costs, high sta-
bility, environmental friendliness, and strong affinity for
arsenic species (Gimenez et al., 2007). In our recent work, we
developed a process to synthesize ultrafine a-Fe2O3 nano-
crystallites of several nanometers by a room temperature
reaction followed with a solvent thermal process at low
temperature of 150 �C, which demonstrated a good adsorption
performance on both As(III) and As(V) species in water (Tang
et al., 2011a,b). However, their ultrafine size caused the diffi-
cult separation from treated water. It had been demonstrated
that magnetic separation could be a more efficient and se-
lective method to separate nanomaterials from aqueous
environment than conventional approaches of centrifugation
or filtration (Raven et al., 1998).
In this work, a superparamagnetic ultrafine magnesium
ferrite (Mg0.27Fe2.50O4) nanoadsorbent was created by doping
Mg2þ into ultrafine a-Fe2O3 nanocrystallite during the hydro-
thermal process. Similar to that demonstrated in Al-doped
iron oxide arsenic adsorbent in our previous work (Li et al.,
2011), the specific surface area of Mg0.27Fe2.50O4 largely
increased due to the Mg-doping, subsequently enhancing its
arsenic adsorption performance on both As(III) and As(V)
species compared with the ultrafine a-Fe2O3 nanoadsorbent
we developed before. Furthermore, a proper amount of w10%
Mg-doping into iron oxide crystal lattice caused a crystal
structure change from the rch a-Fe2O3 phase to a single
magnesium ferrite phase with cubic spinel structure, which
endowed the ultrafine Mg0.27Fe2.50O4 nanoadsorbent a super-
paramagnetic behavior with a high saturation magnetization.
Thus, no magnetic attraction existed when there was no
external magnetic field applied during the water treatment,
which is beneficial to its better dispersion and the subsequent
better contact efficiency with arsenic species in water. After
the water treatment, however, the external magnetic field
applied could induce its easy magnetic separation from
treated water bodies. Then, the ultrafine Mg0.27Fe2.50O4 nano-
adsorbent could be easily recovered by NaOH washing and
reused for arsenic removal. To our best knowledge, no report
was available in literature on the superior arsenic removal
performance andmechanism study of the superparamagnetic
magnesium ferrite nanoadsorbent. With further develop-
ment, this novel superparamagnetic magnesium ferrite
nanoadsorbent may offer a simple single step adsorption
treatment option to remove arsenic contamination from
water without the pre-/post-treatment requirement for cur-
rent industrial practice.
2. Experimental
2.1. Chemicals and materials
All the chemicals used are of analytical reagent grade.
Anhydrous ferric chloride (FeCl3, 99.0 wt%, Chemical Reagent
Co., Ltd, Beijing, P.R. China) and anhydrous magnesium
chloride (MgCl2, 99.0 wt%, Shenyang Guo Yao Technology,
Shenyang, P.R. China) were used as the raw materials. So-
dium hydrate (NaOH, 98 wt%, Tianjin Damao Chemical Re-
agents Development Center, Tianjin, P.R. China) was used as
the precipitation agent, and anhydrous ethanol (C2H5OH,
�99.0%, Yili Chemical Reagent Co., Ltd, Beijing, P.R. China)
was used as the solvent. Concentrated hydrochloric acid
(HCl, 32e38%, Tianda Chemical Reagents Factory, Tianjin,
P.R. China) was used to stabilize the arsenic species after
water treatment. Sodium chloride (NaCl, 99 wt%, Tianjin
Damao Chemical Reagents Development Center, Tianjin, P.
R. China) was used as supporting electrolyte in the electro-
phoretic mobility (EM) study.
2.2. Nanoadsorbent synthesis
The typical synthesis process of the ultrafine magnesium
ferrite nanoadsorbent included a room-temperature reaction
followed with a solvent thermal treatment, similar as that of
ultrafine a-Fe2O3 nanocrystallites we reported previously
(Tang et al., 2011a,b). First, a mixture solution of 0.09 M FeCl3and 0.01 MMgCl2 was obtained by dissolving proper amounts
of FeCl3 and MgCl2 into 70 mL ethanol. Then, 10 mL NaOH
ethanol solution (2.03 M) was added into themixture solution
to obtain the fixed molar ratio of NaOH:FeCl3 of 3:1 and
NaOH:MgCl2 of 2:1 at room temperature, and the reaction
lasted for 1 h under ultrasonic oscillation. Yellow precipitates
were produced during the room temperature reaction, which
were composed of amorphous (Fe, Mg)x(OH)y nanoparticles
confined in NaCl “cage” in situ formed during the reaction.
Next, the suspension was sealed in a Teflon-lined autoclave,
placed in an oven pre-heated at 150 �C for 2 h, and then
cooled to room temperature naturally. During the solvent
thermal treatment, the thermal decomposition of (Fe,
Mg)x(OH)y occurred, and magnesium ferrite nanocrystallites
were produced. During this process, the NaCl “cage” still
existed and could confine the growth of magnesium ferrite
nanocrystallites. Then, the obtained precipitation was
washed with distilled water under ultrasonic oscillation for
three times to remove NaCl. After being dried at 80 �C for 12 h,
the ultrafine magnesium ferrite nanoadsorbent was
obtained.
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2.3. Nanoadsorbent characterization
A D/MAX-2004 X-ray powder diffractometer (Rigaku Corpora-
tion, Tokyo, Japan) was used to analyze the sample’s crystal
structure.Transmissionelectronmicroscopy (TEM)wasusedfor
the morphology observation of nanocrystallites on a JEM 2100
transmission electron microscope (JEOL Corporation, Tokyo,
Japan) operated at 200 kV. The surface area and pore volume of
the sample were measured by N2 adsorptionedesorption
isotherm with an Autosorb-1 Series Surface Area and Pore Size
Analyzers (Quantachrome Instruments, Boynton Beach, FL,
U.S.A.). Prior to the experiment, the sample was dehydrated at
130 �C for 12 h. The relative pressure (P/P0) during 0.1018e0.3056
was used to determine the BET surface area. The pore-size dis-
tribution (PSD) was calculated using the desorption branches of
the N2 adsorption isotherm and the BarretteJoynereHalenda
(BJH) formula (Cheng et al., 1997). The average size and size
polydispersity of thesenanoparticlesdispersed inDIwaterwere
determined by dynamic light scattering with a Malvern Nano
ZS90 Zetasizer (Malvern Instruments Ltd., Malvern, Worcester-
shire, UK). The instrument was calibrated with standard latex
nanoparticles (Malvern Instruments Ltd., Malvern, Worcester-
shire, UK). The FTIR spectra of sampleswere carriedout at room
temperature in transmission mode by Fourier transform
infrared spectroscopy (FTIR, Bruker TENSOR 27, MCT detector)
with the resolution of 4 cm�1. The element composition of the
sample was determined by an inductive coupled plasmamass-
spectrometer (Perkin Elmer-SCIEX ELAN DRCe ICPeMS, Nor-
walk, U.S.A.). Zeta-potential of samples at different pHs was
measured by electrophoretic spectroscopy (JS84H, Shanghai
Zhongchen Digital Instrument Co., Ltd., Shanghai, P.R. China).
The zeta-potential was determined by mixing 100 mg samples
with 200 mL 0.01 M KNO3 solution with pH values adjusted be-
tween1and10byadding0.1MHNO3or0.1MKOH.Themagnetic
properties of samples at room temperaturewere examinedona
Quantum Design MPMS-XL superconducting quantum inter-
ference device (SQUID, Quantum Design, Inc., San Diego, CA,
U.S.A.). The semi-quantitative surface chemical composition
data of samples were examined by X-ray photoelectron spec-
troscopy (XPS) using an ESCALAB250 X-ray photoelectron
spectrometer (Thermo Fisher Scientific Inc., Waltham, MA,
U.S.A.) with an Al K anode (1486.6 eV photon energy, 0.05 eV
photon energy resolution, 300W).
2.4. Batch arsenic adsorption experiments
All the arsenic adsorption experiments were carried out at
w25 �C. During the arsenic removal experiment, the arsenic
solutions were stirred mechanically at 300 rpm by an electric
stirrer (JJ-1, Shanghai Pudong Physical Optical Instrument Fac-
tory Shanghai, P.R. China) to disperse the adsorbent to ensure a
good contact with arsenic contaminations. After recovering the
adsorbent magnetically, one drop of concentrated HCl was
added into the clear solution to avoid the potential oxidation of
As(III) to As(V). As (III) and As (V) concentrations of the aqueous
solutions were determined by an atomic fluorescence spec-
trophotometer (AFS-9800, Beijing Ke Chuang Hai Guang In-
strument Inc., Beijing, P.R. China) with the valence analysis
function. Detailed information on experiment conditions could
be found in the Supplementary Data.
3. Results and discussion
3.1. Crystal structure, composition, and morphology
Fig. 1a shows the XRD pattern of the nanoadsorbent sample.
A crystallized magnesium ferrite phase with cubic spinel
structure was observed, which was very close to the cubic
spinel structure of MgFe2O4 (JCPDS NO. 36-0398) (Candeia
et al., 2006). The bulk composition of the nanoadsorbent
sample was obtained by ICPeMS, and the sample’s chemical
formula was determined as Mg0.27Fe2.50O4, identical to its
surface composition determined by XPS analysis. Thus, the
nanoadsorbent should have a single magnesium ferrite
phase and could not contain the mixture of crystallized iron
oxides (like g-Fe2O3 or Fe3O4) with amorphous Mg(OH)2/
MgO, although g-Fe2O3 or Fe3O4 had similar XRD diffraction
patterns and magnetic behaviors as that of magnesium
ferrite. The Mg/(Fe þ Mg) molar percentage of Mg0.27Fe2.50O4
was determined at w9.7%, just slightly lower than the
starting composition of 10% for raw materials. The insert
image in Fig. 1a shows the diffraction peak shift of
Mg0.27Fe2.50O4 toward a slightly higher diffraction angle,
compared with MgFe2O4. More diffraction peak position
shift data could be found in Table S1 in the Supplementary
Data. This diffraction peak position shift could be attributed
to the different Mg-contents between Mg0.27Fe2.50O4 and
MgFe2O4. Mg2þ has a larger radius of 0.71 �A, compared with
that of Fe3þ (0.63 �A) (Nakagomi et al., 2009). The smaller
amount of Mg-content in Mg0.27Fe2.50O4 resulted in the
decrease of its lattice spacing, compared with that of
MgFe2O4. According to the Bragg equation as demonstrated
in Eq. (1):
2d sin q ¼ l (1)
where d is the lattice spacing, l is the average wavelength of
the X-ray radiation, and q is the diffracting angle. Thus, the
smaller lattice spacing of Mg0.27Fe2.50O4 could cause the in-
crease of 2q, compared with that of MgFe2O4.
Fig. 1b shows the TEM observation of the Mg0.27Fe2.50O4
nanoadsorbent. It is clear that the Mg0.27Fe2.50O4 nano-
adsorbent consisted of ultrafine nanocrystallites. The insert
image of Fig. 1b demonstrates that these ultrafine nano-
crystallites had a narrow size distribution as determined by
the professional image processing and analysis software of
Image-Pro Plus 5.0 on over 10 TEM images by the quantitative
statistical method. By Gaussian fitting, their average size was
determined at w3.7 nm, which was smaller than that of ul-
trafine a-Fe2O3 nanocrystallites without Mg-doping (w4.8 nm)
(Tang et al., 2011a,b). Similar to the synthesis of ultrafine
a-Fe2O3 nanocrystallites, the in situ self-formed NaCl “cage”
was effective in confining the growth of Mg0.27Fe2.50O4 nano-
crystallites. Fig. 1c shows the high resolution TEM (HRTEM)
image of Mg0.27Fe2.50O4 nanocrystallites. The insert SAD
pattern demonstrated that these nanocrystallites had a high
degree of crystallinity, which was in agreement with XRD
analysis result. For example, a set of lattice planes with the
d-spacing at w0.2957 nm could be easily identified on one
nanocrystallite, which corresponded to the (220) plane of
magnesium ferrite.
Fig. 1 e (a) X-ray diffraction pattern, (b) TEM image, and (c) HRTEM image of the Mg0.27Fe2.50O4 nanoadsorbent.
wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 6 2 4e3 6 3 4 3627
3.2. Surface properties
The BET specific surface area of ultrafine Mg0.27Fe2.50O4
nanocrystallites was determined at w438.2 m2/g, about 2.7
times as that of ultrafine a-Fe2O3 nanocrystallites (w162 m2/g)
synthesized with a similar process (Tang et al., 2011a,b).
Compared with ultrafine a-Fe2O3 nanocrystallites (w4.8 nm),
their average particle size (w3.7 nm) only decreased w23%.
Thus, the particle size decrease was not the only reason
for such a large BET specific surface area increase of
Mg0.27Fe2.50O4 nanocrystallites. The substitution of heavy Fe3þ
by lighter Mg2þ could induce internal and surface defects/
pores, which could then further increase the surface area.
Pore size distribution analysis demonstrated that most pores
were mesoporous. The average pore size was w3.89 nm,
which should reflect the inter-particle porosity. The pore
volume was determined to be w0.648 cm3/g. Thus, these
Mg0.27Fe2.50O4 nanocrystallites had a relatively larger surface
area and pore volume, which were beneficial for their arsenic
adsorption capability. Dynamic light scattering results
demonstrated that these nanoparticles formed aggregates in
water. The average size of these aggregates was determined at
w630 nm and their size rangewas fromw450 nm tow820 nm.
Fig. 2a shows the FTIR spectrum of ultrafine Mg0.27Fe2.50O4
nanocrystallites. Compared with that of a-Fe2O3 nano-
crystallites, two new absorption bands at w593 cm�1 and
442 cm�1 occurred in the FTIR spectrum of Mg0.27Fe2.50O4
nanocrystallites, which corresponded to the vibration of
tetrahedral and octahedral complexes, respectively. These
two absorption bands were the indication of the formation of
magnesium ferrite with cubic spinel structure (Pradeep et al.,
2008). Both FTIR spectra had evident HOH stretching
(w3398 cm�1) and bending (w1618 cm�1) vibrations of
physically adsorbed H2O, and the deformation (w1387 cm�1)
and bending (1047 cm�1) vibrations of hydroxyl groups on
metal oxides (Sun et al., 2009; Keiser et al., 1982). The signal
intensities of these vibrations of Mg0.27Fe2.50O4 nano-
crystallites were much stronger than that of a-Fe2O3 nano-
crystallites (see the transmittance data of these FTIR
adsorption bands in Table S2 in the Supplementary Data),
which indicated that more hydroxyl groups existed on their
surface. Fig. 2b compares the zeta potential of ultrafine
Mg0.27Fe2.50O4 nanocrystallites with that of ultrafine a-Fe2O3
nanocrystallites. The isoelectric point (IEP) of Mg0.27Fe2.50O4
nanocrystallites was w pH 5.2, lower than that of a-Fe2O3
nanocrystallites (wpH 6.1). Ultrafine Mg0.27Fe2.50O4 nano-
crystallites were more negatively charged near neutral pH
than a-Fe2O3 nanocrystallites, which indicated that more
surface hydroxyl groups existed on their surface and was in
agreement with the FTIR comparison result. Thus, the larger
specific surface area and the existence of more surface hy-
droxyl groups on ultrafine Mg0.27Fe2.50O4 nanocrystallites
indicated that they could possess an even better arsenic
removal effect by adsorption than a-Fe2O3 nanocrystallites.
3.3. Magnetic properties of magnesium ferritenanocrystallites
The large crystal structure change from the rhomb-centered
hexagonal (rch) a-Fe2O3 phase to the cubic spinel magnesium
ferrite structure caused a dramatic change on their magnetic
properties. Fig. 3 compares the magnetic field-dependent be-
haviors of ultrafineMg0.27Fe2.50O4 nanocrystallites and ultrafine
a-Fe2O3 nanocrystallites. Because of their ultrafine particle size
(w3.7 nm), ultrafine Mg0.27Fe2.50O4 nanocrystallites demon-
strated the typical superparamagnetic behavior with zero
Fig. 2 e (a) IR spectra and (b) zeta potential curves of ultrafine Mg0.27Fe2.50O4 and a-Fe2O3 nanocrystallites.
wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 6 2 4e3 6 3 43628
remanence and zero coercivity (Yang et al., 2009; Qin et al.,
2009; Skumryev et al., 2003), while ultrafine a-Fe2O3 nano-
crystallites had a weak ferromagnetic behavior with a narrow
magnetic hysteresis loop. The saturation magnetization, Ms,
could be obtained by extrapolating the graph of M vs. 1/H to
1/H / 0 (for H > 10 kOe). Ultrafine Mg0.27Fe2.50O4 nano-
crystallites had a highMs value ofw32.9 emu/g, far higher than
that of ultrafine a-Fe2O3 nanocrystallites (w4.5 emu/g). The
superparamagnetic behavior of ultrafine Mg0.27Fe2.50O4 nano-
crystallites could enhance their dispersion in arsenic solution
because no magnetic attraction existed when there was no
external magnetic field, while their high saturation magneti-
zation could make their magnetic separation feasible after the
water treatment.
3.4. Adsorption kinetic studies on As(III) and As(V)adsorption by magnesium ferrite nanocrystallites
Fig. 4a and b show the kinetics of As(III) and As(V) adsorption
onto the ultrafine Mg0.27Fe2.50O4 nanoadsorbent at different
sample loadings in the lab-prepared water samples at neutral
condition (pHw 7.0), respectively.With the increase of sample
loading amount, the equilibrium arsenic concentration in the
treated water samples gradually decreased. With just 0.01 g/L
sample loading, the equilibrium As(III) concentration dropped
from the initial 0.097 mg/L to 0.003 mg/L after the treatment
Fig. 3 e Magnetic field-dependent behaviors of ultrafine
Mg0.27Fe2.50O4 and a-Fe2O3 nanocrystallites.
(Fig. 4a), far below the USEPA limit for arsenic in drinking
water (0.01 mg/L). Similarly, the ultrafine Mg0.27Fe2.50O4
nanoadsorbent demonstrated a strong adsorption effect on
As(V). With just 0.01 g/L sample loading, the equilibrium As(V)
concentration dropped from the initial 0.101 mg/L to zero
(Fig. 4b), representing a 100% removal. Thus, a single step
As(III)/As(V) removal process is feasible by the ultrafine
Mg0.27Fe2.50O4 nanoadsorbent without pre-treatment (oxida-
tion/pH adjustment) and post-treatment pH adjustment.
The adsorption kinetic experimental results could be best
fitted into a pseudo-second-order rate kinetic model (Ho and
McKay, 1999), which had been widely used to describe metal
ion adsorption on different adsorbents (Ho and McKay, 1999;
Martinez et al., 2006). The integrated pseudo-second-order
rate expression could be described by Eq. (2):
t=qt ¼ t=qe þ 1=�Kadq
2e
�(2)
where qe and qt are the amount (mg/g) of arsenic adsorbed at
equilibrium and at time t, respectively, and Kad is the rate
constant of adsorption (g/(mg min)). The As(III) and As(V)
adsorption experimental data fitting results were shown in
Figure S1a and S1b of Supplementary Data, respectively, and
the kinetics parameters obtained in fitting the experimental
data were summarized in Table S3 of Supplementary Data.
The closeness of the square of the correlation coefficients r (r2)
to 1 indicated that the pseudo-second-order rate kinetic
model fitted the experimental data accurately. The rate con-
stant of adsorption (Kad) increased steadily with the increase
of the adsorbent loading for both arsenic species. Kad for the
adsorption of As(V) was higher than that for As(III) under the
similar experimental conditions, indicating that the ultrafine
Mg0.27Fe2.50O4 nanoadsorbent had a faster removal effect on
As(V) than on As(III). Similar result was observed for the
adsorption of As(III) and As(V) onto ferrihydrite surface (Raven
et al., 1998). The accurate fitting of the kinetic experimental
data to the pseudo-second-order rate kinetic model indicated
that the rate-limiting step may be a chemical sorption
involving valence forces through sharing or exchange of
electrons between sorbent and sorbate (Skumryev et al., 2003).
3.5. Equilibrium isotherm studies on As(III) and As(V)adsorption by magnesium ferrite nanocrystallites
The arsenic adsorption capacity of the ultrafine Mg0.27Fe2.50O4
nanoadsorbent at near neutral condition (pH w 7.0) was
Fig. 4 e Arsenic adsorption kinetics in lab-prepared water samples on the ultrafine Mg0.27Fe2.50O4 nanoadsorbent: (a) with
initial As(III) concentration of w0.097 mg/L, (b) with initial As(V) concentration of w0.101 mg/L.
wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 6 2 4e3 6 3 4 3629
investigated by the equilibrium adsorption isotherm study.
Fig. 5a and b demonstrate the equilibrium adsorption isotherm
curves for As(III) and As(V), respectively. As demonstrated in
Fig. 5a, the As(III) adsorption data could be best fitted with the
Freundlich isotherm as given in Eq. (3):
qe ¼ KFCe1=n (3)
where qe is the amount (mg/g) of As(III) adsorbed at equilib-
rium, Ce is the equilibrium As(III) concentration (mg/L) in
water samples, and KF and n are the Freundlich constants of
adsorption. The parameters obtained in fitting the experi-
mental data were summarized in Table S4 of Supplementary
Data. The adsorption capacity of the ultrafine Mg0.27Fe2.50O4
nanoadsorbent for As(III) at the near neutral pH environment
should be larger than 127 mg/g, far higher than that of ultra-
fine a-Fe2O3 nanoparticles (w95 mg/g) (Tang et al., 2011a,b).
The As(V) adsorption data could be best fitted with the Lang-
muir isotherm as given in Eq. (4):
qe ¼ qmKLCe=ð1=KLCeÞ (4)
where qe is the amount (mg/g) of As(V) adsorbed at equilib-
rium, qm is the maximum As(V) adsorption capability amount
(mg/g), Ce is the equilibrium As(V) concentration (mg/L) in
water samples, and KL is the Langmuir constant of adsorption.
The parameters obtained in fitting the experimental datawere
also shown in Table S4 of Supplementary Data. Its maximum
Fig. 5 e The equilibrium arsenic adsorption isotherms on ultra
As(V) adsorption capability was determined at w83 mg/g,
nearly twice as high as that of ultrafine a-Fe2O3 nanoparticles
(Tang et al., 2011a,b).
Because of the very low arsenic guideline limit of 0.01 mg/L
in drinking water, the amount of arsenic that an adsorbent
could adsorb at low equilibrium concentration is more
important than its maximum adsorption capability to esti-
mate its performance for arsenic removal in drinking water.
The inset images in Fig. 5a and b demonstrate the equilibrium
As(III) and As(V) adsorption isotherm curves at near neutral
condition for low equilibrium arsenic concentrations,
respectively. Under such conditions, the amounts of As(III)
and As(V) adsorbed at equilibrium increased with the equi-
librium arsenic concentration increase at a linear relation-
ship, as described by Eq. (5):
qe ¼ KCe þ b (5)
where qe is the amount (mg/g) of arsenic adsorbed at equilib-
rium, Ce is the equilibrium arsenic concentration (mg/L) in
water samples, K (L/g) and b are the adsorption constants. At an
equilibrium arsenic concentration of 0.01 mg/L (the USEPA
standard for drinking water), the amounts of adsorbed As(III)
and As(V) were around 10.1 mg/g and 11.8 mg/g, respectively,
nearly 2.7 and 2.0 times as high as that of ultrafine a-Fe2O3
nanoparticles (Tang et al., 2011a,b). From the above analysis, it
is clear that the amount of As(V) adsorbed at equilibrium was
fine Mg0.27Fe2.50O4 nanoadsorbent: (a) As(III) and (b) As(V).
wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 6 2 4e3 6 3 43630
slightly more than the amount of As(III) adsorbed at equilib-
rium by the ultrafine Mg0.27Fe2.50O4 nanoadsorbent under low
arsenic equilibrium concentration, while it was far less when
the equilibrium arsenic concentration was high. This observa-
tion could be attributed to the different surface charge condi-
tions of As(III) and As(V) species under the neutral pH
environment. Similar phenomena had been observed and
explained in details in our previous work on various arsenic
adsorbents (Cui et al., 2012; Li et al., 2011; Tang et al., 2011a,b).
The equilibrium isotherm adsorption studies demonstrated
that the ultrafine Mg0.27Fe2.50O4 nanoadsorbent had a superior
arsenic adsorption performance than ultrafine a-Fe2O3 nano-
particles we previously reported. Its arsenic adsorption capac-
ity wasmuch higher than that of various traditional adsorbents
(Jeong et al., 2007; Balaji et al., 2005; Dutta et al., 2004).
3.6. Adsorption mechanism studies
The arsenic adsorption mechanism on the ultrafine
Mg0.27Fe2.50O4 nanoadsorbent was investigated by both
macroscopic and microscopic techniques. Fig. 6a and b show
the removal efficiencies of As(III) and As(V) by the ultrafine
Mg0.27Fe2.50O4 nanoadsorbent with different ionic strengths,
respectively. The ionic strength in the solution was adjusted
by the addition of different amounts of NaCl. For both As(III)
and As(V), their removal efficiencies showed no obvious
change with the increase of the ionic strength from pH 1 to pH
13. As suggested by Goldberg and Johnston (2001), the
adsorption of arsenic species will decrease with the increase
of ionic strength if arsenic species form outer-sphere surface
complexes, while the adsorption of arsenic species will not
Fig. 6 e (a) and (b) Ionic strength effects on the arsenic removal
respectively. (c) Zeta-potential curves and (d) FTIR spectra of the
adsorption, respectively.
change or increase with the increase of ionic strength
if arsenic species form inner-sphere surface complexes.
Thus, both As(III) and As(V) adsorptions on the ultrafine
Mg0.27Fe2.50O4 nanoadsorbent followed the inner-sphere
complex mechanism.
The electrophoretic mobility measurement was also used
to examine the arsenic adsorption mechanism. Fig. 6c shows
the zeta potentials of Mg0.27Fe2.50O4, Mg0.27Fe2.50O4 after As(III)
adsorption (1 mg/L), and Mg0.27Fe2.50O4 after As(V) adsorption
(1 mg/L), respectively. The IEP of the ultrafine Mg0.27Fe2.50O4
nanoadsorbent decreased from w pH 5.2 to wpH 4.0 after
As(III) adsorption and to wpH 3.0 after As(V) adsorption. The
IEP of a metal oxide is determined by the protonation and
deprotonation of surface hydroxyl groups. It had been re-
ported that the formation of outer-sphere surface complexes
could not shift the IEP because there is no chemical reaction
between the adsorbate and the adsorbent surface that could
change the surface charge (Frimmel, 1993; Pena et al., 2005;
Sun et al., 2009). Therefore, the shift of IEP provided the evi-
dence of the formation of inner-sphere As(III)/As(V) anionic
charged surface complexes on the ultrafine Mg0.27Fe2.50O4
nanoadsorbent (Frimmel, 1993; Sun et al., 2009).
It had been reported that substitution of hydroxyl groups
(eOH) groups by arsenic species played the key role in their
adsorption mechanism (Goldberg and Johnston, 2001). So the
arsenic adsorption mechanism was further investigated with
the microscopic technique of FTIR spectroscopy. Fig. 6d
demonstrates the FTIR spectra of Mg0.27Fe2.50O4 after As(III)
and As(V) adsorption, respectively. Compared with that of
Mg0.27Fe2.50O4 without arsenic adsorption (see Fig. 2a), the
MeOH bending vibration peak (1047 cm�1) mostly
efficiency of As(III) and As(V) under different pHs,
ultrafine Mg0.27Fe2.50O4 nanoadsorbent after As(III) or As(V)
wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 6 2 4e3 6 3 4 3631
disappeared, and the intensities of the MeOH deformation
vibration peak (1387 cm�1) and the two peaks from physically
adsorbed H2O at 1618 cm�1 and 3698 cm�1 decreased. This
observation indicated that the replacement of eOH and H2O
occurred at during the arsenic adsorption. For As(III) adsorp-
tion, a new peak at 762 cm�1 appeared which corresponded to
the stretching vibration of uncomplexed As(III)eO bond (Sun
et al., 2009). As demonstrated by Pena et al. (2006) on the
arsenic adsorption by TiO2, the uncomplexed As(III)eO bond
of dissolved arsenite species was at 650 cm�1. The red shift of
As(III)eO observed here was caused by the decrease of the
strength of the uncomplexed AseO bond because of the for-
mation of AseOeM bond (Frimmel, 1993; Sun et al., 2009),
indicating the formation of inner-sphere complexes on the
nanoadsorbent surface after As(III) adsorption. For As(V), a
similar result was observed. A newpeak at 823 cm�1 appeared,
which corresponded to n(AseOeM) (Frimmel, 1993). The for-
mation of AseOeM bond indicated that the As(V) adsorption
on the nanoadsorbent surface also followed the inner-sphere
complex mechanism.
XPS analysis had been used in literature to study surface
interactions between adsorbates and adsorbents in the
adsorption process (Zhang et al., 2003; Deliyanni et al., 2006;
Lim et al., 2009; Martinson and Reddy, 2009). The interactions
between arsenic species and the functional groups (eOH) on
the ultrafine Mg0.27Fe2.50O4 nanoadsorbent were analyzed by
examining their signal intensity change. Fig. 7aec demon-
strate the O 1s XPS peak scan over ultrafine Mg0.27Fe2.50O4
nanoadsorbent, and ultrafine Mg0.27Fe2.50O4 nanoadsorbent
after As(III) and As(V) adsorption, respectively. The O 1s XPS
peak spectrum could be best fitted by the combination of three
Fig. 7 e (a)e(c) The surface O 1s spectra of the ultrafine Mg0.27Fe
adsorption, respectively. (d) The surface O 1s spectra of ultrafin
peaks with the center position at 530.1 eV, 531.2 eV, and
532.5 eV, which could be assigned to metal oxide (MeO), hy-
droxyl bonded to metal (MeOH), and adsorbed H2O, respec-
tively (Deng and Ting, 2005; Goh et al., 2009; Mamindy-Pajany
et al., 2009). After As(III) and As(V) adsorption, the area ratio of
MeOH peak decreased sharply from 35.4% to 15.7% and 18.5%,
respectively, indicating the decrease of the amounts of hy-
droxyl group on the adsorbent surface during the arsenic
adsorption. This observation was in agreement with the FTIR
analysis result on the formationofAseOeMbond (Fig. 6d). Fig. 7d
shows the O 1s XPS peak scan over ultrafine a-Fe2O3 nano-
particles. ItsarearatioofMeOHpeakwasonly18.9%, far less than
that of ultrafine Mg0.27Fe2.50O4 nanoadsorbent. This observation
was in agreement with the FTIR analysis on the hydroxyl group
signal (Fig. 2a), and indicated that the ultrafine Mg0.27Fe2.50O4
nanoadsorbent should have superior arsenic adsorption
performance than the ultrafine a-Fe2O3 nanoparticles.
3.7. Arsenic removal from natural water samples
The ultrafine Mg0.27Fe2.50O4 nanoadsorbent demonstrated a
good arsenic removal performance in natural water samples
from Lake Yangzonghai in the Yunnan Province of China even
when large amounts of competing ions were present (see its
water quality data in Table S5 of Supplementary Data). A
much better arsenic removal performance than that of the
ultrafine a-Fe2O3 nanoparticles was observed for it (see
Fig. 8a). The initial As(III) and As(V) concentrations were found
at w0.044 mg/L and w0.071 mg/L, respectively, and the water
sample’s pH value was w7.0. With a low material loading at
0.02 g/L, about 98% arsenic contaminationwas removed by the
2.50O4 nanoadsorbent before and after As(III) or As(V)
e a-Fe2O3 nanoadsorbent.
Fig. 8 e (a) Arsenic removal percentages on natural water samples from Lake Yangzonghai by the ultrafine Mg0.27Fe2.50O4
nanoadsorbent, compared with that by the a-Fe2O3 nanoadsorbent (material loading at 0.02 g/L). (b) Images of ultrafine
Mg0.27Fe2.50O4 and a-Fe2O3 nanoadsorbents dispersed in water w/o external magnetic field (NbFeB: a kind of rare earth
permanent magnet purchased from Shenyang Guo Yao Technology, Shenyang, P.R. China). (c) The arsenic removal
percentage of the regenerated ultrafine Mg0.27Fe2.50O4 nanoadsorbent on natural water samples from Lake Yangzonghai for
5 times.
wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 6 2 4e3 6 3 43632
ultrafine Mg0.27Fe2.50O4 nanoadsorbent, while only 28%
arsenic contamination was removed by the ultrafine a-Fe2O3
nanoparticles. Thus, the ultrafine Mg0.27Fe2.50O4 nano-
adsorbent could effectively remove both As(III) and As(V) from
natural water samples without pre-treatment (oxidation/pH
adjustment) and post-treatment pH adjustment to meet the
USEPA standard for arsenic in drinking water.
3.8. Magnetic separation and adsorbent regeneration/reuse
As demonstrated in Fig. 8b, the ultrafine Mg0.27Fe2.50O4 nano-
adsorbent could be easily separated from treatedwater bodies
at the presence of external magnetic field within 5 min, while
the ultrafine a-Fe2O3 nanoparticles were still dispersed well in
water under the same magnetic field. Thus, magnetic sepa-
ration could be a feasible way to remove the ultrafine
Mg0.27Fe2.50O4 nanoadsorbent from treated water, which is
more efficient than conventional approaches of centrifugation
or filtration. After the separation from water, arsenic species
adsorbed on the ultrafine Mg0.27Fe2.50O4 nanoadsorbent could
be desorbed by washing with 2 M NaOH solution. Over 90%
arsenic species could be desorbed, and the regenerated ul-
trafine Mg0.27Fe2.50O4 nanoadsorbent could be reused for
arsenic removal. Fig. 8c demonstrates the arsenic removal
percentage of the regenerated ultrafine Mg0.27Fe2.50O4 nano-
adsorbent on natural water samples from Lake Yangzonghai
for 5 times. After being regenerated for four times, the re-
generated ultrafine Mg0.27Fe2.50O4 nanoadsorbent could still
keep w70e80% of its arsenic removal capability, which is
desirable for its potential applications in real practice to
reduce the operation cost.
4. Conclusions
In summary, we successfully synthesized a super-
paramagnetic ultrafine magnesium ferrite (Mg0.27Fe2.50O4)
nanoadsorbent in a simple room-temperature precipitation
reaction followed with a hydrothermal process at a low tem-
perature of 150 �C. The in situ self-formed NaCl “cage”
confined the growth of Mg0.27Fe2.50O4 nanocrystallites to an
ultrafine size of w3.7 nm, which endowed a super-
paramagnetic behavior to these Mg0.27Fe2.50O4 nano-
crystallites. The ultrafine Mg0.27Fe2.50O4 nanoadsorbent had a
large BET specific surface area of w438.2 m2/g, and demon-
strated a superior arsenic adsorption performance on both
As(III) and As(V). Its adsorption capacities on As(III) and As(V)
were found to be no less than 127.4 mg/g and 83.2 mg/g,
respectively. The amounts of adsorbed As(III) and As(V)
reached 10.1 mg/g and 11.8 mg/g, respectively, at an equilib-
rium arsenic concentration of just 0.01 mg/L (the USEPA
standard for drinking water). The arsenic adsorption on the
ultrafine Mg0.27Fe2.50O4 nanoadsorbent followed the inner-
sphere complex mechanism, and abundant surface hydroxyl
groups played themajor role in its superior arsenic adsorption
performance. It could effectively remove arsenic contamina-
tion in natural water samples with the presence of large
amounts of competing ions, could be readily separated by
external magnetic field, and could be regenerated and reused
wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 3 6 2 4e3 6 3 4 3633
for arsenic removal. With further development, it may offer a
simple and effective single step treatment option to treat
arsenic contaminated water without the pre-/post-treatment
requirement for current industrial practice.
Acknowledgments
This study was supported by the National Natural Science
Foundation of China (Grant No. 51102246), the Knowledge
Innovation Programof Chinese Academy of Sciences (Grant No.
Y0N5711171), the Knowledge Innovation Programof Institute of
Metal Research, Chinese Academy of Sciences (Grant No.
Y0N5A111A1), and the Youth Innovation Promotion Associa-
tion, Chinese Academy of Sciences (Grant No. Y2N5711171).
Appendix A. Supplementary data
Supplementary data related to this article can be found, in the
onlineversion,athttp://dx.doi.org/10.1016/j.watres.2013.04.023.
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