Carbohydrate Polymers 114 (2014) 521–529

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

Simultaneous removal of Co(II) and 1-naphthol by core–shell

structured Fe3 O4 @cyclodextrin magnetic nanoparticles
Xiaobao Zhang a,b,1 , Yong Wang b,1 , Shitong Yang b,∗,1
a
Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 215123 Suzhou,
PR China
b
School for Radiological and Interdisciplinary Sciences, Soochow University, 215123 Suzhou, PR China

a r t i c l e i n f o a b s t r a c t

Article history: Herein, ␤-cyclodextrin (␤-CD) was introduced on the surfaces of Fe3 O4 particles via the chemical co-
Received 20 April 2014 precipitation approach. The as-prepared Fe3 O4 @CD MNPs can be easily separated from the aqueous phase
Received in revised form 19 July 2014 with a magnet. The removal performance of Fe3 O4 @CD MNPs toward Co(II) and 1-naphthol were investi-
Accepted 13 August 2014
gated by using the batch technique. The maximum sorption capacities of Fe3 O4 @CD MNPs toward Co(II)
Available online 2 September 2014
and 1-naphthol are higher than a series of adsorbent materials. The simultaneous removal of Co(II) and
1-naphthol is achieved via the binding of Co(II) on the external surface sites of Fe3 O4 @CD MNPs and the
Keywords:
incorporation of 1-naphthol into the hydrophobic cavity of surface-coated ␤-CD. The Fe3 O4 @CD MNPs
Fe3 O4 @CD MNPs
Co(II)
exhibit favorable removal performance toward Co(II) and 1-naphthol from the simulated effluent. The
1-Naphthol experimental results herein suggest that Fe3 O4 @CD MNPs can be used as cost-effective material for the
Simulated effluent purification of co-contaminated water systems.
Magnetic separation © 2014 Elsevier Ltd. All rights reserved.

1. Introduction of advantages such as simple device, easy operation, high secu-

rity, high efficiency, low cost and no secondary pollution. A series
With the rapid development of industrial and agricultural of adsorbent materials such as bentonite, fly ash, activated bamboo
production processes, various heavy metal ions and organic con- charcoal, carbon nanotubes, glycine-␤-cyclodextrin and so on have
taminants are discharged into the natural environment. Cobalt been tested for the simultaneous removal of inorganic and organic
(Co(II)) is commonly present in effluents due to its widely appli- pollutants (Diaz-Flores, López-Urías, Terrones, & Rangel-Mendez,
cation in mechanotronics, galvanization, alloy materials, batteries 2009; Jović-Jovičić et al., 2010; Ma, Wang, Huang, & Wang, 2010;
manufacturing, pigment processing, catalyst production, medical Visa, Bogatu, & Duta, 2010; Wang et al., 2010). However, the diffi-
diagnosis, and etc. The ionizable aromatic compounds (e.g., naph- culty for separating these materials from the aqueous phase limits
thol and naphthylamine) are widely present in the wastewaters their application potential in effluent purification. Recently, some
discharged from pesticides, dyestuffs, pharmaceuticals, petro- magnetic composites with high separation convenience have been
chemicals, and etc. The high toxicity and harmful effects of exploited to compensate for this deficiency (Chen et al., 2012a;
these pollutants on ecosystem and human heath have aroused Chen, Zhang, Wang, Li, & Guo, 2012b; Shen, Pan, Zhang, Huang,
widespread concern all over the world (Liu et al., 2003; Wang et al., & Gong, 2012; Singh, Barick, & Bahadur, 2013; Zhu et al., 2011).
2010). In view of this, the significant environmental priority nowa- For instance, Chen et al. (2012a) prepared a thiourea-modified ion-
days is to exploit advanced techniques and adsorbent materials imprinted chitosan/TiO2 (MICT) composite for the simultaneous
for the effective remediation of co-contaminated soil and water removal of Cd(II) and 2,4-dichlorophenol. The experimental results
systems. showed that the MICT composite exhibited a maximum sorption
Among the current techniques for environmental remediation, capacity of 256.41 mg/g toward Cd(II) and a degradation efficiency
the sorption approach has been extensively used due to a series of 98% toward 2,4-dichlorophenol. Singh et al. (2013) reported that
the prepared Fe3 O4 embedded ZnO nanocomposites (Fe3 O4 –ZnO
MSN) via a facile soft-chemical approach showed strong tendency
for the simultaneous removal of Co(II), Pb(II), Hg(II) and As(III)
∗ Corresponding author. Tel.: +86 512 65883945; fax: +86 512 65883945.
from wastewater. In addition, the Fe3 O4 –ZnO MSN exhibited high
E-mail address: shitongyang@suda.edu.cn (S. Yang).
1
Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Educa- photocatalytic activity for degradating organic dyes (rhodamine B,
tion Institutions. methyl orange and methylene blue) and also high efficiency for

http://dx.doi.org/10.1016/j.carbpol.2014.08.072
0144-8617/© 2014 Elsevier Ltd. All rights reserved.
522 X. Zhang et al. / Carbohydrate Polymers 114 (2014) 521–529

capturing bacteria pathogens. From the forgoing literatures, one cooled to room temperature. The Fe3 O4 @CD precipitates were col-
can conclude that magnetic composites exhibit favorable perfor- lected by using an external magnet. Afterward, the wet pastes were
mance for the simultaneous removal of toxic heavy metal ions and washed several times with ethanol and Milli-Q water and then
organic contaminants from aquatic systems. dried in vacuum oven. The control sample of bare Fe3 O4 was syn-
Cyclodextrin (CD) is a kind of torus-shaped cyclic oligosac- thesized in the similar way without the addition of ␤-CD. The zero
charide containing several ␣-1,4-linked d-glucopyranose units point charge (pHzpc ) values of bare Fe3 O4 and Fe3 O4 @CD MNPs are
with an internal hydrophobic cavity. CD possesses an excel- identified to be ∼6.8 and ∼4.0, respectively. Herein, the lower pHzpc
lent capacity to form inclusion complexes with properly sized value of Fe3 O4 @CD MNPs suggests that ␤-CD has been successfully
and structured organic molecules through host–guest interac- grafted on the Fe3 O4 surfaces. According to the N2 -BET method, the
tions. The three types of natural CDs (i.e., ␣-CD, ␤-CD and specific surface area of bare Fe3 O4 and Fe3 O4 @CD MNPs were mea-
␥-CD) and their derivatives (e.g., the randomly methylated-␤-CD sured to be 18.5 and 26.4 m2 /g, respectively. The FTIR spectra were
(RM␤CD), hydroxypropylated-␣-CD (HP␣CD), hydroxypropylated- collected and analyzed to identify the binding mechanism for the
␤-CD (HP␤CD), hydroxypropylated-␥-CD (HP␥CD) and sulfobutyl formation of Fe3 O4 @CD MNPs. The XRD patterns were recorded on
ether-␤-CD (SBE␤CD)) have been produced and used in food, a MAC Science Co. M18XHF diffractometer using Cu K␣ radiation.
cosmetics, catalysis, molecular recognition, drug delivery and envi- TEM images were performed on JEOL-2010 microscope to char-
ronmental protection fields (Badruddoza, Tay, Tan, Hidajat, & acterize the morphology and size distribution of bare Fe3 O4 and
Uddin, 2011; Chen, & Liu, 2010; del Valle, 2004; Fuhrer, Herrmann, Fe3 O4 @CD MNPs. The magnetic measurements were performed in
Athanassiou, Grass, & Stark, 2011; Loftsson, & Brewster, 2011; a MPMS-XL SQUID magnetometer.
Maturi, & Reddy, 2006; Morin-Crini, & Crini, 2013). Among these CD
products, ␤-CD is the most accessible, the lowest-priced and gener-
2.3. Experimental procedure
ally the most useful type (del Valle, 2004). Compared with the ␣-CD
and ␥-CD, the ␤-CD shows much better capacity for the removal
The sorption experiments were performed by using the batch
of phenol from water due to the optimal size matching (Chen
technique in amber EPA vials equipped with Teflon-lined screw
et al., 2012b). In view of this, ␤-CD was selected and introduced
caps. Briefly, the adsorbent suspensions, NaCl electrolyte solution,
on Fe3 O4 magnetic nanoparticles (MNPs) via the chemical co-
Co(II) and 1-naphthol stock solutions were added to achieve the
precipitation approach to prepare core–shell structured Fe3 O4 @CD
required concentrations of individual components. The pH values
MNPs. Herein, the use of natural CD instead of the derivatives
were adjusted to desired values by adding tiny amounts of HCl
can simplify the synthetic procedures and reduce the prepara-
or NaOH solutions. The obtained mixtures were gently oscillated
tion cost of Fe3 O4 @CD MNPs. The properties of bare Fe3 O4 and
for 24 h to achieve sorption equilibrium. Afterward, a permanent
Fe3 O4 @CD MNPs were characterized by using the Fourier transform
magnet was used to separate the solid from the liquid phases.
infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission
The concentration of Co(II) in the supernatants was measured
electron microscopy (TEM), elemental analysis and magnetization
by using the atomic absorption spectroscopy. The concentration
curves. Afterward, batch experiments were conducted to evaluate
of 1-naphthol was measured by using the UV–vis spectropho-
the removal performance of Fe3 O4 @CD MNPs toward Co(II) and 1-
tometer at 332 nm. To improve the determination sensitivity,
naphthol. The application potential of Fe3 O4 @CD MNPs for effluent
the supernatants were basified with 0.1 mol/L NaOH solution to
purification was further evaluated on the basis of the experimental
achieve the dissociation state of 1-naphthol. The sorption percent-
results.
age (S % = (C0 − Ce )/C0 × 100%), sorption amount (qe = (C0 − Ce )V/m)
and the distribution coefficient (Kd = qe /Ce × 100%) of Co(II) and 1-
2. Experimental details naphthol were calculated from the initial concentration (C0 ), the
final concentration (Ce ), the adsorbent mass (m) and the suspension
2.1. Materials and reagents volume (V).

The ␤-cyclodextrin (␤-CD), CoCl2 ·6H2 O, FeCl2 ·4H2 O and

3. Results and discussion
FeCl3 ·6H2 O chemicals were purchased from Sinopharm Chemical
Reagent Co. Ltd. (China). The analytical pure CoCl2 ·6H2 O was dis-
3.1. Characterization
solved in deionized water to obtain the Co(II) stock solution in
a concentration of 200 mg/L. The prepared Co(II) stock solution
Fig. S1 shows the FTIR spectra of bare Fe3 O4 , Fe3 O4 @CD MNPs
were diluted to the required concentrations in the following exper-
and pure ␤-CD. For bare Fe3 O4 (curve A), the peak at 586 cm−1 cor-
iments. The 1-naphthol powders were dissolved in a solution (pH
responds to the stretching vibration of Fe O bond. For Fe3 O4 @CD
∼6.5) containing 100 mg/L NaN3 to prepare the 1-naphthol stock
MNPs (curve B), the characteristic peak of the Fe O bond slightly
solution. Herein, the NaN3 was used as a biocide to prevent the
shifts to ∼580 cm−1 . This variation trend suggests that the Fe O
biodegradation of 1-naphthol (Oleszczuk, Pan, & Xing, 2009; Wang,
bond is involved in the formation of Fe3 O4 @CD MNPs. The spec-
Tao, & Xing, 2009b). In order to ensure complete dissolution, the 1-
trum of Fe3 O4 @CD MNPs (curve B) shows all the characteristic
naphthol concentration was restricted to below 50% of its water
peaks of pure ␤-CD (curve C) with a slight shift. This phenomenon
solubility.
suggests that the ␤-CD moieties have been successfully grafted on
the Fe3 O4 surfaces. Based on the foregoing analysis, the Fe3 O4 @CD
2.2. Preparation and characterization MNPs are formed via the interaction between the functional groups
of ␤-CD and the Fe O bond of the Fe3 O4 cores. As shown in Fig.
The bare Fe3 O4 and Fe3 O4 @CD MNPs were prepared by using S2, the XRD pattern of Fe3 O4 @CD MNPs is similar to that of bare
the chemical co-precipitation method. First, 10 g of FeCl2 ·4H2 O and Fe3 O4 . This phenomenon indicates that the surface coating of ␤-CD
14.5 g of FeCl3 ·6H2 O were dissolved in 200 mL Milli-Q water and does not alter the cubic phase of the Fe3 O4 cores. Detailed dis-
heated to 90 ◦ C. Afterward, 40 mL of ammonium hydroxide (25%) cussions on the FTIR spectra and XRD patterns are described in
solution and 100 mL of NaCl solution containing 1.2 g of ␤-CD were the Supporting information. Fig. S3A and B shows the TEM images
rapidly and sequentially added into the vessel. The mixture was of bare Fe3 O4 and Fe3 O4 @CD MNPs, respectively. It is clear that
vigorous stirred under nitrogen atmosphere for 50 min and then the bare Fe3 O4 particles form a mass of agglomeration, exhibiting
 X. Zhang et al. / Carbohydrate Polymers 114 (2014) 521–529 523

contact time. Herein, such a short time for reaching sorption equi-
librium suggests the occurrence of chemical complexation rather
than physical sorption (Wang, Dong, He, Chen, & Yu, 2009a). Specif-
ically, the fast sorption kinetics procedure is attributed to the rapid
migration of Co(II) from the solution onto the external sites of bare
Fe3 O4 and Fe3 O4 @CD MNPs.
Fig. 1B illustrates the sorption kinetics data of 1-naphthol on
bare Fe3 O4 and Fe3 O4 @CD MNPs. It is interesting to find that 1-
naphthol exhibits distinct sorption trends on the two materials.
Specifically, the sorption of 1-naphthol on bare Fe3 O4 increases
rapidly with increasing contact time and reaches equilibrium after
3 h. Herein, the unrestricted transport of 1-naphthol from the solu-
tion onto the surfaces of bare Fe3 O4 without diffusion resistance
is the acceptable interpretation for the observed sorption kinetics
trend (Zhang, Niu, Cai, Zhao, & Shi, 2010). In contrast, the sorption
kinetics of 1-naphthol on Fe3 O4 @CD MNPs contains two succes-
sive procedures, i.e., an initial fast rate followed by a subsequent
slow rate. The sorption process reaches equilibrium after a con-
tact time of ∼7 h. It is necessary to interpret the sorption trend
of 1-naphthol based on the structural configuration of Fe3 O4 @CD
MNPs. The surface-coated ␤-CD molecules possess a characteristic
truncated-cone structure with hydrophilic hydroxyl sites on the
external surfaces and hydrophobic cylindrical cavity in the interior
(Badruddoza et al., 2011). During the initial stage of contact time,
1-naphthol molecules tend to transfer rapidly from the solution
onto the external surfaces of Fe3 O4 @CD MNPs. With time going
on, the surface-attached 1-naphthol molecules gradually migrate
into the internal cavity due to the hydrophobic interaction, form-
ing strong inclusion complexes. Such slow diffusion will result in
a slow increase in the sorption amount at the later kinetics stage
(Al-Qunaibit, Mekhemer, & Zaghloul, 2005; Bhattacharyya & Gupta,
2008).
The required contact time to reach sorption equilibrium is sig-
nificant for assessing the application potential of an adsorbent
material in wastewater purification. Herein, the short sorption
kinetics procedure suggests that Fe3 O4 @CD MNPs can be poten-
Fig. 1. Sorption kinetics of Co(II) (A) and 1-naphthol (B) on bare Fe3 O4 tially used for continuous effluent purification. After reaching the
and Fe3 O4 @CD MNPs. T = 293 K, pH = 6.5, m/V = 0.5 g/L, CCo(II) = 10 mg/L, sorption equilibrium, the pollutant-loaded Fe3 O4 @CD MNPs can be
C1-naphthol = 50 mg/L, I = 0.01 mol/L NaCl.
easily recovered by using an external magnet and subsequently
regenerated for cyclic utilization. According to the sorption kinetics
no uniform shape and size. In contrast, the Fe3 O4 @CD MNPs dis- data, a shaking time of 24 h is chosen in the following experiments
perse uniformly in quasi-spherical shape with an average size of to ensure complete sorption equilibrium of Co(II) and 1-naphthol
∼200 nm. The obvious differences suggest that the surface-coated on Fe3 O4 @CD MNPs.
␤-CD moieties greatly enhance the dispersion of Fe3 O4 @CD MNPs As shown in Fig. 1, the sorption amounts of Co(II) and 1-naphthol
in solution. Elemental analysis suggests that the grafted amount of on Fe3 O4 @CD MNPs are higher than those on bare Fe3 O4 . Owing
␤-CD on the Fe3 O4 cores is 115 mg/g (i.e., 11.5% in w/w). In addition, to the surface coating of ␤-CD moieties, the specific surface area
the grafted amount of natural organic matter on the solid surfaces of Fe3 O4 @CD MNPs (26.4 m2 /g) is higher than that of bare Fe3 O4
can be quantitatively verified from the magnetization curves (Liu, (18.5 m2 /g). The surface-coated ␤-CD moieties contribute to the
Zhao, & Jiang, 2008; Yang, Zong, Ren, Wang, & Wang, 2012). As higher sorption performance of Fe3 O4 @CD MNPs toward Co(II) and
shown in Fig. S4, the saturation magnetization (Ms ) of bare Fe3 O4 1-naphthol. The TEM images suggest that ␤-CD coating can effi-
and Fe3 O4 @CD MNPs are 77.1 and 67.9 emu/g, respectively. The ciently reduce the aggregation of Fe3 O4 MNPs in solution (Fig.
decreased Ms value of 9.2 emu/g corresponds to a grafted ␤-CD con- S3B). Meanwhile, the surface-coated ␤-CD can also improve the
tent of ∼11.9% (w/w), which agrees well with that calculated from physiochemical stability of Fe3 O4 MNPs in solution. The enhanced
the elemental analysis (11.5% in w/w). Undoubtedly, the Ms value dispersion and stability of Fe3 O4 @CD MNPs can improve the avail-
of Fe3 O4 @CD MNPs is high enough for their separation from the ability of surface hydroxyl sites and the cylindrical cavity for
liquid phase by using a magnet (inset in Fig. S4). binding Co(II) and 1-naphthol.

3.2. Single-solute sorption systems 3.2.2. Effect of pH

Fig. 2 shows the sorption percentage of Co(II) on Fe3 O4 @CD
3.2.1. Effect of contact time MNPs as a function of pH in 0.01 mol/L NaCl electrolyte solution.
Fig. 1A shows the sorption kinetics data of Co(II) on bare It is clear that solution pH plays a significant role in Co(II) sorp-
Fe3 O4 and Fe3 O4 @CD MNPs. One can see that the sorption amount tion procedure. As shown in Fig. 2, the sorption of Co(II) increases
increases rapidly in the initial contact time of 2 h, and then keeps slowly from ∼20% to ∼25% as pH increases from 2.0 to 4.0, then
almost unchanged after 2 h. Note that the transitory time for sepa- increases sharply from ∼25% to ∼98% as pH increases from 4.0 to
rating bare Fe3 O4 and Fe3 O4 @CD MNPs from the aqueous solution 8.0, and finally maintains high level at higher pH range. Herein,
with an external magnet is ignored when calculating the total the observed sorption trend can be interpreted by the relative
524 X. Zhang et al. / Carbohydrate Polymers 114 (2014) 521–529

Fig. 2. Sorption of Co(II) on Fe3 O4 @CD MNPs as a function of solution pH. Inset: Rel-
ative proportion of Co(II) species in solution. T = 293 K, m/V = 0.5 g/L, CCo(II) = 10 mg/L,
I = 0.01 mol/L NaCl.
Fig. 3. Sorption of 1-naphthol on Fe3 O4 @CD MNPs as a function of solution pH.
Inset: Relative proportion of 1-naphthol species in solution. T = 293 K, m/V = 0.5 g/L,
distribution of Co(II) species in solution and the surface properties C1-naphthol = 50 mg/L, I = 0.01 mol/L NaCl.

of Fe3 O4 @CD MNPs in terms of surface charge and dissociation of

functional groups. The inset in Fig. 2 illustrates the relative propor-
1-naphthol species and the protonated Fe3 O4 @CD MNPs partly
tion of Co(II) species in solution as computed by Visual MINTEQ
impedes the insertion of 1-naphthol into the hydrophobic cavity
ver. 3.0 (Gustafsson, 2011). It is clear that Co(II) in solution is
of the surface-coated ␤-CD molecules. In the pH range of 4.0–9.0,
mainly present as the positively charged Co2+ species in the pH
no obvious electrostatic repulsion is present between the neutral
range of 2.0–8.0. The surfaces of Fe3 O4 @CD MNPs are positively
1-naphthol species and the deprotonated Fe3 O4 @CD surfaces with
charged below the pHzpc value of ∼4.0. The electrostatic repul-
negative charge. Consequently, the 1-naphthol molecules can be
sion between Co2+ ions and the protonated sites (i.e., SOH+2 ) on
easily wrapped into the hydrophobic cavity of the surface-coated
Fe3 O4 @CD surfaces results in the low Co(II) sorption percentage
␤-CD molecules, resulting in the sharp increase of sorption per-
in the pH range of 2.0–4.0. In contrast, the surfaces of Fe3 O4 @CD
centage. At pH > 9.0, the anionic 1-naphthol species are strongly
MNPs become negatively charged at pH > 4.0 due to the deproto-
repelled by the negatively charged Fe3 O4 @CD surfaces, leading to
nation reaction (i.e., SOH − H+ ⇔ SO− ). The electrostatic attraction
the reduction of sorption percentage in this pH range. In addi-
between the negatively charged binding sites on Fe3 O4 @CD surface
tion, the Fe3 O4 @CD MNPs are more hydrophilic at higher pH
and the positively charged Co2+ species enhances the formation of
values. This variation trend will increase the affinity of their surface
strong complexes. In addition, the deprotonated surface sites can
hydroxyl sites for water molecules via hydrogen-bond interaction.
improve the dispersion of Fe3 O4 @CD MNPs in solution. This phe-
The surface-retained water molecules can serve as the binding cen-
nomenon will increase the contact area between Co2+ ions and the
ters for other water molecules, leading to the formation of water
binding sites on Fe3 O4 @CD surfaces, which consequently enhances
molecule clusters on Fe3 O4 @CD surfaces (Fontecha-Cámara, López-
the sorption percentage of Co(II) at pH 4.0–8.0. The precipitation
Ramón, Álvarez-Merino, & Moreno-Castilla, 2007). These clusters
curve is illustrated to check whether the formation of hydrox-
hinder the close approach of 1-naphthol molecules to the surfaces
ide precipitates contributes to Co(II) sorption trend. One can see
of Fe3 O4 @CD MNPs and correspondingly decrease the sorption
that Co(II) in solution begins to form Co(OH)2 (s) precipitates at
percentage. Howsoever, the Fe3 O4 @CD MNPs still exhibit good
pH ∼8.3 (Fig. 2). Therefore, the abrupt increase of Co(II) sorption
removal performance toward 1-naphthol from both the acidic and
on Fe3 O4 @CD MNPs at pH < 8.0 is not attributed to the formation
alkaline solutions.
of Co(OH)2 (s) precipitates. At pH > 8.0, Co(II) is mainly present as
Co(OH)2 species with a proportion of CoCO3 and Co(OH)+ species
in solution (inset in Fig. 2). Hence, the high sorption percentage 3.2.3. Effect of ionic strength
of Co(II) on Fe3 O4 @CD MNPs at pH > 8.0 is attributed to the for- To disclose the underlying sequestration mechanisms, the sorp-
mation of Co(OH)2 and CoCO3 precipitates as well as some surface tion trends of Co(II) and 1-naphthol on Fe3 O4 @CD MNPs were
complexes. The experimental result herein suggests that Fe3 O4 @CD investigated in 0.001, 0.01, 0.1 and 0.7 mol/L NaCl solutions, respec-
MNPs possess favorable application potential for the purification of tively. The tested ionic strength range herein covers the salinities of
Co(II)-bearing effluents in a wide pH range. fresh water and seawater. As shown in Fig. 4A, the sorption of Co(II)
As shown in Fig. 3, the sorption percentage of 1-naphthol is greatly affected by ionic strength variation at pH < 4.0, while no
on Fe3 O4 @CD MNPs increases slowly from ∼15% to ∼25% as pH obvious difference can be observed at pH > 4.0. Herein, the ionic
increases from 2.0 to 4.0, then increases sharply from ∼25% to strength-dependent sorption trend at pH < 4.0 is attributed to the
∼85% as pH increases from 4.0 to 9.0, and afterward decreases cation exchange/outer-sphere surface complexation (Yang et al.,
with increasing pH values. This phenomenon can be interpreted 2010). This deduction is supported by the fact that the sorption
by the dissociation property of 1-naphthol in solution and the of Co(II) decreases with increasing ionic strength. At pH > 4.0, the
physicochemical property of Fe3 O4 @CD surface sites. As an ion- ignorable effect of ionic strength suggests the occurrence of inner-
izable aromatic compound, the 1-naphthol species is associated sphere complexation. Furthermore, the formation of Co(OH)2 (s)
with its dissociated constant (pKa ) and the solution pH. Below its precipitates may play an important role in the sequestration pro-
pKa value of 9.34, 1-naphthol is present as nondissociated species cess of Co(II) by Fe3 O4 @CD MNPs at high alkaline pH. The ionic
in neutral form, and above this value 1-naphthol is in present strength-independence of Co(II) sorption at pH > 4.0 suggests that
as dissociated species in anionic form (inset in Fig. 3). At pH Fe3 O4 @CD MNPs are feasible for the purification of Co(II)-bearing
2.0–4.0, the electrostatic repulsion between the nondissociated effluent under various salinity conditions.
 X. Zhang et al. / Carbohydrate Polymers 114 (2014) 521–529 525

Fig. 4. Sorption of Co(II) (A) and 1-naphthol (B) on Fe3 O4 @CD MNPs as a function Fig. 5. Sorption isotherms and model fittings of Co(II) (A) and 1-naphthol (B) on
of ionic strength. T = 293 K, m/V = 0.5 g/L, CCo(II) = 10 mg/L, C1-naphthol = 50 mg/L. Fe3 O4 @CD MNPs. Symbols represent the sorption isotherms, solid lines represent
the fitting of Langmuir model and dash lines represent the fitting of Freundlich
model. pH = 6.5, m/V = 0.5 g/L, I = 0.01 mol/L NaCl.

As shown in Fig. 4B, the sorption of 1-naphthol is not influenced

by ionic strength variation at pH < 9.0, while increases with increas- adsorbates, the surface properties of solid particles as well as the
ing ionic strength at pH > 9.0. As mentioned above, 1-naphthol type and concentration range of electrolyte solution.
is present as nondissociated species in neutral form at pH < 9.0
(see the inset in Fig. 3). Hence, its aqueous solubility and activ- 3.2.4. Sorption isotherms
ity coefficient are not affected by ionic strength variation due to Fig. 5A and B illustrates the sorption isotherms of Co(II) and
the low coordination ability of electronic ions with the nondisso- 1-naphthol on Fe3 O4 @CD MNPs at three different temperatures
ciated species (Chen, Chen, & Zhu, 2008; Yang, Wu, Jing, & Zhu, (viz. 293, 313 and 333 K). Both the sorption amounts of Co(II) and
2008). In view of this, the ionic strength-independent sorption 1-naphthol increase with increasing equilibrium concentration in
at pH < 9.0 arises from the inclusion reaction due to hydropho- solution. Herein, higher concentrations of Co(II) and 1-naphthol
bic effect. At pH > 9.0, repulsive electrostatic interaction occurs can provide greater driving force for overcoming the mass trans-
between the negatively charged Fe3 O4 @CD MNPs and the disso- fer limitation between the aqueous phase and the bulk phase of
ciated 1-naphthol species in anionic form. At higher ionic strength, Fe3 O4 @CD MNPs. In addition, higher concentrations can enhance
the electrostatic repulsion between these two negatively charged the collisions between Co(II)/1-naphthol and Fe3 O4 @CD MNPs,
moieties can be effectively reduced due to the screening effect resulting in the higher sorption amounts (Rathinam, Maharshi,
on the surface charge. This variation trend will create a favorable Janardhanan, Jonnalagadda, & Nair, 2010; Vimala & Das, 2009). Note
electrostatic environment for the close approach of 1-naphthol that both the sorption isotherms of Co(II) and 1-naphthol reveal the
to Fe3 O4 @CD MNPs. In addition, the increase of ionic strength is typical L shape. This phenomenon eliminates the occurrence of sur-
expected to reduce the solubility of 1-naphthol due to the “salt- face (co)precipitation, which is expected to induce an exponential
ing out” effect (i.e., the strong coordination ability of electronic increase of sorption amount with increasing equilibrium concen-
ions with the dissociated species), which enhances the hydropho- tration. For both Co(II) and 1-naphthol, the sorption isotherm is the
bic interaction and thus increases the sorption percentage. In a highest at 333 K and is the lowest at 293 K, suggesting the occur-
previous study, an increase in KCl concentration was reported to rence of endothermic reaction for Co(II) and 1-naphthol binding on
increase the uptake of herbicide diuron on activated carbon fiber Fe3 O4 @CD MNPs.
(Fontecha-Cámara et al., 2007). In contrast, the sorption affinities of 
To deduce the underlying sorption mechanisms, the sorption 
nonionic aromatic compounds to carbon nanotubes were not influ- isotherms are simulated with Langmuir qe = bqmax Ce /(1 + bCe )
enced by ionic strength variation (Chen et al., 2008; Pan, & Xing, and Freundlich (qe = KF Ce n ) models. In the equations, Ce is the resid-
2008). The differences between our study and the literatures are ual concentration of adsorbates in solution (mg/L); qe is the sorption
caused by various factors such as the physiochemical properties of amount after equilibrium (mg/g); qmax is the maximum sorption
526 X. Zhang et al. / Carbohydrate Polymers 114 (2014) 521–529

amount at complete monolayer coverage (mg/g); b is a parameter

that relates to the sorption heat (L/mg); KF represents the sorp-
tion capacity when the equilibrium concentration of adsorbates
equals to 1 (mg1−n /g Ln ) and n represents the sorption dependence
degree. The parameters derived from the model fittings are listed
in Table S1. One can see from the R2 values that Langmuir model
simulates the experimental data better than Freundlich model.
This result implies that chemisorption is the principal driving force
for Co(II) and 1-naphthol binding on Fe3 O4 @CD MNPs (Zhou, Nie,
Branford-White, He, & Zhu, 2009). The small n values (<1) sug-
gest that a nonlinear sorption process occurs on Fe3 O4 @CD MNPs.
Overall, the derived parameters indicate that the sorption of Co(II)
and 1-naphthol on Fe3 O4 @CD MNPs is a favorable and chemisorp-
tion process. Herein, the maximum sorption capacities (i.e., qmax )
of Fe3 O4 @CD MNPs toward Co(II) and 1-naphthol are carefully
compared with other adsorbents to further check the potential
of using this material in effluent purification. From Tables S2 and
S3, one can see that the qmax values of Co(II) and 1-naphthol on
Fe3 O4 @CD MNPs are higher than a series of adsorbent materi-
als. In view of this, Fe3 O4 @CD MNPs can be potentially used in
wastewater disposal with favorable environmental and economic
benefits. More detailed information on the comparation of max-
imum sorption capacities between Fe3 O4 @CD MNPs and other
materials is described in the Supporting information. The ther-
modynamic parameters (H◦ , S◦ , and G◦ ) (Table S4) derived
from the sorption isotherms suggest that the sorption processes
of Co(II) and 1-naphthol on Fe3 O4 @CD MNPs are spontaneous and
endothermic. Detailed information on the thermodynamic analysis
is described in the Supporting information.

3.3. Binary-solute sorption systems

3.3.1. Effect of 1-naphthol concentration on Co(II) sorption

Fig. 6A shows the sorption of Co(II) on Fe3 O4 @CD MNPs as a
Fig. 6. Effect of initial 1-naphthol and Co(II) concentration on the sorption of
function of initial 1-naphthol concentration. It is clear that Co(II) Co(II) (A) and 1-naphthol (B) on Fe3 O4 @CD MNPs. T = 293 K, pH = 6.5, m/V = 0.5 g/L,
sorption percentage increases with increasing 1-naphthol con- I = 0.01 mol/L NaCl.
centration at C1-naphthol < 80 mg/L, while decreases with increasing
1-naphthol concentration at C1-naphthol > 80 mg/L. In general, the
sorption of heavy metal ions on material surfaces can be influenced Co(II) on Fe3 O4 @CD MNPs. A previous study reported that the acid
by the coexisting organic components through various interaction blue 25 dye enhanced the sorption of Zn(II), Ni(II) and Cd(II) on
mechanisms. On one hand, the simultaneous organic matter can Ca(PO3 )2 -modified carbon (Tovar-Gómez et al., 2012). Similarly,
join the solid surface sites and the metal ions in solution. This tetracycline increased the sorption of Cu(II) on montmorillonite
interaction is expected to enhance the sorption of metal ions via due to the stronger affinity of tetracycline-Cu(II) complexes to
the formation of ternary surface complexes (Polubesova, Zadaka, montmorillonite (Wang et al., 2008). However, some dissolved
Groisman, & Nir, 2006). On the other hand, the coexistent organic organic matters significantly reduced the sorption capacities of
matter can compete with metal ions for surface binding sites and Cd(II), Zn(II) and Cu(II) on soils (Ashworth, & Alloway, 2007; Wong,
also complete with surface binding sites for dissolved metal ions Li, Zhou, & Selvam, 2007). The foregoing similarities and differences
(MacKay, & Canterbury, 2005; Wang, Jia, Sun, Zhu, & Zhou, 2008). are attributed to various factors such as the surface properties of
These two interactions will reduce the sorption of metal ions on solid particles, the chemical species of metal ions, the nature of
solid surfaces. Moreover, the dissociation of organic matter can organic matters as well as the solution chemistry conditions.
change the solution pH, which will further influence the physi-
cochemical properties of solid particles, the species of metal ions 3.3.2. Effect of Co(II) concentration on 1-naphthol sorption
and accordingly the sorption of metal ions (Zhou, Wang, Cang, Fig. 6B shows the sorption of 1-naphthol on Fe3 O4 @CD MNPs
Hao, & Luo, 2004). As discussed above, 1-naphthol is captured by as a function of initial Co(II) concentration in solution. One can see
incorporating the hydrophobic phenyl groups into the cavity of that the variation of initial Co(II) concentration exhibits no obvi-
surface-coated ␤-CD molecules, keeping the hydrophilic pheno- ous influence on 1-naphthol sorption. Generally, the coexisting
lic hydroxyl groups outside the cavity. As the initial concentration metal ions can bridge the organic matters and the solid surface
of 1-naphthol increases, more phenolic hydroxyl groups are avail- sites, compress the electric double layer, neutralize the negative
able as active sites to form complexes with Co(II) ions, resulting in charges of organic matters and thereby influence the sequestration
the increase of Co(II) sorption percentage. This experimental phe- of organic matters on solid particles (Hyung, & Kim, 2008; Lu, & Su,
nomenon also points to the formation of type B ternary complexes, 2007). However, the experimental phenomenon herein suggests
i.e., the “ligand-bridging” Fe3 O4 @CD-naphthol-Co(II). However, at that the Co(II) ions in solution are incapable to obstruct the inclu-
higher 1-naphthol concentration, the cavity numbers of the ␤-CD sion of 1-naphthol into the ␤-CD cavities via hydrophobic effect.
molecules are not adequate for the incorporation of 1-naphthol. This result also eliminates the potential formation of type A ternary
The remaining 1-naphthol forms complexes with Co(II) in solution complexes, i.e., the “metal-bridging” Fe3 O4 @CD-Co(II)-naphthol. If
and thereby competitively diminishes the sorption percentage of Co(II) and 1-naphthol were simultaneously retained on Fe3 O4 @CD
 X. Zhang et al. / Carbohydrate Polymers 114 (2014) 521–529 527

MNPs to form type A ternary complexes, the sorption percentage

of 1-naphthol would increase with increasing Co(II) concentration.
Alternatively, one may deduce the formation of type B ternary
complexes, i.e., the “ligand-bridging” Fe3 O4 @CD-naphthol-Co(II).
By integrating the effect of 1-naphthol concentration on Co(II) sorp-
tion (Fig. 6A), one can conclude that Fe3 O4 @CD MNPs can be used
for the simultaneous removal of Co(II) and 1-naphthol in a wide
concentration range.

3.3.3. Effect of addition sequence

The real aquatic system is a complicated and multicomponent
environmental medium. The addition sequences of heavy metal
ions, organic pollutants and the solid materials significantly influ-
ence their sorption trends and species distribution (Floroiu, Davis,
& Torrents, 2001). In view of this, the sorption behaviors of Co(II)
and 1-naphthol on Fe3 O4 @CD MNPs were investigated as a function
of solution pH for the following four sequences: (1) the Fe3 O4 @CD
MNPs, Co(II) and 1-naphthol stock solutions were simultaneously
added into the vials and the resulting mixtures were gently oscil-
lated for 24 h (denoted as batch 1); (2) the Fe3 O4 @CD MNPs and
Co(II) stock solution were pre-equilibrated for 24 h before the
addition of 1-naphthol stock solution (denoted as batch 2); (3)
the Fe3 O4 @CD MNPs and 1-naphthol stock solution were pre-
equilibrated for 24 h before the addition of Co(II) stock solution
(denoted as batch 3); and (4) the Co(II) and 1-naphthol stock
solutions were pre-equilibrated for 24 h before the addition of
Fe3 O4 @CD MNPs (denoted as batch 4). The test herein is of great
significance to check the reversibility of the sorption process and
assess the application potential of Fe3 O4 @CD MNPs in wastewater
disposal.
One can see from Fig. 7A that the sorption of Co(II) on Fe3 O4 @CD
MNPs exhibits discrepant trends for the four different addition
sequences. Specifically, the sorption percentage shows an order
of batch 1 ≈ batch 3 > batch 2 > batch 4. For batch 1 and batch 3, Fig. 7. Effect of addition sequence on Co(II) (A) and 1-naphthol (B) sorption
on Fe3 O4 @CD MNPs. T = 293 K, m/V = 0.5 g/L, CCo(II) = 10 mg/L, C1-naphthol = 50 mg/L,
the Co(II) ions can be sequestrated on the surface hydroxyl sites of
I = 0.01 mol/L NaCl.
Fe3 O4 @CD MNPs and the phenolic hydroxyl groups of 1-naphthol
outside the cavity of surface-coated ␤-CD moieties. For batch 2, the
subsequently added 1-naphthol is expected to desorb some Co(II) batch 1 and batch 3 are higher than those in batch 2 and batch
ions that have been pre-adsorbed on the surfaces of Fe3 O4 @CD 4. To improve the removal efficiency of Fe3 O4 @CD MNPs toward
MNPs. The formation of 1-naphthol-Co(II) complexes in solution Co(II) and 1-naphthol, one should mingle the Fe3 O4 @CD MNPs,
would lead to the slight decline of Co(II) sorption percentage. For Co(II) and 1-naphthol stock solutions simultaneously or premix the
batch 4, the Co(II) in solution is present in the form of soluble 1- Fe3 O4 @CD MNPs and 1-naphthol stock solution before the addition
naphthol-Co(II) complexes instead of the hydrated ions. Compared of Co(II) stock solution.
with the hydrated Co(II) ions, the soluble 1-naphthol-Co(II) com-
plexes with hydrophobic phenyl groups are more difficult to bind
on the hydrophilic surface sites of Fe3 O4 @CD MNPs. This is the ten- 3.4. Simulated effluent purification
tative interpretation for the decrease of Co(II) sorption percentage.
As shown in Fig. 7B, the sorption percentage of 1-naphthol on Owing to the heterogeneity and complexity of aquatic envi-
Fe3 O4 @CD MNPs exhibits an order of batch 1 ≈ batch 3 ≈ batch ronment, the removal performance of Fe3 O4 @CD MNPs toward a
4 > batch 2. For batch 1, batch 3 and batch 4, the hydrophobic phenyl simulated effluent was further tested to corroborate the research
groups of 1-naphthol can easily incorporate into the hydropho- findings obtained from the foregoing experiments. To better reflect
bic cavity of surface-coated ␤-CD molecules, leading to similar the heterogeneous water system, the simulated effluent consists
1-naphthol sorption trends in these three systems. For batch 2, the of 10 mg/L of Cd(II), 10 mg/L of Co(II), 5 mg/L of phosphate, 5 mg/L
surface hydroxyl sites of Fe3 O4 @CD MNPs have been occupied by of As(V), 30 mg/L of methylene blue (MB), 50 mg/L 1-naphthol,
the Co(II) ions before the addition of 1-naphthol. The hydrophilic 10 mg/L of humic acid (HA) and 0.01 mol/L NaCl electrolyte solu-
environment caused by the hydration layer of adsorbed Co(II) ions tion. Typical experiment was performed by adding 200 mL of the
is unfavorable for the sequestration of hydrophobic 1-naphthol. In simulated effluent into a 500 mL beaker. The subsequent procedure
addition, the surface-adsorbed Co(II) ions may reduce the size of the was conducted at pH 6.5 via a similar approach as that in batch
␤-CD cylindrical cavity, which will disturb the host–guest consis- experiments. The analysis results show that the removal percent-
tency and further hinder the inclusion of hydrophobic 1-naphthol. ages of Fe3 O4 @CD MNPs toward Cd(II), Co(II), phosphate, As(V),
Hence, the sorption percentage of 1-naphthol on Fe3 O4 @CD MNPs MB, 1-naphthol and HA are 52%, 57%, 45%, 42%, 50%, 59% and 41%,
in batch 2 is lower than those in the other three addition sequences. respectively. Note that Fe3 O4 @CD MNPs still exhibit good removal
According to the above-mentioned experimental results and performance toward Co(II) and 1-naphthol in the simulated efflu-
analysis, it is clear that the addition sequence of different compo- ent. The results herein can provide more accurate experimental
nents can influence the removal performance of Fe3 O4 @CD MNPs. parameters for evaluate the application potential of Fe3 O4 @CD
Herein, the sorption percentages of both Co(II) and 1-naphthol in MNPs in the actual wastewater treatment.
528 X. Zhang et al. / Carbohydrate Polymers 114 (2014) 521–529

Fig. 8. Schematic illustration for the simultaneous removal of Co(II) and 1-naphthol by Fe3 O4 @CD MNPs.

4. Conclusions synthesis approach, environmental friendliness, favorable removal

efficiency and convenient separation performance, Fe3 O4 @CD
The present study reports the preparation of core–shell struc- MNPs can exhibit great application potential in environmental
tured Fe3 O4 @CD MNPs by using a simple chemical co-precipitation protection field.
method. The raw materials (i.e., iron salts and ␤-CD) used for the
synthesis of Fe3 O4 @CD MNPs are abundant and harmless to the
environment. Consequently, the as-prepared Fe3 O4 @CD MNPs are Acknowledgements
low-cost and eco-friendly. The high dispersion and plentiful bind-
ing sites of Fe3 O4 @CD MNPs contribute to their favorable removal Financial support from the National Natural Science Founda-
performance toward Co(II) and 1-naphthol. The experimental tion of China (No. 41203086), the Jiangsu Provincial Key Laboratory
results derived from the binary-solute system and the simulated of Radiation Medicine and Protection and the Priority Academic
effluent suggest that Fe3 O4 @CD MNPs can support the simulta- Program Development of Jiangsu Higher Education Institutions are
neous removal of Co(II) and 1-naphthol via the binding of Co(II) acknowledged.
on the external surface sites and the incorporation of 1-naphthol
into the hydrophobic cavity of surface-coated ␤-CD molecules Appendix A. Supplementary data
(Fig. 8). According to the disparate sorption trends for differ-
ent addition sequences, one should adopt appropriate addition Supplementary data associated with this article can be
sequence to improve the removal efficiency of Fe3 O4 @CD MNPs. found, in the online version, at http://dx.doi.org/10.1016/j.carbpol.
By considering the abundant and low-cost raw materials, simple 2014.08.072.
 X. Zhang et al. / Carbohydrate Polymers 114 (2014) 521–529 529

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