Synthesis and characterization of magnetic mesoporous Fe3O4@mSiO2-DODGA nanoparticles for adsorption of 16 rare earth elements

Article abstract of DOI:10.1039/c8ra07762b

In this study, novel magnetic mesoporous Fe₃O₄@mSiO₂-DODGA nanoparticles were prepared for efficiently adsorbing and recycling REEs. Fe₃O₄@mSiO₂-DODGA was characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The adsorption behavior of Fe₃O₄@mSiO₂-DODGA was investigated by ICP-OES. The results showed that the content of DODGA in the adsorbent was 367 μmol g^-1. Fe₃O₄@mSiO₂-DODGA exhibited the highest adsorption rates for 15 REEs, except Tm, in a 2 mol L^-1 nitric acid solution. Among these elements, the adsorption rates for Nd, Sm, Eu, Dy, Ho, Yb, Lu, Y and Sc ranged from 85.1% to 100.1%. The desorption rates for all 16 REE ions reached their maximum values when 0.01 mol L^-1 EDTA was used as the eluent. The desorption rates for Nd, Ce, Sm, Eu, Ho, Yb, Lu, Y, and Sc were 87.7-99.8%. Fe₃O₄@mSiO₂-DODGA had high stability in 2 mol L^-1 HNO₃ and could be used five times without significant loss of adsorption capacity. Moreover, these nanoparticles had high selectivity, and their adsorption rate was not affected even in a high-concentration solution of a coexisting ion. Therefore, 8 REE ions (Nd, Sm, Eu, Ho, Yb, Lu, Y, and Sc) were selected for the study of adsorption kinetics and adsorption isotherm experiments. It was demonstrated that the values of Q_e (equilibrium adsorption capacity) for Nd, Sm, Eu, Ho, Yb, Lu, Y, and Sc were 14.28-60.80 mg g^-1. The adsorption of REEs on Fe₃O₄@mSiO₂-DODGA followed the pseudo-second-order kinetic model, Elovich model and Langmuir isotherm model, which indicated that the adsorption process of Fe₃O₄@mSiO₂-DODGA for REEs comprised single-layer adsorption on a non-uniform surface controlled by chemical adsorption. It was concluded that Fe₃O₄@mSiO₂-DODGA represents a new material for the adsorption of REEs in strongly acidic solutions.

Full text of DOI:10.1039/c8ra07762b

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Synthesis and characterization of magnetic
mesoporous Fe3O4@mSiO2–DODGA nanoparticles
for adsorption of 16 rare earth elements†
Cite this: RSC Adv., 2018, 8, 39149
ab
Jingrui Li,ab Aijun Gong,
Fukai Li,ab Lina Qiu,ab Weiwei Zhang,ab Ge Gao,ab
*
Yu Liuab and Jiandi Liab
In this study, novel magnetic mesoporous Fe3O4@mSiO2–DODGA nanoparticles were prepared for efficiently
adsorbing and recycling REEs. Fe3O4@mSiO2–DODGA was characterized by powder X-ray diffraction (XRD),
transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), Fourier transform infrared
spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The adsorption behavior of Fe3O4@mSiO2–
DODGA was investigated by ICP-OES. The results showed that the content of DODGA in the adsorbent was
367 mmol gꢀ1. Fe3O4@mSiO2–DODGA exhibited the highest adsorption rates for 15 REEs, except Tm, in
a 2 mol Lꢀ1 nitric acid solution. Among these elements, the adsorption rates for Nd, Sm, Eu, Dy, Ho, Yb, Lu, Y
and Sc ranged from 85.1% to 100.1%. The desorption rates for all 16 REE ions reached their maximum values
when 0.01 mol Lꢀ1 EDTA was used as the eluent. The desorption rates for Nd, Ce, Sm, Eu, Ho, Yb, Lu, Y, and
Sc were 87.7–99.8%. Fe3O4@mSiO2–DODGA had high stability in 2 mol Lꢀ1 HNO3 and could be used five
times without significant loss of adsorption capacity. Moreover, these nanoparticles had high selectivity, and
their adsorption rate was not affected even in a high-concentration solution of a coexisting ion. Therefore, 8
REE ions (Nd, Sm, Eu, Ho, Yb, Lu, Y, and Sc) were selected for the study of adsorption kinetics and adsorption
isotherm experiments. It was demonstrated that the values of Qe (equilibrium adsorption capacity) for Nd, Sm,
Eu, Ho, Yb, Lu, Y, and Sc were 14.28–60.80 mg gꢀ1. The adsorption of REEs on Fe3O4@mSiO2–DODGA
followed the pseudo-second-order kinetic model, Elovich model and Langmuir isotherm model, which
indicated that the adsorption process of Fe3O4@mSiO2–DODGA for REEs comprised single-layer adsorption
on a non-uniform surface controlled by chemical adsorption. It was concluded that Fe3O4@mSiO2–DODGA
represents a new material for the adsorption of REEs in strongly acidic solutions.
Received 18th September 2018
Accepted 2nd November 2018
DOI: 10.1039/c8ra07762b
rsc.li/rsc-advances
separation and purication of REEs are still the main technical
difficulties and need further industrial processes.
1. Introduction
Therefore, many efforts have been made to develop efficient
methods for extracting and purifying REEs, such as liquid–liquid
extraction,4,5 supported liquid extraction6 or solid-phase extrac-
tion7 procedures. These methods all rely on the association
constants between ligands and REEs. Liquid–liquid extraction is
the most widely used method. The most widely used ligands
include di(2-ethylhexyl)phosphoric acid (HDEHP, P204),8,9 2-eth-
ylhexylphosphonic acid mono(2-ethylhexyl) ester (HEHEHP,
P507),10 and tributyl phosphate (TBP).11 Among the obvious
disadvantages inherent in liquid–liquid extraction are the utili-
zation of large volumes of solvents in the extraction procedure
and the generation of undesired waste. Supported liquid extrac-
tion and solid-phase extraction are relatively environmentally
friendly. The combination of a suitable solid phase and effective
ligands represents one direction for the development of extrac-
tion methods for REEs. Inspired by the above description,
nanosized materials are among the candidates for the solid
phase. Nanosized materials (e.g., silica nanoparticles,12 carbon
nanotubes,13 and magnetic nanoparticles14,15) with a high surface
Rare earth elements (REEs) have many unique physical and
chemical properties, which are widely applied in some modern
technologies, including alloys,1 magnets,2 ceramics,3 networks and
communication devices. The outstanding properties mentioned
above make these devices smaller, lighter and faster in terms of
performance and enable devices to exhibit greater efficiency,
speed, performance and thermal stability. With the dramatic
increase in their global consumption, the requirement for pure
REEs has also risen over the past two decades. At present, although
the extraction of REEs on a large scale has been industrialized, the
aSchool of Chemistry and Biological Engineering, University of Science and Technology
Beijing, Beijing 100083, China. E-mail: Gongaijun5661@ustb.edu.cn; Fax: +86-10-
62334071; Tel: +86-10-82375661
bBeijing Key Laboratory for Science and Application of Functional Molecular and
Crystalline Materials, University of Science and Technology Beijing, Beijing 100083,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra07762b
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area ratio have been utilized in various elds.16 In particular, Gd, Td, Dy, Ho, Er, Tm, Yb, Lu, Y and Sc) were prepared by
magnetic nanoparticles are commonly considered for use as dissolving appropriate amounts of the corresponding Specpure
internal supporting materials in some elds for the following oxides in dilute HNO3. Working solutions were prepared daily
reasons: their easy preparation via solvothermal reactions and by appropriately diluting the stock solutions. NaCl, Na2SO4,
superparamagnetic features for fast separation from complex FeCl3, AlCl3, CuSO4, ZnCl2, NaAc, NaHCO3, NH4NO3, HNO3,
samples in practical applications.17 There are several kinds of H2O2, NH3$H2O, CH2Cl2, ethanol, carbamide, ethylene glycol,
magnetic solid-phase extraction material that are employed for acetic anhydride, oxalyl chloride and tetraethoxysilane (TEOS)
the adsorption of a single REE such as Eu(III) and Ce(III) in an were bought from Sinopharm Chemical Reagent Co., Ltd,
aqueous solution. These materials mainly include mesoporous China. Diglycolic acid, dioctylamine, cetyltrimethylammonium
SBA-15 modied with the Schiff base N,N0-bis(salicylidene)-1,2- bromide (CTAB), and aminopropyltriethoxysilane (APTES) were
ethylenediamine and decorated on Fe3O4 nanoparticles (SBA- purchased from Sigma-Aldrich Chemical Company. High-purity
15-BSEA-Fe3O4-NPs),18 a Ce(III) ion-imprinted polymer (Ce(III)-IIP) water (18.2 MU cm) obtained from a Milli-Q Element system
graed on Fe3O4 nanoparticles supported by SBA-15 mesopores (Millipore, Molsheim, France) was used directly throughout this
(Fe3O4@SBA-15-Ce(III)-IIP),19 a core–shell structured Fe3O4@- study. Plastic and glass containers and all other laboratory
carboxymethylcellulose magnetic composite (Fe3O4@CMC),20 an materials that could come into contact with the samples and
Fe3O4/sepiolite composite,21 an Fe3O4@cyclodextrin magnetic standards were stored in 10% (v/v) nitric acid for 24 h and
composite (Fe3O4@CD-MCs)22 and core–shell structured Fe3- rinsed with high-purity water.
O4@humic acid magnetic nanoparticles (Fe3O4@HA-MNPs).23
In addition, the ligand is also an important aspect of the
design of solid-phase extraction materials. Among the ligands
used for adsorbing REEs, N,N,N0,N0-tetraoctyldiglycolamide
(TODGA)24,25 is among the ligands commonly used for separation.
In this ligand, the functional group diglycolamide (DGA) plays an
important role in coordination with REE ions. Some derivatives
based on the DGA chemical structure have been synthesized and
used industrially for adsorbing REEs and exhibited excellent
enrichment and selectivity.26,27 Unfortunately, no previous work
has been published in which magnetic nanoparticles modied
with a diglycolamide ligand were used for the adsorption of
REEs. There have been reports on the synthesis of magnetic
nanomaterials modied with diglycolamide ligands, including
diglycolamic acid-functionalized chitosan-coated Fe3O4 (Fe@CS-
DGA),28 TODGA-coated magnetite nanoparticles (Fe3O4@-
TODGA),29 and DGA-methacrylate-coated Fe3O4 particles
(Fe3O4@MC-DGA).30 However, these materials were employed for
removing Pb(II) and extracting Am(III) and Pu(IV) in a nitric acid
solution, but not for the adsorption of REEs.
In this study, mesoporous magnetic Fe3O4@mSiO2–
DODGA nanomaterials were prepared by modifying the
surface of mesoporous Fe3O4 particles with a diglycolamide
ligand. In comparison with previously reported materials, 16
REEs other than the radioactive element Pm were used to
assess the adsorption properties of the nal products. Studies
of the inuence of coexisting ions and adsorption kinetics
demonstrated that Fe3O4@mSiO2–DODGA exhibited better
anti-interference performance and a faster adsorption equi-
2.2 Material preparation
Firstly, a magnetic recycling unit was constructed. Uniform Fe3O4
particles prepared via a solvothermal reaction (Scheme 1)31 were
chosen as the inner supporting materials, which were nanosized
and had a much larger specic surface area and super-
paramagnetic properties. Secondly, amine groups were used to
modify the surface of the Fe3O4 particles.32–34 Before this chemical
modication, an SiO2 layer was coated efficiently on the Fe3O4
core to protect the Fe3O4 core from corrosion by the strong acid in
the solution. A further mesoporous SiO2 layer was created on the
surface of Fe3O4@SiO2 to further modify the amino groups to
prepare Fe3O4@mSiO2@NH2. The mesoporous structure
increased the specic surface area of the material to enable
modication with a greater amount of amino groups.
In the structure of TODGA, both carbonyl groups are bound
to N,N-di(n-octyl)amine groups and have low reactivity. There-
fore, N,N-dioctyldiglycolic acid (DODGA-OH) was synthe-
sized.35,36 This compound has a highly active carboxylic acid
group that can be converted into an acyl chloride (DODGA-Cl)
and used to modify the magnetic core. In addition, DODGA-
OH retains half of the n-octyl substituents in TODGA, which
ensures that the functional groups are hydrophobic. Finally,
functional magnetic Fe3O4@mSiO2–DODGA nanomaterials
were obtained by modifying the diglycolamide functional unit
DODGA-Cl with the aminated Fe3O4@mSiO2@NH2 particles.
2.3 Characterization
librium process. Moreover, this highly selective Fe3O4@- Characterization by Fourier transform infrared (FT-IR) spec-
mSiO2–DODGA exhibited high stability and reusability and troscopy was performed with an FT-IR 8400S spectrometer
represents a new solid-phase extraction material for the (Shimadzu, Japan). Spectra were recorded over the range of
adsorption of rare earth elements under strongly acidic and 4000–400 cmꢀ1 in transmission mode, and 32 scans were
high-salinity conditions.
accumulated at a resolution of 4 cmꢀ1. Magnetic properties
were determined with a 7410 vibrating sample magnetometer
(VSM, Lake Shore Cryotronics, Inc., Ohio, USA). Morphological
observations of the nanoparticles were carried out with a JEM-
2010 high-resolution transmission electron microscope (JEOL,
2. Experimental
2.1 Reagents and materials
All reagents were of analytical grade unless otherwise noted. Tokyo, Japan). Thermogravimetric analysis (TGA) was con-
Stock solutions (1.000 g Lꢀ1) of 16 REEs (La, Ce, Pr, Nd, Sm, Eu, ducted using a Mettler Toledo TGA2 analyzer (Switzerland) from
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Scheme 1 Preparation of the functionalized Fe3O4 nanoparticles.
30 to 750 C at a heating rate of 10 C minꢀ1. X-ray diffraction shis reported as ppm (in CDCl3 and DMSO-d6 with TMS as an
(XRD) patterns were recorded in the range of 2q ¼ 20–80ꢁ by internal standard). Mass spectrometric analysis was carried out
step scanning using a Rigaku D/Max-2500 diffractometer using an LCQ Fleet mass spectrometer (Thermo Fisher). An ICP-
(Japan) with Cu Ka radiation. 1H NMR and 13C NMR spectra OES system (715-ES, Varian Medical Systems, USA) was used for
were recorded with a Bruker AV-400 spectrometer with chemical the determination of REEs.
ꢁ
ꢁ
Fig. 1 TEM images of hollow Fe3O4 (a and b), SAED pattern (c), and TEM images of Fe3O4@SiO2 (d) and Fe3O4@mSiO2 (e and f).
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2.4 Adsorption experiments
2.4.3 Stability of Fe3O4@mSiO2–DODGA in HNO3. Fe3-
O4@mSiO2–DODGA was soaked in HNO3 (2.0 mol Lꢀ1) for
different numbers of days. The adsorption rates for 16 rare
earth elements in mixed solutions were measured for different
immersion times of the adsorbent, and their differences were
compared.
2.4.1 Effect of the concentration of HNO3 on the adsorp-
tion of REEs. Batch selection experiments on REEs were carried
out in 10.0 mL glass tubes. To determine the optimum
concentrations of HNO3 for the adsorption of REEs, the inu-
ence of the concentration of HNO3 was examined rst. A series
of REE solutions (containing 16 REEs; the concentration of each
REE was 0.2 mg Lꢀ1) with different concentrations of HNO3
(0.01, 0.05, 0.10, 0.50, 1.0, 2.0, and 3.0 mol Lꢀ1) were prepared.
Then, 10 mg of Fe3O4@mSiO2–DODGA nanoparticles was
added to 10 mL of the REE solution under continuous stirring at
2.4.4 Reusability of Fe3O4@mSiO2–DODGA. To assess the
recyclability of Fe3O4@mSiO2–DODGA for the adsorption of
REEs, 5 cycles of adsorption–desorption tests were conducted.
For the desorption process, Fe3O4@mSiO2–DODGA loaded with
REEs was washed with deionized water and then immersed in
a 0.01 mol Lꢀ1 EDTA solution for 3 h aer it was separated from
the REE solution using a magnet. The adsorbent was washed
with deionized water 3 times prior to the next adsorption cycle.
2.4.5 Effect of coexisting ions on the adsorption of REEs.
The effect of coexisting ions was determined by adding the
corresponding salts of model ions (K+, Na+, Ca2+, Mg2+, Fe3+,
Al3+, Zn2+, Cu2+, SO42ꢀ, Clꢀ, and NO3ꢀ) to the solution of REEs.
2.4.6 Adsorption experiments. Fe3O4@mSiO2–DODGA (20
mg) was introduced into 100 mL solutions of single rare earth
elements with an initial concentration (C0) of 20 mg Lꢀ1. The
experiments were carried out in a water bath, and the solutions
ꢁ
25 C for 24 h. The sorbent was separated with a magnet, and
the content of rare earth ions in the supernatant was deter-
mined by ICP-OES.
2.4.2 Effect of the concentration of EDTA on the desorp-
tion of REEs. Multiple batches of Fe3O4@mSiO2–DODGA
nanoparticles loaded with rare earth ions were collected under
the same conditions. A series of EDTA solutions (0.002, 0.005,
0.01, 0.02, 0.03, and 0.04 mol Lꢀ1) were used to desorb REEs
from these Fe3O4@mSiO2–DODGA nanoparticles under
ꢁ
continuous stirring at 25 C for 3 h.
Fig. 2 XRD patterns (A), FT-IR spectra (B), TGA curves (C) and VSM curves (D) of Fe3O4 nanoparticles.
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crystal. The layer of SiO2 was coated by a sol–gel reaction on the
surface of the Fe3O4 particles. Under the same TEM character-
ization conditions as were used for the hollow Fe3O4 nano-
particles, the mean diameter of the Fe3O4@SiO2 particles
increased from 200 nm to 300 nm. Well-dened core–shell
structures can be clearly observed in a TEM image of Fe3O4@-
SiO2 particles (Fig. 1d). The core with dark contrast represented
an Fe3O4 particle; the outer layer with light contrast represented
the SiO2 shell. The outer layer of mesoporous silica was con-
structed by a sol–gel reaction with the help of the surfactant
CTAB. The outer mesoporous SiO2 layer was lighter in color and
looser in structure than the inner SiO2 layer (Fig. 1e and f). The
construction of the mesoporous structure on Fe3O4 particles
was carried out to increase the specic surface area of the
material. To characterize the mesoporous structure, a BET
experiment was performed. As shown in Fig. S9,† the pore
diameter of Fe3O4@mSiO2–DODGA was about 6 nm. Its specic
surface area was 707.181 m2 gꢀ1. In contrast, no pore structures
existed in Fe3O4@SiO2 (Fig. S10†). Its specic surface area was
16.284 m2 gꢀ1, which was much smaller than that of Fe3O4@-
mSiO2–DODGA.
The crystal structure of Fe3O4 was characterized by XRD. The
X-ray diffraction pattern of Fe3O4 nanoparticles prepared via the
solvothermal method exhibited six typical diffraction peaks
corresponding to (220), (311), (400), (422), (511) and (440)
planes. As shown in Fig. 2A, six peaks were observed by
analyzing the Fe3O4, Fe3O4@mSiO2 and Fe3O4@mSiO2–DODGA
nanoparticles, which indicated that the crystal structure of the
magnetite core was not changed by coating with the SiO2 or
mesoporous silica layer or chemical modication with DODGA.
To characterize the chemical compositions of the bare Fe3O4
nanoparticles, SiO2- and mesoporous silica-coated Fe3O4
nanoparticles, and DODGA-modied Fe3O4 nanoparticles, their
FT-IR spectra were recorded, as shown in Fig. 2B. Firstly, an
absorption band at 606 cmꢀ1 attributed to the characteristic
stretching vibrations of Fe–O bonds in Fe3O4 appeared in the
spectra of these nanoparticles, which indicated that the chem-
ical structure of Fe3O4 was not changed by chemical modica-
tion. Owing to the coatings of SiO2 and mesoporous silica layers
on the surface of the bare Fe3O4 nanoparticles, the strength of
Fig. 3 Effect of different concentrations of HNO3 on the adsorption
rate.
were stirred at 25 ꢁC. At certain time intervals (5, 10, 15, 30, 45,
60, 120, 240, 360, and 480 min), the sorbent was separated with
a magnet and the supernatant was analyzed to determine the
remaining concentration (Ct, mg Lꢀ1) using ICP-OES.
Adsorption isotherms were calculated via batch experiments.
The adsorbent was mixed with aqueous solutions containing
different concentrations of 8 REEs ranging from 1 to 20 mg Lꢀ1
and stirred at 298 K for 24 h. The sorbent was subsequently
separated with a magnet, and the supernatant was analyzed to
determine the remaining concentration (Ce, mg Lꢀ1) using ICP-
OES.
3. Results and discussion
3.1 Characterization of Fe3O4, Fe3O4@SiO2, and
Fe3O4@mSiO2–DODGA
The morphological features of the nanomaterial were charac-
terized by TEM to observe its specic shape and appearance
characteristics. As shown in Fig. 1, the hollow Fe3O4 nano-
particles had spherical shapes and small sizes with a mean
diameter of 200 nm (Fig. 1a and b). Single nanocrystals were
analyzed by SAED, as shown in Fig. 1c. The diffraction patterns
exhibited a distribution comprising a series of points, which
was consistent with a face-centred crystal pattern, and the
diffraction points corresponded to the (220) crystal planes of the
Table 1 Adsorption and desorption rates for 16 rare earth elements
under optimal conditions
Element
AR (%)
DR (%)
Element
AR (%)
DR (%)
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
17
52
44
86
85
90
58
46
17
94
45
88
96
93
64
43
Dy
Ho
Er
Tm
Yb
Lu
Y
93
95
65
65
98
100
95
88
67
98
48
83
98
100
99
93
Sc
Fig. 4 Effect of EDTA concentration on desorption rate.
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the Fe–O stretching band underwent a relative decrease.
Secondly, aer the SiO2 layer was coated, peaks appeared at
1089 cmꢀ1, which were attributed to the stretching vibrations of
Si–O–Si bonds. Thirdly, aer the nal modication with
DODGA, the appearance of two new peaks at 1730 cmꢀ1 and
2955 cmꢀ1 conrmed that DODGA was successfully covalently
bonded to the surface of Fe3O4@mSiO2, and these were attrib-
uted to the stretching vibrations of C]O and C–H bonds.
To quantify the DODGA present on the chemically modied
Fe3O4 nanoparticles, thermogravimetric analysis was used to
estimate the content of DGA ligands. As shown in Fig. 2C, in the
TGA curves of Fe3O4@mSiO2, Fe3O4@mSiO2@NH2 and Fe3-
O4@mSiO2–DODGA in the temperature range of 30–750 ꢁC
there was an obvious change in the loss of mass from Fe3-
O4@mSiO2–DODGA. Owing to the physical adsorption of water
by Fe3O4@mSiO2, there was a slight mass loss during the
heating process. At 750 ꢁC, the weight loss for Fe3O4@mSiO2
was 0.31 wt%. Aer the modication with amino groups and
DODGA ligands of the surface of the magnetic nanoparticles, an
obvious weight loss was detected. DODGA on the nanoparticles
began to decompose thermally at about 200 ꢁC. This process did
not stop until the temperature reached 500 ꢁC. Finally, the
weight loss for Fe3O4@mSiO2–DODGA at 750 ꢁC reached 21.3%,
whereas the weight loss for Fe3O4@mSiO2@NH2 was 8.2%.
According to the results of the TGA analysis, the weight
percentage of DODGA on the modied nanoparticles was about
13.1%, which indicated that the content of the DODGA ligand
Fig.
5 Adsorption rates of Fe3O4@mSiO2–DODGA after being
immersed in an HNO3 solution for different numbers of days.
Table
DODGA
2
Experimental results for reusability of Fe3O4@mSiO2–
Adsorption rate (%)
Element
1
2
3
4
La
Ce
Pr
99
100
99
100
100
99
99
99
100
100
99
99
99
100
99
100
95
95
95
97
95
96
95
93
96
95
94
96
96
95
95
96
92
92
92
94
92
92
92
90
92
92
90
93
94
92
93
93
90
89
90
93
89
90
89
89
89
90
88
91
91
89
91
92
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Y
on the modied adsorbent was approximately 367 mmol gꢀ1
.
The magnetic properties of magnetic nanoparticles are an
important aspect of their properties and ensure the rapid
separation of adsorbents from a complex matrix. VSM was
utilized to investigate the magnetic properties of Fe3O4, Fe3-
O4@mSiO2 and Fe3O4@mSiO2–DODGA nanoparticles. As
shown in Fig. 2D, because it was coated with SiO2 and DODGA
layers the saturation magnetization of Fe3O4 decreased from the
initial value of 58.5 emu gꢀ1 to 37.3 emu gꢀ1. This result for the
magnetic response of Fe3O4@mSiO2–DODGA nanoparticles
showed that they still possessed high magnetization and could
Sc
Table 3 Tolerance limits for coexisting ions
Coexisting ion
Tolerance limit mg Lꢀ1
Tolerance limit mg Lꢀ1
Tolerance limit mg Lꢀ1
Tolerance limit mg Lꢀ1
K+
5000
5000
2000
2000
50
50
50
150
100
200
200
—
—
—
—
50
—
—
50
67
—
—
—
—
18
—
—
5
—
5
—
Na+
1610
200
120
—
—
—
135
—
—
Ca2+
Mg2+
Fe3+
Co2+
Ni2+
Al3+
Zn2+
Cu2+
Ag+
5
—
—
—
—
5
—
—
—
—
19
50
100
—
2ꢀ
SO4
2000
5000
12 000
2000
This work
500
200
200
500
23
Clꢀ
ꢀ
NO3
HPO4
Reference
2ꢀ
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former materials. Firstly, magnetic Fe3O4 particles were chosen
as the inner supporting core. Their excellent super-
paramagnetic characteristics were benecial for their rapid
collection from real samples. Secondly, the nanosized dimen-
sions of this material guaranteed a large specic surface area for
chemical modication for further anchoring the functional
ligand DODGA. Thirdly, DODGA, which has a high coordination
ability for REEs, was chemically bonded to the Fe3O4 particles to
create a bifunctional adsorbent.
3.2.1 Effects of the concentration of HNO3. The nitric acid
concentration affected the formation of metal chelates and
played an essential role in the adsorption of REEs on Fe3O4@-
mSiO2–DODGA.
The adsorption rate (AR%) was calculated by:
AR% ¼ (C0 ꢀ Ce) ꢂ 100/C0
where C0 and Ce are the concentration of the REE in the
aqueous phase before and aer adsorption, respectively.
The adsorption results are shown in Fig. 3. When the
concentration of the HNO3 solution was 0.01 mol Lꢀ1, the
adsorbent displayed different adsorption capabilities for all 16
REEs. All the adsorption rates for each REE were low. With an
increase in the concentration of the HNO3 solution, the
adsorption rates for 15 REEs, other than Tm, all increased. The
adsorption rates of Fe3O4@mSiO2–DODGA for 9 REEs (namely,
Nd, Sm, Eu, Dy, Ho, Yb, Lu, Y and Sc) exceeded 80% when the
concentration of the HNO3 solution was 1.0 mol Lꢀ1. However,
when the concentration of nitric acid was increased to
3.0 mol Lꢀ1, the increase in the adsorption rate of the Fe3-
O4@mSiO2–DODGA nanoparticles for 9 rare earth elements was
Fig. 6 Adsorption kinetics of 8 REEs.
be rapidly and completely separated from the mixture by an
external magnetic eld within 30 s. These three kinds of
nanoparticle displayed no magnetic hysteresis and exhibited
superparamagnetic behaviour, which made them disperse
homogeneously without an external magnet and aggregate
quickly in the presence of
applications.
a magnet during practical
3.2 Adsorption experiments
The material, namely, Fe3O4@mSiO2–DODGA, that was
prepared and used in this study displayed some advantages over
Table 4 Equations of the kinetic models
Kinetic model
Equation
Parameter
Pseudo-rst-order37
Pseudo-second-order38
Intra-particle diffusion39
Elovich40
qt ¼ qe(1 ꢀ eꢀk t
)
qe (mg gꢀ1
)
Amount of REEs adsorbed at equilibrium time
Pseudo-rst-order reaction rate constant
Pseudo-second-order reaction rate constant
1
k1 (minꢀ1
)
k2 (g mgꢀ1 minꢀ1
)
t
1
1
¼
þ
t
2
qt
2k2qe
qe
ki (mg gꢀ1 minꢀ1/2
)
Intra-particle diffusion rate constant
Boundary layer diffusion effect
Initial adsorption rate constant
qt ¼ kit1=2 þ C
C
lnðabÞ ln t
a (mg gꢀ1 minꢀ1
)
)
qt ¼
þ
b (mg gꢀ1 minꢀ1
Parameter related to the surface coverage of the adsorbent
b
b
Table 5 Parameters of pseudo-first-order and pseudo-second-order models
Pseudo-rst-order
Pseudo-second-order
Element
R2
qe (mg gꢀ1
)
k1 (minꢀ1
)
R2
qe (mg gꢀ1
)
k2 (g mgꢀ1 minꢀ1
)
Nd
Sm
Eu
Ho
Yb
Lu
Y
0.9000
0.9755
0.7948
0.9561
0.9761
0.5491
0.9592
0.9106
57.36
26.65
35.39
16.43
33.16
39.29
15.61
13.85
0.0891
0.0665
0.1882
0.0927
0.1052
0.2851
0.1032
0.0881
0.9997
0.9998
1.0000
0.9997
0.9999
0.9998
0.9999
0.9997
61.76
28.01
37.04
17.30
34.60
42.19
16.45
14.56
282.73
123.03
56.79
61.47
90.69
84.57
51.42
53.58
Sc
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Table 6 Parameters of intra-particle diffusion and Elovich models
Intra-particle diffusion
Elovich
a
Element
R2
ki (mg gꢀ1 minꢀ1/2
)
C
R2
(mg gꢀ1 minꢀ1
)
b (mg gꢀ1 minꢀ1
)
Nd
Sm
Eu
Ho
Yb
Lu
Y
0.6592
0.5851
0.5898
0.4838
0.5001
0.8311
0.4985
0.4386
1.4109
0.7398
0.4675
0.3864
0.6897
0.4466
0.3384
0.3277
35.99
14.79
28.68
10.43
22.53
33.57
10.42
8.76
0.9612
0.9266
0.9298
0.8744
0.8899
0.9867
0.8782
0.8181
45.6711
17.5533
89.1700
12.38142
29.0774
284.1745
13.1118
13.0943
0.03711
0.06792
0.10771
0.12171
0.06873
0.12994
0.14076
0.14125
Sc
not obvious. During the adsorption procedure, a high concen- coordination affinities for trivalent actinide ions in comparison
tration of HNO3 could potentially cause damage to instruments. with trivalent lanthanide ions. The adsorption results shown in
Therefore, a 2.0 mol Lꢀ1 HNO3 solution was used in the Table 1 are in agreement with the above conclusion. Further-
following adsorption experiments. In these conditions, the more, according to the literature48,49 cited in the above research
adsorption rates for all 16 rare earth elements ranged from article, DODGA exhibited different adsorption behaviour for
17.3% to 100.1% (Table 1). Among these elements, the REEs when it was used in liquid–liquid extraction. As the
adsorption rates for Nd, Sm, Eu, Dy, Ho, Yb, Lu, Y and Sc ranged functional ligand for the adsorption of REEs, the anchoring of
from 85.12% to 100.10%.
DODGA on Fe3O4@mSiO2–DODGA made it display the same
Moreover, the ligand has two carbonyl groups and an ether adsorption properties. The difference in the amount of each
bond of which the oxygen atoms donate electrons to REE ions. It REE adsorbed by Fe3O4@mSiO2–DODGA proved the above
has been reported that47 nitrogen donor ligands have higher conclusion. Moreover, when each REE had the same
Fig. 7 Kinetic plots for Nd as a representative element: (a) pseudo-first-order, (b) pseudo-second-order, (c) intra-particle diffusion, and (d)
Elovich models.
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Table 7 Comparison of the adsorption capacities of Fe3O4@mSiO2–
DODGA for REEs with those of other magnetic adsorbents
concentration in the solution, the adsorption behaviour of each
REE on Fe3O4@mSiO2–DODGA was competitive. In particular, if
the same amount of Fe3O4@mSiO2–DODGA was used for
adsorption, the amount of each REE adsorbed was different.
3.2.2 Effects of EDTA concentration on desorption rate.
The REEs adsorbed on the Fe3O4@mSiO2–DODGA nano-
particles were recovered by eluting the nanoparticles with an
EDTA solution.
MSPE material
Element
Qmax (mg gꢀ1
)
Reference
SBA-15-BSEA-Fe3O4-NPs
Fe3O4@SBA-15-Ce(III)-IIP
Fe3O4@CMC
Ce(III)
Ce(III)
Eu(III)
Eu(III)
Eu(III)
Eu(III)
Pb(II)
Am(III)
Am(III)
Pu(IV)
Nd(III)
Sm(III)
Eu(III)
Ho(III)
Yb(III)
Lu(III)
Y(III)
49.00
87.42
42.24
30.85
12.69
10.56
70.57
—
18
19
20
21
22
23
28
29
30
Fe3O4/sepiolite
Fe3O4@CD-MCs
Fe3O4@HA-MNPs
Fe@CS-DGA
Fe3O4@MC-DGA
Fe3O4@TODGA
The desorption rate (DR%) was calculated by:
DR% ¼ Cd ꢂ 100/(C0 ꢀ Ce)
—
—
where Cd, C0 and Ce are the concentration of the REE eluted
from Fe3O4@mSiO2–DODGA and the concentrations of the REE
in the aqueous phase before and aer adsorption, respectively.
As shown in Fig. 4, when the concentration of EDTA was
0.01 mol Lꢀ1 the desorption rate for each REE ion reached its
maximum value. The desorption rates for some REEs (namely,
Nd, Ce, Sm, Eu, Ho, Yb, Lu, Y, and Sc) were greater than 85%
(Table 1). When the concentration of EDTA continued to
increase, the desorption rates for the REEs decreased. There-
fore, 0.01 mol Lꢀ1 EDTA was selected as the optimal concen-
tration in the following experiments.
3.2.3 Stability of Fe3O4@mSiO2–DODGA in HNO3. To
determine whether the Fe3O4@mSiO2–DODGA nanoparticles
could withstand contact with HNO3 for the adsorption of REEs,
the relative stability of Fe3O4@mSiO2–DODGA in an HNO3
solution was investigated. Fe3O4@mSiO2–DODGA nano-
particles were immersed in an HNO3 solution with a concen-
tration of 2 mol Lꢀ1 for different times (1, 2, and 3 days). Then,
their adsorption capacities for REEs were tested. As shown in
Fig. 5, aer 3 days the adsorption rate did not decrease signif-
icantly, which implied that Fe3O4@mSiO2–DODGA retained its
adsorption ability. This also indicated that Fe3O4@mSiO2–
DODGA was stable aer long-term contact with 2 mol Lꢀ1 HNO3
and could hence be used for the adsorption of rare earth
elements in a strongly acidic environment.
Fe3O4@mSiO2–DODGA
60.80
27.54
36.86
17.16
34.36
42.15
16.29
14.28
This work
Sc(III)
REEs were investigated. The effects of een kinds of ion
(namely, K+, Na+, Ca2+, Mg2+, Fe3+, Co2+, Ni2+, Al3+, Zn2+, Cu2+,
Ag+, SO42ꢀ, Clꢀ, NO3ꢀ, and HPO42ꢀ) on the adsorption capacity
were investigated. If the content of a coexisting ion was not
higher than the value in Table 3, the adsorption of REEs by
these nanoparticles would not be affected. By comparison with
the concentrations of coexisting ions in other works,18,19,23 it was
obvious that the Fe3O4@mSiO2–DODGA nanoparticles had
excellent anti-interference abilities and the potential to adsorb
rare earth ions in complex samples.
3.2.6 Adsorption kinetics. Considering both the adsorption
rate and the desorption rate of Fe3O4@mSiO2–DODGA for all 16
REEs, the nanoparticles exhibited a high binding capacity for 8
rare earth ions (Nd, Sm, Eu, Ho, Yb, Lu, Y and Sc), and could be
regenerated easily when EDTA was used as the eluent. There-
fore, these 8 REEs were selected for use in the following
adsorption kinetics and adsorption isotherm experiments.
3.2.4 Reusability of Fe3O4@mSiO2–DODGA for adsorption
of REEs. Aer the stability of Fe3O4@mSiO2–DODGA nano-
particles was tested, their reusability was also examined. The
adsorbent was regenerated with 0.01 mol Lꢀ1 EDTA 4 times, and
the adsorption effect aer regeneration was expressed as
a percentage of the original adsorption rate. As shown in Table
2, aer 4 adsorption–desorption cycles the Fe3O4@mSiO2–
DODGA nanoparticles retained 88.25–92.63% of their initial
adsorption rate for REEs.
Several kinds of material used for recycling REEs are listed in
Table S10.† The materials prepared with magnetic particles as
the inner core and a functional ligand are usually employed for
recycling one or two kinds of REE ion. Aer being used for 5 or
more cycles, these materials still exhibited high adsorption
abilities for REE ions, whereas Fe3O4@mSiO2–DODGA dis-
played great advantages over these materials.
3.2.5 Effects of coexisting ions on adsorption of REEs.
Coexisting ions are ubiquitous in real sample systems and
might have a negative effect on the adsorption of REEs. The
effects of coexisting cations and anions on the adsorption of
Fig. 8 Adsorption isotherms for the adsorption of REEs on Fe3O4@-
mSiO2–DODGA.
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Table 8 Equations of the adsorption isotherm models
Isotherm model
Langmuir41
Freundlich42
D-R43
Equation
Parameters
KL (L mgꢀ1
Qm (mg gꢀ1
Kf
)
Constant related to the free energy of adsorption
Langmuir monolayer adsorption capacity
Ce
qe
Ce
1
¼
þ
)
Qmax KLQmax
1
Constant indicative of the relative sorption capacity
Constant indicative of the heterogeneity factor
Maximum adsorption capacity in D-R model
Constant related to the adsorption energy
Constant related to the heat of adsorption
Polanyi potential, Temkin equilibrium binding constant
lnðQeÞ ¼ lnðKf Þ þ lnðCeÞ
n
n
ln Qe ¼ ln Qm ꢀ b32
Qm (mg gꢀ1
)
b (mol2 Jꢀ2
)
Temkin44
Qe ¼ BT ln KT + BT ln Ce
BT ¼ (RT)/bT
KT (L gꢀ1
)
The adsorption rate of a material for REEs is commonly nanoparticles. The third stage was retention of the adsorbate on
considered to be an important criterion for evaluating potential the active sites, which was considered to be a stage of instan-
applications of the adsorbent. The adsorption capacity (Q, mg taneous entrapment. This stage could be regarded as negligible.
gꢀ1) and the equilibrium adsorption capacity (Qe, mg gꢀ1) were
calculated according to the following equation:
The Elovich model was used to describe the adsorption
process on the surface of a non-uniform solid, which was
based on the theoretical assumption that adsorption
increased exponentially with the number of adsorption sites,
which described the kinetics of a chemical adsorption process.
The value of R2 of this model was about 0.900, which indicated
that the adsorption process of REEs tended to comprise
chemical adsorption on an uneven surface, which was
consistent with the structural characteristics of the meso-
porous layer of nanoparticles.
ðC0 ꢀ CtÞV
Q ¼
m
where C0 (mg Lꢀ1) is the initial concentration of REEs in the
aqueous phase; Ct (mg Lꢀ1) is the initial and remaining
concentration of REEs in the aqueous phase at different
sampling intervals; V (L) is the volume of the solution; and m (g)
is the weight of the adsorbent.
As shown in Fig. 6, the adsorption of REEs by Fe3O4@mSiO2–
DODGA could reach adsorption equilibrium in 100 min. It
could be calculated that the maximum adsorption capacity of
Fe3O4@mSiO2–DODGA was 60.80, 27.54, 36.86, 17.16, 34.36,
42.15, 16.29, and 14.28 mg gꢀ1 for Nd, Sm, Eu, Ho, Yb, Lu, Y,
and Sc, respectively.
On the basis of the data that were obtained, four kinetic
models were used to analyze the adsorption behaviour of Fe3-
O4@mSiO2–DODGA for the 8 REEs. According to the equations
in Table 4, the original data were processed and tted. The
corresponding kinetic constants, equilibrium capacities, and
correlation coefficients of each adsorption kinetics model are
listed in Tables 5 and 6, and the kinetic model equations are
listed in the ESI (Tables S1–S4†).
Kinetics plots were obtained by taking Nd as a representative
element and are shown in Fig. 7. The pseudo-rst-order kinetic
model and pseudo-second-order model indicated that adsorp-
tion was determined by diffusion and chemical adsorption,
respectively. Because the value of R2 of the pseudo-second-order
model for each REE was 0.999, this model was a better t than
the pseudo-rst-order kinetic model. This result indicated that
the adsorption of the 8 REEs on the Fe3O4@mSiO2–DODGA
nanoparticles mainly occurred in two ways, i.e., physical and
chemical adsorption. Chemical adsorption determined the
adsorption process.
The adsorption capacity of Fe3O4@mSiO2–DODGA for REEs
was mainly reected by the adsorbed amount. The total
amount of REEs adsorbed by the material was 249.44 mg gꢀ1
(Table 7). The highest adsorbed amount for a single REE was
60.8 mg gꢀ1 for Nd(III), which was close to the reported
amount.28 Other reported magnetic nanomaterials were used
for the adsorption of single rare earth elements, usually Eu or
Ce. In this study, the diglycolamide ligand was used to modify
the magnetic nanomaterials for the further investigation of
the adsorption properties for all 16 REEs and the adsorption
kinetics of 8 REEs. The maximum adsorption capacities (Qmax
)
for 8 REEs of Fe3O4@mSiO2–DODGA in a 2 mol Lꢀ1 nitric acid
solution are listed in Table 7. As shown, Qmax for Eu(III) was
36.86 mg gꢀ1. This value is higher than those of a series of
adsorbent materials such as an Fe3O4/sepiolite composite,21
Fe3O4@CD-MCs22 and Fe3O4@HA-MNPs.23 Fe3O4@mSiO2–
DODGA could be used as an effective adsorbent for the
adsorption of REEs.
Table 9 Isotherm parameters of Langmuir and Freundlich models for
the adsorption of REEs onto Fe3O4@mSiO2–DODGA
Langmuir
Element R2 Qmax (mg gꢀ1
Freundlich
R2
)
KL (L mgꢀ1
)
Kf
n
Aer data processing according to the intra-particle diffu-
sion kinetic model, the plot displayed three typical linear rela-
tionships. Each linear relationship corresponded to a stage of
the adsorption process. The rst stage was surface diffusion,
which explained the diffusion of REEs from the solution to the
surface of Fe3O4@mSiO2–DODGA. The second stage was intra-
particle diffusion of REEs between the Fe3O4@mSiO2–DODGA
Nd
Sm
Eu
Ho
Yb
Lu
Y
0.9327 68.96
0.9987 28.57
0.9981 40.16
0.9892 17.27
0.9911 38.75
0.9900 45.45
0.9919 17.60
0.9983 14.83
1.33
3.43
1.18
1.31
1.09
0.99
1.78
6.41
0.9972 31.79 2.18
0.9366 16.31 3.40
0.9838 26.21 6.48
0.9950 10.27 5.53
0.9828 18.55 1.29
0.9444 17.60 2.06
0.9025
0.9429
8.83 1.52
9.87 2.52
Sc
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Table 10 Isotherm parameters of D-R and Temkin models for the adsorption of REEs onto Fe3O4@mSiO2–DODGA
Temkin
D-R
Element
R2
BT
KT (L gꢀ1
)
R2
b (mol2 Jꢀ2
)
E (kJ mol ꢀ1
)
Nd
Sm
Eu
Ho
Yb
Lu
Y
0.8545
0.9702
0.9665
0.9805
0.9395
0.9650
0.8066
0.8240
9.08
3.89
7.73
2.24
8.06
7.91
3.48
2.30
63.97
153.23
16.24
101.69
12.33
18.79
17.40
82.71
0.9543
0.9574
0.9530
0.9603
0.9578
0.9569
0.9555
0.9666
8.001 ꢂ 10ꢀ9
1.137 ꢂ 10ꢀ8
2.801 ꢂ 10ꢀ8
1.200 ꢂ 10ꢀ8
3.197 ꢂ 10ꢀ8
2.000 ꢂ 10ꢀ8
4.001 ꢂ 10ꢀ8
1.600 ꢂ 10ꢀ8
15.81
13.26
8.45
12.91
7.91
10.00
7.07
11.18
Sc
of adsorption model (Table 8, namely, Langmuir, Freundlich,
Dubinin-Radushkevich (D-R) and Temkin) were utilized to
quantify the adsorption capacity and investigate the specic
adsorption characteristics of Fe3O4@mSiO2–DODGA. The
constants, parameters, and correlation coefficients of the 4
adsorption models are listed in Tables 9 and 10, and the kinetic
model equations are listed in the ESI (Tables S5–S8†).
3.2.7 Adsorption isotherms. With the aim of under-
standing the adsorption mechanism during the adsorption
process, it was necessary to carry out adsorption equilibrium
experiments.
As shown in Fig. 8, the equilibrium adsorption capacity of
Fe3O4@mSiO2–DODGA for REEs increased markedly with an
increase in the initial REE concentration. When the concen-
tration of REEs was 20 mg Lꢀ1, the adsorption capacity reached
its maximum value. This result indicated that the main driving
force generated by the initial concentration of REEs could
signicantly affect the mass transfer of REEs from the solution
to the surface of Fe3O4@mSiO2–DODGA. Therefore, four kinds
Upon comparing the R2 values of the Langmuir and
Freundlich models (Fig. 9), it was obvious that the adsorption
data were more suitable for tting with the Langmuir isotherm
model than with the Freundlich isotherm model, which
demonstrated that the adsorption of REEs by Fe3O4@mSiO2–
Fig. 9 Adsorption model plots for Nd as a representative element: (a) Langmuir, (b) Freundlich, (c) Temkin, and (d) D-R models.
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DODGA comprised single-layer adsorption. Furthermore, the
Langmuir dimensionless separation factor RL was used to
predict the affinity between Fe3O4@mSiO2–DODGA and REEs:
Conflicts of interest
There are no conicts to declare.
RL ¼ 1/(1 + KLC0)
Acknowledgements
The KL values for each REE are listed in Table S5.† Because
all the KL values for the 8 REEs were positive, all the RL values
were in the range of 0–1 for all the tested REE concentrations,
which indicated that the adsorption process for each REE on
Fe3O4@mSiO2–DODGA was favorable.
This work was nancially supported by the National Key R&D
Program of China (No. 2017YFB0702100) and the Major Science
and Technology Program for Water Pollution Control and
Treatment (No. 2017ZX07402001).
When the data were tted with the D-R model, the E values
were obtained from this model to explain that the adsorption
process was mainly dominated by chemical ion exchange or
physical forces.45,46 The E values for Yb and Y were less than
8 kJ molꢀ1, which indicated that physical forces dominated the
adsorption mechanism. The E values for the other REEs were
greater than 8 kJ molꢀ1, which suggested that chemical ion
exchange was the major adsorption mechanism.
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