Preparation of terpyridine-functionalized paramagnetic nickel-zinc ferrite microspheres for adsorbing Pb(II), Hg(II), and Cd(II) from water

Article abstract of DOI:10.1039/d0ra06746f

A paramagnetic microsphere combining special functional groups may be one kind of the most promising methods for heavy metal adsorption, due to their specific separation capacity, selectivity and reusability. In this study, a novel terpyridine-based magnetic solid-phase adsorbent (TPY-M) is successfully constructed. The paramagnetic Ni0.25Zn0.75Fe2O4 microsphere (M) is synthesized and applied as a magnetic core, and is functionalized by terpyridine (TPY) groups. The naked magnetic core and TPY-M are characterized by vibration sample magnetism (VSM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and Fourier-transform infrared spectroscopy (FT-IR) techniques. Some parameters of the TPY-M samples are evaluated as potential adsorbents for heavy metal ions in various aqueous solutions. The adsorption capacities of TPY-M for Pb(ii), Hg(ii) and Cd(ii) were 64.75 mg g-1, 33.94 mg g-1 and 24.64 mg g-1 under given conditions, respectively. In the case of Pb(ii), some influencing factors on the TPY-M adsorbent are investigated, including the pH, adsorption time, and ion concentrations. The adsorbent can be easily regenerated by HCl solution after use. The adsorbent revealed good adsorption performance in some real water samples. This journal is

Full text of DOI:10.1039/d0ra06746f

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PAPER
Preparation of terpyridine-functionalized
paramagnetic nickel–zinc ferrite microspheres for
adsorbing Pb(II), Hg(II), and Cd(II) from water†
Cite this: RSC Adv., 2020, 10, 39468
*
Jie Ma,
Huiling Wang, Manman Zhang, Denghui Li, Lian Liu and Honggao Yang
A paramagnetic microsphere combining special functional groups may be one kind of the most promising
methods for heavy metal adsorption, due to their specific separation capacity, selectivity and reusability. In
this study, a novel terpyridine-based magnetic solid-phase adsorbent (TPY-M) is successfully constructed.
The paramagnetic Ni0.25Zn0.75Fe2O4 microsphere (M) is synthesized and applied as a magnetic core, and is
functionalized by terpyridine (TPY) groups. The naked magnetic core and TPY-M are characterized by
vibration sample magnetism (VSM), X-ray diffraction (XRD), scanning electron microscopy (SEM),
transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and Fourier-transform
infrared spectroscopy (FT-IR) techniques. Some parameters of the TPY-M samples are evaluated as
potential adsorbents for heavy metal ions in various aqueous solutions. The adsorption capacities of TPY-M
for Pb(II), Hg(II) and Cd(II) were 64.75 mg gꢀ1, 33.94 mg gꢀ1 and 24.64 mg gꢀ1 under given conditions,
respectively. In the case of Pb(II), some influencing factors on the TPY-M adsorbent are investigated,
including the pH, adsorption time, and ion concentrations. The adsorbent can be easily regenerated by HCl
solution after use. The adsorbent revealed good adsorption performance in some real water samples.
Received 5th August 2020
Accepted 12th October 2020
DOI: 10.1039/d0ra06746f
rsc.li/rsc-advances
Magnetic materials, applied in the separation eld, show
some advantages of low cost, energy conservation and facile
1 Introduction
separation. The properties of magnetic nanomaterials can be
controlled by synthetic methods and material composition,22,23
which makes them have good application prospects in various
elds.24–27 Recently, the bimetallic ferrites AxB1ꢀxFe2O4 (A, B as
divalent metal ions) have attracted extensive attention due to
their controllable morphologies and magnetism by changing
the types and relative contents of A and B.28 Ni0.25Zn0.75Fe2O4 is
an ideal material for preparing magnetic adsorbents due to its
excellent paramagnetic properties. The solvothermal method is
one of the most commonly used methods for the synthesis of
nanomaterials;29,30 the high temperature and pressure envi-
ronment in a solvothermal system is conducive for obtaining
magnetic nanoparticles with good crystallinity, high specic
magnetism, narrow particle distribution, and good dispersion
and shapes.
Researchers oen modify the surface of materials to improve
their acid/base resistance, adsorption, and selectivity.31–33 The
adsorption interaction between metal ions and functional
groups mainly include ionic bonds and covalent bonds.34 To
enhance the adsorption capacity of ions, we designed
a magnetic adsorbent with terpyridyl groups. Previous studies
have shown that the three nitrogen atoms of the terpyridyl
groups can form a stable chelate with some metals, which has
a stronger chelating force than ordinary coordination. Recently,
terpyridine-based compounds have been widely used in uo-
rescent molecular probes,35 but their application in metal ion
It is well known that heavy metal ions in water pose a serious
threat to public health and lives.1–3 They may cause serious
health problems via transmission through the food chain and
accumulation in the body.4,5 In seawater, heavy metal ions are
also extremely harmful to marine ecosystems.6,7 Therefore,
people have paid more attention to the detection and disposal
of heavy metal ions in various water settings. Many methods
have been established for separating and removing toxic metal
ions from waste-water, which include the sedimentation
method,8 membrane ltration method,9 phytoremediation
method,10 occulation method,11,12 ion-exchange method13,14
and the adsorption method. Among them, the adsorption
method is widely applied with simple operation and low energy
consumption. However, the traditional adsorption materials,
such as activated carbon,15 zeolite,16 resin, clay,17 chitosan18 and
multi-walled carbon nanotubes,19 are difficult to separate and
reuse aer use, which restricts its massive application in water
treatment. In recent years, the functionalized magnetic nano-
materials have aroused great interest due to their unique
magnetic properties, which could solve the two aforementioned
tough problems of traditional adsorbents.20,21
School of Science, University of Shanghai for Science and Technology, Shanghai,
200093, P. R. China. E-mail: majie0203ch@hotmail.com; majie@usst.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06746f
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removal has not been reported. In this paper, Ni0.25Zn0.75Fe2O4
paramagnetic microspheres (M) are functionalized, for the rst
time, by terpyridyl groups. We have prepared a novel magnetic
adsorption material (TPY-M) to separate heavy metal ions from
solution.
2.2 Synthesis and characterization
The schematic shown in Scheme 1 describes the preparation
procedures of the adsorbent. According to our previous
research,28 a nickel–zinc ferrite (denoted as Ni0.25Zn0.75Fe2O4)
nanomaterial (M) was prepared via the same solvothermal
method. In a typical procedure, 2 mmol FeCl3$6H2O, 0.75 mmol
ZnCl2, and 0.25 mmol NiCl2$6H2O were fully dissolved in 40 mL
of ethylene glycol in the autoclaved vessel, and then 30 mL of
ethylene glycol (containing 5 mmol sodium acetate) was quickly
added and stirred for 30 minutes. The sealed autoclave was
placed in a 200 ꢁC oven for 12 hours and then cooled naturally
to room temperature. The black solids were collected with an
external magnet and thoroughly cleaned with DI water and
ethanol to obtain the Ni0.25Zn0.75Fe2O4 microspheres. Secondly,
2 g M was dispersed into 100 mL of ethanol–water mixed solvent
(4 : 1 v/v), then 20 mL TEOS–ethanol solution (1 : 9 v/v) and
1.0 mL ammonium hydroxide (25%) were added dropwise
under mechanical stirring at 45 ꢁC. Aer stirring for 2 hours, the
precipitate was separated, washed and dried to produce HO-M.
Thirdly, the 2 g HO-M samples were mechanically stirred and
dispersed in 150 mL ethanol (containing 30 mL H2O), and
21 mL KH–550-ethanol solution (1 : 6 v/v), and 1.0 mL ammo-
nium hydroxide (25%) was added dropwise under the condi-
tions of mechanical stirring at 45 ꢁC. Aer one hour, the
Nickel–zinc ferrite superparamagnetic microspheres were
synthesized by a solvothermal method. For graing terpyridyl
groups, the magnetic core was pretreated with silica and
amino–silane coupling agent (KH-550) in turn to obtain amino-
magnetic microspheres (NH2-M). The terpyridyl group was from
the 40-(4-bromophenyl)-2,20:60,200-terpyridine (TPY) molecule,
which was synthesized via the reaction of 4-bromobenzaldehyde
and 2-acetyl pyridine. The TPY-M adsorbent was constructed via
NH2-M reacted with TPY. The relative parameters of the TPY-M
adsorbent properties were investigated, including the acidity of
the solution, adsorption time, initial concentration of metal
ions in solution, the type and concentration of eluent, the
selectivity and anti-jamming capacity. The adsorption capac-
ities of the adsorbent for Pb(II) and Hg(II) in solution were better
as compared to that of Cd(II).
2 Materials and methods
2.1 Materials and reagents
ꢁ
reaction temperature of the system was increased to 70 C and
Except for some special requirements, the purity of all chem-
icals was beyond analytical grade. FeCl3$6H2O, ZnCl2, NiCl2-
$6H2O, ammonium hydroxide (25%), ethylene glycol, sodium
acetate, 3-aminopropyltriethoxysilane (KH-550, China), tet-
raethyl orthosilicate (TEOS), sodium iodide (NaI), N,N-dime-
thylformamide (DMF), ethanol (99%), 4-bromobenzaldehyde
and 2-acetylpyridine were purchased from Sinopharm Chemical
Reagent Corporation (Shanghai, China). Pb(II) stock solution
was prepared with Pb (NO3)2$6H2O at 1000 mg Lꢀ1, and was
further diluted to different concentrations and pH using NaOH
and HNO3 (0.1 mol Lꢀ1) according to the working standard. The
ultrapure water produced by the laboratory water purication
system (Hetai, China) was used throughout the experiments.
stirred continuously for one hour. The system was then cooled
to room temperature, and solid samples were collected by an
external magnetic eld. Aer washing and drying, the NH2-M
composite material was prepared.
The 40-(4-bromophenyl)-2,20:60,200-terpyridine molecule was
successfully prepared, according to literature.35 4-Bromo-
benzaldehyde (1 g, 5.4 mmol) and 2-acetylpyridine (1.3 g, 10.8
mmol) were stirred in 120 mL ethanol, followed by the addition
of NaOH (0.22 g, 5.4 mmol) and NH4OH (30 mL), and then the
mixture was reuxed for 36 hours. The white solid TPY was
obtained aer the precipitate was ltered and washed with
ethanol and water at room temperature. The TPY (0.2 g) and
Scheme 1 Diagram of the synthesis process for the TPY-M magnetic microspheres.
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NH2-M microspheres (0.1 g) were dispersed in a three-necked According to the above experimental results, Pb(II) was selected
ask in 50 mL DMF with 0.2 g NaI, 0.45 g K2CO3 and stirred to evaluate the factors affecting the adsorption performance.
continuously at 80 ꢁC for 36 h. Aer the reaction system was
The effects of pH were determined with 10 mg of TPY-M
cooled to room temperature, the precipitate was collected by an adsorbents suspended in a series of 25 mL of 20 ppm Pb(II)
external magnetic eld. The solid samples were washed thor- solutions with different acidities. The pH was adjusted with
oughly with distilled water and ethanol, dried in a vacuum 0.1 mol Lꢀ1 HNO3 or 0.1 mol Lꢀ1 NaOH solution. When the
freeze dryer for 6 hours, and TPY-M adsorbents were obtained mixture was dispersed by oscillator for 30 min, the adsorbent
successfully.
was collected and separated by an external magnetic eld. Pb(II)
The magnetic properties of the materials were investigated stock solution was diluted to different concentrations according
using an EZ-VSM vibrating sample magnetometer (VSM) (Micro to working standards. The effects of concentration were deter-
Sense, USA) in the range of ꢀ8000–8000 Oe at room tempera- mined with 10 mg of TPY-M adsorbents suspended in a series of
ture. The crystalline phase of TPY-M was characterized by X-ray 25 mL of Pb(II) solutions at pH 7 with different Pb(II) concen-
diffraction (XRD) performed on a D8 advance X-ray diffrac- trations. When the mixture was dispersed by oscillator for
tometer (Bruker, German)ꢀw1 ith Cu Ka radiation at 1.540 Aꢁ at 30 min, the adsorbent was collected and separated by an
a scanning rate of 5ꢁ min in the range from 10ꢁ to 80ꢁ. The external magnetic eld. The effects of adsorption time were
morphologies of the as-prepared samples were characterized by determined with 10 mg of TPY-M adsorbents suspended in
eld emission scanning electron microscopy (SEM) with an a series of solutions at pH 7, containing 25 mL of 20 ppm Pb(II).
acceleration voltage at 10 kV (Vega3, Tescan, CZ), and CM-120 Aer vigorous ultrasonic dispersion at room temperature for
transmission electron microscopy equipped with an energy- different periods ranging from 5 to 100 min, the magnetic
dispersive X-ray spectroscopy (EDS) detector, (TEM) with an adsorbent was quickly separated by a strong external magnetic
acceleration voltage about 200 kV (Philips, Netherlands). An eld. The residual concentration of Pb(II) in the liquid was
FTIR spectrometer (Nicolet, USA) collects the Fourier-transform measured by the FAAS. In the adsorption–desorption cycle
infrared spectra (FTIR) in the range of 400–4000 cmꢀ1. The experiment, we separately investigated the analytical ability of
concentrations of metal ions were detected using a ame three desorption solutions of HNO3, HCl and EDTA.
atomic absorption spectrophotometer with an air-acetylene
ame (FAAS, TAS990, China).
3 Results and discussion
3.1 Structural analysis
The XRD patterns of M and TPY-M samples are shown in
Fig. 1(a). The pattern of M is consistent with the XRD data of the
spinel-phase Ni0.25Zn0.75Fe2O4 listed in JCPDS (No. 52-0279),
though the actual molecular formula of the magnetic nucleus
was determined as Ni0.25Zn0.15Fe0.6Fe2O4 by a FAAS spectro-
photometer. The XRD pattern of the TPY-M sample was similar
to the magnetic core, indicating that the crystalline phase of the
modied magnetic substrate remained stable. The diffraction
peak strength of TPY-M was slightly weakened due to the
2.3 Batch adsorption experiments
The adsorption efficiency (E) and the adsorption capacity (q)
were respectively calculated by the following equations:
C0 ꢀ C2
E ¼
ꢂ 100%
(1)
(2)
C0
ðC1 ꢀ C2ÞV
q ¼
M
where C0, C1 and C2 refer to the initial adsorption concentra- amorphous modied layer.
tion, pre-adsorption concentration and post-adsorption
The VSM data indicate that the as-prepared M and TPY-M are
concentration, respectively. V refers to the volume of the paramagnetic, with a saturation magnetization of 82.1 emu gꢀ1
adsorption solution, and M represents the initial amount of and 41.5 emu gꢀ1, respectively, as shown in Fig. 1(b). Due to the
TPY-M.
non-magnetic properties of the SiO2 coating and functionalized
To determine the selectivity of TPY-M adsorbents, 10 mg molecules modied on the surface of M microspheres, the
TPY-M was added to 25 mL solutions of Pb(II), Hg(II), Cd(II), saturation magnetization of TPY-M decreased as compared with
Zn(II), Mn(II), Co(II), Ni(II) and Cr(III) with concentrations of the naked magnetic microsphere core. Meanwhile, the coer-
60 ppm at pH 7, respectively. To study the anti-interference civities were lower than 24 Oe, indicating that the magnetic
ability of the adsorbents, the adsorption experiments were nanoparticles modied by terpyridyl groups still had good
carried out on the above nine ionic coexisting solutions, and paramagnetic character. The as-prepared TPY-M adsorbent had
Pb(II), Hg(II) and Cd(II) coexisting solutions under the same excellent magnetic response characteristics, dispersion and
conditions. When the TPY-M adsorbent was dispersed fully into stability, which ensured its uniform dispersion and rapid
the mixed ions solution by an oscillator for 30 minutes, it was separation in the process of adsorption and desorption. The
separated by an external magnetic eld, and the residual results revealed that the structural and magnetic properties of
concentration of Pb(II), Hg(II) and Cd(II) in the solution was M remained stable during the multi-step modication proce-
determined by FAAS. To investigate the practicability of the TPY- dure, and the above procedure is suitable for preparing func-
M adsorbent, deionized water was replaced with four real water tionalized magnetic nanomaterials.
samples, including distilled water, tap water, river water and
The morphologies of the M and TPY-M samples were charac-
groundwater, as solvents in some of the detection experiments. terized by SEM, as shown in Fig. 2(a and b). The two kinds of
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Fig. 1 XRD patterns (a) and VSM loops (b) of the M and TPY-M samples.
samples are homogeneous spheres with an average size of 461 nm of the material, which ensured the adsorbent maintained excel-
and 477 nm, respectively. The particle size aer coating increased lent dispersion and sufficient magnetic response.
slightly due to the modied layers on the surface. Meanwhile, the
The surface texture and morphology of the M and TPY-M
thin coating layers did not affect the magnetic response properties samples were observed using TEM. We can see the internal
Fig. 2 SEM images and TEM images of M (a and c) and TPY-M (b and d).
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information and composition of microspheres by investigating
TEM images. Fig. 2(c and d) shows that the particles of Ni0.25
-
Zn0.75Fe2O4 and TPY-M samples are spheres composed of many
small particles, which is consistent with the related XRD anal-
ysis results. The obvious light and shadow at the edge of the
microspheres in Fig. 2(d) further conrm that the TPY-M
microspheres are core–shell structured. This indicates that the
core–shell functionalized materials can be prepared via the
above procedures.
The surface element analysis of naked M and TPY-M samples
was conducted by EDS, and the corresponding results are
shown in Fig. 3. Compared with Fig. 3(a), the contents of C
and N elements in Fig. 3(b) greatly increased. This could be
attributed to 40-(4-bromophenyl)-2,20:60,200-terpyridine, which
contains pyridine rings and benzene rings. It reveals that the
terpyridyl groups were successfully graed onto the surface of
the magnetic microspheres. The content of Fe, Ni and Zn
elements changed, which is coincident with the result of FAAS
analysis. This reveals that EDS spectra only can be applied to the
detection of elements in the limited surface layer.
Fig. 4 FTIR spectra of M (a), HO-M (b), NH2-M (c), TPY-M (d), and TPY (e).
1075 cmꢀ1 in (e), indicating that the material was successfully
The FTIR spectra exhibited some important information
about the functional groups of the samples, as shown in Fig. 4.
The peak at 501 cmꢀ1 was attributed to the stretching vibration
peak of the C–Br bond in terpyridine. The absorption peaks at
789 cmꢀ1 and 1589 cmꢀ1 were attributed to the bending
vibrations of the C–H bond and the stretching vibrations of the
C]C bond in the benzene ring, respectively. The absorption
peak at 594 cmꢀ1 appeared in (a), (b), (c) and (d), and belongs to
the vibration absorption of Fe–O groups from ferrite. This
conrmed that all the samples contained the ferrite structure.
The characteristic absorption peaks of O–H groups at
3434 cmꢀ1 and 1634 cmꢀ1 indicate the presence of hydroxyl
groups in the samples. The absorption at 1079 cmꢀ1 belongs to
the exural vibration of Fe–O–Si, while the absorption at
1469 cmꢀ1 is attributed to the exural vibration of N–H.
Comparing (d) and (e), some matching peaks, such as the peak
at 2964 cmꢀ1 in (d) and 3058 cmꢀ1 in (e) pertain to the
stretching vibrations of C–H bonds in the phenyl group, and the
C–N bond in tripyridine appeared at 1154 cmꢀ1 in (d) and
modied with terpyridyl groups.
3.2 Adsorption characteristics investigation
3.2.1 The selectivity and practicality of TPY-M adsorbents.
As shown in Fig. 5(a), in single-metal ionic solution, under the
same conditions, the adsorption capacities of TPY-M adsor-
bents on Pb(II), Hg(II), Cd(II), Mn(II), Ni(II), Co(II), Zn(II) and Cr(III)
were 64.75, 33.94, 24.64, 14, 12.4, 8.2, 5.2 and 0 mg gꢀ1
,
respectively. The results showed that the adsorbent has an
outstanding adsorption capacity for lead, cadmium and
mercury ions but a poor capacity for others. Therefore, TPY-M
can be used as a highly efficient adsorbent for Pb(II), Hg(II) and
Cd(II) in a single-ionic aqueous system. As shown in Table 1, the
unmodied M has a certain adsorption on all tested metal ions
based on its high specic surface area. Compared with the
naked M microspheres, the NH2-M microspheres showed
a difference in the adsorption of ions; the ability improved for
the adsorption of some ions, but the adsorption of the other
Fig. 3 EDS maps of M (a) and TPY-M (b) samples.
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Fig. 5 The adsorbability of common metal ions (a) and the comparison of the adsorption properties in four water samples (b).
ions was signicantly reduced. This indicates that the NH2-M higher adsorption efficiency for Hg(II) in groundwater. This
material possessed chemical adsorption properties. The may be due to the anions in river water and groundwater,
prepared TPY-M material presented the best adsorption which promote the coordination between adsorbents and
capacity toward Pb(II), Hg(II) and Cd(II) ions among the three different metal ions. The results showed that the synthesized
adsorbents, and there was no signicant effect on other metal TPY-M adsorbent can be used to separate Pb(II) and Hg(II) in
ions. The diverse adsorption performance of the adsorbent is real water samples. Compared with the existing results in
attributed to terpyridyl groups, which show different coordi- Table 2, the TPY-M adsorbent has advantages in the adsorp-
nation abilities with metal ions.
tion of Pb(II) and Hg(II).
The effects of coexistent ions were investigated, and the
3.2.2 The effect of adsorption conditions. The effect of pH
relative results are inserted in Fig. 5(a). In the case of the on the adsorption properties was investigated by series of batch
coexistence of multiple ions, the adsorption efficiencies of equilibrium tests with the same parameters; the adsorption
Pb(II), Hg(II) and Cd(II) were 45.2%, 18.6% and 1.2% in the three- efficiency and adsorption capacity of TPY-M within the range of
ions system and 12.48%, 4% and 3.2% in the nine-ions system. pH 1–10 are shown in Fig. 6(a). The adsorption capacity of the
The results revealed that TPY-M still had a good adsorption adsorbent for Pb(II) was weak when the pH changed from 1 to 4,
capacity for Pb(II) and Hg(II), but weak adsorption capacity and and gradually increased when the pH went from 4 to 8. At the
poor anti-interference ability for Cd(II) in the multi-ion envi- pH of 8–10, the formation of insoluble sediments by the
ronment. This indicates that the TPY-M adsorbent can be used hydrolysis reaction resulted in the concentration of residual
for the specic adsorption of Pb(II) and Hg(II) in the multi-ion Pb(II) being further reduced in the system. Aer precluding the
setting.
inuence of the hydrolysis reaction, the adsorption capacity
The adsorption efficiency of TPY-M in real water samples reached a maximum of 67.4 mg gꢀ1 at pH 8. When the pH
reects its practicability. According to the adsorption effi- changed from 9 to 10, metal ions preferentially formed the
ciency of TPY-M toward Pb(II), Hg(II) and Cd(II) ions in Fig. 5(b), hydroxide precipitate as compared to adsorption, which resul-
the adsorption efficiency of TPY-M adsorbents in all actual ted in lower adsorption capacity, although a higher adsorption
water samples is higher than in distilled water. The adsorption efficiency was observed in the detection procedures. Therefore,
efficiencies of TPY-M for Pb(II) in distilled water, tap water, the optimum pH range of the TPY-M adsorbent for Pb(II) is 5–9,
rivers and groundwater were 45.2%, 53.5%, 62.3% and 58.6%,
while for Hg(II) they were 18.6%, 29.2%, 28.6% and 37.1%.
Table 2 Comparison of the adsorption capacities of various metal ion
adsorbents for Hg(II) and Pb(II) ions
Under the same conditions, the adsorbent presented a small
adsorbability on Cd(II). Among them, TPY-M showed a higher
adsorption efficiency for Pb(II) in river water, while it showed
Adsorption
capacities
(mg gꢀ1
Pb(II)
)
Table 1 Comparison of the adsorption capacities of M, NH2-M and
TPY-M
Adsorbent
Hg(II)
Reference
Adsorption capacity (mg gꢀ1
)
Fe3O4–SO3H MNP
MCB
XMCS
Modied microalgae residuals
Modied activated carbons
Fe3O4-GS
108.93
37.99
76.9
—
—
23.03
64.75
—
—
—
62.5
41.0
27.95
33.94
36
37
38
39
40
41
Adsorbent Pb(II) Hg(II) Cd(II) Mn(II) Ni(II) Co(II) Zn(II) Cr(III)
M
NH2-M
TPY-M
18.52 17.90
9.33 8.08
4.52 18.46 11.08 1.93
9.03 10.35 8.97 1.23
33.11 25.56 13.61 7.83
64.75 33.94 24.64 14
12.4
8.2
5.2
0
TPY-M
This study
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Fig. 6 The effects of (a) pH, (b) initial concentration, and (c) contact time on the adsorption of Pb(II). (d) The fitting curve of contact time with the
pseudo-second-order kinetic model (inset: a fitting curve with the pseudo-first-order dynamic model).
which meets the applicable pH conditions for Pb(II) adsorption
The result in Fig. 6(c) demonstrates that the adsorption
from waste solution as specied in the emission standards.
efficiency increased rapidly within 5–20 min, achieved 92.1% at
As shown in Fig. 6(b), when Pb(II) concentration was within 50 min, and nally reached 95.9% at 90 min. This suggests that
the range of 5–30 ppm, the adsorption efficiency of TPY-M the adsorption equilibrium should be achieved in about 50 min.
adsorbent increased rapidly; from 30 to 60 ppm, the adsorption The pseudo-rst-order kinetics can be generally described by
capacity increased slowly and it was almost constant from eqn (3),42 while the pseudo-second-order kinetic model can be
60 ppm to 100 ppm. The maximum adsorption capacity of the described by eqn (4):43
tted curve was 67.81 mg gꢀ1 (R ¼ 0.9926), which is consistent
lg(Qe ꢀ Qt) ¼ lg Qe ꢀ K1t/2.303
(3)
(4)
with the experimental data, indicating that the adsorption
process is via chemisorption. The results show that the
adsorption capacity of TPY-M is related to the content of ter-
pyridyl groups. Based on the molar ratio of TPY : Pb2+ of 1 : 1,
the amount of terpyridine on the magnetic surface was calcu-
lated as about 3.26 ꢂ 10ꢀ4 mol gꢀ1. Because the specic surface
area of TPY-M was detected by the BET instrument as
11.0177 m2 gꢀ1, TPY molecules can be sufficiently graed on the
magnetic microsphere surface. The higher the Pb(II) concen-
tration, the more Pb(II) ions are coordinated with the terpyridyl
groups. The adsorption capacity reaches its maximum when
metal ions occupy all terpyridyl groups. It was concluded that
the adsorption process of TPY-M is chemical adsorption, which
depends on the number and type of functional groups.
2
Qe K2t
Qt ¼
1 þ QeK2t
where Qe is the amount (mg gꢀ1) of adsorbed Pb(II) on the
surface of the adsorbent at equilibrium, and Qt is the amount
(mg gꢀ1) of adsorbed Pb(II) on the surface of the adsorbent at
time t. K1 and K2 are the rst-order rate constant (1 minꢀ1) and
the pseudo-second-order rate constant (g mgꢀ1 minꢀ1),
respectively.
The tting results of the pseudo-rst-order and pseudo-
second-order kinetic models are shown in Fig. 6(d). Compared
with the pseudo-rst-order kinetic model (R1 > 0.7139), the
pseudo-second-order model is more preferable for describing
the adsorption process (R2 > 0.9731). The experimental data
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Fig. 7 The effects of (a) three desorption solutions, (b) different concentrations of HCl, and (c) desorption time on the desorbing efficiency. (d)
The adsorption and desorption efficiency in different cycles of regeneration.
agree well with the pseudo-second-order model, which conrms suggests that a highly acidic environment may destroy the as-
the chemisorption presented in the adsorption process. It synthesized materials.44 The results show that a desorption
further indicates that the adsorption of Pb(II) by TPY-M occurs solution with suitable acidity is very important for the regen-
via chemisorption.
eration of the adsorbents. It can be seen from Fig. 7(c) that the
desorption equilibrium was reached in 2 min.
Renewability is a key indicator for the recycling of adsor-
bents in practical applications. To investigate the reusability of
adsorbents, ve adsorption–desorption cycles were conducted.
As can be seen from Fig. 7(d), aer ve adsorption–desorption
cycles, the adsorption efficiency of the TPY-M adsorbent
remained above 75%, while the desorption efficiency was above
87%. The slight decrease in the adsorption performance of the
TPY-M adsorbent during the recovery process was mainly due to
the loss of the adsorbent, indicating that TPY-M is a stable and
reusable adsorbent.
3.3 Desorption and regeneration
Since the TPY-M material is a chemical adsorbent, hydrogen
ions at high acid concentrations will cause the adsorbent to lose
its ability to bond with metal ions. The results of the study on
three commonly used desorption solutions of HNO3, HCl and
EDTA are shown in Fig. 7(a), indicating that the solution with
the highest desorption efficiency was the HCl solution. We
further studied the effects of the concentration and desorption
time of HCl solution; the corresponding data are shown in
Fig. 7(b) and (c). At a low concentration of HCl, the concentra-
tion of H(I) in the system was insufficient to release all Pb(II)
from the adsorbent. As the concentration of HCl increased, H(I)
gradually replaced all Pb(II) on the adsorbent. Nevertheless, the
4 Conclusion
curve shows that the acid strength is not completely propor- The TPY-M paramagnetic adsorbent, functionalized by terpyr-
tional to the desorption capacity in Fig. 7(b). The highest idyl, has been successfully prepared. The magnetic adsorbent
desorption efficiency of 92.6% was produced by HCl with TPY-M has good dispersion, fast magnetic separation, and can
a concentration of 1.0 mol Lꢀ1. The desorption solution turned be selectively coordinated with metal ions. TPY-M showed
brown when the concentration exceeded 1.5 mol Lꢀ1, which outstanding adsorption properties toward Pb(II), Cd(II) and
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Hg(II) among common metal ions. The adsorbing capacities
were 64.75, 33.94 and 24.64 mg gꢀ1, respectively, at pH 7 and
ionic concentration of 60 ppm. In a system of co-existing ions,
9 I. E. Imiete and N. V. Alekseeva, Reverse osmosis
purication: a case study of the Niger Delta region, Water
Sci., 2018, 32(1), 129–137.
the TPY-M adsorbent displayed superior performances toward 10 M. S. Liphadzi and M. B. Kirkham, Phytoremediation of soil
Pb(II) and Hg(II) as compared to Cd(II). According to the analysis
of the adsorption kinetics, the adsorption mechanism of the
TPY-M adsorbent on metal ions is via chemisorption. Aer ve
contaminated with heavy metals:
a
technology for
rehabilitation of the environment, S. Afr. J. Bot., 2005,
71(1), 24–37.
adsorption–desorption cycles in 1.0 mol Lꢀ1 HCl solution, the 11 A. Hansda, V. Kumar and Anshumali, A comparative review
adsorption efficiency of the TPY-M adsorbent for Pb(II) was
beyond 75% and the desorption efficiency was still above 87%.
This work has veried the important role of terpyridyl groups in
towards potential of microbial cells for heavy metal removal
with emphasis on biosorption and bioaccumulation, World
J. Microbiol. Biotechnol., 2016, 32(10), 14.
the adsorption of heavy metal ions, and also brings forward an 12 A. Pal and A. K. Paul, Microbial extracellular polymeric
important approach to the synthesis of magnetic nano-
substances:
central
elements
in
heavy
metal
adsorbents modied by terpyridyl groups, which should be of
bioremediation, Indian J. Microbiol., 2008, 48(1), 49–64.
great signicance to theoretical research and practical water 13 G. Al-Enezi, M. F. Hamoda and N. Fawzi, Ion exchange
treatment applications.
extraction of heavy metals from wastewater sludges, J.
Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ.
Eng., 2004, 39(2), 455–464.
Conflicts of interest
14 A. A. S. Aboul-Magd, S. A. Al-Husain and S. A. Al-Zahrani,
Batch adsorptive removal of Fe(III), Cu(II) and Zn(II) ions
in aqueous and aqueous organic-HCl media by Dowex
HYRW2-Na Polisher resin as adsorbents, Arabian J. Chem.,
2016, 9, S1–S8.
15 A. Kumar and H. M. Jena, Adsorption of Cr(VI) from aqueous
solution by prepared high surface area activated carbon
from Fox nutshell by chemical activation with H3PO4, J.
Environ. Chem. Eng., 2017, 5(2), 2032–2041.
There are no conicts to declare.
Acknowledgements
The authors are thankful to the Natural Science Foundation of
Shanghai (No. 15ZR1428500) for nancial supports.
16 H. Figueiredo and C. Quintelas, Tailored zeolites for the
removal of metal oxyanions: Overcoming intrinsic
limitations of zeolites, J. Hazard. Mater., 2014, 274, 287–299.
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