Preparation of thermosensitive, calix[4]arene incorporated P(NIPAM-co-HBCalix) hydrogel as a reusable adsorbent of nickel(II) ions

Article abstract of DOI:10.1002/pola.26625

A calix-conjugated thermo-responsive hydrogel containing 15% tetra(5-hexenyloxy)-p-tert-butylcalix[4]arene (HBCalix), P(NIPAM-co-HBCalix), was used to remove nickel(II) ions from water. Both thermo-sensitive properties and the Ni²⁺-adsorption capabilities of the prepared P(NIPAM-co-HBCalix) hydrogels are investigated. Introduction of the monomer HBCalix considerably enhanced the adsorption of Ni²⁺ onto the hydrogel by chelation between hexenyloxy groups and metal ion. When HBCalix units capture Ni²⁺ and forms HBCalix/Ni²⁺ host-guest complexes, the lower critical solution temperature of the hydrogel shifts to a higher temperature due to both the repulsion between charged HBCali/Ni ²⁺ groups and the osmotic pressure within the hydrogel. Adsorption studies were carried out by varying contact time, counter ion and initial concentration of Ni²⁺. The evaluation of adsorption properties showed that the hydrogel exhibited better correlation with Langmuir isotherm model. P(NIPAM-co-HBCalix) could be used repeatedly with little loss in adsorption capacity by simply changing the environmental temperature. This kind of ion-recognition hydrogel is promising as a novel adsorption material for adsorption and separation of Ni²⁺ ions.

Full text of DOI:10.1002/pola.26625

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Preparation of Thermosensitive, Calix[4]arene Incorporated
P(NIPAM-co-HBCalix) Hydrogel as a Reusable Adsorbent
of Nickel(II) Ions
Fanyong Yan, Meng Wang, Donglei Cao, Shanshan Guo, Li Chen
State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Key Lab of Fiber Modification & Functional Fiber of
Tianjin, Tianjin Polytechnic University, Tianjin, 300387 People’s Republic of China
Correspondence to: L. Chen (E-mail: yfany@163.com)
Received 15 November 2012; accepted 29 January 2013; published online 15 March 2013
DOI: 10.1002/pola.26625
ABSTRACT: A calix-conjugated thermo-responsive hydrogel
containing 15% tetra(5-hexenyloxy)-p-tert-butylcalix[4]arene
(HBCalix), P(NIPAM-co-HBCalix), was used to remove nickel(II)
ions from water. Both thermo-sensitive properties and the
Ni21-adsorption capabilities of the prepared P(NIPAM-co-HBCa-
lix) hydrogels are investigated. Introduction of the monomer
HBCalix considerably enhanced the adsorption of Ni21 onto
the hydrogel by chelation between hexenyloxy groups and
metal ion. When HBCalix units capture Ni21 and forms HBCa-
lix/Ni21 host–guest complexes, the lower critical solution tem-
perature of the hydrogel shifts to a higher temperature due to
both the repulsion between charged HBCali/Ni21 groups and
the osmotic pressure within the hydrogel. Adsorption studies
were carried out by varying contact time, counter ion and
initial concentration of Ni21
The evaluation of adsorption
.
properties showed that the hydrogel exhibited better correla-
tion with Langmuir isotherm model. P(NIPAM-co-HBCalix)
could be used repeatedly with little loss in adsorption capacity
by simply changing the environmental temperature. This kind
of ion-recognition hydrogel is promising as a novel adsorption
material for adsorption and separation of Ni21 ions.
2013
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KEYWORDS: adsorption; Calix[4]arene; hydrogel; nickel(II) ions
removal; N-isopropylacrylamide; supramolecular structures
swell well in water or organic solvents.9 Poly(N-isopropyla-
crylamide) (PNIPAM) and its derivative copolymers are
widely studied thermo-sensitive hydrogels as solid-phase
extraction adsorbents. These hydrogels performed a temper-
ature-swing adsorption (TSA) toward metal ions,10,11 such as
PNIPAM hydrogels showed an extractive affinity for Au3112
and Cu21-surfactant complexes.13 The modified PNIPAM
hydrogels can bind heavy metal ions through different
metal-complexing ligands, which interact selectively and
strongly with heavy-metal ions.14–17 Moreover easy handling
and reusability make hydrogels promising materials for
water purification. Thus, there remains a need to discover
more selective ionophores and distribute them in the PNI-
PAM chain.
INTRODUCTION The role of heavy-metal ions in environmental
issues has become increasingly prominent because of their
unique detrimental and non-biodegradable characteristics for
the living organisms.1 Among the toxic ions, the nickel, which
is widely used in modern industry and catalytic processes, has
been shown to exert many negative effects on human health
and environment if it had been discharged without suitable
treatment.2,3 Various techniques (e.g., ion exchange, adsorption,
chemical precipitate, and membrane processes) were used to
remove the nickel(II) ion from wastewater streams. Adsorption
has been shown to be an economically feasible method
because of its high efficiency, easy handling and availability of
adsorbents.4,5 Several different types of materials have been
investigated as adsorbents, among them amorphous silica, acti-
vated carbon, clays, zeolites, and polymers.6–8 However, these
adsorptive materials are either ineffective or nonselective,
when metal ions are present in the wastewater at low concen-
trations. So, the synthesis of adsorbents for the removal of
Ni21 from wastewater is a continuing research objective of
environmental pollution-control processes.
Calixarenes, the third generation of synthetic host com-
pounds, have a remarkable recognition ability toward spe-
cific metal ions through coordination bonds, hydrogen bonds,
and ionic bonds.18,19 Calixarenes with appended or pendant
groups can be thought as useful latent hosts for toxicant rec-
ognition and separation.20,21 When the ion diameter matches
the cavity size of modified calix[4]arenes cavity, the
ion could be captured by the calixarenes as receptors and
Smart synthetic hydrogels are attracting growing research
interests due to their three-dimensional (3D) networks and
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“host-guest” complexes are stably formed. Many advanced
polymer materials for metal ion detection and selective sepa-
ration have been designed by anchoring calixarenes deriva-
tives to a polymeric support or bonding in the polymer
backbone.22 Ulewicz et al.23 studied the polymer inclusion
membranes using calix[4]crown-6 derivatives as ion carriers
(d, J 5 12.5 Hz, 4H), 3.95 (t, J 5 7.5 Hz, 3H), 3.29 (d, J 5 12.5
Hz, 4H), 2.02 (m, 2H), 1.27 (s, 18H), 1.24 (t, J 5 7.5 Hz, 3H),
0.98 (s, 18H); 13C NMR: d 5 150.8, 150.0, 149.9, 146.9,
146.8, 141.4, 133.2, 132.9, 127.9, 127.8, 125.6, 125.5, 125.1,
117.5, 33.9, 31.9, 31.8, 31.2, 23.5, 11.0; FT-IR (KBr, cm21):
m 5 3413, 2961, 2904, 1643, 1486, 1361, 1120.
and investigated their selectivity of Zn21, Cd21, and Pd21
.
Akkus et al. synthesized the polymeric calix[4]arene having
phthalimide groups at the lower rim.24 The order of extract-
ability of metal cations by this derivative is selective in the
sequence: Hg21 > Cd21 > K1 >Co21 > Cu21. The central chal-
lenge in employing calixarenes in adsorptive materials is
how best to secure them onto a polymeric support. Covalent
linkages are ideal, but may require synthesis of the starting
reagent given a suitable precursor is commercially available.
Preparation and Characterization of
P(NIPAM-co-HBCalix) Hydrogels
Cross-linked P(NIPAM-co-HBCalix) hydrogels were prepared
by thermally initiated free-radical copolymerization.27 NIPAM
(3.3942 g), HBCalix (0.5512 g) and crosslinker fully dis-
solved in 20 mL mixed solvent CH2Cl2-CH3CN (v/v, 1/1)
using AIBN as initiator. The polymerization was carried out
ꢀ
at 60 C for 12 h under nitrogen atmosphere. The pure PNI-
PAM hydrogel was prepared under the same condition, but
without any addition of HBCalix. After the gelation was com-
pleted, the P(NIPAM-co-HBCalix) hydrogels obtained was cut
into circular pieces (1.0 cm in diameter) with a perforator.
Hydrogel disks were soaked in ethanol and deionized water
overnight to remove any remaining unreacted monomers or
initiator. This rinsing step was repeated three times and
then stored in Millipure water at 12 ꢀC.28 Before the meas-
urements, the originally swollen hydrogelsꢀ samples were
dried in an over to a constant weight at 40 C for 48 h and
then freeze-dried by FD-1TC freeze dryer (245 ꢀC and 0.1
mbar) for at least 48 h. The samples were kept in a closed
container for characterization and for use in the adsorption
experiment.
In this study, we prepared a novel thermo-sensitive poly
(N-isopropyl acrylamide-co-tetra(5-hexenyloxy)-p-tert-butyl-
calix[4]arene) (P(NIPAM-co-HBCalix)) with good ion-recogni-
tion property by using PNIPAM hydrogel as an actuator and
tetra(5-hexenyloxy)-p-tert-butylcalix[4]arene (HBCalix) as the
ion-signal receptor. When the ambient temperature is lower
than the lower critical solution temperature (LCST), PNIPAM
chain stretching, HBCalix and Ni21 with larger binding coeffi-
cient, more Ni21 ions form complexes with the HBCalix,
which are adsorbed on the hydrogel. Therefore, here we
report the application of P(NIPAM-co-HBCalix) hydrogel as a
novel adsorbent to remove Ni21 from aqueous solutions.
EXPERIMENTAL
The FT-IR analyses of powdered xerogel samples were char-
acterized by Nicolet 560 spectrophotometer. The proper
amount ratio (Sample:KBr 5 1:50) was mixed and grounded
and then compressed into a pellet under a pressure of 11
tones, for about a minute, using a Graseby Specac Model:
15.011. Spectra were obtained in the 4000–400 cm21 wave
number range, at 25 ꢀC and at 4 cm21 spectral resolution.
Proton Nuclear magnetic resonance spectroscopy (1H NMR)
experiments were performed on a Bruker DRX-300 NMR
spectrometer with DMSO-d6 of as solvent and TMS as the in-
Materials
N-Isopropylacrylamide (NIPAM, Tokyo Kasei Kogyo, Japan)
was purified by recrystallization with a cyclohexane-toluene
(v/v, 50/50) mixture solution. Initiator 2,20-azoisobutyroni-
trile (AIBN, Shanghai Siweihe Chemical, China) was
recrystallized from ethanol solution. Cross-linker N,N0-meth-
ylenebisacylamide (BIS, Tianjin Chemical Reagent, China). All
other reagents for synthesis were purchased from commer-
cial suppliers and used without further purification. The
deionized water was prepared by a Millipore Nano Pure
purification system (resistivity higher than 18.2 MX cm21).
ternal standard. Carbon nuclear magnetic resonance (13
C
NMR) spectra was recorded on the Bruker DSX-400 NMR
spectrometer operated at a resonance frequency of 100.47
MH. Scanning electron microscopy (SEM) images were col-
lected on a JEOL JSM-6380 electron microscope.
Synthesis of HBCalix
P-tert-butylcalix[4]arene and HBCalix were synthesized
according to the literature procedures.25,26 The mixture of
p-tert-butylcalix[4]arene (0.05 mol), 6-bromo-1-hexene (0.50
mol), and anhydrous NaH (0.25 mol) in dimethyl sulfoxide
(200 mL) were reacted at room temperature for 72 h under
nitrogen atmosphere. The mixture was treated with HCl
(10%, v/v) and extracted with CHCl3. The organic layer was
separated, dried with MgSO4, and concentrated. The crude
product was purified by column chromatography (silica gel,
dichloromethane: petroleum ether 5 2:1–10:3, v/v) to afford
HBCalix as a white powder in 32% yield.
Metal ions Adsorption and Thermo-Sensitive Behaviors
of P(NIPAM-co-HBCalix) Hydrogels
The ion-recognition and thermo responsive behaviors of
P(NIPAM-co-HBCalix) hydrogels are comprehensively investi-
gated by evaluating their volume-phase transition (VPT)
behaviors in metal ion (Ni21
, , , )
Pb21 Cd21 and Cu21
solutions and deionized water. For this purpose, aqueous sol-
utions (10 mL) containing different amounts of metal ions
(in the range of 1–40 mg L21) were incubated with 0.05 g
dried hydrogels at various designed ambient temperature
values. After the desired adsorption period (up to 180 min),
concentration of metal ion in the aqueous phases was
1H NMR (300 MHz, CDCl3): d 5 7.64 (s, 2H), 7.03 (s, 4H),
6.83 (s, 2H), 6.82 (s, 2H), 6.23(m, 1H), 5.75 (d, J 5 15 Hz,
1H), 5.36 (d, J 5 15 Hz, 1H), 4.52 (d, J 5 2.0 Hz, 2H), 4.28
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FIGURE 1 Schematic representation of the synthesis of HBCalix and its corresponding polymeric P(NIPAM-co-HBCalix) hydrogels.
measured using inductively coupled plasma (ICP, Atomscan
Advantage). A water-bath shaker (model CH-4311) at a
speed of 150 rpm was used throughout the equilibrium
examination.
rate of 150 rpm for 180 min. The concentration of Ni21 ions
in the desorption medium were followed as described
before. After each adsorption–desorption cycle, the P(NIPAM-
co-HBCalix) hydrogels were washed three times in 50 mL of
deionized water to eliminate metal ions that were not bound
to the hydrogel matrix. Rinses took 6 h and were stopped
when the rinse solution contained no Ni21 ions detectible by
ICP mass spectrometry.
The equilibrium amounts of metal ions adsorbed onto the
unit mass of P(NIPAM-co-HBCalix) hydrogels at the designed
temperature T (ꢀC), Q (mg g21), was determined using the
mass balance equation as follows:
Q5½ðC02CeÞ0Vꢁ=m
(1)
RESULTS AND DISCUSSION
where V is the volume of the solution (L), and m is the
weight of the dried P(NIPAM-co-HBCalix) gel (g), respec-
tively; C0 and Ce (mg L21) are the initial and equilibrium
concentrations of the metal ions in the aqueous phase after
adsorption at T (ꢀC). For comparison, similar adsorption
experiments are also carried out with PNIPAM hydrogels. To
obtain the reusability of P(NIPAM-co-HBCalix) hydrogels,
adsorption–desorption cycle was repeated five times using
the same adsorbent in a batch system. Desorption of Ni21
from P(NIPAM-co-HBCalix) hydrogels was achieved by using
10 mL deionized water. The hydrogels were placed in the
desorption medium and shaken in a water bath shaker at a
Characterization Studies
The synthesis route of P(NIPAM-co-HBCalix) hydrogels is
shown in Figure 1. A 3D hydrogel network formed in 30
min. The HBCalix content in the hydrogel was generally
12.2% (W/W). Uncrosslinked HBCalix content was verified
by means of soaking the hydrogel in methanol at ambient
temperature for 1 or 4 days followed by HPLC analysis of
the soak solutions. The hydrogel sample is transparent at
lower temperature 12 ꢀC. By visual appearance the same
hydrogel suddenly turns into white color from transparent
near 32 ꢀC. It is known that the sequential distribution is
mainly depended on the relative reactivity ratio of the
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and ACH2A backbone and ACHANA NIPAM) and 170–180
ppm (CO, NIPAM). The 13C NMR spectra provided further
evidence for the formation of P(NIPAM-co-HBCalix) copoly-
mer between HBCalix and NIPAM.
The cross-section structures of freeze-dried hydrogels by
SEM are shown as Figure 4. The data clearly illustrate the
dependence of interior morphology on the composition ratio
of HBCalix. From the size of the bar, these hydrogel networks
have
a
different porous structure. PNIPAM hydrogel
[Fig. 4(a)] has the smallest one in the range of 25–50 mm,
while P(NIPAM-co-HBCalix) hydrogel [Fig. 4(b)] has been
large continuous interconnected pore size in the range of
70–160 mm. Because of numerous interconnected pores in
the hydrogel network, water molecules can easily diffuse.
P(NIPAM-co-HBCalix) hydrogels could enhance the rate of
the swelling processes. The thermosensitive P(NIPAM-co-
HBCalix) hydrogels have potential applications in the extrac-
tion of metal ions since the network may be able to provide
enough space for the loading and releasing of metal ions.
The formation of macroporous structures of P(NIPAM-co-
HBCalix) can be explained by the following: the network
structure of P(NIPAM-co-HBCalix) hydrogels are different,
mainly due to the HBCalix contains four aromatic rings
linked by methylene bridges, a highly expanded network can
be generated by electrostatic repulsion among hydrophobic
cavities during the polymerization process. Therefore, the
sizes of pores over range from 50 to 160 mm that increase
the interaction between hydrogel and the metal ions and
decrease the diffusion limitation in metal ion’s adsorption.
The similar phenomenon is also reported.10
FIGURE 2 FT-IR spectra of HBCalix, PNIPAM and P(NIPAM-co-
HBCalix) hydrogel.
monomers and the monomer feed ratio.29 Hydrogels contain-
ing more amounts of HBCalix were directly synthesized.
However, these hydrogels had a poor mechanical strength.
The reason may be that too much HBCalix monomers iso-
lated the hydrogel chains and prevented them from forming
hydrogen bonds, which limited the primary source of the
physical cross-links in the hydrogels.30
The FT-IR spectra of P(NIPAM-co-HBCalix) and PNIPAM hydro-
gels as well as HBCalix monomer is shown in Figure 2. Evi-
dently, the FT-IR spectra of all the hydrogels was similar,
although the spectrum of each hydrogel showed slight changes
with different HBCalix contents. Every spectrum showed a
broad band in the range of 3600–3200 cm21, which belongs
to NAH stretching vibration of the NIPAM. A strong band
around 2920 cm21 can be assigned as stretching vibrations of
ACH2 from NIPAM and HBCalix. The typical amide I band
(1650 cm21), consisting of C@O stretch of PNIPAM and amide
II bands (1540 cm21), including NAH vibration are evident in
every spectrum.31 Two bands of CAH vibration at 1386 and
1367 cm21 belong to the divided bands of the symmetric
ACH(CH3)2 group. The existence of HBCalix was evident by
the presence of a strong 1460 cm21 band for C@C skeletal
stretching vibration of phenyl ring32; consisting of CAO asym-
metric stretching vibration in RAOAR0, although this band
was weak due to the low HBCalix content in hydrogels. The
peak intensity at 1120 cm21 increased gradually, which con-
firmed that the HBCalix content increased. All above are im-
portant evidence to prove that P(NIPAM-co-HBCalix)
hydrogels were polymerized successfully.
Single-Component Heavy Metal Ions Adsorption Rate of
P(NIPAM-co-HBCalix) Hydrogels
The effect of contact time on adsorption values of metal ions
on P(NIPAM-co-HBCalix) hydrogel was studied, and the results
are presented in Figure 5. It is observed that P(NIPAM-co-
Solid state 13C NMR spectroscopy provides information
regarding the structure of P(NIPAM-co-HBCalix). Figure 3
displays 13C NMR spectra of P(NIPAM-co-HBCalix) and
PNIPA. The feature signal at 31 ppm corresponding to
P(NIPAM-co-HBCalix) is the carbon of tert-butyl. Solid-state
13C-NMR of PNIPAM shows three broad peaks at 20–26
ppm (ACH3, NIPAM; ACHAbackbone), 30–50 ppm (ACHA
FIGURE 3 13C NMR spectra of PNIPAM and P(NIPAM-co-HBCa-
lix) hydrogel.
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FIGURE 4 SEM images of PNIPAM hydrogel (a), P(NIPAM-co-HBCalix) hydrogel (b) freeze-dried at low temperature.
HBCalix) hydrogels show a significant extraction ability to-
ward transition metal cations Ni21, Cu21, Pb21, Zn21. Where
the adsorption occurred in two steps, the initial adsorption
rates increases sharply during the first 1 h, followed by grad-
ually establishing adsorption equilibrium (plateau values)
within 150 min for all the tested metals. This fast adsorption
equilibrium is most probably due to high complexation and
geometric affinity between functional calix groups and heavy-
metal ions. The equilibrium was reached within 150 min. As
seen in the figure, high adsorption capacity for Ni21 (3.481
mg g21) was achieved with P(NIPAM-co-HBCalix) hydrogels.
Further increase in contact time did not show an increasing
in adsorption. Thus, 150 min shaking time was considered to
be the optimum contact time for adsorption of metal ions on
P(NIPAM-co-HBCalix) hydrogels. In particular, there are no
chelating functional groups on the plain PNIPAM hydrogels,
the adsorption of mental ions on PNIPAM hydrogels was low
at 10 or 50ꢀC.33 This adsorption may be due to diffusion of
metal ions into the matrix and weak interactions between
metal ions and amide moieties of PNIPAM hydrogels. There-
fore, this result means that introducing hexenyloxy groups to
the calixarene platform is essential for creating a high extract-
ability for Ni21 ions.
detecting the volume change in deionized water and in 20
mg L21 Ni21 aqueous solutions. Where VT/V0 is the ratio of
volume of the hydrogel disc at temperature T to that at 21
ꢀC. The temperature, at which the value of VT/V0 decreases
to half of the total change value, is taken as the LCST of the
cross-linked hydrogel. As shown in Figure 6(a,b), the vol-
ume-change trend of PNIPAM hydrogels is almost the same
as that in Ni21 solution, and the LCST of PNIPAM hydrogel in
Ni21 solution does not have any significant change compared
with that in deionized water. The result indicates that the
metal ions scarcely affect the phase transition behaviors of
PNIPAM-based hydrogels without presence of HBCalix recep-
tors in the polymeric networks.
As expected, the temperature-dependent volume-change
trend of P(NIPAM-co-HBCalix) hydrogels in Ni21 solution is
significantly different from that in deionized water. The LCST
of P(NIPAM-co-HBCalix) hydrogel shifts to a higher tempera-
ture in Ni21 solution: the P(NIPAM-co-HBCalix) hydrogel
Ion-Recognition and Thermosensitive Behaviors of
P(NIPAM-co-HBCalix) Hydrogels
In this work, all the hydrogels can be swollen without dis-
solving, suggesting the formation of crosslinked structures. It
is seen that at a low temperature (e.g., <30 ꢀC) the final
hydrogels obtained were macroscopically homogeneous sur-
face and translucent in appearance. When heated up to the
higher temperatures (e.g., >40 ꢀC), the hydrogels became
shrank and cloudy, implying the occurrence of the VPT phe-
nomenon. Re-cooled to the low temperatures, the hydrogels
can be re-swollen and became translucent again. This obser-
vation indicates that all P(NIPAM-co-HBCalix) hydrogels pos-
sess temperature-sensitive properties.
FIGURE 5 Adsorption rates of metal ions on P(NIPAM-co-
HBCalix) hydrogel; initial concentration of metal ions 20 mg/L,
temperature: 21 ꢀC and pH: 6.0.
The Ni21-recognition and thermo responsive characteristics
of hydrogels with different HBCalix contents are studied by
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FIGURE 8 Observed amounts of Ni21 ions adsorbed on the
P(NIPAM-co-HBCalix) hydrogels at 21 ꢀC and 45 ꢀC during the
temperature swing.
counteract the shrinkage of P(NIPAM-co-HBCalix) hydrogels
like ionic polymer networks during the increase of tempera-
ture. Additionally, the osmotic pressure within the hydrogel
due to a Donnan potential also makes the hydrogel swell
more when the temperature is high enough, thereby result-
ing in the shift of the LCST to a higher temperature.
It can be said that the change in the size of hydrogel-net-
work or the hydrophilic/hydrophobic transition has an im-
portant effect on adsorption performance. At temperature
lower than the LCST, P(NIPAM-co-HBCalix) hydrogels swell
and there is small steric hindrance near the HBCalix cavities
but can provide a room to accept Ni21 ions, which lead to
Ni21 ions easily be included into the HBCalix cavities with
the 5-hexenyloxy groups. The swollen network structure can
provide enough space so that the complex of HBCalix cavity
and Ni21 ion is comparatively high at temperature lower
than the LCST. This decrease in adsorption of P(NIPAM-co-
HBCalix) hydrogels can be attributed to the existence of mul-
tifunctional groups in HBCalix such as four aromatic rings
FIGURE 6 Temperature dependence of volume change ratios
of P(NIPAM-co-HBCalix) and PNIPA hydrogels in deionized
water and Ni21 aqueous solution (20 mg/L) (a-b).
shifts to 33.5 ꢀC. Such an LCST shift indicates the HBCalix
receptors in the copolymer hydrogel can recognize and effec-
tively capture Ni21 into their cavities, and then the formation
of supramolecula “host–guest” HBCalix/metal ion complexes.
The repulsion between charged HBCalix/Ni21 complexes
FIGURE 7 Concept of the thermal-responsive adsorption/desorption behavior of P(NIPAM-co-HBCalix) hydrogel towards Ni21 ions.
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is the equilibrium Ni21 concentration in solution (mg L21),
b is the constant eluted to the affinity binding sites (L
TABLE 1 Langmuir and Freundlich Adsorption Isotherm
Constants of P(NIPAM-co-HBCalix)
mg21), KF is the Freundlich constant, and
Freundlich exponent.
n is the
Langmuir Isotherm
Freundlich Isotherm
KF 1/n
R2
Type qm (mg g21
)
b (L g21
)
R2
Table 1 shows the values of the parameters of the two iso-
therms and the related correlation coefficients. Langmuir
model fits better to the produced data than Freundlich
model according to R2 values. Langmuir isotherm assumes
monolayer adsorption onto a surface containing finite num-
ber sites of uniform strategies of adsorption with no trans-
migration of adsorbate in the plane of surface.34 As also
illustrated in Table 1, the value of 1/n is 0.3825, which indi-
cates favorable adsorption.35 Therefore, the Langmuir model
can be applicable in Ni21 ions removal by P(NIPAM-co-HBCa-
lix) hydrogels.
Ni 3.9908
2.8598
0.9936
2.8135 0.3825 0.9881
linked by methylene bridges and hexenyloxy groups, which
are grafted onto the hydrogels. Therefore, more interactions
with the ions take place. When at temperature higher than
the LCST, the leveling off of adsorption amount after a cer-
tain graft yield is due to the densely packed structure of hex-
enyloxy groups which acts as a barrier after a certain value
of grafting that impedes diffusion of ions into the HBCalix
backbone. A schematic illustration of the thermo-responsive
adsorption/desorption behavior of P(NIPAM-co-HBCalix)
hydrogels toward Ni21 ion is shown in Figure 7. This phe-
nomenon verifies that P(NIPAM-co-HBCalix) hydrogels can
be applied to temperature-controlled Ni21 separation.
CONCLUSIONS
Smart thermo-sensitive P(NIPAM-co-HBCalix) hydrogels capa-
ble of recognizing metal ions have been successfully devel-
oped in this study. The prepared P(NIPAM-co-HBCalix)
hydrogels exhibit good Ni21 adsorption characteristics by
the HBCalix units. The LCST of the P(NIPAM-co-HBCalix)
hydrogel shifts to 33.5 ꢀC due to the formation of HBCalix/
Ni21 host–guest chelating sites. The swollen hydrogel can
provide enough space so that the adsorption amount is com-
paratively high at temperature lower than the LCST. The
adsorption of Ni21 onto the hydrogels reached equilibrium
within about 150 min. When at temperature higher than the
LCST, the hydrogel chains shrink more drastically, and it is
so crowded around the HBCalix cavities, the complex
becomes unstable leading to Ni21 desorption. The experi-
mental adsorption data could be better fitted by Langmuir
isotherm model rather than Freundlich model. After five
adsorption–desorption cycles, the adsorption capacity could
be maintained above 90% with no significant loss in adsorp-
tion properties.
Desorption and Repeated Use
To examine the stability of adsorption performance of
P(NIPAM-co-HBCalix) hydrogels by repeating temperature
swing, the temperature was changed repeatedly between 21
and 45 ꢀC, and the adsorbed amount was measured using
the same the P(NIPAM-co-HBCalix) hydrogels. The result is
shown in Figure 8. The amount adsorbed is high at 21 ꢀC
and low at 45 ꢀC, that is, the P(NIPAM-co-HBCalix) adsorbs
and desorbs reversibly. It can be considered that in metal
chelating systems, chelation (i.e., binding of heavy-metal ion
with HBCalix) is completely reversible. After each adsorp-
tion–desorption cycle, the hydrogels were washed several
times with deionized water. The amount adsorbed at 21 ꢀC
seems to decrease somewhat due to the repetition of TSA.
As the adsorption amount is mg g21 level, when the concen-
tration of Ni21 ions is detected by ICP, the volume of solu-
tion will be lost, even the loss is trace amounts, the amount
adsorbed at 21 ꢀC seems to decrease somewhat. The result
showed the feasibility of the P(NIPAM-co-HBCalix) hydrogels
for Ni21 ion recovery.
ACKNOWLEDGMENTS
The authors thank the financial supports from the National Nat-
ural Science Foundation of China (50973084), Science and
Technical Development Foundation of Colleges and Univer-
sities, Tianjin, China (20071214).
Adsorption Isotherms
An equilibrium isotherm is used to characterize the interac-
tion of Ni21 with P(NIPAM-co-HBCalix) hydrogels. This rep-
resents the relationship between the amount of adsorbate
removed from solution and unit of mass of a sorbent, at the
constant temperatures, at equilibrium. In this work, the Ni21
ions initial concentration was varied from 1 to 20 mg L21
with 24 h of contact time to obtain the experimental adsorp-
tion data. Results were fitted by two parameter Langmuir
(eq (2)) and Freundlich (eq (3)) models,
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