Adsorption and desorption of calcium ions by temperature swing with copolymer of thermosensitive and chelating components grafted on porous ethylene vinyl acetate disk

Article abstract of DOI:10.1016/j.reactfunctpolym.2013.09.006

A novel adsorbent composed of a copolymer of a thermosensitive component, N-isopropylacrylamide (NIPAM), and a chelating component, N-(4-vinyl) benzylethylenediamine (VBEDA), was developed, and the adsorption and desorption behaviors of Ca²⁺ ions were investigated. The copolymer of NIPAM and VBEDA was grafted onto the pore surfaces of a porous ethylene vinyl acetate (EVA) disk by a plasma-initiated polymerization method. The copolymerization ratios of VBEDA were 3 and 6 mol%. Although a hydrophilic component, VBEDA, was copolymerized with NIPAM, the transition temperature of the copolymer was almost the same as that of poly(NIPAM). The permeability through the polymer-grafted EVA disk strongly depended on the temperature. It was quite low below the transition temperature of the grafted polymer, and increased drastically when the temperature exceeded the transition temperature. The polymer-grafted EVA disk adsorbed Ca²⁺ ions at low temperatures and the adsorption amount decreased significantly when the temperature exceeded the transition temperature of the grafted polymer, and repeated adsorption and desorption by temperature swings were possible. Furthermore, it was suggested that one Ca ²⁺ ion was captured by three VBEDA groups, and this multi-point coordination bond was destroyed by heating as a result of shrinkage of the grafted poly(NIPAM-co-VBEDA) chains, leading to desorption of Ca²⁺ ion.

Full text of DOI:10.1016/j.reactfunctpolym.2013.09.006

Reactive & Functional Polymers 73 (2013) 1632–1638
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Adsorption and desorption of calcium ions by temperature swing
with copolymer of thermosensitive and chelating components grafted
on porous ethylene vinyl acetate disk
⇑
Shuji Sakohara , Yusuke Kuriyama, Kensuke Kobayashi, Takehiko Gotoh, Takashi Iizawa
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel adsorbent composed of a copolymer of a thermosensitive component, N-isopropylacrylamide
(NIPAM), and a chelating component, N-(4-vinyl)benzylethylenediamine (VBEDA), was developed, and
the adsorption and desorption behaviors of Ca2+ ions were investigated. The copolymer of NIPAM and
VBEDA was grafted onto the pore surfaces of a porous ethylene vinyl acetate (EVA) disk by a plasma-ini-
tiated polymerization method. The copolymerization ratios of VBEDA were 3 and 6 mol%. Although a
hydrophilic component, VBEDA, was copolymerized with NIPAM, the transition temperature of the
copolymer was almost the same as that of poly(NIPAM). The permeability through the polymer-grafted
EVA disk strongly depended on the temperature. It was quite low below the transition temperature of the
grafted polymer, and increased drastically when the temperature exceeded the transition temperature.
The polymer-grafted EVA disk adsorbed Ca2+ ions at low temperatures and the adsorption amount
decreased significantly when the temperature exceeded the transition temperature of the grafted poly-
mer, and repeated adsorption and desorption by temperature swings were possible. Furthermore, it
was suggested that one Ca2+ ion was captured by three VBEDA groups, and this multi-point coordination
bond was destroyed by heating as a result of shrinkage of the grafted poly(NIPAM-co-VBEDA) chains,
leading to desorption of Ca2+ ion.
Received 8 June 2013
Received in revised form 25 September
2013
Accepted 27 September 2013
Available online 9 October 2013
Keywords:
Thermosensitive polymer
Chelating component
Plasma-initiated polymerization
Temperature swing
Adsorption
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
temperature or the lower critical solution temperature (LCST).
Poly(N-isopropylacrylamide) [poly(NIPAM)] is a representative
The removal of harmful ions such as heavy-metal ions from
aqueous solutions has attracted much attention, and various meth-
ods have been proposed. The use of specific adsorbents containing
ligands such as ion-exchange groups or chelating agents has been
developed as a promising technique [1–3]. These specific adsor-
bents have high adsorption capacities because of the strong inter-
actions between ligands and ions. However, these strong
interactions make regeneration of the adsorbents difficult, and acid
solutions are usually used to release the ions from the adsorbents
[3,4]. As a result, acidic secondary wastes are produced. To allevi-
ate this problem, thermosensitive gels containing ligand groups
have attracted special interest as adsorbents in developing envi-
ronment friendly separation processes [5–15].
thermosensitive polymer with a transition temperature of about
32 °C [16]. A thermosensitive gel composed of a cross-linked ther-
mosensitive polymer network swells below the transition temper-
ature of the polymer, and shrinks above the transition
temperature.
In a previous study, we prepared a copolymer of NIPAM and N-
(4-vinyl)benzylethylenediamine (VBEDA), which is a chelating
component [7,8]. The chemical structures of NIPAM and VBEDA
are shown in Fig. 1. The NIPAM-co-VBEDA gel reversibly adsorbed
and desorbed Cu2+ ions by temperature swings. Furthermore, using
the gel prepared by the molecular imprinting technique proposed
by Lu et al. [17], Cu2+ ions were adsorbed selectively from a solu-
tion containing Cu2+, Zn2+, and Ni2+ ions [7]. The mechanism of
adsorption and desorption of Cu2+ proposed in our previous study
is as follows. At a specific temperature, the adsorption sites, i.e. the
ethylendiamine groups of VBEDA, on the gel network capture Cu2+
by forming a multi-point coordination bond. However, when the
temperature is changed, the multi-point adsorption site is dis-
rupted and the coordination bond between Cu2+ and ethylenedia-
mine is broken by expansion or shrinkage of the thermosensitive
gel network. As a result, the adsorption/desorption of Cu2+ by a
Thermosensitive polymers are soluble in water at low temper-
atures. However, they are insoluble above the intrinsic transition
temperature of the polymer because of a transition to hydropho-
bicity. This intrinsic temperature is referred to as the transition
⇑
Corresponding author. Fax: +81 82 424 7720.
E-mail address: sakohara@hiroshima-u.ac.jp (S. Sakohara).
1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2013.09.006

S. Sakohara et al. / Reactive & Functional Polymers 73 (2013) 1632–1638
1633
the polymer chains shrink significantly, and, as a result, the mul-
ti-point adsorption site is disrupted and the coordination bonds
are broken, leading to desorption of Ca2+. In this study, the temper-
ature dependence of the permeability of water through a copoly-
mer-grafted porous EVA disk, the temperature dependence of the
adsorption amount of Ca2+, the adsorption and desorption proper-
ties of Ca2+ under temperature swings, and the adsorption and
desorption mechanism are examined.
2. Experimental
2.1. Synthesis of VBEDA
Fig. 1. Chemical structures of NIPAM and VBEDA.
The chelating component, VBEDA, was synthesized in the same
manner as that used in our previous study [7], which was based on
the method developed by Lu et al. [17]. First, two solutions were
prepared. One was a 4-vinylbenzyl chloride solution (solution-A),
which was prepared by dissolving 10 mmol of 4-vinylbenzyl chlo-
ride (96%, mixture of meta and para isomers; Across Organic, USA)
in 100 mL of dichloromethane (Kanto Chemical Co., Inc., Japan).
The other was an ethylenediamine solution (solution-B), which
was prepared by dissolving 40 mmol of ethylenediamine (Nakarai
Tesque Inc., Japan) in 200 mL of dichloromethane. Next, solution-A
was slowly dropped into solution-B, taking 2 h, under stirring with
a magnetic stirrer at 30 °C. The stirring was continued for 1 d for
complete reaction. The dichloromethane was then removed using
a rotary evaporator. The VBEDA was obtained as a residue. Subse-
quently, the VBEDA was dissolved in 200 mL of methanol, and then
the solution was dropped into 1 L of diethyl ether to crystallize the
VBEDA. Finally, the VBEDA crystals were collected by filtration, and
dried under reduced pressure for about 1 h.
temperature swing is achieved. Several studies of thermosensitive
gel adsorbents containing ionic groups that adsorb metal ions via
the formation of an ionic bond have been reported [5]. The ionic
groups strongly adsorb metal ions, but spontaneous desorption
by the expansion or shrinkage of the gel network is quite difficult
because of the strong interactions. Multi-point adsorption by coor-
dination bonding is a promising way of controlling adsorption/
desorption by temperature swings.
Thermosensitive gel adsorbents have been successfully pre-
pared, and adsorption and desorption by temperature swings have
been achieved, as mentioned above. However, the adsorption rates
of heavy-metal ions on the adsorption sites of the gel network
were extremely low because the diffusion of ions to the adsorption
sites is retarded by the gel network [18,19]. To alleviate this prob-
lem, in this study, we used a thermosensitive polymer adsorbent,
and the adsorption and desorption behaviors of Ca2+, which is a
representative component of hard water, under temperature
swings were investigated. Although the adsorption of Ca2+ with
thermosensitive gels has already been reported, experimental ver-
ification of adsorption/desorption by temperature swings has not
yet been obtained [20,21]. The thermosensitive polymer adsorbent
was prepared by grafting a copolymer of NIPAM and VBEDA on the
pore surfaces of a porous ethylene vinyl acetate (EVA) disk. The
adsorption and desorption mechanism is considered to be as fol-
lows. At temperatures lower than the transition temperature of
poly(NIPAM-co-VBEDA), interaction between Ca2+ and VBEDA
forms a multi-point coordination bond, which is expected to have
octahedral coordination [22], leading to adsorption, as shown in
Fig. 2. At temperatures higher than the transition temperature,
2.2. Preparation of copolymers of NIPAM and VBEDA, and
measurement of their transition behaviors
The copolymer of NIPAM and VBEDA was prepared by radical
polymerization. NIPAM (Kohjin Film & Chemicals Co., Ltd., Japan)
was purified by crystallization from hexane. The polymerization
was carried out using N,N,N0,N0-tetramethylethylenediamine
(TEMED, Kishida Chem. Co., Ltd., Japan) and ammonium peroxodi-
sulfate (APS, Kanto Chem. Co., Inc., Japan) as the polymerization
accelerator and initiator, respectively. NIPAM, VBEDA, and TEMED
of the desired concentrations were dissolved in distilled water, and
dissolved oxygen was removed by purging with nitrogen gas. Then,
Fig. 2. Schematic diagram of adsorption and desorption mechanism of Ca2+
.

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S. Sakohara et al. / Reactive & Functional Polymers 73 (2013) 1632–1638
Table 1
Table 2
Preparation condition of poly(NIPAM-co-VBEDA).
Preparation condition of polymer-grafted EVA disk.
Concentration
(mol/m3)
NIPAM
VBEDA
Grafted amount
(g-polymer/g-EVA)
(mol/m3)
(mol/m3)
Monomer: N-isopropylacrylamide (NIPAM)
Co-monomer: N-(4-vinyl)benzylethylenediamine
(VBEDA)
1000 1000
Disk-A
Disk-B
1000
1000
30
60
0.239
0.215
30
60
Accelerator: N,N,N’,N’-tetramethylethylenediamine
(TEMED)
Initiator: Ammonium peroxodisulfate
6
6
2.3. Preparation of poly(NIPAM-co-VBEDA) grafted on pore surfaces in
disk-type porous ethylene vinyl acetate
2
2
an APS solution of the desired concentration, from which the dis-
solved oxygen had already been removed, was added to the mix-
ture. The polymerization was carried out at 10 °C for 6 h under a
nitrogen atmosphere. The copolymerization ratios of VBEDA were
3 and 6 mol%. The preparation conditions are shown in Table 1.
The prepared copolymer was purified by dialysis over a period of
one week, using a dialysis membrane (Cellu Step T3, nominal
MWCO: 12,000–14,000, Membrane Filtration Products, Inc.) The
molecular weight of poly(NIPAM) prepared in our previous study
[23] under the same polymerization condition as that in this study
was more than 1 ꢀ 107. Therefore, it was considered that the
molecular weights of these copolymers prepared in this study were
also very high, more than 1 ꢀ 107, although it was not measured
because it was difficult to measure them by a gel permeation
chromatography.
Since it is well known that the transition temperature of poly(-
NIPAM) increases on copolymerization with a hydrophilic compo-
nent, the transition temperature of the prepared copolymer,
poly(NIPAM-co-VBEDA), was estimated from the temperature
dependence of the transmittance through the polymer solution,
in the same manner as reported in our previous article [24]. An
aqueous solution of a thermosensitive polymer is transparent be-
low the transition temperature of the polymer because the poly-
mer molecules are hydrophilic and soluble in water. However,
when the solution is heated above the transition temperature,
the solution becomes milky white because the polymer molecules
become hydrophobic, and therefore insoluble in water. The transi-
tion temperature is therefore usually estimated based on the
changes in transmittance through the polymer solution with tem-
perature. The transmittance was measured at 600 nm using a spec-
trophotometer equipped with a temperature control system (V-
530, Japan Spectroscopy Co., Ltd.) The polymer concentration was
0.5 wt.%.
Poly(NIPAM-co-VBEDA) was grafted on the pore surfaces of a
porous EVA disk using a plasma-initiated polymerization method.
The plasma-initiated graft polymerization technique has previ-
ously been used to graft polymers on the surfaces of polymer sub-
strates [25,26]. The porous EVA (thickness: 4 mm) was kindly
supplied by the Hitachi Chemical Co., Ltd., (Japan). The procedure
was basically the same as that used in our previous study [27].
Fig. 3 shows a schematic diagram of the argon plasma treatment
apparatus. A scanning electron microscopy (SEM) photograph of
the inner structure of the porous EVA is also shown in Fig. 3. The
porosity of the porous EVA is about 80%, and the average pore size
is about 60 lm, although the pore size distribution is quite wide. A
piece of the porous EVA (25 ꢀ 25 mm) was suspended in a reactor
by a fine glass hook. In this work, the direct plasma treatment, in
which the sample EVA was placed in the plasma zone, was used
because it is effective in the modification of the sample surface
[28,29]. First, the reactor was evacuated fully with a vacuum pump.
Then, argon gas was introduced into the evacuated reactor, and the
pressure in the reactor was adjusted to about 10 Pa by controlling
the flow rate of the argon gas. Then, the porous EVA was treated for
1 min with a radio-frequency argon plasma operated at 40 W and a
frequency of 13.56 MHz. After the treatment, the porous EVA was
detached from the glass hook by shaking the reactor lightly. Then,
the monomer solution containing NIPAM and VBEDA of the desired
composition, from which oxygen had previously been removed by
purging with nitrogen gas, was poured into the reactor under re-
duced pressure to introduce the monomer solution into the pores
of the porous EVA. The porous EVA was immersed in the monomer
solution for the desired period at 25 °C under a nitrogen atmo-
sphere. The polymer-grafted EVA was cut into a disk of diameter
20 mm and washed with distilled water. The weight of the grafted
copolymer was evaluated from the difference between the masses
of the disk before and after grafting of the copolymer. The
Fig. 3. Schematic diagram of argon plasma treatment apparatus.

S. Sakohara et al. / Reactive & Functional Polymers 73 (2013) 1632–1638
1635
solution, and m is the weight of the copolymer grafted on the por-
ous EVA disk.
The adsorption of Ca2+ on the porous EVA disk without the
grafted copolymer was also examined. However, the adsorption
amount was very small, and was difficult to estimate precisely.
Therefore, in these experiments, adsorption on the porous EVA disk
used as a supporting material was neglected.
The desorption experiments were performed without pressuriz-
ing because the permeability was high. The procedure for the
desorption experiments was as follows. Ca2+ ions were adsorbed
on the polymer-grafted EVA disk by permeating with CaCl2 solu-
tion (concentration of Ca2+: 30 mg/L), using the method described
above. Then, the disk was set in the apparatus, which had previ-
ously been heated to the desired temperature above the transition
temperature of the grafted copolymer, and CaCl2 solution (concen-
tration of Ca2+: 30 mg/L) heated at the same temperature was per-
meated through the disk. Then, the concentration of Ca2+ in the
solution permeated through the disk was measured in the same
manner as described above.
In the repeated adsorption and desorption experiments with
temperature swings, the adsorption and desorption procedures de-
scribed above were repeated several times.
Fig. 4. Schematic diagram of apparatus for measuring permeability through
polymer-grafted EVA disk.
3. Results and discussion
copolymerization ratios of VBEDA grafted on the porous EVA disks
were 3 and 6 mol%. The grafted polymer weights are shown in
Table 2.
3.1. Transition behavior of poly(NIPAM-co-VBEDA)
Fig. 5 shows the temperature dependences of the transmittance
through solutions of copolymers of NIPAM and VBEDA prepared
with two different compositions. The results for poly(NIPAM) are
also shown for comparison. The transmittance of poly(NIPAM)
changed drastically at around 32 °C, and those of the copolymers
of NIPAM and VBEDA also changed drastically at almost the same
temperature. Usually, the transition temperature of poly(NIPAM)
increases on its copolymerization with a hydrophilic component.
However, for poly(NIPAM-co-VBEDA) solutions, the transition tem-
perature scarcely changed, even when 6 mol% VBEDA was copoly-
merized with NIPAM. Although the reason is not yet clear, it is
considered that the benzene ring in the VBEDA causes this
phenomenon.
2.4. Measurement of permeability through polymer-grafted EVA disk
Fig. 4 shows a schematic diagram of the apparatus used to mea-
sure the permeability of water through the polymer-grafted EVA
disk. The disk was set in the apparatus, and distilled water was per-
meated through the disk by pressurizing with nitrogen gas at the
desired temperature. Since the grafted copolymer chains expand
in the pores at temperatures lower than the transition temperature
of the grafted copolymer, the permeation resistance of water
through the pores is large. The distilled water was therefore pres-
surized to achieve permeation. The permeability through the poly-
mer-grafted EVA disk, k, was defined by the following equation:
D
l
P
L
F ¼ Ak
ð1Þ
3.2. Temperature dependence of permeability of water through
polymer-grafted EVA disk
where F is the flow rate of water permeated through the polymer-
grafted EVA disk, A is the cross-sectional area of the polymer-
Fig. 6 shows the temperature dependences of water permeabil-
ity for two polymer-grafted EVA disks prepared with different
grafted EVA disk,
DP is the pressure difference between the two
sides of the disk, L is the thickness of the disk, and
l is the viscosity
of water at the experimental temperature.
2.5. Ca2+ adsorption/desorption experiments
The Ca2+ adsorption experiments were performed using the
same apparatus as that used for the permeation experiments,
shown in Fig. 4. The polymer-grafted EVA disk was set in the appa-
ratus. Then, CaCl2 aqueous solution of the desired concentration
was permeated through the disk in the same manner as in the
measurements of the permeability of water, described above; the
permeated solution was collected at desired time intervals, and
the concentration of Ca2+ in the collected sample was measured
by chelatometric titration. The adsorption amount, Q, was calcu-
lated using the following equation:
ðC0 ꢁ CÞV
Q ¼
ð2Þ
m
where C0 and C are the concentrations of Ca2+ in the solution before
and after adsorption, respectively, V is the volume of the sample
Fig. 5. Temperature dependence of transmittance through solution of poly(NIPAM-
co-VBEDA).

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S. Sakohara et al. / Reactive & Functional Polymers 73 (2013) 1632–1638
Fig. 7. Time course of adsorption amount of Ca2+ on disk-A.
Fig. 6. Temperature dependence of permeability of water through polymer-grafted
EVA disk.
polymer compositions, i.e. 3 and 6 mol% VBEDA. These are denoted
by disk-A and -B, respectively. The permeabilities through these
disks were quite low below the transition temperature of the
grafted copolymer, around 32 °C, and increased drastically when
the temperature exceeded the transition temperatures. At temper-
atures lower than the transition temperature, the grafted copoly-
mer chains expand in the pores, leading to a large resistance to
permeation of water through the pores. However, at temperatures
higher than the transition temperature, the copolymer chains
shrink, leading to a small permeation resistance through the pores.
The permeability through disk-A was smaller than that through
disk-B, although the difference between the weights of the copoly-
mer grafted on these disks was not large. The reason has not yet
been clarified. Furthermore, the distribution of the grafted copoly-
mer in the porous EVA disk was not examined because detection of
the grafted copolymer was quite difficult. From the SEM photo-
graph of a section of the disk, it seems that the copolymer was
not grafted on the central part of the disk. These results will be
investigated further. Furthermore, in our apparatus, the size of
the plasma zone is not enough for the sample size of EVA. This im-
plies that it is difficult to treat uniformly the sample EVA. It will
also affect the uniformity of the grafted amount of the copolymer
on the sample EVA.
Fig. 8. Temperature dependence of equilibrium adsorption amounts for disk-A and
-B.
3.4. Effect of temperature and VBEDA concentration on equilibrium
adsorption amount
Fig. 8 shows the temperature dependences of the equilibrium
adsorption amount for disk-A and -B. The equilibrium adsorption
amount decreased with increasing temperature, and above the
transition temperature of the grafted copolymer, it was very small.
This implies that the adsorption sites, i.e. the ethylenediamine
groups that adsorb Ca2+ ions were hindered by the shrunk polymer,
making it difficult to form complexes of Ca2+ and VBEDA. At a low-
er temperature, around 20 °C, the adsorption amount on disk-B,
which was prepared with 6 mol% VBEDA, was almost double that
on disk-A, which was prepared with 3 mol% VBEDA. In this temper-
ature region, the grafted polymer chains expand, and it is easy to
adsorb Ca2+ on the adsorption sites, and the equilibrium adsorption
amount was almost proportional to the copolymerized composi-
tion of VBEDA.
It is difficult to know the copolymerization ratio of VBEDA in
the grafted polymer. If it is assumed that the copolymerization ra-
tio of VBEDA in the grafted copolymer is the same as that in the
monomer solution, i.e., the mixture of NIPAM and VBEDA, the
amount of VBEDA in the copolymer grafted on disk-A is
0.0127 mmol. In Fig. 8, the adsorption amount of Ca2+ at 20 °C on
disk-A was 0.00381 mmol. Therefore, in this condition, the amount
of used adsorption sites in the grafted copolymer, in other words,
3.3. Time course of adsorption amount of Ca2+
Fig. 7 shows the time courses of the adsorption amounts of
Ca2+ obtained using permeation rates of about 20 and 12 mL/
min, which were controlled by the applied pressure. Disk-A, with
the copolymer with 3 mol% VBEDA, was used. The concentration
of Ca2+ in the solution was 30 mg/L, and the temperature was
20 °C, which was lower than the transition temperature of the
grafted copolymer. The adsorption amount of Ca2+ increased with
time, and reached equilibrium within 12 and 25 min for these
two permeation rates, i.e. 20 and 12 mL/min, respectively. The
equilibrium adsorption amounts obtained using these two perme-
ation rates were almost the same. Furthermore, the equilibrium
adsorption amount was measured at the permeation rate of
5 mL/min. The value was also same as those obtained at the per-
meation rates of 20 and 12 mL/min. These results suggest that the
adsorption rate by the polymer is quite high compared with that
by the gel [19], as expected, and that the equilibrium adsorption
amount might be the same as that measured under the stationary
condition.

S. Sakohara et al. / Reactive & Functional Polymers 73 (2013) 1632–1638
1637
Fig. 11. Adsorption and desorption by temperature swings.
Fig. 9. Relationship between amount of VBEDA in grafted copolymer and adsorp-
tion amount of Ca2+
.
the amount of VBEDA used to form the complex with Ca2+ was
0.0114 mmol because three VBEDA is used to form complex with
Ca2+ according to the adsorption mechanism. This result implies
that 90% of the adsorption sites were used. However, it is consid-
ered to be too high. The copolymerization ratio of VBEDA in the
grafted copolymer might be larger than that in the monomer
solution.
ated volume reached 800 mL, i.e. until the adsorption amount
reached the equilibrium adsorption amount, the temperature
was kept at 20 °C. Then, the temperature was increased to 60 °C,
which is higher than the transition temperature of the grafted
copolymer. For both disks, by increasing the temperature, the
adsorption amount decreased, i.e. desorption of Ca2+ occurred. It
is considered that the multi-point adsorption sites shown in
Fig. 2 are disrupted and the coordination bonds are broken as a re-
sult of shrinkage of the grafted copolymer chains on heating above
the transition temperature of the grafted copolymer.
Fig. 11 shows the changes in the adsorption amount as a result
of temperature swings between 20 and 60 °C for disk-A and -B. The
permeated volume for adsorption was 800 mL, and that for desorp-
tion was 700 mL. For both disks the reversible adsorption and
desorption were observed. The remaining adsorbed amount after
desorption on both disks were almost the same. As a result, the dif-
ference between the adsorption and desorption amounts for disk-B
was significantly larger than that for disk-A, and 75% of the ad-
sorbed amount was desorbed in the case disk-B.
Fig. 9 shows the relationship between the copolymerized
amount of VBEDA (
the grafted amount of polymer and the adsorption amount of
Ca2+
mol). In estimating the copolymerized amount of VBEDA,
lmol) in the grafted polymer estimated from
(l
it was assumed that the copolymerized ratio of VBEDA was the
same as the ratio of VBEDA in the monomer solution. The adsorp-
tion amount of Ca2+ is proportional to the amount of VBEDA, and
the slope is close to 1/3. This implies that the complex is formed
with one Ca2+ and three VBEDA groups, and that the adsorption
mechanism shown in Fig. 2 is appropriate.
QmaxKCeq
1 þ KCeq
Q ¼
ð3Þ
4. Conclusion
3.5. Desorption of Ca2+ and adsorption/desorption by temperature
swings
A copolymer of NIPAM and VBEDA was successfully grafted on a
porous EVA disk. The permeability of the solution through the disk
strongly depended on the temperature because the grafted copoly-
mer expanded at temperatures below the transition temperature
of the grafted copolymer, and shrank at temperatures higher than
the transition temperature. The adsorption amount of Ca2+ also
strongly depended on the temperature, and decreased significantly
at temperatures higher than the transition temperature of the
grafted copolymer. The repeated adsorption and desorption of
Ca2+ by temperature swings was possible. It is suggested that one
Ca2+ was captured by three VBEDA groups. The thermosensitive
polymer adsorbent proposed in this study is a promising candidate
as an adsorbent for developing environmentally friendly separa-
tion processes.
Fig. 10 shows the changes in the adsorption amount obtained
by changing the temperature for disk-A and -B. Until the perme-
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