Efficient removal of pentachlorophenol from aqueous solution by 4-: Tert -butylcalix[8]arene modified thermally sensitive hydrogels

Article abstract of DOI:10.1039/c8ra00392k

We prepared poly(N-isopropylacrylamide-co-4-tert-butylcalix[8]arene) (PNIPAM-TBCX) hydrogels by copolymerization of N-isopropylacrylamide (NIPAM) with 4-tert-butylcalix[8]arene (TBCX) to capture hazardous pentachlorophenol (PCP) from aqueous solution. Adsorption experiments showed that the adsorption capacities of PNIPAM-TBCX hydrogels reached 1.96, 2.08 and 2.02 mg PCP per 1 g of hydrogel, while the molar percentage ratio of TBCX in the hydrogels was as low as 0.5%, 0.7% and 1%. The equilibrium adsorption of PCP on the hydrogels was studied using different adsorption models. In addition, the PNIPAM-TBCX hydrogel still retained its performance when regenerated several times by immersing in water at 323 K.

Full text of DOI:10.1039/c8ra00392k

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PAPER
Efficient removal of pentachlorophenol from
aqueous solution by 4-tert-butylcalix[8]arene
modified thermally sensitive hydrogels
Cite this: RSC Adv., 2018, 8, 6840
ab
c
ab
ab
ab
Ying Guo, Bing Liao, Kun Wang, Yangyang Zhao, Qiwen Yong,
Hongwei Zhao and Hao Pang *a
ab
We prepared poly(N-isopropylacrylamide-co-4-tert-butylcalix[8]arene) (PNIPAM-TBCX) hydrogels by
copolymerization of N-isopropylacrylamide (NIPAM) with 4-tert-butylcalix[8]arene (TBCX) to capture
hazardous pentachlorophenol (PCP) from aqueous solution. Adsorption experiments showed that the
adsorption capacities of PNIPAM-TBCX hydrogels reached 1.96, 2.08 and 2.02 mg PCP per 1 g of
hydrogel, while the molar percentage ratio of TBCX in the hydrogels was as low as 0.5%, 0.7% and 1%.
The equilibrium adsorption of PCP on the hydrogels was studied using different adsorption models. In
addition, the PNIPAM-TBCX hydrogel still retained its performance when regenerated several times by
immersing in water at 323 K.
Received 13th January 2018
Accepted 2nd February 2018
DOI: 10.1039/c8ra00392k
rsc.li/rsc-advances
1
3
14
15
nanotubes, cobalt ferrite and alumina nanoparticles,
1
. Introduction
16
17
chitin and hydrogels. Among these multifarious adsorption
Pentachlorophenol (PCP) is a widely distributed pollutant. It materials, smart hydrogel is unique in that it can be easily
was commonly used as a wood preservative and it is hard to regenerated. The high swelling ratio and specic surface area,
18
19
degrade, meanwhile it could be derived from atmospheric connected porous structure, thermo-sensitive property and
20
oxidation of a highly persistent, globally distributed organic feasible modication made the PNIPAM hydrogel widely used
1
21
22
pollutant, hexachlorobenzene. Long-term exposure to PCP, in adsorption and drug delivery.
23
even at low levels, can cause serious damage to the liver,
The calixarene (CX) is a macrocyclic cavitand compound
kidneys, blood, reproductive organs and nervous system. PCP with two portals that can form complexes with small ions and
is a hydrophobic ionizable organic compound due to its high organic molecules by hydrogen bonding, van der Waals' force,
2
octanol–water partition coefficient (log k ¼ 5.25) and low pK
hydrophobic interactions or shape complementation. There-
value (4.5). It exists in the presence of both ionic and molecular fore, the CX and its derivatives show high supramolecular
ow
a
3
species in aqueous solution.
recognition and enrichment capabilities with organic mole-
Various biological, chemical and physical techniques have cules and ions. By combining the merits of p-sulfonated calix
24
4
been developed to remove PCP, such as microbe degradation,
[8]arene and the single-walled carbon nanohorns, a highly
5
3
biological and synthetic enzyme catalysis, photocatalytic sensitive dual-signalling electrochemical sensing platform for
6
7
25
oxidation, electrochemical processes, dechlorination with Fe/ a organic molecule aconitine has been developed. Based on
8
Ni nanomaterials, and adsorption strategies. Because of its the conformation of calix[4]arene bearing diverse tails, a novel
facile operation and high efficiency, the adsorption method uorometric chemosensor displaying selective response toward
+
26
attracts the attention of many researchers. The adsorption Ag among different metal ions and a calix-conjugated
would take place if the kinetic energy of adsorbate is not suffi- thermo-responsive hydrogel used to remove nickel(II) from its
27
cient to overcome the potential barrier of the residual free force aqueous solution have been prepared. When PCP dissolved in
ꢀ
1

eld on the adsorbent surface. Thus, high special surface area is neutral ionized water, at low concentrations from 6 mg L to
ꢀ
1
essential to adsorb PCP. Several types of materials have been 20 mg L , the ratio of molecular and ionized PCP species
used as adsorbents, such as natural products (eggshells, corn ranges from 0.84 to 1.54, and the pH value of PCP aqueous
wastes, biochar and so forth), boron nitride and carbon solution is from 4.57 to 4.31. The ratio of molecular and ionized
PCP species increases with the decreasing of pH value for PCP
9
10
11
12
aqueous solution. The poly(N-isopropylacrylamide) (PNIPAM)
hydrogel absorbs more the molecular than the ionized PCP
species due to hydrophobic interaction. Thus, the poly(N-iso-
a
Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of
Chemistry, Chinese Academy of Sciences, Guangzhou 510650, People's Republic of
China. E-mail: panghao@gic.ac.cn
b
propylacrylamide) (PNIPAM) hydrogel was hydrophobically
University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China
c
modied to enhance the adsorption for PCP under the
Guangdong Academy of Sciences, Guangzhou 510650, Republic of China
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condition of the pH value of 3 in the published literature. The
above mentioned method complicated the purication of arene. Referring to the dual acylation method of 1,1 -bi-2-
sewage and caused a secondary pollution. The others reported naphthol and the single acylation method of 1,3-dihydrox-
RSC Advances
17
2.2.2 Synthesis of O(1)-(4-pentenoyl)-p-tert-butylcalix[8]
0
3
0
3
1
that some adsorbents could be used to remove PCP from ybenzene, we synthesized O(1)-(4-pentenoyl)-p-tert-butylcalix
aqueous solution without adjusting pH value, but difficult to [8]arene (Fig. 1). A suspension of 1.30 g (ca. 1.0 mmol) 4-tert-
9,28
recycle. Taking into account that PCP exists in the presence of butylcalix[8]arene and 1.05 g (ca. 1.01 mmol) triethylamine in
ionic and molecular species in aqueous solution and CX can 15 mL of trichloromethane was cooled to 273 K. Over a period of
bind different polar substances, we synthesized a novel thermo- 20 min, a solution of 1.19 g (1.0 mmol) of 4-pentenoyl chloride
sensitive hydrogel (PNIPAM-TBCX). The PNIPAM-TBCX hydro- in 10 mL trichloromethane was added. Then the mixture was
gel was comprised of NIPAM and 4-tert-butylcalix[8]arene, and warmed to room temperature naturally and stirred for two
the calixarene with rigid benzene ring was proposed to increase additional hours to ensure complete reaction. The ltrate was
the porous size and surface area of the hydrogel and to enhance obtained by pumping ltration, and washed with HCl aq.
the interaction between the hydrogel and PCP. In addition, the (5 wt%), NaOH aq. (5 wt%) and water, resulting in an almost
thermo-sensitive PNIPAM-TBCX hydrogel could be simply colorless trichloromethane solution. This trichloromethane
regenerated by immersing it in the water at temperature higher solution was dried over magnesium sulfate, ltered and
than its lower critical solution temperature.
concentrated by rotary evaporation to yield skin-pink solid.
In this study, we have synthesized the PNIPAM-TBCX
2.2.3 Synthesis of the PNIPAM-TBCX hydrogels. Poly(N-
hydrogels with different 4-tert-butylcalix[8]arene contents. The isopropylacrylamide-co-4-tert-butylcalix[8]arene)
(PNIPAM-
adsorption kinetics of PCP on the hydrogels and the reusability TBCX) hydrogels (Fig. 1) were synthesized via aqueous
of the hydrogels were investigated. We also discussed the micellar polymerization. A xed molar ratio of NIPAM (0.5 g)
interactions between the hydrogels and PCP to reveal the and BIS (10 : 1) were dissolved in 4.5 g deionized water, and
adsorption mechanism.
then different amounts of O(1)-(4-pentenoyl)-p-tert-butylcalix[8]
arene (0.5–1 mol% of NIPAM) were added into the mixture. The
solution was degassed by bubbling nitrogen for 20 min. Then,
sodium dodecyl sulfate (SDS, 35 mass times of O(1)-(4-pente-
noyl)-p-tert-butylcalix[8]arene) was added, and the hydrophobic
monomers were solubilized within the surfactant micelles. Aer
adding APS and TEMED as a redox initiator, the polymerization
proceeded at 323 K for 24 h. The prepared hydrogels were sliced
into akes with the thickness of 3–4 mm, and immersed in
trichloromethane for 24 h, in ethanol for 24 h and in deionized
water for 7 days (replaced water every 8 h) in succession to
thoroughly leach out unreacted monomers and other unreacted
ingredients. The dried hydrogels were obtained by vacuum
drying under 323 K to constant weight.
2. Experimental
2.1 Materials
All experiments were performed with deionized water at a neutral
pH. N-Isopropylacrylamide (NIPAM) (98%, Aladdin Co., Ltd,
Shanghai, China) was puried via recrystallization from n-hexane
and dried under vacuum at room temperature. p-tert-Butylphe-
0
0
0
nol, N,N -methylenebis(acrylamide) (BIS), N,N,N ,N -tetramethy-
lenediamine (TEMED), ammonium persulfate (APS), sodium
dodecyl sulfate, sodium hydroxide, triethylamine, 4-pentenoyl
chloride were all of analytical grade, and used without further
purication.
2.2.4 Adsorption experiments. The batch adsorption
experiments were conducted by adding the PNIPAM-TBCX
xerogels into PCP solutions of different concentrations at 303
K for 24 hours shaking. All PCP solutions were obtained by
2.2 Experiments
2.2.1 Synthesis of 4-tert-butylcalix[8]arene. We synthesized dissolving PCP methanol solution in deionized water.
4
-tert-butylcalix[8]arene (Fig. 1) according to the literature
To evaluate the adsorption kinetic characteristics, 0.49 g
2
9
ꢀ1
procedure. A slurry prepared from 50 g (ca. 0.33 mol) of p-tert- PNIPAM-TBCX hydrogels were added in 300 mL 0.01 mmol L
butylphenol, 17.5 g (ca. 0.55 mol) of paraformaldehyde, and PCP solution with magnetic stirring. At intervals, 1.0 mL PCP
ꢀ
1
1
.0 mL (0.01 mol) of 10 mol L sodium hydroxide in 300 mL of solution was withdrawn and analyzed via high performance
o-xylene were placed in a 1 L, round-bottomed, three-necked liquid chromatography.
ask tted with a Dean–Stark water collector and a magnetic
For the determination of the equilibrium adsorption

stirrer. The air in the ask was replaced with nitrogen. The isotherm, 10 mg PNIPAM-TBCX hydrogels were added into 7 mL
reaction mixture was heated to reux over 2 h and kept reux PCP solution of different concentrations and were shaken for
for 4 h. Then the mixture was placed to cool down to room 24 h.
temperature, and was ltrated to collect precipitate. The crude
2.2.5 Desorption experiments. The adsorption–desorption
product was washed, successively, with 400 mL portions of process was used to investigate hydrogels' regeneration. The
toluene, ether, acetone and water, then dried under reduced hydrogel saturated with PCP was immerged into a batch of
pressure. It was subsequently dissolved in ca. 800 mL of deionized water for 24 h at 323 K to regenerate. The regenerated
boiling chloroform. The chloroform solution was concen- hydrogel was used for the next adsorption process, and the
trated to ca. 600 mL and cooled to room temperature. The same adsorption–desorption procedure was performed in the
precipitate was collected by ltration to yield a colorless succeeding cycles.
powder.
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Fig. 1 Synthesis scheme of 4-tert-butylcalix[8]arene (a), O(1)-(4-pentenoyl)-p-tert-butylcalix[8]arene (b), PNIPAM-TBCX hydrogels and dia-
grammatic sketch of the PNIPAM-TBCX hydrogels in PCP aqueous solution.
2
.3 Analysis and characterization methods
3
. Results and discussion
The concentration of PCP solution was detected by ultraviolet
spectrometer (Shimadzu UV-1800) and high performance liquid
chromatography (Shimadzu) using a chromatographic column
3.1 Characterization
1
13
The H and C NMR spectra of 4-tert-butylcalix[8]arene and O(1)-
4-pentenoyl)-p-tert-butylcalix[8]arene were shown in Fig. 2 and 3,
respectively.
-tert-Butylcalix[8]arene H NMR (400 MHz, CDCl , ppm):
(
(
InertSustain C18) with a UV determine wavelength 220 nm. The
constituent of mobile phase was acetonitrile/water/acetic acid ¼
5/25/1. The functional groups of hydrogels were analyzed by the
Fourier Transform Infrared Spectroscopy (FTIR, Bruker Tensor
1
4
3
7
d9.63 (8H, s, OH), d7.18 (16H, s, ArH), d4.38–4.35 (8H, d,
1
3
1
13
CH
NMR (400 MHz, CDCl
46.6).
O(1)-(4-Pentenoyl)-p-tert-butylcalix[8]arene
2
(a)), d3.52–3.43 (8H, d, CH
2
(b)), d1.25 (72H, s, CH
3
).
C
27) with KBr pellet pressing method. H and C NMR spectra
3
, ppm): d(31.5, 32.3, 125.5, 128.7, 144.7,
were recorded on a DRX-400 MHz (Bruker) nuclear magnetic
resonance (NMR) spectrometer. The morphology of hydrogels
was observed by FEI-quanta-200F scanning electron microscope.
The BET surface area was detected by Micromeritics ASAP 2020.
The investigation of equilibrium swelling ratio (SR) was
carried out under isothermal conditions ranging from 273 to
1
1
H
NMR (400
MHz, CDCl , ppm): d7.19 (s, ArH), d5.82 (s, CH), d5.08 (CH ),
3
2
d3.71 (CH ), d2.48 (CH ), d2.27 (CH ), d5.82 (CH), d1.26 (s, CH ).
2
2
2
3
1
3
C NMR (400 MHz, CDCl , ppm): d(31.3, 34.3, 116.2, 136.1,
3
142.2, 148.3, 171.2).
323 K. The swelling equilibrium was reached by immerging
Fig. 4 showed the FTIR spectra of PNIPAM-TBCX hydrogels.
hydrogels into deionized water until constant weight at
different temperatures. The swelling ratio (SR) was calculated as
The carboxyl group stretching peak of ester appeared at
ꢀ1
ꢀ1
1
3
659 cm , and the C–O–C of ester at 1180 cm . Both
617 cm and 719 cm belonged to phenolic hydroxyl group,
SR ¼ m
h x h x
/m , where m and m represented the weight of
ꢀ1
ꢀ1
hydrogel and xerogel, respectively.
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1
Fig. 2 H NMR spectra of 4-tert-butylcalix[8]arene and O(1)-(4-pen-
tenoyl)-p-tert-butylcalix[8]arene.
Fig.
5
Swelling ratios of different PNIPAM-TBCX hydrogels as
a function of temperature. -, PNIPAM-TBCX-0.5% hydrogel; , PNI-
PAM-TBCX-0.7% hydrogel; , PNIPAM-TBCX-1% hydrogel.
the above data indicated that the PNIPAM-TBCX hydrogels were
synthesized.
3
.2 Swelling ratios analysis
Fig. 5 showed the temperature dependence of the swelling ratio
SR) for the PNIPAM-TBCX hydrogels. It could be seen that
(
hydrogels with higher calixarene content exhibited lower SR. As
the temperature rose, van der Waals' forces and hydrogen
bonds between water and amide broke up, polymer chains
shrank, and the hydrogels shrank macroscopically. At last, SR
approached zero in the high temperature region. From 273 K to
3
1
20 K, the SR of PNIPAM-TBCX-0.5% hydrogel went down from
2.32 to only 1.33. The SR curves had a sharp turn at 303 K,
1
3
Fig. 3
C NMR spectra of 4-tert-butylcalix[8]arene and O(1)-(4-
pentenoyl)-p-tert-butylcalix[8]arene.
which pointed that the LCST of PNIPAM-TBCX hydrogels was
around 303 K.
3.3 Morphology and specic surface area analysis
Fig. 6 showed the SEM results of the cryodesiccated PNIPAM-
TBCX hydrogels with different TBCX contents under 2000 and
500 magnications. It could be seen that all the PNIPAM-TBCX
hydrogels exhibited porous and net-shaped structures, and the
pores and grids became lager with the increase of TBCX, which
was consistent with the pore structure parameters of the
PNIPAM-TBCX hydrogels, as showed in Table 1. Within the
PNIPAM-TBCX hydrogel, PNIPAM was cross-linked with TBCX
which had rigid aromatic ring, providing larger porous and
three-dimensional network structure with larger gap. However,
too much calixarene contents would result in bad dispersing in
the hydrogel, which would cause the porous structure of
hydrogel to become agglomerated and inhomogeneous, as
showed in the SEM images of PNIPAM-TBCX hydrogel-1%.
Fig. 4 Infrared spectra of the PNIPAM-TBCX hydrogels.
ꢀ1
and 3077 cm belonged to C–H of calixarene. The PNIPAM 3.4 Effects of calixarene contents on the adsorption
segment showed the characteristic of N–H stretching vibration performance of hydrogels
ꢀ
1
ꢀ1
at 3286 cm , N–H in-plane bending vibrations at 1545 cm
Fig. 7 showed the removal efficiencies of PCP adsorbed by the
PNIPAM-TBCX hydrogels. Although PNIPAM-TBCX-1% hydro-
gel had higher BET surface area and pore volume than PNIPAM-
ꢀ1
and C–H stretching vibration at 2973, 2938 and 2878 cm . All
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Fig. 6 SEM images of the PNIPAM-TBCX hydrogels. A, B, C are the SEM results of PNIPAM-TBCX-0.5% hydrogel, PNIPAM-TBCX-0.7% hydrogel,
0
0
0
PNIPAM-TBCX-1% hydrogel under 2000 magnifications, and A , B , C are their SEM results under 500 magnifications.
Table 1 Surface area, pore volume and pore size of the PNIPAM-TBCX hydrogels
BET surface
area (m g
2
ꢀ1
a
3
ꢀ1
b
)
Pore volume (cm g
)
Pore size (nm)
ꢀ
ꢀ
ꢀ
3
3
2
PNIPAM-TBCX-0.5% hydrogel
PNIPAM-TBCX-0.7% hydrogel
PNIPAM-TBCX-1% hydrogel
17.8
5.16
28.2
6.28 ꢁ 10
5.88 ꢁ 10
2.69 ꢁ 10
2.29
18.9
69.6
a
Single point adsorption total pore volume of pores less than 433.68 nm diameter at P/P
0
¼ 0.995–0.997, where P/P
0
means the relative pressure of
b
nitrogen gas's actual and saturation pressure. BJH adsorption average pore diameter.
TBCX-0.7% hydrogel, its removal efficiency of PCP was lower 800 min. When the adsorptions reached equilibrium, the
than that of PNIPAM-TBCX-0.7% hydrogel. It was probably adsorption capacity of the PNIPAM-TBCX-0.7% hydrogel was
ꢀ
1
because that the conglomerations of TBCX in the PNIPAM- 2.08 mg g , which was higher than that of PNIPAM-TBCX-0.5%
ꢀ1
TBCX hydrogel-1% decreased the contact sites between TBCX hydrogel (1.96 mg g ) and PNIPAM-TBCX-1% hydrogel
ꢀ1
and PCP, thereby affecting the interaction between them and (2.02 mg g ). The adsorption capacity for PCP reached 1.52,
ꢀ1
reducing the removal efficiency of PCP. Compared with the 1.96 and 2.25 mg g by poly(N-isopropylacrylamide-co-butyl
17
TBCX-0.7% hydrogel, the TBCX-0.5% hydrogel had higher BET acrylate) hydrogels at pH value of 3 in literature, and the molar
surface area and pore volume but lower removal efficiency of percentage ratios of butyl acrylate in the hydrogels were 0%, 5%
PCP. The possible reason was that the lower TBCX contents in and 11%, respectively. Although the PNIPAM-TBCX hydrogels in
the PNIPAM-TBCX-0.5% hydrogel led to fewer binding sites of this study had the similar adsorption capacity for PCP, their
TBCX and PCP.
molar percentage ratios of TBCX were much lower, only 0.5%,
.7% and 1%.
Pseudo-rst-order and pseudo-second-order models were
0
32
33
3.5 Adsorption kinetics
used to investigate the PNIPAM-TBCX hydrogel adsorption
kinetics with eqn (1) and (2), respectively.
The adsorption kinetic experiments were performed to deter-
mine the equilibrium time of PCP adsorption by the PNIPAM-
TBCX hydrogels (Fig. 8). The removal rates of PCP by the PNI-
PAM-TBCX-0.5% hydrogel and PNIPAM-TBCX-1% hydrogel
stayed almost constant aer 600 min, but the removal rate of
PCP by the PNIPAM-TBCX-0.7% hydrogel kept rising until
ln (q
e
ꢀ q
t
) ¼ ln q
e
ꢀ k
1
t
(1)
(2)
t
1
1
¼ k
þ
t
2
q
t
2
q
e
q
e
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the adsorption rate of PCP from solution onto the surface of
PNIPAM-TBCX hydrogels decreased with the raising of TBCX
content in the hydrogels. This may be because that higher TBCX
content resulted in higher hydrophobicity of the surface of the
hydrogels and lower diffusion rate of charged PCP species from
the aqueous phase to the surface of the hydrogels. The value of
e 2
q and k indicated that although the adsorption rate increased
with the decrease of TBCX content in the hydrogels, the
adsorption capacity of PNIPAM-TBCX-0.7% hydrogel was higher
than PNIPAM-TBCX-0.5% hydrogel and PNIPAM-TBCX-1%
hydrogel. Taking into account the fracture morphology of the
PNIPAM-TBCX hydrogels, we speculated that in addition to the
TBCX content, the porous structure of the hydrogels could also
inuence the adsorption capacity. Compact and uniform
porous structure and higher TBCX contents both gave better
adsorption capacity, but higher TBCX contents could result in
agglomerated and inhomogeneous porous structure. Thus, it
explained the highest adsorption capacity of PNIPAM-TBCX-
Fig. 7 Effects of TBCX contents on the removal of PCP by the PNI-
PAM-TBCX hydrogels.
0.7% hydrogel.
3.6 Adsorption equilibrium isotherm of PCP removal
Equilibrium isotherms were used to describe the experimental
data of equilibrium adsorption (Fig. 9). It was seen that the
adsorption capacity of the PNIPAM-TBCX-0.7% hydrogel was
higher than that of PNIPAM-TBCX-0.5% hydrogel and PNIPAM-
TBCX-1% hydrogel. When the PCP equilibrium concentration
ꢀ
1
was 10.0 mg L , the adsorption capacities of PNIPAM-TBCX-
.5% hydrogel, PNIPAM-TBCX-0.7% hydrogel and PNIPAM-
0
Fig. 8 Removal of PCP by PNIPAM-TBCX hydrogels with different
TBCX contents against the adsorption time. -, PNIPAM-TBCX-0.5%
hydrogel; C, PNIPAM-TBCX-0.7% hydrogel; :, PNIPAM-TBCX-1%
hydrogel.
ꢀ
1
where q
t
and q
e
(mg g ) are the adsorption capacity at time t
(min) and (g
and equilibrium, respectively, and
mg min ) are the pseudo-rst-order and pseudo-second-
k
1
k
2
ꢀ
1
ꢀ1
order rate constants, respectively.
2
From the values of R tted by pseudo-rst-order and
pseudo-second-order model in Table 2, we found that the
adsorption kinetic data were tted better by the pseudo-second-
order model than pseudo-rst-order model. That k increased in
the order of PNIPAM-TBCX-0.5% hydrogel > PNIPAM-TBCX-
2
Fig. 9 Adsorption isotherm of PCP on the PNIPAM-TBCX hydrogels.
, PNIPAM-TBCX hydrogel-0.5%; , PNIPAM-TBCX hydrogel-0.7%;
-
,
0
.7% hydrogel > PNIPAM-TBCX-1% hydrogel, illustrated that PNIPAM-TBCX hydrogel-1%. The dashed line represented the Lang-
muir isotherm and the solid line represented the Freundlich isotherm.
Table 2 Pseudo-first-order and pseudo-second-order adsorption rate constants for adsorption of PCP on PNIPAM-TBCX hydrogels
Pseudo-rst-order kinetic model Pseudo-second-order kinetic model
ꢀ1
ꢀ1
2
ꢀ1
ꢀ1
ꢀ1
2
Isotherm models constants
q
e
(mg g
)
K
1
(min
)
R
q
e
(mg g
)
K
2
(g mg min
)
R
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ3
PNIPAM-TBCX-0.5% hydrogel
PNIPAM-TBCX-0.7% hydrogel
PNIPAM-TBCX-1% hydrogel
1.81
1.86
1.84
6.89 ꢁ 10
6.09 ꢁ 10
4.64 ꢁ 10
0.9434
0.8101
0.8821
2.08
2.25
2.17
4.31 ꢁ 10
2.89 ꢁ 10
2.64 ꢁ 10
0.9996
0.9935
0.9941
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ꢀ
1
TBCX-1% hydrogel were 6.49, 7.27 and 6.91 mg g , respec- favorable (0 < R
L
<1), linear (R
L
¼ 0) or unfavorable (R
L
> 1). By
tively. The results showed that the adsorption capacity of dealing with the eqn (3), the RL values were amongst the
PNIPAM-TBCX hydrogel-0.7% was 1.12 times of PNIPAM-TBCX favorable range in all processes (0 < R <1). The PNIPAM-TBCX-
L
2
hydrogel-0.5% and 1.05 times of PNIPAM-TBCX hydrogel-1%. 0.7% hydrogel had higher value of rL than the other two
This was probably because proper content of TBCX in the hydrogels, which probably indicated that there were more
hydrogels could bind PCP, but excess TBCX led to the porous TBCX-PCP complexes formed on its surface. This might be
structure of the hydrogels agglomerated and inhomogeneous.
because the PNIPAM-TBCX-0.7% hydrogel had higher content
We used Langmuir and Freundlich isotherm model to t the of TBCX than the PNIPAM-TBCX-0.5% hydrogel, and exhibited
adsorption data. The Langmuir model was originally developed the more homogeneous phase structure than the PNIPAM-
to represent chemisorption on a set of distinct localized TBCX-1% hydrogel.
34
adsorption sites, in which the homogeneity of the adsorbent is
In consideration of the inhomogeneous phase structure of
a prerequisite. The Langmuir isotherm can be represented as the PNIPAM-TBCX hydrogels, it was supposed that the adsorp-
the following form (3): tion of PCP on the hydrogels was multilayer. Referring to
35
37
multilayer adsorption isotherm of gas adsorbed on solid, it
was speculated that the multilayer adsorption isotherm of
adsorbate or solute adsorbed on the adsorbent from solution
could expressed in the following eqn (6), based on the
assumption that the competitive adsorption of solvent on the
adsorbent relative to could be ignored. The adsorption data
were tted in Fig. 10.
q
mon L e
K C
q
e
¼
(3)
1
þ K
L e
C
ꢀ1
c
e
is the equilibrium PCP concentration (mg L ), q
e
is the
ꢀ1
saturated adsorbance (mg g ), KL is the Langmuir constant
related to the adsorption energy, qmon is the Langmuir constant
related to the adsorption capacity and the amount of adsorbate
at monolayer coverage.
kqmonc
e
The empirical isotherm, Freundlich equation can be repre-
sented as the following form (4):
qe ¼
ꢀ
(6)
h
ðk ꢀ 1Þc
e
ðcmax ꢀ c Þ 1 þ
e
c
max
1
q
e
¼ K
f
C
en
(4)
e
where c , k, qmon and cmax was the equilibrium PCP concentra-
ꢀ1
tion (mg L ), the ratio of the adsorption time of the adsorbate
directly bounded to the surface of the adsorbent to the
where K
f
and 1/n are Freundlich constants in relation to
adsorption capacity and energy distribution of the adsorption
sites, respectively.
The parameter values of the Langmuir and Freundlich
isotherm model were shown in Table 3. The equilibrium
adsorption data of PCP on different PNIPAM-TBCX hydrogels
were tted to Freundlich isotherm model better than Langmuir
2
L
isotherm model. And r
value of the Langmuir equation was
lower than 0.99, indicating the inhomogeneous nature of the
PNIPAM-TBCX hydrogels, and was consistent with SEM results.
The essential features of Langmuir adsorption isotherm can
be expressed in terms of a dimensionless constant called
36
separation factor or equilibrium parameter (RL), which is
dened by the following relationship (5):
1
R
L
¼ 1
(5)
þ K
L 0
C
Fig. 10 The multilayer adsorption isotherm of PCP on the PNIPAM-
ꢀ
1
where C is the initial dye concentration (mg L ). The R value TBCX hydrogels. -, PNIPAM-TBCX hydrogel-0.5%; C, PNIPAM-TBCX
indicates the shape of the isotherm to be irreversible (R
0
L
hydrogel-0.7%; :, PNIPAM-TBCX hydrogel-1%.
L
¼ 0),
Table 3 Langmuir and Freundlich constants for adsorption of PCP on the PNIPAM-TBCX hydrogels
Absorbents
Isotherm models
Langmuir
Constants
PNIPAM-TBCX-0.5%
PNIPAM-TBCX-0.7%
PNIPAM-TBCX-1%
ꢀ
1
q
K
R
mon (mg g
)
8.18
0.477
0.0566–0.349
0.7246
4.32
9.47
0.399
0.0698–0.402
0.9162
4.62
11.5
0.168
0.159–0.615
0.732
3.16
ꢀ
1
L
(L mg
)
L
2
r
L
1
ꢀ1/n
1/n ꢀ1
Freundlich
K
1
F
(mg
kg
g
)
ꢀ1
/2n (kg g
)
0.176
0.8729
0.196
0.9842
0.340
0.8722
r
F
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Table 4 The constants of the derived function q
e
¼ kqmon
c
e
/[(cmax
ꢀ
24 h, immerged into deionized water at 320–323 K for another
c
e
)(1 + (k ꢀ 1)c
e
/cmax)] fitted by matlab for adsorption of PCP on
24 h to desorb PCP. The adsorption–desorption cycle was
PNIPAM-TBCX hydrogels
repeated several times. The results were illustrated in Fig. 11.
The regeneration rate of the PNIPAM-TBCX-0.7% hydrogels was
still 95% aer ve circles. The results indicated that the PNI-
PAM hydrogels modied by 4-tert-butylcalix[8]arene had
reversible adsorption–desorption of PCP by adjusting
temperature.
PCP desorption from PNIPAM-TBCX hydrogels at tempera-
ture over LCST was probably related to several factors. When the
temperature surpassed the LCST, the delicate hydrophilic/
hydrophobic balance in the network structure was disrupted,
so the hydrogel dehydrated and shrank. As the hydrogel shrank,
exposed surface decreased, thus resulting in the decrease of
adsorptive sites. TBCX and NIPAM compacted together and
weakened the interaction between themselves. Moreover,
higher temperature led to higher solubility of PCP in aqueous
solution. Thus, the regeneration of PNIPAM-TBCX hydrogels
could be easily achieved by changing the solution temperature.
Absorbents
PNIPAM-
PNIPAM-
PNIPAM-
TBCX-1%
Constants
TBCX-0.5%
TBCX-0.7%
q
mon
6.061
7.275
5.867
ꢀ1
(mg g
)
k
295
121.4
126.9
124.7
136.9
67.57
c
max
ꢀ1
(
r
m2 g L
)
0.9079
0.9658
0.9353
4. Conclusions
We synthesized the thermal-sensitive PNIPAM-TBCX hydrogels
via aqueous micellar polymerization. The TBCX modied
PNIPAM-TBCX hydrogels exhibited considerable adsorption
capacity for PCP in neutral aqueous solutions. The parameter
values tted by Langmuir, Freundlich and the derived multi-
layer adsorption equation indicated inhomogeneous nature of
the hydrogels and the mixed chemical and multilayer adsorp-
tion of PCP. Aer several cycles of adsorption and desorption,
the regeneration rates of the PNIPAM-TBCX-0.5% and PNIPAM-
TBCX-0.7% hydrogels were still over 80%. Accordingly, the
PNIPAM-TBCX hydrogels could be a highly efficient and easily
regenerated adsorbent to remove pollutant in wastewater.
Fig. 11 Adsorption/desorption circulation behavior of PCP as a func-
tion of time and temperature.
adsorption time of the adsorbate adsorbed to the rst layer, the
ꢀ
1
adsorption amount of unit mass (mg g ) when a single
molecular layer was formed, the minimum concentration of the
solution at the time of saturated adsorption, respectively.
The parameter values of the multilayer adsorption equation
for solution were shown in Table 4. The equilibrium adsorption
data of PCP on different PNIPAM-TBCX hydrogels were tted to
the multilayer adsorption equation better than Langmuir
isotherm model, which indicated that the PCP adsorption on
the hydrogels probably included chemisorption and multilayer
adsorption. The values of qmon tted by the multilayer adsorp-
tion equation were lower than that from Langmuir isotherm
model, which further strengthened the above point of view. In
addition, from the values of k, it could be seen that the
adsorption time of the adsorbate directly bounded to the
surface of the adsorbent was the two orders of magnitude larger
than that of the adsorbate adsorbed to the rst layer, which
indicated that the rst layer adsorption belonged to chemical
adsorption and played a major role.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We appreciated the nancial support to this study by the
Guangdong Academy of Sciences Project (No. 2017GDASCX-
0705).
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