Accepted Manuscript Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes Wenqi Song, Yuyang Liu, Liwei Qian, Luying Niu, Liqun Xiao, Yu Hou, Yan Wang, Xiaodong Fan PII: DOI: Reference:
S1385-8947(15)01582-X http://dx.doi.org/10.1016/j.cej.2015.11.039 CEJ 14436
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
8 September 2015 8 November 2015 13 November 2015
Please cite this article as: W. Song, Y. Liu, L. Qian, L. Niu, L. Xiao, Y. Hou, Y. Wang, X. Fan, Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.11.039
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Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes
Wenqi Song, Yuyang Liu
Liwei Qian, Luying Niu, Liqun Xiao, Yu Hou, Yan Wang, Xiaodong Fan
4 5 6
The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education and Key Laboratory of Macromolecular Science and Technology of Shaanxi Province, School of Science, Northwestern Polytechnical University, Xi’an, 710072, P. R. China
7 8 9
HIGHLIGHTS • A novel hyperbranched polymeric ionic liquid (hb-PIm+PF6-) was synthesized.
• The hb-PIm+PF6- exhibited high adsorption capacity toward anionic dyes.
• It selectively adsorbed the anionic dye from a mixture of cationic and anionic dyes.
A hydrophobic hyperbranched polymeric ionic liquid (hb-PIL) with an imidazolium (Im+)-salt backbone
(hb-PIm+PF6-) was proposed for the efficient adsorption of anionic dyes. For this purpose, a hydrophilic hb-PIL was
first synthesized via the thiol–ene addition polymerization by the “A2+B3” method. Then, the anion exchange
reaction of hb-PIm+Cl- with KPF6 afforded the target hb-PIm+PF6- adsorbent. hb-PIm+PF6- has high adsorption
capacity towards anionic dyes. In contrast, it exhibit less adsorption amount toward cationic dyes. By using congo
red (CR) as the model adsorbate, the adsorption mechanism of hb-PIm+PF6- was investigated in detail by Fourier
transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray, and transmission electron
microscopy analyses. The adsorption behavior of hb-PIm+PF6- towards CR was analyzed by two kinetic and four
isotherm models. These results indicate the characteristic structure of hb-PIm+PF6- was responsible for its high
adsorption capacity for CR. The main driving force for the CR adsorption was electrostatic attraction, carried out
by the anion-exchange between CR and counter-ion PF6- of hb-PIm+PF6-. The presence of the cavities formed by
the branched chains provided storage sites for CR binding. In addition, the selective adsorption of hb-PIm+PF6-
towards anionic dye could be used to purify cationic solution containing anionic dye.
Keywords: Hyperbranched polymer; Polymeric ionic liquid; Adsorption mechanism; Selective adsorption
28 Corresponding author. E-mail: [email protected]
The purification of industrial wastewater has become a crucial subject in the environmental field. Wastewater
from textile, paper, plastic, and leather industries contain a large amount of dyes [1,2], which are low
biodegradability and are harmful to the environment . For example, Congo red (CR) is typical acid dye and
commonly used to give wool and silk red color with yellow fluorescence . However, CR can be metabolized to
benzidine, a human carcinogen [5,6]. Therefore, it is necessary to remove these dyes from the colored wastewater
prior to discharge. Many techniques such as photocatalytic degradation , electrochemical method , biological
treatment , and adsorption  have been used to treat the colored water. The adsorption method is of low initial
cost, ﬂexibility, and operational simplicity [10,11]. Therefore, designing novel functional adsorbents for efficient
adsorption of dyes is of significant interest [12,13,14,15,16,17].
Since numerous dyes are ionic (i. e., cationic or anionic) , the electrostatic attraction between an adsorbent
and an adsorbate could markedly enhance the adsorption ability of the adsorbent [12,13,14,15,16]. For example,
Gao et al.  synthesized carboxylic hyperbranched polyglycerol grafted Fe3O4/SiO2 magnetic adsorbent
(Fe3O4/SiO2/HPG-COOH). The electrostatic interaction between the surface -COOH groups of the adsorbent and
the adsorbates caused it to exhibit a high adsorption capacity of 0.6 mmol g-1 (i. e., 245 mg g-1) for cationic dye
methyl violet, and this was more than twice of the previous magnetic adsorbent. Similarly, Zhou et al.  also
used multicarboxylic hyperbranched polyglycerol to modify a calcined mesoporous silica SBA-15 and produced
hybrid adsorbent SBA/HPG-COOH. The saturated adsorption capacity of SBA/HPG-COOH towards cationic dye
methylene blue was 0.5 mmol g-1 (i. e., 187 mg g-1), which is 10 times more than the amount adsorbed by
Polymeric ionic liquids (PILs)  exhibit potential application in the adsorption of adsorbates including dyes
[14,15,16,19,20,21,22,23], because of their ionic moieties, which can electrostatically attract adsorbates. For
example, Yuan et al.  synthesized PIL poly(quaternary ammonium salt)-grafted PVBC microspheres
(PVBC-g-PDMAPMA+Br-), whose adsorption capacity towards phenol was
The ion exchange between phenol and the counter-ion Br- was responsible for the adsorption. Imidazolium (Im+)
salt-based PILs (PIms) have been used to adsorb anionic dyes, because their polymer chains bear cationic groups.
The structure of the positive aromatic rings of PIms can provide strong electrostatic force as well as the weak
interaction (i. e., π–π stacking ) for adsorbates binding. For instance, Kong et al.  synthesized a 2
2.23 mmol g-1 (i. e., 210 mg g-1).
hydrophobic linear poly(3-ethyl-1-vinylimidazolium bis(trifluoromethanesulfonyl)imide) polymer (PIm+TFSI-),
whose adsorption capacity towards acid dye methyl blue was 476 mg g-1. The electrostatic attraction between PILs
and adsorbates play an important role in the adsorptions. Obviously, the exposed IL moieties of PILs to the
adsorbates (i.e., availability of the IL moieties of PILs for adsorbates ) are prerequisite for the interaction. So far,
the availability of the IL moieties was usually improved by increasing their porosity and specific surface area of
According to the above, if ionic liquid (IL) moieties are introduced into the backbone of a hyperbranched
polymer (HBP) , the cavities are produced naturally around the IL groups . Thus, the ionic groups on the
HBP backbone were more accessible for guest molecules with counter charges, leading to the efficient adsorption
of guest molecules around ionic groups by electrostatic attraction. The encapsulation of the cavities of a HBP was
previously confirmed [26,27,28]. Therefore, a hyperbranched PIL (hb-PIL) may exhibit high adsorption capacity.
However, to the best of our knowledge, this has not yet been explored. Herein, we report a novel hb-PIL adsorbent,
where IL groups are located on the HBP backbone. In this study, the hb-PIL with an Im+ backbone (hb-PIm+Cl-)
was synthesized by the “A2+B3” thiol–ene click polymerization (Scheme 1). The anion exchange of hb-PIm+Cl-
with KPF6 afforded a hydrophobic adsorbent hb-PIm+PF6-. The adsorption capacity and mechanism of hb-PIm+PF6-
for anionic dyes were investigated in detail.
Scheme 1 Synthesis route of the hyperbranched polymeric ionic liquid hb-PIm+PF63
2,2-azobisisobutyronitrile (AIBN, 98%) and trimethylolethane (TME, 97%), Acros; 3-mercaptopropionic acid
(>99%), Sigma-Aldrich; N-vinyl imidazole (VIm, >98%), TCI. Potassium hexafluorophosphate (KPF6, 99%),
Aladdin Chemistry. Co. Ltd; ethylene glycol (EG) and chloroacetic acid (CAA), Sinopharm Chemical Reagent Co.
Ltd; para-toluenesulfonic acid (p-TSA), Tianjin Kemiou Chemical Reagent Co. Ltd (Tianjin city, China); congo red
(CR), methyl orange (MO), acid Fuchsin (AF), thymol blue (TB), methyl violet (MV), and malachite green (MG),
Tianjin Hongyan Chemical Co. Ltd (Tianjin City, China); and methylene blue (MeB), SERVA Electrophoresis
GmbH. Co. Ltd. AIBN was recrystallized from methanol prior to use.
Nuclear magnetic resonance (1H NMR) spectra were recorded using a Bruker Ascend 400 MHz spectrometer
in CDCl3 or D2O. Fourier transform infrared (FTIR) spectra of the samples were recorded using a NICOLET iS10
spectrometer (Nicolet, USA) in the range 4000–400 cm-1. Polymer samples for FTIR observation were prepared by
casting thin polymer films on a KBr holder. UV–vis spectra were recorded using a spectrophotometer UV-2550
model (Shimadzu, Japan). The BET surface area and total pore volume of the adsorbent were characterized by
using N2 adsorption-desorption isotherms with a Trestar 3020 surface area analyzer (Micromeritics, USA). The
surface morphologies and compositions of the samples were investigated using a field emission scanning electron
microscope (SEM, TEI Quanta 600 FEG, USA) equipped with an electron dispersive X-ray analysis (EDX) detector.
To observe the interior morphologies of the samples, they were first embedded in EPON 812 epoxy resin, followed
by curing at 60 °C for 48 h. The ultrathin sections (~50 nm) of the embedded specimens were cut with an
ultramicrotome and were then observed by transmission electron microscopy (TEM, HITACHI H-7650, Japan).
The Z-average hydrodynamic diameter of sample in water was characterized by using a Zetasizer Nano-ZS
dynamic light scattering (DLS) device (Malvern, Britain).
2.3 Synthesis of di-thiol A2 monomer (EGDMPA)
EG (2.0 g, 32 mmol), MPA (10.0 g, 94 mmol), and p-TSA (0.60 g, 3 mmol) were successively dissolved in 80
mL of toluene. The mixture was heated to reflux under Dean-Stark condition in an oil bath at 120 °C for 4 h, and
was further allowed to cool to room temperature, diluted with 150 mL of dichloromethane, and then washed
successively with 4% aqueous sodium carbonate solution and water. The crude product was further purified by 4
silica-gel column chromatography. The final liquid product was obtained by vacuum drying at 20 °C. Yield: 78%.
(KBr, cm-1): 2575 (S–H), 1746, (C=O).
2.4 Synthesis of tri-chloride B3 precursor (TMECA)
H NMR (400 MHz, CDCl3, ppm): 1.70 (2H, –SH), ~2.72–2.82 (8H, COCH2CH2SH), 4.35 (4H, CH2OCO); FTIR
TME (2.40 g, 20 mmol), CAA (7.00 g, 74 mmol), and p-TSA (0.38 g, 2 mmol) were successively dissolved in
80 mL of toluene. Under the condition of Dean-Stark, reaction mixture was heated to reflux in an oil bath of 120 °C
for 4 h. The purification process of the obtained product was similar to that of EGDMA. Yield: 81% wt. 1H NMR
(400 MHz, DMSO-d6, ppm): 0.99 (3H, –CH3), 4.10 (6H, CH2OCO), 4.45 (6H, COCH2Cl). FTIR (KBr, cm-1): 1748
(C=O), 790 (C–Cl).
2.5 Synthesis of triene ionic liquid B3 monomer (TMEAVIm+Cl-)
TMEAVIm+Cl- was synthesized by stirring the mixture of TMECA (4.0 g, 11 mmol) and VIm (5.0 g, 53 mmol)
in 6 mL of ethanol at 60 °C for 18 h. The product was precipitated thrice from the ethanol solution using the mixed
dichloromethane/ethyl acetate (200 mL/200 mL) solvents. After drying at 30 °C in vacuum, a slight yellow
amorphous product was obtained (yield, 64%.). 1H NMR (400 MHz, D2O, ppm): ~0.77–0.99 (3H, –CH3),
~4.02–4.22 (6H, CH2OCO), ~5.06–5.17 (6H, COCH2N+), ~7.10–5.40 (9H, –CH=CH2), ~9.06–7.26 (9H,
imidazolium ring +NCHCHNCH). FTIR (KBr, cm-1): ~3136–3050 (sp2 C–H), ~1653–1360 (imidazolium ring,
–CH=CH2), 920 (–CH=CH2).
2.6 Synthesis of hb-PIm+Cl-
hb-PIm+Cl- was synthesized by the thiol–ene addition polymerization between di-thiol monomer EGDMPA
and triene monomer TMEAVIm+Cl- using AIBN as the initiator. The detailed feed composition is listed in Table 1.
Briefly, TMEAVIm+Cl- was dissolved in ethanol and added to a DMF solution of EGDMPA and AIBN. Then, the
polymerization was carried out at 60 °C for 24 h. After completing the reaction, the resulting mixture was allowed
to cool to room temperature and then precipitated in 50 mL of THF. The crude product was then dissolved in 5 mL
water and dialyzed (molecular weight cut off: 3500) against water for two days. The dialyzed product was
lyophilized and kept in a glassware under vacuum for further characterization.
2.7 Synthesis of hb-PIm+PF6-
hb-PIm+PF6- was synthesized by the anion exchange reaction of hb-PIm+Cl- with KPF6. Briefly, a solution of 1
g of KPF6 in 100 mL water was dropwise added to a solution of 1g of hb-PIm+Cl- in 100 mL water. The mixture 5
was then stirred vigorously at 40 °C. After 24 h, the produced precipitate was washed several times with deionized
water until the complete removal of the halide ions, which was confirmed by silver nitrate solution . After
lyophilizing, a white powder was obtained.
2.8 Adsorption experiments
A certain amount of hb-PIm+PF6- was added to the CR solution of specific concentration and volume. Then,
the mixture was shaken at 25 °C and 100 rpm. At a predetermined interval, the adsorbent was removed by filtration
using a 450 nm membrane ﬁlter, and the CR concentration of the filtrate was analyzed by measuring its UV–vis
absorbance at 496 nm. The adsorption capacity at time t (h) and equilibrium was defined as qt (mg g-1) and qe (mg
g-1), respectively. The removal efficiency (R) was defined as the percentage ratio of the CR amount removed by the
adsorption in all the CR. They were determined by the following equations. qt
(C0,CR Ct ,CR ) madsorbent
(C0,CR Ce,CR )
(C0,CR Ce,CR ) C0,CR
where C0,CR (mg L-1) is the CR initial concentration; Ct,CR (mg L-1) and Ce,CR (mg L-1) are the liquid-phase
concentrations of CR at time t (h) and at equilibrium, respectively, VCR (L) is the volume of the used CR solution,
and madsorbent is the weight of hb-PIm+PF6-.
3. Results and discussion
3.1 Synthesis of A2- and B3-types monomers
The synthesis routes of A2- and B3-types monomers are shown in Schemes 1A–C. Monomer EGDMPA was
synthesized via the O-esterification reaction  of EG and MPA. Monomer TMEAVIm+Cl- was synthesized via
two steps: the O-esterification reaction of TME with CAA to afford the tri-chloride precursor TMECA and the
haloalkylation reaction  of TMECA with VIm to afford the B3 monomer. To avoid the formation of insoluble
gels during the following thiol–ene addition polymerizations because of rigid structures , EG and TME as the
building units were introduced into the monomer structures expected to provide the resulting hb-PILs with flexible
alkyl chains. Moreover, solvent EtOH was required for the haloalkyation reaction (Scheme 1C), because the high
viscosity of the reaction system could result in an incomplete alkylation. The structures of the obtained A2 and 6
B3-type monomers were confirmed by 1H NMR and FTIR measurements. Monomer EGDMPA was easily soluble
in DMF, whereas monomer TMEAVIm+Cl- is only slightly soluble. Monomer TMEAVIm+Cl- is soluble in water
and EtOH; however, monomer EGDMPA isn’t.
3.2 Synthesis of the hyperbranched polymeric ionic liquid adsorbent
Hydrophilic hb-PIm+Cl- was prepared by “A2+B3” thiol–ene addition polymerization based on the radical
mechanism [33,34] (Scheme 1D). For the synthesis, the molar A2/B3 feed ratio was set at 2.1/1. Preliminary study
showed that mixed EtOH/DMF solvent was suitable for the synthesis; however, the composition of EtOH/DMF
composition exerted an evident effect on the structure of the resulting hb-PIm+Cl- (Table 1). Fig. 1 shows the 1H
NMR spectra of hb-PIm+Cl- synthesized at different EtOH/DMF compositions. In Fig. 1, the peaks at δ ~3.60–3.35
and δ 3.00–2.20 were assigned to the protons of ＞N CH2CH2SCH2CH2COO– units (labeled as e, f, g, and h
protons, respectively), confirming the thiol–ene reaction by β-addition. The peak at δ ~1.25–1.00 ppm was assigned
to the proton of –SCH(N)CH3 (j) from the α-addition of the thiol–ene reaction. The peaks at δ ~9.50–7.10 and
~7.10–5.30 were assigned to the protons of Im+ rings (N+CHNCH2CH2) and the unreacted double bonds,
respectively. The assignment of the other proton peaks of hb-PIm+Cl are as follows: δ ~5.26–5.03 (c COCH2N+, k
–SCHN–), δ ~0.95–~0.73 (a –CH3) and δ ~4.50–4.00 (b, i –COOCH2). It was worth noting that the reaction
between the double bonds might existed under the radical mechanism. However, it was difficult to directly
recognize the relative proton peaks from the 1H NMR spectra, because of the overlap of the peaks of multiprotons.
However, for a double bond of the B3 monomer, the ratios of their possible reactions are shown by Eq. (4),
X X X homo X C C 100%
where Xβ and Xα are the ratio of thiol–ene reaction by β- and α-addition, respectively; Xhomo and XC=C are the ratio of
reaction of double bonds and unreacted double bond of B3 monomer, respectively. The ratios of their different
reactions were calculated by the integration ratio of their specific protons (Table 1), and the results are listed in
Regardless of β- or α-addition of thiol–ene reaction, a branched polymer was conducive to form. However, the
reaction between the double bonds could cause side reactions such as crosslinking or linear propagation. Therefore,
a decrease in the content of Xhomo and XC=C was beneficial to constructing HBP. By comparison of the Xβ, Xα, Xhomo,
and XC=C values (Table 1), the composition of solvents was found to exert an evident impact on the structure of 7
hb-PIm+Cl-. At VEtOH/VDMF = 4:1, XC=C and Xhomo of hb-PIm+Cl--1 were 14% and 8%, respectively. However, at
VEtOH/VDMF =1:1, XC=C and Xhomo of hb-PIm+Cl--4 were found to be 0%, indicating that the variation in the solvent
composition reduced the side reaction of the double bonds and availed to form a hyperbranched structure.
188 189 190
Fig. 1 1H-NMR spectra of samples hb-PIm+Cl 1–4 in D2O
To further analyze the structure of samples hb-PIm+Cl--1–4, the actual A2 and B3 unit ratio ([A2]/[B3]) of the
samples were estimated by comparing the integration area of the proton signals of i and d (Fig. 1), and the results
are listed in Table 1. If there was no intramolecular thiol–ene reaction and other side reactions, the obtained
hyperbranched architecture should be perfect, where the actual numbers of A2 and B3 units should follow [A2] =
2[B3] +1 at a molar feed [A2]/[B3] ratio of 2.1/1 . Therefore, the actual [A2]/[B3] ratio of a HBP could exhibit its
degree of branching . The larger [A2]/[B3] ratio of a sample is, the higher will be its degree of branching. As
listed in Table 1, the actual [A2]/[B3] values of all obtained samples were <2. However, sample hb-PIm+Cl--4 did
not show the structure signal from the side reaction, and its [A2]/[B3] value was the highest in these samples. The
Z-average hydrodynamic diameter of hb-PIm+Cl--4 in water (2.5 mg mL-1) was determined by DLS and was found
to be 9.3 nm with a particle dispersion index of 0.248, indicating the macromolecular scale of the hb-PIm+Cl--4. 8
Therefore, sample hb-PIm+Cl--4 should be a polymer with a relatively highly branched structure and was therefore
used in the following studies.
Synthesis conditions and structural data of hb-PIm+ClSynthesis conditionsa Code
VEtOH b VDMF
[ A2 ] [ B3 ]
hb-PIm Cl -3
a) AIBN content was fixed at 2.5 wt. b) the solvent mixture composition. c) the total monomer concentration. d)
XC=C = I7.10~5.30/I9.50~7.10 × 100%; Xβ = 1.5I3.60~3.35/I9.50~7.10 × 100%; Xα = I1.25-1.00/I9.50~7.10 × 100%; where I is the
integral area of the relative proton peak. The actual [A2]/[B3] values were calculated by the integral ratio of
COOCH2CH2COO (i) of the A2 unit to the imidazolium ring (d) of B3 unit, [A2]/[B3] = 9Ii/4Id=9(I4.0~4.50－2/3Id)/4Id
After the anion exchange reaction of hb-PIm+Cl--4 with KPF6 (Scheme 1E), a hydrophobic product
hb-PIm+PF6- was obtained and used as the adsorbent for the subsequent adsorption studies.
3.3 Effect of adsorption conditions on the adsorption properties of the hb-PIm+PF6- adsorbent
3.3.1 Effect of contact time
To assess the adsorption ability of sample hb-PIm+PF6-, CR was used as the model dye and its molecular
structure is shown in Fig. S1. Fig. 2 shows the adsorption capacity (qt) of hb-PIm+PF6- towards CR versus contact
time (t). As shown in Fig. 2, under higher weight ratio conditions of CR to hb-PIm+PF6- (C0,CRVCR/madsorbent =
3750/1, mg/g, curve a) and at the contact time of 1 h, q1h was 529 mg g-1, and for contact time >20 h, q24h increased
to 1862 mg g-1. After 24 h, the qt of hb-PIm+PF6- towards CR showed less change with time. At a lower ratio of CR
to hb-PIm+PF6- (C0,CRVCR/madsorbent = 500/1, mg/g, Curve b in Fig. 2), over the contact time of 15 h, qt was close to
500 mg g-1. These results exhibit that the adsorption of hb-PIm+PF6- toward CR tended to achieve a dynamic
equilibrium after 24 h. Therefore, in the following adsorption studies, a contact time of 24 h was selected.
3.3.2 Effect of adsorbent dosage
Fig. 3 shows the effect of adsorbent dosage on the qe and R values of hb-PIm+PF6. As shown in Fig. 3, at a
hb-PIm+PF6- dosage of 37 mg L-1, 67.1% of CR was removed from the solution, and the qe value was 1813 mg g-1, 9
indicating that hb-PIm+PF6- is an excellent adsorbent for the removal of CR. When the dosage of hb-PIm+PF6- was
increased to 76 mg L-1, 92.5% of CR was removed; however, the qe value decreased to 1216 mg L-1. When
hb-PIm+PF6- amount was further increased to >125 mg L-1, all the CR was almost removed from the solution;
however, the qe values decreased with increasing amount of hb-PIm+PF6-.
Fig. 2 Adsorption kinetics of hb-PIm+PF6- toward CR at 25 °C. (a) C0,CR = 300 mg L-1, VCR= 25 mL, madsorbent = 2
mg, (b) C0,CR =100 mg L-1, VCR =10 mL, and madsorbent = 2 mg.
Fig. 3 Effect of hb-PIm+PF6- dosage on the adsorption capacity and removal efficiency of CR for a contact time of
24 h at 25 °C. (C0,CR = 100 mg L-1, VCR = 10 mL).
3.3.3 Effect of initial dye concentration
To further assess the maximum adsorption capacity of hb-PIm+PF6- towards CR, several adsorption
experiments were carried out at C0,CR values from 100 to 600 mg L-1 at 25 °C in 25 mL solutions, and the results are
shown in Fig. 4. As seen from Fig. 4, at the C0,CR values of 100 and 150 mg L-1, the qe values of hb-PIm+PF6- were 10
1150 and 1622 mg g-1, respectively. At these adsorption amounts, 96.7 and 91.3% of CR were removed from the
two solutions, respectively. When C0,CR increased from 200 to 350 mg L-1, the qe values increased from 1714 to
1905 mg g-1, but the R value decreased from 71.2 to 48.3%. When C0,CR increased from 400 to 600 mg L-1, the qe
values exhibited less change with C0,CR, and at C0,CR = 600 mg L-1, only 27.7% of CR was removed. These indicate
that the maximum adsorption capacity of hb-PIm+PF6- towards CR was 1993 mg L-1 at 25 °C, which was much
higher than those of the previously reported adsorbents for CR (Table S1) [36,37,38,39,40,41].
Fig. 4 Effect of C0,CR on the adsorption capacity and removal efficiency at 25 °C. (madsorbent = 2 mg, VCR = 25 mL)
3.4 Adsorption mechanism of hb-PIm+PF6-
3.4.1 SEM and TEM analyses
To investigate the mechanism of CR adsorbed into (or onto) the hb-PIm+PF6-, the samples were obtained from
point 1# of Fig. 3 and points 2# and 3# of Fig. 4 and labeled as HP-CR500, HP-CR1160, and HP-CR1950,
respectively, where the number exhibit their adsorption capacities. These samples were investigated by SEM, EDX,
TEM, and FTIR analyses. Fig. 5 shows the SEM and EDX images of the samples hb-PIm+PF6-, HP-CR500,
HP-CR1160, and HP-CR1950. As shown in Figs. 5a and 5b, the surface morphology of sample HP-CR500 is
similar to that of hb-PIm+PF6-, indicating that there was not evident CR aggregation on the surface of hb-PIm+PF6-
at an adsorption capacity of ~500 mg g-1. However, the surface morphologies of samples HP-CR1160 and
HP-CR1950 were different from that of hb-PIm+PF6- (Figs. 5c and 5d), probably caused by the CR aggregation on
the surface of hb-PIm+PF6-, indicating that some CR was adsorbed onto the surface of hb-PIm+PF6- at higher
adsorption amount of CR. 11
Fig. 5 SEM images of hb-PIm+PF6- (a), HP-CR500 (b), HP-CR1160 (c) and HP-CR1950 (d), and the corresponding
EDX images (a′)–(d′).
Fig. 6. TEM images of ultrathin cross-sections (~50 nm) of samples hb-PIm+PF6- (a), HP-CR500 (b), HP-CR1160
(c) and HP-CR1950 (d).
To further investigate the CR distribution inside hb-PIm+PF6-, ultrathin sections of samples hb-PIm+PF6-,
HP-CR500, HP-CR1160, and HP-CR1950 were prepared using an epoxy resin matrix for TEM observation, and the
results are shown in Fig. 6. The relatively dark regions in the Figure were assigned to the samples. Compared to the
blank sample hb-PIm+PF6-, the colors of samples HP-CR500, HP-CR1160, and HP-CR1950 successively deepened,
attributing to the presence of CR inside the samples. This is because an election-rich substance could weak light
transmission. Furthermore, as shown in Figs. 6a and 6b, the light color parts inside the region of samples
HP-CR500 and HP-CR1160 were clearly observed, probably because of the pores of the samples themselves,
whereas sample HP-CR1950 exhibited fewer lighter spots, indicating that at lower adsorption capacity, the
adsorbed CR distributed mainly in the polymer matrix. At higher adsorption capacity, the adsorbed CR might fill 12
the pores of samples or anchor on the surface of these samples. Especially, as shown in Figs. 6c and 6d, the edge
regions of samples HP-CR1160 and HP-CR1950 show evident CR aggregation. This observation was in agreement
with that from the SEM results, indicating that CR was first adsorbed into the polymer matrix in the adsorption
process, and with increasing adsorption capacity, CR began to fill the pores and aggregate on the surface of
hb-PIm+PF6-. The adsorbed CR distribution in the polymeric matrix indicates the position of the accommodation of
molecules. In fact, the encapsulation of cavities of HBP was confirmed previously [23, 24]. Therefore, these results
suggest that the cavities formed by hyperbranched structure play an important role in high adsorption capacity of
hb-PIm+PF6- towards CR. The low surface area (1.65 m2 g-1) of hb-PIm+PF6- (N2 adsroption-desorption curve
shown in Fig. S2) futher confirmed that its high adsorption capacity should depend on the cavities formed by
hyperbranched hyperbranched structure.
Parameters calculated form the adsorption kinetic models a Pseudo-first-order: ln(qe qt ) ln qe k1t qe,exp a Dye -1 (mg g ) qe,cal (mg g-1) k1 (h-1) R 12
Pseudo-second-order: t qt 1 k2 qe2 t qe qe,cal (mg g-1)
k2 (g mg-1 h-1) 10-4
C0,dye=300 mg L . qe,exp and qe,cal are the experimental and calculated maximum adsorption capacity, respectively;
R is the correlation coefﬁcient; k1 (h-1) and k2 (mg g-1 h-1) are the rate constants of the pseudo first-order and the
pseudo-second-order kinetics, respectively.
3.4.2 Adsorption Kinetics
To investigate the adsorption mechanism of hb-PIm+PF6- towards CR, the data of curve a in Fig. 2 were
analyzed by using pseudo-first-order and pseudo-second-order models . Their fitted plots are shown in Figs. S3
and S4, respectively, and the results are listed in Table 2. The calculated qm values from the fitted curves by
pseudo-first-order and pseudo-second-order kinetic models were 1404 and 2003 mg g-1, respectively; whereas the
experimental qe value was 1859 mg g-1. This means that the result from seudo-second-order kinetic model was
closer to experimental qe value. The correlation coefﬁcient (R2) values of the two fitted curves by the
pseudo-first-order and pseudo-second-order models were 0.9856 and 0.9985, respectively. These results means that
pseudo-second-order model could describle the adsorption process better. The result indicates the chemisorption
nature of hb-PIm+PF6- towards CR [43,44]. This is further confirmed by EDX (Figs. 5a′– 5d′) and FTIR (Fig. S5) 13
analyses. Regardless of the results from EDX or FTIR measurement, an increase in the adsorption capacity of CR
decreased the PF6- content, probably because of the anion exchange between CR anions and PF6- counter-ions. As a
result, CR was bound to the backbone of the hyperbanched polymer, whereas PF6- was removed. Therefore, the
chemisorption involved the strong electrostatic interaction between the adsorbent and CR.
The adsorption data of hb-PIm+PF6- toward dyes MO and AF (the structures shown in Fig. S1) were also
analyzed by using pseudo-first-order and pseudo-second-order models (Figs. S6-S11) and the results are listed in
Table 2. The results reveals that the adsorptions of hb-PIm+PF6- toward dyes MO and AF could also well follow the
pseudo-second kinetic model.
3.4.3 Adsorption Isotherms
To investigate the adsorption mechanism of hb-PIm+PF6- toward CR, the data of Fig. 4 were further analyzed
by the Langmuir and Freundlich isotherm models , and their fitted plots are shown in Figs. S12 and S13,
respectively, and the results are shown in Table 3. As listed in Table 3, from the Langmuir model, the maximum
adsorption capacity (qm) of 2050 mg g-1 was obtained, and the R2 value of the fitting was 0.9996, whereas, the KF
from the Freundlich model is 1181 mg g-1, which was evidently lower than the experimental value, indicating the
adsorption of hb-PIm+PF6- towards CR conformed well to the Langmuir model. Langmuir isotherm model is based
on monolayer adsorption. Therefore, the adsorption of hb-PIm+PF6- towards CR might be mainly related to site
adsorption. For adsorbent hb-PIm+PF6-, the main site characteristics are the cationic Im+ rings of the hyperbranched
backbone and the cavities formed by the hyperbranched structure. To investigate the role of the cationic Im+ groups
of the hyperbranched backbone in the adsorption, three cationic dyes (MeB, MV, and MG) (the structures shown in
Fig. S1) were selected as the guest molecules. At C0,dye=400 mg L-1, the adsorption capacities of hb-PIm+PF6-
towards cationic MeB, MV, and MG were about 45, 86 and 75 mg g-1, respectively, whereas at C0,dye = 10 mg L-1,
these cationic dyes were not at all adsorbed by hb-PIm+PF6-. Such a low adsorption ability of hb-PIm+PF6- towards
the cationic dyes may be caused by the electrostatic repulsion between the cationic Im+ rings and the cationic dyes.
However, for other anionic dyes such as MO and AF, at C0,dye = 400 mg L-1, the adsorption capacities were 679 and
2136 mg g-1, respectively (their isotherms shown in Figs. S14 and S15), indicating that the electrostatic interaction
of polymer backbones with dyes is the main driving force for the adsorption. This result was also confirmed by the
FTIR (Fig. S4) and EDX (Fig. 5) spectra, and the cavities provide a storage site for the interaction. Owing to the
fact that the cavities formed by hyperbranched chains should be at the molecular level, a site may accommodate a 14
CR molecule. Therefore, the adsorption could follow the Langmuir model. Of course, as shown in Figs. 5c-d and
Figs. 6c-d,, CR shows evident aggregation on the surfaces of hb-PIm+PF6- (Figs. 5c-d and Figs. 6c-d), and the
presence of pores inside the adsorbed samples was not very evident, indicating that it may be multilayer adsorption
on the surfaces and inside the pores of hb-PIm+PF6-. Moreover, the R2 value of the fitting by the Freundlich model
also reached to 0.9590. Therefore, the adsorption of hb-PIm+PF6- may be heterogeneous and follow more than one
mechanism [44,45]. However, as a whole, the adsorption conformed better to the Langmuir model. Therefore, both
the cavities and the IL moieties of the hyperbranched adsorbent played an important role in the adsorption of CR.
Parameters calculated form the adsorption isotherm models Parameter a
Isotherm model qe,exp Langmuir
Ce qe 1 qm K L Ce qm
(mg g )
ln qe 1 n ln Ce ln K F
qe B ln KT B ln Ce
qe Ce qm K S K S qe
(mg g )
1/n RT2 KT B
(L mg ) -2
(KJ mol )
RS,H2 qm,H KS,H
(mg g ) (L mg )
qe,exp and qe,cal are the experimental and calculated maximum adsorption capacity, respectively; R2 is the correlation
coefﬁcient; KL is the adsorption equilibrium constant; KT is the equilibrium binding constant corresponding to the
maximum binding energy; KS,H and KS,L are the adsorption equilibrium constant for “high-affinity” and the
“low-affinity” binding sites, respectively. KF and 1/n are the characteristic constants representing the adsorption
capacity and adsorption intensity of the system, respectively; B is the Termkin constant related to heat of adsorption.
Further, the CR adsorption data of Fig. 4 were also analyzed by the Temkin  and Scatchard  isotherm
models, and their fitted plots are shown in Figs. S16 and S17, respectively, and the results are listed in Table 3. The
Temkin model takes account of the interaction between adsorbing species and adsorbate. Therefore, as listed in
Table 2, high R2 value (0.9622) of the fitted Temkin model indicates that the adsorption of hb-PIm+PF6- towards
CR follow chemisorption and is in agreement with above mentioned mechanism. In the Sactchard plot (Fig. S17),
the presence of a deviation from the linearity on the plot (two inflection) points out the presence of more than one
type of binding sites mainly contributing to the adsorption , indicating that two main affinity binding sites exist
in the adsorption process. The high-affinity binding sites correspond to the anion exchange sites of PF6-, and the
low-affinity binding sites might be attributed to the weak interactions such as π–π stacking or H-bonding sites.
The results from isotherm analysis of the adsorption data of hb-PIm+PF6- toward dyes MO and AF (Figs.
S18-S25) are shown in Table 3. The results reveals that the adsorptions of hb-PIm+PF6- toward MO and AF also
conform to the Langmuir and Termkin models well.
Standard thermodynamic parameters of the dye adsorption a Dye
C0,dye (mg L-1)
ΔG (KJ mol-1) at different temperatures c
(KJ mol-1 K-1)
a), the adsorption condition: madsorbent = 2 mg, V = 25 mL, contact time = 24 h; b), R is the correlation coefﬁcient;
c), the values of ΔH and ΔS are determined by plotting ln(qe/Ce) against 1/T based on the equation ln(qe/Ce) = ΔH
/RT– ΔS/R, where T is the temperature (K); the value of parameter ΔG at a certain temperature is calculated by
equation ΔG = –RTln(qe/Ce), where qe (mg g-1) and Ce (mg L-1) are the equilibrium amount of dyes adsorbed on the
adsorbent and equilibrium concentration of dyes in the solution, respectively, R is the universal gas constant (8.314
3.4.4 Adsorption thermodynamics
Thermodynamic analysis of an adsorption process provides information on its spontaneity [47,48,49]. The
changes in the standard Gibbs energy (ΔG), enthalphy (ΔH), and entropy (ΔS) of dyes CR, MO and AF are
calculated from experimental data obtained at different temperatures (Fig. S26). The curves by plotting ln(qe/Ce)
against 1/T are shown in Fig. S27, and the calculated results are listed in Table 4. As seen from Table 4, all the ΔG
values are negative, indicating the spontaneous nature of dyes adsorbed on hb-PIm+PF6-. The negative values of ΔH 16
indicated exothermic nature of the adsorption process , which is in agree with experiment results that lower
temperatures lead to higher adsorption capacity. The negative values of ΔS corresponds to a decrease in degree of
freedom of the adsorbed species.
Fig. 7. UV–Vis spectra of aqueous solutions of TB (a), MeB (b), TB + MeB mixture (c) and TB + MeB after the
addition of hb-PIm+PF6 (d) at 25 °C for 24 h. Experimental conditions: Vdye = 6 mL, C0,dye = 25 μmol L-1, and
madsorbent = 2 mg.
387 388 389 390 391
Fig. 8 Effect of NaCl concentration (a) and initial pH (b) on the removal efficiency of CR at 25 °C. (madsorbent = 2 mg, VCR = 10 mL, C0,CR = 100 mg L-1, the initial pH of CR solution is adjusted by using 1 mol L-1 HCl solution)
3.5 Adsorption in salted or acid CR solutions
Dye solutions in the textile dyeing process usually contain NaCl and acid, because NaCl promotes the dye
adsorption of textile fibers  and acid promotes the acid dye dyeing . Fig. 8 shows effect of the NaCl
concentration and acid pH of the CR solution on the removal efficiency at C0,CR of 100 mg L-1. As seen from Fig. 8,
when NaCl concentration is up to 4000 mg L-1, nearly all the CR (R >98%) can be removed, indicating the presence 17
of NaCl produce less influence on the CR adsorption. In addition, as seen from Fig. 8, it was found that in the
initial pH range from 2.3 to 6.8 (the natural pH0 of CR solution is 6.7~6.8 [6,51]), pH variation had less influence
on R values.
A hydrophilic hyperbranched polymer ionic liquid, hb-PIm+Cl-, was synthesized via the thiol–ene addition
polymerization by the “A2+B3” method. The anion exchange of hb-PIm+Cl- with KPF6 afforded a hydrophobic
adsorbent hb-PIm+PF6-. The as-prepared hb-PIm+PF6- effectively adsorbed anionic dyes such as CR, AF, and MO
from their aqueous solutions, whereas very less amount of cationic dyes such as MeB, MV, and MG were
adsorbed by hb-PIm+PF6- under the similar conditions. The selective adsorption could be used to separate a
mixture of cationic and anionic dyes. By using CR as the adsorbate, the adsorption mechanism of hb-PIm+PF6-
was investigated in detail. The results demonstrate that the characteristic structure of hb-PIm+PF6- was
responsible for its high adsorption capacity towards CR. The main driving force of the CR adsorption was
electrostatic attraction, which occurred by the anion-exchange between CR and the PF6- of hb-PIm+PF6-, and the
cavities formed by the branched chains provide storage sites for the interaction.
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