Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes

Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes

Accepted Manuscript Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes Wenqi Song, Yuyang Li...

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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

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes

2 *,

3

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.

10

• The hb-PIm+PF6- exhibited high adsorption capacity toward anionic dyes.

11

• It selectively adsorbed the anionic dye from a mixture of cationic and anionic dyes.

12 13

ABSTRACT

14

A hydrophobic hyperbranched polymeric ionic liquid (hb-PIL) with an imidazolium (Im+)-salt backbone

15

(hb-PIm+PF6-) was proposed for the efficient adsorption of anionic dyes. For this purpose, a hydrophilic hb-PIL was

16

first synthesized via the thiol–ene addition polymerization by the “A2+B3” method. Then, the anion exchange

17

reaction of hb-PIm+Cl- with KPF6 afforded the target hb-PIm+PF6- adsorbent. hb-PIm+PF6- has high adsorption

18

capacity towards anionic dyes. In contrast, it exhibit less adsorption amount toward cationic dyes. By using congo

19

red (CR) as the model adsorbate, the adsorption mechanism of hb-PIm+PF6- was investigated in detail by Fourier

20

transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray, and transmission electron

21

microscopy analyses. The adsorption behavior of hb-PIm+PF6- towards CR was analyzed by two kinetic and four

22

isotherm models. These results indicate the characteristic structure of hb-PIm+PF6- was responsible for its high

23

adsorption capacity for CR. The main driving force for the CR adsorption was electrostatic attraction, carried out

24

by the anion-exchange between CR and counter-ion PF6- of hb-PIm+PF6-. The presence of the cavities formed by

25

the branched chains provided storage sites for CR binding. In addition, the selective adsorption of hb-PIm+PF6-

26

towards anionic dye could be used to purify cationic solution containing anionic dye.

27

Keywords: Hyperbranched polymer; Polymeric ionic liquid; Adsorption mechanism; Selective adsorption

28 Corresponding author. E-mail: [email protected] 1

29

1. Introduction

30

The purification of industrial wastewater has become a crucial subject in the environmental field. Wastewater

31

from textile, paper, plastic, and leather industries contain a large amount of dyes [1,2], which are low

32

biodegradability and are harmful to the environment [3]. For example, Congo red (CR) is typical acid dye and

33

commonly used to give wool and silk red color with yellow fluorescence [4]. However, CR can be metabolized to

34

benzidine, a human carcinogen [5,6]. Therefore, it is necessary to remove these dyes from the colored wastewater

35

prior to discharge. Many techniques such as photocatalytic degradation [7], electrochemical method [8], biological

36

treatment [9], and adsorption [10] have been used to treat the colored water. The adsorption method is of low initial

37

cost, flexibility, and operational simplicity [10,11]. Therefore, designing novel functional adsorbents for efficient

38

adsorption of dyes is of significant interest [12,13,14,15,16,17].

39

Since numerous dyes are ionic (i. e., cationic or anionic) [1], the electrostatic attraction between an adsorbent

40

and an adsorbate could markedly enhance the adsorption ability of the adsorbent [12,13,14,15,16]. For example,

41

Gao et al. [12] synthesized carboxylic hyperbranched polyglycerol grafted Fe3O4/SiO2 magnetic adsorbent

42

(Fe3O4/SiO2/HPG-COOH). The electrostatic interaction between the surface -COOH groups of the adsorbent and

43

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

44

methyl violet, and this was more than twice of the previous magnetic adsorbent. Similarly, Zhou et al. [13] also

45

used multicarboxylic hyperbranched polyglycerol to modify a calcined mesoporous silica SBA-15 and produced

46

hybrid adsorbent SBA/HPG-COOH. The saturated adsorption capacity of SBA/HPG-COOH towards cationic dye

47

methylene blue was 0.5 mmol g-1 (i. e., 187 mg g-1), which is 10 times more than the amount adsorbed by

48

nonmodified SBA-15.

49

Polymeric ionic liquids (PILs) [18] exhibit potential application in the adsorption of adsorbates including dyes

50

[14,15,16,19,20,21,22,23], because of their ionic moieties, which can electrostatically attract adsorbates. For

51

example, Yuan et al. [22] synthesized PIL poly(quaternary ammonium salt)-grafted PVBC microspheres

52

(PVBC-g-PDMAPMA+Br-), whose adsorption capacity towards phenol was

53

The ion exchange between phenol and the counter-ion Br- was responsible for the adsorption. Imidazolium (Im+)

54

salt-based PILs (PIms) have been used to adsorb anionic dyes, because their polymer chains bear cationic groups.

55

The structure of the positive aromatic rings of PIms can provide strong electrostatic force as well as the weak

56

interaction (i. e., π–π stacking [24]) for adsorbates binding. For instance, Kong et al. [16] synthesized a 2

2.23 mmol g-1 (i. e., 210 mg g-1).

57

hydrophobic linear poly(3-ethyl-1-vinylimidazolium bis(trifluoromethanesulfonyl)imide) polymer (PIm+TFSI-),

58

whose adsorption capacity towards acid dye methyl blue was 476 mg g-1. The electrostatic attraction between PILs

59

and adsorbates play an important role in the adsorptions. Obviously, the exposed IL moieties of PILs to the

60

adsorbates (i.e., availability of the IL moieties of PILs for adsorbates ) are prerequisite for the interaction. So far,

61

the availability of the IL moieties was usually improved by increasing their porosity and specific surface area of

62

PILs

[14,15,23].

63

According to the above, if ionic liquid (IL) moieties are introduced into the backbone of a hyperbranched

64

polymer (HBP) [25], the cavities are produced naturally around the IL groups [26]. Thus, the ionic groups on the

65

HBP backbone were more accessible for guest molecules with counter charges, leading to the efficient adsorption

66

of guest molecules around ionic groups by electrostatic attraction. The encapsulation of the cavities of a HBP was

67

previously confirmed [26,27,28]. Therefore, a hyperbranched PIL (hb-PIL) may exhibit high adsorption capacity.

68

However, to the best of our knowledge, this has not yet been explored. Herein, we report a novel hb-PIL adsorbent,

69

where IL groups are located on the HBP backbone. In this study, the hb-PIL with an Im+ backbone (hb-PIm+Cl-)

70

was synthesized by the “A2+B3” thiol–ene click polymerization (Scheme 1). The anion exchange of hb-PIm+Cl-

71

with KPF6 afforded a hydrophobic adsorbent hb-PIm+PF6-. The adsorption capacity and mechanism of hb-PIm+PF6-

72

for anionic dyes were investigated in detail.

73 74

Scheme 1 Synthesis route of the hyperbranched polymeric ionic liquid hb-PIm+PF63

75

2. Experimental

76

2.1 Materials

77

2,2-azobisisobutyronitrile (AIBN, 98%) and trimethylolethane (TME, 97%), Acros; 3-mercaptopropionic acid

78

(>99%), Sigma-Aldrich; N-vinyl imidazole (VIm, >98%), TCI. Potassium hexafluorophosphate (KPF6, 99%),

79

Aladdin Chemistry. Co. Ltd; ethylene glycol (EG) and chloroacetic acid (CAA), Sinopharm Chemical Reagent Co.

80

Ltd; para-toluenesulfonic acid (p-TSA), Tianjin Kemiou Chemical Reagent Co. Ltd (Tianjin city, China); congo red

81

(CR), methyl orange (MO), acid Fuchsin (AF), thymol blue (TB), methyl violet (MV), and malachite green (MG),

82

Tianjin Hongyan Chemical Co. Ltd (Tianjin City, China); and methylene blue (MeB), SERVA Electrophoresis

83

GmbH. Co. Ltd. AIBN was recrystallized from methanol prior to use.

84

2.2 Characterizations

85

Nuclear magnetic resonance (1H NMR) spectra were recorded using a Bruker Ascend 400 MHz spectrometer

86

in CDCl3 or D2O. Fourier transform infrared (FTIR) spectra of the samples were recorded using a NICOLET iS10

87

spectrometer (Nicolet, USA) in the range 4000–400 cm-1. Polymer samples for FTIR observation were prepared by

88

casting thin polymer films on a KBr holder. UV–vis spectra were recorded using a spectrophotometer UV-2550

89

model (Shimadzu, Japan). The BET surface area and total pore volume of the adsorbent were characterized by

90

using N2 adsorption-desorption isotherms with a Trestar 3020 surface area analyzer (Micromeritics, USA). The

91

surface morphologies and compositions of the samples were investigated using a field emission scanning electron

92

microscope (SEM, TEI Quanta 600 FEG, USA) equipped with an electron dispersive X-ray analysis (EDX) detector.

93

To observe the interior morphologies of the samples, they were first embedded in EPON 812 epoxy resin, followed

94

by curing at 60 °C for 48 h. The ultrathin sections (~50 nm) of the embedded specimens were cut with an

95

ultramicrotome and were then observed by transmission electron microscopy (TEM, HITACHI H-7650, Japan).

96

The Z-average hydrodynamic diameter of sample in water was characterized by using a Zetasizer Nano-ZS

97

dynamic light scattering (DLS) device (Malvern, Britain).

98

2.3 Synthesis of di-thiol A2 monomer (EGDMPA)

99

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

100

mL of toluene. The mixture was heated to reflux under Dean-Stark condition in an oil bath at 120 °C for 4 h, and

101

was further allowed to cool to room temperature, diluted with 150 mL of dichloromethane, and then washed

102

successively with 4% aqueous sodium carbonate solution and water. The crude product was further purified by 4

103

silica-gel column chromatography. The final liquid product was obtained by vacuum drying at 20 °C. Yield: 78%.

104

1

105

(KBr, cm-1): 2575 (S–H), 1746, (C=O).

106

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

107

TME (2.40 g, 20 mmol), CAA (7.00 g, 74 mmol), and p-TSA (0.38 g, 2 mmol) were successively dissolved in

108

80 mL of toluene. Under the condition of Dean-Stark, reaction mixture was heated to reflux in an oil bath of 120 °C

109

for 4 h. The purification process of the obtained product was similar to that of EGDMA. Yield: 81% wt. 1H NMR

110

(400 MHz, DMSO-d6, ppm): 0.99 (3H, –CH3), 4.10 (6H, CH2OCO), 4.45 (6H, COCH2Cl). FTIR (KBr, cm-1): 1748

111

(C=O), 790 (C–Cl).

112

2.5 Synthesis of triene ionic liquid B3 monomer (TMEAVIm+Cl-)

113

TMEAVIm+Cl- was synthesized by stirring the mixture of TMECA (4.0 g, 11 mmol) and VIm (5.0 g, 53 mmol)

114

in 6 mL of ethanol at 60 °C for 18 h. The product was precipitated thrice from the ethanol solution using the mixed

115

dichloromethane/ethyl acetate (200 mL/200 mL) solvents. After drying at 30 °C in vacuum, a slight yellow

116

amorphous product was obtained (yield, 64%.). 1H NMR (400 MHz, D2O, ppm): ~0.77–0.99 (3H, –CH3),

117

~4.02–4.22 (6H, CH2OCO), ~5.06–5.17 (6H, COCH2N+), ~7.10–5.40 (9H, –CH=CH2), ~9.06–7.26 (9H,

118

imidazolium ring +NCHCHNCH). FTIR (KBr, cm-1): ~3136–3050 (sp2 C–H), ~1653–1360 (imidazolium ring,

119

–CH=CH2), 920 (–CH=CH2).

120

2.6 Synthesis of hb-PIm+Cl-

121

hb-PIm+Cl- was synthesized by the thiol–ene addition polymerization between di-thiol monomer EGDMPA

122

and triene monomer TMEAVIm+Cl- using AIBN as the initiator. The detailed feed composition is listed in Table 1.

123

Briefly, TMEAVIm+Cl- was dissolved in ethanol and added to a DMF solution of EGDMPA and AIBN. Then, the

124

polymerization was carried out at 60 °C for 24 h. After completing the reaction, the resulting mixture was allowed

125

to cool to room temperature and then precipitated in 50 mL of THF. The crude product was then dissolved in 5 mL

126

water and dialyzed (molecular weight cut off: 3500) against water for two days. The dialyzed product was

127

lyophilized and kept in a glassware under vacuum for further characterization.

128

2.7 Synthesis of hb-PIm+PF6-

129

hb-PIm+PF6- was synthesized by the anion exchange reaction of hb-PIm+Cl- with KPF6. Briefly, a solution of 1

130

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

131

was then stirred vigorously at 40 °C. After 24 h, the produced precipitate was washed several times with deionized

132

water until the complete removal of the halide ions, which was confirmed by silver nitrate solution [29]. After

133

lyophilizing, a white powder was obtained.

134

2.8 Adsorption experiments

135

A certain amount of hb-PIm+PF6- was added to the CR solution of specific concentration and volume. Then,

136

the mixture was shaken at 25 °C and 100 rpm. At a predetermined interval, the adsorbent was removed by filtration

137

using a 450 nm membrane filter, and the CR concentration of the filtrate was analyzed by measuring its UV–vis

138

absorbance at 496 nm. The adsorption capacity at time t (h) and equilibrium was defined as qt (mg g-1) and qe (mg

139

g-1), respectively. The removal efficiency (R) was defined as the percentage ratio of the CR amount removed by the

140

adsorption in all the CR. They were determined by the following equations. qt 

141

qe 

142 143

R

(C0,CR  Ct ,CR ) madsorbent

(C0,CR  Ce,CR )

(C0,CR  Ce,CR ) C0,CR

madsorbent

(1)

VCR

(2)

VCR

100%  (1 

Ce,CR C0,CR

) 100%

(3)

144

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

145

concentrations of CR at time t (h) and at equilibrium, respectively, VCR (L) is the volume of the used CR solution,

146

and madsorbent is the weight of hb-PIm+PF6-.

147

3. Results and discussion

148

3.1 Synthesis of A2- and B3-types monomers

149

The synthesis routes of A2- and B3-types monomers are shown in Schemes 1A–C. Monomer EGDMPA was

150

synthesized via the O-esterification reaction [30] of EG and MPA. Monomer TMEAVIm+Cl- was synthesized via

151

two steps: the O-esterification reaction of TME with CAA to afford the tri-chloride precursor TMECA and the

152

haloalkylation reaction [31] of TMECA with VIm to afford the B3 monomer. To avoid the formation of insoluble

153

gels during the following thiol–ene addition polymerizations because of rigid structures [32], EG and TME as the

154

building units were introduced into the monomer structures expected to provide the resulting hb-PILs with flexible

155

alkyl chains. Moreover, solvent EtOH was required for the haloalkyation reaction (Scheme 1C), because the high

156

viscosity of the reaction system could result in an incomplete alkylation. The structures of the obtained A2 and 6

157

B3-type monomers were confirmed by 1H NMR and FTIR measurements. Monomer EGDMPA was easily soluble

158

in DMF, whereas monomer TMEAVIm+Cl- is only slightly soluble. Monomer TMEAVIm+Cl- is soluble in water

159

and EtOH; however, monomer EGDMPA isn’t.

160

3.2 Synthesis of the hyperbranched polymeric ionic liquid adsorbent

161

Hydrophilic hb-PIm+Cl- was prepared by “A2+B3” thiol–ene addition polymerization based on the radical

162

mechanism [33,34] (Scheme 1D). For the synthesis, the molar A2/B3 feed ratio was set at 2.1/1. Preliminary study

163

showed that mixed EtOH/DMF solvent was suitable for the synthesis; however, the composition of EtOH/DMF

164

composition exerted an evident effect on the structure of the resulting hb-PIm+Cl- (Table 1). Fig. 1 shows the 1H

165

NMR spectra of hb-PIm+Cl- synthesized at different EtOH/DMF compositions. In Fig. 1, the peaks at δ ~3.60–3.35

166

and δ 3.00–2.20 were assigned to the protons of >N CH2CH2SCH2CH2COO– units (labeled as e, f, g, and h

167

protons, respectively), confirming the thiol–ene reaction by β-addition. The peak at δ ~1.25–1.00 ppm was assigned

168

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

169

~7.10–5.30 were assigned to the protons of Im+ rings (N+CHNCH2CH2) and the unreacted double bonds,

170

respectively. The assignment of the other proton peaks of hb-PIm+Cl are as follows: δ ~5.26–5.03 (c COCH2N+, k

171

–SCHN–), δ ~0.95–~0.73 (a –CH3) and δ ~4.50–4.00 (b, i –COOCH2). It was worth noting that the reaction

172

between the double bonds might existed under the radical mechanism. However, it was difficult to directly

173

recognize the relative proton peaks from the 1H NMR spectra, because of the overlap of the peaks of multiprotons.

174

However, for a double bond of the B3 monomer, the ratios of their possible reactions are shown by Eq. (4),

175

X   X   X homo  X C C  100%

176

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

177

reaction of double bonds and unreacted double bond of B3 monomer, respectively. The ratios of their different

178

reactions were calculated by the integration ratio of their specific protons (Table 1), and the results are listed in

179

Table 1.

(4)

180

Regardless of β- or α-addition of thiol–ene reaction, a branched polymer was conducive to form. However, the

181

reaction between the double bonds could cause side reactions such as crosslinking or linear propagation. Therefore,

182

a decrease in the content of Xhomo and XC=C was beneficial to constructing HBP. By comparison of the Xβ, Xα, Xhomo,

183

and XC=C values (Table 1), the composition of solvents was found to exert an evident impact on the structure of 7

184

hb-PIm+Cl-. At VEtOH/VDMF = 4:1, XC=C and Xhomo of hb-PIm+Cl--1 were 14% and 8%, respectively. However, at

185

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

186

composition reduced the side reaction of the double bonds and availed to form a hyperbranched structure.

187

188 189 190

Fig. 1 1H-NMR spectra of samples hb-PIm+Cl 1–4 in D2O

191

To further analyze the structure of samples hb-PIm+Cl--1–4, the actual A2 and B3 unit ratio ([A2]/[B3]) of the

192

samples were estimated by comparing the integration area of the proton signals of i and d (Fig. 1), and the results

193

are listed in Table 1. If there was no intramolecular thiol–ene reaction and other side reactions, the obtained

194

hyperbranched architecture should be perfect, where the actual numbers of A2 and B3 units should follow [A2] =

195

2[B3] +1 at a molar feed [A2]/[B3] ratio of 2.1/1 [31]. Therefore, the actual [A2]/[B3] ratio of a HBP could exhibit its

196

degree of branching [35]. The larger [A2]/[B3] ratio of a sample is, the higher will be its degree of branching. As

197

listed in Table 1, the actual [A2]/[B3] values of all obtained samples were <2. However, sample hb-PIm+Cl--4 did

198

not show the structure signal from the side reaction, and its [A2]/[B3] value was the highest in these samples. The

199

Z-average hydrodynamic diameter of hb-PIm+Cl--4 in water (2.5 mg mL-1) was determined by DLS and was found

200

to be 9.3 nm with a particle dispersion index of 0.248, indicating the macromolecular scale of the hb-PIm+Cl--4. 8

201

Therefore, sample hb-PIm+Cl--4 should be a polymer with a relatively highly branched structure and was therefore

202

used in the following studies.

203 204

Table 1

205

Synthesis conditions and structural data of hb-PIm+ClSynthesis conditionsa Code

VEtOH b VDMF

hb-PIm+Cl- d

Conc. c

XC=C





Xhomo

mol L-1

%

%

%

%

[ A2 ] [ B3 ]

hb-PIm+Cl--1

4:1

1.02

14

63

15

8

0.93

hb-PIm+Cl--2

3:1

0.78

0

58

10

32

1.02

hb-PIm Cl -3

2:1

0.78

0

67

29

4

1.80

hb-PIm+Cl--4

1:1

0.78

0

50

50

0

1.85

+

-

206

a) AIBN content was fixed at 2.5 wt. b) the solvent mixture composition. c) the total monomer concentration. d)

207

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

208

integral area of the relative proton peak. The actual [A2]/[B3] values were calculated by the integral ratio of

209

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

210

= 9(I4.0~4.50-2/3I9.50~7.10)/4I9.50~7.10.

211 212

After the anion exchange reaction of hb-PIm+Cl--4 with KPF6 (Scheme 1E), a hydrophobic product

213

hb-PIm+PF6- was obtained and used as the adsorbent for the subsequent adsorption studies.

214

3.3 Effect of adsorption conditions on the adsorption properties of the hb-PIm+PF6- adsorbent

215

3.3.1 Effect of contact time

216

To assess the adsorption ability of sample hb-PIm+PF6-, CR was used as the model dye and its molecular

217

structure is shown in Fig. S1. Fig. 2 shows the adsorption capacity (qt) of hb-PIm+PF6- towards CR versus contact

218

time (t). As shown in Fig. 2, under higher weight ratio conditions of CR to hb-PIm+PF6- (C0,CRVCR/madsorbent =

219

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

220

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

221

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

222

500 mg g-1. These results exhibit that the adsorption of hb-PIm+PF6- toward CR tended to achieve a dynamic

223

equilibrium after 24 h. Therefore, in the following adsorption studies, a contact time of 24 h was selected.

224

3.3.2 Effect of adsorbent dosage

225

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

226

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

227

indicating that hb-PIm+PF6- is an excellent adsorbent for the removal of CR. When the dosage of hb-PIm+PF6- was

228

increased to 76 mg L-1, 92.5% of CR was removed; however, the qe value decreased to 1216 mg L-1. When

229

hb-PIm+PF6- amount was further increased to >125 mg L-1, all the CR was almost removed from the solution;

230

however, the qe values decreased with increasing amount of hb-PIm+PF6-.

231 232

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

233

mg, (b) C0,CR =100 mg L-1, VCR =10 mL, and madsorbent = 2 mg.

234 235

Fig. 3 Effect of hb-PIm+PF6- dosage on the adsorption capacity and removal efficiency of CR for a contact time of

236

24 h at 25 °C. (C0,CR = 100 mg L-1, VCR = 10 mL).

237 238

3.3.3 Effect of initial dye concentration

239

To further assess the maximum adsorption capacity of hb-PIm+PF6- towards CR, several adsorption

240

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

241

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

242

1150 and 1622 mg g-1, respectively. At these adsorption amounts, 96.7 and 91.3% of CR were removed from the

243

two solutions, respectively. When C0,CR increased from 200 to 350 mg L-1, the qe values increased from 1714 to

244

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

245

values exhibited less change with C0,CR, and at C0,CR = 600 mg L-1, only 27.7% of CR was removed. These indicate

246

that the maximum adsorption capacity of hb-PIm+PF6- towards CR was 1993 mg L-1 at 25 °C, which was much

247

higher than those of the previously reported adsorbents for CR (Table S1) [36,37,38,39,40,41].

248 249

Fig. 4 Effect of C0,CR on the adsorption capacity and removal efficiency at 25 °C. (madsorbent = 2 mg, VCR = 25 mL)

250 251

3.4 Adsorption mechanism of hb-PIm+PF6-

252

3.4.1 SEM and TEM analyses

253

To investigate the mechanism of CR adsorbed into (or onto) the hb-PIm+PF6-, the samples were obtained from

254

point 1# of Fig. 3 and points 2# and 3# of Fig. 4 and labeled as HP-CR500, HP-CR1160, and HP-CR1950,

255

respectively, where the number exhibit their adsorption capacities. These samples were investigated by SEM, EDX,

256

TEM, and FTIR analyses. Fig. 5 shows the SEM and EDX images of the samples hb-PIm+PF6-, HP-CR500,

257

HP-CR1160, and HP-CR1950. As shown in Figs. 5a and 5b, the surface morphology of sample HP-CR500 is

258

similar to that of hb-PIm+PF6-, indicating that there was not evident CR aggregation on the surface of hb-PIm+PF6-

259

at an adsorption capacity of ~500 mg g-1. However, the surface morphologies of samples HP-CR1160 and

260

HP-CR1950 were different from that of hb-PIm+PF6- (Figs. 5c and 5d), probably caused by the CR aggregation on

261

the surface of hb-PIm+PF6-, indicating that some CR was adsorbed onto the surface of hb-PIm+PF6- at higher

262

adsorption amount of CR. 11

263

264 265

Fig. 5 SEM images of hb-PIm+PF6- (a), HP-CR500 (b), HP-CR1160 (c) and HP-CR1950 (d), and the corresponding

266

EDX images (a′)–(d′).

267

268 269

Fig. 6. TEM images of ultrathin cross-sections (~50 nm) of samples hb-PIm+PF6- (a), HP-CR500 (b), HP-CR1160

270

(c) and HP-CR1950 (d).

271 272

To further investigate the CR distribution inside hb-PIm+PF6-, ultrathin sections of samples hb-PIm+PF6-,

273

HP-CR500, HP-CR1160, and HP-CR1950 were prepared using an epoxy resin matrix for TEM observation, and the

274

results are shown in Fig. 6. The relatively dark regions in the Figure were assigned to the samples. Compared to the

275

blank sample hb-PIm+PF6-, the colors of samples HP-CR500, HP-CR1160, and HP-CR1950 successively deepened,

276

attributing to the presence of CR inside the samples. This is because an election-rich substance could weak light

277

transmission. Furthermore, as shown in Figs. 6a and 6b, the light color parts inside the region of samples

278

HP-CR500 and HP-CR1160 were clearly observed, probably because of the pores of the samples themselves,

279

whereas sample HP-CR1950 exhibited fewer lighter spots, indicating that at lower adsorption capacity, the

280

adsorbed CR distributed mainly in the polymer matrix. At higher adsorption capacity, the adsorbed CR might fill 12

281

the pores of samples or anchor on the surface of these samples. Especially, as shown in Figs. 6c and 6d, the edge

282

regions of samples HP-CR1160 and HP-CR1950 show evident CR aggregation. This observation was in agreement

283

with that from the SEM results, indicating that CR was first adsorbed into the polymer matrix in the adsorption

284

process, and with increasing adsorption capacity, CR began to fill the pores and aggregate on the surface of

285

hb-PIm+PF6-. The adsorbed CR distribution in the polymeric matrix indicates the position of the accommodation of

286

molecules. In fact, the encapsulation of cavities of HBP was confirmed previously [23, 24]. Therefore, these results

287

suggest that the cavities formed by hyperbranched structure play an important role in high adsorption capacity of

288

hb-PIm+PF6- towards CR. The low surface area (1.65 m2 g-1) of hb-PIm+PF6- (N2 adsroption-desorption curve

289

shown in Fig. S2) futher confirmed that its high adsorption capacity should depend on the cavities formed by

290

hyperbranched hyperbranched structure.

291 292

Table 2

293

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

294

Pseudo-second-order: t qt  1 k2 qe2  t qe qe,cal (mg g-1)

k2 (g mg-1 h-1) 10-4

R22

CR

1859±56

1404

0.175

0.9856

2003

2.06

0.9985

MO

665±14

509

0.275

0.9426

741

7.23

0.9957

AF

2094±202

1939

0.208

0.9338

2485

1.07

0.9932

a

-1

C0,dye=300 mg L . qe,exp and qe,cal are the experimental and calculated maximum adsorption capacity, respectively;

295

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

296 297

pseudo-second-order kinetics, respectively.

298

3.4.2 Adsorption Kinetics

2

299

To investigate the adsorption mechanism of hb-PIm+PF6- towards CR, the data of curve a in Fig. 2 were

300

analyzed by using pseudo-first-order and pseudo-second-order models [42]. Their fitted plots are shown in Figs. S3

301

and S4, respectively, and the results are listed in Table 2. The calculated qm values from the fitted curves by

302

pseudo-first-order and pseudo-second-order kinetic models were 1404 and 2003 mg g-1, respectively; whereas the

303

experimental qe value was 1859 mg g-1. This means that the result from seudo-second-order kinetic model was

304

closer to experimental qe value. The correlation coefficient (R2) values of the two fitted curves by the

305

pseudo-first-order and pseudo-second-order models were 0.9856 and 0.9985, respectively. These results means that

306

pseudo-second-order model could describle the adsorption process better. The result indicates the chemisorption

307

nature of hb-PIm+PF6- towards CR [43,44]. This is further confirmed by EDX (Figs. 5a′– 5d′) and FTIR (Fig. S5) 13

308

analyses. Regardless of the results from EDX or FTIR measurement, an increase in the adsorption capacity of CR

309

decreased the PF6- content, probably because of the anion exchange between CR anions and PF6- counter-ions. As a

310

result, CR was bound to the backbone of the hyperbanched polymer, whereas PF6- was removed. Therefore, the

311

chemisorption involved the strong electrostatic interaction between the adsorbent and CR.

312

The adsorption data of hb-PIm+PF6- toward dyes MO and AF (the structures shown in Fig. S1) were also

313

analyzed by using pseudo-first-order and pseudo-second-order models (Figs. S6-S11) and the results are listed in

314

Table 2. The results reveals that the adsorptions of hb-PIm+PF6- toward dyes MO and AF could also well follow the

315

pseudo-second kinetic model.

316

3.4.3 Adsorption Isotherms

317

To investigate the adsorption mechanism of hb-PIm+PF6- toward CR, the data of Fig. 4 were further analyzed

318

by the Langmuir and Freundlich isotherm models [45], and their fitted plots are shown in Figs. S12 and S13,

319

respectively, and the results are shown in Table 3. As listed in Table 3, from the Langmuir model, the maximum

320

adsorption capacity (qm) of 2050 mg g-1 was obtained, and the R2 value of the fitting was 0.9996, whereas, the KF

321

from the Freundlich model is 1181 mg g-1, which was evidently lower than the experimental value, indicating the

322

adsorption of hb-PIm+PF6- towards CR conformed well to the Langmuir model. Langmuir isotherm model is based

323

on monolayer adsorption. Therefore, the adsorption of hb-PIm+PF6- towards CR might be mainly related to site

324

adsorption. For adsorbent hb-PIm+PF6-, the main site characteristics are the cationic Im+ rings of the hyperbranched

325

backbone and the cavities formed by the hyperbranched structure. To investigate the role of the cationic Im+ groups

326

of the hyperbranched backbone in the adsorption, three cationic dyes (MeB, MV, and MG) (the structures shown in

327

Fig. S1) were selected as the guest molecules. At C0,dye=400 mg L-1, the adsorption capacities of hb-PIm+PF6-

328

towards cationic MeB, MV, and MG were about 45, 86 and 75 mg g-1, respectively, whereas at C0,dye = 10 mg L-1,

329

these cationic dyes were not at all adsorbed by hb-PIm+PF6-. Such a low adsorption ability of hb-PIm+PF6- towards

330

the cationic dyes may be caused by the electrostatic repulsion between the cationic Im+ rings and the cationic dyes.

331

However, for other anionic dyes such as MO and AF, at C0,dye = 400 mg L-1, the adsorption capacities were 679 and

332

2136 mg g-1, respectively (their isotherms shown in Figs. S14 and S15), indicating that the electrostatic interaction

333

of polymer backbones with dyes is the main driving force for the adsorption. This result was also confirmed by the

334

FTIR (Fig. S4) and EDX (Fig. 5) spectra, and the cavities provide a storage site for the interaction. Owing to the

335

fact that the cavities formed by hyperbranched chains should be at the molecular level, a site may accommodate a 14

336

CR molecule. Therefore, the adsorption could follow the Langmuir model. Of course, as shown in Figs. 5c-d and

337

Figs. 6c-d,, CR shows evident aggregation on the surfaces of hb-PIm+PF6- (Figs. 5c-d and Figs. 6c-d), and the

338

presence of pores inside the adsorbed samples was not very evident, indicating that it may be multilayer adsorption

339

on the surfaces and inside the pores of hb-PIm+PF6-. Moreover, the R2 value of the fitting by the Freundlich model

340

also reached to 0.9590. Therefore, the adsorption of hb-PIm+PF6- may be heterogeneous and follow more than one

341

mechanism [44,45]. However, as a whole, the adsorption conformed better to the Langmuir model. Therefore, both

342

the cavities and the IL moieties of the hyperbranched adsorbent played an important role in the adsorption of CR.

343 344

Table 3

345

Parameters calculated form the adsorption isotherm models Parameter a

Isotherm model qe,exp Langmuir

(mg

g-1)

RL2 -1

CR

MO

AF

1993±58

679±43

2136±175

0.9996

0.9937

0.9986

Ce qe  1 qm K L  Ce qm

qe,cal

(mg g )

2050

769

2287

KL

(L mg-1)

0.091

0.025

0.056

Freundlich

RF2

0.9590

0.9897

0.9185

1181

122

522

0.094

0.315

0.278

0.9622

0.9558

0.9652

339.3

0.343

0.828

175

151

430

ln qe  1 n ln Ce  ln K F

Termkin

qe  B ln KT  B ln Ce

Scatchard

qe Ce  qm K S  K S qe

KF

-1

(mg g )

1/n RT2 KT B

-1

(L mg ) -2

-2

(KJ mol )

RS,H2 qm,H KS,H

0.9664

0.9914

0.8475

-1

2323

958

2321

-1

0.098

0.052

0.051

0.9973

0.9133

0.9729

(mg g ) (L mg )

RS,L2 qm,L

(mg g-1)

2252

956

2397

KS,L

(L mg-1)

0.031

0.011

0.040

346

a

qe,exp and qe,cal are the experimental and calculated maximum adsorption capacity, respectively; R2 is the correlation

347

coefficient; KL is the adsorption equilibrium constant; KT is the equilibrium binding constant corresponding to the

348

maximum binding energy; KS,H and KS,L are the adsorption equilibrium constant for “high-affinity” and the

349

“low-affinity” binding sites, respectively. KF and 1/n are the characteristic constants representing the adsorption

350

capacity and adsorption intensity of the system, respectively; B is the Termkin constant related to heat of adsorption.

351

15

352

Further, the CR adsorption data of Fig. 4 were also analyzed by the Temkin [42] and Scatchard [46] isotherm

353

models, and their fitted plots are shown in Figs. S16 and S17, respectively, and the results are listed in Table 3. The

354

Temkin model takes account of the interaction between adsorbing species and adsorbate. Therefore, as listed in

355

Table 2, high R2 value (0.9622) of the fitted Temkin model indicates that the adsorption of hb-PIm+PF6- towards

356

CR follow chemisorption and is in agreement with above mentioned mechanism. In the Sactchard plot (Fig. S17),

357

the presence of a deviation from the linearity on the plot (two inflection) points out the presence of more than one

358

type of binding sites mainly contributing to the adsorption [42], indicating that two main affinity binding sites exist

359

in the adsorption process. The high-affinity binding sites correspond to the anion exchange sites of PF6-, and the

360

low-affinity binding sites might be attributed to the weak interactions such as π–π stacking or H-bonding sites.

361

The results from isotherm analysis of the adsorption data of hb-PIm+PF6- toward dyes MO and AF (Figs.

362

S18-S25) are shown in Table 3. The results reveals that the adsorptions of hb-PIm+PF6- toward MO and AF also

363

conform to the Langmuir and Termkin models well.

364 365

Table 4

366

Standard thermodynamic parameters of the dye adsorption a Dye

C0,dye (mg L-1)

ΔG (KJ mol-1) at different temperatures c

ΔH c

ΔS c

(KJ mol-1)

(KJ mol-1 K-1)

298 K

308 K

318 K

328 K

R2 b

CR

500

0.985

-35.065

-0.103

-4.391

-3.227

-2.014

-1.353

MO

300

0.959

-27.565

-0.084

-2.455

-2.132

-1.002

-0.045

AF

400

0.936

-25.079

-0.064

-5.929

-5.673

-5.026

-4.023

367

a), the adsorption condition: madsorbent = 2 mg, V = 25 mL, contact time = 24 h; b), R is the correlation coefficient;

368

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

369

/RT– ΔS/R, where T is the temperature (K); the value of parameter ΔG at a certain temperature is calculated by

370

equation ΔG = –RTln(qe/Ce), where qe (mg g-1) and Ce (mg L-1) are the equilibrium amount of dyes adsorbed on the

371

adsorbent and equilibrium concentration of dyes in the solution, respectively, R is the universal gas constant (8.314

372

J/mol K);

373

3.4.4 Adsorption thermodynamics

2

374

Thermodynamic analysis of an adsorption process provides information on its spontaneity [47,48,49]. The

375

changes in the standard Gibbs energy (ΔG), enthalphy (ΔH), and entropy (ΔS) of dyes CR, MO and AF are

376

calculated from experimental data obtained at different temperatures (Fig. S26). The curves by plotting ln(qe/Ce)

377

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

378

values are negative, indicating the spontaneous nature of dyes adsorbed on hb-PIm+PF6-. The negative values of ΔH 16

379

indicated exothermic nature of the adsorption process [48], which is in agree with experiment results that lower

380

temperatures lead to higher adsorption capacity. The negative values of ΔS corresponds to a decrease in degree of

381

freedom of the adsorbed species.

382 383

Fig. 7. UV–Vis spectra of aqueous solutions of TB (a), MeB (b), TB + MeB mixture (c) and TB + MeB after the

384

addition of hb-PIm+PF6 (d) at 25 °C for 24 h. Experimental conditions: Vdye = 6 mL, C0,dye = 25 μmol L-1, and

385

madsorbent = 2 mg.

386

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

392

Dye solutions in the textile dyeing process usually contain NaCl and acid, because NaCl promotes the dye

393

adsorption of textile fibers [50] and acid promotes the acid dye dyeing [2]. Fig. 8 shows effect of the NaCl

394

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,

395

when NaCl concentration is up to 4000 mg L-1, nearly all the CR (R >98%) can be removed, indicating the presence 17

396

of NaCl produce less influence on the CR adsorption. In addition, as seen from Fig. 8, it was found that in the

397

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

398

on R values.

399

4. Conclusions

400

A hydrophilic hyperbranched polymer ionic liquid, hb-PIm+Cl-, was synthesized via the thiol–ene addition

401

polymerization by the “A2+B3” method. The anion exchange of hb-PIm+Cl- with KPF6 afforded a hydrophobic

402

adsorbent hb-PIm+PF6-. The as-prepared hb-PIm+PF6- effectively adsorbed anionic dyes such as CR, AF, and MO

403

from their aqueous solutions, whereas very less amount of cationic dyes such as MeB, MV, and MG were

404

adsorbed by hb-PIm+PF6- under the similar conditions. The selective adsorption could be used to separate a

405

mixture of cationic and anionic dyes. By using CR as the adsorbate, the adsorption mechanism of hb-PIm+PF6-

406

was investigated in detail. The results demonstrate that the characteristic structure of hb-PIm+PF6- was

407

responsible for its high adsorption capacity towards CR. The main driving force of the CR adsorption was

408

electrostatic attraction, which occurred by the anion-exchange between CR and the PF6- of hb-PIm+PF6-, and the

409

cavities formed by the branched chains provide storage sites for the interaction.

410 411 412 413 414

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415

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