Carboxymethylated Cellulose Fibers as Low-Cost and Renewable

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Carboxymethylated Cellulose Fibers as Lowcost and Renewable Adsorbent Materials Jian Wang, Miao Dang, Chao Duan, Wei Zhao, and Kai Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03697 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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Industrial & Engineering Chemistry Research

1

Carboxymethylated Cellulose Fibers as Low-cost and

2

Renewable Adsorbent Materials

3

Jian Wang a, b, Miao Danga*, Chao Duana, Wei Zhaoa, Kai Wanga

4

a. College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science &

5

Technology, Xi’an 710021, China

6

b. National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi

7

University of Science & Technology, Xi’an 710021, China

8 9

ABSTRACT: In recent years, decontamination of heavy metals from the water body is an important

10

topic in China due to the rapid industrialization and it is of both fundamental and practical interest to

11

develop eco-friendly, renewable, and degradable materials for this purpose. In this study,

12

cellulose-based adsorbent materials were prepared, based on the carboxymethylated modification

13

method. The carboxymethylated cellulose fiber (CMF) material, with high carboxyl content and

14

desirable fiber morphology, was demonstrated, and its application to remove heavy metal ions, such as

15

copper ions, via filtration-adsorption process, was investigated. Results showed that the CMF-based

16

adsorbent material displayed excellent adsorption capacity and regenerated property, due to its high

17

carboxyl content and accessibility to copper ions. Besides, the CMF retained good fiber morphology

18

due to a mild carboxymethylation on fibers surface (exterior and interior), which has great potential to

19

form commercial renewable filtering material for copper ions removal.

20

KEYWORDS: Carboxymethylated cellulose fiber, Adsorbent material, Wastewater treatment, Copper

21

ions removal

22 23

1. INTRODUCTION

24

With the rapid development of copper ions mining operations and copper ions plating facilities, the

25

discharge of wastewater containing copper ions into the environment significantly increases 1. Copper

26

ions have specific function in animal metabolism, however, the excessive ingestion of copper ions can

27

lead to serious toxicological concerns, such as cramps, vomiting, convulsion or even death 2. Therefore,

28

it is necessary to treat copper ions-containing wastewater prior to discharging it.

29

A wide range of treatment technologies have been developed to remove heavy metal ions from the

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Page 2 of 21

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wastewater, including chemical precipitation, ion exchange, adsorption, membrane separation etc. 1, 3-5.

31

So far, chemical precipitation has been considered to be the most practical method since it is low-cost

32

and easy-to-handle. However, the chemical precipitation would generate large volumes of low density

33

sludge and result in dewatering and disposal problems 6. Recently, the adsorption method has been

34

recognized as an effective and economic technique for removal of heavy metal ions from wastewater

35

since the adsorption process offers flexibility in design and operation, and can produce high-quality

36

treated effluent in many cases. Activated carbon (AC) adsorbents, the most typical adsorbents, are

37

under extensive studies 7-9. Membrane filtration technology shows great potential for heavy metal ions

38

removal owing to its high efficiency, easy operation and space saving features, and there is a large body

39

of literature on the topic of membrane filtration

40

materials as low cost, eco-friendly, and renewable adsorbents for wastewater treatment are of practical

41

interest.

10-13

. Furthermore, economic, green and sustainable

42

Cellulose is one of the natural, bio-based materials with eco-friendly and low cost features. In recent

43

years, one of the emerging areas in the cellulose industry is the development of adsorbent materials

44

which make good use of cellulose, cellulose derivatives and cellulose composites14-16. Much effort in

45

cellulose-based adsorbent materials has been focused on studies of increasing the charge of cellulose.

46

Many widely reported chemical pretreatments17-20 (i.e. TEMPO-mediated oxidation, periodate-chlorite

47

oxidation or carboxymethylation) can introduce carboxylate groups onto the cellulose fibers for the

48

charge improvement of cellulose, among which, carboxymethylation is the most economic and facile

49

method. Moreover, the most important thing is that carboxymethylation is the only technology of

50

industrialization to date. Hence, this work focuses on the method of carboxymethylation.

51

Carboxymethyl cellulose (CMC) has been widely investigated as a kind of adsorption material

52

Although CMC has high adsorption capacity, its recycling from wastewater is challenging due to its

53

water solubility. In this project we focused on the preparation of carboxymethylated cellulose fibers

54

(CMF) that were obtained through a mild carboxymethylation on the cellulose fibers surface (exterior

55

and interior). In this way, the morphology of fibers could be maintained so that the CMF would be

56

easily recycled in the subsequent wastewater treatment process. The characteristics of CMF were

57

investigated. Subsequently, the CMF adsorbent filter material was prepared and its adsorption

58

performance to remove copper ions from synthetic waste water was studied.

59

2. EXPERIMENTAL SECTION


21, 22

.

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2.1 Materials. Non-refined, bleached softwood kraft pulp fibers (SKF; MCC paper Yinhe Co., Ltd.,

61

China) were used as native cellulose material, and the chemical composition of SKF was generously

62

provided by MCC paper Yinhe Co., Ltd.. Reagent grade sodium chloroacetate (MCA), sodium

63

hydroxide (NaOH), hydrochloric acid (HCl), sodium chloride (NaCl), sodium bicarbonate (NaHCO3),

64

ethanol and methyl red indicator were purchased from Jinquan chemical additive Co., Ltd. in China.

65

All chemicals were used as received. Milli-Q water and deionized water were used throughout this

66

work.

67

Table 1. Chemical Composition of SKF Hemicellulose

Sample

Cellulose/ %

SKF

Ash content/%

Lignin / %

87.6

1.3

Xylose/%

Mannose%

5.12

7.18

0.6%

68

2.2 Preparation of CMF and CMF-based Filter Material. SKF boards were torn into small pieces

69

and soaked in deionized water overnight. The wet pulp was disintegrated using a deflaker, then filtered

70

and dried at 50 °C. The dried pulp was mixed with a sodium monochloroacetate (MCA) solution (g of

71

MCA : g of pulp fiber : g of H2O = 1.5 : 1 : 2.6) in a household mixer for 10 min and then placed in a

72

60 °C water bath for 4 h for impregnation. After that, a sodium hydroxide (NaOH) solution (g of

73

NaOH : g of pulp fiber : g of H2O = 1.25 : 1 : 2) was added to the reaction vessel and mixed for 10 min.

74

The obtained pulp mixture was kept for 24 h at room temperature, and then soaked and washed with 70,

75

80, 90 and 100 wt% ethanol in sequence until it was alkali-free. Then the yield of the mixture of

76

water-soluble carboxymethyl cellulose (CMC, completely carboxymethylated without fiber

77

morphology)

78

carboxymethylated with good fiber morphology), so called CMC/CMF mixture, expressed as a

79

percentage, was calculated relative to the raw fiber material using Equation (1):

and

water-insoluble

carboxymethylated

cellulose

fibers

(CMF,

partially

80

Yield of product (%) =

(1)

81

where ma is the oven dry weight of raw material (g), and mb is the oven dry weight of product (g).

82

Finally, the obtained CMC/CMF mixture was further soaked and washed with distilled water to

83

remove water-soluble CMC, and the yield of the residual CMF was also calculated according to

84

Equation (1).

85

The obtained CMF was filtered on a glass funnel by suction filtration to prepare the CMF-based



86

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filter material, and the final product was air-dried before the adsorption studies.

87

2.3 Characterizations

88

Carboxyl Content of Pulp. The carboxyl groups (-COOH) are acidic groups introduced onto

89

cellulose chains, which indicate the ion-exchange capacity of the corresponding pulp, i.e., the ability to

90

absorb metallic cations during the treatment. The carboxyl contents of SKF and CMF were determined

91

according to the TAPPI T 237 with some modifications, because the carboxyl content of CMF was

92

much higher than that of the normal pulp. Specifically, the volume of sodium bicarbonate-sodium

93

chloride solution added to the test specimen was increased from 50.00 mL to 100.00 mL. The carboxyl

94

content in milliequivalents (meq) per 100 g of oven-dried pulp was calculated according to the

95

modified Equation (2):

96

,

= − + ×

× ×

!

(2)

97

where A is volume in mL of 0.010 N HCl consumed in titration of 25 mL of the pulp filtrate (mL), B is

98

volume in mL of 0.010 N HCl consumed in titration of 25 mL of the sodium bicarbonate-sodium

99

chloride solution (mL), C is weight of water in the pulp pad, i.e., weight of wet pad with the deduction

100

of the oven-dried weight of the sample (g), N is normality of HCl used in titration, W is weight of

101

oven-dried test specimen (g), 100 is volume of sodium bicarbonate-sodium chloride solution added to

102

the test specimen (mL), 400 is derived as 4 × 100, where 4 is a factor to account for 25 mL aliquot

103

taken for titration, and 100 is to express the result on 100 g of pulp.

104

FQA Analysis. The morphological properties of fibers, such as the length, width, fines content, were

105

measured using the Fiber Quality Analyzer (FQA, Morfi Compact, Techpap Co. Ltd., France).

106

Well-mixed dilute suspension (40 mg/L) of fiber sample was used for the measurement, which included

107

approximately 5000 analyzed fibers in each suspension.

108

Water Retention Value (WRV). The WRV of samples was measured according to SCAN-C 62:00.

109

Samples were soaked in distilled water for 10 min at room temperature, then put them into the

110

centrifuge tubes with sieves and centrifuged for 15 min at the speed of 3000 r/min to remove excess

111

water. After centrifugation, samples were weighted as m0. Subsequently, the samples were dried in an

112

air oven to the constant weight recorded as m1. The WRV can be calculated according to the following

113

Equation (3)

114

W=

" # #

× 100%


(3)

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115

where m0 is the weight of sample after centrifugation (g) and m1 is the weight of sample after drying

116

(g).

117

Pore Volume. The pore volume of samples before and after carboxymethylation were measured

118

based on the Brunauer–Emmett–Teller (BET) analysis of nitrogen absorption isotherms by using a

119

Belsorp-Max volumetric gas adsorption instrument (Bel Japan, Inc., Osaka, Japan).

120

Microscopic Imaging

121

Optical Microscopy. Cellulose fiber dispersions were observed with an optical microscope (DMB5,

122

Motic Co. Ltd., China). Samples were dyed with a few drops of Herzbery solution prior to imaging.

123

Scanning Electron Microscopy (SEM). Surface morphology images of fibers were recorded on a

124

SEM (S-4800, HITACHI, Tokyo, Japan) operating at an accelerating voltage of 3.0 kV. Freeze-dried

125

fiber samples were used, and sputter-coated with gold to a thickness of 8 nm before analysis.

126

X-ray Diffraction Analysis (XRD). Crystalline analysis of oven-dried fiber samples was performed

127

on a XRD analyzer (D/MAX-2200PC, Rigaku Denki Co. Ltd., Japan) operating at the Cu Kα radiation

128

(λ=0.154 nm) with 40 kV and 40 mA with a 2θ ranging from 5 ° to 60 ° at a step size of 0.02 °.

129

Fourier Transform Infrared Spectroscopy (FT-IR). The chemical bonds and molecular structure

130

of fiber samples were analyzed using a FTIR (VERTEX 70, Bruker Optics Corporation; Germany). The

131

samples were dried in an oven at 60 ºC to remove the moisture. 5 mg of fiber sample was mixed with

132

potassium bromide (KBr) and finely ground before the mixture was pressed to be a transparent pellet.

133

The samples were measured in the transmission of a wavelength number range from 4000 to 400 cm-1.

134

2.4 Adsorption Kinetics. Adsorption kinetics was studied by filtering quantitative volume of copper

135

ions-containing aqueous solution (100 ppm) through the CMF-based filter material for a predetermined

136

time. The adsorption experiment proceeded in the modified filtrate flow rate-adjustable equipment is

137

shown in Fig 1.



Page 6 of 21

138 139

Fig. 1 Flow Rate-adjustable Filter Equipment for Adsorption Experiment.

140

All the solutions were analyzed using an inductively coupled plasma spectrometer (IRIS Intrepid,

141

ICP-AES, America). The adsorption capacity at time t was calculated from the mass balance expression

142

in Equation (4) according to the literature 23, 24:

143

*" + ,-

() =

(4)

144

where Co and Ct are initial concentration and the concentration at time t, respectively, V is the solution

145

volume, and m is the dry mass of filter material.

146 147 148

The time-dependence of the adsorption is fitted with the pseudo-second-order kinetic model as described in literature 25, 26, which is shown as follows: )

+

=.

/ / 0

+ 1 0

(5)

149

where qt and qe are the adsorption capacities at time t and equilibrium, respectively, and k2 is the rate

150

constant of the pseudo-second-order model. The equilibrium isotherms for the adsorption are fitted by

151

Langmuir and Freundlich isotherm equations27. The Langmuir equation can be expressed as:

152

20

0

=

20

3

+

3 4 5

(6)

153

where qe is the amount of adsorbed material at equilibrium (mg/g), Ce is the equilibrium

154

concentration in solution (mg/L), qm is the maximum capacity of adsorbent (mg/g), and KL is the

155

“affinity parameter” or Langmuir constant (L/mg). The linear form of Freundlich equation, which is an

156

empirical equation derived to model the multilayer adsorption, can be represented as follows:

157 158

ln ( =

8

ln 9 + ln :;

(7)

where qe and Ce are defined as above, KF is Freundlich constant (L/mg), and n is the heterogeneity


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159 160

factor. 2.5 Recycling Performance. The copper ions saturated CMF filter material was soaked in a 0.01 N

161

HCl solution for 10 minutes, and was then washed with deionized water until the filtrate turned to be

162

neutral (no precipitation occurred with the addition of silver nitrate solution). Subsequently, the CMF

163

filter material was air-dried to constant weight before the subsequent adsorption study.

164

3. RESULTS AND DISCUSSION

165

3.1 Characterization of the As-prepared CMF

166

3.1.1 Yield of CMF and CMC/CMF Mixture Relative to the Raw Fiber Material. Shown in Fig.

167

2 are the yields of CMF and CMC/CMF mixture obtained from the carboxymethylation of SKF relative

168

to the raw fiber material, respectively. It shows that the yield (relative to the raw fiber material) of

169

CMC/CMF mixture, a heterogeneous fiber complex consisting of water- soluble CMC and water-

170

insoluble fibrous CMF, gradually rose and exceeded 100 wt% (due to the retention of a fraction of

171

CMC in ethanol rinsing) with the increase of the carboxymethylation extent under the conditions

172

studied. However, it can be inferred from Fig. 2 that the CMF/CMC fraction reduced from 1.75 to 0.60

173

with the increase of the carboxymethylation extent. As Tejado et al. reported28, fibers would break apart

174

and large amounts of CMC become soluble in water above 3 mmol/g of carboxylate content.

175

Consequently, the increased yield of water-soluble CMC resulted in the decreasing of the CMF/CMC

176

fraction as the carboxymethylation degree increased, and especially as the dosage of MCA exceeded 10

177

g (the corresponding yield of CMF was 87.6 wt% relative to the raw fiber material and the CMF/CMC

178

fraction was 1.17). Based on the abovementioned results, the MCA amount was optimized to be 10 g

179

during the carboxymethylation and the resultant CMF under the optimum condition was used for the

180

following characterization and adsorption study. In literature, the heterogeneity of carboxymethylation

181

in various solvent systems, such as water and ethanol, has already been discussed and the results

182

supported that CMC can be readily dissolved in distilled water and then extracted with 95 % (v/v)

183

ethanol as precipitated solid29-31.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Yield (relative to the raw fiber material)/%


184

Page 8 of 21

200 180

CMF CMC/CMF mixture

160 140 120 100 80 60 2.5

5.0

7.5

10.0

12.5

15.0

17.5

The dosage of MCA /g

185

Fig. 2 Yields of CMF and CMC/CMF Mixture Relative to the Raw Fiber Material After

186

Carboxymethylation.

187

3.1.2 Morphology and WRV. The comparison of morphology and water retention ability between

188

SKF and CMF is shown in Table 2. The average length of fibers decreased slightly from 1.790 mm to

189

1.485 mm after the carboxymethylation, while the average width of fibers increased from 30.5 µm to

190

37.9 µm and the fines content increased from 2.84 % to 5.29 %. The morphological change of fibers

191

could be attributed to the carboxymethylation, in which the original fiber structure was weakly

192

destroyed and shortened. Consequently, CMF still maintained a good fiber morphology, although the

193

fiber length slightly decreased and the fines content increased a little bit. The destruction of original

194

fiber structure, along with the enhanced water-swollen ability and the extended transverse section of

195

fibers improved the fiber accessibility and the reaction sites accordingly. Sim et al.

196

fibers can absorb large amounts of water as individual micro fibrils swollen in all three

197

Secondary-layers (S layers) and the width of swollen S-layers was up to 5-10 times more after the

198

carboxymethylation. Furthermore, compared to SKF, the WRV of CMF dramatically increased from

199

78.63 wt% to 289.87 wt%. As Kekäläinen et al. 33 reported, the WRV can indirectly represent the total

200

pore volume inside the fibers so that the increased WRV indicated the expansion of water-storage pore

201

volume of fibers. This inference was further confirmed by the pore volume test results (The pore

202

volume of fiber increased from 0.025 cm3/g to 0.041 cm3/g). Aarne et al.34 reported the CMC

203

adsorption on fibers which could introduce the carboxylic acid groups in Na-form to fibers, and the

204

WRV of the CMC- adsorbed fibers increased in that case too. However, the CMC adsorption on fibers

205

was lack of the stability compared to the CMF. The WRV and pore volume results further supported the

206

conclusion that more hydrophilic carboxyl groups and open pores were available on the swollen CMF


32

reported that the

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207

sample due to the carboxymethylation. Therefore, not only were there some good water pockets on

208

fibers, but also improved accessibility and increased reaction sites on fibers, resulting from the

209

carboxymethylation

210

for heavy metal ions removal and the result showed that the NaOH treatment of lignocellulosic

211

materials can cause swelling and lead to an increase in internal surface area, thus improving the heavy

212

metal ions adsorption and removal.

213

32

. Wan et al.

35

reviewed several chemically modified plant wastes as adsorbents

Table 2. Morphology and WRV Properties of SKF and CMF Samples Length

Width

Fines content

WRV

Pore volume

/ mm

/ µm

/%

/%

/*10-2cm3/g

SKF

1.790±0.021

30.5±0.6

2.84±0.53

78.63±1.21

2.5±0.2

CMF

1.485±0.032

37.9±0.4

5.29±0.65

289.87±1.36

4.1±0.1

Sample

214

3.1.3 XRD and FTIR. The XRD patterns of SKF and CMF are illustrated in Fig. 3. As seen in SKF

215

sample, two broad peaks and one intense peak respectively appeared at around 14.8°, 16.4° and 22.5°,

216

which indicated the typical XRD pattern of cellulose I 36. The CMF sample showed an intense peak at

217

about 20.1° indicating that the native cellulose I was mercerized and transformed into the cellulose II

218

due to the high concentration of NaOH in carboxymethylation of SKF

219

fibers would cause intra- and inter crystalline swelling, which results in the irreversible change of

220

cellulose crystalline structures. From the insert table in Fig. 3, the SKF sample, with regular crystalline

221

structure, showed a much higher crystallinity degree (54.69 %) than CMF sample (9.07 %). The low

222

crystallinity degree of CMF was most likely caused by the effect of NaOH on the cellulose structure in

223

the carboxymethylation. The cleavage of hydrogen bonds between fibers when soaked in NaOH

224

solution increased the distance between cellulose molecules, which could remarkably facilitate the

225

substitution of MCA molecules on the cellulose, as compared to the cellulose without alkalization

226

As a result, the NaOH treatment largely increased the carboxyl group content. Moreover, it also

227

increased the fiber accessibility. Sim et al.

228

heavily disrupt the crystalline structure of cellulose fibers, hence significantly improving the solvent

229

accessibility to the fibers.

39

37

. The alkaline treatment of

38

.

also reported that the highly alkaline condition would



Page 10 of 21

230

Sample

Crystallinity/%

SKF

54.69

CMF

9.07

SKF

CMF

0

5

10 15 20 25 30 35 40 45 50 55 60 65

2Theta /°

231

Fig. 3 X-ray Diffractograms of Air-dried SKF and CMF.

232

The presence of abundant carboxyl groups introduced to fiber after carboxymethylation was also

233

verified by the FTIR results. As shown in Fig. 4, the infrared spectrum of fibers after

234

carboxymethylation exhibited no new adsorption peaks. The bands at 3440 cm−1 and 2920 cm−1 for

235

both SKF and CMF sample corresponded to the presence of the stretching vibration of –OH and C–H,

236

respectively 40. The peak at 1060 cm−1 was considered to be the characteristic of the C–O–C stretching

237

of the polysaccharide skeleton. Compared to SKF sample, the CMF sample exhibited stronger

238

adsorption bands of COO– at wavelengths of 1610 cm-1 (asymmetric stretching vibration) and 1420

239

cm-1 (symmetric stretching vibration), which provides the evidence of more carboxyl groups introduced

240

to the CMF 41. In the study of carboxymethylated cellulose hydrogel beads, Yang et al.

241

the bands at 1422 and 1608 cm-1 could be assigned to the symmetric and asymmetric stretching

242

vibration of COO-, respectively. From the insert table in Fig. 4, the CMF sample showed much higher

243

carboxyl content (104.28 meq/100g) than SKF (4.07 meq/100g), which supported the fact that more

244

carboxyl groups were introduced on CMF after carboxymethylation.

42

Sample Carboxyl content /meq/100 g SKF 4.07 CMF 104.28

SKF CMF

C=O 4000

245 246

3500

3000

2500

2000

Wavenumber /cm

1500

1000

500

-1

Fig. 4 FTIR Spectrum of Fiber before and after Carboxymethylation.


reported that

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247

3.1.4 Microscopic Imaging Analyses. Shown in Fig. 5 are the LM and SEM images of SKF and

248

CMF. As seen, the native SKF had regular structure and smooth surface (Fig. 5a), however, some

249

highly swollen balloons appeared on the fibers after the carboxymethylation (Fig. 5b).

250

Carboxymethylation proceeded in a heterogeneous manner so that different sized balloons on fibers

251

were generated, which was caused by the heterogeneous damage of fiber intrinsic structure resulting in

252

different levels of swelling in aqueous system. The obtained hydrophilic balloons remarkably improved

253

the water adsorption capacity of CMF as water pockets. It is well known that the wood cell wall can be

254

divided into the middle lamella (ML), the primary wall (P), and the secondary wall (S)

255

with high lignin content, had been completely disintegrated in the pulping process, thereby, the primary

256

wall turned into the outer layer of the SKF in reserve. As shown in the magnified SEM images in Fig.

257

5c, the SKF had regular fibers with nearly intact primary wall on the fiber surface. In the process of

258

carboxymethylation, the solvent penetrated into the fiber through the permeable primary wall resulting

259

in the dissolution of S2 wall by fragmentation. According to the study of Le Moigne et al. 45, the S2

260

wall was the easiest to dissolve, compared to the external walls which contain a larger amount of

261

non-cellulosic components. As the inner structure of fiber was gradually filled with solvent, the S1 wall

262

swelled transversely and subsequently the primary wall broke (as seen in Fig.5d) in one or more spots

263

under the swelling pressure, then rolled up forming threads (the thin lines laid down along the balloon

264

surface) and collars (rearrangement of the rolled primary wall around the fiber diameter)

265

several different sized balloons were formed. The surface of the unswollen sections of CMF (actually a

266

region between two balloons) demonstrated periodic vertical wrinkles (as indicated in the magnified

267

SEM images in Fig. 5d) indicating that the secondary layer of the fiber cell wall was exposed and a

268

fraction of wood cell walls of fibers were even broken into pieces (Fig. 5d) under the maximum

269

swelling stress. The swelling process of wood fibers was thoroughly investigated by Le Moigne et al. 46,

270

and they mentioned that the S2 wall could be fully dissolved and hold by the S1 wall (as the membrane

271

of the balloons), while the primary wall would form threads and collars surrounding the balloons.


43, 44

. The ML,

46

. Finally,


272

273 274

Fig. 5 LM and SEM Images of SKF and CMF: (a) Optical Micrographs of SKF; (b) Optical

275

Micrographs of CMF; (c) SEM Images of SKF; (d) SEM Images of CMF.

276

3.2 Removal of Copper Ions by CMF-based Filter Material by Adsorption

277

3.2.1 Adsorption Kinetics. The kinetic parameters and coefficient of determination (R2) of the first

278

and second adsorption for pseudo-second-order kinetic model are calculated and listed in Table 3. The

279

pseudo-first-order model fitted the experimental data with a R2 value of 0.9999 and the equilibrium

280

adsorption capacity obtained by fitting curves was very close to the actual adsorption capacity. As seen

281

in Fig. 6, the adsorption of copper ions onto CMF-based filter material was a highly efficient process,

282

which reached the adsorption equilibrium in 5 min. The results of the first and second adsorption

283

process showed that the equilibrium adsorption capacity of CMF decreased from 64.15 mg/g to 53.22

284

mg/g. Nonetheless, the adsorption rate was nearly constant. Therefore, the fast adsorption rate of the

285

CMF-based filter material for copper ions revealed its advantage when treating large amounts of copper

286

ions in a short contact time in a continuous flow system.

287

Table 3 Pseudo-second-order Kinetic Parameters for the Adsorption of CMF-based Filter Material for

288

Copper Ions.


Page 12 of 21

Page 13 of 21

Parameter

k

qe

q

g/mg/min

mg/g

mg/g

The first adsorption

0.0426

64.23

64.15

0.9999

The second adsorption

0.0527

53.28

53.22

0.9999

r² Type

70 60 50

1.8 1.6

40

1.4 1.2

30

1.0

t /q

q t/mg/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6

20

0.4

The first adsorption The second adsorption

0.2

10

0.0 0

0

10

20

30

40

50

60

70

80

90 100

t /min

0

10

20

30

40

50

60

70

80

90 100

t/min

289 290

Fig. 6 Adsorption Kinetics and Pseudo-second-order Curves of the CMF-based Filter Material in the

291

First and Second Adsorption for Copper Ions.

292

3.2.2 Adsorption isotherms. The equilibrium isotherms for the adsorption of copper ions by

293

CMF-based filter material at the temperature of 30 ºC in pH 6 are shown in Fig. 7. It can be noticed

294

that the adsorption capacity was increased with increasing equilibrium concentration and eventually

295

reached a saturated value, showing a concave curve tended to be a plateau. The equilibrium data were

296

fitted by Langmuir and Freundlich isotherm equations, respectively. The values of qm and KL were

297

determined by the slope and intercept of the linear plots of Ce /qe versus Ce (Fig. 8a) and the values of

298

KF and n were determined by the slope and intercept of the linear plot of lnqe versus lnCe (Fig. 8b). The

299

correlative isotherm parameters were summarized in Table 4. It can be seen that the Langmuir isotherm

300

shows a better fit to experimental data in comparison to the Freundlich isotherm due to the much higher

301

values of the coefficient of determination (R2) of the Langmuir isotherm, which indicated that the

302

adsorbed copper ions formed a monolayer coverage on the adsorbent surface47, 48. In other words, the

303

adsorption of copper ions accumulated on the adsorbent should have taken place at binding sites (or



304

functional groups) on the CMF surface which should be accounted for monolayer sorption.

305

Table 4 Langmuir and Freundlich Model Parameters for Copper Ions Adsorption by the CMF-based

306

Filter Material. Langmuir model

Adsorbent

Freundlich model

qm (mg/g)

KL (L/mg)

R2

KF(L/mg)

n

R2

68.03

0.0720

0.993

1.1548

3.344

0.909

CMF

65 60 55

qe/mg/g

50 45 40 35 30 25

0

20

40

307

60

80

100

Ce/mg/L

308

Fig. 7 Equilibrium Isotherms for Copper Ions Adsorption by CMF-based Filter Material at 30 ºC and

309

Initial pH 6. 1.8

4.2

(a)

1.6

(b) 4.0

1.4 1.2

3.8

1.0

lnqe

C e/qe/(mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

0.8

3.6

0.6 3.4

0.4 0.2 0

20

40

310

60

80

100

3.2 2.0

2.5

3.0

Ce/(mg/L)

3.5

4.0

4.5

5.0

lnCe

311

Fig. 8 The Langmuir (a) and Freundlich (b) isotherm plots for Copper Ions adsorption by the

312

CMF-based Filter Material.

313

3.2.3 Recycling of CMF. Shown in Fig. 9 are the results of adsorption capacity of CMF filter

314

material in different regeneration times. It shows the adsorption capacity of fresh CMF filter material

315

was 64.15 mg/g, whereas its adsorption capacity decreased to 53.22 mg/g after the first-time

316

regeneration. The decreased adsorption capacity after regeneration was closely interrelated to the

317

treatment of filter material by hydrochloric acid (HCl). Before the acid treatment, the carboxyl groups

318

on the surface of fibers were in the form of carboxylate due to the alkaline condition in the CMF

319

preparation. The adsorption of heavy metal ions occurred as shown below:


Page 15 of 21

320

2RCOONa + M2+ → (RCOO)2M + 2Na+

321

When the copper ions-loading CMF filter material was treated with HCl, the linkage between CMF

322

and copper ions was cut off, and the adsorption sites on the surface of CMF were protonated, leaving

323

the copper ions in the acid aqueous phase rather than being adsorbed onto the fiber

324

first-time regeneration, the CMF, with reduced accessibility, was preserved in the protonated state even

325

though being washed by distilled water, thus its adsorption capacity would decrease. According to the

326

report from Kumar et al. 49, HCl treated rice husk showed lower adsorption capacity for cadmium than

327

the untreated counterpart. Low et al.

328

sorption capacity than the HCl-treated one.

50

(8)

35

. After the

reported that the NaOH-treated spent grain showed higher

329

The adsorption capacity of the CMF filter material for copper ions after fifth regeneration merely

330

decreased with 1.57 mg/g compared to that after the first regeneration, which displayed relatively

331

stable adsorption performance. This minor change from the second-use to the fifth-use in the

332

adsorption performance of the CMF filter material indicated that the reusability of CMF and its

333

application as efficient adsorbents for heavy metal ions removal in wastewater treatment are feasible

334

and promising. 70

64.15

60

53.22

53.15

53.02

52.88

52.65

1

2

3

4

5

50

q/mg/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30 20 10 0

0

335

Regeneration times

336

Fig. 9 The Adsorption Capacity of Regenerated CMF Filter Material.

337

3.3 Concept of Preparation of the CMF Adsorbent Material and Its Application to Copper Ions

338

Removal via Adsorption. The preparation and adsorption process of the CMF filter material is shown

339

in Fig. 10. The pristine SKF was initially alkalified with NaOH to swell the fibers. Subsequently, the

340

alkali-treated fibers were etherified with MCA to introduce carboxymethyl groups onto the fibers. On

341

one hand, CMF, with hydrophilic carboxymethyl groups on fibers, could remarkably improve the

342

hydrophilicity of the fibers 51 and facilitate the complexation between fibers and copper ions during the



343

follow-up wastewater treatment. The complexation induced by the large number of carboxyl groups

344

from carboxymethyl groups can rapidly result in stable coordination bonds between carboxyl groups

345

and copper ions 52-54, thus, the carboxyl-rich CMF is a good candidate for copper ions removal. On the

346

other hand, CMF, maintaining good fiber morphology, is readily made into a filter material. The length

347

of fibers was negligibly shortened because the mild carboxymethylation, while the transverse

348

heterogeneous swelling of CMF occurring in water led to balloon-like structure on fibers 55. Moreover,

349

in comparison to the ordinary fibers in water, the water-swollen CMF showed better accessibility to

350

copper ions and provided more reaction sites for complexation, which also contributes to the improved

351

adsorption capacity of CMF. Consequently, CMF filter material could be a promising one for

352

wastewater treatment due to its high carboxyl content as well as its good water-swollen structure.

353 354

Fig. 10 Schematic Illustration for the Preparation of the CMF Filter Material and Its Application to

355

Copper Ions Removal via the Adsorption Process.

356

4. CONCLUSIONS

357

In this work, carboxymethylated cellulose fiber (CMF), with high carboxyl content and reasonable

358

fiber morphology, was prepared and used to fabricate an adsorbent filter material. The characteristics of

359

the as-prepared CMF and the adsorption performance of the as-prepared CMF filter material were

360

investigated. Results showed that the optimized CMF sample (carboxymethylation at 10 g MCA) had

361

much higher carboxyl content and reasonable fiber morphology (larger fiber width and similar fiber


Page 16 of 21

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


362

length) compared to the original fibers. The CMF-based adsorbent filter material exhibited excellent

363

adsorption capacity for copper ions and good recycling performance. The adsorption equilibrium was

364

reached within 5 min due to its high adsorption rate.

365

■ AUTHOR INFORMATION

366

Corresponding Author

367

*E-mail: [email protected]

368 369

Notes

370

■ ACKNOWLEDGMENTS

371

This work was financially supported by the Science and Technology Department of Shaanxi Province

372

(2017GY-184) and the Department of Education of Shaanxi Province (No. 15JS015)

373

■ REFERENCES

374 375

The authors declare no competing financial interest.

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