Synthesis of novel chitosan-based flocculants with amphiphilic structure and its application in sludge dewatering: Role of hydrophobic groups

Journal Pre-proof Synthesis of novel chitosan-based flocculants with amphiphilic structure and its application in sludge dewatering: Role of hydrophobic groups Yongzhi Liu, Huaili Zheng, Yongjun Sun, Jie Ren, Xinyu Zheng, Qiang Sun, Shaojie Jiang, Wei Ding PII:

S0959-6526(19)34220-9

DOI:

https://doi.org/10.1016/j.jclepro.2019.119350

Reference:

JCLP 119350

To appear in:

Journal of Cleaner Production

Received Date: 3 May 2019 Revised Date:

14 November 2019

Accepted Date: 16 November 2019

Please cite this article as: Liu Y, Zheng H, Sun Y, Ren J, Zheng X, Sun Q, Jiang S, Ding W, Synthesis of novel chitosan-based flocculants with amphiphilic structure and its application in sludge dewatering: Role of hydrophobic groups, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/ j.jclepro.2019.119350. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

Graphical abstract

1

Synthesis of novel chitosan-based flocculants with amphiphilic structure

2

and its application in sludge dewatering: Role of hydrophobic groups a,b

a,b

*, Yongjun Sun c,d, Jie Ren a,b, Xinyu Zheng a,b, Qiang Sun a,b,

3

Yongzhi Liu

4

Shaojie Jiang a,b, Wei Ding a,b

5

a

6

Chongqing 400044, China

7

b

8

Education, Chongqing University, Chongqing 400045, PR China

9

c

College of Urban Construction, Nanjing Tech University, Nanjing, 211800, China

10

d

Jiangsu Key Laboratory of Industrial Water-Conservation & Emission Reduction, College of

11

Environment, Nanjing Tech University, Nanjing, 211800, China

, Huaili Zheng

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University,

Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, Ministry of

12

*Corresponding author: Huaili Zheng, Tel &Fax: +86 023 65120827, Email: [email protected]

13

Abstract

14

Natural polymeric flocculants as sludge conditioners have received great attention due to

15

their non-toxic and biodegradable advantages. However, the application of amphiphilic

16

natural flocculants as sludge conditioners is rarely reported. In this work, the novel

17

chitosan-based flocculants (CS-g-PAO) with amphiphilic structure were synthesized by

18

ultraviolet light initiated method for the enhancement of sludge dewatering performance. The

19

solubility of CS-g-PAO was significantly improved in a wide pH range (2.0-10.0). Various

20

characterization methods were used to investigate the structure, thermal stability and surface

21

morphology of copolymers, suggesting the successful synthesis of CS-g-PAO. In dewatering

22

experiments, CS-g-PAO showed superior dewatering performance, and filter cake moisture

23

content could be decreased to 77.98% from 95.14%. Moreover, the results of extracellular

24

polymeric substances (EPS) components indicated that hydrophobic effect of CS-g-PAO was

25

beneficial to remove the protein in S-EPS and TB-EPS, which had the crucial influence on 1

26

dewatering performance. Furthermore, the dewatering mechanism was discussed based on the

27

effects of zeta potential, apparent viscosity, surface tension and flocs structure on

28

dewaterability. Results revealed that CS-g-PAO had strong hydrophobic association effect and

29

surface activity. In addition, enhanced charge neutralization and hydrophobic interaction

30

played a synergetic role in improving the dewaterability of CS-g-PAO.

31

Key words: Sludge dewatering; Chitosan-based amphiphilic flocculants; Hydrophobic effect;

32

Dewaterability; Dewatering mechanism

33

1 Introduction

34

As one of the by-products of sewage treatment, the output of sewage sludge increases

35

gradually with the development of urban and economy. Sludge has a high water content

36

(>95%) and contains complex extracellular polymeric substances (EPS), which greatly

37

increases the sludge volume and the difficulty of sludge dewatering (Anjum. et al., 2016).

38

Due to the compressible characteristic of sludge, it is easy to cause the close of cake voids

39

and the poor sludge dewaterability under the high pressure of mechanical dewatering (Wang.

40

et al., 2019). Therefore, the pretreatment processes prior to mechanical dewatering, such as

41

microwave, ultrasonic, heat and flocculation, are commonly used to enhance the sludge

42

dewaterability (Liu. et al., 2019; Niu. et al., 2013; Xu. et al., 2019; Zhang. et al., 2015).

43

Among these methods, flocculation is widely used due to its simple operation and efficient

44

dewatering performance (Wei. et al., 2018). During dewatering process, flocculants can

45

interact with sludge colloidal particles by charge neutralization and bridging effect to achieve

46

the destruction of the stable sludge system and the compression of sludge EPS components,

47

and then improve sludge dewaterability. In addition, the concentration of flocculant not only

48

affects the sludge conditioning effect, but also affects the consumption of flocculant. Among

49

various flocculants, organic flocculants are widely used in sludge dewatering due to its low

50

dose and a wide pH range. Generally, the concentration of the organic flocculant is suitably

51

between 0.1 wt. % and 0.3 wt. %. However, study found the addition of organic flocculants

52

easily caused the sticky gel phenomenon, making it difficult to further improve the sludge 2

53

dewaterability (Lv. et al., 2014). In order to develop the enhanced organic flocculants for

54

sludge dewatering, many methods including the hydrophobic modification of organic

55

flocculants have been investigated (Chen. et al., 2018).

56

Ejtemaei. et al. (2019) found that the cationic surfactant (CTAB) as dewatering aid

57

significantly improved the sludge filtration rate by the hydrophobic attraction and charge

58

neutralization effects. Based on the Laplace equation, the hydrophobic property of surfactant

59

reduced the capillary pressure of water in sludge pores and improved the dewaterability

60

(Vaziri Hassas. et al., 2014). Generally, surfactants tend to produce large amounts of foam in

61

using process, which is detrimental to the water environment (Peng. et al., 2019). Besides, in

62

order to obtain superior dewatering performance, surfactant needs be used in combination

63

with traditional flocculants during dewatering process. These problems can be solved by

64

introducing the surface active monomer that has amphiphilic functional group and

65

polymerizable double bond. The amphiphilic flocculants can be prepared by the

66

copolymerization reaction of surface active monomers with acrylamide monomers. In our

67

previous reports, the amphiphilic organic flocculants showed significant improvement of the

68

dewatering performance compared to traditional organic flocculants (Liao. et al., 2014).The

69

amphiphilic flocculant can change the molecular conformation of the polymer in solution and

70

increase the possibility of polymer chains interacting with polluted particles (Zhao. et al.,

71

2018). In addition, the hydrophobic property of flocculant reduces the hydrophilicity of flocs,

72

which makes flocs easier to separate from water and settle. However, it cannot be neglected

73

that there are many environmental problems for organic flocculants because some derivatives

74

and intermediates are hazardous and non-biodegradable to water environment and human

75

health.

76

Natural polymeric flocculants have attracted increasing attention in the application of

77

sludge dewatering due to their advantages of environmentally friendly, nontoxic and

78

biodegradable

79

N-acetyl-D-glucosamine and D-glucosamine, has abundant hydroxyl and amino groups.

80

Under acid conditions, these amino groups can be protonated, which facilitates the removal of

(Filho.

et

al.,

2018).

Chitosan

3

(CS),

a

natural

copolymer

of

81

anionic contaminants from water (Sun. et al., 2019a; Tang. et al., 2020). Moreover, many

82

chitosan derivatives can be prepared by a series of chemical reactions on the hydroxyl and

83

amino of CS chain. Due to the unique structure, biocompatibility and availability, CS has

84

been used for various fields including sludge dewatering (Shi. et al., 2019; Sun. et al., 2019b;

85

Zeng. et al., 2008). However, CS has poor solubility at pH > 6.5 conditions and low positive

86

charge density, resulting in poor sludge dewatering performance, which hinders its application

87

in sludge dewatering (Yang. et al., 2016). In order to improve CS properties, many

88

modification methods have been studied. Grafting copolymerization is a simple and effective

89

method, in which the amino groups of chitosan react with vinyl monomers under mild

90

conditions (Azmeera. et al., 2017). And, grafting copolymerization as one of chemical

91

modification methods was extensively used to realize the combination of the advantages of

92

natural flocculants and organic flocculants (Agbovi and Wilson, 2018). Yang. et al. (2014)

93

successfully synthesized the CS-based flocculants by grafting copolymerization, and the

94

flocculation experiments indicated the graft flocculants showed excellent flocculation

95

performance on the removal of turbidity and humic acid. Although CS-based graft flocculants

96

have been widely used for the removal of turbidity, dye, heavy metal and other pollutants,

97

there is no research on the flocculants of chitosan based amphiphilic flocculants as sludge

98

conditioner. In this work, a small amount of acrylamide (AM) was grafted onto chitosan

99

backbone to increase the molecular weight of flocculants. Acryloyloxyethyl dimethylbenzyl

100

ammonium chloride (AO) was grafted on chitosan backbone as the multifunctional monomer,

101

which not only significantly enhanced cationic degree of flocculants, but also could introduce

102

the hydrophilic functional groups (quaternary ammonium) and hydrophobic functional groups

103

(benzyl) on molecular chains.

104

In conclusion, the objective of this work is to synthesize a series of CS-based amphiphilic

105

flocculant (CS-g-PAO) with enhanced sludge dewatering function. Sludge dewatering

106

efficiency of flocculants was investigated by taking specific resistance to filtration (SRF),

107

filter cake moisture content (FCMC), capillary suction time (CST), compressibility and flocs

108

surface properties as evaluation indexes. Furthermore, the change of EPS components during 4

109

dewatering was analyzed and the dewatering mechanism was revealed.

110

2 Materials and methods

111

2.1 Chemical materials

112

Chitosan (CS, 95% deacetylated, viscosity 100-200 mpa.s) was purchased from Shanghai

113

Macklin Biochemical Co., Ltd. Acrylamide (AM, AR grade) and polyacrylamide (PAM, AR,

114

molecular weight 300 thousands) were supplied by Chengdu Chron Chemical CO., Ltd.

115

Acryloyloxyethyl dimethylbenzyl ammonium chloride (AO, 80 %) was obtained from Wuhan

116

Yihuacheng Technology CO., Ltd. The initiator 2,2′-azobis[2-(2-imidazolin-2-yl)propane]

117

dihydrochloride (VA-044, AR) and the commercial poly(acrylamide- acryloxyethyltrimethyl

118

ammonium chloride) (CPAD) were purchased from Chongqing Lanjie Material CO., Ltd.

119

Albumin from bovine serum (BR, 96%), brilliant Blue G (AR), glucose anhydrase (AR) and

120

anthrone (AR) were purchased from Shanghai Aladdin reagent Co. LTD, China. Other

121

chemicals including sulfuric acid (98%), acetic acid (98%) and ethyl alcohol (95%),

122

phosphoric acid (85%), HCl, NaOH, and NaCl (AR) were supplied from Chongqing

123

Chuandong Chemical CO., Ltd. All solutions were prepared with distilled water in the

124

experiment.

125

2.2 Preparation of CS-g-PAO flocculants

126

3.0 wt. % CS solution was obtained by CS fully dissolved in 1.0 wt. % acetic acid solution

127

with vigorous stirring. The total monomer molality was determined as 2.0 mmol/g. A

128

predetermined molar ratio of AO and CS (nAO :nCS ) was added to the reaction vessel of Pyrex

129

glass. The detailed quantities of AM, AO and CS are as follows: nAO :nCS =1:2 (AM: 5.0 g,

130

AO: 1.0 mL, CS:1.0 g), nAO :nCS =1:1 (AM: 4.9 g, AO: 1.7 mL, CS: 0.9 g), nAO :nCS =2:1

131

(AM: 4.5 g, AO: 3.6 mL, CS: 0.9 g), nAO :nCS =3:1 (AM: 4.7 g, AO: 3.3 mL, CS: 0.6 g). The

132

certain volume water was added to the reaction vessel, followed by fully mixing the solution.

133

Then, the oxygen in the reaction vessel was eliminated by blowing N2 (99.0%) at ambient 5

134

temperature for 10 min. After that, the predetermined amount of VA-044 was added to the

135

mixed solution. Finally, the reaction vessel was sealed and placed on the rotated platform so

136

that the reaction vessel can be evenly illuminated by UV light (500 W, light intensity

137

11.7×102 µw/cm2) until the desired reaction time (Fig. S1). The obtained gel product after

138

polymerization reaction was purified using excess acetone and ethanol absolute. Soxhlet

139

extraction apparatus (the 1:1 formamide-acetic acid mixture) and ethanol absolute were used

140

to remove the copolymer of organic monomer and homopolymer (PAM and PAO), then the

141

pure graft copolymer was obtained (Liu. et al., 2018). The final products were dried in

142

vacuum oven at 60 ± 5 °C.

143

2.3 Characterization of CS-g-PAO flocculants

144

The Fourier transform infrared (FTIR) spectra were obtained using KBr pellets on Nicolet

145

iS50 infrared spectrometer (Thermo Fisher Scientific Company, China). The wave numbers

146

were 500-4000 cm-1. The nuclear magnetic resonance (1H NMR) spectra were acquired by

147

Advance-500 NMR spectrometer (Bruker Company, Germany) with tetramethylsilane as an

148

internal standard and deuteroxide (D2O) as solvent. The X-ray diffraction (XRD) spectra were

149

obtained by X-ray diffractometer PANalytical X’Pert Powder (Spectris Pte. Ltd, Netherlands).

150

The Raman spectra were acquired by LabRAM HR Evolution spectrometer at 532 nm

151

excitation

152

thermogravimetric/differential scanning calorimetry (TG/DSC) measurement were conducted

153

on a TGA/DSC1/1600LF instrument (Mettler, Switzerland) under nitrogen atmosphere from

154

20 to 600 °C. The heating rate was 10 °C·min-1. The scanning electron microscopy (SEM)

155

images were acquired by FEI Nova Nano SEM (FEI, America) instrument to illustrate the

156

surface morphology of materials.

157

2.4 Sludge properties

158

wavelength

(HORIBA

Jobin

Yvon

S.A.S,

France).

The

Surplus sludge was obtained from the mud-storage basin in Jingkou Sewage Treatment 6

159

Plant of Chongqing, China. The influent COD and NH3-N concentration were 159-330 mg/L

160

and 30-90 mg/L. The sewage was treated by the improved oxidation ditch process and the

161

average daily sewage treatment capacity is 30,000 tons in the plant. The collected sludge was

162

stored in refrigerator of 4 °C and used within 5 days. The initial sludge parameters are listed

163

in Table 1.

164

Table 1. The characteristics of raw sludge.

165

Parameters

Value

Moisture content (%)

98.80 ± 0.20

TSS (g/L)

8.97 ± 0.55

VSS (g/L)

6.19 ± 0.13

COD (mg/L)

73.45 ± 1.50

pH

6.88 ± 0.10

Zeta potential (mV)

-18.50 ± 0.35

CST (s)

95.1 ± 0.5

2.5 Dewatering test

166

The detailed properties of flocculants used in dewatering test were listed in Table 2.

167

Predetermined amount of flocculant was added to the 200 mL sludge sample. The dewatering

168

tests were performed on the paddle apparatus (TA6-1, Wuhan Hengling Technology Co., Ltd,

169

China). Stirring procedure was programmed to rapidly mix with 200 r/min for 50 s, followed

170

by slowly stirring 50 r/min for 2 min. Finally, the sludge after flocculation was settled for 30

171

min. The zeta potential of the supernatant was measured by a ZETA potential analyzer (Nano

172

ZS90, Malvern Instruments Ltd., UK). Apparent viscosity was measured using a DV-2+PRO

173

rotated viscometer (Shanghai Nirui Intelligent Technology Co., Ltd, China) at 25 °C. Surface

174

tension was measured by DuNouy Ring method using automatic surface & interface

175

tensiometer model A101/A101Plus (USA KINO Industry Co., Ltd) at 25 °C. SRF, FCMC,

176

CST, compressibility and EPS components were determined according to the methods

177

described in the following subsections. 7

178 179 180 181 182

Table 2. The properties of flocculants used in sludge dewatering test. Types of

Molar ratio of

Intrinsic

Grafting

Cationic degree

flocculants

AO:CS

viscosity a

efficiency b

(mmol/g)

(mL/g)

(%)

CS-g-PAM

-

452. 1638

92.8

-

CPADc

-

410.8610

-

1.6907

CS-g-PAO#1

1:2

358.4920

80.3

0.5611

CS-g-PAO#2

1:1

437.9189

89.0

1.2532

CS-g-PAO#3

2:1

409.4794

92.0

1.5086

CS-g-PAO#4

3:1

389.4015

94.5

1.6934

183

a

184

b

185

c

186

2.5.1 Determination of dewatering performance

187

Intrinsic viscosity (η) was caculated according to Mark-Houwink equation. The detail of grafting efficiency measurment was shown in Text S3.

CPAD was the traditional flocculant used in sludge dewatering.

The measurement methods of SRF, FCMC, compressibility and CST are described in Text

188

S1. In addition, all dewatering experiments were repeated three times.

189

2.5.2 EPS extraction and analysis

190

2.5.2.1 EPS extraction

191

The slightly modified heat extraction method was used to extract S-EPS, LB-EPS and

192

TB-EPS. Firstly, the 50 mL sludge suspension was centrifuged at 4000 rpm for 15 min, and

193

the obtained supernatant was S-EPS. Secondly, the sludge pellet after above centrifugation 8

194

was suspended in 50 ml of 0.05% NaCl solution, sonicated at 40 KHz for 10 min and then

195

centrifuged at 8000 rpm for 15 min. The acquired supernatant was LB-EPS. Finally, the

196

sludge pellet was resuspended in 50 ml of 0.05% NaCl solution, sonicated at 40 KHz for 10

197

min, heated at 80 °C for 1 h and then centrifuged at 4000 rpm for 15 min. The obtained

198

supernatant was TB-EPS.

199

2.5.2.2 EPS analysis

200

All EPS would be filtered through a 0.45 µm hydrophobic membrane prior to subsequent

201

protein (PN), polysaccharide (PS) and humic acid (HA) testing. The analytic methods

202

(colorimetric methods) and the calibration curves of PN, PS and HA are described in Text S2

203

and Fig. S2-S4.

204

The excitation emission matrix (EEM) fluorescence spectroscopy was used to analyze the

205

organic matters with fluorescence characteristics (F-7000, 5J1-0004, Hitachi Limited, Japan).

206

Prior to test, the sludge supernatant was filtered through a 0.45 µm acetate fiber

207

microfiltration membrane. The main parameters in the test of EEM were: excitation

208

wavelength (EX) range of 200-500 nm, scanning interval of 4 nm; emission wavelength (Em)

209

range of 250-600 nm, scanning interval of 3 nm; scanning speed of 6000 nm/min; voltage of

210

700 V.

211

2.5.3 Characterization and size distribution of sludge flocs

212

Sludge flocs were freeze-dried using vacuum freezing drying oven at -70 °C for 48 h

213

(FD-1A-50, Shanghai Yuming Instrument Co., Ltd, China). SEM (FEI Nova Nano SEM,

214

America) were used for the characterization of flocs. The size distribution of sludge flocs was

215

measured using a laser particle size analyzer (Mastersizer, APA2000; Malvern, UK). The

216

procedure of floc formation, fragmentation and regrowth were: a rapid agitation of 200 r/min

217

for 60 s and a slow stir at 30 r/min for 5 min, followed by a fragmentation stage at 450 r/min

218

for 2 min and a regrowth stage at 30 r/min for 8 min. 9

219

3 Result and discussion

220

3.1 Solubility property of flocculants

221

The water-solubility of flocculants is an important parameter for evaluating whether

222

flocculants can fully extend in water and interact with pollutant particles. Due to the

223

intermolecular and intramolecular hydrogen bonding, CS has poor water-solubility, especially

224

in neutral and alkaline environments (Gamage and Shahidi, 2007). The hydrophobic

225

modification of flocculants may affect its water-solubility and application. Therefore, as

226

shown in Table 3, the solubility property of CS and CS-g-PAO#1-4 under different pH

227

conditions was investigated. Obviously, the water solubility of CS-g-PAO#1-4 were greatly

228

improved compared to CS, especially in the alkaline environments (pH=6.0-10.0). The

229

phenomenon could be attributed to the introduction of quaternary ammonium groups on the

230

CS backbone, weakening the hydrogen bonding of CS. Moreover, with the increase of the

231

molar ratio (nAO: nCS), there was no significant difference in the water solubility of

232

CS-g-PAO#1-4. The result suggested that the effect of quaternary ammonium salt groups on

233

the solubility of CS-g-PAO was greater than that of the hydrophobic group (benzyl groups).

234

Therefore, in this study, the hydrophobic modification of flocculants by introducing the

235

multifunctional monomer (AO) did not deteriorate its water-solubility. The satisfactory

236

water-solubility property of CS-g-PAO was helpful to improve flocculation/dewatering

237

performance in practical application.

238

Table 3. Solubility property of flocculants under different pH conditions a. pH

2.0

4.0

6.0

7.0

8.0

10.0

CS

+

±

-

-

-

-

CS-g-PAO#1

+

+

+

+

+

+

CS-g-PAO#2

+

+

+

+

+

+

CS-g-PAO#3

+

+

+

+

+

±

CS-g-PAO#4

+

+

+

+

±

±

10

239

a

240

concentration of the flocculants were 2 mg/mL. And, the solution pH was adjusted to 2.0-10.0 using HCl (0.1

241

mol/L) and NaOH (0.1 mol/L)).

242

3.2 Characterization

243

3.2.1 FTIR, 1H NMR and XRD spectra

+: soluble; ±: partially soluble; -: insoluble (The products were dissolved in deionized water for

and the

244

Fig. 1a shows the mechanism of graft copolymerization. Fig. 1b-c show the FTIR, 1H NMR

245

and XRD spectra of CS, PAO and CS-g-PAO, respectively. As shown in Fig. 1b, the

246

characteristic peaks at 895, 1076, 1158, 1397, 2878 and 3442 cm-1 in the spectrum of CS were

247

assigned to the stretching vibration of the 6-membered ring, C-OH, C-O-C, C-H, -CH2 groups

248

of chitosan, respectively (Razmi. et al., 2019). The above adsorption peaks were also

249

observed on the spectra of CS-g-PAO. The characteristic peaks appearing at 1604, 1647 cm-1

250

were ascribed to the primary amine vibration in CS, but the peaks disappeared on the spectra

251

of CS-g-PAO, suggesting the grafting reaction probably occurred on primary amine groups

252

(Fig. 1a). Moreover, the new characteristic peaks appearing at 775 and 1653 cm-1 in the

253

spectra of CS-g-PAO were assigned to the vibration of benzene skeleton and carbonyl (-C=O)

254

groups of AO (Liao et al., 2014).

255

In Fig. 1c, the peaks at δ=3.26 and δ=1.92-2.22 ppm in the 1H NMR spectra of CS-g-PAO

256

were the proton signals of H1-H6 from CS. The peaks at 1.61, 2.18, 3.62, 3.49, 3.08 4.57 and

257

7.54 ppm, were ascribed to the proton signals of Ha, Hb, Hc, Hd, He, Hf and Hg in AM and AO,

258

respectively. Moreover, the 1H NMR spectra of CS-g-PAO were similar to that of PAO (Liao

259

et al., 2014). And, the intensity of characteristic signals derived from PAO increased with the

260

increase of nAO: nCS (except CS-g-PAO#3). The results indicated that as the monomer ratio

261

increased, the amount of organic polymer grafted increased gradually, which was consistent

262

with the results of cationic degree and grafting efficiency of CS-g-PAO (Table 1). The

263

phenomenon of inconsistency for CS-g-PAO#3 probably was due to the relatively high

264

intrinsic viscosity of CS-g-PAO#3, which caused its inadequate dissolution in D2O solvent, 11

265

and thus the intensity of characteristic signals became weak. In conclusion, the results of

266

FTIR and 1H NMR spectra indicated the successfully grafting of AM and AO onto CS

267

backbone.

268

Furthermore, the evidences of the structure changes in products are observed in the XRD

269

spectra (Fig. 1d). The intense diffraction peak at 2θ=20.15° was attributed to the crystal form

270

II of CS (Dong. et al., 2014). The XRD pattern of PAO showed a higher but wider diffraction

271

peak at 2θ=19.94-22.17° than the characteristic peak of CS, indicating that the crystallinity of

272

CS was obviously higher than that of PAO (Sun. et al., 2017). After graft copolymerization,

273

the diffraction peaks of CS-g-PAO became even broader and weaker due to the deterioration

274

of original ordered structure. In addition, with the increase in feeding the molar ratio of AO

275

and CS, the diffraction peak became gradually stronger and wider, which was consistent with

276

the result of 1H NMR spectra.

277

278 279 280 281

Fig. 1. The graft copolymerization reaction scheme (a), FTIR (b), 1H NMR (c) and XRD (d) spectra.

12

282

3.2.2 Raman spectra results

283

Raman spectra of CS, PAO and CS-g-PAO#4 are shown in Fig. 2. In the Raman spectrum

284

of CS, the bands at 897 and 1024 cm-1 were assigned as the ring breathing of CS. Meanwhile,

285

the characteristic bands at 1316, 1670, 1949, 2514, 2869 and 3318 cm-1 were attributed to the

286

vibrations of δ(C-N), ν(C-O), ν(C-H), ν(-CH2), δ(C-H), and ν(-NH2) in CS, respectively

287

(Zając. et al., 2015). In the Raman spectrum of PAO, these characteristic bands at 1004, 1448,

288

1609, 2932 and 3059 cm-1 were ascribed to the stretching vibration of -N+, -CH2, -C=O, C-H

289

and benzyl group in PAO, respectively (Deval. et al., 2014). Compared with the Raman

290

spectra of CS and PAO, the main characteristic peaks of CS and PAO were all found in the

291

Raman spectrum of CS-g-PAO#4, and these peaks were labeled in Fig. 2. Based on the

292

analysis of Raman spectra, results further confirmed the successful fabrication of CS-g-PAO.

293 294

Fig. 2. Raman spectra of precursor component and synthesized copolymer: a) CS, b) PAO, c)

295

CS-g-PAO#4.

296

3.2.3 TG/DSC analysis

297

In order to investigate the thermal stability of copolymer, the thermal analysis results of CS,

298

PAO and CS-g-PAO are indicated in Fig. S5. And, the thermal stability of polymer is

299

described in Text S4.

13

300

3.2.4 SEM and EDS mapping analyses of CS, PAO and CS-g-PAO

301

As shown in Fig. 3, the SEM and EDS with elemental mapping images are used to

302

investigate the surface morphology and surface elements of CS, PAO and CS-g-PAO#4. The

303

elements including C, N, O and Cl on the surface of the three samples were measured. In Fig.

304

3a, CS demonstrated a comparatively smooth and layered surface structure. In Fig. 3b, PAO

305

demonstrated an irregular and folds surface structure. After the grafting copolymerization,

306

CS-g-PAO showed a porous and uneven structure, and a relatively large surface area as shown

307

in Fig. 3c. The phenomenon indicated that the surface morphology of CS was obviously

308

changed and destroyed due to the introduction of PAO. The fractal dimensions (Fig. S6) of CS,

309

PAO and CS-g-PAO were the slopes of corresponding fitting straight-line, which were 1.505,

310

1.666 and 2.207, respectively. The result indicated that CS-g-PAO had a larger surface area

311

than CS and PAO. Moreover, it could be found from EDS mapping images that the elements

312

of C, N and O existed in the three samples, whereas the element of Cl only was observed in

313

the EDS mapping images of PAO and CS-g-PAO. In addition, the elemental weight percent of

314

N in CS-g-PAO (19.91 wt%) was higher than that of CS (12.98 wt%), indicating the

315

successful introduction of PAO on CS backbone.

316

14

317 318

(a)

319

320 321

(b)

15

322

323 324

(c)

325

Fig. 3. SEM images and EDS with elemental mapping images of CS (a); PAO (b); CS- g-PAO

326

(c).

327

3.3 Sludge dewatering test

328

3.3.1 Effect of CS-g-PAO category on sludge dewaterability

329

Generally, the internal structure of flocculants has great influence on sludge dewatering

330

performance. The grafting efficiency (GE) represents the amount of organic monomers

331

grafting onto the backbone of natural polymer, which greatly affects the amount of functional

332

groups. Therefore, the effects of CS-g-PAO#1-4 with different grafting efficiency (Table 2) on

333

dewaterability were investigated. As shown in Fig. 4a, the SRF and FCMC after conditioning

334

with CS-g-PAO#1-4 initially decreased with the increase of dosage, then slowly increased to a

335

stable value after reaching the optimal dosage. At low dosage, the linear chains of flocculants 16

336

could not effectively interact with the dispersed sludge particles resulting in poor

337

dewaterability. At overdose, the restabilizaiton effect occurred due to the charge reversal of

338

the negatively charged sludge colloid particles, resulting in the increase of electrostatic

339

repulsion and the decrease of dewaterability (Liu et al., 2018). With the increase of GE, the

340

optimal dewatering performance was gradually improved (80.98%, 80.02%, 79.52% and

341

77.98% of FCMC; 1.61×1012, 1.30×1012, 8.98×1011 and 3.83×1011 cm/g of SRF) and the

342

corresponding optimal dose was 10, 8, 6 and 6 mg/g TSS, respectively. Obviously, the

343

dewatering efficiency of CS-g-PAO#4 with highest GE was superior to CS-g-PAO#1-3, which

344

demonstrated that the dewatering performance was proportional to the GE.

345

Besides SRF and FCMC, CST were the important parameters for evaluating sludge

346

dewaterability. Fig. 4b shows that as dosage increased, CST rapidly dropped, and the CST

347

values of CS-g-PAO#1-4 reached the minimum values of 25.8, 15.5, 12.0, 4.9 s, respectively,

348

when the optimal doses were 10, 8, 6, 6 mg/g TSS, respectively. Jonathan. et al. (2016)

349

reported that sludge exhibited a good dewaterability when CST value was lower than 20 s.

350

Based on above results, the optimal CST values of CS-g-PAO#1-4 were all less than 20 s,

351

which demonstrated that CS-g-PAO#1-4 had the effective sludge dewatering performance. In

352

addition, CS-g-PAO#4 showed the lowest CST value. The phenomenon further confirmed that

353

the excellent dewatering performance of CS-g-PAO#4, which was in line with the results of

354

SRF and FCMC. Due to the introduction of the multifunctional monomer AO, the cationic

355

degree (CD) and hydrophobic groups of CS-g-PAO would increase with the increase of GE.

356

In addition, the difference of bridging effect among CS-g-PAO#1-4 could be neglected

357

because of the similar intrinsic viscosity of CS-g-PAO#1-4. Therefore, it could be assumed

358

that charge neutralization and hydrophobic effects played an important role in the dewatering

359

process.

17

360 361

Fig. 4. Effect of CS-g-PAO category on SRF and FCMC (a) and CST (b).

362

3.3.2 Effect of flocculant category on sludge dewaterbility

363

In order to further investigate the effects of cationic and hydrophobic groups on dewatering

364

performance, CS-g-PAM (with similar η to CS-g-PAO#4) and CPAD with hydrophilic

365

quaternary ammonium groups (with similar η and CD to CS-g-PAO#4) were chosen as the

366

contrast samples. CPAD is representative of traditional organic flocculants used in sludge

367

dewatering. In this work, CPAD was purchased from Chongqing Lanjie Material CO., Ltd.

368

Therefore, it is necessary to compare the sludge dewatering effects of CS-g-PAO and CPAD.

369

Fig. 5a shows the effect of flocculant category on dewatering performance. With the increase

370

of dosage, SRF and FCMC exhibited the similar change trend of decline-climb-plateau.

371

CS-g-PAO#4 showed better dewaterability (77.98%, 3.83×1011 cm/g) than CPAD and

372

CS-g-PAM (80.10%, 1.09×1012 cm/g for CPAD, 80.98%, 1.86×1012 cm/g for CS-g-PAM) in

373

reducing SRF and FCMC. The SRF of sludge after CS-g-PAO#4 flocculation was about 34%

374

of the SRF after CPAD flocculation. The result indicated that sludge dewatering effect of

375

CS-g-PAO#4 was much better than CPAD. Moreover, the optimal dosage of CS-g-PAO#4 (6

376

mg/g TSS) was lower than that of CS-g-PAM (8 mg/g TSS), which could be concluded that

377

the hydrophobic modified flocculant CS-g-PAO#4 showed superior dewatering performance

378

to CS-g-PAM and CPAD. In addition, when dosage was excessive, the negatively charged

379

sludge particles would undergo charge reversal, causing the destabilized colloidal particles to

380

become re-stabilized state, which rapidly deteriorated the dewatering performance (Chai. et

381

al., 2014). In Fig. 5a, the FCMC of sludge after conditioning with CS-g-PAO#4 was kept 18

382

between 77.98% and 79.19% with the increase of dosage from 6 to 14 mg/g TSS. On the

383

contrast, the FCMC of sludge after conditioning with CPAD increased from 80.10% to 82.05%

384

with the change of dosage from 6 to 14 mg/g TSS. Therefore, the result indicated that

385

CS-g-PAO#4 had a wider flocculation window than CPAD, and CS-g-PAO#4 was less prone

386

to restabilizaiton.

387

Furthermore, CST and compressibility of sludge were used to evaluate dewatering

388

performance. In Fig. 5b, the optimum CST values for CS-g-PAO#4, CPAD and CS-g-PAM

389

were 4.5, 15.5 and 30.5 s, respectively, when the flocculant dosage was 6, 6 and 8 mg/g TSS,

390

respectively. In addition, the compressibility coefficient of sludge after conditioning with

391

CS-g-PAO#4 at optimum dosage (0.54) was lower than raw sludge (0.64). The decrease of

392

CST and compressibility meant that sludge dewaterability and filterability were improved.

393

However, the compressibility coefficient after CPAD and CS-g-PAM conditioning (0.63 for

394

CPAD; 0.89 for CS-g-PAM) were approximately same or higher than the compressibility

395

coefficient of raw sludge (0.64). Generally, the simple charge neutralization and bridging

396

effect of polymer could cause the formation of sticky gel-like sludge and the closure of

397

channels and voids in sludge filter cake, and then worsen the filterability and dewaterability

398

of CPAD and CS-g-PAM. The above results suggested that the hydrophobic groups of

399

CS-g-PAO were beneficial to the improvement of sludge filterability and dewaterability.

400

19

401 402

Fig. 5. Effect of flocculants category on FCMC and SRF (a), CST (b) and compressibility (c).

403

3.3.3 Effects of flocculant category on zeta potential, apparent viscosity and surface tension

404

To verify the difference of the three flocculants in charge neutralization effect and

405

hydrophobic property, it is necessary to analyze the effects of flocculant category on zeta

406

potential, apparent viscosity and surface tension (Wang. et al., 2019). In Fig. 6a, with the

407

increase of CPAD and CS-g-PAD#4 dosages, zeta potentials rapidly climbed and shifted from

408

the negative potential to the positive potential. This phenomenon indicated that charge

409

neutralization effect played a crucial part in sludge dewatering for CPAD and CS-g-PAO#4,

410

which caused the negatively charged sludge particles being neutralized. However, the zeta

411

potentials after conditioning with CS-g-PAM always maintained in the negative potential

412

region, which was attributed to the weak charge neutralization of nonionic CS-g-PAM. In

413

addition, the supernatant zeta potentials after conditioning with CS-g-PAO#4 were lower than

414

that of conditioning with CPAD when dosage was excessive (≥ 6 mg/g TSS). The result

415

indicated that the restabilization effect of CS-g-PAO#4 was weaker than PAO at excessive

416

dose. Furthermore, it was worth mentioning that the zeta potentials after conditioning with

417

CPAD and CS-g-PAO#4 (6 mg/g TSS) were 0.55 and -3.99 mV, respectively. The result

418

indicated that charge neutralization was the main dewatering mechanism of CPAD, so that the

419

zeta potential closed to the isoelectric point (0 mV) at optimal dose (Sillanpää. et al., 2018).

420

Similarly, the zeta potential of CS-g-PAO#4 at optimal dose was below 0 mV, indicating that

421

other dewatering mechanisms of CS-g-PAO#4 were involved except simple charge

422

neutralization and bridging effect. 20

423

Apparent viscosity and surface tension are the important parameters for evaluating the

424

hydrophobic association effect and surface activity of amphiphilic polymer (Lv et al., 2014).

425

As shown in Fig. 6b, the apparent viscosities of CS-g-PAM, CPAD, CS-g-PAO#3 and

426

CS-g-PAO#4 gradually increased with the increase of copolymers concentration. The

427

phenomenon was attributed to the effect of molecular association in the molecular chains of

428

the copolymers. As copolymers concentration increased, the intramolecular association

429

changed into intermolecular association, which enhanced the hydrodynamic volume of the

430

copolymer and increased the apparent viscosity. Besides, when copolymer concentration

431

exceeded 5 mg/mL, CS-g-PAO#3 and CS-g-PAO#4 showed the higher apparent viscosities

432

than CS-g-PAM and CPAD. And, the maximum apparent viscosities of CS-g-PAM, CPAD,

433

CS-g-PAO#3 and CS-g-PAO#4 (8 mg/mL) were 28.10, 26.43, 39.54 and 45.70 mPa/s,

434

respectively. Generally, the structure of amphiphilic copolymer tended to show the

435

cross-linking network structure due to the hydrophobic association effect of hydrophobic

436

functional groups, which could increase the apparent viscosity of copolymer and the

437

possibility of collision between flocculants and sludge particles. Furthermore, the correlation

438

between the maximum apparent viscosity and optimum SRF was shown in the inset of Fig. 6b.

439

The result indicated dewaterability was positively correlated with apparent viscosity. Hence,

440

the results confirmed that CS-g-PAO had the strong hydrophobic association effect, which

441

was beneficial to the improvement of dewaterability.

442

In Fig. 6c, CS-g-PAO#3 and CS-g-PAO#4 showed stronger surface activity than CPAD and

443

CS-g-PAM. Besides, the minimum surface tensions of CS-g-PAM, CPAD, CS-g-PAO#3 and

444

CS-g-PAO#4 were 71.10, 70.02, 67.20 and 65.86 mN/m, respectively. The results indicated

445

CS-g-PAO had the balanced hydrophilic groups and hydrophobic groups in solution, thereby

446

reducing the surface tensions of copolymers in solution (Jiang. et al., 2014; Yang. et al., 2010).

447

And, the correlation between the minimum surface tension and optimum SRF was shown in

448

the inset of Fig. 6c. The result indicated the stronger the surface activity, the better the

449

dewaterability. Moreover, studies found that the property of amphiphilic flocculants was

450

favorable for forming more hydrophobic regions in solution and enhancing the filterability of 21

451

sludge (in section 3.3.5). In conclusion, the hydrophobic association effect and strong surface

452

activity of CS-g-PAO had greatly effect on the improvement of dewatering performance.

453

Compared to the traditional organic flocculant, the introduction of CS improved the

454

biodegradable property of organic flocculant. In addition, the multifunctional monomer AO

455

with the hydrophilic cationic group and hydrophobic group enhanced the charge

456

neutralization and hydrophobic association effects of CS-g-PAO during sludge dewatering.

457

458 459

Fig. 6. Effect of flocculants category on zeta potential (a), apparent viscosity (b) and surface

460

tension (c).

461

3.3.4 Effect of flocculant dosage on EPS components

462

In addition to the direct parameters reflecting dewaterability mentioned above, EPS as the

463

indirectly parameter can also influence dewatering efficiency. And, the change of EPS

464

compositions during dewatering process exerts a significant influence on sludge

465

dewaterability and dewatering mechanism (Christensen. et al., 2015). Hence, the impact of

466

EPS components including proteins (PN), polysaccharide (PS) and humic acid (HA) on the 22

467

dewatering performance were investigated.

468

3.3.4.1 Proteins, polysaccharide and humic acid

469

Fig. 7a-c show the changes of PN, PS and HA contents after conditioning with different

470

flocculants as the dosage increases. The initial contents of PN, PS and HA in different EPS

471

fractions (S-EPS, LB-EPS and TB-EPS) were 0.613, 0.824 and 0.401 mg/g TSS, 1.060, 1.286

472

and 2.304 mg/g TSS, 2.446, 5.223 and 6.994 mg/g TSS, respectively. Correspondingly, the

473

initial PN, PS and HA contents in different EPS followed the order: TB-EPS > LB-EPS >

474

S-EPS, which was consistent with the published works (Dai. et al., 2018). As shown in Fig.

475

7a-c, with the increase of dose, the PN content of the three EPS fractions firstly decreased and

476

then increased. The phenomenon was in line with the change trend of dewatering performance

477

discussed previously. The decrease of PN content caused the destruction of sludge

478

configuration, thereby releasing more water and enhancing dewatering performance. In

479

addition, CS-g-PAO#4 was superior to CPAD and CS-g-PAM in removing PN. The maximum

480

removal of PN in S-EPS, LB-EPS and TB-EPS were 61.827% (0.234 mg/g TSS), 36.336%

481

(0.675 mg/g TSS) and 69.013% (0.758 mg/g TSS), respectively. The satisfactory PN removal

482

rate of CS-g-PAO#4 was due to the strong charge neutralization and hydrophobic association

483

effect, which could effectively compress the negatively charged EPS layer and closely interact

484

with PN components in EPS (Yan. et al., 2019). Moreover, the relatively high removal rate of

485

PN in S-EPS and TB-EPS indicated that the removal of PN in S-EPS and TB-EPS played an

486

important part in improving dewaterability. However, the relatively poor removal effect of PN

487

in LB-EPS might be a limiting factor for further improving dewaterability.

488

As shown in Fig. 7d-f, the content of PS exhibited a similar change trend with the PN as

489

the increase of dosage. The removal effect of CS-g-PAO#4 on PS was more pronounced than

490

that of CPAD and CS-g-PAM. And, the maximum removal rate of PS in S-EPS, LB-EPS and

491

TB-EPS were 33.783%, 29.826% and 42.263%, respectively. The results indicated that

492

CS-g-PAO#4 improved the removal rate of PS because the similar structure between PS and

493

CS enhanced the compatibility and interaction of PS with flocculant chains. However, the 23

494

removal rate of PS by CS-g-PAO#4 was lower than the removal rate of PN by CS-g-PAO#4.

495

Moreover, when the dosage of CS-g-PAO#4was more than 6 mg/g TSS, there was no obvious

496

advantage compared with CPAD. The phenomenon suggested that the relationship of between

497

PN contents and dewaterability was more closely related to PS (You. et al., 2017).

498

Furthermore, Fig. S7 shows the content of HA in EPS prior to conditioning and after

499

conditioning with the three flocculants at corresponding optimal dosage. The initial HA

500

content in S-EPS, LB-EPS and TB-EPS were 0.4014, 2.3045 and 6.99474 mg/g TSS,

501

respectively. Obviously, CS-g-PAO#4 showed the best performance in removing HA in

502

LB-EPS. The phenomenon indicated that the hydrophobic effect of CS-g-PAO#4 greatly

503

improve the removal rate on HA in LB-EPS.

504

505 506

Fig. 7. Effect of flocculants category on proteins (a-c) and polysaccharide (d-f) in S-, LB- and

507

TB-EPS fractions.

508

3.3.4.2 EEM analysis

509

In order to further verify the change of EPS components, EEM fluorescence spectroscopy

510

was used to analyze the PN and HA. The fluorescent peaks at λEX /Em = 240-290/300-350 nm

511

are ascribed to protein-like substances. The fluorescent peaks at λEX /Em = 350-440/370-510

512

nm are ascribed to humic acid. As shown in Fig. S8, the intensity of these peaks all decreased 24

513

after treatment with flocculants. The PN peaks in LB-EPS and TB-EPS were more obvious

514

than the PN peaks in S-EPS due to the low PN concentration in S-EPS. However, the intensity

515

of the protein-like and HA peaks after CPAD and CS-g-PAM conditioning was stronger than

516

CS-g-PAD#4. The results were consistent with the change of EPS components.

517

3.4 Sludge dewatering mechanism

518

3.4.1 FTIR spectra of flocs

519

In order to evaluate the internal interaction between functional groups of flocculant and

520

sludge particles, the FTIR spectra of raw sludge, CS-g-PAO#4 and sludge floc were shown in

521

Fig. S9. In the spectrum of raw sludge, the characteristic peaks at 1030, 1400, 1655 and 2359

522

cm-1 was ascribe to the stretching vibration of -C-OH, COO-, C=O and O=C=O of sludge

523

(Wei-Hua. et al., 2014). In the spectrum of sludge floc, those peaks shifted a little and

524

appeared at 1034, 1399, 1647 and 2359 cm-1, respectively. Furthermore, the adsorption peak

525

intensity of -CH3 groups attributed to quaternary ammonium groups of CS-g-PAO at 2934

526

cm-1 was weakened in the spectrum of sludge floc. And, the adsorption peak attributed to

527

benzene groups at 771 and 1452 cm-1 in the spectrum of CS-g-PAO shifted to 668 and 1457

528

cm-1 in the spectrum of sludge floc (Liao et al., 2014). The results indicated that the quaternary

529

ammonium groups and benzene groups of CS-g-PAO successfully aggregated with the sludge

530

particles during dewatering process, which caused the shift and attenuation of characterization

531

bands of sludge and CS-g-PAO.

532

3.4.2 SEM images of flocs

533

In Fig. 8, SEM images of sludge floc were used to characterize flocs morphology.

534

Obviously, the surface of sludge flocs after conditioning with CS-g-PAO appeared more

535

crevices and holes. The phenomenon was attributed to the abundant hydrophobic regions of

536

CS-g-PAO (according to the results of surface tension), which was helpful to form the more

537

drainage channels in sludge flocs due to the incompatibility between flocculants and sludge 25

538

particles. These drainage channels could greatly improve sludge filterability and

539

compressibility. On the contrast, the surface of sludge flocs after treatment with CS-g-PAM

540

and CPAD exhibited an irregular massive structure. The phenomenon suggested that the

541

simple charge neutralization and bridging effect of CS-g-PAM and CPAD easily caused the

542

closure of sludge voids and poor dewatering performance, which was in line with the result of

543

dewatering experiment.

544

545 546

Fig. 8. SEM images of raw sludge (a); floc after conditioning with CS-g-PAO#4 (b); CPAD (c)

547

and CS-g-PAM (d).

548

3.4.2 Summary of dewatering mechanism

549

According to the above discussions, the dewatering mechanism schematic of CAD-g-PAO

550

was shown in Fig. S10. On one hand, the positively charged quaternary ammonium groups

551

and amino groups in CS-g-PAO enhanced the charge neutralization effect during dewatering, 26

552

causing the stabilized sludge system more easily to destabilize, and the layer structure of EPS

553

was compressed to release more water. On the other hand, the hydrophobic groups of

554

CS-g-PAO could increase the hydrophobic association effect, which significantly increased

555

the absorbed amount of sludge particles and PN by increasing the collision possibility

556

between flocculant with sludge fractions. As shown in Fig. 9a, the size of sludge floc

557

gradually increased at the floc formation phase (0-5 min), and rapidly decreased at the

558

breakage stage (5-7 min) due to the vigorous agitation, and then floc size slowly increased

559

when stirring speed was 30 r/min. The floc size under CS-g-PAO#4 treatments was all larger

560

than that with the treatment of CS-g-PAM and CAPD at the breakage and regrowth stages,

561

indicating the sludge flocs after the treatment of CS-g-PAO#4 were more resistant to

562

mechanical breakage. Moreover, as shown in Fig. 9b-d, the average flocs size (d50) after the

563

treatment of CS-g-PAO#4 (754.537 µm) was higher than CS-g-PAM and CPAD (618.792 and

564

274.468 µm), suggesting that the sludge particles and EPS components could be efficiently

565

aggregated by the hydrophobic segments of CS-g-PAO, thereby forming larger sludge flocs.

566

In addition, strong surface activity of CS-g-PAO increased hydrophobic regions, which

567

contributed to forming more drainage channels in flocs interior, thereby enhancing sludge

568

filterability. In conclusion, the charge neutralization and hydrophobic properties of CS-g-PAO

569

played a synergetic role in improving dewaterability and filterability of sludge.

570

27

571 572

Fig. 9. Change in particle size of sludge flocs with time (a), and sludge floc size distribution

573

of CS-g-PAM (b), CPAD (c), CS-g-PAO#4 (d).

574

Conclusion

575 576

A chitosan-based amphiphilic flocculant, CS-g-PAO, was synthesized in this work through the graft copolymerization of AM and AO using UV initiated method for sludge dewatering.

577

The characterization results of FTIR, 1H NMR, XRD and Raman spectra confirmed the

578

successful synthesis of CS-g-PAO. SEM analysis showed that the porous and uneven

579

surface structure of CS-g-PAO.

580

The dewatering performance CS-g-PAO#4 was superior to CPAD and CS-g-PAM, and

581

CS-g-PAO#4 had a wide dewatering window. When CS-g-PAO#4 dose was 6 mg/g TSS,

582

the best dewatering efficiency was obtained and the minimum values of FCMC, SRF and

583

CST were 77.98%, 3.83×1011 cm/g and 4.9 s, respectively.

584

The changes of EPS components suggested the removal of PN in S-EPS and TB-EPS

585

played an important role in improving dewaterability of CS-g-PAO.

586

The results of zeta potential, apparent viscosity, surface tension and flocs structure

587

confirmed the synergetic role of enhanced charge neutralization and hydrophobic effect

588

in improving sludge dewaterability.

589

Acknowledgements

590

This work was supported by the National Natural Science Foundation of China (Project 28

591

No.21477010),

592

cstc2018jszx-cyzd0053) and Graduate Research and Innovation Foundation of Chongqing,

593

China (Project No.CYB18041).

594

References:

595

Agbovi, H.K., Wilson, L.D., 2018. Design of amphoteric chitosan flocculants for phosphate

596 597 598

Chongqing

Science

and

Technology

Committee

(Project

and turbidity removal in wastewater. Carbohyd. Polym. 189, 360-370. Anjum, M., et al., 2016. Wastewater sludge stabilization using pre-treatment methods. Process Saf. Environ. 102, 615-632.

599

Azmeera, V., et al., 2017. Solution and microwave assisted synthesis of β-Cyclodextrin

600

grafted polyacrylamide: Water treatment and In-vitro drug release study. Int. J. Biol.

601

Macromol. 104, 1204-1211.

602 603 604 605 606 607

Chai, S.L., et al., 2014. A review on application of flocculants in wastewater treatment. Process Saf. Environ. 92 (6), 489-508. Chen, L., et al., 2018. Characterization and sludge dewatering performance evaluation of the photo-initiated cationic flocculant PDD. J. Taiwan Inst. Chem. E. 93, 253-262. Christensen, M.L., et al., 2015. Dewatering in biological wastewater treatment: A review. Water Res. 82 (SI), 14-24.

608

Dai, Q., et al., 2018. Research on dewaterability and properties of sewage sludge under

609

modified phosphogypsum and acetic acid pretreatments. Bioresource Technol. 264,

610

268-276.

611

No.

Deval, V., et al., 2014. Molecular structure (monomeric and dimeric) and hydrogen bonds in

612

5-benzyl 2-thiohydantoin studied by FT-IR and FT-Raman spectroscopy and DFT

613

calculations. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 132,

614

15-26.

615 616 617

Dong, C., et al., 2014. Flocculation of algal cells by amphoteric chitosan-based flocculant. Bioresource Technol. 170, 239-247. Ejtemaei, M., et al., 2019. Synergistic effects of surfactant-flocculant mixtures on ultrafine 29

618 619

coal dewatering and their linkage with interfacial chemistry. J. Clean. Prod. 232, 953-965. Filho, C.M.C., et al., 2018. Synthesis, characterization and sorption studies of aromatic

620

compounds by hydrogels of chitosan blended with β-cyclodextrin- and

621

PVA-functionalized pectin. RSC Adv. 8 (26), 14609-14622.

622 623 624

Gamage, A., Shahidi, F., 2007. Use of chitosan for the removal of metal ion contaminants and proteins from water. Food Chem. 104 (3), 989-996. Jiang, X., et al., 2014. Decolorization of anionic dye solutions using the hydrophobically

625

modified polyelectrolytes containing beta-cyclodextrin moieties. Chem. Eng. J. 253,

626

183-189.

627 628 629

Jonathan, W.C.W., et al., 2016. Improved dewatering of CEPT sludge by biogenic flocculant from Acidithiobacillus ferrooxidans. Water Sci. Technol. 73 (4), 843. Liao, Y., et al., 2014. UV-initiated polymerization of hydrophobically associating cationic

630

polyacrylamide modified by a surface-active monomer: a comparative study of synthesis,

631

characterization, and sludge dewatering performance. Ind. Eng. Chem. Res. 53 (27),

632

11193-11203.

633

Liu, X., et al., 2019. Microwave pretreatment of polyacrylamide flocculated waste activated

634

sludge: Effect on anaerobic digestion and polyacrylamide degradation. Bioresource

635

Technol. 290, 121776.

636

Liu, Y.Z., et al., 2018. Synthesis of a cationic polyacrylamide by a photocatalytic

637

surface-initiated method and evaluation of its flocculation and dewatering performance:

638

nano-TiO2 as a photo initiator. RSC Adv. 8 (50), 28329-28340.

639

Lv, S., et al., 2014. Synthesis of starch-g-p(DMDAAC) using HRP initiation and the

640

correlation of its structure and sludge dewaterability. Carbohyd. Polym. 103, 285-293.

641

Niu, M., et al., 2013. Correlation of physicochemical properties and sludge dewaterability

642

under chemical conditioning using inorganic coagulants. Bioresource Technol. 144,

643

337-343.

644 645

Peng, W., et al., 2019. An overview on the surfactants used in ion flotation. J. Mol. Liq. 286, 110955. 30

646 647 648 649 650 651 652

Razmi, F.A., et al., 2019. Kinetics, thermodynamics, isotherm and regeneration analysis of chitosan modified pandan adsorbent. J. Clean. Prod. 231, 98-109. Shi, C., et al., 2019. Synthesis, Characterization, and Sludge Dewaterability Evaluation of the Chitosan-Based Flocculant CCPAD. Polymers-Basel. 11 (1), 95. Sillanpää, M., et al., 2018. Removal of natural organic matter in drinking water treatment by coagulation: A comprehensive review. Chemosphere. 190, 54-71. Sun, Y.J., et al., 2019a. Performance evaluation of chitosan-based flocculants with good pH

653

resistance and high heavy metals removal capacity. Sep. Purif. Technol. 215, 208-216.

654

Sun, Y., et al., 2019b. Novel chitosan-based flocculants for chromium and nickle removal in

655 656

wastewater via integrated chelation and flocculation. J. Environ. Manage. 248, 109241. Sun, Y.J., et al., 2017. Plasma-initiated polymerization of chitosan-based

657

CS-g-P(AM-DMDAAC) flocculant for the enhanced flocculation of low-algal-turbidity

658

water. Carbohyd. Polym. 164, 222-232.

659

Tang, X., et al., 2020. The role of sulfonated chitosan-based flocculant in the treatment of

660

hematite wastewater containing heavy metals. Colloids and Surfaces A: Physicochemical

661

and Engineering Aspects. 585, 124070.

662 663 664 665 666 667

Vaziri Hassas, B., et al., 2014. Ultrafine coal dewatering: Relationship between hydrophilic lipophilic balance (HLB) of surfactants and coal rank. Int. J. Miner. Process. 133, 97-104. Wang, H., et al., 2019. Combined use of inorganic coagulants and cationic polyacrylamide for enhancing dewaterability of sewage sludge. J. Clean. Prod. 211, 387-395. Wei, H., et al., 2018. Coagulation/flocculation in dewatering of sludge: A review. Water Res. 143, 608-631.

668

Wei-Hua, L.I., et al., 2014. Analysis of Poly-β-Hydroxyalkonates (PHA) During the

669

Enhanced Biological Phosphorus Removal Process Using FTIR Spectroscopy.

670

Spectroscopy & Spectral Analysis. 34 (6), 1512-1517.

671 672 673

Xu, X., et al., 2019. Study on ultrasonic treatment for municipal sludge. Ultrason. Sonochem. 57, 29-37. Yan, Z., et al., 2019. Insights into the interactions between triclosan (TCS) and extracellular 31

674 675 676

polymeric substance (EPS) of activated sludge. J. Environ. Manage. 232, 219-225. Yang, R., et al., 2016. A review on chitosan-based flocculants and their applications in water treatment. Water Res. 95, 59-89.

677

Yang, Z., et al., 2014. Evaluation of a novel chitosan-based flocculant with high flocculation

678

performance, low toxicity and good floc properties. J. Hazard. Mater. 276, 480-488.

679 680 681 682 683

Yang, Z.L., et al., 2010. Synthesis and characterization of hydrophobically associating cationic polyacrylamide. Chem. Eng. J. 161 (1-2), 27-33. You, G., et al., 2017. Insights into the short-term effects of CeO2 nanoparticles on sludge dewatering and related mechanism. Water Res. 118, 93-103. Zając, A., et al., 2015. Determination of N-acetylation degree in chitosan using Raman

684

spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.

685

134, 114-120.

686 687

Zeng, D., et al., 2008. Application of a chitosan flocculant to water treatment. Carbohyd. Polym. 71 (1), 135-139.

688

Zhang, W., et al., 2015. Insights into the respective role of acidification and oxidation for

689

enhancing anaerobic digested sludge dewatering performance with Fenton process.

690

Bioresource Technol. 181, 247-253.

691 692

Zhao, C., et al., 2018. Evaluation of a novel dextran-based flocculant on treatment of dye wastewater: Effect of kaolin particles. Sci. Total Environ. 640, 243-254.

693

32

A novel chitosan-based flocculant with amphiphilic structure was synthesized. The water solubility of the amphiphilic flocculant was improved in a wide pH range. The amphiphilic flocculant showed superior dewatering efficiency under low dosage. All results confirmed the role of charge neutralization and hydrophobic attraction.

Author Contribution Statement Yongzhi Liu: Conceptualization, Investigation, Methodology, Visualization, WritingOriginal Draft. Huaili Zheng: Resources, Writing-Review & Editing, Supervision, Project administration. Yongjun Sun: Writing-Review & Editing, Validation, Supervision. Jie Ren: Methodology, Software. Xinyu Zheng: Formal analysis, Writing-Review & Editing. Qiang Sun: Writing- Reviewing and Editing. Shaojie Jiang: Project administration. Wei Ding: Writing-Review & Editing.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: