Preparation of octopus-like lignin-grafted cationic polyacrylamide flocculant and its application for water flocculation

Journal Pre-proof Preparation of octopus-like lignin-grafted cationic polyacrylamide flocculant and its application for water flocculation

Nian Chen, Weifeng Liu, Jinhao Huang, Xueqing Qiu PII:

S0141-8130(19)39391-2

DOI:

https://doi.org/10.1016/j.ijbiomac.2019.12.245

Reference:

BIOMAC 14273

To appear in:

International Journal of Biological Macromolecules

Received date:

18 November 2019

Revised date:

28 December 2019

Accepted date:

28 December 2019

Please cite this article as: N. Chen, W. Liu, J. Huang, et al., Preparation of octopus-like lignin-grafted cationic polyacrylamide flocculant and its application for water flocculation, International Journal of Biological Macromolecules(2018), https://doi.org/10.1016/ j.ijbiomac.2019.12.245

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© 2018 Published by Elsevier.

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Preparation of octopus-like lignin-grafted cationic polyacrylamide flocculant and its application for water flocculation Nian Chen†, Weifeng Liu*†, Jinhao Huang†, Xueqing Qiu *†‡ † School of Chemistry and Chemical Engineering, Guangdong Engineering Research

Road 381, Guangzhou, Guangdong, 510640, China

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Center for Green Fine Chemicals, South China University of Technology, Wushan

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‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Road 381, Guangzhou, Guangdong, 510640, China

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*Corresponding authors

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E-mail: [email protected] (W. Liu); [email protected] (X. Qiu).

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Tel.: +86-020-87114722.

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Abstract

synthesized

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High efficiency lignin-grafted cationic polyacrylamide (L-CPA) flocculant was via

“grafting

methylacryloyloxyethyltrimethyl

to”

method

ammonium

using

chloride

acrylamide

(DMC)

and

(AM), enzymatic

hydrolysis lignin (EHL) as raw materials. The linear pre-polymer of cationic polyacrylamide (CPA) terminated with chlorine was first synthesized and then grafted onto EHL via reactions of chlorine with phenolic hydroxyl groups in lignin molecules. The synthesized L-CPA could self-assemble into octopus-like nanospheres with CPA segments dissolved in water and hydrophobic lignin skeletons concentrated in the core, which endowed the L-CPA with excellent flocculation efficiency for kaolin

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suspension under faintly acid, neutral or alkalescent conditions (pH=5-9). Only a small dosage between 4.0~4.5 mg/L of L-CPA was needed for flocculation of the kaolin suspension. The charge neutralization and bridging effect was proposed for the flocculation mechanism of the lignin-grafted cationic polyacrylamide. The octopus-like L-CPA was cheap, environmentally friendly and technically feasible,

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showing a great prospect in wastewater treatment.

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Keywords: Lignin, Polyacrylamide, Water Flocculation, Self-assembly

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1. Introduction

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The global water crisis has become one of the most immediate threats restricting human development.[1-4] The increasing water pollution has brought unprecedented

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challenges to the wastewater treatment.[5] At present, the most commonly used

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organic flocculant for water treatment is polyacrylamide (PAM).[6-7] The linear ultra-high molecular weight cationic and anionic PAM are widely used in water treatment because of their high flocculation efficiency. However, disadvantages for the conventional ultra-high molecular weight PAM are also obvious, such as high viscosity,[8] long dissolution time, low efficiency for charge utilization as well as the high production cost etc. Therefore, developing new PAM flocculants with high performance but low cost is one of the pivotal issues to improve the wastewater treatment technology.[9-10] In order to solve these problems, researchers proposed hyperbranched chain microstructure for cationic PAM to reduce the viscosity and improve the cation

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utilization efficiency. For example, Wang et al. synthesized hyperbranched cationic PAM via semibatch reversible addition-fragmentation chain transfer (RAFT) polymerization.[11-12] Compared with commercial linear cationic PAM with ultra-high molecular weight (7,000-10,000 kD) and high cation degree (25-35%), the hyperbranched cationic PAM possessed lower molecular weight (2,000-3,500 kD) and

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less cationicity (10-25%), but the flocculation efficiency was comparable or even better, attributing to the hyperbranched chain microstructure with cation units

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concentrated at the branching chain ends. The superiority of hyperbranched cationic

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PAM was demonstrated, however, the “living” controlled radical polymerization is

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technically difficult, costly and of low production efficiency.

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In recent years, the shortage in petrochemical resource has driven the hot

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attention to biomass resources for chemical industry due to their merits of nontoxicity, biodegradability, easy availability and environmental friendliness.[13] So far, many

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biomass resources have been tried to synthesize water treatment agent, such as chitosan,[14-17] cellulose,[18-20] starch,[21-22] xylan,[23] and lignin[24-25] etc, among which, lignin has attracted great interest. As the second largest biomass resource in plants after cellulose, lignin is regarded as one of the most abundant green resources available to humans in the 21st century. Lignin is a natural renewable aromatic biopolymer having three-dimensional hyperbranched structure and many reactive functional groups.[26-27] Recently, lignin is found great potential applications in functional materials.[28-35] In addition, lignin is able to adsorb polar organics, mesh colloidal particles and suspended particles,[36] which can be used as a natural

Journal Pre-proof flocculant material. Li et al.[37] synthesized lignin/diallyl dimethyl ammonium chloride/acrylamide terpolymer via normal free radical polymerization to treat humic acid (HA)/kaolin suspension, however, the flocculation efficiency was poor. Zu et al.[38] synthesized a lignin-based star copolymer via Steglich esterification and subsequent RAFT polymerization with DMC and applied it in the flocculation of

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kaolin suspension. The efficiency of turbidity removal reached 95% with 6 mg/L dosage of the lignin-based star copolymer.

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The lignin-based flocculants reported previously were mainly synthesized by

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free radical polymerization via “grafting from” strategy.[39-40] However, as well known,

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lignin is a good scavenger for free radicals,[41-43] which actually inhibits the free

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radical polymerization and leads to a low grafting efficiency. Therefore, the chain

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microstructure of the lignin-grafted products from the “grafting from” strategy is uncontrollable and some are even blends instead of the grafted products. The resulting

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lignin-grafted products usually exhibit quite poor flocculation performance, especially when comparing with the commercial cationic PAM. In this work, instead of the “grafting from” route, we successfully applied the “grafting to” strategy in the synthesis of high performance lignin-grafted cationic PAM flocculant. At first, a linear cationic polyacrylamide (CPA) prepolymer terminated with chlorine was synthesized via normal free radical polymerization. It was then grafted onto enzymatic hydrolysis lignin (EHL) to form L-CPA via efficient reaction of chlorine with phenolic hydroxyl groups in lignin molecules. The chemical structures of the prepolymer and lignin-grafted products were characterized. The

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effects of the monomer ratio, grafting ratio and the value of pH on the flocculation performance of the lignin-grafted products were investigated. It was demonstrated that the synthesized L-CPA could form a special type of octopus-like self-assembly structure with CPA segments dissolved in water and hydrophobic lignin skeletons in the core, which endowed the L-CPA with excellent flocculation efficiency.

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2. Materials and methods

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2.1. Materials

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Enzymatic hydrolysis lignin (EHL) was provided by Shandong Longli

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Bio-Technology (China) and was purified by alkali-acid treatment. Acrylamide (AM,

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99%), [2-(methacryloyloxy)ethyl]trimethyl ammonium chloride (DMC, 75%) in 75 wt% aqueous solution and the initiator ammonium persulfate (APS) were purchased

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from Aladdin Industrial Corporation. The chain transfer agent, 4-chloromethyl styrene

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(CS, 90%), was provided by Energy Chemical. Acetone and HCl (AR, 37%) were purchased from Guangzhou Chemical Reagent Factory while ethanol was from Damao Chemical Company, Tianjin. Commercial grade kaolin clay (11 μm) was from Aladdin. The pH of kaolin suspension was adjusted by 0.1 mol/L NaOH (Fuchen Chemical Company, Tianjin) and HCl. Polyaluminium chloride (PAC) was provided by Macklin. Deionized water was used as solvent in this study. 2.2. Synthesis of linear cationic polyacrylamide (CPA) with chlorinated chain ends

The CPA was prepared by free radical polymerization using AM and DMC.

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Specially, AM and DMC were firstly added into a 150 ml three-necked flask according to the recipe in Table 1, then a certain amount of CS was added as the chain transfer agent. Nitrogen was then introduced into the flask for 30 minutes to discharge the air. The flask temperature was stabilized at 60 °C and APS was finally added as the initiator. The reaction was carried on for 10 h at 60 °C under mechanical stirring

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and nitrogen atmosphere. After the reaction, the solution was poured into a beaker containing ethanol to extract the product. The obtained white substance was then

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freeze-dried to get the purified CPA, which was then used for the synthesis of lignin

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grafted products. The prepolymer was labeled according to the molar ratio of

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AM/DMC in feed, e.g. CPA-3 means the molar ratio of AM/DMC in feed was 3:1.

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Table 1. Formulations for the synthesis of cationic polyacrylamide prepolymer (CPA).

DMC/g

CS/g

APS/g

Yield

CD/%*

6.900

0.063

0.245

68.9%

25.8

7.100

6.900

0.074

0.280

70.7%

22.5

CPA-5

8.875

6.900

0.084

0.316

81.1%

20.0

CPA0

7.100

6.900

0

0.280

83.6%

22.5

AM/g

CPA-3

5.325

CPA-4

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Name

*CD means theoretical cationic degree calculated by the amount of feed.

2.3. Synthesis of lignin-grafted cationic polyacrylamide (L-CPA) flocculant

CPA, EHL and 1 mol/L NaOH solution were added into a three-necked flask in a

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certain proportion (Table 2). The reaction temperature was set at 80 °C. After 4 hours of reaction, the solution was poured into a 250 mL beaker to cool down. Then triple volume of acetone was added into the beaker to extract the product. The product was washed with acetone three times and then dissolved in deionized water, dialyzed, freeze-dried to get the purified L-CPA. The grafting product was named by Lx-CPA-y,

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e.g. L2-CPA-3 means the mass ratio of CPA-3 to EHL in feed was 2:1.

CPA/g

EHL/g

L2-CPA-3

2.000

1.000

L2-CPA-4

2.000

1.000

L2-CPA-5

2.000

L1-CPA-4

1.000

L2-CPA0

CD/% 17.2

99.9%

15.0

81.6%

13.3

1.000

93.6%

11.2

3.000

1.000

95.7%

16.9

2.000

1.000

-

15.0

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77.9%

1.000

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L3-CPA-4

Grafting yield

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Name

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Table 2. Formulations for the synthesis of lignin-grafted cationic polyacrylamide flocculant.

*CD means theoretical cationic degree calculated by the amount of feed.

2.4. Flocculation sedimentation test Kaolin suspension of 0.5 g/L was used as simulated wastewater to evaluate the flocculation performance of L-CPA. 10 mg polyaluminum chloride (PAC) was first added into 1 L kaolin suspension. The L-CPA solution of 2.0 g/L was then added into the kaolin suspension. The suspension was immediately stirred at a rapid speed of 200

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rpm for 2 min, followed by a slower stirring at 40 rpm for 10 min. After stirring, the supernatant of kaolin suspension was collected and the transmittance was measured using a Dispersion Stabilizer (Formulation Company, France). 0.1 mol/L of hydrochloric acid and sodium hydroxide were used to adjust the pH of suspension solution so as to study the influence of pH on the flocculation performance.

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2.5. Characterizations The Fourier transform infrared spectra (FTIR) of EHL, CPA and L-CPA were

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recorded on a Bruker Tensor 27 FTIR spectrometer. The samples were ground and

H NMR spectra of EHL, CPA and L-CPA were obtained with a Bruker High

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1

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dried at 50 °C for 12 h by infrared oven before analysis to remove the absorbed water.

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Performance Digital NMR Spectrometer (Avance Neo 500MHZ), in which the EHL

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was dissolved in DMSO-d6 and the CPA and L-CPA were dissolved in D2O-d6 at room temperature. Approximately 40 mg of above-mentioned samples were dissolved in

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0.55 ml of the deuterated solvent.

The phenolic hydroxyl in lignin was determined by aqueous phase potentiometric titration according to our previous reported method.[44] The molecular weight and distribution of CPA were determined by gel permeation chromatography (PL GPC 50 plus). NaNO3 solution (0.1 mol/L) was used as the flow phase at the rate of 0.8 ml/min at 30 °C. Polyethylene oxide (PEO) was used as the standard with k = 12.5×10-3 mL/g, α = 0.78. The viscosity of L-CPA was determined by HAAKE MARS III rotational rheometer equipped with a plate rotor. Multifunctional X-ray Photoelectron Spectroscopy/ESCA (XPS, Axis Ultra DLD)

Journal Pre-proof was used to characterize the distribution of elements. Monochromated Al Kα source was used as the X illuminant in vacuum condition of 5×10-9 toor. A nanoparticle analyzer SZ-100Z (HORIBA, Japan) was used to measure the Zeta potential of CPA and L-CPA. The Particle Charge Detector (PCD-04 Travel) was used to test the surface potential. A Malvern laser granulator was used to measure the particle size.

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The samples were prepared with a concentration of 2.0 g/L, each sample was measured three times for average. The surface morphologies of L2-CPA-4 and floc

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were investigated by scanning electron microscopy (HITACHI UHR FE-SEM

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SU8200). Before test, the sample was dried and placed on the conductive adhesive.

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The morphology of L2-CPA-4 was also observed by field emission transmission

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electron microscope (Thermo Fisher Scientific FE-TEM Talos F200S). Firstly, the

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L2-CPA-4 was dissolved in deionized water to form a suspension with the concentration of 1 mol/L. Then the dispersion was aspirated with a capillary and

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dropped onto the copper network to be air dried.

3. Results and discussion

3.1. Synthesis and characterization of L-CPA The prepolymer of linear cationic polyacrylamide was synthesized via normal free radical polymerization using AM and DMC monomers. 4-Chloromethyl styrene was introduced as the chain transfer agent to give the chlorinated chain end, as shown in Scheme 1. The molecular weight of CPA increased as the feeding molar ratio of AM/DMC increased, as shown in Table 1 and Fig. S1. Specially, when the molar ratio of AM/DMC increased from 3 to 4, the weight-averaged molecular weight increased

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sharply from 32.7 kDa to 63.0 kDa. The steric-hindrance effect of the cationic

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monomer DMC played a major role in limiting the molecular weight growth.[45]

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Scheme 1. Schematic diagram of the synthesis process.

CPA-3

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CPA-4

Mn (kDa)

CPA-5

Mw (kDa)

PDI

15.0

32.7

2.2

24.8

63.0

2.5

27.3

71.3

2.6

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Sample Name

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Table 3. The molecular weight distribution of CPAs

The lignin-grafted cationic polyacrylamide (L-CPA) was then synthesized by the “grafting to” strategy via the reaction of chlorine in CPA with phenolic hydroxyl groups in EHL under the base condition (Scheme 1). The structures of EHL, CPA and L-CPA were first characterized by FT-IR, as shown in Fig. 1a. The absorption peak at 3430 cm-1 and 1600 cm-1 were attributed to -OH and -COOH groups (asymmetric stretching ), respectively.[46] The -CH stretching from the -CH2 group was observed at

Journal Pre-proof 2936 cm-1 and 2874 cm-1.[47] The new peak at 1566 cm-1 was due to the vibration of C-N group which was only found in CPA-4 and L2-CPA-4. In addition, the new peak at 1480 cm-1 was attributed to the bending vibration of -CH2-N+(CH3)3 group of DMC.[48-49] The stretching bands at 1513 and 1452 cm−1 in the EHL came from the aromatic rings.[50] However, when EHL was grafted with CPA-4, the 1513 cm−1 peak

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transferred to 1535 cm−1 in L2-CPA-4, suggesting the structure variation on the

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aromatic rings after grafting reaction.

aromatioc rings

3.21 1.17

-p

1513 1452

L2-CPA-4 1566 CN stretching

2.95

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Transmittance

EHL

L2-CPA-4

CPA-4

1535

3.78

2936 2874 3430 OH stretching

3500

1480

CH stretching 1600 COOH stretching

EHL

+ -CH2N (CH3)3

3000 2000 1500 -1 Wavenumbers (cm )

1000

8

7

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4000

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CPA-4

(a)

6

5 4 3 Chemical shift (ppm)

2

1

0

(b)

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Fig. 1. (a) FT-IR spectra of EHL, CPA-4 and L2-CPA-4. (b) 1H-NMR spectra of EHL, CPA-4 and L2-CPA-4.

The 1H-NMR spectra of EHL, CPA-4 and L2-CPA-4 were shown in Fig. 1b. The peaks at δ 3.45 in EHL and δ 3.56 in CPA-4 and L2-CPA-4 corresponded to the protons in aliphatic/aromatic methoxy groups.[51] The peak at δ 1.17 for all samples was assigned to the protons in methyl group (-CH3). In CPA-4, the peak at δ 3.78 was assigned to the protons in the methylene group of -CH2-N+. The sharp peak at δ 3.21 was assigned to the protons in the three equivalent methyl groups of -N+(CH3)3 introduced by the DMC comonomer.[52-53] Compared with CPA-4, the signal at δ 3.21

Journal Pre-proof migrated to δ 2.95 and weakened significantly, and the peak at δ 3.78 also weakened mostly in L2-CPA-4, suggesting the structure variation after the grafting reaction. The phenolic hydroxyl content in EHL determined by aqueous phase potentiometric titration was about 1.05 mmol/g, and the value was reduced to around 0.63 mmol/g in L2-CPA-4, verifying the reaction of the phenolic hydroxyl groups after grafting. Both

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the FT-IR analysis and the 1H-NMR spectra analysis confirmed the successful grafting

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600

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Na

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N

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C

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of EHL by CPA.

500

400

300

EHL Cl

CPA-4 L2-CPA-4

200

100

0

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Binding Energy(eV)

Fig. 2. XPS spectra of EHL, CPA-4 and L2-CPA-4.

The grafting of CPA on lignin was further demonstrated by the XPS analysis, as shown in Fig. 2. Chlorine element was obviously found in CPA-4, but there was no chlorine in lignin and lignin-grafted product (L2-CPA-4), as the chlorine atoms in CPA reacted with the phenolic hydroxyl groups in lignin after grafting reaction. Meanwhile, the content of nitrogen element in L2-CPA-4 decreased in comparison with that in CPA-4, also suggesting successful grafting of CPA onto lignin as no obvious nitrogen element was found in lignin.

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CPAM L2-CPA-3

0.20

L2-CPA-4 L2-CPA-5

Viscosity(Pa.s)

0.16

L1-CPA-4 L3-CPA-4

0.12

0.08

0.04

0.00 0

200

400 600 Shear rate (1/s)

800

1000

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polyacrylamide).

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Fig. 3. Rheological property of L-CPA (CPAM represent the commercial cationic

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As shown in Fig. 3, the shear viscosity of commercial CPAM and L-CPAs

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decreased with the shear rate. In addition, the shear viscosity of L-CPAs was lower

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than that of CPAM, showing that the as-prepared flocculants were easier to dissolve in

100

Zeta (mV)

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50

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water, which is beneficial for its practical application.

0

L1-CPA-4 L2-CPA-4 EHL L2-CPA-5 L2-CPA-3 L3-CPA-4

CPA-3 CPA-4 CPA-5

-50

-100

-150

-200

Fig. 4. Zeta potential of CPA and L-CPA at pH of 7.

The zeta potential of the prepolymers CPA-3, CPA-4 and CPA-5 were verified to be positive, as shown in Fig. 4. The absolute value of the zeta potential decreased in CPA as the feeding ratio of cationic monomer decreased, which was consistent with

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the theoretical cationic degree in Table 1. Interestingly, after grafting onto lignin, the grafted product L-CPA all exhibited negative zeta potential. This was due to the strong electronegativity of lignin itself. The absolute value of the zeta potential for L-CPA decreased as grafting ratio increased (L2-CPA-4 vs. L3-CPA-4). When the grafting ratio (CPA/EHL ratio) was fixed at 2:1, the absolute value of L-CPA increased as the

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cationic degree of CPA decreased. 3.2. Flocculation performance of L-CPA

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For the flocculation sedimentation test, the concentration of kaolin suspension

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was kept at 0.5 g/L. PAC was used as the coagulant with a certain dosage of 10 mg/L.

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The surface potential of kaolin particles was negative (-484 mV). When PAC was

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added into the suspension, the surface potential became positive (579 mV), and the

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kaolin suspension did not sedimentate, as the function of PAC was to adsorb kaolin particles by electrical neutralization and form weak aggregation.[54-55] Obvious

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flocculation phenomenon was observed when L-CPA was added after PAC. As shown in Fig. 5a, the higher light transmittance meant the lower turbidity and better flocculation efficiency. Only a small dosage between 4.0~4.5 mg/L of L2-CPA-4 was needed for flocculation and sedimentation of the kaolin suspension. When the dosage of L-CPA was kept at 4 mg/L, the maximum transmittance from L2-CPA-3, L2-CPA-4 and L2-CPA-5 were 92%, 90% and 82% (Fig. 5b), respectively, suggesting that less electronegativity (higher cationic degree in the grafted CPA segment) was beneficial for the flocculation. The maximum light transmittance from L3-CPA-4 (65%) was much smaller than those from L1-CPA-4 and L2-CPA-4 (90%). The grafting ratio of

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L3-CPA-4 was higher than L1-CPA-4 and L2-CPA-4, in other words, the lignin fraction in L3-CPA-4 was the smallest. The result suggested that the lignin fraction in the grafted product played a special role in the flocculation process. The light transmittance after flocculation by the control sample L2-CPA0 was the poorest (Fig. 5b), suggesting that the product prepared from the prepolymer without chlorine chain

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end exhibited quite poor flocculation performance. The water became very clear after flocculation by the lignin-grafted product, as visually demonstrated in Fig. 5c. To the

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best of our knowledge, comparing with other lignin-derived flocculants shown in Fig.

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6,[38, 56-58, 60] the L-CPA synthesized via “grafting to” strategy in this work required the

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minimum dosage, showing the excellent flocculation efficiency.

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60

40

80

60

40 L2-CPA-3 L2-CPA-4

20

L2-CPA-4(3.5mg/L)

20

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Transmittance (%)

80

100

Transmittance (%)

lP

100

L2-CPA-5 L1-CPA-4

L2-CPA-4(4.0mg/L)

0 0

10

20

30 Time (min)

40

50

L3-CPA-4

0

L2-CPA-4(4.5mg/L) 60

L2-CPA0

0

(a)

10

20

30 Time (min)

40

50

60

(b) After

Before

(c) Fig. 5. (a) Evolution of the transmittance with time during the flocculation and sedimentation test of L2-CPA-4 at the pH value of 7. (b) Evolution of the transmittance with time during the

Journal Pre-proof flocculation and sedimentation test of L-CPA at the pH value of 7 in the dosage of 4.0 mg/L. (c) Photo demonstration before and after flocculation and sedimentation by L2-CPA-4. 400

Our work [10] [56] [57]* [58]* [59] [38] [60] [61]

350

Dosage (mg/L)

300 250 200 150 100

0 20

40

60 80 Time (min)

100

120

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0

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50

Fig. 6. Comparison with literature-reported biomass derived flocculation results.10, 38, 56-61 The

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flocculation time. ([ ]* means centrifugal time)

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ordinate is the dosage needed for 80% transmittance, and the x-coordinate is the corresponding

The influence of the pH value on the flocculation efficiency was also studied. Fig.

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7 shows the transmittance after flocculation by L2-CPA-4 under different pH values.

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The transmittance maintained relatively high level in the pH range of 5~9, but reduced sharply at a strong acid (pH=3) or base condition (pH=11). This was because in the strong base condition, the lignin components in L-CPA was soluble, while in the strong acid condition, the lignin component in L-CPA aggregated and precipitated, which both prevented L-CPA from flocculation of colloidal part. In addition, the deprotonation of -NH2 groups in the CPA chains and deprotonation of phenolic hydroxyl and carboxyl groups in the lignin component by OH- in the strong base environment led to stronger electro-negativity, which also weakened the flocculation performance of L-CPA.[62-63]

Journal Pre-proof 100

100

80

60

Transmittance (%)

Transmittance (%)

80

15min 30min 45min

40 pH=3 pH=5 pH=7 pH=9 pH=11

20

0 0

10

20

30 Time (min)

40

50

60

40

20

0

60

2

3

4

5

(a)

6

pH

7

8

9

10

11

12

(b)

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Fig. 7. Flocculation and sedimentation performance of L2-CPA-4 under different pH conditions, (a)

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evolution of the transmittance with time; (b) comparison of the light transmittance at specified

-p

times.

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To further reveal the flocculation mechanism, the dispersion pattern of L-CPA in

lP

water was investigated. Although L-CPA seemed to dissolve in water easily and fast (Fig. 8a), the Marvin particle size test on L-CPA aqueous solution showed that the

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average particle size of L-CPA in water was around 200 nm (Fig. 8). TEM analysis

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verified that the L-CPA existed as micellar particles of around 50~100 nm in water (Fig. 9a & 9b). This was because that lignin was not soluble in neutral water, but after grafting with CPA, the hydrophilicity of lignin increased, leading to the fast dispersion of L-CPA with a relatively small particle size. Interestingly, when the L-CPA solution was naturally volatilized, the SEM image of the dried L-CPA powder disclosed that it formed uniform spherical particles with an average particle size of 1~2 μm (Fig. 9c), which was 5~10 times of that detected from the TEM and Marvin tests. This suggested that during the water evaporation, the small L-CPA micelles coalesced to form larger particles. The TEM and SEM analysis revealed that the

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lignin-grafted product L-CPA exhibited similar properties to amphiphilic block copolymers. It could self-assemble in water to form octopus-like hydrophilic micelles with hydrophilic CPA segments dissolved in water and hydrophobic lignin skeletons wrapped in the core part, as illustrated in Scheme 2.

L2-CPA-3 L2-CPA-4 L2-CPA-5

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L1-CPA-4 L3-CPA-4

0

100

200

re

-p

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Distribution (%)

(a)

300

400

500

600

lP

Size (d.nm)

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Fig. 8. The size distribution of L-CPAs. Inserted (a) is the photograph of L2-CPA-4 in water.

(a)

(b)

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(d)

Fig. 9. TEM images of L-CPA (a) in 200 nm scale and (b) in 100 nm scale. SEM images of (c)

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L2-CPA-4 and (d) floc.

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Scheme 2. Ilustration for the self-assembling of L-CPA.

Fig. 10. Diagram illustration of flocculation mechanism.

Fig. 9d shows the floc morphology from the kaolin suspension. The floc presented in an irregular shape with loosely coagulated kaolin particles of several micrometers. The flocculation model of L-CPA was elucidated in Fig. 10. After adding PAC into the kaolin suspension, the PAC coagulants could adsorb on the surface of kaolin particles due to the electrostatic adsorption. When the L-CPA

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solution was added, the electronegative lignin part in L-CPA could adsorb the electropositive PAC coagulants, and meanwhile, the covalently-connected cationic CPA chain segments could reach the negatively charged surface of kaolin particles, through which the L-CPA acted as bridges to connect the kaolin particles, leading to the shrink and subsequent collapse of flocs. However, when excess L-CPA was added,

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too strong electronegativity induced by lignin would enhance the repulsion between the kaolin particles and lignin-grafted flocculants, leading to reduced flocculation

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neutralization and bridging effect.

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efficiency. In one word, the flocculation of L-CPA was realized by charge

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4. Conclusion

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A highly efficient lignin-grafted cationic polyacrylamide flocculant (L-CPA) was synthesized via “grafting to” strategy. The linear prepolymer of cationic polyacrylamide (CPA) with chlorinated chain ends was firstly obtained by free radical polymerization

using

acrylamide

(AM)

and

methylacryloyloxyethyltrimethyl

ammonium chloride (DMC) as monomers and 4-chloromethyl styrene as the chain transfer agent. The CPA prepolymer was then grafted onto enzymatic hydrolysis lignin (EHL) via the reaction of chlorine with phenolic hydroxyl groups in lignin molecules. The L-CPA exhibited excellent flocculation capability for high turbidity kaolin suspension under faintly acid, neutral or alkalescence conditions (pH=5-9). Comparing with other lignin-derived flocculants, the L-CPA synthesized via “grafting

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to” strategy in this work required the minimum dosage, showing the best flocculation efficiency. It was demonstrated that L-CPA could self-assemble in water to form octopus-like hydrophilic micelles with CPA segments dissolved in water and hydrophobic lignin skeletons wrapped in the core part, which endowed the L-CPA with excellent flocculation efficiency. The charge neutralization and bridging effect

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was proposed for the flocculation mechanism of L-CPA. Lignin is a biological macromolecule, and importantly, lignin plays a crucial role in the flocculation

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performance of the grafted product L-CPA in this work. The key technique for the

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flocculation performance of L-CPA lies on the hyperbranched amphiphilic structure of

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lignin which allowed the L-CPA to self-assemble in water to form octopus-like

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hydrophilic micelles. As an environmentally friendly, cheap, nontoxic and technically

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feasible flocculant, the octopus-like L-CPA has a great prospect in waste-water

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treatment.

Acknowledgements

The authors gratefully thank the National Natural Science Foundation of China (21706082), the Science and Technology Program of Guangzhou (201707020025, 201804010140), and Natural Science Foundation of Guangdong Province (2017B090903003, 2018B030311052) for the financial support.

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Author Statement Chen Nian: Validation, Investigation, Data Curation, Writing-Original Draft Liu Weifeng: Conceptualization, Methodology, Project administration, Supervision, Writing-Review & Editing Huang Jinhao: Formal analysis, Visualization

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Qiu Xueqing: Resources, Supervision, Funding acquisition

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Highlights:  A novel flocculant of L-CPA was synthesized via “grafting to” strategy.  Linear prepolymer CPA was prepared by introducing CS as chain transfer agent.

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 The L-CPA synthesized in this work exhibited excellent flocculation efficiency for kaolin suspension under faintly acid, neutral or

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alkalescent conditions.