Multi-branch strategy to decorate carboxyl groups on cellulose

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Multi-branch strategy to decorate carboxyl groups on cellulose nanocrystals to prepare adsorbent/flocculants and Pickering emulsions Mei-Li Song, Hou-Yong Yu, Lu-Min Chen, Jiaying Zhu, YanYan Wang, Juming Yao, Zhuanyong Zou, and Kam Chiu Tam ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06671 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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ACS Sustainable Chemistry & Engineering

Multi-branch strategy to decorate carboxyl groups on cellulose nanocrystals to prepare adsorbent/flocculants and Pickering emulsions Mei-Li Song†, Hou-Yong Yu †*, ‡, §, Lu-Min Chen†, Jia-Ying Zhu†, Yan-Yan Wang†, Ju-Ming Yao†, Zhuanyong Zou‡, Kam Chiu Tam§*

†

The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of

Ministry of Education, College of Materials and Textile, Zhejiang Sci-Tech University, Xiasha Higher Education Park Avenue 2 No. 928, Hangzhou 310018, China. ‡

Key Laboratory of Clean Dyeing and Finishing Technology of Zhejiang Province, Shaoxing

University, 508# Huancheng West Road, Shaoxing 312000, P.R. China. §

Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of

Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1.

E–mail addresses: E-mail: [email protected]; [email protected]. *Corresponding authors. Tel: +86-571-86843618; Fax: +86-571-86843619; Tel: 1-519-888-4567x38339; Fax: 1-519-888-4347 1

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ABSTRACT: A simple multi-branch strategy was employed to increase the carboxyl contents on CNC surface. The effects of various sequential grafting of ascorbic acid or citric acid on the morphology, microstructure, thermal stability, dye adsorption capability (methylene blue) and coagulation–flocculation capacity (model Kaolin suspension) of the functionalized CNCs were investigated. Cellulose nanocrystals with multi-carboxyl groups (CNC-g-AA-g-CA) showed better thermal stability (Tmax=359.3oC), and possessed the highest carboxylic groups of 4.073mmol/g, which led to a high absolute zeta potential value up to 47.7 mV. Furthermore, the CNC-g-AA-g-CA exhibited excellent coagulation−flocculation capability to kaolin suspension with a turbidity removal rate of 91.07% and good cationic dye (methylene blue) removal rate of 87.8%, indicating that the CNC-g-AA-g-CA can be used as excellent adsorbent and efficient flocculants. Besides, CNC-g-AA-g-CA have good stabilizing effects on soybean oil/water Pickering emulsions and the resultant Pickering emulsion volume can remain for 30 days or longer. KEYWORDS: Cellulose nanocrystals, Multi-branch, Carboxyl content, Adsorption performance, Flocculation performance

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

INTRODUCTION Cellulose nanocrystals (CNCs) possess many excellent properties, such as high

crystallinity, high specific surface area, modulated size/morphology, more functional groups, biodegradability and biocompatibility1-6. These systems have good development prospects in many applications, such as adsorbents

6,7,

flocculants

emulsions 10, particularly in water treatment

6-9.

8,9

and green stabilizers for Pickering

It has been reported that the ideal stabilizers

for Pickering emulsions required high crystallinity index, smaller size11,12 and suitable charge density or charged surface groups, the CNCs with amphiphilic character can form stable emulsions by the interactions between the hydrophobic crystalline cellulose chains and oil, as well as the interactions between the hydrophilic carboxyl groups of CNCs and water13. With the rapid development of economy and population, a large number of textile enterprises have emerged, generating a large amount of waste dye solutions, and organic pollutants that could be harmful to human health

14.

If people drink contaminated water containing organic

pollutants, it can cause chronic poisoning, resulting in nausea, vomiting, memory loss, insanity, and may even lead to death

14.

Thus, an effective method to purify wastewater is

urgently needed. Currently, the main methods for treating wastewater consist of adsorbents, flocculants or using microorganism to degrade pollutants in water

15-17.

The common

flocculants, such as petroleum-based synthetic polymers can treat wastewater containing suspended particles, dyes as well as heavy metals, but it is not biodegradable, hence it is harm to soil, causing secondary pollution to the environment 18. Compared to common flocculants, the biodegradable CNCs with abundant functional groups can address these challenges in wastewater treatment. Due to the strong electrostatic adsorption between anionic carboxyl

3



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groups and cationic dye or suspended particles (namely charge neutralization mechanism) in wastewater, the carboxylated CNCs with charged groups are ideal adsorbents-flocculants for uses in wastewater treatment in textile enterprises, tannery and paper mill 4. Recently, carboxylated CNC with charged groups has been successfully fabricated via the various methods, such as ammonium persulfate (APS) oxidation13, two-step 2,2,6,6-tetramethylpiperidine-1-oxyl

(TEMPO)

radical

oxidation14,

periodate–chlorite

oxidation20,21, hydrochloric/nitric acid hydrolysis22 and citric/hydrochloric acid hydrolysis23. For ammonium persulfate (APS) oxidation, the morphology/size of the carboxylated CNCs could be controlled by the types of cellulose materials and reaction conditions, the yield carboxyl contents are low (carboxyl content was 0.91mmol/g) 13. Batmaz et al 14 reported that carboxylated CNCs fabricated by the two-step TEMPO oxidation possessed a higher carboxyl content of 2.1mmol/g, with an improved adsorption capacity for cationic dyes with a reported qmax of 769 mg dye/g CNC). However, only the C6 carboxylate groups were produced on the cellulose surface via TEMPO radical oxidation, and this limited the further oxidation of the CNC

surface.

Suopajärvi

et

al.20 successfully

prepared

dicarboxyl

CNCs

via

periodate–chlorite oxidation for the treatment of urban wastewater yielding a reduction in the turbidity of 40−80% (initial turbidity of 156−175 NTU). However, the periodate–chlorite oxidation required expensive, toxic periodate and a low carboxyl content of 1.75 mmol/g was reported

20,24,25.

In addition, the glycosidic ring will be cleaved by the the oxidation reaction,

which may reduce the length and stiffness of the molecular chains of the cellulose nanocrystals 26,27. Recently, carboxylated CNCs with intact glycosidic rings (carboxyl content was 1.75mmol/g) were prepared by citric/hydrochloric acid hydrolysis in our previous work 25

4


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and they displayed good adsorption performance for wastewater1,4,5,28. The carboxyl contents on CNC surface play a critical role in dye adsorption and flocculation capability in wastewater treatment. However, only limited research to increase the surface carboxyl group contents of these CNCs for wastewater treatment was reported. In this study, simple multi-branch strategy was employed to modify CNCs from microcrystalline cellulose (MCC), which endowed the CNC with a dendritic morphology containing more terminal functional groups yielding a higher carboxyl contents on the CNC surface. As expected, CNC-g-AA-g-CA with multi-branched structure was designed by the hydrolysis of MCC using hydrochloric acid as the catalyst with ascorbic acid (AA) and citric acid (CA) as the branching groups. The effects of various sequential grafting of ascorbic acid or citric acid on the morphology, microstructure, thermal stability, dye adsorption capability (methylene blue) and coagulation–flocculation capacity (model Kaolin suspension) of the functionalized

CNCs

were

investigated.

The

multi-branched

CNCs

with

super

adsorption/coagulation−flocculation capability, good reusability after alcohol-washing could address the limitations of expensive flocculant for wastewater treatment. In addition, another potential application in soybean oil/water Pickering emulsions with multi-branched CNCs were evaluated.

EXPERIMENTAL SECTION Materials. Commercial microcrystalline cellulose (MCC) was purchased from Sinopharm Chemical Reagent Co., Ltd. Hydrochloric acid was provided by Huadong medical Limited by Share Ltd. Citric acid(CA), anhydrous calcium chloride, anhydrous ethanol, tetrahydrofuran

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and N, N-dimethylformamide (DMF) were purchased from Hangzhou Gaojing Fine Chemical Industry Co., Ltd. Ascorbic acid (AA) was purchased from Aladdin biochemical Polytron Technologies Inc. Methylene blue (MB) was provided by Tianjin Yongda Chemical Reagent Co., Ltd. Dimethyl sulfoxide (DMSO) was purchased from Macklin company. Kaolin suspension (superfine size of 11 μm) was purchased from Aladdin Chemistry Co., Ltd. All the water

used

was

deionized

water.

The

carboxylated

CNCs

were prepared

via

citric/hydrochloric acid hydrolysis from MCC as reported previously4. Preparation of ascorbic acid grafted CNC (CNC-g-AA). The detailed grafting reaction process (Scheme 1) is described as follows: CNC-g-AA was fabricated by the condensation polymerization between carboxyl groups of carboxylated CNCs (3.0g) and hydroxyl groups of ascorbic acid (9.0g) under mechanical stirring (750 r/min) at 80 °C for 4 h with the addition of several drops of hydrochloric acid (6 M) as catalysis. Subsequently, the suspension was repeatedly washed by centrifugation with deionized water until its pH approached 7, and the CNC-g-AAs were freeze-dried for 48 h. Preparation of citric acid grafted CNC-g-AA (CNC-g-AA-g-CA). CNC-g-AA-g-CAs were prepared by the graft polymerization between hydroxyl groups of CNC-g-AA (3g) and carboxyl groups of citric acid with adding several drops of hydrochloric acid as catalyst (Scheme 1). After reacting at 80 °C for 4h, the CNC-AA-g-CA suspension was repeatedly washed by centrifugation with deionized water until its pH value approached 7. Finally, the CNC-g-AA-g-CA products were freeze-dried for 48 h.

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Scheme 1. Schematic preparation process of CNC, CNC-g-AA, CNC-g-AA-g-CA.



CHARACTERIZATION The surface morphology of carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA were

examined by field emission scanning electron microscopy (FE-SEM, JSM-5610, JEOL, Japan) with an acceleration voltage of 1.0 kV at room temperature. 0.01wt% carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA were used to prepared the SEM samples. The morphology was determined by analyzing more than 200 rod-shaped nanoparticles to calculate their average length, width and size distribution). Transmission Electron Microscope (TEM) of powdered CNCs, CNC-g-AA and CNC-g-AA-g-CA was examined by using a Transmission electron microscopy (Philips CM10) at an acceleration voltage of 60 keV. Infrared spectra were recorded at room temperature on a FTIR spectrometer (Nlcolet iS50, Thermo Electron Corp., USA). The freeze-dried samples were mixed with KBr (dried in an oven at 150 °C for 10 min) and analyzed over a spectral width of 4000-400 cm-1. 7



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The crystallinity and crystal characteristics of MCC, the carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA were determined using an X-ray powder diffractometer (XRD, ARL X’TRA, Thermo Electron Corp) with a monochromatic Cu Kα radiation at λ = 1.54056 Å (40 kV, 40mA) in the range of 2θ=5−80°with a scanning rate of 5° min−1. The XRD patterns of the samples were plotted using the obtained data to calculate the crystallinity using Eq. (1)29 I I  X  crystallinity  =  c a   100%  Ic 

(1)

where Ic is the peak intensity at 2θ=22.7° and Ia is the crystal strength of the amorphous phase at 2θ=18-19°. The crystal dimension of different planes of the samples were calculated according to the Scherrer equation (Eq. (2)): 4,29 D hkl =

K Bhkl cos 

(2)

where Dhkl is the crystalline dimension of the hkl crystal planes; K=0.94, λ represent the diffraction wavelength (λ=1.54 Å); and Bhkl is the full width at half-maximum (FWHM) of the reflection peak at the corresponding peak angle θ of the corresponding crystal plane. The zeta potentials of carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA in water (0.01 wt %) were determined using a Nano ZS Malvern Zetasizer. Three samples were prepared from the stock suspension of 2.5 mg/mL and diluted to 0.01wt%. The zeta potential was tested three times at 25oC to determine the average zeta potentials. The pH was adjusted from 3 to 10, the zeta potential of samples with CaCl2 and without CaCl2 was examined. Thermogravimetric analysis (TGA) were performed in the PyrisDiamond I, (PerkinElmer Corp.) under dynamic nitrogen atmosphere at a heating rate of 20 ° C / min over

8


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a test temperature range of 30 to 800 ° C. The thermal stability of the sample was determined by analyzing the TGA and drivative thermogravimetry (DTG) curves. The activation energy (Ea) was obtained by analyzing the TGA based on the Horowitz and Metzger relationship (Eq. (3)): 4,30

  Wo   Ea ㏑ ㏑   = 2   WT   RTS

(3)

where W0 is the initial weight of the samples; WT is the residual weight of the sample at temperature T; Ts is the temperature determined at 37.202% weight loss; θ is T-Ts; R is the gas constant. X-ray photoelectron spectroscopy (XPS) measured by K-Alpha (Thermo Fisher Scientific) operated in the constant analyzer energy (CAE) mode with a 1 eV of energy step size and an Al Ka X-ray source (1486.6 eV). In addition, the thermal stability of the samples was further studied by heating carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA samples at different temperatures. The samples on glass slides were placed in an oven and photographed at 100, 120, 140, 160, 180, and 200 oC, respectively (5 min for each temperature). Oil/water emulsions were prepared by first dispersing different amounts of CNC-g-AA-g-CA samples powder (0.1-1.2% w/v) in water at a pH of 3.0 (pH was adjusted using 1 M HCl). Oil was then added to the dispersion (10% v/v of final emulsion) to prepare the emulsions, and the mixtures were emulsified using a homogenizer (IKA T 18 D S25) for 8 min. The optical microscope (Nikon ECLIPSE TS100, Shanghai Puqi Photoelectric Technology co., LTD) was used to examine the microstructure of the emulsion, and imaged 9



Page 10 of 34

with a digital camera fitted to microscope. The average diameter of droplets was determined from more than 100 droplets for three separate emulsion samples. The emulsion storage stability was evaluated by recording the emulsion at 1 and 7 days, respectively. The carboxyl content was determined by conductivity titration. The dried sample (20 mg) was added to a 50 ml of 0.01M hydrochloric acid, and the mixture was stirred for 15h to yield a well-dispersed suspension. Then, 0.1 M NaOH was added dropwise into the mixture to adjust the pH to 7. The carboxyl content of the sample was determined from the conductivity and pH curves. The charge density of the CNC suspension was determined by the addition of a cationic polyelectrolyte. The degree of substitution (DS) for the samples by the electric conductivity titration according to Eq. (4), and from FTIR spectra according to Eq. (5) as follows33:

 COOH  = DS 

V 2  V 1 CNaOH w

0.5  COOH   10-3 (1-m  0.5  COOH   10-3 )

 n

n(V2  V1 )C NaOH 2000 w  m(V2  V1 )C NaOH

DS = 0.5(I1725 / I1060)

(4)

(5)

where the COOH is carboxyl group content of CNC samples, (V2-V1) is the volume of NaOH (mL) required to deprotonate the carboxylic acids groups, w is the mass of the samples, m, n were the values correspond to the molecular weight increase for the samples and the molecular weight of a backbone unit, respectively. The radio of the intensity of carbonyl peak (1725 cm-1) to that of the peak (1060 cm-1) related to backbone structure of cellulose can be calculated as the degree of substitution values (DS). The suspension stability test was carried out on the carboxylated CNCs, CNC-g-AA 10


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and CNC-g-AA-g-CA, and their dispersion in water was visually examined. The photographs of samples suspension (5 mg/mL) were taken at 2, 24, 96 hours, respectively. In addition, the dispersibility in other solvents was determined by observing the dispersion of carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA in DMSO, DMF and THF. The sample suspensions in different solvents were prepared by ultrasonic dispersion for about 30 min at below 5 oC, and pictures at 2 and 24 hours were recorded. The hydrophilicity of carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA was tested by measuring the water contact angle using the SL200B, USA Kino Industry Co., Ltd. A drop of water (10 μL) was placed on a CNC film (1 cm in diameter, 1.0 mm in thickness), and the contact angle was measured at a defined ambient temperature. Each sample was tested three times and its average was determined. Kaolin suspension (1000mg/L) was used to test the flocculation performance of carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA. First, the pH of the kaolin suspension was adjusted to 7 (using 0.1 M hydrochloric acid solution and 0.1 M sodium hydroxide solution). Then CaCl2 (30 mg) as coagulant was introduced to the 100 ml kaolin suspension under vigorous stirring (200 rpm for 3 min). Different dosages (20-80mg/L) of the CNCs were added to suspension under slow agitation (300 rpm for 7 min). After 1h, the turbidity of the supernatant was measured using a turb550 turbidimeter, and the influence of pH on the flocculation performance was evaluated. Dye

adsorption

experiments

for

the

carboxylated

CNCs,

CNC-g-AA

and

CNC-g-AA-g-CA were tested by adding 7.5 ml of dye (400mg/L) and samples suspension to 20 mL vials and then the mixture was stirred at 500 rpm for 30 min. The mixtures were

11



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diluted ten times, centrifuged at 7500 rpm for 10 min. The residual concentration of methylene blue (MB) in the supernatant was tested using an ultraviolet-visible spectrophotometer, which was determined by the absorbance of the corresponding dye molecules at the wavelength of 664 nm. The dye removal rate on the surface of samples determined using Eq. (6): 4 qe 

(Co  Ce)v m

(6)

where qe is the amount of dye adsorbed by 1g of samples (mg/g); Ce is the equilibrium concentration of the free dye molecules in the solution (mg/L); Co is the initial dye concentration (mg/L); V is the volume of the solution (L): M is the mass of samples (g). The influence of sample dosages (2.5-20 mg/mL) on the adsorbent performance was studied under the dye concentration of 500 mg/L, pH=7 and a temperature of 25 °C. In addition, the effect of different pHs (4, 5, 6, 7, 8, 9, 10) on the adsorbent performance were studied by maintaining dye concentration of 500 mg / L and 20 mg/mL of adsorbent at 25oC.



RESULTS AND DISCUSSION The morphological characterization and geometrical dimensions of the carboxylated

CNCs, CNC-g-AA and CNC-g-AA-g-CA was determined by SEM and TEM. In Figure 1, the carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA displayed typical rod-like morphology. The lengths of the carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA were 251.1, 282.5 and 309.4 nm, respectively (inserts of Figure 1). The diameter showed a slight increase with sequential surface modifications, which suggested that the CNC-g-AA and multi-branched CNC-g-AA-g-CA were successfully produced. Surprisingly, good dispersion of rod-like

12


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nanocrystals was found for multi-branched CNC-g-AA-g-CA, while the CNC-g-AA exhibited a slight agglomeration. This further supports that multi-branched CNC-g-AA-g-CA possessed more carboxyl groups than carboxylated CNCs and CNC-g-AA, resulting in stronger electrostatic repulsion between rod-like nanocrystals.

Figure 1 FE-SEM images of (a) CNC, (b) CNC-g-AA, (c) CNC-g-AA-g-CA and their corresponding TEM images (a’, b’, c’), (d) their corresponding length and diameter measured from FE-SEM images.

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Table 1. Aspect ratio, carboxyl content, conductometric DS, FTIR DS and Zeta potential for CNC, CNC-g-AA and CNC-g-AA-g-CA

Sample

Carboxyl content

Conductometric

FTIR

Zeta potential

（mmol/g）

DS

DS

（mV）

Aspect ratio

CNC

9.695±7.361

1.491±0.101

0.139

0.127

-33.3

CNC-g-AA

9.608±5.925

0.652±0.105

0.117

0.096

-23.0

CNC-g-AA-g-CA

9.321±5.630

4.073±0.203

0.952

0.813

-47.7

The chemical structure of MCC and carboxylated CNCs, CNC-g-AA and multi-branched CNC-g-AA-g-CA were analyzed by Fourier transform infrared spectroscopy (FTIR) as shown in Figure 2(a). It was observed that the peaks at 3341, 2898, 1429, 1376 cm-1 were attributed to the stretching vibration of O-H, and C-H, bending vibration of H-C-H and O-C-H and the deformation vibration of C-H, respectively, representing the characteristic peaks of cellulose29,32,33. These peaks did not change significantly after surface modification, indicating no obvious change in the chemical structure between CNCs and MCC. Compared to MCC, an absorption peak at 1725 cm-1 was observed for carboxylated CNCs, CNC-g-AA and multi-branched CNC-g-AA-g-CA, which was associated to the carboxyl groups from the grafting of AA and carboxyl groups of CA

28,29.

In Figure 2(c), a first decrease and then

increase in the normalized carboxyl peak intensity of the CNC samples were observed, meanwhile carboxyl group content was reduced from 1.491 mmol/g for the carboxylated CNCs to 0.652 mmol/g for CNC-g-AA, however it increased significantly to 4.073 mmol/g for multi-branched CNC-g-AA-g-CA (Table1). These observations confirmed the successfully sequential modification of the cellulose nanocrystals with AA and CA segments.

14


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Figure 2 (a) FT-IR spectra of MCC, CNC, CNC-g-AA and CNC-g-AA-g-CA; (b) FT-IR spectra in range from 1550 to 1800 cm-1; (c) Normalized peak intensities at 1725 cm−1; (d) XRD spectra of MCC, CNC, CNC-g-AA and CNC-g-AA-g-CA. Figure 2(d) depicts the XRD patterns of the carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA. Clearly, the XRD patterns of all the samples possessed the characteristic diffraction peaks of cellulose I crystal structure, such as peak at 14.9 °, 16.5 °, 20.5 °, 22.7 ° and 34.5 ° assigned to the typical reflectance of cellulose I at 1I̅ 0, 110, 012, 200, and 004, respectively 26,34. These results suggest that the acid hydrolysis and chemical modification did not alter their original morphology and crystal integrity. In addition, with sequential surface modification, no obvious change in the average crystallite size was observed, however the crystallinity was increased from 89.77% for cellulose nanocrystals to 91.19% for 15



Page 16 of 34

multi-branched CNC-g-AA-g-CA (Table 2), which showed that the crystalline transformation of CNCs occurred during the grafting reaction7, indicating the amorphous region of cellulose undergoes some damage again to form cellulose crystals during the reaction of high temperature and weak acid environment35, which causes the slightly increase of crystallinity of CNC-g-AA-g-CA. Table 2. Average crystallite size and crystallinity for the MCC, CNC, CNC-g-AA and CNC-g-AA-g-CA. Sample

X(crystallinity) (%)

D200 (nm)

D1Ī0 (nm)

D110 (nm)

MCC

82.23

6.9

5.4

6.8

CNC

89.77

6.4

4.8

6.0

CNC-g-AA

92.68

6.2

4.7

5.9

CNC-g-AA-g-CA

91.19

6.1

4.6

5.8

The graft reactions among the CNC, CNC-g-AA and CNC-g-AA-g-CA can be verified by using XPS technique. Figure 3 showed that XPS C 1 s region from XPS spectra of the CNC, CNC-g-AA and CNC-g-AA-g-CA were dominated by peaks C–O and C=O, centered at 287.1 and 288.3 eV, respectively29. The change of the peak (centered at 287.1 and 288.3 eV) area and the new peak C=C (centered at 284.2 eV) from AA segments appeared in Figure 3(c), (d), indicating their successful grafting reaction. Moreover, the C=O peak areas from carboxyl groups exhibited first reduction and then increase, further proving the highest contents of carboxyl groups for CNC-g-AA-g-CA samples.

16


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Figure 3 XPS (a) spectra of CNC, CNC-g-AA and CNC-g-AA-g-CA; C1 s of (b) CNC; (c) CNC-g-AA; (d) CNC-g-AA-g-CA. The TGA and DTG curves between 30 to 800 oC of carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA are shown in Figure 4. The initial decomposition temperature (T0) and maximum decomposition temperature (Tmax) are summarized in Table 3. Compared to T0 of MCC (315.1°C), carboxylated CNCs displayed slightly lower T0 (294.1 °C) due to the presence of unstable carboxyl groups that facilitate the degradation of glycosidic chains36. Moreover, T0 value of CNC-g-AA (313.0 °C) was higher than carboxylated CNCs, which might be related to the conversion of carboxyl to hydroxyl groups. However, the Tmax and T0 of the multi-branched CNC-g-AA-g-CA was 17.4 and 7.4 °C higher than that of CNC, respectively, which could be associated to the higher crystallinity. Moreover, similar trend in 17



Page 18 of 34

the Tmax and Ea of samples are shown in Table 3. Compared to the previously reported Tmax values of CNC samples (31% H2SO4 hydrolysis of ultrafine cellulose hydrolysis of cotton linter pulp Carex meyeriana Kunth

39,

38,

37,

64% H2SO4

TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized

APS (ammonium persulfate)-oxidized viscose fibers

hydrolysis of lyocell fibers

40),

13,

APS

the higher thermal stability was observed for the

multi-branched CNC-g-AA-g-CA reported in this study.

Figure 4 TGA (a), DTG (b) curve of the MCC, CNC, CNC-g-AA and CNC-g-AA-g-CA, and their Tmax values in this work and CNC samples produced by 31% H2SO4 hydrolysis of ultrafine

cellulose37,

64%

H2SO4

hydrolysis

of

cotton

pulp38,

linter

(2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized Carex meyeriana Kunth,

39

TEMPO

APS (ammonium

persulfate)-oxidized viscose fibers 13, APS hydrolysis of lyocell fibers 40 (c) Estimated value.

18


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During the melt extrusion or injection molding process, the polymer is usually subjected a higher temperature, and visual observation was used to evaluate the quality of the product4. As evidence from Figure 5, the thermal stability of the cellulose was tested by heating the samples in air, where the original state of the carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA were white at 100 °C to 140 °C. As the temperature was raised to 160 °C, CNC and CNC-g-AA displayed a slight yellowish color, while the color of CNC-g-AA-g-CA was still close to its original state. The CNC-g-AA-g-CA began to displayed a slight discoloration when the temperature exceeded 180 °C, and at 200 °C, the color of CNC and CNC-g-AA-g-CA was dark yellow and light yellow respectively. The results showed that the surface groups influenced the thermal stability of CNC nanomaterials, and the thermal stability of multi-branched CNC-g-AA-g-CA was enhanced. Table 3. Thermal parameters for the MCC, CNC, CNC-g-AA and CNC-g-AA-g-CA. Sample

T0 (oC)

Tmax (oC)

Ea (kJ· mol-1)

MCC

315.1

354.1

241.7

CNC

294.1

351.9

270.4

CNC-g-AA

313.0

379.0

367.1

CNC-g-AA-g-CA

311.5

359.3

318.9

19



Figure 5 Qualitative observations on the CNC, CNC-g-AA and CNC-g-AA-g-CA samples heated at different temperatures in air. The effects of zeta potentials for the different samples are shown in Table 1. Compared to carboxylated CNCs (-33.3mV), the absolute value of zeta potential of samples decreased initially for CNC-g-AA (-23.0mV) and then increased for CNC-g-AA-g-CA (-47.7 mV) with sequential surface modification. CNC-g-AA-g-CA displayed the most negative zeta potentials, indicating the ionization of and presence of more carboxyl groups. In general, the aqueous suspension dispersion stability of carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA was strongly dependent on the zeta potentials29,41. Thus, the suspension stability of CNC-g-AA-g-CA in water (pH=7) was superior to those of other samples (Figure 6 (a)). Aqueous suspension stability of MCC, CNC, CNC-g-AA and CNC-g-AA-g-CA with different pH values were tested (Figure S1), which showed that MCC, CNC, CNC-g-AA and CNC-g-AA-g-CA under neutral or alkaline conditions exhibited better aqueous suspension stability than acidic aqueous solution. In Figure 6 (b), CNC-g-AA-g-CA displayed good suspension stability after sonication in four polar solvents (H2O > DMSO > DMF > THF) for

20


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2 h. After 24 h, CNC-g-AA-g-CA maintained a good stability in these solvents except in THF. CNC-g-AA-g-CA had good dispersion stability in both water and organic solvents, which will be beneficial in some applications.

Figure 6 (a) Aqueous suspension (pH=7) stability of MCC, CNC, CNC-g-AA and CNC-g-AA-g-CA, (b) suspension stability of CNC-g-AA-g-CA in water, DMSO, DMF and THF. The

water

contact

angles

of

MCC,

carboxylated

CNCs,

CNC-g-AA and

CNC-g-AA-g-CA are shown in Figure 7, where MCC, carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA possessed a value of 43.05°, 33.14°, 0° and 0°, respectively. Due to more carboxyl groups, and smaller CNC, the hydrophilicity of CNC will be enhanced, hence it is expected that the water contact angle of CNC was smaller than MCC. In addition, CNC-g-AA and CNC-g-AA-g-CA showed the greatest hydrophilicity (0°), which was mainly due to the higher hydroxyl and carboxyl contents of CNC-g-AA and CNC-g-AA-g-CA. The greatest hydrophilicity would improve the interface compatibility between samples and hydrophilic polymers (polyvinyl alcohol, and polyurethane) 4,13.

21



Figure 7 Water contact angle for MCC, the CNC, CNC-g-AA and CNC-g-AA-g-CA. During the flocculation process, the turbidity of wastewater is usually dependent on flocculant and coagulant (CaCl2) content, and other environmental factors, such as pH and ionic strength4,42. To improve coagulation-flocculation performance, the dosage of the CaCl2 coagulant was selected at 300 mg/L since cationic CaCl2 particles displayed strong electrostatic adsorption for the anionic kaolin (simulated wastewater) through charge neutralization to induce the flocculation of suspended particles 4. Therefore, the effect of different dosages and environmental pH values of various CNC samples (carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA) on the turbidity of kaolin suspension were investigated (Figure 8(a) and (b)).

22


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Figure 8 Effect of (a) different dosages (pH=7) and (b) pH values on coagulation-flocculation property of CNC samples assisted with CaCl2 to kaolin (1000 mg/L); Zeta potentials (c) (without CaCl2), (d) (with CaCl2) of the CNC samples at different pH values. Clearly, the turbidity decreased gradually at 20-40 mg/L, and it was slightly increased at 40-80 mg/L, which indicated that the kaolin solution displayed the lowest turbidity at 40mg/L. Compared to CNC (81.71%) and CNC-g-AA (77.21%), the highest turbidity removal of 95.4% was reported for CNC-g-AA-g-CA, suggesting the better coagulation-flocculation capability due to more anionic carboxyl groups on its surface. Indeed, more anionic groups (e.g. carboxyl groups) on the CNC surfaces enhanced the electrostatic adsorption due to charge neutralization. Figure 8(b) shows a gradual reduction in turbidity removal of kaolin solution with coagulation-flocculation of all the CNC samples at the same dosage, where the 23



turbidity reduction was enhanced with increasing pH values. Moreover, at the same condition, CNC-g-AA-g-CA showed a higher turbidity reduction rate about 95.71% compared to CNC (88.57%) and CNC-g-AA (87.11%) at pH=10, indicating that more COO− ions formed by the deprotonation of carboxyl groups on the CNC-g-AA-g-CA at the alkaline environment for the charge neutralization. The zeta potential values of carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA were −33.3, −23.0, and −47.7 mV (Table 1) at pH=7, respectively. Thus, CNC-g-AA-g-CA could flocculate the suspended particles by charge neutralization due to more carboxyl contents of CNC-g-AA-g-CA (Table 1), providing more binding sites to flocculate the suspended particles.

Figure 9 (a) Dye removal rate and qe for the CNC; (b) CNC-g-AA; (c) CNC-g-AA-g-CA; (d) the dye removal rate at different pH with 17.5 mg / mL of samples.

24


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The textile production contained a variety of ionic or dispersed dyes, which needed to be removed in the wastewater treatment process. It has been reported that the dosage and pH were important factors to affect its adsorption performance4,20,21. Therefore, the effect of dosage and pH on the dye removal rate of methylene blue (MB) for carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA were examined (Figure 9). From Figure 9 (a)-(c), the dye removal rate of various CNC samples increased gradually with increasing dosage of CNC adsorbent. The dye removal rate of the CNC-g-AA-g-CA adsorbent approached a maximum value of 88.83% at 20 mg/mL, which was higher than 85.91% for CNC and 77.17% for CNC-g-AA, indicating that a large amount of carboxyl groups on the CNC-g-AA-g-CA surface provided more active sites for binding MB molecules. Interestingly, with the increase of adsorbent dosages, the qe of the sample decreased. As the ratio of CNC-g-AA-g-CA to dye molecules increased, the unsaturation degree of active sites of CNC-g-AA-g-CA increased. When the pH of solution was increased, the dye removal rate reached 95.01% at pH=10 due to the successive deprotonation of the adsorbents (carboxylated CNCs, CNC-g-AA and CNC-g-AA-g-CA) that greatly increased the electrostatic attraction between positively charged MB molecules and negatively charged surface of various CNC adsorbents. When pH was increased from 5.0 to 10.0, the adsorption capacity of CNC-g-AA-g-CA was exceeded 85%, showing that CNC-g-AA-g-CA was effective over a wide range of pH. In order to show another potential application, CNC-g-AA-g-CA was selected to prepare the CNC-stabilized Pickering emulsion based on the mixture of oil and water, and the effect of different CNC-g-AA-g-CA contents on emulsion droplet size, O/W emulsion stability were investigated. Figure S2 shows the good dispersion of CNC-g-AA-g-CA on the oil-water interface contributed to the stability of oil-in-water emulsions and relatively efficient surface coverage (Supporting Information). It has been reported that remained crystalline regions

25



Page 26 of 34

(high crystallinity) and surface hydrophilic groups render the CNC samples with amphiphilic characteristics toward oil/water4,13. The influence of CNC-g-AA-g-CA content on the storage stability was determined by visual inspection. From Figure S3 (a), we observed that the emulsion droplets displayed a good dispersion in water before storage. After 7 days (Figure S3 (b)), all the emulsion displayed various degrees of phase separation, although the emulsion volume of CNC-g-AA-g-CA was increased. The emulsion volume remained at 30 days

or

longer

(Figure S3

(c)),

suggesting

the

excellent

storage

stability

for

CNC-g-AA-g-CA-stabilized o/w emulsions. CONCLUSIONS The CNC-g-AA-g-CA with multi-branched structure (more carboxyl groups) was successfully extracted from hydrolysis of MCC by using hydrochloric acid as the catalyst, citric acid and ascorbic acid as the branching groups. The CNC-g-AA-g-CA with multi-branched structure possessed increased crystallinity, high carboxyl content and good suspension stability. In addition, compared to the carboxylated CNCs, CNC-g-AA-g-CA exhibited higher turbidity removal rate and dye removal, which was attributed to more surface carboxyl groups, where the CNC-g-AA-g-CA provided more active sites for charge interactions between the dye molecules and the flocculant. The multi-branched cellulose nanocrystals could potentially be applied in many fields, such as wastewater treatment, food emulsion and medical applications. 

ASSOCIATED CONTENT

Supporting Information The images showing the suspension stability of CNC, CNC-g-AA, CNC-g-AA-g-CA, under acidic aqueous condition pH=2 of different time (a0h), (a24h), (a96h); pH=5 of different time

26


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(b0h), (b24h), (b96h); under alkaline aqueous condition pH=10 of different time (c0h), (c24h), (c96h); pH=13 of different time (d0h), (d24h), (d96h) (Figure S1). Optical microscopic images of emulsions with (a) 0.1, (b) 0.2, (c) 0.4, (d) 0.8, (e) 1.2 w/v% of CNC-g-AA-g-CA (Figure S2). Photographers of emulsions (oil:water = 1:9) after storage for (a) 0 day, (b) 7 day, (c) 30 day (CNC-g-AA-g-CA concentrations from left to right were 0.1, 0.2, 0.4, 0.8 and 1.2 w/v%, respectively) (Figure S3). 

AUTHOR INFORMATION

Corresponding Author *Hou-Yong Yu (H.Y. Yu); Tel.: 86 571 86843618; E–mail addresses: [email protected]. *Kam Chiu Tam; Tel: 1-519-888-4567x38339; Fax: 1-519-888-4347. E–mail addresses: [email protected]. ORCID Hou-Yong Yu: 0000-0002-6543-5924 Kam Chiu Tam: 0000-0002-7603-5635 Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENTS The work is supported by international cooperation of Prof. Jaromir Marek and Key

Program for International S

&T Innovation

Cooperation

Projects of China [2016YFE0131400], Opening Project of Key Laboratory of Clean Dyeing and Finishing Technology of Zhejiang Province [Project Number: 1811], Candidates of Young and Middle Aged Academic Leader of Zhejiang Province, “521” Talent Project of Zhejiang Sci-Tech University, and the Young Elite Scientists Sponsorship

27



Program by CAST. The research funding from CelluForce and FP Innovations facilitated research on CNC. K.C.T. wishes to acknowledge funding from CFI and NSERC. 

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For Table of Contents Use Only

Simple multi-branch strategy was employed to increase carboxyl contents on CNC and their promising applications as adsorbent/flocculants and Pickering emulsions.

34


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Multi-branch strategy to decorate carboxyl groups on cellulose

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