High Aspect Ratio Carboxylated Cellulose Nanofibers Cross-linked to

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Functional Nanostructured Materials (including low-D carbon)

High Aspect Ratio Carboxylated Cellulose Nanofibers Crosslinked to Robust Aerogels for Superabsorption -flocculants: Paving Way from Nano-Scale to Macro-Scale Duan-Chao Wang, Hou-Yong Yu, Xuemeng Fan, Jiping Gu, Shounuan Ye, Juming Yao, and Qing-Qing Ni ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04211 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Applied Materials & Interfaces

High Aspect Ratio Carboxylated Cellulose Nanofibers Crosslinked to Robust Aerogels for Superabsorption−flocculants: Paving Way from Nano−Scale to Macro−Scale Duanchao Wang†, Houyong Yu*†‡, Xuemeng Fan†, Jiping Gu†, Shounuan Ye†, Juming Yao†, Qingqing Ni†§ †

The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of

Ministry of Education, National Engineering Lab for Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech University, Xiasha Higher Education Park Avenue 2 No.928, Hangzhou 310018, China ‡

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,

Donghua University, Shanghai 201620, China §

Department of Mechanical Engineering & Robotics, Shinshu University, Tokida, Ueda

386−8576, Japan *Corresponding author. Tel: 86 571 86843618; fax: 86 571 86843619. E−mail addresses: [email protected] (Houyong Yu)

Keywords: cellulose nanofiber, aerogel, physical crosslinking, mechanical properties, absorption−flocculants, copper ion adsorption.

Abstract: Charged nanocellulose (NC) with high aspect ratio (larger than 100) extracted from animal or bacterial cellulose and chemical crosslinked NC aerogels have great promising applicability in material science, but facile fabrication of such NC aerogels from plant cellulose by physical crosslinking still remains a major challenge. In 1

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this work, carboxylated cellulose nanofiber (CNF) with highest aspect ratio of 144 was extracted from wasted ginger fibers by a simple one−step acid hydrolysis. Our approach could easily make the carboxylated CNF assemble into robust bulk aerogels with tunable densities and desirable shapes on a large scale (3D macropores to mesopores) by hydrogen bonds. Excitingly, these CNF aerogels had better compression mechanical properties (99.5 kPa at 80% strain) and high shape recovery. Moreover, the CNF aerogels had strong coagulation−flocculation ability (87.1%), removal efficiency of MB dye uptake (127.73 mg/g) and moderate Cu2+ absorption capacity (45.053 mg/g), which were due to assistance mechanisms of charge neutralization, network capture effect and chain bridging of high aspect ratio carboxylated CNF. This provided a novel physical crosslinking method to design robust aerogels with modulated networked structures to be a general substrate material for industrial applications such as superabsorbent, flocculation, oil−water separation, and potential electrical energy storage materials.



INTRODUCTION

Porous ultralightweight aerogels with extremely low density and low thermal conductivity have a wide range of applications in insulation, catalyst carriers, tissue engineering, and battery electrodes, which was obtained by using freeze−drying and thus replace liquid components of the gel with gases 1. However, aerogels based on silica and other inorganic nanoparticles often show fragility to limit their applications 2-3. NC aerogels extracted from renewable cellulose are believed as an economic and eco−friendly alternative to replace them, due to controlled surface chemistry, excellent mechanical strength and most abundant cellulose raw material in the world. As we know, NC can be classified as cellulose nanocrystals (CNC), cellulose nanofibers (CNF) and bacterial cellulose (BC) 4, their larger aspect ratio generally induce stronger rigidity or 2


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mechanical strength of NC. As a result, the CNF and BC with larger aspect ratio was used to fabricate high−strength aerogels. But CNC 5-6 usually has low length (500 nm) were not detected due to technical limitations.

Table 1. Average Dimensions, Carboxyl Contents and Zeta Potential of Samples Dimensions(nm) Carboxyl

Zeta

Content

Potential

(mmol/g)

(mV)

0

−21.7 ± 5.2

Aspect

SSA

Ratio

(m2/g) a

S−CNF 2540 ± 350 30.5 ± 15.1

83

85.26

9−CNF 3720 ± 600 34.4 ± 13.3

108

102.24

1.17 ± 0.10 −34.6 ± 3.2

7−CNF 3690 ± 550 25.6 ± 10.2

144

139.57

1.18 ± 0.10 −36.0 ± 3.0

5−CNF 3070 ± 530 25.3 ± 10.4

121

146.62

1.13 ± 0.10 −32.6 ± 3.1

Sample

a

Length

Diameter

SSA: specific surface area.

In addition, Figure 2(a1−2) show a larger fiber block in S−CNF sample, which may be due to the residual fruit gum, lignin and hemicellulose in untreated ginger fibers to hinder further hydrolysis via sulfuric acid. However, uniform interlaced fiber network with free−bump was observed at 9−CNF (Figure 2(b1−2)), 7−CNF (Figure 2(c1−2)) and 5−CNF (Figure 2(d1−2)), they were very different from the NC extracted from other plants

4

and this broad range of nanofiber regularity was believed to provide

excellent compressive resistance of CNF aerogels (discussed below). It indicated that C6H8O7/HCl mixed acid hydrolysis can extract CNF with high aspect ratio, compared 8


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with traditional sulfuric acid method. Moreover, according to FE−SEM images, the ratio of mixed acid significantly affected the morphologies and dimensions of CNF. The lower ratio of C6H8O7/HCl generally resulted in shorter nanofiber for 5−CNF. In order to determine clear dimensions of the CNF, Atomic Force Microscopy (AFM) images were used to observe different CNF samples (Figure 2e and Supporting Information Figure S2), and obtained aspect ratio data was recorded in Figure 2f and Table 1. Clearly, CNF prepared by mixed acid exhibited larger aspect ratio than that of S−CNF. Especially, well dispersed 7−CNF (Figure 2e−f) had the highest aspect ratio of 144. In general, organic citric acids are less acidic and inorganic hydrochloric acids are more acidic. As the proportion of organic citric acid rose, the inorganic hydrochloric acid content dropped to only 10%, which led to an increase in the diameter of the 9-CNF. But the average length of 9-CNF closed to 7-CNF, resulting in the highest aspect ratio for 7-CNF among all the CNF samples. Figure 4 shows the carboxylated CNF aspect ratio was also larger than BC and NC from other groups

21-30

. It should be noted that

mixed acid hydrolysis could also give CNF more carboxyl groups to improve CNF dispersion, which can be supported by the results from zeta potential and carboxyl contents (see below).

9



Figure 2. FE−SEM images of aerogels pore structure and high magnification for (a1−2) S−CNF, (b1−2) 9−CNF, (c1−2) 7−CNF, (d1−2) 5−CNF, (e−f) AFM images and statistical data of dimensions for 7−CNF (S-CNF, 9-CNF, 5-CNF images in Supporting Information Figure S2). AFM images size: 10μm×10μm, 5μm×5μm.

Figure 3. Barrett–Joyner–Halenda (BJH) pore size distribution of aerogels.

10


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Figure 4. Aspect ratio of CNF samples in this work, BC−1

21

, BC−2

22

, acetobacter

xylinum* 23, cotton stalks 24, banana peels 25, α-cellulose powder* 26, commercial CNF* 27

, banana waste pulp* 28, wood 29, wheat straw 30. * Estimated value.

Chemical Structure and Suspension Stability. CNF as absorption−flocculation agents require high surface charge (groups) and long polymer chains or large aspect ratio, which could enhance charge neutralization ability and bridging effect of CNF with dye particles 31. Thus, the surface functional groups of CNF prepared by mixed acid and sulfuric acid hydrolysis were tested by Fourier transform infrared (FTIR) spectroscopy. Figure 5a shows characteristic peaks of cellulose in the FTIR spectra, such as O−H stretching vibration at 3341 cm-1, C−H tensile vibration at 2898 cm-1, H−C−H and O−C−H internal bending vibration at 1429 cm-1, and C−H deformation vibrations at 1376 cm-1

32

. For S−CNF, peaks at 1238, 1183 and 661 cm-1 were assigned to

asymmetric and symmetrical stretching of S=O, S−O stretching vibration, respectively. It clearly indicates the presence of sulfuric acid ester groups (−SO3H) on the S−CNF due to the occurrence half−ester reaction of sulfuric acid with cellulose 33. However, a new ester carbonyl peak at 1735 cm-1 appeared in the carboxylated CNF, indicating that 11



an esterification reaction took place between hydroxyl groups on NC and carboxyl groups of citric acid to produce carboxyl groups (−COOH) on the prepared CNF surface. Further, normalized intensity of the carboxyl band (1735 cm-1) （Figure 5b） shows that high mixed acid ratio can cause stronger peak intensity of carboxyl groups in CNF sample, which were also confirmed by test results from carboxyl content titration (conductivity method) and zeta potential (Table 1). The 7−CNF showed the highest carboxyl group content of 1.18 mmol/g and largest negative zeta potential of −36.0mV. Indeed, more carboxyl groups of NC would lead to larger surface charge density and absolute zeta potential value. From above detailed results, the C6H8O7/HCl=7/3 was the optimum preparation condition to get CNF with ultra−high aspect ratio and carboxyl groups. Moreover, the 7−CNF as model sample showed robust absorption−flocculation effect of kaolin and dyes (MB and MO) in stimulated wastewater.

Figure 5. (a) FTIR spectra of S−CNF, 9−CNF, 7−CNF and 5−CNF, (b) normalized peak intensities of 9−CNF, 7−CNF and 5−CNF.

Table 2. Density, Porosity, Water Absorption Capacity and Thermal Parameters of Aerogels

12


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Sample

Density 3

(mg/cm )

Absorption

Porosity (%)

Capacity (g/g)

T0 (oC) Tmax (oC)

S−CNF

25.6 ± 9.6

90.85

16.58 ± 3.65

132.1

199.0

9−CNF

23.3 ± 9.1

98.81

29.94 ± 2.06

169.2

338.8

7−CNF

20.3 ± 6.2

98.40

27.62 ± 2.11

180.7

345.7

5−CNF

19.0 ± 8.3

98.17

19.22 ± 1.98

158.6

209.4

Aerogel Structural and Mechanical Properties. Excitingly, these carboxylated CNF with high aspect ratio were physically crosslinked to aerogels after freeze−drying (Insert photos in Figure 6a), because high aspect ratio of CNF allowed them to entangle with each other. On the other hand, more hydrogen bonds between carboxyl groups of CNF and aligned or crosslinked CNF could strengthen 3D network structures of aerogels. These CNF with low density and high porosity did not require additional chemical modification to be self−crosslinked into aerogels with high mechanical properties and excellent shape recovery (Figure 6a−c). The low density CNF aerogels could be tailored within 10.7 to 32.4 mg/cm3 according to weight ratio of CNF suspensions, meanwhile the aerogels owned high porosity of more than 98 % and high water absorption capacity of 16.58−29.94 g/g. Shape recovery ability of CNF aerogels was evaluated by cyclic compression tests after 5 cycles in air, and they could keep ∼80% of their original height after being compressed 20%, but the shape recovery ratio reduce with increasing compressive strain to 80%. Hydrogen bonds provided a "temporary shaping" effect. After deformation under external force, the original hydrogen bonds were destroyed and new hydrogen bonds were formed at the new position. These newly generated hydrogen bonds could temporarily resist rebound, and when they accumulated to a certain amount, 13



the aerogel would recover a certain shape ratio. However, when the high deformation occurred, the macropores inside the aerogel were destroyed, and only new hydrogen bonds could no longer recover a large proportion of the shape 34.Surprisingly, according to digital photos the aerogel did not break during 80% compression and showed good integrity (Figure 6c). Also, the final ultralightweight CNF aerogel can stand on the top of a Green bristlegrass without even bending the grass burr or heads (insert in Figure 6b). More importantly, the compressive stresses of carboxylated 7−CNF aerogel were about 99.5 kPa at 80% strain and only 20.3 mg/cm3, which were better than other works (Figure 7). Especially, 7−CNF aerogels showed the highest compressive stress of 241.77 kPa at 90% strain than other CNF aerogels, suggesting that higher aspect ratio, broad range of nanofiber regularity, entanglement/crossed structure and carboxyl groups were indeed beneficial to highly structural stability of robust CNF aerogels for widely potential applications. The possible mechanical enhancement mechanism for the high aspect ratio carboxylated CNF aerogels is shown in Figure 6d.

14


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Figure 6. (a) Shape recovery ratio of CNF aerogels under different compressive strains (insert photos of four kind CNF aerogels), (b) compressive stress−strain curves of aerogels from 0 to 95% strain in air (insert photo of 7−CNF aerogel standing on top of Green bristlegrass), (c) Photos of compressed aerogels 0% to 80% strain, (d) possible microscopic crosslinking mode and mechanical enhancement mechanism of carboxylated CNF aerogels.

15



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Figure 7. An overview of compressive stress vs density of various CNF aerogels reported in the literatures. Hexagons: 1, CNF aerogels aerogels

35

13

. 2, CNF aerogels

. 4, PAN−Si aerogels 2. 5, PVA/CNF aerogels

PVA/CNF/GONS aerogels

38

36

19

. 3, CNF

. 6, NFC aerogels

. 8, hydrophobic modified CNF aerogels

39

37

. 7,

. T, this work

7−CNF aerogels.

Thermal stability. After the dyeing process, the most wastewater temperature from textile enterprises is in the range of 30−200 oC, thus thermal stability of CNF aerogels is an important parameter for potential adsorbents−flocculants. Figure 8a−b show the thermo−gravimetric analysis (TGA) and differential thermogravimetric (DTG) curves, T0 and Tmax are listed in Table 2. All the CNF samples gave two degradation peaks, but the degradation peaks moved to high temperature region compared to S−CNF. The T0 and Tmax of the S−CNF aerogels were 132.1oC and 199.0oC, respectively. The T0 and Tmax of the CNF prepared by the mixed acid method were in the range of 158.6−180.7 o

C and 209.4−345.7oC, respectively. The carboxylated CNF aerogels prepared by

C6H8O7/HCl mixed acid method showed better thermal stability than the CNF prepared by sulfuric acid method. 7−CNF exhibited the best thermal stability among the 16


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carboxylated CNF samples, the Tmax increased by 345.7 oC compared to 199.0 oC for S−CNF, 240−300.0 oC for other carboxylated CNF cellulose−silica nanocomposite aerogels

41

40

and 315 − 346.0 oC for

. Indeed, S-CNF prepared by sulfuric acid

hydrolysis can be thermally degraded because less energy was needed to eliminate the sulfuric acid in sulfated anhydroglucose units. During the process, it was released at a lower temperature, which ultimately reduced the degradation temperature of S-CNF significantly. The relatively high thermal stability of the carboxylated CNF benefited from lower degree of damage to the crystalline region by mixed acid method

32

. This

suggests that the CNF prepared by this mixed acid method have a significant advantage in thermal performance compared to S−CNF, and suitable C6H8O7/HCl mixed acid can be used to obtain the high thermal stability of CNF.

Figure 8. (a) TGA and (b) DTG curves of S−CNF, 9−CNF, 7−CNF and 5−CNF aerogels.

Figure 9a shows that the dispersion stability of 7−CNF after standing 4 hours was significantly higher than other carboxylated samples and S−CNC, due to strong repulsive interaction between charged carboxyl contents. Moreover, the surface of the CNF prepared by the mixed acid method had a large number of functional carboxyl groups (Figure 9b), which might have excellent adsorption (adsorption-flocculation) capability to kaolin, methylene blue (MB), methyl orange (MO), and copper ions, and 17



these adsorption (adsorption-flocculation) properties will be evaluated in detail below (Figure 9c).

Figure 9. (a) Photos of CNF suspension (0h to 4h), coagulation−flocculation of kaolin and absorption effect on dyes, (b) schematic and mechanism of synthetic steps, (c) possible mechanisms of carboxylated CNF aerogels for kaolin flocculation, adsorptions of dyes (MB, MO) and heavy metal ions.

Coagulation−Flocculation Behavior. This study carboxylated CNF samples with surface carboxyl groups can lose H+ in aqueous solution to be negatively charged. They can be used as flocculation agents to treat wastewater with CaCl2 as assisted coagulant that can effectively reduce the turbidity of the wastewater 42. Figure 10a shows that the coagulation−flocculation treatment of model wastewater (500 mg/L kaolin suspension) with CNF samples were tested, the turbidity of the kaolin suspension reduced from 930 18


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to 121 NTU with addition of only 300 mg/L CaCl2 after half an hour. Clearly, cationic CaCl2 particles can induce electrostatic absorption with anionic kaolin through charge neutralization mechanism. For saving cost, dosage of coagulant was chosen at 50 mg/L CaCl2 to study coagulation−flocculation effect of various dosages of CNF (2−6 mg/L) on the turbidity of the kaolin suspension at pH=7. It was found that the residual turbidity of kaolin suspension decreased with the increasing CNF dosages to 4 mg/L, and did not increase at 6 mg/L. 6 mg/L of 7−CNF could greatly decreased residual turbidity from 386 NTU to 47.7 NTU (Figure 10b), indicating most efficient flocculation ability to kaolin suspensions. Moreover, at the same dosages, the carboxylated CNF showed higher turbidity reduction efficiency of 85.6−87.1% than 73.6% for S−CNF, 40−80% for 10−50 mg/L nanofibrillated dicarboxylated cellulose with 25 mg/L coagulant (ferric sulphate) cellulose with alum (62.5 mg/L) (PAC),

close

to

91%

for

44

43

, 55.5−72.2% for anionic dialdehyde

and 79% for commercial polyaluminum chloride

CPAM

flocculant

42

.

The

above

excellent

coagulation−flocculation effect of the carboxylated CNF might be due to charge neutralization mechanism caused by electrostatic absorption among anionic CNF carboxyl groups, anionic kaolin and cationic CaCl2 particles, bridging and network capture mechanism induced by high aspect ratio. The more carboxyl or sulfate groups of CNF can provide more absorption sites to flocculate more suspended particles through three above mechanisms. The zeta potential values of CNF samples (Table 1) were consistent with surface groups and flocculation effect.

19



Figure 10. Coagulation performance of (a) CaCl2 to the kaolin and (b) coagulation−flocculation property of CNF with different dosages assisted with CaCl2 to kaolin suspension.

Dye absorption of MB. There are many textile enterprises in the world, while the resultant industrial effluents/wastewater involves not only suspended particles (kaolin), many kinds of dyes are also existed in the wastewater. Adjusting the CNF dosage to test its percentage of decolorization (dye removal) can predict the amounts and cost of adsorbent needed per unit of the treated dye solution. So the effect of the dosages (2−16 mg/mL) of CNF on MB removal rate and dye uptake (qe) was studied under optimized conditions of pH=7. Figure 11a−d show the increase of dye removal rate with the increasing CNF dosages and finally occurrence of the absorption plateau at 12 mg/mL. 7−CNF performed the best absorption properties of 94.55% dye removal and 127.73 mg/g dye uptake that higher than 85.5% and 110.11 mg/g for 9−CNF, 91.1% and 119.33 mg/g for 5−CNF, 52.55% and 14.56 mg/g for S−CNF. 7−CNF with availability of more unsaturated binding sites can absorb the most active dyes because of its surface most carboxyl groups and biggest surface area (Table 1). The amount of carboxyl groups can provide more active absorption sites, so in the same amount can have better dye 20


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removal ability. However, the decreased dye uptake (qe) with increasing CNF dosages were due to unsaturated CNF carboxyl groups as active adsorbent sites resulted from the ratio of CNF/dye molecules increased 45.

Figure 11. Dye removal and uptake (qe) of (a) S−CNF, (b) 9−CNF, (c) 7−CNF and (d) 5−CNF.

Generally, industrial dye wastewater is not neutral and environmental pH affects the protonation or deprotonation process of −COO− and −OSO3− on the CNF surface and thus affect dye removal capacity. Figure 12 gives a slight increase in dye removal of all the CNF samples with increasing pH value from 4 to 10, because the alkaline conditions favored the protonation of more carboxyl group and increase the active absorption sites 46

. At the same pH value, 7−CNF exhibited a higher dye removal of 50−92% than other

CNF samples. Especially, all the carboxylated CNF samples exceeded 80−90% of MB 21



Page 22 of 35

dye removal at pH=10, which were larger than 38% for S−CNF due to low surface area and sulfate groups. The carboxylated CNF with wide pH range (pH=4−10) was applied in the wastewater treatment of textile/dye and municipal effluents.

Figure 12. 3D plots of effect of different pH values on MB dye removal for CNF samples.

Absorption Kinetics Study. The absorption rate and absorption mechanism were generally studied by absorption kinetics based on the curve of dye absorption capacity with increasing absorption time. As shown in Figure 13a, qt increased fast during the initial absorption time less than 60 min, and then slowly improved from 60 to 120 min, and finally reached equilibrium about 120 min. qt was almost no increase after 120 min. Unfortunately, compared to carboxylated CNF fabricated by mixed acid method, S−CNF showed lower absorption capacity of 36.969 mg/g with increasing absorption time from 0 to 240 min, hinting slow absorption rate. Moreover, two dynamic models of pseudo−first−order

(Supporting

Information

Eq.4)

and

pseudo−second−order

(Supporting Information Eq.5) were used to further explore absorption mechanism according to these absorption data 47-48. 22


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The data in Figure 13a are linearly fitted using Eq. 5 and Eq. 6 (Supporting Information) to obtain Figure 13b (pseudo−first−order model fitting curve) and Figure 13c (pseudo−second−order model fitting curve). The relevant kinetic parameters are calculated and recorded in Table 3. By comparing R2 (correlation coefficient), the CNF absorption kinetics of this study was closer to pseudo−second−order kinetics. Then the theoretical qe values were calculated from the pseudo−second−order model, which were basically consistent with the experimental equilibrium absorption capacity of absorbed MB (127.73 mg/g). Therefore, we can conclude that the MB absorption onto CNF was mostly controlled by the chemical absorption likely ascribed to exchange or sharing of electrons between cation groups of dye and anion carboxyl or sulfate groups of CNF samples

47

. Furthermore, Figure 14 shows that carboxylated CNF in this work had

higher maximum theoretical qe of 132.979 mg/g than CNF extracted from other works 46, 49-50

.

Figure 13. Comparison of absorption kinetic models of MB on different CNF: (a) effect 23



Page 24 of 35

of contact time on dye absorption of CNF, (b) pseudo−first−order model and (c) pseudo−second−order model.

Table 3. Absorption Kinetic Parameters for MB Absorption of CNF. Models

Pseudo−first−order

Pseudo−second−order

Parameters

S−CNF

9−CNF

7−CNF

5−CNF

R2

0.92133

0.90400

0.84773

0.89028

K1(min 1)

0.0179

0.0296

0.0313

0.0296

qe(mg/g)

21.905

50.726

28.214

35.354

R2

0.99172

0.99948

0.99956

0.99942

K2(g/mg/min)

0.0019

0.0028

0.0075

0.0072

qe(mg/g)

36.969

124.224

132.979

126.582

−

Figure 14. Maximum theoretical qe of CNF in this work, biochar

49

, macroalga

46

,

pistachio 50, luffa 51, cotton 52 and quince seed mucilage (QSM) 53.

Dye removal of MO. To determine relationship among network capture mechanism, bridging function of physical absorption/flocculation and carboxylated CNF high aspect ratio, the anionic MO as model dye was utilized to study uptake capacity of 24


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carboxylated CNF through only network capture mechanism since the anionic CNF had no electrostatic absorption with anionic MO. Excitedly, the carboxylated CNF with high aspect ratio showed relatively high MO uptake capacity of 38−47% (Figure 15a), indicating good network capture ability and bridging function of MO dye. It suggests that such ginger CNF with high aspect ratio have great potential applications in robust flocculation in the future.

Figure 15. Effect of CNF dosage on (a) MO dye removal and (b) absorption capacities of different CNF for Cu ions. (c) Cu ion absorption capacity in this work, macroalga 54, activated carbon

55

, commercial biochar 56, cellulose acetate

graphene oxide

59

57

, metal ion adsorbent

58

,

and ion-imprinted poly(polyethylenimine/hydroxyethyl acrylate) 25



hydrogel 60.

Heavy metal ion (Cu2+) absorption capacity. Ideal adsorbents−flocculants generally required adsorb high concentrations of heavy metals from textile/dye and municipal effluents. Figure 15b clearly found that carboxylated CNF had a strong absorption capacity of heavy metal ions (Cu2+ as model ion), whereas S−CNF showed lower absorption capacity of 11.168 mg/g. Among the CNF samples, the 7−CNF exhibited highest Cu2+ removal rate of 90.11% and absorption capacity of 45.053 mg/g than 80.04% and 40.019 for 9−CNF, 59.73% and 29.863 mg/g for 5−CNF. This can be explained that more metal−binding functional groups (carboxyl groups) contents on the 7−CNF was deprotonated to bring their own negative charges to absorb Cu2+ ions in the stimulate wastewater. In Figure 15c, compared with other literature works, this work carboxylated CNF had better Cu2+ absorption than CNF extracted from other works 54-60. One-step as-produced CNF extracted from the biomass waste showed excellent absorption capacity in heavy metal ion, and its application to environmental protection was undoubtedly an important contribution to the sustainable development of nature. From Figure 16, an interesting phenomenon was found that CNF aerogel (7−CNF aerogel as model sample) could absorb MB dye from the bottom of the silicone oil. CNF aerogel did not show absorption when it was just in contact with the silicone oil. However, when it came into contact with MB dye, a rapid adsorption phenomenon occurred, and eventually MB dye was removed from the bottom of the silicone oil, indicating good ability of CNF aerogel to separate oil and water.

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Figure 16. Digital photos of the removal of methylene blue (100 mg/L) below silicone oil with the 7−CNF aerogel.



CONCLUSION In summary, this study had completed a comprehensive and systematic research on

the impact of NC hydrolysis conditions, aspect ratio, and surface charge on their performance. We used an innovative method of C6H8O7/HCl mixed acid hydrolysis to prepare carboxylated CNF aerogels with 3D network structure from macropores and mesopores, where these carboxylated CNF had high aspect ratios of 144, aligned in a large area and abundant surface carboxyl groups, enabling them to entangle or physically crosslinked into aerogels with robust compressive strength and high shape recovery. Physically crosslinked CNF showed excellent mechanical properties over chemically crosslinked NC and other traditional aerogels. Especially, 7−CNF aerogels have better mechanical compressive properties (99.5 kPa at 80% strain, 241.77 kPa at 90% strain) at low density of 20.3 mg/cm3. At the same time, 7−CNF aerogels also had strong coagulation−flocculation ability (87.1%), high MB dye uptake (127.73 mg/g), excellent Cu2+ absorption capacity (45.053 mg/g) and fast oil−water separation capability. We focus on studying the high aspect ratio and rich carboxyl groups of CNF improve the mechanical properties and absorption properties of “all cellulose” aerogels, and first obtain carboxylated CNF based 3D aerogels with adjustable density, robust mechanical properties and ideal shape from nano−scale to macro−scale, which will 27



provide broad prospects in both scientific research and commercial potential production/applications in wide field of gas collector, supercapacitors, absorbents, separators and pressure sensors.



ASSOCIATED CONTENT

Supporting Information Characterizations of FE−SEM, TEM, AFM, FTIR spectroscopy, TGA, zeta potential test, density, SSA and pore size distribution, porosity, water absorption capacity, compression performance test, carboxyl content analysis, flocculation of kaolin study, dye absorption and absorption kinetics for MB, network capture for MO and copper ion absorption study. TEM images of S−CNF, 9−CNF, 7−CNF and 5−CNF, AFM images of S−CNF, 9−CNF and 5−CNF.



ACKNOWLEDGMENTS

The work is funded by Key Program for International S&T Innovation Cooperation Projects of China (2016YFE0131400), “521” Talent Project of Zhejiang Sci−Tech University, State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (LK1713), and Candidates of Young and Middle Aged Academic Leader of Zhejiang Province.



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High Aspect Ratio Carboxylated Cellulose Nanofibers Cross-linked to

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