New Approach for Single-Step Extraction of Carboxylated Cellulose

Research Article pubs.acs.org/journal/ascecg

New Approach for Single-Step Extraction of Carboxylated Cellulose Nanocrystals for Their Use As Adsorbents and Flocculants Hou-Yong Yu,*,†,‡ Dong-Zi Zhang,† Fang-Fang Lu,† and Juming Yao*,†,‡ †

The Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education, College of Materials and Textiles, and ‡National Engineering Lab for Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China

ACS Sustainable Chem. Eng. 2016.4:2632-2643. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 09/01/18. For personal use only.

S Supporting Information *

ABSTRACT: A simple approach was developed to isolate cellulose nanocrystals (CNCs) with carboxylic groups from microcrystalline cellulose (MCC). The effect of reaction time on the morphology, microstructure, and thermal stability of isolated CNCs was investigated. The rod-like CNCs with size of 200−250 nm in length and about 15−20 nm in width were obtained by one-step citric/hydrochloric acid (C6H8O7/HCl) hydrolysis of MCC. The CNCs extracted at 4 h showed the highest carboxylic group content which led to a high absolute zeta potential value up to 46.63 mV. Moreover, these CNCs may be used as cationic dye adsorbent (methylene blue) and efficient flocculants with excellent coagulation−flocculation capability to kaolin suspension with a turbidity removal of 99.5%. KEYWORDS: Cellulose nanocrystal, Carboxylic groups, Adsorption, Flocculation effect

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application as efficient adsorbents and flocculants in wastewater treatment fields. Recently, 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) mediated oxidation,21−24,16 periodate−chlorite oxidation,18,25−29 and ammonium persulfate (APS) oxidation30,31 have been used to produce carboxylated CNCs. The preparation steps and reaction time for the different methods are shown in Table S1. In the first method, the preparation of carboxylated CNCs needs two steps: the extraction of CNCs (sulfuric acid hydrolysis of cellulose) and then TEMPO-mediated oxidation of CNCs with multiple post-treatment steps.16,22−24 Surface carboxylated CNCs with different sizes and degrees of oxidation were prepared by TEMPO-mediated oxidation of cellulose, and they showed higher adsorption capacity for cationic dyes (769 mg dye/g CNC) than that of unmodified CNCs.16,17 But the TEMPO-mediated oxidation method still had many shortcomings, such as long oxidation time, toxic TEMPO reagents (leading to environmental issues),32 and limited oxidation at C6 primary hydroxyl groups in CNCs.16,33 For periodate-chlorite carboxylation of CNC, the periodate was first employed to oxidize the C2 and C3 hydroxyl groups in the cellulose to be two aldehyde groups, and then obtain carboxylated CNCs with two carboxyl groups by using chlorite.18,28,29 Suopajärvi et al. reported that the dicarboxylic cellulose nanofibrils were able to flocculate municipal wastewater, and provide a turbidity reduction of 40−80%

INTRODUCTION Cellulose nanocrystals (CNCs) have great potential in highperformance bionanocomposites,1−3 aerogels for oil/water separation,4−6 protein immobilization,7,8 antimicrobial packagings,1,2,9 drug delivery,10,11 and metallic reaction templates,12,13 although the use of CNCs in environmental engineering applications (wastewater treatment) is much less explored. In the world, there are a large number of textile enterprises, so a large amount of dye wastewater must be treated. Also, common petroleum-based synthetic polymers such as flocculants can treat some industrial effluents (wastewater) containing lots of suspended particles, dyes, and heavy metals,14−19 but they are not readily biodegradable and hold potential toxicity to soil, which cause secondary pollution to the environment.14,15,18 CNCs derived from environmentally friendly and renewable cellulose have the ability to solve this problem because they have high specific surface area of 480 m2/g and surface functional groups (easy modification) to provide many active sites for immobilization of dyes or heavy metal ions, while the excellent mechanical strength and high crystallinity and hydrophilicity of CNCs can resist high-pressure environments (real water purification) and chemical corrosion. 20 However, some challenges for using CNCs in water purification are related to agglomeration due to formation of tightly hydrogen-bound networks of CNCs (numerous hydroxyl groups on their surface).1,2 Introduction of anionic groups (carboxyl groups) on the CNC surface (carboxylated CNCs) can loosen the hydrogen-bound network, as well as increase electrostatic repulsion and adsorption ability, which will broaden their © 2016 American Chemical Society

Received: January 20, 2016 Revised: March 25, 2016 Published: March 29, 2016 2632

DOI: 10.1021/acssuschemeng.6b00126 ACS Sustainable Chem. Eng. 2016, 4, 2632−2643

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Route for Fabricating Carboxylated CNCs

(initial turbidity of 156−175 NTU).18 The CNCs with low carboxyl content of below 0.5 mmol/g show slightly lower turbidity reduction and flocculation efficiency than that of the CNCs with high carboxyl content of 1.75 mmol/g. However, this two-step oxidation needs the expensive and toxic periodate as well as disintegration process with high energy consumption (highpressure homogenizer).18,29,31 Furthermore, the glycosidic rings will be subsequently split after the oxidation reaction, which may decrease molecular chain lengths or rigidity of the CNCs.27,32 More recently, carboxylated CNCs with yields of 14−81% were successfully extracted from various cellulosic sources by one-step APS hydrolysis, but this method requires time-consuming alkaline pretreatments and long reaction times of 16−24 h.30,31 As described above, the CNCs with anionic groups (variable charge densities), tunable hydrophilicity, and no damage of rigidity will have robust potentials in wastewater treatment fields. Nevertheless, little research is focused on the preparation of such CNCs for this specific application. In this work, a simple and versatile citric/hydrochloric acid hydrolysis is reported for extracting anionic carboxylated CNCs without damage in molecular chain lengths, in which the hydronium ions (H3O+) from HCl or RCOOH dissociation can not only hydrolyze the

amorphous domains of microcrystalline cellulose (MCC) but also catalyze the esterification of hydroxyl groups on the exposed MCC chains with carboxyl groups of C6H8O7 (Scheme 1). The effects of reaction conditions such as hydrolysis time, temperature, and acid-to-MCC ratio on the morphology, microstructure, and thermal stability of the carboxylated CNCs were investigated. Moreover, dye adsorption ability (methylene blue) and coagulation−flocculation capacity (model Kaolin suspension and dye effluent) for different CNC samples were studied.

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EXPERIMENTAL SECTION

Materials. Commercial microcrystalline cellulose (MCC, about 10 μm of size) was purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial MCCs without long-time pretreatment show high purity, crystallinity, and chemical reactivity. Thus, MCCs are chosen as starting materials for preparing CNCs. Hydrochloric acid was provided by Hangzhou Shuanglin Chemical Reagent Company. Citric acid and N,N-dimethylformamide (DMF) were purchased from Hangzhu Gaojing Fine Chemical Industry Co., Ltd. Dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and methylene blue (MB) were purchased from Guoyao Chemical Reagent Co., Ltd. Kaolin suspension (superfine size of 11 μm) was purchased from Aladdin Chemistry Co., Ltd. Commercial-grade cationic polyacrylamide flocculant (CPAM, the 2633


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ACS Sustainable Chemistry & Engineering molecular weight of 12 million, charge density of 1.12 ± 0.02 mmol/g, and ζ-potential values of 25 mV) was purchased from Aladdin Chemistry Co., Ltd. All the reagents were analytical grade and used as received without further purification. All the solutions were prepared with deionized water. Preparation of Carboxylated CNCs. A novel citric/hydrochloric acid hydrolysis method was carried out for the preparation of carboxylated CNCs, and the preparation steps were illustrated in Figure 1.

water (100 mL) and 0.01 M NaCl (10 mL) and the mixture was sufficiently stirred to prepare a well-dispersed suspension. Then, 0.1 M HCl was added dropwise into the mixture to set the pH value in the range of 2.5−3.0. A 0.015 M NaOH solution was added at a rate of 0.1 mL/min up to pH = 11 by using a pH stat. The carboxyl group contents of the sample were determined from the conductivity and pH curves. The charge density of CNC suspension was obtained by titrating with a cationic polyelectrolyte (poly DADMAC) through a particle charge detector reported by Suopajärvi et al.18 The sulfate contents of CNCs-S was measured according to previously reported methods:35 First, the CNCs-S aqueous suspension (0.1 wt %, 50 g) and 0.05 M aqueous NaCl solution (1 mL) were added into a 100 mL threeneck flask, then an ORION combination electrode (ROSS) and a thermometer were inserted through the side-necks. In addition, the electrode should be calibrated with both pH = 4 and 7 buffers before each measurement. Next, the suspension was held at 25 °C and stirred under argon gas flow (minimize carbon dioxide interference) until the reading of the pH measurement (PB10, Sartorius, Germany) was stable. Finally, the aqueous NaOH solution (0.0011 M) was added dropwise from a microburet and the pH recorded after each drop. The equivalence points in the titrations were determined using Gran’s Plot procedure.36 In our titrations, titration of the unhydrolyzed MCC gave an equivalence-point volume translating into sulfate contents. Thus, the sulfate contents on the CNCs-S were calculated from the additional base required for neutralization. The formate content of CNCs-F was determined by pH titration through our previous methos.37 First, the CNC suspension was prepared by adding CNCs-F powders (0.1 g) into 30 mL deionized water, after being stronger sonication treatment, the pH value of the suspension was adjusted to 4.5 by adding dropwisely the hydroxylamine hydrochloride solution (0.43 g in 20 mL deionized water). The obtained suspension was stirred at ambient temperature for 24 h. By the ending, by titrating 0.01 M NaOH until the pH of the mixture reached a constant of 4.5, and the formate content was determined by recording the consumption of NaOH. The dispersibility of carboxylated CNCs in water was determined by visual inspection. Photographs of the CNCs suspensions (5 mg/mL) were taken at 2, 24, and 96 h after sonication. The suspension dispersibility for the acidic CNCs, the CNCs neutralized by centrifugations with deionized water (CNCs-4H), and the CNCs neutralized with NaOH was also studied. In addition, the dispersibility of other solvents was probed by observing the suspension stability in DMSO, DMF, and THF as a function of storage time. CNCs suspensions (each 15 mL with a concentration of 5 mg/mL) were prepared in different solvents with ultrasonic dispersion of 30 min under ice bath. Pictures were taken immediately at 2 and 24 h. The contact angle was used to evaluate the hydrophobicity of carboxylated CNCs by using SL200B, USA Kino Industry Co., Ltd. A drop of water (10 μL) was placed on a disk (1.0 cm of diameter, 1 mm of thickness) of the CNCs, and the contact angle was determined at the ambient temperature. The crystallinity and crystalline dimensions were measured by X-ray powder diffractometer (XRD, ARL X’TRA, Thermo Electron Corp) using a monochromatic Cu Kα radiation at λ = 1.54056 Å (40 kV, 40 mA) in the range of 2θ = 5−50° with a scanning rate of 2° min−1. The crystallinity (XC) of microcrystalline cellulose and CNCs was calculated according to the Segal equation:26,27,30,33

Figure 1. Preparation steps for extracting the carboxylated CNCs from raw materials. The detailed extraction process was described as follows: The CNCs were fabricated by 90% citric acid (135 mL, 3 M)/10% hydrochloric acid (15 mL, 6 M) (v/v) hydrolysis of MCC (3.0 g) under different reaction time with mechanical stirring (750 rev/min) in a 250 mL flask. Subsequently, after cooling to room temperature, the resultant suspension was repeatedly washed by successive centrifugations with deionized water until approximate neutrality. Then the CNCs were freeze-dried for 48 h and denoted as CNCs-2H, CNCs-3H, CNCs-4H, CNCs-5H, and CNCs-6H according to different reaction time. In addition, the effect of the acid-to-MCC ratio (liquid-to-solid ratio) (20:1; 40:1; 1:50; 60:1; 80:1 g/mL), C6H8O7/HCl ratios (9:1; 7:3; 5:5) and the temperature (70, 80, 90 °C) on the morphologies of the CNCs was studied, and the codes of the CNCs were shown in Figure S1. For comparison, MCC was hydrolyzed with 64 wt % sulfuric acid (50 °C for 1h) with acid-to-MCC ratio of 20 g/mL and formic/hydrochloric acid (90 °C for 3h) with acid-to-MCC ratio of 50 g/mL. Finally the obtained products were denoted as CNCs-S and CNCs-F, respectively.1,2,34 The CNCs-F with formate content of 580 mmol/kg and CNCs-S with amount of sulfate half-ester groups (around 50 mmol/kg) were determined by using oxime reaction and conductivity titration. Characterization. The morphologies of carboxylated CNCs were observed by field emission scanning electron microscopy (FE-SEM, JSM-5610, JEOL, Japan) with a acceleration voltage of 2.0 kV at room temperature. Aqueous suspensions of approximately 0.01 wt % nanocrystals were dispersed in an ultrasonic bath (model S7500, Branson) with ice with ultrasonic dispersion for 30−60 min and then deposited on a silicon wafer. The CNCs dimensions (length (L) and width (W)) were measured. Over 200 rodlike nanocrystals were analyzed to determine the average length, width, and size distribution. Infrared spectra were recorded at room temperature on a FTIR spectrometry (Nicolet 5700, Thermo Electron Corp., USA). All freezedried samples were prepared as KBr pellets and analyzed using a spectral width ranging from 4000−400 cm−1 with a 2 cm−1 resolution and an accumulation of 32 scans. The ζ-potential of CNCs in water (0.01 wt %) was determined with a Nano ZS Malvern Zetasizer. 2.5 mg/mL aqueous suspensions of the CNCs were prepared and then diluted to the 0.01% aqueous suspensions, and measurements were performed in triplicate at 25 °C. The average zeta potential values of CNCs-S (sulfate groups) and CNCs-F (formate groups, almost without charged group) were −22.0 and −1.7 mV, respectively. The carboxyl group contents were determined by the electric conductivity titration method.33 A dried sample (0.1 g) was added into

XC = (I(200) − I(amorphous))/I(200) × 100%

(1)

where I200 is the overall intensity of the peak at 2θ = 22.7°, and I(amorphous) represents the intensity of the baseline at 2θ = 18°. Crystalline dimensions of different planes of the samples were calculated according to the Scherrer equation:1,30,33

Dhkl =

Kλ Bhkl cos θ

(2)

where Dhkl is the average crystalline width of a specific plane; λ represents the wavelength of incident X-rays (λ = 0.15418 nm); 2634


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ACS Sustainable Chemistry & Engineering θ is the center of the peak; and β1/2 (in radius) represents the full width at half-maximum (FWHM) of the reflection peak. The thermal stability was evaluated by thermogravimetric analysis (TGA, Pyris Diamond I, PerkinElmer Corp). Samples of ca. 10 mg were heated from 30 to 600 °C under dynamic nitrogen atmosphere at a heating rate of 20 °C min−1. Activation energies (Ea) were obtained from TGA data by applying Horowitz and Metzger method as equation below:7

⎡ ⎛ W ⎞⎤ Eθ ln⎢ln⎜ 0 ⎟⎥ = a 2 ⎢⎣ ⎝ WT ⎠⎥⎦ RTs

C6H8O7/HCl ratio of 9:1 were more uniform and had relatively higher carboxyl contents, thus this kind of CNCs was used for further investigation. Size and Shape Evaluation of Carboxylated CNCs. Figure 2 shows FE-SEM images of carboxylated CNCs prepared under different reaction time, and their geometrical dimensions are presented in Table 1. All CNC images exhibited the typically rod-like morphology with length of about 200−260 nm and width of about 15−20 nm. A slight change in dimension of can be found for the CNCs prepared under different reaction time, i. e. the average length was gradually decreased with the increased reaction time. As shown in Figure 2 and Table 1, CNCs-2H possessed an average length of 249.8 nm. With the increase of the reaction time, the average length was decreased to 204.3 nm for CNCs-6H samples. High magnification FE-SEM results (Shanghai Jiao Tong University) can also confirm the change trendy of size of the CNCs (Figure S4). These can be attributed to the further removal of the amorphous region during the longer time hydrolysis on CNCs-6H samples. Further, compared to individual CNCs-2H, relatively less aggregation was observed for CNCs-4H, benefiting from relatively large electrostatic repulsion between CNCs-4H with more carboxyl groups (this will be discussed later). The effects of reaction time on the yields of the CNC were also summarized in Table 1. It is clearly shown that the yields from 78−90% can be achieved for CNCs in this work, which was obviously larger than that for sulfuric acid hydrolysis.34,39 Among all samples, the high yield up to 89.53% of CNCs-2H was benefited from the use of short processing time, which was obviously larger than 78.78% for CNCs-6H. Mixed acid could penetrates into the inner layers of the cellulose network and hydrolyzes the amorphous regions of cellulose chains, meanwhile the crystalline regions of cellulose were more resistant to mixed acid hydrolysis due to strong hydrogen bonding between adjacent cellulose molecules compared to the less compact amorphous regions.1,2,34,40,41 Chemical Structures of Carboxylated CNCs. Figure 3 shows the FT-IR spectra of MCC and carboxylated CNCs prepared under different reaction time. It was observed that O−H stretching vibrations around 3341 cm−1, C−H stretching vibrations at 2898 cm−1, H−C−H and O−C−H in-plane bending vibrations at 1429 cm−1, and the C−H deformation vibrations at 1376 cm−1 represented characteristic peaks of celluloses.30,34 These peaks match well in both the spectra of the MCC and CNC samples and no significant difference can be observed, suggesting the unaltered the conformation of the cellulose structure in the transformation from MCCs to CNCs. However, compared with MCC, a new carbonyl peak at 1735 cm−1 occurred, showing the existence of carboxyl groups (−COOH) on the CNCs surface, which were formed by esterification reaction between the hydroxyl groups of cellulose and the carboxyl groups of citric acid. Compared with the esterification in this study, the process of sulfuric acid-hydrolysis of MCC induced a sulfate half-ester reaction between sulfuric acid and cellulose, which leaded to occurrence of sulfate halfester groups on the surface of CNCs,34,35,39,40 while the CNCs prepared by hydrochloric acid hydrolysis only had hydroxyl groups.34,40 Moreover, the normalized intensity of carboxylic groups band (1735 cm−1) are given in Figure 3b, it shows that the peak intensity for carboxyl groups (1735 cm−1) was changed, indicating the CNCs prepared under different reaction time had variable carboxyl contents. It deduces that the reaction time will have an influence on the carboxyl contents of CNCs. In order to quantitatively obtain the content of carboxyl groups on the

(3)

Where W0 is initial weight of polymer; WT is the residual weight of polymer at temperature T; Ts is the temperature determined at 37.202% weight loss; θ is T − Ts. R is the gas constant. In addition, samples of the CNCs types were heated at different temperature in air to study thermal stability. The samples on glass slide put on the oven and take photos on 100, 120, 140, 160, 180, and 200 °C (each temperature for 5 min). The coagulation-flocculation of CNCs was estimated by using Kaolin suspension (concentration of 1000 mg/L) as simulated wastewater. Before flocculation, the pH of kaolin suspension was adjusted to 7 (the 0.1 M hydrochloric acid solution and 0.1 M sodium hydroxide solution was used to adjust the pH of kaolin suspension). Then 30 mg of CaCl2 as coagulant was put into 100 mL of kaolin suspension with vigorous stirring (200 rpm for 3 min), and subsequently, a certain amount of CNCs was added with slow stirring (30 rpm for 7 min). After the mixtures were settled for 30 min, the turbidity of the supernatant can be measured with a Turb550 turbiditor. For dye adsorption studies, stock solutions of MB and adsorbents (CNCs-F, CNCs-S, or CNCs-4H) were prepared to evaluate the adsorption of MB on CNCs. For each experiment, 7.5 mL dye and a certain amount of CNC suspension was put into 20 mL vial and the resulting mixture was stirred at 500 rpm for 30 min. Then the mixture from the vial was centrifuged at 7500 rpm for 10 min, and small samples in the centrifuge tube were diluted (30−100×). The residual concentration of MB in the supernatant was measured by using a UV−vis spectrophotometer (Hitachi U-2900, Japan), which could be determined by corresponding to the absorption values of free dye molecules according to calibration curve of absorbance versus the concentration of MB at 664 nm. The dye uptake (qe) on the CNC surface was calculated using eq 4:

qe =

(C0 − Ce)V m

(4)

where qe is the amount of dye adsorbed for 1 g of CNCs (mg/g), Ce is the equilibrium concentration of free dye molecules in the solution (mg/L), C0 is the initial dye concentration (mg/L), V is the volume of solution (L), and m is the mass of CNCs (g). The effect of adsorbent dosage was studied by varying the concentration of CNCs from 2.5 to 20 mg/mL while keeping the dye concentration constant at 500 mg/L, the pH at 7 and the temperature at 25 °C. In addition, the effect of different pH values(4, 5, 6, 7, 8, 9, and 10) was studied by performing the dye adsorption test at 25 °C with the dye concentration at 500 mg/L and the adsorbent dosage of 17.5 mg/mL. COD and BOD measurements were carried out by COD Trak II and BOD Trak II, respectively.

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RESULTS AND DISCUSSION It is well-known that the dimensions and crystallinity of carboxylated CNCs depend on the particular hydrolysis conditions (reaction temperature, reaction time, and solid-to-liquid ratio) and sources of cellulose.30,31,38 In this study, by optimizing the preparation conditions (Table S2, Figure S1−S3), CNCs with relatively high carboxyl contents were obtained at the reaction temperature of 80 °C and solid-to-liquid ratio of 1:50 g/mL (Figure S3). It should be noted that the CNCs under different C6H8O7/HCl ratios also have been prepared and investigated. According to experimental results, the CNCs obtained under the 2635


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ACS Sustainable Chemistry & Engineering

Figure 2. FE-SEM images of (a) CNCs-2H, (b) CNCs-3H, (c) CNCs-4H, (d) CNCs-5H, (e) CNCs-6H, and the corresponding particle size distribution (a′, b′, c′, d′, e′) of CNCs measured from FE-SEM images.

reaction time, the carboxyl contents of CNCs were increased, and the highest content was achieved for CNCs-4H (1.39 mmol/g), which was due to more carboxyl groups formed with the

CNCs surface, an electric conductivity titration method was conducted.33 The results of carboxyl group contents of CNCs are presented in Table 1. It was observed that with the increasing 2636


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Table 1. Yield, Average Geometrical Dimensions, Carboxyl Contents, and Zeta Potential for the Carboxylated CNCs Prepared under Different Reaction Times dimensions (nm) sample

yield (%)

length

diameter

carboxyl content (mmol/g)

charge density (mmol/g)

zeta potential (mV)

CNCs-2H CNCs-3H CNCs-4H CNCs-5H CNCs-6H

89.5 87.1 82.5 79.2 78.8

249.8 ± 22.4 236.3 ± 24.2 215.8 ± 23.2 206.8 ± 26.7 204.3 ± 23.1

15.6 ± 2.5 19.1 ± 2.9 15.9 ± 3.0 18.9 ± 2.4 21.3 ± 2.8

1.15 ± 0.06 1.32 ± 0.07 1.39 ± 0.10 1.21 ± 0.08 1.10 ± 0.05

1.12 ± 0.05 1.31 ± 0.08 1.40 ± 0.12 1.23 ± 0.08 1.09 ± 0.06

−37.5 −42.1 −46.6 −35.1 −33.5

Figure 3. (a) FT-IR spectra of MCC and carboxylated CNCs prepared under different reaction time. (b) Normalized peak intensities at 1735 cm−1.

Figure 4. (a) Images for MCC and carboxylated CNC suspension stability in H2O. (b) Photographs of CNCs-4H dispersions in H2O, DMSO, DMF, and THF (from left to right).

lengthening of esterification reaction. However, as reaction time was prolonged, a decrease of carboxyl contents was observed for CNCs-5H (1.21 mmol/g) and CNCs-6H (1.10 mmol/g), which was caused by the acid hydrolysis of esterified part of cellulose.38 This phenomenon corresponded with the FT-IR results. Zeta Potential Evaluation and Suspension Stability of Carboxylated CNCs. As a result of the presence of carboxyl groups on their surface, CNCs were changed to negatively charged nanoparticles, and the zeta potential was usually used to measure the charged extent of nanoparticles in aqueous suspension. The effect of reaction time on zeta potential of CNCs is presented in Table 1. As we can see, the change trend of the zeta potential for CNCs was consistent with that of carboxyl contents, implying the carboxyl groups had a direct influence in the negatively charged surface of CNCs due to the ionization of the carboxyl groups. With the increase of reaction time, the absolute zeta potential values of CNCs first was increased and then decreased, and the largest absolute value (46.63 mV) was achieved for the CNCs-4H, which indicated the strongest repulsive force between CNCs-4H brought by the highest

carboxyl contents. In general, the dispersible stability of CNCs in water was strongly dependent on the zeta potential of CNC suspensions.34,35,42 Therefore, the suspension stability of CNCs-4H was better than that of the other CNCs (Figure 4a), due to the highest carboxylic contents on the CNCs-4H. Moreover, the CNCs-4H neutralized with centrifugations with deionized water and NaOH showed better suspension stability than acidic CNCs (Figure S5). As shown in Figure 4b, the CNCs-4H had good suspension stability in these four solvents with different polarity (H2O > DMSO > DMF > THF) after 2 h of sonication. Twenty-four hours later, CNCs-4H still possessed relatively good suspension stability in these solvents except THF. The good dispersion of the CNCs-4H in aqueous and organic solvents will broaden their application based on hydrophilic and hydrophobic polymers.39 Hydrophilic Property of Carboxylated CNCs. In order to further study the surface properties, contact angle measurements were carried out. The MCC and the CNCs possessed relatively hydrophilic surfaces with contact angles of 41.4°, 32.9°, 26.0°, 23.0°, 30.0°, and 33.9°, respectively (Figure 5), due to the 2637


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Table 2. Average Crystallite Size and Crystallinity for the MCC and Carboxylated CNCs Prepared under Different Reaction Times

a

sample

D1I̅0 (nm)

D110 (nm)

D200 (nm)

Xca (%)

MCC CNCs-2H CNCs-3H CNCs-4H CNCs-5H CNCs-6H

5.4 4.9 4.8 4.8 4.8 4.8

6.7 5.8 5.9 5.9 5.8 5.8

6.9 6.2 6.1 6.1 6.1 6.1

82.6 88.9 91.3 91.4 90.7 90.9

XC was calculated from the XRD patterns.

Comparison in Thermal Stability of Carboxylated CNCs and Other CNC Products. Thermal stability of the CNCs is an important parameter for nanoreinforcements in melt-processed composites.1 The thermal behavior of the MCC, CNCs-S, and carboxylated CNCs prepared under different reaction time was studied by thermal gravimetric analysis (TGA). Figure 7a and b shows the TGA and differential thermal gravimetric (DTG) curves. Onset degradation temperature (T0) and maximum decomposition temperature (Tmax) are listed in Table 3. Interestingly, carboxylated CNCs exhibited a slightly lower T0 (298.8 °C) than that of the MCC, due to the presence of carboxyl groups on their surface. This might be due to the disruption in hydrogen bonding of original cellulose on insertion of carboxyl group.7,43 Moreover, the more carboxyl groups on nanocrystals had, the lower T0 and Tmax the CNCs showed. However, the values of T0 (283.4− 293.8 °C) and Tmax (355.3−377.9 °C) for the carboxylated CNCs were interestingly higher by more than 100 °C, as compared to CNCs-S (194.8 and 244.3 °C). This could be explained by that the elimination of sulfuric acid in sulfated anhydroglucose units required less energy, and thus they could be released at much lower temperatures during the thermal degradation process.7,34 It implies that the CNCs prepared by this method showed more significant advantage in the thermal property compared to the CNCs-S. Figure 7c shows the plot of ln[ln(W0/WT)] vs θ for the main stage of thermal degradation of MCC, CNCs-S, and CNCs, which can be used to calculate the average apparent activation energy (Ea). A higher Ea suggests a faster degradation rate for the materials during the thermal degradation process.7 Table 3 clearly demonstrates that the Ea values of CNCs samples were decreased with the increasing hydrolysis reaction time, and the lowest Ea was found for CNCs-4H. It indicates that the CNCs-4H showed the slowest degradation rate, which was

Figure 5. Tests of static contact angle (water) for MCC and carboxylated CNCs.

hydrophilic hydroxyl and carboxylic groups on their surface. In addition, the contact angles of the CNCs were decreased with the increase of reaction time and the smallest contact angle was obtained for CNCs-4H, due to their most carboxylic groups. The more CNCs-4H hydrophilic property not only could improve the suspension stability, and their surface more carboxylic groups would induce steric stabilization between them when they were dispersed in water-soluble polymer matrices39 but also endow the CNCs with excellent biocompatibility and open wide application in the biomedical and tissue engineering.22,29,30 Crystal Structure and Crystallinity of Carboxylated CNCs. Generally, common acid hydrolysis and modification can only occur at the surface of CNCs, while the original morphology and the integrity of the crystals can be preserved.22,39 Figure 6 gives similar evidence from WAXD patterns that all the CNCs exhibited similar patterns to that of MCC, such as characteristic diffraction peaks at 2θ = 14.9°, 16.5°, 20.5°, 22.7°, and 34.5° assigning to 1I0̅ , 110, 012, 200, and 004 reflection planes of typical cellulose I, respectively.26,27,34 It indicates that the CNC samples remained cellulose I crystal structure of MCC. Table 2 illustrates that the crystallinities (XC) of the CNCs were higher than that of MCC (Table 2). With the increase of reaction time, the crystallinities of CNCs were slightly increased to maximum value, and then slightly reduced. Indeed, the long hydrolysis time would result in a slight reduction in the crystallinity, resulting from the removal of amorphous cellulose as well as partial destruction in the crystalline regions.

Figure 6. (a) XRD patterns of MCC and carboxylated CNCs prepared under different reaction times, (b) normalized peak intensities at 1I0̅ , 110, and 200. 2638


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Figure 7. TGA (a), DTG (b) curves, the plot of ln[ln(W0/WT)] vs θ (c) of MCC and carboxylated CNCs, qualitative observations on two CNCs samples heated at different temperature in air (d).

concentration of the surface groups in CNCs contributed to heat resistance (or thermal stability) of the nanomaterials. Further, the reinforcing efficiency of the different CNCs in the mechanical property of PLA was evaluated (Figure S6). Compared to neat PLA, PLA/CNCs-4H possessed a significantly higher tensile strength and Young’s modulus (improved by 203.4%) than PLA/CNCs-F (116.7%) and PLA/CNCs-S (94.4%)) with the same loading levels. It also confirms our assumption that this method will not disturb molecular chain lengths or rigidity of CNCs. Coagulation-Flocculation Behavior (Kaolin Suspension As Model Wastewater) of Carboxylated CNCs and Other CNC Products. It has been reported that the dosage of coagulant (CaCl2) show great contribution to the turbidity removal of wastewater in the flocculation process. Figure 8a shows an obvious reduction in the turbidity by adding 300 mg/mL CaCl2. It indicates that electrostatic adsorption between the anionic kaolin and cationic CaCl2 particles (namely charge neutralization mechanism) can induce effective flocculation of suspended particles (inset photos in Figure 8a). It should be noted that dosage of coagulant (CaCl2) was selected at 300 mg/mL due to saving cost, and the kaolin with CaCl2 showed stable negative zeta potential values, indicating their good adsorption with cationic particles (as we reported previously14,15). Figure 8b shows the turbidity change of kaolin suspension with the different CNC flocculants assisted with CaCl2 coagulant at pH = 7 (avoiding equipment corrosion), in which the effect of CNC dosage on the coagulation-flocculation performance was studied. After the coagulation-flocculation with different CNCs,

Table 3. Thermal Parameters for the MCC and Carboxylated CNCs Prepared under Different Reaction Times sample

T0 (°C)

Tmax (°C)

Ea (kJ/mol)

MCC CNCs-2H CNCs-3H CNCs-4H CNCs-5H CNCs-6H CNCs-S CNCs-F

298.8 293.7 288.9 283.4 290.7 293.8 194.8 338.2

357.5 372.6 355.3 353.2 363.9 377.9 243.3 359.4

201.75 216.77 194.09 175.36 181.73 192.97 26.92 245.6

more suitable for the melt-processing techniques. Also, Figure 7c shows that Ea of CNCs-S was lower than CNCs-4H, suggesting that CNCs-S exhibited slower degradation rate, due to effect of the sulfate groups as flame retardants.27,28,37 During melt extrusion or injection molding, the polymers are usually kept at a higher temperature for a period of time, and the color of the obtained products is used to judge the quality of the products after melting process. The thermal stability of the CNCs was qualitatively investigated by heating two kinds of CNCs in air (the pictures in Figure 7d). The images show that the CNCs-4H were close to white in their pristine state at 100 °C, while the CNCs-S was becoming yellow at the same temperature. As the heating temperature increased to 160 °C for 10 min, the CNCs-4H presented a dark yellow while an near-black was observed for CNCs-S, indicating that darkening of CNCs-4H was not considerable until a temperature of ca. 180 °C, as compared with CNCs-S. This result suggests that the species and 2639


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Figure 8. (a) Coagulation performance of CaCl2 with 300 mg/L to the kaolin suspension (1000 mg/L) at pH = 7, (b) coagulation-flocculation property of CNC with different dosages assisted with CaCl2 to kaolin suspension at pH = 7, (c) turbidity of CNC assisted with CaCl2 at different pH to the kaolin suspension, and (d) zeta potential of the CNCs with or without CaCl2 in the pH range from 3 to 10.

Compared to CNCs-S and CNCs-F, the CNCs-4H showed high turbidity removal with constant values from pH 4 to 10. It is easy to believe that more carboxyl groups on CNCs-4H were deprotonized to form COO− groups under alkaline conditions, which was beneficial to obtain favorable coagulation-flocculation ability to kaolin suspension due to charge neutralization effect. This result was supported by the change in zeta potential of three kinds of CNCs with or without CaCl2 (Figure 8d). Clearly, negative zeta potential values in the whole test pH were found for all the CNC suspensions with or without CaCl2. With addition of CaCl2, the zeta potential of CNCs-4H and CNCs-S showed constant zeta potential values on the negative side, in comparison with that of pure CNC suspensions. Due to few charged groups, lower absolute zeta potential value was observed for CNCs-F with or without CaCl2, and thus weak coagulation-flocculation performance to kaolin suspension (Figure 8c). In contrast, the CNCs-4H showed larger absolute zeta potential value, indicating their excellent adsorption with cationic particles. Dye Adsorption Analysis of Carboxylated CNCs and Other CNC Products. In textile production, there are many ionic dyes or disperse dyes in the wastewater. The removal of dye is an important factor for the wastewater turbidity. The effect of adsorbent dosage on the percentage dye removal and dye uptake (qe) was investigated by varying the amounts of CNC (adsorbent) from 2.5 to 20 mg/mL at initial dye concentration of 500 mg/L. Figure 9a−c shows that the dye removal of all the CNC samples was increased gradually with an increase of adsorbent dosage, approaching a plateau at 18 mg/mL. The maximum dye removal of 92.8% achieved at CNCs-4H dosage of 18 mg/mL (considering the dosage saving), which was higher than 37.8% for CNCs-F and 62.6% for CNCs-S. Also, dye

all the residual turbidities of kaolin suspension decreased with the increasing dosage up to 40 mg/L. When the CNC dosage exceeded to 40 mg/L, the turbidities rose slightly. At the same dosage of 40 mg/L, the coagulation-flocculation of CNCs-4H and CaCl2 exhibited higher turbidity removal of 95.4% than 82.9% for CNCs-S and 34.3% for CNCs-F (inset photos in Figure 8b). The enhanced flocculation performance of the CNCs was dependent on the surface chemical groups. According to the charge neutralization mechanism, the electrostatic adsorption among anion groups of CNCs (carboxyl groups, sulfate groups), the anionic kaolin and cationic CaCl2 particles would induce the flocculation of suspended particles.14 When the kaolin suspension and the coagulant CaCl2 are fixed, the more charged groups or absolute zeta potential values of the CNCs have, the more active flocculation sites of samples have. Zeta potential values for CNCs-S (sulfate groups),34 CNCs-F (formate groups, almost without charged group),37 and CNCs-4H (carboxyl groups) were −22.0, −1.7, and −46.6 mV, respectively. Thus, more carboxyl contents (more anion groups) of CNCs-4H at similar specific surface area (Asp) [see Table S3] can act as active flocculation sites to flocculate more suspended particles through charge neutralization mechanism. Moreover, the turbidity of kaolin suspension with treatment of 20 mg/L CNCs-4H was reduced to 20 NTU, which was between 36.0 NTU for commercial polyaluminum chloride (PAC) and 15.8 NTU for CPAM flocculant.14 Generally, environmental wastewater shows different pH values, which will affect the performance of CNC flocculants on suspended particles. The flocculation performance of combinations of CaCl2 (300 mg/L) with three kinds of CNCs with same dosage (40 mg/L) in the pH range 4−10 is shown in Figure 8c. 2640


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Figure 9. (a) Effect of CNC-F dosage on dye removal percent and qe, (b) effect of CNC-S dosage on dye removal percent and qe, (c) effect of CNC-4H dosage on dye removal percent and qe, and (d) effect of pH on dye removal for three kinds of CNCs. (inset) Photos of dye before and after adsorption with CNCs at pH = 7.

Figure 10. (a) Turbidity, CODCr, and BOD5 of commercial CPAM and CNC-4H suspensions assisted with CaCl2 to the dye effluent from a textile company. (b) Photos of the dye effluent before and after coagulation-flocculation.

range from 4 to 10, and the CNCs-4H exhibited higher dye removal of 72−93% (an obvious color change shown in inset photos), indicating that CNCs-4H can be used to treat textile/ dye effluents or paper mill/municipal effluent within a wide range of pH. Coagulation-Flocculation Ability of Carboxylated CNCs and Commercial Flocculants. In order to demonstrate the flocculation effect of CNCs-4H on the actual application, the dye effluent from a textile company was used to evaluate the coagulation-flocculation ability. Figure 10 illustrates that the initial turbidity of the dye effluent was about 98.5 NTU, the turbidities of supernatant were reduced to 58.3 for CPAM and 3.8 NTU for CNCs-4H/CaCl2, respectively. The turbidity reduction, chemical oxygen demand (CODCr), and biochemical oxygen demand (BOD5) removals of CPAM were about 40.8%,

removal ability of CNCs-4H in this study (92.3% at 20 mg/mL adsorbent in initial dye concentration of 200 mg/L) was better than that of carboxylated CNCs prepared by TEMPO oxidation.16 It further indicates that at the same adsorbent dosage, many carboxyl groups on CNCs-4H can serve as more active sites for the same number of dye molecules.16,17 Interestingly, dye uptake (qe) for all the CNCs decreased with the increase of adsorbent dosage, which was ascribed to the unsaturation of active sites on the CNCs because of the increase in the ratio of CNCs-4H to the dye molecules.16 It is well-known that environmental pH will influence the surface charges of the adsorbents through the protonation or deprotonation of chemical groups, such as such as −COO− (CNCs-4H) and −OSO3− (CNCs-S). Figure 9d shows that all the CNCs presented a slight increase in dye removal at pH 2641



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5.1%, and 5.0%, respectively. On the contrary, CNCs-4H showed higher turbidity reduction of 80.9%, meanwhile the CODCr and BOD5 removals of 36.5% and 37.0%, respectively. Inset photos shows that CNCs-4H formed small flocks which slowly settled on the beaker bottom, and thus a clear supernatant was observed (inset photos in Figure 10), compared to original dye effluent and the supernatant treated by the CPAM. In conclusion, compared to commercial CPAM, the carboxylated CNCs products (CNCs-4H) showed larger turbidity removal, but CODCr and BOD5 removals were slightly improved.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: 86 571 86843618. Fax: 86 571 86843619. E-mail addresses: [email protected] (H.-Y.Y.). *E-mail: [email protected] (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is funded by the National Natural Science Foundation of China (51403187), Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ14E030007, the public technology research plan of Zhejiang Province, China under Grant No. 2015C33111, and “521” Talent Project of Zhejiang Sci-Tech University and Top Priority Discipline of Zhejiang Province in Zhejiang Sci-Tech University (2013YBZX01).

CONCLUSIONS

In summary, the rod-like CNCs with carboxylic groups were successfully extracted by simple C6H8O7/HCl hydrolysis of MCC. The optimal CNC samples with increased crystallinity, highest carboxylic contents and best suspension stability were achieved at the hydrolysis time of 4 h (CNCs-4H). In addition, CNCs-4H possessed a relatively better thermal stability compared to CNCs-S. Moreover, in comparison with other two kinds of CNCs, CNCs-4H showed remarkable coagulationflocculation performance to kaolin suspension, cationic dyes and dye effluent from a textile company. This was attributed to the higher content of carboxyl groups which could provide more active flocculation/adsorption sites to most of components in effluents based on charge neutralization mechanism. Furthermore, such negatively charged CNCs were significantly superior to the commercial CPAM and can be further widely applied to the many wastewater treatment fields.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00126. The experimental process for preparation of PLA/CNC nanocomposite, mechanical test, and the preparation steps for extracting the carboxylated CNCs (Figure S1). FE-SEM images of (a) CNCs-A1, (b) CNCs-A2, (c) CNCs-A3, (d) CNCs-A4, (e) CNCs-A5, (f) CNCs-B6, (g) CNCs-B7, (h) CNCs-B8 (Figure S2). FT-IR spectra of MCC and carboxylated CNC prepared under different solid-to-liquid ratio (a) and reaction temperature (b), normalized peak intensities at 1735 cm−1 under different solid-to-liquid ratio (c), and reaction temperature (d) (Figure S3). FE-SEM images and the corresponding particle size distribution of prepared CNCs (a, a′) CNC2H, (b, b′) CNC-4H, and (c, c′) CNC-6H (Figure S4). The images showing the suspension stability of carboxylated CNCs (a) CNCs neutralized by NaOH (pH = 7), (b) CNCs neutralized by successive centrifugations with deionized water (stronger sonication, pH = 7), (c) acidic CNCs (pH = 4) (Figure S5). Tensile strength and Young’s modulus for neat PLA and the nanocomposites reinforced with 10 wt % three CNC types (PLA/CNCs-F, PLA/CNCs-S, and PLA/CNCs-4H) (Figure S6). Comparison of isolated CNCs in this work with other conventional techniques (Table S1). The yield, average geometrical dimensions, carboxyl content, and zeta potential for the CNCs prepared under different reaction conditions (Table S2). The specific surface area (Asp) for the CNCs-F, CNCs-S, and CNCs-4H (Table S3) (PDF) 2642


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New Approach for Single-Step Extraction of Carboxylated Cellulose

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