Sustainable, Reusable, and Superhydrophobic Aerogels from

Research Article pubs.acs.org/journal/ascecg

Sustainable, Reusable, and Superhydrophobic Aerogels from Microfibrillated Cellulose for Highly Effective Oil/Water Separation Sukun Zhou,† Pengpeng Liu,† Meng Wang,† Hong Zhao,‡ Jun Yang,*,† and Feng Xu† †

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China College of Light Chemistry and Textile Engineering, Qiqihar University, Qiqihar 161006, China

‡

S Supporting Information *

ABSTRACT: The development of sustainable and efficient absorbents for oil and organic pollutants cleaning is an attractive and challenging work. Here, novel superhydrophobic microfibrillated cellulose aerogels (HMFCAs) with high lipophilicity, ultralow density (≤5.08 mg/cm3), superior porosity (≥99.68%) as well as extremely high mechanical stability were successfully prepared from microfibrillated cellulose aerogels (MFCAs) via a facile and environmentally friendly silanization reaction in liquid phase. The superhydrophobicity of the as-prepared HMFCAs (water contact angle as high as 151.8°) was attributed to the formation of polysiloxane on the surface of HMFCAs by the silanization reaction. The HMFCAs exhibited excellent oil/water selective absorption capacity with oil absorption up to 159 g/g. The reusability experiment showed that the adsorption capacity still exceeded 92 g/g for pump oil after 30 absorption cycles, demonstrating its superior reusability. Our work paves the way for the development of sustainable and efficient absorbents toward oils and organic pollutant removal applications. KEYWORDS: Microfibrillated cellulose aerogel, Surface modification, Superhydrophobicity, Oil absorption, Reusability

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INTRODUCTION As a result of oil leakage and industrial wastewater discharge, water pollution has attracted considerable attention because of its disastrous effect on the environment and ecosystems.1,2 Various methods including burning, hydrocarbon degrading, and physical absorption have been developed to solve this problem.3−5 Among these approaches, physical absorption via absorbents is the most promising way, because it is low cost and it does not generate byproducts. A series of absorbents made from mineral materials,6,7 carbon fibers,8 and polydivinylbenzene9 have been reported. However, they usually suffer from shortcomings such as low absorption capacities, complicated fabrication processes, environmental incompatibility, and insufficient buoyancy. These disadvantages and the demand for novel, efficient, and sustainable absorbents drive the researchers to find better alternatives. Nanocellulose aerogels as absorbents have attracted considerable research interests due to their high absorption capacities (more than 40 times by weight), environmental friendliness, biodegradability and sustainability.10−12 Besides, the nanocellulose aerogels also possess high compression strength and high flexibility,10,13,14 which could affect the reusability of the aerogels. These properties make them possible to be used as oil and organic pollutant absorbents. Despite the advantages of nanocellulose aerogels mentioned above, one challenge must be addressed before the nanocellulose aerogels can really be used as ideal oil and organic pollutant absorbents. The challenge stems from the inherent hydrophilicity of the nanocellulose © 2016 American Chemical Society

aerogels, which leads to the poor oil/water selectivity and the collapse of the porous structure of cellulose aerogels. A possible method to transfer the hydrophilicity of nanocellulose aerogels to hydrophobicity is modification of nanocellulose surface by hydrophobic molecules. In recent years, the nanocellulose aerogels are mostly hydrophobization modified by chlorosilanes using chemical vapor deposition method.15 Hsieh et al.3 treated the hydrophilic nanocellulose aerogels with triethoxyl(octyl)silane through vapor deposition and obtained hydrophobic aerogels. Similarly, Sun et al.16 made the vapor of methyltrimethoxysilane diffused into the skeleton of the nanocellulose aerogels to modify the surface of the aerogels. Although the hydrophobicity of nanocellulose aerogels was improved by using these chemical vapor deposition methods, the grafting distribution within the aerogels was not homogeneous.10,17 Moreover, the preparation conditions such as initial quantity of reagent, reaction temperature, pressure, and time of the chemical vapor deposition reactions need to be controlled accurately and the strict conditions restrict the large-scale production of the hydrophobic nanocellulose aerogels.2 To solve these problems, Tingaut et al.10 fabricated hydrophobic nanocellulose aerogels by freeze-drying of a nanocellulose suspension treated by methyltrimethoxysilane. However, the structure and properties Received: May 18, 2016 Revised: July 29, 2016 Published: October 10, 2016 6409

DOI: 10.1021/acssuschemeng.6b01075 ACS Sustainable Chem. Eng. 2016, 4, 6409−6416

Research Article

ACS Sustainable Chemistry & Engineering of the aerogels might be significantly changed by the silylation agent in the reaction.10 There were some works which reported the hydrophobization of cellulose in liquid phase, but they just focused on the enhancement of the cellulose dispersibility in nonpolar solvents and hydrophobic polymers,18,19 not the cellulose aerogels. Herein, we report a simple and environmentally friendly method to modify the surface of nanocellulose aerogels in liquid phase. We immersed blocky microfibrillated cellulose aerogels (MFCAs) in ethanol/methyltriethoxysilane (MTES) solution, followed by vacuum-drying to obtain superhydrophobic and oleophilic MFC aerogels (HMFCAs). Compared with the chemical vapor deposition method,20−22 the silylating agent (MTES) solubilized in the ethanol could diffuse into the pores of the HMFCAs uniformly and avoids the heterogeneity of the obtained HMFCAs. Besides, we used the nontoxic ethanol as the silylating agent dispersant which could be removed by simple vacuum-drying without collapse of the nanostructure, which is different from the method reported by Xiang et al.2

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Table 1. Amounts of Reagents Used in the Modification Reaction ethanol (mL) water (mL) MTES (mL)

HMFCA-1

HMFCA-2

HMFCA-3

40 0.5 1

40 0.5 2

40 0.5 3

with energy dispersive X-ray spectroscopy (EDS). The specific surface areas of the aerogel were determined by a Gemin V (Micromeritics, Norcross, GA) at the temperature of liquid nitrogen. Densities (ρgel) of the aerogels were calculated by weighing the aerogels and measuring their volumes. The weight of the aerogels was measured by an analytical balance (readability 0.0001 g) and the dimensions of the aerogels were measured by a digital caliper at five different positions. The porosities (P) were calculated by the densities (ρgel) of the aerogels and the densities (ρsolid) of the solid using eq 1

P(%) = 100 × (1 − ρgel /ρsolid )

(1)

Fourier transform infrared spectra (FTIR) were conducted on an infrared spectrophotometer (Nicolet iN10-MX, ThermoScientific) and X-ray Diffraction (XRD) patterns were recorded on a D8-Advance Xray Diffraction Analyzer. Mechanical Properties. Compression tests were measured by a Zwick Testing System equipped with a 50 N loading cell. Each aerogel was cut into a cylinder shape (20 mm in diameter, 10 mm in height). The aerogel was compressed with a speed of 2 mm/min to 75% of its original thickness. Five replicates were measured for each sample. Wettability and Liquid Absorption Capacity. The surface wettability of HMFCAs was evaluated by water contact angle (WCA) measurement (OCA20) equipped with a high-speed camera. Water droplets (1 μL) were deposited on the surface of the aerogels and the static contact angles were measured immediately, and the dynamic contact angles were measured by using the Wilhelmy plate method. For liquid absorption test, cylindrical HMFCAs (30 mm in diameter, 10 mm in height) were immersed into 50 mL various oils (or organic solvents) for a certain time and then picked out for measurements. The HMFCAs filled with liquids were weighed after the aerogels were wiped with a filter paper to remove excess liquids. The liquid absorption capacity was calculated using eq 2

EXPERIMENTAL SECTION

Materials. A softwood kraft pulp was provided by Northwood Pulp and Timber Limited (Canada). 2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO), sodium phosphate, sodium chlorite, sodium hypochlorite, ethanol, MTES, and other chemicals were analytical grade and used without further purification. Preparation of MFCAs. MFC suspensions were isolated from softwood kraft pulp according to a previously published method.23 Briefly, the pulp was oxidized by TEMPO system and then the TEMPO-oxidized cellulose was fibrillated by a high pressure homogenizer (APV-2000, 100 MPa) for 30 min. Thereafter, the obtained MFC suspensions (0.2 wt %) were frozen in liquid nitrogen and subjected to freeze-drying using a freeze-dryer (Scientz-10N, China). Thus, the microfibrillated cellulose aerogels (MFCAs) were obtained. Preparation of HMFCAs. The obtained blocky MFCAs (30 mm in diameter, 10 mm in height) were immersed into 40 mL ethanol which contained 0.5 mL water and different amounts of MTES (1 mL, 2 mL, 3 mL). The pH of the mixture was adjusted to 2 with acetic acid (99.5%) and kept stirring at 60 °C for 90 min. Then the pH of the mixture was adjusted to 7.5 with ammonia (25.0%) and stirred for another 60 min. Thereafter, the aerogels were rinsed with ethanol and methanol thoroughly to remove the residual reactants and byproducts. Finally, the aerogels were vacuum-dried at 105 °C for 2 h and thus the modified aerogels (HMFCAs) were obtained (Scheme 1). Aerogels with different MTES loadings were coded as HMFCA-1, HMFCA-2, and HMFCA-3, respectively. The specific amounts of reagents used in the modification reaction were shown in Table 1. Characterization. The morphology of the aerogels was observed by scanning electron microscope (SEM, Hitachi, SU8010) equipped

C=

W1 − W0 W0

(2)

where W0 and W1 are the weights of HMFCAs before and after absorption, respectively. Reusability and Durability. To evaluate the reusability of the HMFCAs as absorbents, absorption and regeneration of the aerogels were carried out in 30 consecutive cycles. The oil-absorbed aerogels were rinsed with toluene and dried under vacuum for 5 h at 105 °C for reuse. The durability of HMFCAs was evaluated by Soxhlet extraction of the samples for 15 h, using three solvents: water, ethanol, and chloroform, respectively. After extraction, the HMFCAs were vacuumdried for 12 h at 105 °C. The liquid absorption capacities and WCAs of the aerogels were re-evaluated.

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Scheme 1. HMFCAs Preparation: (a, d) MFCAs and HMFCAs, (b, c) Water Contact Angle of MFCAs and HMFCAs

RESULTS AND DISCUSSION Morphology. The morphologies of the aerogels before and after silanization treatment were observed by SEM, and the result showed that the cellulose nanofibers self-aggregated into porous or sheetlike structures in all the aerogels. The similar highly porous microstructures in MFCAs and HMFCAs (Figure 1a−d) indicated that the MTES treatment did not change the porous structures of the pristine MFCAs. The highly porous structures endowed the HMFCAs with high liquid absorption capacities. After MTES modification, some polysiloxane particles with diameters about 200 nm appeared on the surface of the modified aerogels as shown in Figure 1f− 6410


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Figure 1. SEM images of the aerogels: (a, e) MFCAs, (b, f) HMFCA-1, (c, g) HMFCA-2, (d, h) HMFCA-3.

Scheme 2. Reaction between MTES and Cellulose

cellulose aerogels.3,10 The change of the porosity was negligible during the surface modification process, and they exhibited high porosity up to 99.71%. Compared to the MFCAs, the HMFCAs showed higher BET surface areas, because the formation of the polysiloxane particles roughed the surface of the cellulose nanofibers, leading to the higher BET surface areas. Chemical Structure. The successful modification could be demonstrated by the appearance of silane typical peaks in the HMFCAs spectra (Figure 2a). The bands at 750 cm−1 (δ (C− H)) and 2980 cm−1 (ν (C−H)) were assigned to the vibrations characteristic of the CH3 in silane. Besides, Si−CH3 bending vibration characteristic at 1250 cm−1 was observed, and the intensity of these bands increased with the increasing amount of MTES in the reaction. The characteristic peaks of Si−O−Si bonds in the siloxane overlapped with the C−O bonds of cellulose (1000−1130 cm−1).25 XRD patterns of the MFCAs and HMFCAs are shown in Figure 2b. The MFCAs retained cellulose Iβ crystalline structure with characteristic XRD peaks at 2θ = 14.6°, 16.8° and 22.7°, which were also observed in the patterns of HMFCAs, demonstrating that the cellulose Iβ crystalline structure was preserved during the silanization reaction. While the intensity of the peaks for the cellulose Iβ crystalline structure were weakened in the HMFCAs, indicating that the MFCAs were coated with polysiloxane. A new peak observed in HMFCAs patterns at 2θ = 10.5°, revealed the formation of polysiloxane particles. Mechanical Properties. The mechanical properties of the aerogels are important for the oil/water separation application. As shown in Figure 3, all the aerogels could tolerate high compression strains without damage, which are superior to the brittle resorcinol−formaldehyde and silica-based aerogels.26−28 The stress−strain curves of the samples exhibited two distinct regions: a linear elastic region below 50% strain and a

h. This phenomenon was different from those reported in other papers, such as the formation of polysiloxane nanofibers,1 the widening of cellulose fibers,24 and the lateral expansion of the cellulose.2 The formation mechanism of the polysiloxane particles is schematically presented in Scheme 2. The small amount of water in the solution led to the hydrolysis of MTES, then the obtained silanols would react with the hydroxyl groups on the surface of MFCAs or other silanols. The reaction between the silanols and the hydroxyl groups resulted in a covalently attached silane layer on the surface of the MFCAs, and the formation of the polysiloxane particles was attributed to the self-polymerization of silanols. The EDS spectra of the HMFCAs (Figure S1) showed the peaks of silicon, and the relative silicon weight percentage (Table 2) in the modified aerogels increased with the increasing of MTES loadings, confirming the successful silanization reaction on the surface of MFCAs. Density, Porosity and BET Surface Area. As shown in Table 2, the density, porosity and BET surface area of the MFCAs were consistent with data in previous reports.2 Compared with the pristine MFCAs, the densities of HMFCAs increased due to the formation of the polysiloxane particles, but they were still lower than those for other reported hydrophobic Table 2. Density, Porosity, BET Surface Area, and Si Content of the Aerogels

sample MFCAs HMFCA-1 HMFCA-2 HMFCA-3

density (mg/cm3) 3.41 4.58 4.81 5.08

± ± ± ±

0.21 0.24 0.31 0.29

porosity (%)

BET surface area (m2/g)

Si content relative atomic % by EDS

± ± ± ±

94.8 158.4 179.7 195.5

0 2.137 4.754 6.352

99.79 99.71 99.70 99.68

0.02 0.02 0.02 0.02

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Figure 2. FTIR spectra (a) and XRD patterns (b) of MFCAs and HMFCAs.

The oleophilic of aerogels were also tested by dripping gasoline drops and water drops on the surface of the MFCAs and HMFCA-3 at the same time (Figure 4b, c). The MFCAs

Figure 4. (a) Water contact angle of MFCAs and HMFCAs. (b, c) Water (dyed with methyl blue) and gasoline (dyed with Susan III) dropped on the surface of MFCAs and HMFCA-3. (d) MFCAs sinking into water. Meanwhile, the HMFCA-3 floated on the surface of water.

Figure 3. Compressive stress−strain curves of MFCAs and HMFCAs.

densification region above 50% strain. In the linear elastic region, the stress increased slowly with the gradually shrinking of the aerogels, while in the densification region, the stress increased sharply due to the continuous reduction in the pore volume.28 At 70% stain, the compressive stresses for the MFCAs, HMFCA-1, HMFCA-2, and HMFCA-3 were 138.16, 152.53, 165.80, and 190.67 kPa, respectively, which were higher than those for other reported hydrophobic cellulose-based aerogels.3,29 Compared with MFCAs, the HMFCAs showed enhanced compressive stress, which could be ascribed to the cross-linked Si−O−Si bonds in the polysiloxane. Wettability. The wettability of the aerogels was characterized by the static and dynamic contact angles (WCA) of the aerogels surfaces, and the results were summarized in Table S1. The unmodified MFCAs exhibited hydrophilic property, as the aerogel could absorb water drop immediately. On the contrary, the modified HMFCAs were highly hydrophobic, exhibiting WCA above 135.6° depending on the dosage of MTES. The HMFCA-3 exhibited a WCA up to 151.8°, which was higher than most of the reported hydrophobic cellulose materials.2,3,10,22,29,30 Moreover, the contact angel hysteresis and sliding angle for HMFCA-3 were less than 10° (Table S1), demonstrating the superhydrophobic property of the aerogels. According to previous research, the surface wettability is ruled by the surface energy and the surface roughness.31−35 Herein, the surface energy of the aerogels was decreased by the silanization reaction, and the formation of the polysiloxane particles endowed the HMFCAs with high surface roughness.

instantaneously absorbed the gasoline and water, demonstrating their amphiphilic property, which was consistent with the previously reported data.36 The HMFCA-3 absorbed gasoline, while water drops remained on the surface, revealing the hydrophobicity and lipophilicity of the modified aerogel. When the pristine MFCAs and hydrophobic HMFCA-3 were placed on the surface of water, the unmodified aerogel sank into water immediately, while the modified aerogel floated on the surface of water (Figure 4d). Owing to their hydrophobicity and lipophilicity, the HMFCAs are potential candidates for the separation of oils and organic pollutants from water. When the HMFCA-3 was placed in gasoline/water mixture solution (dyed with Sudan III), it selectively absorbed the gasoline, leaving the clean water (Figure 5, Movie S1). Besides, the HMFCAs exhibited selective absorption ability toward organic solvent than water by adsorption 1, 2-dichloroethane from the bottom of water (Figure 5, Movie S2). The HMFCA-3 could maintain its shape and float on the water surface after absorbing the 1, 2dichloroethane due to its mechanical stability and low density. These results demonstrated that the HMFCAs could be ideal absorbents for cleaning up oil and organic pollutants. Liquid Absorption Capacity. The absorption capacities of the HMFCAs for various oils and organic solvents can be defined as the weight of absorbed liquid per weight of 6412


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Table 3. Comparison of Various Absorbents Prepared in the Published Papers year

absorbents

2011

TiO2 coated nanocellulose aerogel N-doped graphene framework carbon nanofiber aerogels from BC CNT/GO sponges carbon fiber aerogel from raw cotton PVA/cellulose nanofibril aerogels silylated nanocellulose sponges silylated chitin sponges cotton PVF sponge carbon aerogel from winter melon silylated bacterial cellulose aerogels carbon aerogels from waste newspapers cellulose nanofibril aerogels PE fiber and silica composite aerogels MFC aerogels

2012 2013 2013 2013

Figure 5. Removal of gasoline and 1, 2-dichloroethane (dyed with Sudan III) from the surface and bottom of water with HMFCA-3.

2014 2014 2014 2014 2014 2014

HMFCAs. In general, the hydrophobic aerogels showed high absorption capacities (up to 116−260 times its own weight) for the oils and organic solvents (Figure 6a), and the absorption capacities of HMFCAs increased with the densities of the liquids (Figure 6b). Various absorbents fabricated in recent years and their absorption capacities are listed in Table 3. The absorption capacities of HMFCAs were much higher than those for other reported absorbents from synthetic polymers (14−57 g/g),37 cellulose fibers (20−50 g/g),38 nanocellulose-based absorbents (20−185 g/g),2,10,30,39,40 chitin sponges (29−58 g/ g),1 carbon aerogels made from cellulose (29−192 g/g),33,41,42 and silica aerogels (16 g/g),43 but they were lower than those for graphene and carbon nanotube aerogels, which showed absorption capacities up to 200−743 g/g.44,45 However, the synthesis procedures for the graphene and carbon nanotube aerogels were complicated and costly, which limited their industry applications.

2015 2016 2016 2016

a

absorption capacity (g/g)

costa

ref

20−40

×

30

200−600

××

44

106−312

×

29

215−743 50−192

×× ×

45 41

44−96

√

39

49−102

√

10

29−58 20−50 14−57 16−50

√ √√ √√ ×

1 38 37 33

86−185

√

2

29−51

×

42

28−46 16

× ×

40 43

116−260

√√

this work

“×” high, “× ×” very high, “√” low, “√ √” very low.

The oils and organic solvents were absorbed by physical absorption and they were stored in the pores of the aerogels.

Figure 6. (a) Absorption capacities of the hydrophobic HMFCA-3 for various oils and organic solvents. (b) Absorption capacities for different oils and organic solvents as a function of the liquid density. (c) Volume absorption capacities of the hydrophobic HMFCA-3 for various oils and organic solvents. (d) Absorption kinetics curves of the HMFCA-3. 6413


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Figure 7. (a) Absorption capacities and water contact angles of HMFCA-3. (b) Oils absorption capacities and water contact angles of HMFCA-3 after leaching with water, ethanol, and chloroform.

silanization process, the as-prepared aerogels displayed superhydrophobicity (water contact angle up to 151.8°) and excellent absorption capacities for various oils and organic solvents (260 g/g for chloroform). The HMFCAs could not only selectively absorb oils and organic solvents from polluted water, but also exhibited superior recyclability with at least 30 times, revealing their great potential application in oil−water separation materials. This low cost, facile, and environmentally friendly preparation method opens up new opportunities for the fabrication of advanced absorbents with oils and organic pollutants cleaning properties.

The volume-based absorption capacities of the HMFCAs could reach up to around 90% (Figure 6c), indicating that almost all of the pores in HMFCAs were filled with oils or organic solvents. The HMFCAs exhibited slightly lower volume-based absorption capacities for oils compared with other organic solvents because of the high viscosity of the oils. The absorption kinetics curves of HMFCAs in different oils or organic solvents are shown in Figure 6d. It is found that HMFCAs needs about 25 s to reach absorption equilibrium in motor oil and mineral oil, while they need less time in other organic solvents. Because the viscosity of liquid has significant effect on the movement velocity of the liquid molecules, the liquid with higher viscosity needs more time to reach absorption equilibrium, while liquid with lower viscosity needs less time to reach absorption equilibrium. The absorption capacities of HMFCA-1 and HMFCA-2 were shown in Figure S2. The HMFCA-1 and HMFCA-2 exhibited lower absorption capacities than that for the HMFCA-3 due to the lower hydrophobicity. Reusability and Durability. After ten absorption/ desorption cycles, 151 g/g absorption capacity was achieved for pump oil, which was 98% for its original absorption capacity, and the water contact angle of HMFCAs decreased slightly from 151.8° to 149.6° (Figure 7a). The HMFCAs still maintained high hydrophobicity (124.9°) and high absorption capacity (92 g/g for pump oil), even after 30 absorption/ desorption cycles, revealing the great reusability of HMFCAs. The durability of HMFCAs was evaluated by leaching the samples with different solvents. The water contact angles and oil absorption capacities of HMFCAs after leaching are shown in Figure 7b. The water contact angle of HMFCAs decreased from 151.8° to around 120° due to the partial removal of polysiloxane particles at the aerogel surface during the leaching experiments. The absorption capacities for motor oil and pump oil decreased from 158 to around 115 g/g, and 154 to around 110 g/g, respectively, revealing the good durability of the HMFCAs for the practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01075. EDS spectra of the aerogels; absorption capacities of HMFCA-1 and HMFCA-2 for various oils and organic solvents; static and dynamic water contact angles of HMFCAs (PDF) Movie showing the removal of gasoline with HMFCA-3 from the water surface (AVI) Movie showing the removal of 1,2-dichloroethane with HMFCA-3 from the bottom of water (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Fundamental Research Funds for the Central Universities (BLYJ201619), the Chinese Ministry of Education (113014A), Beijing Municipal Commission of Education (20131002201), and the State Key Laboratory of Pulp and Paper Engineering (201254).

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CONCLUSION In conclusion, mechanically robust aerogels with superhydrophobicity, low density (≤5.08 mg/cm3), and high porosity (≥99.68%) were simply fabricated from micrifibrillated cellulose via a facile freeze-drying process followed by a silanization reaction. Owing to the successfully introduction of polysiloxane particles on the surface of the aerogels during the

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