Highly compressible and hydrophobic anisotropic aerogels for

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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Highly Compressible and Hydrophobic Anisotropic Aerogels for Selective Oil/Organic Solvent Absorption Xuexia Zhang,†,§ Hankun Wang,*,† Zhiyong Cai,‡ Ning Yan,§,∥ Minghui Liu,† and Yan Yu*,⊥ †

Institute of New Bamboo and Rattan Based Biomaterials, International Center for Bamboo and Rattan, Beijing, 100102, P. R. China Forest Product Laboratory, USDA, Madison, Wisconsin 53726, United States § Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON M5S3B3, Canada ∥ Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON M5S3E5, Canada ⊥ College of Material Engineering, Fujian Agriculture and Forestry University, Fuzhou, P. R. China

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by CALIFORNIA STATE UNIV FRESNO on 11/30/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Cellulose-based aerogels show great potential as absorbents for oil and chemical spill cleanup due to their low density and excellent absorption capacity. However, the hydrophility and inferior mechanical properties have often limited their practical applications. In this study, highperformance biomass-based aerogels were prepared by freeze-casting aqueous suspensions of polyvinyl alcohol and cellulose nanofibrils in the presence of hydrolyzed methyltrimethoxysilane sol. Successful silylation on the substrate surface was confirmed by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, thermal stability, and water contact angle measurements. Freeze-casting successfully assembled a highly aligned interconnected porous structure, resulting in the prepared aerogels with high modulus and strength in the aligned direction (along the freezing direction) and outstanding compression flexibility in the perpendicular direction (transverse to the freezing direction). The ultralow density (10.2 kg/m3), high hydrophobicity (water contact angle of 140°), and good compressive recovery (84% recovery of its original thickness after 100th compression tests) allow the aerogel to absorb oils and organic solvents 45−99 times higher than its own weight. Meanwhile, good reusability was also observed with an absorption capacity greater than 84% after 35 absorption−squeezing cycles. The novel aerogels prepared in this study are expected to have great potential for application in treating oil and chemical spills. KEYWORDS: Aerogel, Anisotropy, Silylation process, Freeze-casting, Oil absorption

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tions.7 Hence, the development of affordable aerogel materials with excellent properties for oil spill cleanup is a especially desirable. Very recently, aerogel based on cellulose nanofibrils (CNFs) has become a very promising candidate for oil absorption applications, due to their large specific surface area, high porosity, low density, and natural biodegradability.8 However, a challenge for cellulose aerogel absorbent materials is their hydrophilicity (i.e., poor oil/water selective absorption) and inferior mechanical properties (i.e., poor shape recovery from deformation). Different strategies have been used to improve the hydrophobicity and oleophilicity of cellulose aerogels, such as chemical vapor deposition process (CVD),9 atom layer deposition (ALD),10 esterification,11 and sol−gel.12 Among

INTRODUCTION Water pollution from oil spills and chemical leaks is a serious environmental problem that also leads to severe economic consequences. Therefore, there is an urgent need to develop more advanced technologies for the cleanup of oil pollutants.1 Using porous absorbent materials for oil recovery is more attractive than other conventional methods (such as in situ burning, chemical treatments, mechanical methods, and biodegradation), because of their high efficiency, low cost, and minimal effects adverse to the environment.2,3 Aerogels, a class of highly porous and lightweight solid materials, display promising potential as ideal absorbents due to their large specific surface area and high porosity.4 Although many advanced and efficient carbon-based aerogels based on carbon nanotubes (CNTs)5 or graphene6 have been reported to show extremely high absorption capacity and stable recyclability, high cost, complicated and energy-consuming processes involved in their fabrication restrict their practical applica© XXXX American Chemical Society

Received: July 26, 2018 Revised: September 28, 2018 Published: October 31, 2018 A

DOI: 10.1021/acssuschemeng.8b03554 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

resulting bamboo-extracted CNF suspension was stored at 4 °C before future utilization. Preparation of PVA/CNF Solutions. PVA solution was prepared by dissolving PVA (5 g) in 100 mL of water at 85 °C for 12 h. The PVA solution (6 g, 0.05 g/mL), CNF suspension (50 g, 0.6 wt %), and a desired amount of DI water with a total concentration in water of 0.6 wt % were mixed together in a flask and stirred for 1 h. The weight ratio between PVA and CNF was 1:1. Preparation of Hydrophobic and Anisotropic Aerogels by Freeze-Casting. First, the obtained PVA/CNF suspension was adjusted to a pH of 4 with 0.5 M hydrochloric acid (HCl) solution. The polysiloxanes sol was prepared separately by dropwise adding MTMS (2.45 g, 40 mmol/g(CNF)) in water (80 mL, pH = 4), and then stirred for 10 min. The freshly prepared solution was added dropwise into the PVA/CNF suspension and stirred at room temperature for 2 h. Silylated PVA/CNF suspensions were frozen using a custom-built freeze-caster as shown in Figure S2. The directional freezing of suspensions was achieved by pouring them into rectangular Teflon molds placed on a copper block, which was immersed in an ethanol/ liquid-nitrogen bath to create a uniaxial thermal gradient from bottom upward, and then covered with a Teflon lid. The suspensions were frozen from the bottom to top, while the ethanol/liquid-nitrogen level was kept up to the height of the copper block. Once completely frozen, the samples were immediately transferred to a freeze-dryer (FreeZone plus 2.5 L, Labconco Corp., Kansas, MO, U.S.A) and freeze-dried at −80 °C under vacuum for 48 h. Thereafter, the silylated samples were taken out and placed in an oven for 3 h at 90 °C to ensure complete silylation. The aerogels were sealed in plastic bags for further characterizations. Density and Porosity. Apparent densities were calculated by measuring their masses and dimensions. The porosity was calculated using the following equation

these methods, the gas phase reactions (e.g., CVD and ALD) were widely used. However, the strict preparation conditions and inhomogeneous grafting distribution within the aerogel hamper its large-scale production for industry applications.13 To address these problems, Zhang et al.14 proposed a synthetic process by directly freeze-drying CNF suspensions in the presence of acid-hydrolyzed methyltrimethoxysilane (MTMS) sols, yielding hydrophobic, flexible, and ultralightweight aerogels. In the process, the silylation altered the microstructure of CNF aerogels remarkably, switching the regular porous cellular structure to a random three-dimensional (3D) structure assembly of thin sheets after modification. Although the obtained aerogels exhibited good recyclability using the rinsing-drying recovery method, it is time-consuming and energy intensive. Alternatively, mechanical squeezing appears to be a simpler, faster, and more cost-effective method to recover the absorbed oil in terms of practical applications.15 To date, the majority of the reported oil absorbents based on CNF still suffer from poor compression robustness, resulting in dramatically reduced absorption capacity after mechanical squeeze.16 Thus, the preparation of mechanically robust and hydrophobic aerogel materials that can withstand multiple squeezing−absorption cycles remains to be a challenge. Freeze-casting is a simple and cost-effective technique for creating a highly anisotropic porous structure with superior mechanical performance.17,18 Typically, this procedure begins with the unidirectional freezing of an aqueous suspension, followed by the unidirectional growth of ice crystals, and the aligned porous structure replicates the shapes of the unidirectional grown ice crystals.19 In this study, we prepared anisotropic aerogels with a lamellar structure through a facile silylation process and freeze-casting technique, using biodegradable polyvinyl alcohol (PVA) and CNF as starting materials. To the best of our knowledge, this is the first time that the mechanical properties and hydrophobicity of CNF aerogels have been simultaneously optimized by freeze-casting PVA/CNF suspensions, together with hydrolyzed methyltrimethoxysilane (MTMS) sols. The obtained aerogels absorbed a wide range of organic solvents and oils with absorption capacities of 45−99 times their own weights, and exhibited outstanding reusability with oil sorption retention of over 84% after 35 cycles of absorption−squeezing tests.

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ij ρ yz Porosity = jjjj1 − zzzz × 100% j ρs z{ k

(1)

where ρ and ρs are the densities of the aerogel and solid materials, respectively. Considering the aerogel as a composite of cellulose, PVA, and polysiloxane, the density of the solid materials (ρs) was calculated according to eq 2 based on the solid density of each component and their weight ratios used in the formulation ρs =

1 wCNF ρCNF

+

wPVA ρPVA

+

wpoly(MTMS) ρpoly(MTMS)

(2)

where w and ρ are the weight fractions and solid densities of different components, and ρCNF, ρPVA, and ρpoly(MTMS) were fixed at 1500, 1267, and 1900 kg/m3, based on data in the literature.9,14 Characterizations. The morphology of obtained CNF was investigated through AFM (Bruker, Dimension ICON). For SEM observation, the aerogel samples were first frozen by liquid nitrogen to prevent deformation, then cut with a razor blade either parallel or perpendicular to the direction of freeze-casting, sputter-coated with platinum, and imaged using a scanning electron microscope (FESEM, XL30, FEI, USA). The BET specific surface area was determined by nitrogen physisorption using a Micrometric ASAP 2460 automated system. Fourier transform infrared (FT-IR) spectra were recorded by a Nicolet IS10 FT-IR spectrometer (Thermo Fisher Scientific, USA) in the range of 400−4000 cm−1 with a resolution of 4 cm−1. The surface compositions of aerogel samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Escalab 250Xi, US) employing Al Kα (1486.6 eV) as the radiation source. The surface spectra were gathered in the range of 0−1100 eV at a takeoff angle of 90°. Thermal stability measurements were carried out using a theromogravimetric analyzer (TGA, Q 50 TA Instruments, USA) from 30 to 700 °C at a 10 °C/min heating rate under N2 protection. The surface wettability of aerogels was measured by static contact angle analysis using a contact angle goniometer (OCA20,

MATERIALS AND METHODS

Materials. Moso bamboo (Phyllostachys pubescens Mazel) culms of four years were obtained from a plantation located in Zhejiang province, China. Polyvinyl alcohol (PVA, MW ∼ 95 000 g/mol) and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 98%) were purchased from Sigma-Aldrich. Sodium chlorite, acetic acid, potassium hydroxide, methyltrimethoxysilane (MTMS, 98%), and other chemicals were obtained from Aladdin Chemistry Co. Ltd. and used as received without any purification. Deionized (DI) water was used in all experiments. Preparation of CNFs. The bamboo powders successively went through a series of chemical pretreatments including acidified sodium chlorite (75 °C for 1 h, 5 times), 2 wt % potassium hydroxide (90 °C, 2 h), 5 wt % potassium hydroxide (90 °C, 2 h), and acidified sodium chlorite (75 °C, 1 h), to remove lignin and hemicellulose. To facilitate nanofibrillation, the purified samples were then oxidized with sodium hypochlorite using TEMPO as a catalyst under neutral conditions (60 °C, 48 h).20 The samples were thoroughly washed and filtrated. The oxidized samples were dispersed in water with a concentration of 0.6 wt % and then nanofibrillated using a high-pressure microfluidizer (Microfluidizer, M-110EH-30, Microfluidics Corp., USA) five times through interaction chambers of 200 and 87 μm at 10 000 psi. The B

DOI: 10.1021/acssuschemeng.8b03554 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Mechanism scheme for the synthesis of modified PVA/CNF aerogels.

Figure 2. Characterization of the unmodified and modified PVA/CNF aerogels: (a) FITR spectra, (b) XPS wide scan spectra, with inserted images of Si 2p narrow scan spectrum of MTMS-modified aerogel, and (c) TGA curves. Dataphysics Instrument, Germany). The volume of the water droplet was fixed at 4.0 μL, and five different positions were tested. Compressive Properties and Shape Recovery Ability. The compression tests were performed using an Instron 5848 testing system (Instron Co. USA) equipped with a 50 N load. Cubic samples of ∼20 mm side length were compressed either parallel or perpendicular to the freeze-casting direction, after they had been conditioned at 50% relative humidity and 23 °C for at least 24 h. The stress−strain curves were measured with a strain rate of 5 mm/min to 80% strain. Compression modulus was determined from the slope of the initial linear region of the stress−strain curves. A 100-cycle loading−unloading cyclic test was performed by measuring stress− strain curves at a strain rate of 200 mm/min. Three replicates were measured for each group. After the sample had been unloaded, the shape recovery ability was assessed according to the following formula

S(%) = 100 − εfinal

diesel, pump oil, corn oil, mineral, and motor oil) and solvents (acetone, ethanol, toluene, hexane, chloroform, and DMSO) for 2 min to reach equilibrium. Then, the soaked aerogels were taken out and weighed after the excess surface oil/solvent of aerogels was removed by filter paper. Absorption capacity (Q, g/g) was calculated according to following equation Q (g/g) =

W1 − W0 W0

(4)

where W0 and W1 are the weights of the aerogels before and after absorption, respectively. All the measurements were carried out in triplicate, and the average values were presented as the final results. Aerogel reusability was evaluated by simply manual squeezing due to its convenient operation and environmental friendliness. Absorption and squeezing of the aerogels were tested for 35 cycles using gasoline as the model oil.

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(3)

RESULTS AND DISCUSSION Structural Characterization. The original unmodified aerogels were fabricated by a freeze-casting technique, taking PVA and CNF as the starting materials. PVA is an inexpensive polymer that can be water-soluble, biocompatible, and

where εfinal is the strain at the final position when load detected reached 0 N. Oil/Solvent Absorption Capacity and Reusability of Aerogel. To explore the oil or solvent absorption capacity of the aerogels, the samples were weighed and immersed in various oils (gasoline, C

DOI: 10.1021/acssuschemeng.8b03554 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. SEM images of the unmodified (a, b) and MTMS-modified (c, d) aerogels at both cross and longitudinal sections.

the MTMS-modified PVA/CNF aerogels, which suggested the presence of silicon on the substrate surface.23 The peak of Si 2p at 102.6 eV was assigned to covalent bonds of Si−O−C, which confirmed that polysiloxane was chemically bonded onto the PVA/CNF surface through dehydration reaction. The TGA curves of the unmodified and silylated PVA/CNF aerogels are shown in Figure 2c, and the degradation data are summarized in Table S1. Silylation modification did not change the thermal stability of the aerogels until around 200 °C. However, beyond 200 °C, the MTMS-modified aerogels showed improved thermal stability compared to their unmodified counterparts. After treatment with MTMS, higher values in both T30% (397.7 °C vs 254.1) and Tmax (268.0 vs 248.64 °C) were observed for the modified aerogels compared to the unmodified samples (Table S1). The improved thermal stability should be due to the rigid polysiloxane coating that could retard the thermal decomposition of the modified aerogels.24 The morphologies of the aerogels before and after silylation were studied by examining their cross and longitudinal sections. As shown in Figure 3, the macropores made up with interconnected CNF sheets were clearly observed in the cross sections (Figure 3a,c), and the alignment of these pores was confirmed as the layered structure could be clearly observed in the longitudinal section (Figure 3b,d) regardless of the modification. The distinctive morphologies at the two sections indicated the anisotropic structure of the aerogels, and it was well preserved after modification, which overcome the limitation that the aqueous silylation process might significantly change the microstructure of MTMS-modified aerogels reported by the previous study.14 This is mainly due to the employed freeze-casting technique, in which laminar ice crystals grow along the temperature gradient from the bottom upward during freezing, producing aerogels with an aligned lamellar structure after sublimation.17 The aligned porous structure offers numerous microchannels for liquid transformation and storage, which are beneficial for oil treatments.25 The freeze-cast aerogels have densities of 6.6 and 10.2 kg/m3 for the original unmodified and MTMS-modified PVA/CNF

biodegradable, exerting significant function in manufacturing the sustainable aerogel.21 CNF was isolated from bamboo powder using the TEMPO-oxidized pretreatment, followed by microfluidization. The morphology of the as-prepared CNFs was characterized by AFM imaging (Figure S1), revealing nanofibrils with widths ranging from 10 to 30 nm. To synthesize modified aerogels, the PVA/CNF suspension was directly freeze-dried in the presence of acid-hydrolyzed MTMS sols using the freeze-casting technique. The mechanism for the hydrophobic modification of aerogels is illustrated in Figure 1. First, MTMS was acid-hydrolyzed in water, yielding reactive silanol groups (Si-OH). Then the silanol sol was added dropwise into the PVA/CNF suspension, and two reactions simultaneously occurred during the mixture stage: (i) the silanol groups may directly react with the hydroxyl groups of the substrate, thereby forming Si−O−C bonds; (ii) the silanol groups may also condense themselves on the substrate surface, forming rigid polysiloxanes with stable Si−O−Si bonds. Subsequently, the silylated suspension was unidirectionally frozen using a custom-built freeze-caster (Figure S2). Finally, polysiloxanes formed rigid layers on the substrate surface through dehydration reactions when the water was removed during the freeze-drying stage. Due to the simplicity of the freeze-casting process, aerogels may be easily shaped into cylinders, cubes, and cuboids (Figure S3). The hydrophobic modification was characterized by FTIR, XPS, and TGA techniques. Figure 2a shows the FTIR spectra of aerogels before and after silylation. After MTMS modification, stretching vibrations of Si-OH at ca. 920 cm−1 increased significantly, and new bands at the stretching vibrations of Si-C and/or Si-O-Si at ca.770 cm−1 and −CH3 deformation vibrations at ca. 1272 cm−1 of the polysiloxanes appeared, which indicated the successful formation of polysiloxanes on the substrate surface as expected.22 The interfacial bonds between polysiloxanes and PVA/CNF substrate were further confirmed by XPS analysis, as shown in Figure 2b. Compared to unmodified aerogels, two peaks at 154.08 and 103.08 eV, attributed to Si 2s and Si 2p, respectively, were intensified remarkably in the spectrum of D

DOI: 10.1021/acssuschemeng.8b03554 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering hybrid aerogels, respectively (Table 1). Both aerogels exhibited ultralight and highly porous (∼99.5%) characteristics. The

Table 2. Mechanical Properties of the Unmodified and MTMS-Modified PVA/CNF Aerogels

Table 1. Structural Characteristic of the Unmodified and MTMS-Modified PVA/CNF Aerogels

samples p-unmodified a-unmodified p-MTMS-modified a-MTMS-modified

density samples

ρa (kg/m3)a

ρs (kg/m3)b

porosity (%)

BET surface area (m2/g)

unmodified MTMS-modified

6.6 10.2

1375 1782

99.5 99.4

28.4 23.4

a ρa is the apparent density of the aerogels. bρs is the density of the solid scaffold.

compression modulus (kPa) 2.5 120.7 6.4 80.0

± ± ± ±

0.4 8.5 0.2 10.7

maximum stress at 80% strain (kPa) 11.0 56.0 13.0 40.3

± ± ± ±

2.1 4.2 1.4 6.8

thickness recovery at 80% strain (%) 71.5 60 83.5 55.8

± ± ± ±

4.9 2.6 2.1 5.4

anisotropic microstructure of the aerogel, as the uniform vertical alignment of sheets can bear high load in the aligned direction, which is consistent with other anisotropic aerogels and foams shown in the previous reports.28,29 Although the freeze-casted aerogels exhibited lower compressive modulus and strength in the perpendicular direction, their compression recovery properties were found to be much better in this direction (Table 2 and Figure S4). Interestingly, using a simple freeze-casting technique, the unmodified PVA/CNF aerogels could recover up to 60% and 75% of their original thickness in the aligned and perpendicular directions at a large deformation of 80%, while the aerogels without freeze-casting showed no recovery (Figure S4). After modification, the PVA/CNF aerogels exhibited good flexibility in the perpendicular direction with a recovery of 86% of its original height after 80% compression strain, whereas an obvious buckling effect and low recovery (56%) were observed in the aligned direction. Furthermore, the MTMS-modified aerogels can even bear 100 cyclic loading−unloading compression tests in the perpendicular direction at a reduced strain of 70% (Figure 5). Rapid shape recovery of the modified aerogel was observed during the unloading process, and no significant decrease in the maximum stress and thickness recovery was observed after 100 compression cycles, highlighting that the aligned porous structure was strong enough to withstand these cyclic compressions (Figure 5b,c, Movie S1). From these results, we speculated that the remarkable compressibility and

specific surface area was measured by the BET method, and a slight decrease of BET surface area was noted (28.40 m2/g for original aerogels and 23.41 m2/g for modified aerogels), as shown in Table 1. Compressive Properties. Good mechanical properties and shape recovery are crucial for the collection of absorbed oils by the simple squeezing method.26 Since the aerogels had an anisotropic structure, compression was performed in both aligned (i.e., along the freezing direction) and perpendicular (i.e., perpendicular to the freezing direction) directions. The “a-” and “p-” letter represents the aerogels compressed in the “aligned direction” and the “perpendicular direction”, respectively, as shown in Figure 4a. The compressive stress− strain curves of aerogels compressed in two directions are shown in Figure 4b, and the mechanical properties are summarized in Table 2. Under compression in the axial direction, aerogels exhibited the typical three-regime curves, including a linear elastic deformation stage at low strain, a plastic yielding plateau, and a final densification with a rapid rise of stress at high strain,27 while no apparent linear regime prior to the plateau was observed when compressed in the perpendicular directions (see the inserted images in Figure 4b). The compressive modulus and maximum stress at 80% strain were both much lower in the perpendicular directions (Table 2). This mechanical anisotropy is mainly due to the

Figure 4. Compressive properties of PVA/CNF aerogels: (a) Schematic illustration of the PVA/CNF aerogels compressed in two different directions, in which “a-” and “p-” stand for “aligned to freezing direction” and “perpendicular to freezing direction”, respectively; (b) stress−strain curves for the unmodified and MTMS-modified PVA/CNF aerogels compressed in two directions. E

DOI: 10.1021/acssuschemeng.8b03554 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 5. (a) Schematic illustration of the changes in the aligned lamellar structure for modified PVA/CNF aerogels with cyclic compression in the perpenducular direction. (b) Cyclic compressive stress−strain curves and (c) maximum stress and thickness recovery of the aerogels under a cyclic compression test.

flexibility of the aerogels were related to their anisotropic microstructure and hydrophobicity, and the illustration is shown in Figure 5a. During compression, stress was spread over through the interconnected pore walls and the strain was absorbed by the deformation of numerous pores. When the applied load was released, these squeezed pores could recover back to their original shape driven by the absorbed strain as well as the repulsive interactions between alkyl groups of polysiloxanes coating. Therefore, the aerogel could withstand large compressive strain without structural collapse. The high compressibility and flexibility make the MTMS-modified aerogels suitable for the oil cleaning applications requiring excellent shape recovery ability. Absorption Capacity and Reusability. Since the modified PVA/CNF aerogels exhibited combined advantages of ultralightweight, hydrophobicity, and compression recovery properties, we further explored their potential use as selective oil absorbents. Effective oil/water selectivity is a crucial property for oil absorbents, particularly for oil spill cleaning in an aqueous environment.30 After silylation, the MTMSmodified aerogels became hydrophobic as water formed droplets on the surface and maintained its round shape with a high contact angle of 140° for more than 10 min (Figure 6a). These tiny water droplets do not move when aerogels were upside down, indicating the formation of a Cassie impregnating wetting area.31 By dipping the modified aerogels into a mixture of oil/solvent and water, the aerogels could selectively remove the floating gasoline (Figure 6c and Movie S2), as well as the sunk chloroform (Figure 6d and Movie S3) completely within minutes. It is important to note that the oil/solventabsorbed aerogel continued to float on the water surface

without oil/solvent release, further confirming the high hydrophobicity and selectivity of the modified aerogels. This phenomenon is beneficial for facile pickup of absorbents in practical applications. Moreover, the modified PVA/CNF aerogels exhibited excellent absorption capacities, ranging from 45 to 99 g/g, toward a wide range of oils and solvents (Figure 6e). This should be attributed to the uniform alignment of numerous pores, not only allowing quick liquid absorption but also providing a large volume for the storage of the absorbed liquids. Significantly, the MTMS-modified aerogels have shown much higher absorption capacity than many recently reported aerogels and sponges, such as lignin/agarose/PVA aerogels (20−40 times),25 polymeric aerogels (25−53 times),32 chemical grafted NCF aerogels (20−46.6 times),33 cottonbased aerogels (30−59 times),23 winter melon carbon aerogels (16−50 times),34 and bacterial cellulose/silica aerogels (8−14 times).35 Nevertheless, the absorption capacity of the modified aerogels is still lower than that of carbon aerogels (50−190 times)36 and graphene aerogel (100−250 times).26 However, these efficient carbon absorbents involve a sophisticated preparation procedure and demanding production conditions, and thereby are expensive. Reusability of the absorbents and the recoverability of pollutants are key criteria for oil/solvents cleanup, because most pollutants may be useful. Here, the absorption− squeezing process has been demonstrated for oil collection. Figure 7a and Movie S4 display the process of the absorption− squeezing cycles. Interestingly, the squeezed aerogels cannot recover their original shape. However, they can almost recover their original shape after reabsorbing oil within a few seconds. F

DOI: 10.1021/acssuschemeng.8b03554 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 7. (a) Rapid shape recovery of the MTMS-modified aerogel after absorbing gasoline. (b) Reusability for absorption of gasoline with a compression−squeezing method.

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CONCLUSIONS In summary, high-performance biomass-based aerogels were successfully prepared by combining a facile aqueous silylation process with a freeze-casting technique. The modification process was achieved by freeze-casting a PVA/CNF suspension with the presence of MTMS-hydrolyzed sol. Owing to their low density (10.2 kg/m3), high porosity (99.4%), improved compressive recovery properties, and hydrophobic characteristics, the modified aerogels possess high absorption capacity of 45−99 times of their own weights, high oil/water selective sorption, and excellent reusability with absorption retention of more than 84% after 35 absorption−squeezing cycles. This is obviously time-efficient and environmentally friendly. Therefore, such aerogels are highly promising as absorbent materials to be used for oil spill control and environmental protection.

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Figure 6. (a) Water contact angle of the modified aerogels taken at 30 s and 10 min. (b) Hydrophobicity of the modified aerogel, and water has been dyed with blue color for easier visualization. (c, d) Images showing the selective removal of gasoline from the water surface and chloroform from under water. (e) Absorption capacities of various oils and solvents.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b03554. Figure S1: Optical images of as-prepared CNF suspension at 0.5 wt %, and AFM height image of diluted 0.1 wt % CNF suspension. Figure S2: Schematic illustration of a custom-made freeze-caster. Figure S3: Optical images of aerogels with diverse shapes. Figure S4: Optical images of unmodified and MTMS-modified PVA/CNF aerogels recovering their original dimensions after compressed to 80% strain aligned or perpendicular to the freezing direction. Table S1: TGA data for unmodified and MTMS-modified aerogels (PDF) Movie S1: Cyclic compression test for MTMS-modified PVA/CNF aerogel in the perpendicular direction (AVI) Movie S2: Removal of gasoline from the water surface using MTMS-modified PVA/CNF aerogel (AVI)

By compressing the aerogels in the perpendicular direction for oil collection, the absorption capacity of the aerogels slightly decreased from 58 to 49 times after 35 cycles, with less than 16% loss in the absorption capacity, indicating excellent reusability (Figure 7b). This is maybe attributed to excellent flexibility and elasticity associated with the aligned porous structure of aerogels via the freeze-casting technique.25 One main problem is that only ca. 57% of the absorbed oil can be collected by the squeezing process due to the oleophilicity of the modified aerogels. But compared to other methods, such as rinsing (solvent extraction), combustion (burning), and distillation (heating), squeezing is competitive because of its simplicity and sustainability.30 G

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Movie S3: Removal of chloroform from water using MTMS-modified PVA/CNF aerogel (AVI) Movie S4: Reusability for absorption of gasoline using MTMS-modified PVA/CNF aerogel with a compression−squeezing method (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-10-84789812 (Y.Y.). *E-mail: [email protected]. Phone: +86-10-84789909 (H.W.). ORCID

Xuexia Zhang: 0000-0002-1625-1897 Hankun Wang: 0000-0003-0338-1809 Ning Yan: 0000-0001-9060-8623 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the SFA 948 project (2015-4-48), the National Key R&D Program of China (2017YFD0600804), the Basic Scientific Research Funds of International Center for Bamboo and Rattan (1632018016), and the China Scholarship Council.

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DOI: 10.1021/acssuschemeng.8b03554 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.8b03554 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX