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Highly Compressible Wood Sponges with a Spring-Like Lamellar Structure as Effective and Reusable Oil Absorbents Hao Guan, Zhiyong Cheng, and Xiaoqing Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05763 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 1, 2018

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ACS Nano

Highly Compressible Wood Sponges with a Spring-Like Lamellar Structure as Effective and Reusable Oil Absorbents

Hao Guan, Zhiyong Cheng, and Xiaoqing Wang*

Department of Wood Modification, Research Institute of Wood Industry, Chinese Academy of Forestry, Xiangshan Road, Haidian District, Beijing 100091, P. R. China

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ABSTRACT Aerogels derived from nanocellulose have emerged as attractive absorbents for cleaning up oil spills and organic pollutants due to their light weight, exceptional absorption capacity, and sustainability. However, the majority of the nanocellulose aerogels based on the bottom-up fabrication process still lacks sufficient mechanical robustness because of their disordered architecture with randomly assembled cellulose nanofibrils, which is an obstacle to their practical application as oil absorbents. Herein, we report an effective strategy to create anisotropic cellulose-based wood sponges with a special spring-like lamellar structure directly from natural balsa wood. The selective removal of lignin and hemicelluloses via chemical treatment broke the thin cell walls of natural wood, leading to a lamellar structure with wave-like stacked layers upon freeze-drying. A subsequent silylation reaction allowed the growth of polysiloxane coating on the skeleton surface. The resulting silylated wood sponge exhibited high mechanical compressibility (reversible compression of 60%) and elastic recovery (~99% height retention after 100 cycles at 40% strain). The wood sponge showed excellent oil/water absorption selectivity with a high oil absorption capacity of 41 g g-1. Moreover, the absorbed oils can be recovered by simple mechanical squeezing, and the porous sponge maintained a high oil-absorption capacity upon multiple squeezing-absorption cycles, displaying excellent recyclability. Taking advantage of the unidirectional liquid transport of the porous sponge, an oil-collecting device was successfully designed to continuously separate contaminants from water. Such an easy, low-cost, and scalable top-down approach holds great potential for developing effective and reusable oil absorbents for oil/water separation.

KEYWORDS: cellulose nanofibers, compressibility, elasticity, oil absorption, recyclability, wood sponges

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The ever-increasing industrial wastewater discharge and frequent oil spill accidents occurring during oil exploitation and transportation have led to severe water pollution and caused growing environmental and ecological concerns.1-3 Thus, it is imperative to find ways to deal with oily wastewater. Various approaches, including gravity separation, centrifugation, flotation, bioremediation, in situ burning, and electrochemical processes have been applied to separate oil from water, but these conventional methods have limitations of low separation efficiency, secondary pollution, and high cost.4,5 Physical absorption using absorbents is one of the most promising approaches for wastewater treatment because of its ease of operation, cost-effectiveness,

and

eco-friendliness.

Among

the

numerous

oil

absorbents,

three-dimensional (3D) porous materials with special wettability have received considerable attention because of their highly porous structure, large specific surface area, and high absorption capacity. In recent years, 3D porous materials, such as synthetic polymers (e.g., polyurethane and melamine),6-8 silicone sponges,9-11 and carbon aerogels (e.g., carbon nanotubes and graphene),12-14 have been developed to separate oil from water. However, such materials usually suffer from either a complex fabrication process, poor mechanical properties, or environmental incompatibility. These drawbacks of conventional 3D oil absorbents are driving the research community to seek more efficient and sustainable alternatives. Given the natural abundance, low cost, biodegradability, and environmental friendliness, biomasses have attracted increasing research interest as promising building blocks to fabricate 3D aerogels (also called sponges or foams). A number of biomass-derived aerogels have been successfully developed as high efficient oil absorbents based on bacterial cellulose,15,16 nanocellulose,17-21 cotton,22 lignin,23 and chitin.24 In particular, cellulose aerogels based on cellulose nanofibrils (CNFs) extracted from plant cell walls have received the most attention for their attractive properties, such as light weight, high porosity, high surface area, flexibility, and tunable surface chemistry. With appropriate chemical functionalization, CNF-based aerogels can be converted from inherently hydrophilic to hydrophobic and display excellent oil/water selectivity.21 However, extracting CNFs from plant cell walls usually involves chemical or enzymatic pretreatments, and mechanical disintegration due to strong interfibril 3


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hydrogen bonding; moreover, assembling nanocellulose building blocks into the final aerogels is also complex.25,26 The complicated fabrication procedures and extensive energy consumption hamper large-scale application of such bottom-up approaches. More importantly, the majority of the CNF-based aerogels still lack sufficient mechanical robustness; their oil-absorption capacities often degrade upon multiple squeezing-absorption cycles,21,27,28 which is undesirable for oil recovery and reuse of the sorbent materials from a practical application viewpoint. Creating mechanically durable aerogel-type absorbents derived from CNFs for oil capture and recovery remains a challenge. As a renewable source, wood represents a 3D hierarchical scaffold of hollow fibers with cell walls consisting of stiff cellulose microfibrils embedded in a soft amorphous matrix of hemicelluloses and lignin.29 Taking advantage of the porous and hierarchical structure, wood has recently been explored to develop advanced functional materials for emerging applications in the areas of energy-saving buildings,30 energy storage,31 wastewater treatment,32 and solar steam generation.33 Highly compressible, anisotropic wood aerogels (up to 95% porosity) with aligned cellulose nanofibers have recently been developed by selectively removing the lignin-hemicellulose matrix in the cell walls via a chemical treatment.34 Such a top-down approach is easy and scalable and represents a promising direction for developing high-quality aerogel materials directly from wood. More recently, strong, mesoporous, and hydrophobic 3D wood/epoxy biocomposites were successfully fabricated to separate oil from water using delignified wood as the template.35 Such wood/epoxy biocomposites showed oil absorption as high as 15 g g-1, and had a much higher compressive yield strength and modulus than cellulose-based foams or aerogels. This demonstrates the feasibility of exploiting the attractive wood scaffold for oil/water separation. Nevertheless, the porous and hierarchical structure of wood still holds great potential to be optimized for creating efficient and recyclable oil absorbents. Herein, using low-density thin-walled balsa wood as the starting material, we propose an effective approach to fabricate mechanically compressible, highly porous, hydrophobic wood sponges with a special layered structure for effective oil/water separation. As illustrated schematically in Figure 1, the original honeycomb-like cellular structure of natural wood can 4


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be readily converted into a spring-like lamellar structure by selectively removing lignin and hemicelluloses via a two-step chemical treatment, resulting in a highly porous wood sponge (essentially cellulose) with well-preserved structural directionality of the cellulose nanofibers in the cell wall. The spring-like lamellar structure is believed to confer sufficient mechanical compressibility and elasticity to the wood sponge, which could sustain repeated squeezing without structural failure. In a subsequent step, the intrinsically hydrophilic wood sponge was functionalized with a silylating agent to create a hydrophobic but oleophilic coating by chemical vapor deposition (CVD), which is beneficial for maximally retaining the highly porous structure of the sponge. This hydrophobic wood sponge can selectively absorb oils from oil/water mixtures and shows a rather high oil absorption capacity of 41 g g-1. Moreover, the absorbed oils can be readily recovered by simple mechanical squeezing. The wood sponge maintained high oil-absorption capacity after multiple squeezing-absorption cycles, demonstrating excellent recyclability. Such mechanically resilient, scalable wood sponges hold great promise as effective and reusable oil absorbents for selective oil/water separation.

Figure 1. Schematic illustration of the fabrication of highly compressible wood sponges with a spring-like lamellar structure directly from natural balsa wood for selective oil/water separation.

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RESULTS AND DISCUSSION Morphology and Structure. Balsa wood with a low density (~92 mg cm-3) was selected as the starting material to prepare the wood sponges. As shown in Figure 2a, natural balsa wood has a honeycomb-like cellular structure with thin cell walls and high porosity from the cross-sectional view, while vertically aligned fiber tracheids and large-lumen vessels can be clearly seen in the longitudinal sections (Figure S1). The wood cell wall is comprised mainly of cellulose, hemicelluloses, and lignin, which intertwine with each other to provide necessary mechanical integrity to the bulk wood. Such a 3D hierarchical wood scaffold with special structural anisotropy shows great potential for further functionalization. A simple delignification process using acidified NaClO2 aqueous solution was initially adopted to selectively remove the lignin component in wood to fabricate the wood sponges. After delignification, the original yellowish wood block became totally white, indicating successful removal of the dark-colored lignin with the colorless polysaccharides left behind (Figure 2b). Removal of lignin was confirmed by Fourier transform-infrared (FT-IR) analysis of the natural wood and delignified wood, where characteristic lignin peaks at 1,593, 1,505, and 1,462 cm-1 (aromatic skeletal vibrations) disappeared after the chemical treatment (Figure 2d). Note that the hemicellulose-related peaks at 1,736 and 1,235 cm-1 remained during the treatment, indicating the selectivity of the NaClO2 solution for removing the lignin fraction. The removal of lignin and the retention of hemicelluloses were verified quantitatively by a chemical composition analysis (Figure 2e). The delignification process also caused significant changes in the morphology and structure of the bulk wood. As shown in Figure 2b, despite a well-preserved external dimension, the delignified wood exhibited extensive delamination of the internal structure due to breakage of the cell walls by the chemical extraction, which presumably occurred along the relatively weak ray parenchyma that runs in the radial direction of the bulk wood. However, the honeycomb-like cellular structure was essentially preserved after the delignification process. High-resolution scanning electron microscopic (SEM) images reveal the changes in the nanostructure of the cell wall (Figure 2b). After removing the lignin, the original densely packed cell wall evolved into a highly loosened skeleton with numerous nanopores generated 6


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in the wall. The well-aligned cellulose nanofibers in the secondary cell wall (diameter, 20–30 nm) were distinctly exposed (Figure S2). The formation of numerous nanoscale pores due to delignification was supported by the Brunauer–Emmett–Teller analysis through physisorption of N2 (Figure S3). The specific surface area (SSA) increased notably from 3.3 m2 g-1 in the natural wood to 21.6 m2 g-1 in the delignified wood. The mass loss due to lignin removal resulted in an approximate 50% decrease in density, and the delignified wood possessed porosity of 97.1% (Table S1). The increased surface area and porosity were expected to reduce the transverse rigidity of the cell walls, thus contributing to the flexibility of the delignified wood. However, while the delignified wood could be compressed to 60% of its original thickness, it did not completely recover after releasing the stress with substantial plastic deformation (~30% residual strain) (Figure S4). This plastic deformation was probably due to the irreversible collapse of the cell walls during compression, as the honeycomb-like cellular structure was still mostly preserved in the delignified wood. Therefore, the delignification process alone was insufficient to make highly compressible wood sponges.

Figure 2. Morphology and structure of different wood samples. (a) Photograph of the natural wood and the scanning electron microscopic (SEM) image showing the honeycomb-like porous structure. (b) Photograph of the delignified wood and the SEM image exhibiting delamination of the internal structure with exposed cellulose nanofibers in the cell wall. (c) Photograph of the wood sponges and a SEM image revealing a spring-like lamellar structure 7


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with numerous wave-shaped stacked layers. (d) Fourier transform-infrared spectra of the different wood samples. (e) Relative content of cellulose, hemicelluloses, and lignin in different wood samples obtained from the chemical composition analysis.

To further optimize the structure, an additional chemical treatment with NaOH solution was used to remove the residual hemicellulose in the delignified wood. The massive removal of hemicelluloses can be deduced from the entire absence of the hemicellulose-related peaks at 1,736 and 1,235 cm-1 in the FT-IR spectra (Figure 2d). The chemical composition analysis also revealed a nearly complete removal of hemicelluloses in the wood sponges, which consisted primarily of cellulose (Figure 2e). As a result, extraction of the matrix components eventually resulted in highly porous wood sponges comprised of a cellulose skeleton, with a low density of ~30 mg cm-3 and a high SSA of 23.4 m2 g-1 (Table S1). After removing the lignin and hemicelluloses, the original honeycomb-like structure disappeared completely and evolved into a spring-like lamellar structure with numerous wave-like stacked layers (Figure 2c). This finding suggests that the additional chemical treatment with NaOH facilitated breakage of the thin cell walls, leading to the formation of the lamellar structure. The thin lamellas were likely crumpled into wave-like layers by the ice crystals during freeze-drying. A close-up view of a single layer reveals the isolated cellulose nanofibers that were previously embedded in the lignin and hemicellulose matrix. The preferential orientation of the cellulose nanofibers along the axial direction can be clearly visualized on the longitudinal section of the wood sponge, indicating that the alignment of the cellulose nanofibers in the cell wall was well preserved after the chemical treatment (Figure S5). Note that the cellulose Iβ crystalline structure of the natural wood was not altered during the treatment (Figure S6). This special spring-like lamellar structure was expected to contribute to high mechanical flexibility. The mechanical compressibility of the porous wood sponge along the layer-stacking direction is demonstrated in Figure 3a. The wood sponge can bear a compressive strain as high as 60% and can completely recover its original height after release of the stress, displaying excellent mechanical compliance in sharp contrast to the delignified wood. Figure 3b presents the compressive stress-strain curves for the wood sponge with maximum 8


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compressive strains of 20%, 40%, and 60%. Two distinct regions were observed during loading of the sample, including a linear elastic region below ~30% strain and a subsequent densification region with the stress being increased sharply. The initial linear elastic region reflects the elastic deformation of the cellulose skeleton in the wood sponge, and the subsequent densification region was associated with eliminating air voids and the mutual collisions between the stacked layers upon further loading. The strain can decrease to zero when the stress is released, indicating complete volume recovery without plastic deformation, which is consistent with the observation in Figure 3a. Similar compressive stress-strain behavior has also been reported in other aerogel-like materials.15,36,37 A cyclic compression test with 100 loading-unloading cycles was carried out on the wood sponge at a constant strain of 40% to evaluate its fatigue resistance (Figure 3c). The results revealed small plastic deformations with height retention (expressed as the percentage of the original height) of ~93% after 100 compression cycles, highlighting the structural robustness of the wood sponges. This was also reflected in the energy dissipation of the different cycles. As shown in Figure 3d, the energy loss coefficient decreased from 0.54 on the first cycle to 0.25 on the 100th cycle, which was relatively low compared with other aerogel materials.38,39 The good mechanical compressibility and fatigue resistance of the wood sponge was attributed to its special spring-like lamellar structure with numerous large pores between the curved layers, which allow large deformation without localized structural collapse. Similar wave-shaped lamellar architecture has also been reported in a carbon-graphene composite exhibiting super-elasticity, high compressibility, and superior fatigue resistance.40 Note that the wood sponge with aligned cellulose nanofibers exhibited obviously anisotropic mechanical properties (Figure S7). When compressed along the fiber-alignment direction (parallel to the stacked layers), the wood sponge was easily crushed due to buckling of the oriented cellulose nanofibers, which is totally distinct from the high compressibility when the stress was applied along the layer-stacking direction.

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Figure 3. Mechanical compressibility and fatigue resistance of the wood sponge. (a) Photographs of the wood sponge showing its reversible compressibility along the layer-stacking direction. (b) Stress-strain curves of the wood sponge under compression at different maximum strain values of 20%, 40%, and 60%, respectively. (c) Stress-strain curves of wood sponge under cyclic compression at the maximum strain of 40%. (d) Height retention and energy loss coefficient of the wood sponge during different cycles derived from the stress-strain curves in (c).

Silylation of Wood Sponges. The wood sponge is an ideal candidate for highly efficient extraction of organic pollutants and oils due to its high porosity, mechanical flexibility, and stability. However, because of their amphiphilicity, the as-prepared wood sponges absorbed both water and silicone oil drops (Figure 4a), making them unsuitable for selective removal of oils from oil/water mixtures. A simple CVD process with methyltrimethoxysilane (MTMS) as a silylating agent was thus applied to tune the wettability. The highly porous, spring-like lamellar structure of the wood sponge was well retained after silylation (Figure S8). Successful silylation of the cellulose skeleton was revealed by the energy dispersive X-ray (EDX) mapping image (Figure 4d), in which silicon is quite homogeneously distributed in the cellulose skeleton, suggesting rather uniform coverage of the siloxane compounds on the skeleton surface. The longitudinal SEM image of the silylated wood sponge clearly shows the growth of particle-like polysiloxane coatings on the surface of the stacked layers (Figure S9). 10


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Silylation of the porous wood sponges was further confirmed by the appearance of the characteristic silane peaks at 755 cm-1 (δ(C-H)) and 1,255 cm-1 (Si-CH3 bending) in the FT-IR spectrum, as well as by the existence of Si-O and Si-C in the XPS spectra of the modified sample (Figure S10a and b). Possible reaction between MTMS and the cellulose skeleton surface are illustrated in Figure S10c. A cross-linked polymeric structure could be formed by vertical polymerization of MTMS under humid conditions.41

Figure 4. Wettability and mechanical compressibility of the silylated wood sponge (SWS). Photographs of water and oil droplets on the (a) pristine and (b) SWS, respectively. (c) Photographs of the SWS floating on the water surface in contrast with the pristine wood sponge (WS) sinking in the water. (d) Energy dispersive X-ray spectroscopic maps showing the elemental distribution of C, O, and Si on the SWS cross-section. (e) Stress-strain curves of the SWS under compression at different maximum strains of 40% and 60%, respectively. (f) Stress–strain curves of the SWS under cyclic compression at the maximum strain of 40%. (g) Height retention and energy loss coefficient of the SWS during different cycles derived from the stress-strain curves in (f).

The silylation treatment made the wood sponges quite hydrophobic, with a water contact 11


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angle of 151°, while the silicone oil quickly penetrated into the substrate, exhibiting both hydrophobic and oleophilic properties of the silylated wood sponge (Figure 4b). The inner surface of the modified sponge also displayed a high water contact angle of 134° (Figure S11), indicating rather homogeneous silylation throughout the entire structure. This was attributed to the highly porous structure that facilitated efficient diffusion and penetration of MTMS into the sample during CVD. Due to its hydrophobicity, the silylated wood sponge readily floated on the water surface as opposed to immediate sinking of the pristine sample (Figure 4c). Interestingly, the silylation treatment made the wood sponges more mechanically robust against repeated compression/release, as evidenced by a height retention of ~99% after 100 compression cycles and a small reduction in energy dissipation during cyclic compression (Figure 4e–g). The enhanced fatigue resistance of the modified wood sponge was presumably due to the polysiloxane coating growing on the skeleton surface, which appeared to reinforce the cellulose skeleton and meanwhile might induce repulsive interactions between the polysiloxane methyl groups present at the cellulose surface.18 Note that the silylation treatment had little effect on the density and surface area of the sponge, as it maintained its highly porous structure (Table S1).

Oil Absorption Capacity and Reusability. The silylated wood sponge (SWS) could be used as an ideal absorbent for cleaning up oil spillage and organic pollutants due to its hydrophobicity, high porosity, and mechanical robustness. As shown in Figure 5a and Video S1, when a small piece of SWS was placed in the silicone oil/water mixture, it selectively absorbed the silicone oil (dyed with Oil Red-O), leaving behind the clean water. After absorption, it still floated on the water surface, allowing easy collection and recycling. The SWS could also readily remove dichloromethane from the bottom of the water, while maintaining its shape and structure due to its mechanical robustness (Figure 5b and Video S2). The absorption capacity of the SWS was defined as the mass of absorbed liquid per unit mass of dry absorbent. As shown in Figure 5c, the SWS exhibited a high absorption capacity of 16–41 times its own weight for a wide range of oils and organic solvents, and the absorption capacity generally increased with the density of the liquids. Note that the volume-based 12


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absorption capacities of the SWS were greater than 70%, indicating that the majority of the pores in the sample were filled with oil or solvent. The high absorption capacity of the SWS was attributed to its high porosity, which provides sufficient space for storing the absorbed liquids. The absorption capacity of our SWS was comparable or even superior to many reported absorbents (Table S2), including polyurethane sponge (15–25 g g-1),6 biomass-derived synthetic polymer aerogel (20–40 g g-1),23 chitin sponge (29–58 g g-1),24 and a fiber/silica composite aerogel (up to 16 g g-1).28 Nevertheless, it is still distinctly lower than that of aerogel-based absorbents derived from graphene (200–600 g g-1),12 carbon nanotubes (80–180 g g-1),14 and nanocellulose (49–102 g g-1).18 However, preparing these highly efficient absorbents involves either costly raw materials or complex synthetic procedures that limit their industrial applications.

Figure 5. Oil absorption performance of the silylated wood sponge (SWS). Removal of (a) silicone oil on the water surface and (b) dichloromethane from the water bottom with the SWS. (c) Mass-based absorption capacities of the SWS for various oils and organic solvents. The two dashed lines represent the theoretical volume-based absorption capacity (v/v) corresponding to the case where the sample is nominally 90% or 70% filled with the oils or 13


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organic liquids. (d) Photographs showing recovery of silicone oil from the SWS by finger squeezing. (e) Cyclic absorption capacities of the SWS for silicone oil.

Reusability of the SWS and the recoverability of the absorbed liquids are crucial for its practical application as an oil/chemical absorbent. As shown in Figure 5d and Video S3, the swollen SWS fully absorbed with silicone oil was directly squeezed by hand to discharge the embedded oils, and the squeezed sample rapidly recovered its original shape upon release without any deformation due to its exceptional compressibility and elasticity. The absorption capacity of the SWS subjected to cyclic squeezing and absorption was evaluated (Figure 5e). Only a slight decrease in adsorption capacity from 24.5 to 23.0 g g-1 was found after ten compression-absorption cycles, indicating stable absorption performance and good reusability of the SWS used as the absorbent. This was in sharp contrast with many nanocellulose-based aerogels lacking sufficient mechanical resiliency as absorbents, whose oil-absorption capacity often degrades after multiple compression-absorption cycles. Only a few methods, such as distillation and solvent extraction, can be used to recover oil; however, these methods are usually complex, time-consuming, and low efficient, restricting their scalable applications.21 Compared with the nanocellulose-based absorbent materials based on a bottom-up fabrication process, our wood sponges derived from a top-down approach have some attractive features, such as ease of preparation, desirable absorption capacity, good reusability, and high feasibility for large-scale synthesis, highlighting their potential for oil/water separation.

Continuous Oil Separation. The prepared SWS exhibited anisotropic liquid transport properties. As shown in Figure S12, when a dyed hexane droplet was dropped on the top surface of the SWS with vertically aligned cellulose nanofibers, it was immediately absorbed and penetrated through the entire thickness to the backside of the sample along the channel direction. In contrast, when a droplet was placed on the transverse surface of the SWS, it spread quickly along the channel direction, whereas the depth of the liquid penetration perpendicular to the direction of fiber alignment was small, indicating anisotropic liquid transport, with much higher transport speed along the channel direction. This unidirectional 14


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liquid transport along the direction of the aligned fibers was attributed to the capillary action produced in the wood sponge microchannels. An oil-collecting device was thus designed by taking advantage of the unidirectional liquid transport of the SWS, which was used as a filter to realize continuous separation of oils from the water surface (Figure 6a). A glass tube connected to a pipe was inserted into the SWS along the direction of the fibers (Figure 6b). The other side of the pipe was connected to a collecting bottle that was connected to a vacuum pump. As shown in Figure 6c and Video S4, when the SWS was dipped into the water-hexane (dyed with Oil Red-O) mixture, the hexane on the surface was rapidly and continuously pumped through the SWS filter into the pipe under vacuum with a flux rate of 84.7 L h-1 g-1 and was eventually collected in the bottle. No water was pumped into the bottle due to the hydrophobicity of the SWS filter, indicating good selectivity during oil/water separation. Notably, the separation speed depended on the viscosity of the liquid. As shown in Figure S13, hexane, petroleum ether, and toluene with low viscosities were quickly pumped through the SWS filter with a flux rate higher than 79 L h-1 g-1, while a low flux rate (6.8 L h-1 g-1) was observed for the high viscosity motor oil. This result suggests that the oil-collecting device based on the modified wood sponge can be used to continuously separate contaminants from water. In this way, oil/water separation and oil collection can be achieved simultaneously, thus saving the complex and time-consuming process of oil recovery from the absorbents. This oil-collecting device allows in situ collection of contaminants from a water surface effectively, which can be used as portable equipment for dealing with oil or organic solvent leakages.

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Figure 6. Continuous separation of oil from water. Photographs of (a) the oil-collecting device and (b) the silylated wood sponge used as a filter. (c) Photographs showing continuous separation of hexane (dyed with Oil Red-O) from the water surface using the oil-collecting device.

CONCLUSIONS In summary, we have demonstrated an effective top-down approach to fabricate highly porous, mechanically resilient, and hydrophobic wood sponges directly from natural balsa wood by selectively removing lignin and hemicelluloses followed by freeze-drying and a subsequent silylation reaction. The obtained SWS possessed a special spring-like lamellar structure with numerous wave-like stacked layers. Such a lamellar structure conferred high mechanical compressibility and elasticity to the anisotropic wood sponge, which could sustain repeated squeezing along the layer-stacking direction without structural failure. These mechanical properties, together with low density, high porosity, and hydrophobic/oleophilic features, endowed the SWS with high oil absorption capacities up to 41 g g-1 and excellent recyclability by simple mechanical squeezing. Continuous separation of contaminants from water was also demonstrated based on an oil-collecting device, with the SWS acting as a filter. Such mechanically resilient, sustainable wood sponges hold great potential as effective and 16


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reusable oil absorbents for cleaning up oil spillage and organic pollutants. The top-down approach demonstrated here is facile, low-cost, and scalable, which could provide guidelines for developing cellulose scaffolds with special mechanical properties directly from natural wood.

MATERIALS AND METHODS Materials and Chemicals. Balsa wood (Ochroma pyramidale) with a density of ~92 mg cm-3 was used to fabricate the wood sponges. The wood samples were cut into the dimension of 15 × 15 × 10 mm3 (radial × tangential × longitudinal) and oven-dried at 80 °C for 12 h before use. Sodium chlorite (NaClO2, 80%), acetic acid, sodium hydroxide, and methyltrimethoxysilane (MTMS, 98%) were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). All of these chemicals were used as received without further purification. Ethanol, n-hexane, petroleum ether, acetone, toluene, dimethyl sulfoxide, dichloromethane, chloroform, and silicone oil were purchased from Beijing Chemical Works (Beijing, China). Motor oil and olive oil were purchased from local stores. Deionized (DI) water was used as the solvent to process the wood. Preparation of Wood Sponges. The wood sponges were prepared by selectively removing the lignin and hemicellulose fractions from the cell wall, followed by freeze-drying. Specifically, the wood samples were initially delignified using an aqueous solution of 2 wt% NaClO2 buffered with acetic acid at pH 4.6 for 6 h at 100 °C. Then, the delignified samples were further extracted with an 8% NaOH solution at 80 °C for 8 h to remove the hemicelluloses fraction. The treated samples were carefully rinsed in ethanol-water solutions to remove the remaining chemicals. Finally, the samples were freeze-dried at −56 °C for 36 h using a freeze-dryer (Scientz-10N, Ningbo Scientz Biotechnology Co., Ltd, Ningbo, Zhejiang, China) after being frozen at −15 °C for 6 h, resulting in the formation of highly porous wood sponges. Porous materials based on direct freeze-drying of the delignified wood were also prepared for comparison. Silylation of Wood Sponges. The wood sponges were hydrophobically modified with a silylating agent (MTMS) via CVD. Specifically, two small open bottles containing 2 mL of 17


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MTMS and 2 mL of DI water respectively were placed in a desiccator together with the as-prepared wood sponges. The desiccator was tightly sealed and heated in an oven at 70 °C for 2 h. Then, the SWS were placed in a vacuum oven at 60 °C for 24 h to remove the unreacted MTMS. Characterization. The morphology and structure of the wood sponges was characterized by field emission scanning electron microscopy (Hitachi SU-8010, Tokyo, Japan) equipped with an energy dispersive X-ray spectroscopic detector for mapping. FT-IR spectra were recorded on a Nicolet 6700 spectrometer (Thermo Scientific, Waltham, MA, USA) over the scan range of 400–4,000 cm-1. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250Xi spectrometer (Thermo Scientific, USA) with a monochromatic Al Kα X-ray source. Acid-insoluble lignin content in the samples was determined with a 72 wt% sulfuric acid solution according to the GB/T 2677.8-94 standard; holocellulose and α-cellulose contents were measured in accordance with the GB/T 2677.10-1995 and GB/T 744-1989 standards, respectively. N2 adsorption-desorption isotherms were determined at 77K with an ASAP 2020 instrument (Micromeritics Instrument Corp., Norcross, GA, USA). The specific surface area (SSA) was calculated according to the Brunauer–Emmett–Teller method. The apparent volumetric mass density of the wood sponges was calculated by weighing the sponges and measuring their volumes. The porosity of the sponges was calculated based on the density of the sponge and the solid scaffold (cellulose) according to a previous method.18 The mechanical compressibility of the wood sponges was evaluated using a universal testing machine (CTM 2050, Shanghai XieQiang Instrument Manufacturing Co., Ltd, Shanghai, China) equipped with a 25 N load cell. Water contact angles were measured using a contact angle meter (JC2000D, Shanghai Zhongchen Powereach Co., Shanghai, China) at room temperature. Liquid Absorption Capacity and Reusability. The absorption capacities of the SWSs were evaluated toward a wide variety of organic solvents and oils. In a typical absorption test, the weighed samples were immersed in 50 mL of solvent or oil for 10 min to reach the mass absorption equilibrium, and then picked out for weight measurements. The weight gain (g g-1), denoted as the mass of absorbed solvent or oil (g) per unit mass of dry absorbent (g), was 18


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used to evaluate oil absorption capacity. At least three replicates were measured to calculate the average value. The reusability of the SWSs as absorbents was evaluated by mechanical squeezing. The sponge was first dipped into silicone oil to reach maximum absorption, and the absorbed oil in the sponge (stored oil) was measured. Then, the absorbed oil was recovered by simple finger squeezing, and the sponge was weighed again to determine the mass of the residual oil trapped in the sample. A total of 10 absorption-squeezing cycles was performed.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM images of the natural balsa wood, delignified wood, wood sponge and silylated wood sponge; N2 adsorption-desorption isotherms and XRD patterns; compressive stress-strain curves of the delignified wood and wood sponge; FT-IR and XPS spectra of the silylated wood sponge; photographs of water contact angle and anisotropic liquid transport in the silylated wood sponge; flux rates of various oils and organic solvents based on the oil-collecting device; density, SSA, and porosity of various samples; comparison of various 3D porous absorbents; descriptions of Videos S1-S4 (PDF) Video S1 (AVI) Video S2 (AVI) Video S3 (AVI) Video S4 (AVI)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. ORCID Xiaoqing Wang: 0000-0002-7460-0004 19


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ACKNOWLEDGMENTS This study was financially supported by the National Key Research and Development Program of China (2017YFD0600202) and the National Natural Science Foundation of China (NSFC grants 31570554).

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