Flexible Underwater Oleophobic Cellulose Aerogels for Efficient Oil/Water Separation.

Hybrid cellulose/N,N'-methylene bisacrylamide/graphene oxide (GO) aerogels with high flexibility and underwater oleophobicity were fabricated via the NaOH/urea solvent system. The as-prepared aerogels demonstrated low density, high porosity, and good flexibility. Underwater oleophobicity is attributed to the abundant hydrophilic groups in the aerogel skeleton, rough surface, and homogeneous distribution of GO. The samples were shaped into the membrane and filtered for oil/water separation by gravity. The separation efficiency over membrane-shaped CG1 was 99.8% with a permeate flux of 22,900 L/(m2·h). Moreover, excellent reusability and durability were observed under long-term tests and corrosive conditions.

Introduction

Recently, industrial oily wastewater has become an urgent threat to our marine ecosystem and public health.[1,2] In this context, tremendous efforts have been dedicated in developing effective and eco-friendly oil/water separation techniques, such as heat treatment, coalescers, bioremediation, depth filters, and absorption process.[3,4] Among these methods, physical absorption and filtration are regarded as the optimal approaches because of their low operation cost and high removal efficiency. Traditional oil/water separation materials can be divided into two kinds: inorganic particles (e.g., zeolites, activated carbon, and silica) and polymer-derived materials.[5−7] The inorganic materials are subjected to many drawbacks, such as poor recyclability, insufficient buoyancy, and limited capacities. In contrast, the organic materials are used more widely in the oil and organic pollutant cleaning.[8,9] For example, the commercial available polyurethane sponges and polypropylene fibers showed excellent oil-sorption capacities.[10,11] However, these synthetic polymers suffer from environmental incompatibility and complicated fabrication procedures.[12−14] There still exists a growing demand for the development of sustainable, low-cost, and efficient oil/water separation materials.[15]

Bio-based cellulose has embarked considerable research interests because of its renewability, biodegradability, and abundance in nature.[16−20] Especially, the cellulose aerogels and membranes have demonstrated great potential in wastewater purification.[21,22] Unfortunately, the cellulose aerogels require further surface modification to improve oil/water selectivity.[23] Enormous attention has been paid to the fabrication of oleophilicity/hydrophobicity materials (termed oil-moving).[24−26] To change the inherent hydrophilicity, chemical vapor deposition (CVD) is frequently adopted to achieve surface decoration and hydrophobicity. However, modifiers mainly involve toxic chlorosilanes or fluorine reagents.[27−29] For instance, Jiang and Hsieh diffused the vapor of triethoxyl(octyl)silane into the cellulose skeleton after freeze-drying to prepare hydrophobic aerogels.[26] Zhou et al. functionalized TEMPO-oxided nanocellulose via methyltriethoxysilane. The water contact angle could reach to 147°, and the oil adsorption capacity was up to 185 g/g.[30] In addition, the CVD process is diffusion-controlled, and the grafting distribution between the outer layer and inner layer might be nonuniform. In this case, constructing composite materials through the one-pot method is a general way to improve the homogeneity. Tingaut and co-workers prepared aerogels via directly freeze-drying a mixed cellulose/methyltrimethoxysilane suspension.[24] The silylated aerogels displayed good homogeneity and could collect a large range of oils with absorption capacities up to 100 times of their own weight. Xiao and co-workers doped graphene on the cellulose sponge through electrospinning process, and the hybrid sponge had a high oil removal efficiency.[31] The main disadvantage of oil-moving materials is that the absorbed oils could pollute or plug the aerogels and lead to secondary pollution.[32]

Although the surface engineering of cellulose aerogels is widely explored, the majority of prior works are focused on the hydrophobic modification.[33,34] Alternatively, the cellulose aerogels with hydrophilicity/oleophobicity are largely ignored, which could stop the oil from flowing through while allowing water to permeate.[35,36] The abundant hydroxyl groups make cellulose an ideal candidate for constructing hydrophilic materials. The studies concerning underwater oleophobic modification are relatively scarce, and conventional cellulose-derived oleophilic materials face the problems of poor mechanical property and need to be surface-coated on substrates.[37−39] Lu et al. coated the cellulose hydrogel prepared from the lithium hydroxide/urea system on stainless mesh. The mesh showed a separation efficiency of 99.0% and a flux of 38,064 L/(m2·h).[40] Other reports employed hydrophilic polymers to construct hybrid aerogels and achieve under superoleophobicity. For instance, Deng et al. sulfonated the cellulose nanofiber aerogel to acquire superoleophobicity.[41] Chen et al. incorporated nanofibrillated cellulose into the chitosan matrix to prepare the superoleophobic hybrid aerogel.[42] Li et al. deposited nanocellulose on cellulose esters through the additive printing method to prepare all-cellulose membranes.[13] The prepared membranes can be applied to the separation of oil/water nanoemulsions.

Herein, we describe a cellulose aerogel, with high flexibility and low density, where the cellulose matrix is functionalized by graphene oxide (GO) and N,N′-methylene bisacrylamide (MBA) to create underwater oleophobicity. GO is selected because of its large amounts of oxygen-containing groups, and MBA acts as a cross-linker. The cellulose is dissolved in a green NaOH/urea aqueous solution, which could improve the homogeneity of composite aerogels. Such modified aerogels demonstrate underwater oleophobicity because of the rough surface and the presence of huge amount of hydrophilic groups. The aerogels exhibit good oil/water separation performance and long-term stability. Moreover, the outstanding flexibility makes the aerogels easily fabricated into various configurations, according to specific requirements. This work sheds lights on the development of efficient bio-polymer materials toward oil removal.

Results and Discussion

Fabrication and Structure of Composite Aerogels

Cellulose is chosen as the aerogel skeleton because of its abundance, sustainability, biodegradability, and eco-compatibility. To achieve underwater oleophobicity and improve mechanical stability, GO and MBA are integrated into an aerogel network. As illustrated in Scheme , cellulose aerogels are prepared in a green and facile NaOH/urea solvent system. The overall preparation procedures involve three steps. First, the NaOH/urea solution was used to dissolve cellulose. This solvent system had an excellent cellulose dissolving ability.[17,43] Then, GO and MBA were added, and the homogenous mixture was put into a mold. MBA acted as cross-linkers to connect −OH groups over cellulose and MBA terminal C=C groups.[44] MBA could lock cellulose chains and enhance mechanical performance of composite aerogels. From the uniform color of cellulose/GO/MBA suspension and hydrogels (Scheme ), it can be referred that there is a good dispersion of GO in the aerogel matrix. This is attributed to the hydrogen-bonding interactions between cellulose and GO, which prevented the aggregation of GO.[45] At higher GO loadings (e.g., 5 wt %), sedimented particles would appear in the mixed suspension. Finally, the cellulose/GO/MBA hydrogels were washed to be neutral, kept for 12 h, and transferred into aerogels via freeze-drying.

Scheme 1

Schematic Illustration of the Fabrication Process of Cellulose Aerogels

The photos of the as-prepared aerogels are present in Figure a. The color is getting darker with the increasing GO addition. The measured density of the aerogels was about 0.01 g/cm–3, and the porosity of all aerogels was ≥97%. In particular, the cellulose/MBA/GO aerogels had a remarkable recoverability (Video S1, Supporting Information). The highly compressed aerogels could recover to its pristine height within a short time after releasing the loading. The good recoverability facilitates the reuse process and endows the aerogels with deformability. X-ray diffraction (XRD) patterns of cellulose/MBA (CG0) and cellulose/MBA/GO hybrid aerogels are displayed in Figure b. As observed, the cellulose aerogels prepared from NaOH/urea solution show peaks at 2θ = 12.0, 19.9, and 21.2°, indicating that the transformation from native Type I cellulose to Type II cellulose.[43,46] Upon GO and MBA addition, the crystallinity of the aerogels just changed slightly. There is no significant diffraction peaks of GO, which might be ascribed to the low content and high dispersion.[45]Figure c records the Fourier-transform infrared (FT-IR) spectra of the cellulose aerogels. The peaks located around 3450 and 2890 cm–1 are attributed to the stretching vibrations of −OH groups and C–H bonds, respectively.[47] The characteristic stretching vibration signals of C=C groups (1620 cm–1) in MBA could not be found in the CG0 sample, providing evidence for the cross-linking reactions between cellulose and MBA. Moreover, the stretching vibrations of the −OH groups decreased obviously after the introduction of GO, suggesting that the formation of hydrogen bonds occurred.[48]

Figure 1

Digital photos (a), XRD patterns (b), and FT-IR spectra (c) of cellulose aerogels.

Next, the surface wetting properties in air of the as-prepared aerogels were investigated. Both the water droplet and oil droplet quickly penetrated into the samples (Figure S1, Supporting Information). The contact angles were near 0°. This phenomenon proves that all aerogels possessed superoleophilicity and superhydrophilicity in air, which is associated with the abundant hydroxyl and amino groups in the aerogel surface. The underwater wetting behavior was further explored. Upon wetting the aerogels in water, the modified aerogels obtained oleophobicability. The oil (dichloromethane) droplets could bead up on the aerogel surface (Figure a and Video S2, Supporting Information).[49] The water-wetting process constructed a water layer in the aerogel surface and protects the surface from oil to enter. The water layer at the oil/solid surface is formed by the hydrophilic groups and water molecules, increasing the oil contact angles. The static oil contact angles in water are summarized in Figure b–f. With the increasing GO content in the range of 0.25–1 wt %, the oil contact angles increased gradually. When the GO loading is ≥1 wt %, the influence of GO on the contact angles is limited. Therefore, CG1 was utilized as the optimal sample for the following experiments.

Figure 2

Optical view of the oil droplets (red) placed on the surface of CG1 under water (a), and the static contact angle of an oil droplet under water on the surface of CG0 (b), CG0.25 (c), CG0.5 (d), CG1 (e), and CG2 (f).

The morphologies of the CG0 and CG1 aerogels were explored to understand the oleophilic to oleophobic transition. As shown in Figure a,b, the CG0 sample without GO addition had an interconnected porous structure. As comparison (Figure c,d), the introduction of GO obviously improved the roughness of aerogel surface. It is suggested that the GO fragments attached and wrapped around the cellulose chain through hydrogen-bonding effects. This structure could transfer load from fragile GO sheets to the cellulose skeleton, thus enhancing the mechanical performance.[44]

Figure 3

Scanning electron microscopy images of CG0 (a,b) and CG1 (c,d) aerogels.

Oil/Water Separation Tests

The oil/water separation experiments were performed with CG1. No other external force except gravity was adopted during the separation. Because of the isotropy and good flexibility, the water-wetted CG1 aerogel can be compressed into a membrane or be twisted into a filter for hexane/water separation. As depicted in Figure a, the mixture was poured into the upper tube, and the oil (hexane) was effectively blocked. In contrast, the water passed through the membrane within 12 s, exhibiting a flux of 22,900 L/(m2·h). The calculated separation efficiency is 99.8%. The membrane could still hold the oil column steadily after the water penetration. It is suggested that the water layer constructed in the interface generated a repulsive force to oil and was unfavorable for the oil to replace the water. The stability of CG1 sample promoted the oil/water separation process, which is important for practical application.[10] The excellent mechanical properties offered the composite aerogels with desirable performance under deformation. By bending and twisting the CG1 aerogel into a filter, it was sucked into the bottom of a funnel. The separation process could be finished within 150 s (Figure b), which is slower than that of membrane-shaped. This is because the filter is denser, and the porosity is substantially restricted. The twisted filter-shaped CG1 could quickly recover to its pristine shapes without cracks or structural fatigue by immersing into water (Figure S2, Supporting Information).

Figure 4

Photographs illustrated the oil/water separation by membrane-shaped CG1 (a), filter-shaped CG1 (b), the separation efficiencies of CG1 for different kinds of oils (c), and the reusability of CG1 for 10 cycles (d).

The massive hydrophilic groups, combined with the rougher surface, contributed to the underwater oleophobicity. Oil would be repelled upon contacting the water layer.[50] However, water could penetrate through the aerogel network with little resistance. Apart from hexane, the separation efficiencies for other kinds of oils are listed in Figure c. To measure the stability of the membrane-shape CG1, the oil/water separation was repeated for 10 cycles. The efficiency was still maintained beyond 99.0%, and the flux holds steady (Figure d). To verify the durability, the CG1 sample was tested under corrosive situations (e.g., 1 M HCl, 1 M NaOH, and 1 M NaCl), and the separation efficiencies remained higher than of 98.5%.

Conclusions

The development of sustainable and efficient materials for oily wastewater cleaning is challenging. In this work, composite cellulose/MBA/GO aerogels with different GO contents were prepared via a green and facile NaOH/urea system. The abundant hydrophilic groups over cellulose, GO, and MBA, together with the rough microstructure formed within the aerogel matrix, leading to the stable underwater oleophobicity. The aerogels also exhibited high flexibility and can be shaped into various configurations such as membrane, filter, and foam. The separation efficiency over CG1 was 99.8% and maintained stable after 10 cycles and even in corrosive alkaline, acidic, and salty conditions. The aerogels are potential for practical separation of oil/water mixtures.

Experimental Section

Materials

Nature graphite power (3.85 μm in size) was provided by Qingdao Jinrilai Graphite Company, China. Cotton linter pulp was used as the cellulose source and supplied by Nantong Cellulose Fibers Company, China. The α-cellulose content in the pulp was ≥96%. MBA (≥99.0%) was obtained from Macklin Reagent Company.

Preparation of the Composite Aerogels

GO was fabricated by Hummer’s method. In the preparation of aerogels, a solution consisted of NaOH/urea/H2O (with a mass ratio of 7:12:81) was precooled to −12 °C for 2 h. Then, the cotton linter was dissolved in the solution under vigorous stirring. The solution had a cellulose content of 2.5 wt %, and the bubbles in the solution were removed by centrifugation. Subsequently, MBA (1 wt %) and GO suspension with different GO contents (0.25, 0.5, 1, 2 wt %) were mixed with the cellulose solution. After 10 min of ultrasonication treatment, the acquired hydrogels were washed by deionized water until neutral. Finally, the composite hydrogels were kept in static for 12 h and freeze-dried for 48 h at −51 °C to obtain aerogels. The resulting aerogels were denoted as CG, where x refers to the concentration of GO suspension. The cellulose/MBA aerogel (designated as CG0) was fabricated as the same procedures without the addition of GO.

Characterization

XRD (Rigaku Ultima IV) experiments were performed to explore the crystalline patterns of the as-prepared aerogels. Fourier transform infrared spectra (FT-IR, Thermo Electron Nicolet-360, USA) were carried out to investigate the interactions among chemical groups. Field-emission scanning electron microscopy (FE-SEM) was conducted by JSM-7600F (JEOL, Tokyo, Japan) to reveal the sample morphologies with an operating voltage of 5 kV. The wettability of the aerogels was measured by observing the contact angles of oil droplets under water with the contact angle analysis system (JC2000D1).

In addition, the density of the aerogels was calculated as followswhere m0 stands for the dry weight of aerogel, and v refers to the volume of aerogel.

Before porosity measurements, all aerogels were immersed into ethanol solution for 24 h and then vacuumed the ethanol out. The porosity of the aerogels was determined as followswhere V and m0 are the volume and weight of aerogel, respectively. ρ indicates the density of cellulose (1.53 g/cm3).

The recoverability of the aerogels was measured as followswhere H and H0 represent the final height of the aerogel after deforming force is removed and the original height of the aerogel.

Oil/Water Separation Experiments

Oil/water mixtures were prepared by mixing 10 mL of oil and 10 mL of water under shaking. The separation tests were performed by fixing the compressed and water-wetted aerogels between two glass tubes. The whole process was driven by gravity. The separation efficiency is calculated as followswhere m1 and m2 are the masses of water before and after separation, respectively.

Flux (F) is obtained as followswhere V (L) is the permeate volume of water at t (h). S (m2) indicates the effective area of the aerogel contacting the oil/water mixture.

Moreover, oil/water separation performance was also tested by stuffing water-wetted aerogels in the narrow opening of a funnel (with a diameter of 6 mm). For the long-term stability test, the separation performance was repeated for 10 cycles. The durability of the aerogels in corrosive solutions was explored by immersing the aerogels into 20 mL of different reagents (e.g., 1 M HCl, 1 M NaOH and 1 M NaCl) for 24 h. The immersed aerogels were dried and used for separation tests.