Renewable, Biomass-Derived, Honeycomblike Aerogel As a Robust

Sep 19, 2017 - The disposal of oily wastewater has attracted extensive attention worldwide these days. Emerging environmentally friendly materials wit...
0 downloads 0 Views 7MB Size
Download PDF
Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10307-10316

Renewable, Biomass-Derived, Honeycomblike Aerogel As a Robust Oil Absorbent with Two-Way Reusability Jingxian Jiang, Qinghua Zhang,* Xiaoli Zhan, and Fengqiu Chen Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture Technology, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, P. R. China

Downloaded via UNIV OF CAMBRIDGE on July 8, 2018 at 20:17:22 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The disposal of oily wastewater has attracted extensive attention worldwide these days. Emerging environmentally friendly materials with large capacity and high selectivity that can effectively absorb oil and organic solvents from water or realize oil/water separation are in high demand. Herein, we demonstrated the facile fabrication of a sustainable, ecofriendly, biomass-derived, honeycomblike aerogel, taking lignin, agarose, and poly(vinyl alcohol) (PVA) as basic ingredients. The aerogel possessed porous three-dimensional (3D) cellular structure with tunable low density (ρ < 0.052 g cm−3) and featured good flexibility and compressibility. The modified aerogel, which was able to achieve switching from the absorption of oil and organic solvents to desorption just by altering the medium pH, was obtained simply through immersing the original aerogel into a solution of the synthesized copolymer containing pH-responsive component poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA). The absorption capacity of the modified aerogel for oil and organic solvents was in the range of 20−40 times its own weight, which was also adjustable via controlling the concentration of starting materials. The reusability of the modified aerogel could be carried out by both manual squeezing and pH-induced desorption, further broadening its application fields. The successful design of the biomass-derived modified aerogel with two-way reusability could provide new thoughts for the design of multifunctional oil absorbents, also giving efficient and sustainable options for water treatment and environmental protection. KEYWORDS: Biomass, Aerogel, Oil absorption and desorption, pH-Responsive, Reusability

■

INTRODUCTION

Sorption is one of the most widely used approaches for wastewater treatment due to its fascinating advantages, such as cost-effectiveness, easy operation, and ecofriendliness.16−18 For the past few years, various advanced absorbent materials have been created to remove oil or other contaminants from aqueous systems.2 Powder sorbents modified by simple methods like sawdust, activated carbon, metal oxides, minerals, and silica nanoparticles can be used for water remediation to some extent, but their disadvantages like relatively low efficiency and difficulty in recycling and reusing heavily limit their application.19−22 Three-dimensional (3D) porous materials have shown great potential in settling water problems because of their unique porous structure and presentative large specific surface area.23−25 Synthetic polymers such as polyurethane, melamine-formaldehyde resin, and polypropylene form one important kind of 3D porous material used for water treatment.26−28 Carbon nanotube (CNT) and graphene 3D absorbents also compose another vital class of 3D absorbent materials.29−31 These 3D absorbents show exceptional comprehensive properties like excellent flexibility and compressibility, extremely high absorption capacity, and stable recyclability.32,33 However, the sophisticated preparation

In recent years, increasingly aggravated environmental problems, especially the water pollution issue, have gradually become a severe threat to the long-term sustainable evolution of human society.1−3 Oil spill accidents frequently occuring along with the process of oil exploitation, transportation, and storage and heavy discharge of wastewater generated from industry and daily life compose the major causes of water pollution. Therefore, the disposal of oily wastewater has become a worldwide challenge, and several approaches have been proposed and employed to extract oil from water or realize oil/water separation.4−6 Traditional remediation methods including centrifugation, sedimentation, floatation, biodegradation, in situ burning, and electrochemical process are always limited in the purification of oily water due to their low efficiency, high cost, and difficulty in large-scale application as well as tendency to generate secondary pollution.7−9 Among strategies to deal with oily wastewater, the membrane filtration technique has aroused extensive attention on account of its continuous operation ability and relatively high oil/water separation efficiency.10−13 However, the membrane separation process still faces some hindrances like the fixed operating environment and fouling occurring on the membrane surfaces and pores that always brings about a decrease in filtration flux and separation selectivity.14,15 © 2017 American Chemical Society

Received: July 12, 2017 Revised: September 4, 2017 Published: September 19, 2017 10307

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering

and Sudan I were bought from Aladdin Chemistry Co. Ltd. Stearyl methyl acrylate (SMA; ⩾95%) was provided by Juhua Group Corp. 3(Trimethoxysilyl)propyl methacrylate (KH-570), hydrochloric acid (36.0−38.0%), glutaraldehyde solution 25%, and n-butyl acetate were obtained from Sinopharm Chemical Reagent Co., Ltd. Organic reagents hexane, tetrahydrofuran (THF), toluene, oleic acid, dichloromethane, liquid paraffin, and chloroform were all received from Sinopharm Chemical Reagent Co., Ltd. Vacuum pump oil GS-1 was bought from Beijing Sifang Special Oil Factory. And, 2,2-azobis(isobutyronitrile) (AIBN) was recrystallized from ethanol. Distilled water was used in the whole experiment process. Preparation of Biomass-Derived Aerogel. Agarose (6.4 g) was added in a 250 mL round-bottom flask containing 153.6 g of distilled water and resolved under mechanical stirring at 120 °C for 20 min. PVA 1788 (6.4 g) was quickly dissolved into 153.6 g of distilled water at 90 °C in another separate 250 mL round-bottom flask. Then, these two kinds of viscous, clear, and transparent liquids were mixed together into a 500 mL round-bottom flask at 120 °C accompanying vigorous mechanical stirring. After the liquid system was cooled to 90 °C, lignin (3.2 g) dissolved in 76.8 g of distilled water was poured into it with continuous stirring, and the color of the solution turned brown. Then the pH of the mixing solution was adjusted to 4 by hydrochloric acid (1 M) and a certain amount of glutaraldehyde solution (1 wt %) was dropped in. Subsequently, the prepared molds placed at room temperature were filled with this uniform mixture and liquid in molds was gradually cooled to room temperature to form brown hydrogel. The obtained hydrogel was put at room temperature for 12 h to achieve adequate cross-linking. After that, the resulting hydrogel samples were precooled in a lyophilizer at a condenser temperature of −70 °C for 8 h and then freeze-dried under vacuum (1 Pa) for 2 days to fabricate biomass-derived aerogels. Synthesis of Copolymer p(SMA-DMAEMA-KH570). Copolymer p(SMA-DMAEMA-KH570) was synthesized through free radical polymerization method. Certain amounts of SMA (5.7 g, 16.86 mmol), DMAEMA (3.8 g, 24.17 mmol), KH570 (0.5 g, 2.01 mmol), and AIBN (0.15g, 0.91 mmol) were added into 25 g of n-butyl acetate placed in a three-necked, round-bottomed flask equipped with magnetic stirring and a condenser tube. The mixture was then heated to 80 °C and reacted under nitrogen atmosphere for 6 h. After that, the product was extracted by precipitation in methanol. Subsequently, the product was put into a vacuum oven and dried at 60 °C for 12 h and then carefully stored in a sealed wide-mouth bottle. Preparation of Modified Biomass-Derived Aerogel. The preparation of modified biomass-derived aerogel was through a facile dip-coating method. First, the as-prepared copolymer p(SMADMAEMA-KH570) was dissolved in tetrahydrofuran solution, and the concentration was adjusted to 3 wt %. Then the constructed biomass-derived aerogels in cylindrical shapes with a height of 10 mm and a diameter of 25.5 mm were immersed into a beaker of 100 mL of the tetrahydrofuran mixture for 1 h and then the aerogel was heattreated in an 80 °C oven for 3 h to realize complete reaction between the copolymer and the aerogel. Finally, aerogels prepared by method above were ready for later characterization. Characterization. The FT-IR spectra of the copolymer was recorded by a Nicolet 5700 Fourier transform infrared (FTIR) instrument. The copolymer was dissolved in tetrahydrofuran solution in advance and then casted onto KBr disks for analysis. The microstructure of the aerogels was investigated through scanning electron microscopy (SEM). The SEM was operated on an S-570 scanning electron microscope (Hitachi) with an accelerating voltage of 5 kV. All the aerogel samples were treated by gold sputtering before analyzing. A mercury porosimeter (AutoPore IV 9500, Micromeritics, USA) was employed to measure the porosity of the original unmodified aerogel and the modified aerogel. The chemical structure of aerogels was studied by the attenuated total reflection/Fourier transform infrared (ATR-FTIR, Nicolet 5700, ThermoFisher). Here, 32 scans were taken for each spectrum at a resolution of 4.0 cm−1. The surface wettability of aerogel samples was measured by static contact angle analysis using a CAM200 optical contact angel meter (KSV Co., Ltd.) at room temperature. Five different spots were tested on each

procedure, excessively high cost, rigorous production conditions, and environmental problems generated during the synthesis process as well as the disposal of the deserted materials are inevitable difficulties in their practical usage, further raising serious challenges.6 Therefore, there exists an imperious demand for sustainable, low-cost, and ecofriendly materials equipped with excellent selectivity, high absorption capacity, and good recyclability, which are suitable for practical, large-scale water remediation. Biomass-derived aerogels have been recognized as a good choice. Biomass materials like cellulose, lignin, agarose, and chitosan are sustainable, low-cost, and naturally abundant.25,34−39 Based on several studies focusing on biomass-derived absorbents, the exploitation of this kind of material should be highlighted meaningfully.40−44 It is fundamentally interesting to find that magnetic responsivity and superhydrophobicity can be easily aggregated onto aerogels, which further improves their practical application potential in water treatment.45−48 In addition, stimuliresponsive two-dimensional materials have been deeply investigated and applied to smart oil/water separation.49−51 Therefore, bestowing stimuli-responsive properties upon biomass-derived aerogels seems to be a fantastic method to realize controllable absorption and desorption under different conditions. For responsive absorbent materials, the combustion and squeezing methods for recovery of them can be replaced by just altering certain variables in the surroundings, like light, temperature, or pH, which will effectively avoid the destruction of the absorbents themselves and be applicable to both elastic and inelastic absorbents.52−54 And, it is worth noting that limited research in this area has been reported. Here in this study, we prepared biomass-derived, honeycomblike aerogels through a facile sol−gel method and freezedrying technique, using biodegradable lignin,55 poly(vinyl alcohol), and agarose as starting materials.41 Subsequently, the modified aerogels were acquired via dip-coating the original aerogels into a solution of synthesized copolymer containing pH-responsive poly(2-(dimethylamino)ethyl methacrylate) (pDMAEMA) chains. The obtained 3D aerogel possessed porous cellular structure with tunable density (ρ < 0.052 g cm−3) and featured good compressibility and flexibility. The superhydrophobicity and superoleophilicity ensured modified aerogels high selectivity toward oil and water, thus they could absorb oil from water regardless of oil densities and realize oil/ water separation. The absorption capacity of the modified aerogels for various oil and organic solvents was in the range of 20−40 times their own weight and was adjustable by regulating the concentration of feed solutions. Apart from the manual squeezing method, the intriguing property to desorb oil or organic solvents through varying the pH of aqueous solution provided a new approach for recovery of absorbents, which could effectively avoid the limitation of elasticity of the material and damage of the material itself. And to the best of our knowledge, this is the first attempt to realize pH-induced oil desorption onto a biomass-derived aerogel. The modified aerogels exhibited proper reusability which is vital in practical application. Therefore, these charming characteristics of the sustainable, biomass-derived aerogels are beneficial to constructing powerful absorbents for water remediation.

■

EXPERIMENTAL SECTION

Materials. Lignin, alkali, and agarose were purchased from SigmaAldrich and used as received. Poly(vinyl alcohol) (PVA) 1788, 87.0− 89.0% alcoholysis, 2-(dimethylamino)ethyl methacrylate (DMAEMA) 10308

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering aerogel sample, and the average value was recorded as the final result. The surface compositions of aerogel samples were analyzed by X-ray photoelectron spectroscopy (XPS, PerkinElmer Phi1600 ESCA system) employing Mg Kα (1254.0 eV) as the radiation source. The surface spectra was gathered in the range of 0−1100 eV at a takeoff angle of 90°. The mechanical properties of aerogels were evaluated by means of a compression test performing on a universal material testing machine (Zwick/Roell Z020) equipped with a 2.5 kN load. And, the as-prepared cylindrical aerogel samples used for the compression test possessed a height of around 44 mm and a diameter of around 25.5 mm. The samples were compressed at a rate of 10 mm/min for each setting strain. Oil/Water Separation Properties. The aerogel sample was investigated for its separation properties by absorbing toluene and chloroform from water, respectively. The separation capability was also tested by fixing an aerogel sample in a simple filtering apparatus to separate a chloroform/water mixture. Measurement of Oil/Solvent Absorption Capacity. In order to explore the absorption capacity of the as-prepared aerogels, testing liquids including tetrahydrofuran, toluene, oleic acid, dichloromethane, chloroform, and hexane were employed in the absorption measurement. A piece of aerogel sample was preweighed and then immersed in an organic liquid at room temperature for 1 min to reach the absorption equilibrium. Then the aerogel sample was taken out of the organic liquid and dripped for 30 s to get rid of the excessive oil/ solvent. After that, the saturated aerogel sample was weighed immediately to avoid the influence of liquid evaporation, and the value was recorded. The oil/solvent absorption capacity k of the aerogel sample was calculated by the following equation:

k = (m − m0)/m0

Depending on the shape of templates, cylinder, cube, heart-, star-, and flower-shaped aerogels could be created (Figure S1). Then the modified aerogel was finally obtained by a simple dipcoating procedure with a tetrahydrofuran solution of synthesized copolymer p(SMA-DMAEMA-KH570). The firm combination between the copolymer and the aerogel was attributed to the hydrolytic condensation of the silane coupling agent KH570. The KH570 side chain in the copolymer was hydrolyzed by the water in air to form silanol that could perform dehydration−condensation reactions with abundant hydroxyl groups in the biomass-derived aerogel at high temperature. The covalent bonding between the copolymer and the aerogel bulk was subsequently produced, guaranteeing their firm combination in turn. Relative analysis of the synthesized copolymer p(SMA-DMAEMA-KH570) is shown in the Supporting Information (Figure S2 and S3). Also, FTIR spectra of lignin and agarose used in the fabrication process are shown in the Supporting Information (Figure S4). Attenuated total reflection/Fourier transform infrared (ATRFTIR) spectroscopy was exerted to study the chemical structure of the original unmodified aerogel and modified aerogel. ATR-FTIR spectra of these two kinds of aerogels are shown in Figure 1. The absorption peak at 3276 cm−1 in the

(1)

where m0 (g) is the weight of the initial dry aerogel sample with no oil/solvent absorbed and m (g) is the weight of the saturated aerogel with oil/solvent absorbed. Three pieces of aerogel samples were determined in the measurement of oil/solvent absorption capacity for each kind of oil/solvent and weight values were recorded separately. Their average values were presented as the final results. Reusability of Aerogel. The reusability of the aerogels was tested by simple direct squeezing because it is a convenient, environmentally friendly way that is easy to perform. Moreover, pH-triggered desorption was also applied as an innovative method to realize reusability. And, the absorbed oil was chloroform. The aerogel samples tested after squeezing or pH-triggered desorption were used for the next absorption/squeezing or absorption/pH-triggered desorption cycle. The absorption capacity retention (R, %) was calculated as follows:

R = MCO/(Ms − M 0)

Figure 1. ATR-FTIR spectra of the original unmodified aerogel and the modified aerogel.

spectra of the original unmodified aerogel was derived from the stretching of −OH groups. For the modified aerogel, the intensity of the −OH stretching peak decreased, indicating the successful adhesion of the copolymer to some extent. The peak at 1727 cm−1 corresponding to the CO stretching vibration in the spectrum of the modified aerogel was much more intensified than that of original unmodified aerogel, which was ascribed to the carbonyl stretching of the synthesized copolymer. The peaks at 2916 and 2848 cm−1 were stretching vibrations of −CH3 and −CH2, which were stronger for the modified aerogel than the original one, mainly attributed to alkyl chain from stearyl methyl acrylate. These apparent differences in the ATR-FTIR spectra of original and modified aerogels indicated the successful modification by the simple dip-coating method. Scanning electron microscopy (SEM) was employed to investigate the morphologies of the original unmodified biomass-derived aerogel and the modified aerogel. And images were shown in Figure 2. From the macroscopic perspective, the original and modified aerogels had self-standing, three-dimensional (3D) structure. It is also worth noting that there was no obvious difference in their microstructures (Figure 2a and d), suggesting that the simple and mild modification method

(2)

Where MCO is the mass of absorbed oil for each cycle, MS is the mass of the saturated aerogel that reached absorption equilibrium, and M0 is the mass of the original dry aerogel. Three aerogel samples were tested, and 10 cycles of absorption/squeezing were performed for each sample. Another three samples were tested, and 5 cycles of absorption/pH-triggered desorption were performed for each sample. The results listed were the average values of the whole experiments.

■

RESULTS AND DISCUSSION The original unmodified aerogel was constructed through a sol−gel method and freeze-drying technique, taking lignin, PVA, and agarose as starting materials. Among these materials, PVA is one kind of unique organic polymer that can be biodegraded, exerting its significant function in manufacturing the sustainable aerogel. The formation of the 3D porous biomass-derived aerogel relied greatly on the abundant hydroxyl groups in the feedstock molecules and the addition of the cross-linking agent glutaraldehyde (Scheme S1). The subsequent freeze-drying process was utilized to produce aerogel with porous cellular and three-dimensional structure. 10309

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. SEM spectra of aerogels: (a−c) original unmodified aerogel, (d−f) modified aerogel. The scale bars are noted inside each image.

Figure 3. (a) Digital pictures of aerogel during a compressing cycle with a compressive strain of 40%. (b) Compressive stress−strain curves of aerogel at the strain of 20%, 40%, and 60%, respectively. (c) Cyclic compressive stress−strain curves of aerogel at a stress strain of 40% for 10 cycles.

through polymer dip-coating had limited influence on the morphology and structure of original aerogel. In fact, through the dip-coating approach, the synthesized polymer was attached to the aerogel both on the outer surface and in the inside pore walls. The effect of polymer coating on the microstructure of aerogels was negligible, particularly on an intrinsic unsmooth surface. Both aerogels had cellular structure with honeycomblike pores irregularly distributed, and the pore sizes were in the range of 10−60 μm. And the porosities of the original unmodified aerogel and the modified aerogel were 87.58% and 86.73%, respectively. As can be seen from the SEM images (Figure 2c and f), each pore had thin walls surrounded and fibrous filaments embedded, and none of the pores inside the aerogels were blocked, especially in the modified aerogel. The formation of the micron-sized pores and nanosized fibrous filaments might be ascribed to the physical and chemical crosslinking among molecules of lignin, agarose, and PVA. The honeycomblike pores with multiple structures not only constructed the porous microstructure of the original and

modified aerogels but also played vital roles in forming their absorption capacities and good mechanical properties. Resisting deformation and recovering their original shapes were attractive capabilities of 3D materials. In order to investigate the mechanical property of our prepared aerogel, a compressive test was performed. Figure 3a exhibited the digital pictures of a compressing and releasing process when the applied strain was set as 40%. The aerogel was deformed in the process of compression, and when the force was withdrawn, it almost recovered back to its original shape. Successive cyclic compressive stress−strain curves were presented in Figure 3b to reveal its compressibility and flexibility. When the compressive strain was set as less than 20%, there existed an initial linear elastic regime in the stress−strain curves, ascribed to the elastic bending of the cellular structure. Subsequently, the compressive stress of our prepared aerogel gradually increased along with the additional compressive strain. A steep increase in the stress−strain curve could be clearly observed when the compressive strain was adjusted to 60%, which reflected a densification region owing to the close packing of 10310

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. Digital pictures of (a) unmodified and modified aerogel when placed on water and modified aerogel when forced into water by tweezers. (b and c) Water and oil droplet (chloroform dyed with Sudan I) on the surface of unmodified and modified aerogels and water and oil contact angles of them, respectively. (d) Absorption of toluene floating on the water surface by modified aerogel. (e) Absorption of chloroform sank into the bottom of water by modified aerogel. (f) Chloroform/water mixture separation process by modified aerogel in a common filtering device. Toluene and chloroform were dyed by Sudan I.

pseudospherical form with a static contact angle of ∼150°. Due to the three-dimensional porous structure and the superhydrophobic nature of the modified aerogel, the water droplet on it was supported both by the aerogel surface itself and the air trapped in it. And the area ratio of droplet/solid interface was much smaller than that of droplet/air interface. When it was placed on the water surface, the modified aerogel floated on the water surface. And when being partly or totally forced into water, the modified aerogel presented a silver mirrorlike surface, which was due to the continuous air layer formed between the superhydrophobic surface and water. Once the external force was withdrawn, the modified aerogel would immediately emerge from water. The unmodified and modified aerogel both presented superoleophilicity toward low surface energy droplets, such as toluene. Once the toluene droplet touched the aerogel, it spread out and was absorbed into the aerogel immediately, even in less than a second. The hydrophilic and superoleophilic nature of unmodified lignin-based aerogel should account for its ineffectiveness in oil/water separation. The modified aerogel was endowed with good superhydrophobicity and superoleophilicity, promising its effectiveness in water remediation aiming to eliminate oily pollution. To demonstrate its usage in water remediation, the modified aerogel was applied to remove toluene and chloroform from water. As can be seen from Figure 4d, the modified aerogel could absorb toluene (dyed with Sudan I) on the surface of water quickly, leaving no oil droplet remaining. By forcing the

the cellular structure. In addition, a maximal stress was exhibited at around 120 kPa at the strain of 60%. The strong mechanical properties endowed our prepared aerogels with great potential to construct 3D multifunctional biomass-derived materials. Furthermore, cyclic stress−strain curves of our prepared aerogel were recorded going through 10 cycles of the compressing−releasing process at the strain of 40%. From these curves in Figure 3c, small deformation in the aerogel could be discovered after the first cycle, but no apparent increase in deformation was observed in the second to tenth cycles. Moreover, there was no obvious decrease in the maximum compressive stress of aerogels during the cyclic test. Therefore, compressibility and flexibility which our prepared aerogel possessed might promise multiple functionalities, such as good recyclability, easy manipulation, convenient storage, and transportation. Surface wettability is always identified as one of the most significant properties of materials and greatly affected by surface chemical composition and morphology. Figure 4a−c exhibited the surface wettability differences between the original unmodified aerogel and the modified aerogel. The unmodified aerogel had a hydrophilic nature and water droplets could be absorbed into it within a short time. When placing the original aerogel on the surface of water, it absorbed water gradually and finally sank beneath the water surface on account of its low density, porous structure, and hydrophilic nature. However, as for the modified aerogel, water droplets could maintain a 10311

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering modified aerogel into water, it showed a silver mirrorlike surface on one hand and swiftly absorbed chloroform that sank to the bottom of water into its 3D bulk on the other hand (Figure 4e, Supporting Movie 1). Soon afterward, the chloroform could be completely removed by taking the oilloaded aerogel out of the water system. In other words, the modified aerogel could separate oil from the water phase regardless of the densities of oils, which suggested its wide potential in absorbing various oils with densities greater or less than that of water. In addition, a piece of modified aerogel was fixed in a commonly used filtering apparatus as shown in Figure 4f. A mixture of chloroform dyed with Sudan I and water colored with methyl blue at a volume ratio of 1:1 was poured into the upper receiver of the filter. Obviously, this liquid mixture was divided into two layers, oil with higher density sank to the bottom and water with lower density formed the upper layer. The chloroform layer contacted the aerogel surface before the water layer, and because of the superoleophilic nature, the modified aerogel could absorb chloroform and let it permeate through. However, the distinct difference in the wettability of modified aerogel toward oil and water guaranteed its barrier action of water. Figure 4f clearly showed that chloroform dyed with Sudan I was collected in the receiving flask beneath containing no blue droplets that represented water. And, blue water was gathered on the surface of the modified aerogel in the upper receiver without permeating through it. That is, the modified aerogel could also be used as a membrane for oil/water separation. It should be noted that the resistance originated by the 3D porous aerogel with certain thicknesses might have adverse effects on the rate of the filtration process. Therefore, the oil/water separation process realized by the modified aerogel could not be finished at once. According to the different volumes of the oil/water mixture, it would take different amounts of time to accomplish the separation process. As above-mentioned, the superhydrophobic/superoleophilic aerogel was obtained by immersing the original aerogel in a solution composed of copolymer p(SMA-DMAEMA-KH570) and THF solvent. The copolymer p(SMA-DMAEMA-KH570) possessed a pH-responsive feature, attributed to the protonation or deprotonation of pDMAEMA chains in different pH medium. In a high-pH medium, pDMAEMA chains deprotonated and exhibited collapsed conformation. Under this circumstance, collapsed pDMAEMA chains and exposed SMA chains created a synergetic effect in constructing a superhydrophobic surface of the modified aerogel. While in a low-pH medium, pDMAEMA chains entirely protonated and were highly stretched because of the electrostatic repulsion effect between polymer chains. The exposing pDMAEMA chains helped to form a more hydrophilic surface. Oil absorption and desorption experiments were used to test the aerogel’s pHresponsiveness. The modified aerogel could abstract chloroform from water because of its superhydrophobicity and superoleophilicity. As Figure 5 illustrated, if we forced a piece of saturated aerogel into water with a pH of 7 (Supporting Movie 2), even a droplet of oil would not drop from the aerogel. It is worth noting that the tweezers only played the role of holding the aerogel, no squeezing effect was applied. However, if the modified aerogel was put into an aqueous solution with a pH of 1 (Supporting Movie 3), the chloroform dyed with Sudan I would be dropwise expelled from the aerogel. The protonated, stretched and exposed pDMAEMA chains made the modified aerogel more hydrophilic, prone to absorb water from the

Figure 5. Oil desorption of modified aerogel with medium pH of (a) 1 and (b) 7, respectively.

aqueous system. The oil dripping from the aerogel was due to the repulsive force between the polar and nonpolar liquids. On account of the particularity of the 3D structure, the outer surface of the aerogel was surrounded by the acidic solution at first, then the outer surface wettability was changed, absorbing water from aqueous system and expelling oil from itself. The wettability of the interior of the aerogel was gradually varied when it touched with the water that the aerogel took in. Therefore, in acidic solution, accompanying with the outside-in change of the wettability, the oil desorption was a gradual process. XPS analysis could offer sufficient evidence that wettability changes were radically ascribed to the protonation and deprotonation of tertiary amine groups in pDMAEMA chains of the synthesized copolymer. The experiment and analysis suggested that the absorption and desorption of oil/ organic solvents could be manipulated by varying the medium pH, further highlighting the applicability of the modified biomass-derived aerogel. X-ray photoelectron spectroscopy (XPS) analysis further confirmed the chemical composition of aerogels before and after modification. Figure 6a displayed the XPS results of original unmodified aerogel, modified aerogel, and modified aerogel treated with acid water solution. Obviously, there existed significant difference among these spectra. First of all, the O/C atomic ratio was decreased from 0.39 of the original unmodified aerogel to 0.20 of the modified aerogel, which was due to the coating of the copolymer p(SMA-DMAEMAKH570). In addition, the N element appeared in the spectrum of the modified aerogel belonging to the DMAEMA component in the synthesized copolymer, indicating the successful coating of the functional copolymer. The highresolution N 1s spectra are shown in Figure 6b and c. The N 1s peak appeared in the XPS spectrum of the modified aerogel at 399 eV, assigned to the tertiary amine in the pDMAEMA component. But in the spectrum of aerogel after treating, two peaks at 399 and 402 eV were observed, corresponding to tertiary amine and quaternary amine, respectively, suggesting the protonation of the pDMAEMA chains in the synthesized copolymer. The protonated and deprotonated pDMAEMA chains played a crucial part in manipulating the surface wettability and oil desorption property of the modified aerogel under different conditions. 10312

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6. (a) XPS spectra of original unmodified aerogel, modified aerogel, and aerogel treated with acid (pH = 1). N 1s spectra of (b) modified aerogel and (c) aerogel treated with acid (pH = 1).

Figure 7. Oil absorption capacity of (a) 4 wt % aerogel and (b) 2 wt % aerogel, respectively. Different kinds of oil and organic solvents including tetrahydrofuran, toluene, oleic acid, dichloromethane, chloroform, hexane, vacuum pump oil, and liquid paraffin are used as in the test. (c) 10 cycles of oil absorption capacity retention by absorption/squeezing of modified aerogel for chloroform. (d) 5 cycles of oil absorption capacity retention by absorption/pH-triggered desorption of modified aerogel for chloroform.

Practically, absorption capacity is an important parameter to appraise the performance of absorbents. Different from twodimensional (2D) materials, 3D materials often possess higher absorption capacity benefiting from their porous microstructure. When immersed into liquids, absorbents were

penetrated by solvents and then saturated within a short time. Various oils and solvents with diverse densities including tetrahydrofuran, toluene, oleic acid, dichloromethane, chloroform, hexane, vacuum pump oil, and liquid paraffin were utilized to investigate the absorption capacity of the modified 10313

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering

stable. And after 10 cycles, the absorption capacity for the modified aerogel remained at about 73.71% of the first cycle, showing good absorption stability and reusability. pH-Induced desorption was a labor-saving method to realize cyclic utilization of our prepared modified aerogel. As shown in Figure 7d, there existed an apparent decrease in oil absorption capacity in the second cycle, and the absorption capacity retention was at around 72.68%. But for the third, forth, and fifth cycle, the absorption capacity remained almost stable with the value of about 66%. That is, the modified aerogel is also equipped with the property of recycling by means of pHinduced desorption, and the absorption capacity after 5 cycles was good. Depending on different service environment, reusable methods could be selected preferably, indicating extensive application fields. Therefore, these two methods to help modified aerogels accomplish their reusability endowed them with promising prospects in water treatment.

aerogel. The modified aerogels were immersed into oils and solvents separately for 1 min, and then, the saturated aerogels were taken out of the liquids, getting rid of the excessive oil/ solvent by dripping at the same time. The results of absorption capacity (g g−1) for eight kinds of oils and solvents were presented in Figure 7a; for tetrahydrofuran, toluene, oleic acid, dichloromethane, chloroform, hexane, vacuum pump oil, and liquid paraffin, the specific values were calculated to be 7.63, 13.20, 14, 19.64, 21, 10.75, 11.83, and 9.6 g g−1, respectively. As what was mentioned above, density of the as-prepared aerogels could be modulated by the concentration of feed solution. The absorption capacity of aerogels has direct relation with their densities or the concentration of feed solution. When adjusted to half the original concentration, the feed solution was used to construct the aerogel with lower density and improved absorption capacity. The absorption capacity of the modified aerogel (Figure 7b) was determined to be 22.15, 23.40, 20.07, 31.46, 40.23, 18.39, 21.93, and 18.1 g g−1 for tetrahydrofuran, toluene, oleic acid, dichloromethane, chloroform, hexane, vacuum pump oil, and liquid paraffin, respectively. These results indicated that the as-prepared modified aerogel had different absorption capacities toward diverse oils and solvents according to their densities, and the absorption capacity of the modified aerogel could be adjusted by controlling its density through setting the concentration of feed solution. This unique character promised broader application possibilities of the modified aerogels. In addition, the as-prepared modified aerogel showed much higher absorption capabilities than powder sorbents19,21 and other reported three-dimensional porous materials such as macroporous Fe/C nanocomposites (4−10 times),24 functionalized polyurethane foams (∼13 times),56 and PDMS nanocomposite foam.57 In addition, the absorption capacities of the as-prepared modified aerogel was also comparable to that of the hydrophobic nanocellulose aerogels,58 the fluorine-free cellulose nanofibril aerogels,59 and the biomass-derived porous carbonaceous aerogel.60 Combined with oil−water separation ability and oil absorption capacity, reusability is an equally significant criterion for oil sorbents. For the modified aerogels we prepared in this work, there were two approaches to realize oil desorption, manually squeezing and pH-induced discharging. These two methods were much more convenient, controllable, costeffective, and eco-friendly than other recycling methods. In the case of evaluating the aerogel’s reusability by manually squeezing, chloroform was applied as an oil representative. The modified aerogel was first immersed into chloroform to realize absorption equilibrium. Subsequently, the saturated aerogel was squeezed and the desorbed oil was harvested. Then the desorbed modified aerogel was used for another cycle. Relative experimental data are depicted in Figure 7c. We could find that the absorption capacity for the second cycle retained 82.16% of the first cycle, and the third cycle kept about 83.1% of the first cycle. The decrease in absorption capacity might be ascribed to the remaining oil restricted in the biomass-derived aerogel. Multiple functional groups in the biomass-derived aerogel played a positive role in absorbing oil; that is, oil was easy to combine with the biomass-derived aerogel. When manually squeezing was not completely enough, a certain amount of absorbed oil might remain in the aerogel, leading to a decrease in absorption capacity for the next cycle. As can be seen from Figure 7c, absorption capacity for the fourth cycle reduced again, turning out to be approximately 68.55% of the first cycle, while for the subsequent cycles, the absorption capacity was

■

CONCLUSIONS In this study, novel biomass-derived aerogels were successfully constructed using a facile sol−gel method and freeze-drying technique. The modification process was achieved by dipcoating the original aerogel with synthesized copolymer p(SMA-DMAEMA-KH570). The original aerogel possessed low density, porous cellular structure, three-dimensional selfsupporting configuration, and good compressibility and flexibility. The modified aerogel was further equipped with superhydrophobicity and superoleophilicity, and could accomplish efficient oil/water separation as well as abstract oil/ organic solvents from water. The salient property to realize oil absorption/desorption by varying the medium pH broadened the aerogel’s application. The absorption capacity of the modified aerogel was around 20−40 times its own weight, and this could be adjusted by the concentration of the feed solutions. In addition, the reusability of the modified aerogel was tested by means of manual squeezing and pH-induced desorption, and more than 70% of its full absorption capacity was maintained after 10 cycles or more than 60% was maintained after 5 cycles respectively, which also revealed a prominent performance of as-prepared oil absorbent. In summary, the properties mentioned above endowed modified aerogels with great comprehensive characteristics, offering them conspicuous application potential in large-scale oil pollutant removal.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02333. Supposed schematic of cross-linked chemical structure for the biomass-derived aerogel; Optical image of aerogels with diverse shapes; Chemical structure of synthesized copolymer p(SMA-DMAEMA-KH570); FTIR spectrum of synthesized copolymer p(SMADMAEMA-KH570); FTIR spectra of (a) lignin and (b) agarose (PDF) Process of absorption of chloroform (AVI) Process of oil desorption in water medium of pH 7 (AVI) Process of oil desorption in water medium of pH 1 (AVI) 10314

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering

■

(17) Chen, B.; Ma, Q.; Tan, C.; Lim, T.; Huang, L.; Zhang, H. Carbon-Based Sorbents with Three-Dimensional Architectures for Water Remediation. Small 2015, 11 (27), 3319−3336. (18) Maleki, H. Recent advances in aerogels for environmental remediation applications: A review. Chem. Eng. J. 2016, 300, 98−118. (19) Bhatnagar, A.; Hogland, W.; Marques, M.; Sillanpaa, M. An overview of the modification methods of activated carbon for its water treatment applications. Chem. Eng. J. 2013, 219, 499−511. (20) Ivshina, I. B.; Kuyukina, M. S.; Krivoruchko, A. V.; Elkin, A. A.; Makarov, S. O.; Cunningham, C. J.; Peshkur, T. A.; Atlas, R. M.; Philp, J. C. Oil spill problems and sustainable response strategies through new technologies. Environ. Sci.: Processes Impacts 2015, 17 (7), 1201− 1219. (21) Toyoda, M.; Inagaki, M. Heavy oil sorption using exfoliated graphite. New application of exfoliated graphite to protect heavy oil pollution. Carbon 2000, 38 (2), 199−210. (22) Zang, D.; Liu, F.; Zhang, M.; Gao, Z.; Wang, C. Novel superhydrophobic and superoleophilic sawdust as a selective oil sorbent for oil spill cleanup. Chem. Eng. Res. Des. 2015, 102, 34−41. (23) Sun, H.; Xu, Z.; Gao, C. Multifunctional, Ultra-Flyweight, Synergistically Assembled Carbon Aerogels. Adv. Mater. 2013, 25 (18), 2554−2560. (24) Chu, Y.; Pan, Q. Three-Dimensionally Macroporous Fe/C Nanocomposites As Highly Selective Oil-Absorption Materials. ACS Appl. Mater. Interfaces 2012, 4 (5), 2420−2425. (25) Wu, X.; Wen, T.; Guo, H.; Yang, S.; Wang, X.; Xu, A. BiomassDerived Sponge-like Carbonaceous Hydrogels and Aerogels for Supercapacitors. ACS Nano 2013, 7 (4), 3589−3597. (26) Ruan, C.; Ai, K.; Li, X.; Lu, L. A Superhydrophobic Sponge with Excellent Absorbency and Flame Retardancy. Angew. Chem., Int. Ed. 2014, 53 (22), 5556−5560. (27) Zhang, X.; Li, Z.; Liu, K.; Jiang, L. Bioinspired Multifunctional Foam with Self-Cleaning and Oil/Water Separation. Adv. Funct. Mater. 2013, 23 (22), 2881−2886. (28) Li, J.; Xu, C.; Zhang, Y.; Wang, R.; Zha, F.; She, H. Robust superhydrophobic attapulgite coated polyurethane sponge for efficient immiscible oil/water mixture and emulsion separation. J. Mater. Chem. A 2016, 4 (40), 15546−15553. (29) Li, L.; Li, B.; Zhang, J. Dopamine-mediated fabrication of ultralight graphene aerogels with low volume shrinkage. J. Mater. Chem. A 2016, 4 (2), 512−518. (30) Chabot, V.; Higgins, D.; Yu, A.; Xiao, X.; Chen, Z.; Zhang, J. A review of graphene and graphene oxide sponge: material synthesis and applications to energy and the environment. Energy Environ. Sci. 2014, 7 (5), 1564−1596. (31) Turco, A.; Malitesta, C.; Barillaro, G.; Greco, A.; Maffezzoli, A.; Mazzotta, E. A magnetic and highly reusable macroporous superhydrophobic/superoleophilic PDMS/MWNT nanocomposite for oil sorption from water. J. Mater. Chem. A 2015, 3 (34), 17685−17696. (32) Liu, Y.; Ma, J.; Wu, T.; Wang, X.; Huang, G.; Liu, Y.; Qiu, H.; Li, Y.; Wang, W.; Gao, J. Cost-Effective Reduced Graphene Oxide-Coated Polyurethane Sponge As a Highly Efficient and Reusable OilAbsorbent. ACS Appl. Mater. Interfaces 2013, 5 (20), 10018−10026. (33) Wu, L.; Li, L.; Li, B.; Zhang, J.; Wang, A. Magnetic, Durable, and Superhydrophobic Polyurethane@Fe3O4@SiO2@Fluoropolymer Sponges for Selective Oil Absorption and Oil/Water Separation. ACS Appl. Mater. Interfaces 2015, 7 (8), 4936−4946. (34) Song, X.; Chen, Y.; Rong, M.; Xie, Z.; Zhao, T.; Wang, Y.; Chen, X.; Wolfbeis, O. S. A Phytic Acid Induced Super-Amphiphilic Multifunctional 3D Graphene-Based Foam. Angew. Chem., Int. Ed. 2016, 55 (12), 3936−3941. (35) Gao, S.; Li, X.; Li, L.; Wei, X. A versatile biomass derived carbon material for oxygen reduction reaction, supercapacitors and oil/water separation. Nano Energy 2017, 33, 334−342. (36) Wang, J.; Nie, P.; Ding, B.; Dong, S.; Hao, X.; Dou, H.; Zhang, X. Biomass derived carbon for energy storage devices. J. Mater. Chem. A 2017, 5 (6), 2411−2428.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qinghua Zhang: 0000-0003-1350-6388 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21476195, 21576236, and 21676248) and Zhejiang Provincial Major Project of Science & Technology for Award (No. 2014C13SAA10006).

■

REFERENCES

(1) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Mariñas, B. J.; Mayes, A. M. Science and technology for water purification in the coming decades. Nature 2008, 452 (7185), 301− 310. (2) Ge, J.; Zhao, H.; Zhu, H.; Huang, J.; Shi, L.; Yu, S. Advanced Sorbents for Oil-Spill Cleanup: Recent Advances and Future Perspectives. Adv. Mater. 2016, 28 (47), 10459−10490. (3) Du, R.; Zhao, Q.; Zheng, Z.; Hu, W.; Zhang, J. 3D SelfSupporting Porous Magnetic Assemblies for Water Remediation and Beyond. Adv. Energy Mater. 2016, 6 (17), 1600473. (4) Jernelov, A. How to defend against future oil spills. Nature 2010, 466 (7303), 182−183. (5) Wang, B.; Liang, W.; Guo, Z.; Liu, W. Biomimetic superlyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature. Chem. Soc. Rev. 2015, 44 (1), 336−361. (6) Ma, Q.; Yu, Y.; Sindoro, M.; Fane, A. G.; Wang, R.; Zhang, H. Carbon-Based Functional Materials Derived from Waste for Water Remediation and Energy Storage. Adv. Mater. 2017, 29 (13), 1605361. (7) Buist, I.; McCourt, J.; Potter, S.; Ross, S.; Trudel, K. In situ burning. Pure Appl. Chem. 1999, 71 (1), 43−65. (8) Broje, V.; Keller, A. A. Improved Mechanical Oil Spill Recovery Using an Optimized Geometry for the Skimmer Surface. Environ. Sci. Technol. 2006, 40 (24), 7914−7918. (9) Jadhav, S. R.; Vemula, P. K.; Kumar, R.; Raghavan, S. R.; John, G. Sugar-Derived Phase-Selective Molecular Gelators as Model Solidifiers for Oil Spills. Angew. Chem., Int. Ed. 2010, 49 (42), 7695−7698. (10) Yang, H.; Hou, J.; Chen, V.; Xu, Z. Surface and interface engineering for organic − inorganic composite membranes. J. Mater. Chem. A 2016, 4 (25), 9716−9729. (11) Zhang, R.; Liu, Y.; He, M.; Su, Y.; Zhao, X.; Elimelech, M.; Jiang, Z. Antifouling membranes for sustainable water purification: strategies and mechanisms. Chem. Soc. Rev. 2016, 45 (21), 5888−5924. (12) Zhang, G.; Jiang, J.; Zhang, Q.; Gao, F.; Zhan, X.; Chen, F. Ultralow Oil-Fouling Heterogeneous Poly(ether sulfone) Ultrafiltration Membrane via Blending with Novel Amphiphilic Fluorinated Gradient Copolymers. Langmuir 2016, 32 (5), 1380−1388. (13) Wang, J.; Geng, G. Simple and eco-friendly fabrication of superhydrophobic textile for oil/water separation. Environ. Technol. 2016, 37 (13), 1591−1596. (14) Zhao, X.; Su, Y.; Dai, H.; Li, Y.; Zhang, R.; Jiang, Z. Coordination-enabled synergistic surface segregation for fabrication of multi-defense mechanism membranes. J. Mater. Chem. A 2015, 3 (7), 3325−3331. (15) Zhang, G.; Jiang, J.; Zhang, Q.; Zhan, X.; Chen, F. Amphiphilic poly(ether sulfone) membranes for oil/water separation: Effect of sequence structure of the modifier. AIChE J. 2017, 63 (2), 739−750. (16) Gupta, S.; Tai, N.-H. Carbon materials as oil sorbents: a review on the synthesis and performance. J. Mater. Chem. A 2016, 4 (5), 1550−1565. 10315

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316

Research Article

ACS Sustainable Chemistry & Engineering (37) Zhang, J.; Li, B.; Li, L.; Wang, A. Ultralight, compressible and multifunctional carbon aerogels based on natural tubular cellulose. J. Mater. Chem. A 2016, 4 (6), 2069−2074. (38) Udoetok, I. A.; Wilson, L. D.; Headley, J. V. Self-Assembled and Cross-Linked Animal and Plant-Based Polysaccharides: Chitosan− Cellulose Composites and Their Anion Uptake Properties. ACS Appl. Mater. Interfaces 2016, 8 (48), 33197−33209. (39) Chaudhary, J. P.; Vadodariya, N.; Nataraj, S. K.; Meena, R. Chitosan-Based Aerogel Membrane for Robust Oil-in-Water Emulsion Separation. ACS Appl. Mater. Interfaces 2015, 7 (44), 24957−24962. (40) Bi, H.; Yin, Z.; Cao, X.; Xie, X.; Tan, C.; Huang, X.; Chen, B.; Chen, F.; Yang, Q.; Bu, X.; Lu, X.; Sun, L.; Zhang, H. Carbon Fiber Aerogel Made from Raw Cotton: A Novel, Efficient and Recyclable Sorbent for Oils and Organic Solvents. Adv. Mater. 2013, 25 (41), 5916−5921. (41) Chaudhary, J. P.; Nataraj, S. K.; Gogda, A.; Meena, R. Bio-based superhydrophilic foam membranes for sustainable oil−water separation. Green Chem. 2014, 16 (10), 4552−4558. (42) Wang, G.; He, Y.; Wang, H.; Zhang, L.; Yu, Q.; Peng, S.; Wu, X.; Ren, T.; Zeng, Z.; Xue, Q. A cellulose sponge with robust superhydrophilicity and under-water superoleophobicity for highly effective oil/water separation. Green Chem. 2015, 17 (5), 3093−3099. (43) Wang, J.; Zhao, D.; Shang, K.; Wang, Y.; Ye, D.; Kang, A. H.; Liao, W.; Wang, Y. Ultrasoft gelatin aerogels for oil contaminant removal. J. Mater. Chem. A 2016, 4 (24), 9381−9389. (44) Dehabadi, L.; Mahaninia, M. H.; Soleimani, M.; Wilson, L. D. Miscanthus Biomass for the Sustainable Fractionation of Ethanol Water Mixtures. ACS Sustainable Chem. Eng. 2017, 5 (4), 2970−2980. (45) Chen, N.; Pan, Q. Versatile Fabrication of Ultralight Magnetic Foams and Application for Oil−Water Separation. ACS Nano 2013, 7 (8), 6875−6883. (46) Zhao, W.; Huang, X.; Wang, Y.; Sun, S.; Zhao, C. A recyclable and regenerable magnetic chitosan absorbent for dye uptake. Carbohydr. Polym. 2016, 150, 201−208. (47) Zhang, Y.; Yang, X.; Wang, Z.; Long, J.; Shao, L. Designing Multifunctional 3D Magnetic Foam for Effective Insoluble Oils Separation and Rapid Selective Dye Removal towards Wastewater Remediation. J. Mater. Chem. A 2017, 5 (16), 7316−7325. (48) Li, L.; Zhang, J. Superamphiphobic, Magnetic, and Elastic Silicone Sponges with Excellent Temperature Stability. Adv. Mater. Interfaces 2016, 3 (23), 1600517. (49) Li, J.; Zhou, Y.; Jiang, Z.; Luo, Z. Electrospun Fibrous Mat with pH-Switchable Superwettability That Can Separate Layered Oil/Water Mixtures. Langmuir 2016, 32 (50), 13358−13366. (50) Kota, A. K.; Kwon, G.; Choi, W.; Mabry, J. M.; Tuteja, A. Hygro-responsive membranes for effective oil−water separation. Nat. Commun. 2012, 3, 1025. (51) Zhou, Y.; Li, J.; Luo, Z. Toward Efficient Water/Oil Separation Material: Effect of Copolymer Composition on pH-Responsive Wettability and Separation Performance. AIChE J. 2016, 62 (5), 1758−1771. (52) Zhu, H.; Chen, D.; Li, N.; Xu, Q.; Li, H.; He, J.; Lu, J. Graphene Foam with Switchable Oil Wettability for Oil and Organic Solvents Recovery. Adv. Funct. Mater. 2015, 25 (4), 597−605. (53) Zhu, H.; Yang, S.; Chen, D.; Li, N.; Xu, Q.; Li, H.; He, J.; Lu, J. A Robust Absorbent Material Based on Light-Responsive Superhydrophobic Melamine Sponge for Oil Recovery. Adv. Mater. Interfaces 2016, 3 (5), 1500683. (54) Zhang, W.; Liu, F.; Liu, G.; Gan, W.; Zhang, M.; Yu, H.; Di, X.; Wang, Y.; Wang, C. Stimulus-Responsive Smart Foam with Dual Wettability for Transfer and Controllable Release of Carbon Tetrachloride. Adv. Mater. Interfaces 2016, 3 (14), 1600100. (55) Xu, X.; Zhou, J.; Nagaraju, D. H.; Jiang, L.; Marinov, V. R.; Lubineau, G.; Alshareef, H. N.; Oh, M. Flexible, Highly Graphitized Carbon Aerogels Based on Bacterial Cellulose/Lignin: Catalyst-Free Synthesis and its Application in Energy Storage Devices. Adv. Funct. Mater. 2015, 25 (21), 3193−3202. (56) Calcagnile, P.; Fragouli, D.; Bayer, I. S.; Anyfantis, G. C.; Martiradonna, L.; Cozzoli, P. D.; Cingolani, R.; Athanassiou, A.

Magnetically Driven Floating Foams for the Removal of Oil Contaminants from Water. ACS Nano 2012, 6 (6), 5413−5419. (57) Gupta, R.; Kulkarni, G. U. Removal of Organic Compounds from Water by Using a Gold Nanoparticle-Poly(dimethylsiloxane) Nanocomposite Foam. ChemSusChem 2011, 4 (6), 737−743. (58) Korhonen, J. T.; Kettunen, M.; Ras, R. H. A.; Ikkala, O. Hydrophobic Nanocellulose Aerogels as Floating, Sustainable, Reusable, and Recyclable Oil Absorbents. ACS Appl. Mater. Interfaces 2011, 3 (6), 1813−1816. (59) Mulyadi, A.; Zhang, Z.; Deng, Y. Fluorine-Free Oil Absorbents Made from Cellulose Nanofibril Aerogels. ACS Appl. Mater. Interfaces 2016, 8 (4), 2732−2740. (60) Wang, Z.; Jin, P.; Wang, M.; Wu, G.; Dong, C.; Wu, A. BiomassDerived Porous Carbonaceous Aerogel as Sorbent for Oil-Spill Remediation. ACS Appl. Mater. Interfaces 2016, 8 (48), 32862−32868.

10316

DOI: 10.1021/acssuschemeng.7b02333 ACS Sustainable Chem. Eng. 2017, 5, 10307−10316