Renewable, Biomass-Derived, Honeycomblike Aerogel As a Robust

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A renewable biomass-derived honeycomb-like aerogel as robust oil absorbent with double way reusability Jingxian Jiang, Qinghua Zhang, Xiaoli Zhan, and Fengqiu Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02333 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Sustainable Chemistry & Engineering

A renewable biomass-derived honeycomb-like aerogel as robust oil absorbent with double 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. Email: [email protected] ABSTRACT: The disposal of oily wastewater has attracted extensive attention worldwide these days. Emerging environment-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, eco-friendly biomass-derived honeycomb-like aerogel, taking lignin, agarose and polyvinyl 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 with good flexibility and compressibility. The modified aerogel, which was able to achieve the 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 solution of the synthesized copolymer containing pH-responsive component 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 double way reusability could provide new thought 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 Recent years, increasingly aggravated environmental problem, especially the water pollution issue, has gradually become a severe threat to the long-term sustainable evolution of human society.1-3 Oil spill accidents frequently occurred along with the process of oil exploitation, transportation and storage, and heavy discharge of waste water generated from industry and daily life, are composed of the major causes of water pollution. Therefore, the disposal of oily waste water 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, difficulty in large-scale application as well as tendency to generate secondary pollution.7-9 Among strategies to deal with oily wastewater, membrane filtration

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technique has aroused extensive attention on account of their continuous operation ability and relatively high oil/water separation efficiency.10-13 However, membrane separation process still faces some hindrances like fixed operating environment and fouling occurred on the membrane surfaces and pores that always brings about decrease in filtration flux and separation selectivity.14-15 Sorption is one of the most widely used approaches for wastewater treatment due to its fascinating advantages, such as cost-effectiveness, easy operation, and eco-friendliness.16-18 For the past few years, various advanced absorbent materials have been created to remove oil or other contaminants from aqueous system.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 have also composed 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 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 eco-friendly materials equipped with excellent selectivity, high absorption capacity and 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 natural abundant.25, 34-39 Based on several studies focusing on biomass-derived absorbents, the exploitation of this kind of materials 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, stimuli-responsive two-dimensional material has been deeply investigated and applied into smart oil/water separation.49-51 Therefore, bestowing stimuli-responsive property 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 variable in surroundings, like light, temperature or pH, which will effectively avoid the destruction of absorbents themselves and be applicable to both elastic and inelastic absorbents.52-54 And it is worth noting that limited researches in this respect have been reported. Here in this study, we prepared biomass-derived honeycomb-like aerogels through facile sol-gel method and freeze-drying technique, using biodegradable lignin,55 polyvinyl 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 pDMAEMA chains. The obtained 3D aerogel possessed porous cellular structure


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with tunable density (ρ＜0.052 g cm-3), and featured with good compressibility and flexibility. The superhydrophobicity and superoleophilicity ensured modified aerogels high selectivity towards 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 manual squeezing method, the intriguing property to desorb oil or organic solvents through varying the pH of aqueous solution provided new approach for recovery of absorbents, which could effectively avoid the limitation of elasticity for the material and the damage of material itself. And to the best of our knowledge, this was the first attempt to realize pH-induced oil desorption onto biomass-derived aerogel. The modified aerogels exhibited proper reusability which was 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 Sigma-Aldrich and used as received. Poly(vinyl alcohol) (PVA) 1788, 87.0-89.0% alcoholysis, 2-(dimethylamino)ethyl methacrylate (DMAEMA) and SudanⅠwere 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%, 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-azobisisobutyronitrile (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 kind 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 (1M) 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 pre-cooled in a lyophilizer at a condenser temperature of -70°C for 8 h and then freeze-dried under vacuum (1 Pa) for two 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



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reacted under nitrogen atmosphere for 6 h. After that, the product was extracted by precipitating 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(SMA-DMAEMA-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 heat-treated 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). 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). 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 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 compressing 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 property. The aerogel sample was investigated for their 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 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 pre-weighed 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


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calculated by the following equation: k = (m-m0)/m0 (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, environment-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 next absorption/squeezing or absorption/pH-triggered desorption cycle. The absorption capacity retention (R, %) was calculated as follows: R = MCO/(Ms-M0) (2) Where MCO is the mass of absorbed oil for each cycle, MS is the mass of the saturated aerogel 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 sol-gel method and freeze-drying technique, taking lignin, PVA and agarose as starting materials. Among these materials, polyvinyl alcohol (PVA) is one kind of unique organic polymer that can be biodegradable, 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. Depending on the shape of templates, cylinder, cube, heart-, starand flower-shaped aerogels could be received (Figure S1). Then the modified aerogel was finally obtained by a simple dip-coating procedure with a tetrahydrofuran solution of synthesized copolymer p(SMA-DMAEMA-KH570). The firmly 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 reaction 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 firmly combination in turn. Relative analysis of the synthesized copolymer p(SMA-DMAEMA-KH570) was exhibited in Supporting Information (Figure S2 and S3). Also, FTIR spectra of lignin and agarose used in the fabrication process were shown in Supporting Information (Figure S4). Attenuated total reflection/fourier transform infrared (ATR-FTIR) 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 were shown in Figure 1. Absorption peak at 3276 cm-1 in the spectra of original unmodified aerogel was derived from the stretching of –OH groups. For



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the modified aerogel, the intensity of –OH stretching peak was decreased, indicating the successful adhesion of the copolymer to some extent. The peak at 1727 cm-1 corresponding to – 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 simple dip-coating method.

Figure 1. ATR-FTIR spectra of the original unmodified aerogel and the modified aerogel 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’s also worth noting that there was no obvious difference in their microstructures (Figure 2a and d), suggesting that the simple and mild modification method 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 honeycomb-like 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 nano-sized fibrous filaments


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might ascribe to the physical and chemical crosslinking among molecules of lignin, agarose and PVA. The honeycomb-like 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.

Figure 2. SEM spectra of aerogels: (a-c) original unmodified aerogel, (d-f) modified aerogel. The scale bar are noted inside. 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, ascribing to the elastic bending of the cellular structure. Subsequently, the compressive stress of our prepared aerogel gradually increased along with the augment of the compressive strain. A steeply 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 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 as it going through 10 cycles of 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 cycle. Moreover, there was no obvious decrease in the maximum compressive stress of aerogel during the cyclic test. Therefore, compressibility and flexibility which our prepared aerogel possessed might promise multiple functionalities, such as good recyclability,



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easy manipulation, convenient storage and transportation.

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. Surface wettability is always identified as one of the most significant properties of materials and greatly affected by surface chemical composition and morphology. Figure 4(a-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 droplet could maintain pseudo-spherical form onto it 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 being placed on the water surface, the modified aerogel remained floating on the water surface. And when being partly or totally forced into water, the modified aerogel presented a silver mirror-like 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 droplet, such as toluene. Once the toluene droplet touched the aerogel, it spread out and was absorbed into 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


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endowed with good superhydrophobicity and superoleophilicity, promising its usage in water remediation aiming to eliminate oily pollution.

Figure 4. Digital pictures of (a) unmodified and modified aerogel when placing on water and modified aerogel when forcing into water by tweezers (b-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 floated 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. 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 SudanⅠ) on the surface of water quickly, leaving no oil droplet remaining. By forcing the modified aerogel into water, it showed a silver mirror-like surface on one hand, and swiftly took chloroform that sank to the bottom of water into its 3D bulk on the other hand (Figure 4e, Supporting Movie 1). Soon afterwards, the chloroform could be completely gotten rid of by taking the oil-loaded aerogel out of the water system. In other words, the modified aerogel could separate oil from water phase regardless of the densities of oils, which suggested its wide usage in absorbing various oils with densities greater or less than that of water. In addition, a piece of modified aerogel was fixed in a common-used filtering apparatus



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as Figure 4f shown. A mixture of chloroform dyed with SudanⅠand water colored with methyl blue at a volume ratio of 1:1 was poured into the upper receiver of 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. Chloroform layer contacted the aerogel surface before 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 SudanⅠwas collected in the receiving flask beneath containing no blue droplet that was 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 thickness 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 volume of the oil/water mixture, it would take different time to accomplish the separation process. As abovementioned, 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, attributing to the protonation or deprotonation of pDMAEMA chains at different pH medium. In a high-pH medium, PDMAEMA chains deprotonated and exhibited collapsed conformation. Under this circumstances, collapsed PDMAEMA chains and exposed SMA chains created synergetic effect in constructing a superhydrophobic surface of the modified aerogel. While in a low-pH medium, PDMAEMA chains entirely protonated and 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 experiment was used to test the aerogel’s pH-responsiveness. 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’s 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 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


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aerogel.

Figure 5. Oil desorption of modified aerogel with medium pH of (a) 1 and (b) 7 respectively. 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-DMAEMA-KH570). In addition, N element appeared in the spectrum of the modified aerogel belonged to the DMAEMA component in the synthesized copolymer, indicating the successful coating of the functional copolymer. The high-resolution N 1s spectra were exhibited in Figure 6(b-c). N 1s peak was appeared in XPS spectrum of the modified aerogel at 399 eV, assigning to tertiary amine in 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 crucial part in manipulating the surface wettability and oil desorption property of the modified aerogel under different conditions.



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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). Practically, absorption capacity is an important parameter to appraise the performance of absorbents. Different from two-dimensional (2D) materials, 3D materials often possess higher absorption capacity benefited 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 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


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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 What’s more, 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 aerogels59 and the biomass-derived porous carbonaceous aerogel.60

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. Combined with oil-water separation ability and oil absorption capacity, reusability is an equal significant criterion for oil sorbents. For the modified aerogels we prepared in this work, there existed two approaches to realize oil desorption, manually squeezing and pH-induced discharging. These two methods were much more convenient, controllable, cost-effective and eco-friendly than other recycle ways. 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 furthest and the desorbed oil was harvested. Then the desorbed modified aerogel was used for another cycle. Relative experimental data were 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



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kept about 83.1 % of the first cycle. The decrease in absorption capacity might be ascribed to the remanent 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, certain amount of absorbed oil might remain in the aerogel, leading to a decrease in absorption capacity for 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 maintained stable. And after 10 cycles, absorption capacity for the modified aerogel remained at about 73.71 % of the first cycle, showing a 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 almost remained stable with the value of about 66%. That is, the modified aerogel also equipped with the property of recycling by means of pH-induced 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 prospect in water treatment.

CONCLUSIONS In this study, novel biomass-derived aerogels were successfully constructed using facile sol-gel method and freeze-drying technique. The modification process was achieved by dip-coating the original aerogel with synthesized copolymer p(SMA-DMAEMA-KH570). The original aerogel possessed low density, porous cellular structure, three-dimensional self-supporting 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 aerogels application. The absorption capacity of the modified aerogel was around 20 to 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 property mentioned above endowed modified aerogels with great comprehensive characteristics, offering them conspicuous application potential in large-scale oil pollutants removal.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. 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(SMA-DMAEMA-KH570);


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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).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS 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).

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Abstract Graphic

Synopsis The robust oil absorbent with double way reusability aiming at water remediation was based on renewable biomass-derived honeycomb-like aerogel.


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Renewable, Biomass-Derived, Honeycomblike Aerogel As a Robust

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