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A renewable and biodegradable all-biomass material for the separation of oil from water surface
Surface & Coatings Technology 372 (2019) 84–92
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
A renewable and biodegradable all-biomass material for the separation of oil from water surface ⁎
T
⁎
Fajun Wang , Ting Xie, Wei Zhong, Junfei Ou, Mingshan Xue , Wen Li School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Renewable materials Biodegradation Superoleophilicity Oil/water separation Loofah sponge
In this work, superhydrophobic and superoleophilic loofah sponge (LS) was prepared by a facile, inexpensive, eco-friendly and sustainable dip-coating method. This method uses LS as the porous skeleton, carnauba wax (CW) and rice bran wax (RW) as the coating material and ethyl acetate as the solvent. The wax layer forms particle/film composite micro-structures, which makes the superhydrophobic coating have excellent stability against soaking of water and various oils, and high resistance to corrosive mediums. The superhydrophobic and superoleophilic LS can selectively separate oil from water surface with desirable properties, such as high separation capacity (> 9.5 g/g), efficiency (> 91%), and reusability (> 10 cycles). In addition, the used SLS can be reused for the recovery of waxes and LS substrate. Furthermore, the LS substrate can be biodegraded in the natural ecological cycle. The present work provides a new idea and solution for developing sustainable, renewable and biodegradable all-biomass oil/water separation materials.
1. Introduction
cellulosic paper were used as substrates to prepare green separation materials [11–17]. However, inorganic nanoparticles and/or synthesized low-surface-energy reagent were still used in these materials. For example, Cheng et al. fabricated a biodegradable oil/water separation material composed of epoxidized soybean oil, stearic acid, ZnO and filter paper [15]. Gu et al. constructed a superhydrophobic material using polylactic acid nonwoven fabric as substrate for oily wastewater treatment [1]. The polylactic acid fabric is biodegradable. However, SiO2 nanoparticles and PS microspheres were used. Cheng et al. used cellulose nanocrystal to create roughness on cotton fabric substrate [2]. However, the surface chemical modification with hexadecyltrimethoxysilane was inescapable. The frequently-used nanoparticles, including ZnO, TiO2 and SiO2, would also pollute environment. In addition, various low surface energy reagents, such as stearic acid, perfluorooctanoic acid, alkyl ketene dimer (AKD), polydimethylsiloxane (PDMS), perfluorooctyl triethoxysilane and others were still used [18–22]. These synthesized low surface energy reagents are not renewable materials. Therefore, it is still a great challenge to develop true sense of renewable and biodegradable separation materials. Very recently, loofah sponge (LS), Luffa cylindrica, with a natural porous structure was modified and employed as a novel material for oil/water separation [23–26]. However, all of these modification methods used synthetic low-surface-energy reagents, including
The frequent oil spill accidents and continuous discharge of oily waste-waters bring hazardous substances into the ocean, which have caused serious threatens to both environment and marine ecosystem [1,2]. The poisonous compounds in oil leakage can even have serious effects on human health through the food chain [3]. Functional materials with surface superhydrophobicity and superoleophilicity, as well as porous structures have received extensive attention due to their reported excellent oil/water separation properties [4–6]. Until now, considerable oil/water separation materials with superwettability have been proposed, including sponges, metal foams and meshes, fabrics and textiles, aerogel and so forth [6–10]. Although these materials exhibited excellent oil/water separation properties, the dispose of these materials after they were used is still a challenging task, because they cannot be degradable in environment. Conventionally, the used oil/water materials were burned or discarded to the environment, which not only caused environmental pollution, but also wasted resources. From the point of view of sustainable development and eco-friendliness of the separation materials, all-biomass based materials with renewability and biodegradability are of great significance. In recent years, some renewable and biodegradable oil/water separation materials, such as cotton fabric, chitosan sponge, and
Abbreviations: LS, loofah sponge; SLS, superhydrophobic loofah sponge; CW, carnauba wax; RW, rice bran wax; CA, contact angle; WCA, water contact angle; OCA, oil contact angle ⁎ Corresponding author. E-mail addresses: [email protected] (F. Wang), [email protected] (M. Xue). https://doi.org/10.1016/j.surfcoat.2019.05.002 Received 2 April 2019; Received in revised form 30 April 2019; Accepted 2 May 2019 Available online 03 May 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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2. Materials and methods
polyhedral oligomeric silsesquioxane (POSS) [23], polyurea adhesive [24], FAS-17 and stearic acid [25,26]. These low-surface-energy reagents are neither renewable nor biodegradable. Therefore, in addition to using natural porous materials as the substrate, the genuine renewable and biodegradable oil/water separation materials must use natural low-surface-energy materials as modifiers. Inspired by lotus leaf, the plant generates a layer of epicuticular wax with low surface energy on its surface [27]. However, the epicuticular wax of lotus leaf is not abundant, and its purification process is complex, which limits the extensive application in separation materials. On the other hand, as a typical example of natural wax, carnauba wax (CW) and beeswax have been used to prepare superhydrophobic surfaces due to their potential applications in paper industry, timber and food packaging [28–30]. These waxes are natural materials with low surface energy and possess lots of advantages, such as low cost, abundant, commercial available, easy fabricating, renewable, environmentally friendly and so on. To the best of our knowledge, all-biomass oil/water separation materials using natural LS as porous substrate and natural waxes as low-surface-energy substances have not been reported. Herein, LS was selected as a three dimensional (3D) porous substrate for the preparation of oil/water separation material. Two types of waxes, namely, CW and rice bran wax (RW) were selected as coating materials for hydrophobic modification of LS. All raw materials are derived from plants [see Fig. 1(a)]. They are cheap, abundant, renewable and fluorine-free. The superhydrophobic LS (SLS) was prepared via a simple and rapid dip-coating method [see Fig. 1(b)] without using any organic nanoparticles and synthesized low-surface-energy reagents. The obtained SLS exhibits superhydrophobicity and superoleophilicity simultaneously after immersing and can be used in oil/water separation [see Fig. 1(c)]. The SLS can selectively adsorb a variety of oils from water surface with high oil absorption capacity, high oil/water separation efficiency and reusability. In addition, the SLS shows high resistant to water, oils and corrosive mediums at ambient conditions. Furthermore, both the wax coating and LS substrate can be reused after separated in hot ethyl acetate [Fig. 1(d)]. Particularly, the LS can be discarded in the environment directly after a long period of use due to its biodegradability. This work might provide a novel strategy for further designing and developing a new generation of renewable and biodegradable oil/water separation materials.
2.1. Materials Loofah sponge (LS) was purchased from local market. Carnauba wax (CW) and rice bran wax (RW) were purchased from Shanghai Jinbaidi Co. Ltd. Hexadecane was purchase from Shanghai Aladdin Biochemical Technology Co. Ltd. Kerosene, gasoline and diesel were supplied by China Petrochemical Corporation Ltd. Deionized water was prepared in laboratory. Ultra-high active cellulase (filter paper activity: 200 U/g) was kindly provided by Huaian Biomass Green Energy Co. Ltd. 2.2. Preparation of superhydrophobic LS The preparation route was illustrated in Fig. 1(a)–(e). 200 mL of ethyl acetate was preheated to a temperature of 70 °C. Subsequently, 0.1 g of CW and 0.1 g of RW pellets were added successively into hot EA under stirring until a transparent solution was formed. The solution was transferred to a water bath under a constant temperature of 50 °C. An opaque wax emulsion was formed slowly. Then, a piece of LS sample (2.5 cm × 3.5 cm) was immersed into the emulsion for 5 s. The sample was taken out from the emulsion and dried at ambient condition. The superhydrophobic LS sample was obtained after three cycles of immersing and drying. 2.3. Characterization of modified LS An optical contact angle measuring instrument (Krüss DSA30, Germany) was used for the measurement of contact angles (CAs or CA) and sliding angles (SAs or SA) for water and hexadecane. The volume of the liquid used was 8 μL. An FE-SEM (QUANTA F250, USA) was used to observed the surface micro-structures of samples. An FTIR spectrometry was used to characterize the chemical composition of the samples before and after modification. 2.4. Stability measurement Stabilities against water and different types of oils immersing were measured by soaking the sample in various liquids respectively for a
Fig. 1. Schematic illustrations of (a) all of the raw materials come from plants; (b) dip-coating; (c) oil/water separation process; and (d) recovery of waxes and disposal of LS. 85
Fig. 2. (a)–(c) WCA and WSA of wax modified LS sample surface as a function of immersing cycles: (a) single wax of RW; (b) single wax of CW; (c) mixed wax of CW and RW. (d)–(f) Stability of wax modified LS sample against water immersing: (d) single wax of RW; (e) single wax of CW; (f) mixed wax of CW and RW.
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Subsequently, 10 mL of cellulose enzyme aqueous solution (with a concentration of 10 mg/mL) was added into each degradation solution. Then, the solutions were shaken at 37 °C using a shaking table. Finally, the LS sample was taken out after a period of time and washed with deionized water. The biodegradation rate (Dr) was calculated by [15]:
period of time. Then, CAs and SAs of the sample for water were measured. In addition, corrosive aqueous solutions with pH values from 1 to 14 were prepared using hydrochloric acid and sodium hydroxide. The CAs and RAs of the sample were tested using the above aqueous solutions, and the stability of the sample in corrosive medium was evaluated through the changes of CAs and SAs.
Dr = 2.5. Oil/water separation properties
where Wa and Wb are the weight of LS after and before biodegradation. Before weighting, the LS sample was dried at 100 °C until achieving a constant weight.
3.5 wt% NaCl aqueous solution was used to mimic sea water. Four kinds of oils, namely, n-hexadecane, kerosene, gasoline and diesel, were selected to mimic spilled oils. The oil absorption capacity (Kac) of SLS was calculated by [8,31]:
K ac =
3. Results and discussion
msa − m 0 m0
3.1. Influence of wax type on the surface wettabilities
where msa is the weight of SLS saturated by oil and m0 is the weight of SLS. A beaker containing 500 ML 3.5 wt% NaCl aqueous solution and 10 g oil was used for the oil/water separation experiment. Oil was dyed red using oil red O for the purpose of distinguishing it from water. Oil floated on the surface of water due to its lower density. A piece of SLS was placed on the surface of oil and agitated repeatedly to make it in full contact with oil. Then, the SLS was removed after it is filled with oil. The adsorbed oil in the SLS was recovered using a high speed centrifuge (TGL-18MS) at 3500 rpm for 5 min. The oil/water separation efficiency (Kse) was calculated by [8,31]:
Kse =
Wb − Wa × 100% Wb
The main components of LS are cellulose, hemicellulose and lignin, which contain a large number of hydrophilic groups, so the surface of the untreated LS is hydrophilic [34–36]. Whereas CW and RW are hydrophobic natural material composed of straight-chained esters, alcohols, and hydroxy-fatty acids (C24, C28, C32, C34, etc.) [15,18]. The surface wettability of a single wax (RW or CW) modified LS was studied at first. The influence of immersing cycle on the surface WCA and WSA of RW modified LS is summarized in Fig. 2(a). One can see that CA increases while SA decreases with the increase of immersing cycles. However, RW modified LS cannot reach superhydrophobicity. As a comparison, the single CW modified LS exhibits superhydrophobicity after 3 times of immersing [Fig. 2(b) and (e)]. In addition, the RW coating is very stable against water immersing [see Fig. 2(d)]. However, the CW coating modified sample shows poor stability against water immersing [Fig. 2(e)]. For the mixed wax modified LS, the sample shows a water CA of 151.7° and a water SA of 7.8° after 3 immersing cycles, which demonstrates the superhydrophobicity. Fig. 2(f) shows that the mixed wax modified LS is very stable against water immersing. Therefore, the superhydrophobic LS (SLS) modified with mixed wax was used for further investigation. The digital photos of hexadecane and water droplets on the surface of pristine LS and SLS were shown in Fig. 3(a) and (b). One can see that both water and oil droplets spread on the surface of pristine LS [see Fig. 3(a)], which indicates the hydrophobic and oleophilic nature of the pristine LS. Water droplets show ball-like shapes on the surface of SLS, while hexadecane droplets are absorbed in the pores of the SLS [see Fig. 3(b)]. Dynamic water impact test also shows that freely falling water droplet from a height of about 3 cm bounces on the surface of LS, without penetrating into the pores of LS [see Supplementary Material (SM), Movie S1]. When a pristine LS and a SLS are placed on water surface simultaneously, the pristine LS is completely submerged by water, while the SLS floats freely on water surface [see SM, Fig. S1(a) and (b)]. Fig. S1(c) and (d) shows a hexadecane droplet is manipulated by a needle to approach to the surface of SLS. The droplet is immediately sucked into the pores of SLS as soon as it makes contact with the surface of SLS [see Fig. S1(e) and (f)]. The SLS possesses desirable
ma − mb mb
where ma and mb are the weight of oil after and before separation. After separation, the oil adsorbed by SLS was collected by centrifugation. The oil absorption kinetics of SLS was studied by testing the relationship between oil absorption weight (Wt) and oil absorption time (t). The oil absorption kinetics is analyzed by pseudo-first-order kinetic model [31–33]:
ln(W − Wt ) = ln W − kt where W is the maximum oil adsorption of the SLS, Wt is the weight of adsorbed oil in SLS at time t, k is the rate constant related to the speed of adsorption, and t is time of adsorption. 2.6. Recover of waxes and biodegradation of LS When the SLS reaches its life span, the wax layer on the surface of LS fiber can be recovered by dissolving the waxes in hot EA at 70 °C [see Fig. 1 and (a)–(e) and (g)]. The obtained EA solution of waxes can be used in another preparation cycle, thus achieving the reuse of waxes. The biodegradation experiment was carried out in an aqueous solution with a pH value of 4.2. A batch of dewaxed LS samples were put into a reaction kettle containing 0.2 wt% H2SO4 aqueous solution, and pretreated under 150 °C for 2 h before biodegradation. The pretreated LS samples were put into the degradation solutions respectively.
Fig. 3. Digital Photo of water and oil (n-hexadecane) droplets added on the surface of (a) pristine LS and (b) SLS. The insets show the corresponding OCA and WCA measurement. Water is dyed blue with methylene blue and oil is dyed red with oil red O. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. SEM images of LS samples prepared with different immersing cycles at different magnification. (a) and (b) pristine LS (or 0 cycle); (c) and (d) modified LS (1 cycle); (e) and (f) modified LS (2 cycles); (g) and (h) modified LS (3 cycles).
properties, such as porosity, superoleophilicity, superhydrophobicity and good buoyancy, which makes it suitable for oil/water separation. 3.2. Influence of wax type on the surface microstructures The surface morphologies of different wax modified LS sample surfaces with different immersing cycles were carefully investigated. The morphologies of single RW and CW modified LS sample surface were shown in Fig. S2 and S3, respectively. The surface microstructures of mixed wax modified LS sample were shown in Fig. 4. One can see that LS consists of three-dimensional interconnected fibers at low magnification [see SM, Fig. S4(a)]. The pores between fibers are irregular in shape and range in size from 200 μm to several millimeters. After the modification of wax, there is no obvious change in fiber diameter and pore size of LS [see SM, Fig. S4(b)–(d)]. However, the surface morphology of LS fiber changes significantly at high magnification with increasing immersing cycles. It is observed that the surface of pristine LS is uneven, it has lots of bumps and potholes [Fig. 4(a) and (b)]. The bumps and potholes are completely covered by a layer of wax coating after one cycle of immersing. The wax coating becomes rougher and rougher with increasing immersing cycles [Fig. 4(c) and (d)]. After three times of immersion, the surface of LS exhibits a water CA > 150°, showing its superhydrophobicity [Fig. 4(g) and (h)].
Fig. 5. FT-IR spectra of wax mixtures, pristine LS and superhydrophobic LS.
A schematic diagram [see SM, Fig. S5] was used to explain the different of micro-structures and stability of different wax coatings, namely, single CW coating, single RW coating and mixed wax coating of CW and RW. For single CW modified LS surface, a particle-like coating was formed [see SM, Fig. S5(a) and Fig. S2]. As discussed in FT-IR measurement result (see Fig. 5), the interaction between wax coating and LS fiber substrate is weak due to physical binding. Therefore, single CW modified LS sample is not stable against water immersing [see Fig. 2(e)]. For single RW modified LS surface [see SM, Figs. S5(b) and S1], the coating is mainly composed of a layer of wax film. The RW film is continuous and tightly coated on the fiber surface of LS, which ensures the stability of the sample during water immersing process [see Fig. 2(d)]. However, there are only a few protrusions on the surface of this film, and the roughness is not enough to endow the surface with superhydrophobicity. For mixed wax modified surface [see SM, Figs. S5(c) and 4(c)–(h)], a desirable CW/RW composite film is formed. The lower layer of the composite film is continuous and covers LS fiber surface completely. Whereas the upper surface of the composite film contains lots of particle-like protrusions that firmly embedded in the film of lower wax. As a result, the composite film shows high stability against water immersing. The SLS surface was prepared at 50 °C, this temperature is helpful to form a particle/film composite coating. Fig. S6 shows the appearance of CW and RW solution when temperature naturally decreases to room temperature. In the present case, our sample is prepared at 50 °C, RW dissolves by ethyl acetate completely
3.3. FTIR spectra analyses The FT-IR spectra of wax mixture, pristine LS and superhydrophobic LS samples are displayed in Fig. 5. The absorption peaks of wax mixture at 2913 cm−1 and 2841 cm−1 are assigned to the asymmetric and symmetric stretching vibrations of methylene (CH2) of natural waxes [37–39]. The peak at 1735 cm−1 corresponds to the stretching vibration of carbonyl group (C]O), which is due to the presence of carboxylic acids, ketones and/or ester groups in natural waxes. The peak at 1465 cm−1 is assigned to the asymmetric bending vibrations of CeH2 and CeH3 groups. The peak at 1165 cm−1 is attributed to the symmetric stretching vibration of ester group (–COO). The peak at 720 cm−1 is attributed to the long carbon chain compounds [38]. The pristine LS exhibits a board absorption peak at 3313 cm−1 is attributed to the hydroxy groups existing on the surface of LS. The peaks at 1026 cm−1 and 894 cm−1 can be attributed to the stretching vibrations of CeO and CeC groups of cellulose [38]. In addition, the FT-IR spectrum of the modified LS is very similar to that of the pristine LS, except the existence of characteristic peaks of methylene groups (2913 cm−1 and 2841 cm−1), which indicates that there is no chemical reaction between natural wax and LS substrate [24]. 88
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Fig. 6. (a) stability of SLS against oil immersing; (b) CA and SA measurement of SLS for aqueous solutions with different pH values.
kinetics is analyzed by pseudo-first-order kinetic model. Plots of ln (W − Wt) versus time are shown in the inset of Fig. 8(c). The linear fit result of the plots is also depicted in the inset of Fig. 8(c). The rate constant (K) corresponds to the slope of the fitting line. The higher the absolute value of the slope of the line, the faster the oil absorption. One can see that the SLS exhibits highest adsorption rate for hexadecane, which can be interpreted by the fact that hexadecane is less volatile than other oils. The oil absorption capacity (g/g) of the SLS for different kinds of oil was influenced by both the density and the volatility of the oils. The greater the density of the oil, the greater the oil absorption capacity of the SLS. Additionally, the evaporation of oil will reduce the actual oil absorption capacity of the SLS. Both the density and the boiling point (boiling range) of gasoline is the lowest [see SM, Table S1]. Therefore, the SLS has the lowest adsorption capacity for gasoline [see Fig. 8(c)]. As a comparison, the density of hexadecane is slightly less than kerosene and diesel, but its boiling point is much higher than that of kerosene and diesel. As a result, the SLS has the largest adsorption capacity for hexadecane [see Fig. 8(c)]. The influence of temperature on the absorption capacities of the SLS for different kinds of oils was measured and the result was shown in Fig. 8(d). One can see that the absorption capacities for all of the four kinds of oils show a slight decrease with increasing temperature. For example, the absorption capacitors for kerosene are 11.4, 10.8 and 10.5 g/g at 10, 25 and 40 °C, respectively. The decrease of absorption capacities with increasing temperature is interpreted by the fact that the density of the oil becomes smaller and the volatility becomes larger with the increase of temperature. Moreover, the SLS could continuously float on the surface of 20 wt% NaCl solution for a week, which demonstrates its stability against the continuous contact of salty water. Fig. S7 (see SM) depicts the separation efficiencies for different oil/water mixtures with different NaCl concentration are above 92%. This can be attributed to the following two reasons. Firstly, both RW and CW are chemical inertness. Secondly, the surface superhydrophobicity of the wax coating could reduce the direct contact area between the wax coating and the salt solution. The above two factors make the SLS have good salt water erosion resistance [13].
while CW particles suspended in EA. As a result, a particles/film-like composite wax coating was formed on the surface of LS fiber [see Figs. 4(f) and (h), S5(c)]. 3.4. Stability investigation The SLS was designed for oil/water separation. Therefore, the SLS must be stable against contacting with both water and oil. One can see that SLS is very stable against water immersing from Fig. 2(f). The high stability of SLS against water contacting can be attributed to the following two aspects. On the one hand, both CW and RW cannot be wet by water. On the other hand, the adhesion force between wax coating layer and LS fiber substrate is strong. From the SEM images [see Fig. 4(f) and (h)], it can be observed that the micro-protrusions of wax are firmly anchored on the fiber surface of LS. In addition, the SLS is also stable against oil immersing, including kerosene, hexadecane, diesel and gasoline. The samples remain superhydrophobicity after being soaked in these oils respectively for 5 days [see Fig. 6(a)]. Furthermore, when corrosive solutions with different pH values (pH = 1–6: HCl solution; pH = 7: 3.5 wt% of NaCl solution and pH = 8–14: NaOH solution) are used as measuring liquid, the surface of SLS still exhibits high CA (> 150°) and low SA (< 10°), which demonstrates that the SLS possesses excellent stability against the contact of corrosive solutions and can be used in a variety of water environments [Fig. 6(b)] [31]. 3.5. Separation of oil from the water surface Fig. 7 illustrates the oil/water separation properties using the superhydrophobic and superoleophilic LS. n-Hexadecane was used to mimicking the oil spill on water surface. The oil was dyed red for good observation. One can see that the oil is adsorbed by the SLS immediately when the SLS is just in contact with oil [Fig. 7(b) and (c)]. There is no oil left on water surface after removing the SLS from water [Fig. 7(e) and (f)], which demonstrates the excellent oil/water separation performance of our SLS. Besides, the SLS can also separate kerosene, diesel and gasoline from water surface with absorption capacitor larger than 10 g/g [Fig. 8(a)]. Additionally, after 10 cycles of separation, the absorption capacitor of SLS reduces < 10%, showing its excellent recyclability. Another important parameter for an oil/water separation material is oil separating efficiency [Fig. 8(b)]. The adsorbed oil in the pores of SLS can be readily collected using a simple centrifugal separation method for reuse. The separation efficiency is above 91% for all of the four kinds of oils even after 10 cycles of separation. Fig. 8(c) shows the relationship between the weight (Wt, g/g) of adsorbed oil and time (t). The SLS sample exhibits fast oil adsorption rate and achieves the saturation oil absorption < 15 s. The oil absorption
3.6. Recovery, recycle and biodegradation of the SLS The SLS becomes a solid waste after long oil/water separation cycles. Based on the 3R principle (namely, reduce, reuse and recycle) [40], we should make full use of the waste and reduce waste discharge. In the present case, the wax coating can be recovered by hot EA [see Fig. 1(d)]. The recycled wax solution can be used again. In addition, the skeleton structure of the LS after dewaxing still exists, so it can be modified again due to the simple preparation process. Thus, both wax and LS can be recycled. Fig. 9 shows the weight loss of the dewaxed LS 89
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Fig. 7. Separation of oil from water surface using the SLS sample. Hexadecane was used to simulate oil on water surface and dyed red to distinguish it from water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
with increasing biodegradation time. One can see that the weight losses of both the pretreated and untreated LS increase with biodegradation time, which demonstrates their biodegradability. Natural LS is
composed of cellulose, hemicellulose and lignin, so they can be biodegraded without causing any pollution [34–36]. However, the cellulosic polymers of LS are surrounded by complex hemicellulose-lignin
Fig. 8. (a) Absorption capacitor and (b) separation efficiency for different kind of oils as a function of separation cycles; (c) absorption capacitor as a function of time, the inset shows the linear fit result using pseudo-first-order kinetic model; (d) absorption capacitor for different kind of oils as a function of temperature. 90
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Acknowledgements The authors acknowledge with pleasure the financial support of this work by the Natural Science Foundation of China (Grant Nos. 51662032 and 11864024) and the Natural Science Foundation of Jiangxi Province (No. 20161 BAB206112). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.05.002. References [1] J.C. Gu, P. Xiao, P. Chen, L. Zhang, H.L. Wang, L.W. Dai, L.P. Song, Y.J. Huang, J.W. Zhang, T. Chen, Functionalization of biodegradable PLA nonwoven fabric as superoleophilic and superhydrophobic material for efficient oil absorption and oil/ water separation, ACS Appl. Mater. Interfaces 9 (2017) 5968–5973. [2] Q.Y. Cheng, C.S. Guan, M. Wang, Y.D. Li, J.B. Zeng, Cellulose nanocrystal coated cotton fabric with superhydrophobicity for efficient oil/water separation, Carbohydr. Polym. 199 (2018) 390–396. [3] Y. Liu, B. Zhan, K.T. Zhang, C. Kay, T. Stegmaier, Z.W. Han, L.Q. Ren, On-demand oil/water separation of 3D Fe foam by controllable wettability, Chem. Eng. J. 331 (2018) 278–289. [4] B. Wang, W.X. Liang, Z.G. Guo, W.M. Liu, Biomimetic super-lyophobic and superlyophilic materials applied for oil/water separation: a new strategy beyond nature, Chem. Soc. Rev. 44 (2015) 336–361. [5] A. Matin, U. Baig, M.A. Gondal, S. Akhtar, S.M. Zubair, Facile fabrication of superhydrophobic/superoleophilic microporous membranes by spray-coating ytterbium oxide particles for efficient oil-water separation, J. Membrane Sci. 548 (2018) 390–397. [6] Y. Yang, H. Yi, C.Y. Wang, Oil absorbents based on melamine/lignin by a dip adsorbing method, ACS Sustain. Chem. Eng. 3 (2015) 3012–3018. [7] K. Hou, Y. Jin, J.H. Chen, X.F. Wen, S.P. Xu, J. Cheng, P.H. Pi, Fabrication of superhydrophobic melamine sponges by thiol-ene click chemistry for oil removal, Mater. Lett. 202 (2015) 99–102. [8] O. Oribayo, X.S. Feng, G.L. Rempel, Q.M. Pan, Synthesis of lignin-based polyurethane/graphene oxide foam and its application as an absorbent for oil spill clean-ups and recovery, Chem. Eng. J. 323 (2017) 191–202. [9] X. Zhao, L.X. Li, B.C. Li, J.P. Zhang, A.Q. Wang, Durable superhydrophobic/superoleophilic PDMS sponges and their applications in selective oil absorption and in plugging oil leakages, J. Mater. Chem. A 2 (2014) 18281–18287. [10] H.Y. Mi, X. Jing, H.X. Huang, X.F. Peng, L.S. Turng, Superhydrophobic graphene/ cellulose/silica aerogel with hierarchical structure as superabsorbers for high efficiency selective oil absorption and recovery, Ind. Eng. Chem. Res. 57 (2018) 1745–1755. [11] Q.Y. Cheng, M.C. Liu, Y.D. Li, J. Zhu, A.K. Du, J.B. Zeng, Biobased super-hydrophobic coating on cotton fabric fabricated by spraycoating for efficient oil/water separation, Polym. Test. 66 (2018) 41–47. [12] C.P. Su, H. Yang, H.P. Zhao, Y.L. Liu, R. Chen, Recyclable and biodegradable superhydrophobic and superoleophilic chitosan sponge for the effective removal of oily pollutants from water, Chem. Eng. J. 330 (2017) 423–432. [13] Xuejie Yue, Jiaxin Li, Tao Zhang, Fengxian Qiu, Dongya Yang, Mengwei Xue, In situ one-step fabrication of durable superhydrophobic-superoleophilic cellulose/LDH membrane with hierarchical structure for efficiency oil/water separation, Chem. Eng. J. 328 (2017) 117–123. [14] D.L. Zang, M. Zhang, F. Liu, C.Y. Wang, Superhydrophobic/superoleophilic corn straw fibers as effective oil sorbents for the recovery of spilled oil, J. Chem. Technol. Biotechnol. 91 (2016) 2449–2456. [15] Q.Y. Cheng, X.P. An, Y.D. Li, C.L. Huang, J.B. Zeng, Sustainable and biodegradable superhydrophobic coating from epoxidized soybean oil and ZnO nanoparticles on cellulosic substrates for efficient oil/water separation, ACS Sustain. Chem. Eng. 5 (2017) 11440–11450. [16] D. Aslanidoua, I. Karapanagiotis, C. Panayiotoua, Superhydrophobic, superoleophobic coatings for the protection of silk textiles, Prog. Org. Coat. 97 (2016) 44–52. [17] I. Karapanagiotis, D. Grosu, D. Aslanidou, K.E. Aifantis, Facile method to prepare superhydrophobic and water repellent cellulosic paper, J. Nanomater. 2015 (2015) 219013. [18] Y. Huang, D.K. Sarkar, X. Grant Chen, Superhydrophobic aluminum alloy surfaces prepared by chemical etching process and their corrosion resistance properties, Appl. Surf. Sci. 356 (2015) 1012–1024. [19] J. Yang, Z.Z. Zhang, X.H. Xu, X.T. Zhu, X.H. Men, X.Y. Zhou, Superhydrophilic–superoleophobic coatings, J. Mater. Chem. 22 (2012) 2834–2837. [20] L. Ejenstam, L. Ovaskainen, I. Rodriguez-Meizoso, L. Wågberg, J.S. Pan, A. Swerin, P.M. Claesson, The effect of superhydrophobic wetting state on corrosion protection-the AKD example, J. Colloid Interface Sci. 412 (2013) 56–64. [21] C.L. Zhou, Z.D. Chen, H. Yang, K. Hou, X.J. Zeng, Y.F. Zheng, J. Cheng, Natureinspired strategy toward superhydrophobic fabrics for versatile oil/water separation, ACS Appl. Mater. Interfaces 9 (2017) 9184–9194.
Fig. 9. Weight loss of the pretreated and untreated LS as a function of degradation time.
composite structure, which impedes the access of cellulase [41,42]. At present, pretreatment is an essential step for the biodegradation of lignocellulose biomass into biofuels (ethanol) [43]. Dilute acid pretreatment can destroy the structure of lignocellulose, hydrolyze hemicellulose, and make the biodegradable cellulose more accessible to microorganisms and enzymes. The weight loss of the LS caused by pretreatment was as high as 15.6% and increases sharply from 15.6% to 27.1% within a day. This can be explained by the fact that in the early stage of biodegradation, the cellulose content in the LS is relatively high, and it is easy to be degraded by cellulase because of pretreatment. The higher the weight loss of the LS due to cellulase hydrolysis, the lower the cellulose content left in the LS. As a consequence, the weight loss of the LS increases slowly with further increasing time. Hence, the weight loss rate of untreated LS is larger than that of the pretreated one in the late stage of hydrolysis (from 1 to 10 day), which can be interpreted by the higher cellulose content remained in the untreated LS. Cheng et al. reported the hydrolytic degradation of epoxidized soybean oil modified superhydrophobic cellulosic filter paper [15]. However, the degradation process is carried out under chemical conditions without the use of biological enzymes, so it is not a technically biodegradation. In addition, it takes up to 70 days to reach a degradation rate of about 15%. In the present case, the weight loss of untreated LS was > 15%, while that of the pretreated LS was > 30% after only 10 days of biodegradation. Hence, our oil/water separation material is more conducive to the recycling of materials.
4. Conclusions In summary, a renewable and biodegradable material using natural LS as substrate and natural waxes as coating was fabricated via a simple dip-coating method. All raw materials are natural ingredients derived from plants. The waxes modified LS exhibits both superoleophilicity and superhydrophobicity. The coating composing of two types of wax (CW and RW) possesses a particles/film-like composite micro-structure, which can guarantee the reasonable stability of the coating. The obtained SLS selectively separated different oils from water surface with oil/water separation capacity larger than 9.5 (g/g) (taking the lowest value for example) and separation efficiency higher than 91%, as well as good reusability (> 10 cycles). Importantly, the wax coating and LS substrate can be separated easily in hot ethyl acetate, thus achieving the reuse of both wax and LS. Particularly, the LS can be biodegradated. This work provides a new idea and method to develop oil/water separation materials by employing all-biomass as raw materials.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
A renewable and biodegradable all-biomass material for the separation of oil from water surface ⁎
T
⁎
Fajun Wang , Ting Xie, Wei Zhong, Junfei Ou, Mingshan Xue , Wen Li School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Renewable materials Biodegradation Superoleophilicity Oil/water separation Loofah sponge
In this work, superhydrophobic and superoleophilic loofah sponge (LS) was prepared by a facile, inexpensive, eco-friendly and sustainable dip-coating method. This method uses LS as the porous skeleton, carnauba wax (CW) and rice bran wax (RW) as the coating material and ethyl acetate as the solvent. The wax layer forms particle/film composite micro-structures, which makes the superhydrophobic coating have excellent stability against soaking of water and various oils, and high resistance to corrosive mediums. The superhydrophobic and superoleophilic LS can selectively separate oil from water surface with desirable properties, such as high separation capacity (> 9.5 g/g), efficiency (> 91%), and reusability (> 10 cycles). In addition, the used SLS can be reused for the recovery of waxes and LS substrate. Furthermore, the LS substrate can be biodegraded in the natural ecological cycle. The present work provides a new idea and solution for developing sustainable, renewable and biodegradable all-biomass oil/water separation materials.
1. Introduction
cellulosic paper were used as substrates to prepare green separation materials [11–17]. However, inorganic nanoparticles and/or synthesized low-surface-energy reagent were still used in these materials. For example, Cheng et al. fabricated a biodegradable oil/water separation material composed of epoxidized soybean oil, stearic acid, ZnO and filter paper [15]. Gu et al. constructed a superhydrophobic material using polylactic acid nonwoven fabric as substrate for oily wastewater treatment [1]. The polylactic acid fabric is biodegradable. However, SiO2 nanoparticles and PS microspheres were used. Cheng et al. used cellulose nanocrystal to create roughness on cotton fabric substrate [2]. However, the surface chemical modification with hexadecyltrimethoxysilane was inescapable. The frequently-used nanoparticles, including ZnO, TiO2 and SiO2, would also pollute environment. In addition, various low surface energy reagents, such as stearic acid, perfluorooctanoic acid, alkyl ketene dimer (AKD), polydimethylsiloxane (PDMS), perfluorooctyl triethoxysilane and others were still used [18–22]. These synthesized low surface energy reagents are not renewable materials. Therefore, it is still a great challenge to develop true sense of renewable and biodegradable separation materials. Very recently, loofah sponge (LS), Luffa cylindrica, with a natural porous structure was modified and employed as a novel material for oil/water separation [23–26]. However, all of these modification methods used synthetic low-surface-energy reagents, including
The frequent oil spill accidents and continuous discharge of oily waste-waters bring hazardous substances into the ocean, which have caused serious threatens to both environment and marine ecosystem [1,2]. The poisonous compounds in oil leakage can even have serious effects on human health through the food chain [3]. Functional materials with surface superhydrophobicity and superoleophilicity, as well as porous structures have received extensive attention due to their reported excellent oil/water separation properties [4–6]. Until now, considerable oil/water separation materials with superwettability have been proposed, including sponges, metal foams and meshes, fabrics and textiles, aerogel and so forth [6–10]. Although these materials exhibited excellent oil/water separation properties, the dispose of these materials after they were used is still a challenging task, because they cannot be degradable in environment. Conventionally, the used oil/water materials were burned or discarded to the environment, which not only caused environmental pollution, but also wasted resources. From the point of view of sustainable development and eco-friendliness of the separation materials, all-biomass based materials with renewability and biodegradability are of great significance. In recent years, some renewable and biodegradable oil/water separation materials, such as cotton fabric, chitosan sponge, and
Abbreviations: LS, loofah sponge; SLS, superhydrophobic loofah sponge; CW, carnauba wax; RW, rice bran wax; CA, contact angle; WCA, water contact angle; OCA, oil contact angle ⁎ Corresponding author. E-mail addresses: [email protected] (F. Wang), [email protected] (M. Xue). https://doi.org/10.1016/j.surfcoat.2019.05.002 Received 2 April 2019; Received in revised form 30 April 2019; Accepted 2 May 2019 Available online 03 May 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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2. Materials and methods
polyhedral oligomeric silsesquioxane (POSS) [23], polyurea adhesive [24], FAS-17 and stearic acid [25,26]. These low-surface-energy reagents are neither renewable nor biodegradable. Therefore, in addition to using natural porous materials as the substrate, the genuine renewable and biodegradable oil/water separation materials must use natural low-surface-energy materials as modifiers. Inspired by lotus leaf, the plant generates a layer of epicuticular wax with low surface energy on its surface [27]. However, the epicuticular wax of lotus leaf is not abundant, and its purification process is complex, which limits the extensive application in separation materials. On the other hand, as a typical example of natural wax, carnauba wax (CW) and beeswax have been used to prepare superhydrophobic surfaces due to their potential applications in paper industry, timber and food packaging [28–30]. These waxes are natural materials with low surface energy and possess lots of advantages, such as low cost, abundant, commercial available, easy fabricating, renewable, environmentally friendly and so on. To the best of our knowledge, all-biomass oil/water separation materials using natural LS as porous substrate and natural waxes as low-surface-energy substances have not been reported. Herein, LS was selected as a three dimensional (3D) porous substrate for the preparation of oil/water separation material. Two types of waxes, namely, CW and rice bran wax (RW) were selected as coating materials for hydrophobic modification of LS. All raw materials are derived from plants [see Fig. 1(a)]. They are cheap, abundant, renewable and fluorine-free. The superhydrophobic LS (SLS) was prepared via a simple and rapid dip-coating method [see Fig. 1(b)] without using any organic nanoparticles and synthesized low-surface-energy reagents. The obtained SLS exhibits superhydrophobicity and superoleophilicity simultaneously after immersing and can be used in oil/water separation [see Fig. 1(c)]. The SLS can selectively adsorb a variety of oils from water surface with high oil absorption capacity, high oil/water separation efficiency and reusability. In addition, the SLS shows high resistant to water, oils and corrosive mediums at ambient conditions. Furthermore, both the wax coating and LS substrate can be reused after separated in hot ethyl acetate [Fig. 1(d)]. Particularly, the LS can be discarded in the environment directly after a long period of use due to its biodegradability. This work might provide a novel strategy for further designing and developing a new generation of renewable and biodegradable oil/water separation materials.
2.1. Materials Loofah sponge (LS) was purchased from local market. Carnauba wax (CW) and rice bran wax (RW) were purchased from Shanghai Jinbaidi Co. Ltd. Hexadecane was purchase from Shanghai Aladdin Biochemical Technology Co. Ltd. Kerosene, gasoline and diesel were supplied by China Petrochemical Corporation Ltd. Deionized water was prepared in laboratory. Ultra-high active cellulase (filter paper activity: 200 U/g) was kindly provided by Huaian Biomass Green Energy Co. Ltd. 2.2. Preparation of superhydrophobic LS The preparation route was illustrated in Fig. 1(a)–(e). 200 mL of ethyl acetate was preheated to a temperature of 70 °C. Subsequently, 0.1 g of CW and 0.1 g of RW pellets were added successively into hot EA under stirring until a transparent solution was formed. The solution was transferred to a water bath under a constant temperature of 50 °C. An opaque wax emulsion was formed slowly. Then, a piece of LS sample (2.5 cm × 3.5 cm) was immersed into the emulsion for 5 s. The sample was taken out from the emulsion and dried at ambient condition. The superhydrophobic LS sample was obtained after three cycles of immersing and drying. 2.3. Characterization of modified LS An optical contact angle measuring instrument (Krüss DSA30, Germany) was used for the measurement of contact angles (CAs or CA) and sliding angles (SAs or SA) for water and hexadecane. The volume of the liquid used was 8 μL. An FE-SEM (QUANTA F250, USA) was used to observed the surface micro-structures of samples. An FTIR spectrometry was used to characterize the chemical composition of the samples before and after modification. 2.4. Stability measurement Stabilities against water and different types of oils immersing were measured by soaking the sample in various liquids respectively for a
Fig. 1. Schematic illustrations of (a) all of the raw materials come from plants; (b) dip-coating; (c) oil/water separation process; and (d) recovery of waxes and disposal of LS. 85
Fig. 2. (a)–(c) WCA and WSA of wax modified LS sample surface as a function of immersing cycles: (a) single wax of RW; (b) single wax of CW; (c) mixed wax of CW and RW. (d)–(f) Stability of wax modified LS sample against water immersing: (d) single wax of RW; (e) single wax of CW; (f) mixed wax of CW and RW.
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Subsequently, 10 mL of cellulose enzyme aqueous solution (with a concentration of 10 mg/mL) was added into each degradation solution. Then, the solutions were shaken at 37 °C using a shaking table. Finally, the LS sample was taken out after a period of time and washed with deionized water. The biodegradation rate (Dr) was calculated by [15]:
period of time. Then, CAs and SAs of the sample for water were measured. In addition, corrosive aqueous solutions with pH values from 1 to 14 were prepared using hydrochloric acid and sodium hydroxide. The CAs and RAs of the sample were tested using the above aqueous solutions, and the stability of the sample in corrosive medium was evaluated through the changes of CAs and SAs.
Dr = 2.5. Oil/water separation properties
where Wa and Wb are the weight of LS after and before biodegradation. Before weighting, the LS sample was dried at 100 °C until achieving a constant weight.
3.5 wt% NaCl aqueous solution was used to mimic sea water. Four kinds of oils, namely, n-hexadecane, kerosene, gasoline and diesel, were selected to mimic spilled oils. The oil absorption capacity (Kac) of SLS was calculated by [8,31]:
K ac =
3. Results and discussion
msa − m 0 m0
3.1. Influence of wax type on the surface wettabilities
where msa is the weight of SLS saturated by oil and m0 is the weight of SLS. A beaker containing 500 ML 3.5 wt% NaCl aqueous solution and 10 g oil was used for the oil/water separation experiment. Oil was dyed red using oil red O for the purpose of distinguishing it from water. Oil floated on the surface of water due to its lower density. A piece of SLS was placed on the surface of oil and agitated repeatedly to make it in full contact with oil. Then, the SLS was removed after it is filled with oil. The adsorbed oil in the SLS was recovered using a high speed centrifuge (TGL-18MS) at 3500 rpm for 5 min. The oil/water separation efficiency (Kse) was calculated by [8,31]:
Kse =
Wb − Wa × 100% Wb
The main components of LS are cellulose, hemicellulose and lignin, which contain a large number of hydrophilic groups, so the surface of the untreated LS is hydrophilic [34–36]. Whereas CW and RW are hydrophobic natural material composed of straight-chained esters, alcohols, and hydroxy-fatty acids (C24, C28, C32, C34, etc.) [15,18]. The surface wettability of a single wax (RW or CW) modified LS was studied at first. The influence of immersing cycle on the surface WCA and WSA of RW modified LS is summarized in Fig. 2(a). One can see that CA increases while SA decreases with the increase of immersing cycles. However, RW modified LS cannot reach superhydrophobicity. As a comparison, the single CW modified LS exhibits superhydrophobicity after 3 times of immersing [Fig. 2(b) and (e)]. In addition, the RW coating is very stable against water immersing [see Fig. 2(d)]. However, the CW coating modified sample shows poor stability against water immersing [Fig. 2(e)]. For the mixed wax modified LS, the sample shows a water CA of 151.7° and a water SA of 7.8° after 3 immersing cycles, which demonstrates the superhydrophobicity. Fig. 2(f) shows that the mixed wax modified LS is very stable against water immersing. Therefore, the superhydrophobic LS (SLS) modified with mixed wax was used for further investigation. The digital photos of hexadecane and water droplets on the surface of pristine LS and SLS were shown in Fig. 3(a) and (b). One can see that both water and oil droplets spread on the surface of pristine LS [see Fig. 3(a)], which indicates the hydrophobic and oleophilic nature of the pristine LS. Water droplets show ball-like shapes on the surface of SLS, while hexadecane droplets are absorbed in the pores of the SLS [see Fig. 3(b)]. Dynamic water impact test also shows that freely falling water droplet from a height of about 3 cm bounces on the surface of LS, without penetrating into the pores of LS [see Supplementary Material (SM), Movie S1]. When a pristine LS and a SLS are placed on water surface simultaneously, the pristine LS is completely submerged by water, while the SLS floats freely on water surface [see SM, Fig. S1(a) and (b)]. Fig. S1(c) and (d) shows a hexadecane droplet is manipulated by a needle to approach to the surface of SLS. The droplet is immediately sucked into the pores of SLS as soon as it makes contact with the surface of SLS [see Fig. S1(e) and (f)]. The SLS possesses desirable
ma − mb mb
where ma and mb are the weight of oil after and before separation. After separation, the oil adsorbed by SLS was collected by centrifugation. The oil absorption kinetics of SLS was studied by testing the relationship between oil absorption weight (Wt) and oil absorption time (t). The oil absorption kinetics is analyzed by pseudo-first-order kinetic model [31–33]:
ln(W − Wt ) = ln W − kt where W is the maximum oil adsorption of the SLS, Wt is the weight of adsorbed oil in SLS at time t, k is the rate constant related to the speed of adsorption, and t is time of adsorption. 2.6. Recover of waxes and biodegradation of LS When the SLS reaches its life span, the wax layer on the surface of LS fiber can be recovered by dissolving the waxes in hot EA at 70 °C [see Fig. 1 and (a)–(e) and (g)]. The obtained EA solution of waxes can be used in another preparation cycle, thus achieving the reuse of waxes. The biodegradation experiment was carried out in an aqueous solution with a pH value of 4.2. A batch of dewaxed LS samples were put into a reaction kettle containing 0.2 wt% H2SO4 aqueous solution, and pretreated under 150 °C for 2 h before biodegradation. The pretreated LS samples were put into the degradation solutions respectively.
Fig. 3. Digital Photo of water and oil (n-hexadecane) droplets added on the surface of (a) pristine LS and (b) SLS. The insets show the corresponding OCA and WCA measurement. Water is dyed blue with methylene blue and oil is dyed red with oil red O. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. SEM images of LS samples prepared with different immersing cycles at different magnification. (a) and (b) pristine LS (or 0 cycle); (c) and (d) modified LS (1 cycle); (e) and (f) modified LS (2 cycles); (g) and (h) modified LS (3 cycles).
properties, such as porosity, superoleophilicity, superhydrophobicity and good buoyancy, which makes it suitable for oil/water separation. 3.2. Influence of wax type on the surface microstructures The surface morphologies of different wax modified LS sample surfaces with different immersing cycles were carefully investigated. The morphologies of single RW and CW modified LS sample surface were shown in Fig. S2 and S3, respectively. The surface microstructures of mixed wax modified LS sample were shown in Fig. 4. One can see that LS consists of three-dimensional interconnected fibers at low magnification [see SM, Fig. S4(a)]. The pores between fibers are irregular in shape and range in size from 200 μm to several millimeters. After the modification of wax, there is no obvious change in fiber diameter and pore size of LS [see SM, Fig. S4(b)–(d)]. However, the surface morphology of LS fiber changes significantly at high magnification with increasing immersing cycles. It is observed that the surface of pristine LS is uneven, it has lots of bumps and potholes [Fig. 4(a) and (b)]. The bumps and potholes are completely covered by a layer of wax coating after one cycle of immersing. The wax coating becomes rougher and rougher with increasing immersing cycles [Fig. 4(c) and (d)]. After three times of immersion, the surface of LS exhibits a water CA > 150°, showing its superhydrophobicity [Fig. 4(g) and (h)].
Fig. 5. FT-IR spectra of wax mixtures, pristine LS and superhydrophobic LS.
A schematic diagram [see SM, Fig. S5] was used to explain the different of micro-structures and stability of different wax coatings, namely, single CW coating, single RW coating and mixed wax coating of CW and RW. For single CW modified LS surface, a particle-like coating was formed [see SM, Fig. S5(a) and Fig. S2]. As discussed in FT-IR measurement result (see Fig. 5), the interaction between wax coating and LS fiber substrate is weak due to physical binding. Therefore, single CW modified LS sample is not stable against water immersing [see Fig. 2(e)]. For single RW modified LS surface [see SM, Figs. S5(b) and S1], the coating is mainly composed of a layer of wax film. The RW film is continuous and tightly coated on the fiber surface of LS, which ensures the stability of the sample during water immersing process [see Fig. 2(d)]. However, there are only a few protrusions on the surface of this film, and the roughness is not enough to endow the surface with superhydrophobicity. For mixed wax modified surface [see SM, Figs. S5(c) and 4(c)–(h)], a desirable CW/RW composite film is formed. The lower layer of the composite film is continuous and covers LS fiber surface completely. Whereas the upper surface of the composite film contains lots of particle-like protrusions that firmly embedded in the film of lower wax. As a result, the composite film shows high stability against water immersing. The SLS surface was prepared at 50 °C, this temperature is helpful to form a particle/film composite coating. Fig. S6 shows the appearance of CW and RW solution when temperature naturally decreases to room temperature. In the present case, our sample is prepared at 50 °C, RW dissolves by ethyl acetate completely
3.3. FTIR spectra analyses The FT-IR spectra of wax mixture, pristine LS and superhydrophobic LS samples are displayed in Fig. 5. The absorption peaks of wax mixture at 2913 cm−1 and 2841 cm−1 are assigned to the asymmetric and symmetric stretching vibrations of methylene (CH2) of natural waxes [37–39]. The peak at 1735 cm−1 corresponds to the stretching vibration of carbonyl group (C]O), which is due to the presence of carboxylic acids, ketones and/or ester groups in natural waxes. The peak at 1465 cm−1 is assigned to the asymmetric bending vibrations of CeH2 and CeH3 groups. The peak at 1165 cm−1 is attributed to the symmetric stretching vibration of ester group (–COO). The peak at 720 cm−1 is attributed to the long carbon chain compounds [38]. The pristine LS exhibits a board absorption peak at 3313 cm−1 is attributed to the hydroxy groups existing on the surface of LS. The peaks at 1026 cm−1 and 894 cm−1 can be attributed to the stretching vibrations of CeO and CeC groups of cellulose [38]. In addition, the FT-IR spectrum of the modified LS is very similar to that of the pristine LS, except the existence of characteristic peaks of methylene groups (2913 cm−1 and 2841 cm−1), which indicates that there is no chemical reaction between natural wax and LS substrate [24]. 88
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Fig. 6. (a) stability of SLS against oil immersing; (b) CA and SA measurement of SLS for aqueous solutions with different pH values.
kinetics is analyzed by pseudo-first-order kinetic model. Plots of ln (W − Wt) versus time are shown in the inset of Fig. 8(c). The linear fit result of the plots is also depicted in the inset of Fig. 8(c). The rate constant (K) corresponds to the slope of the fitting line. The higher the absolute value of the slope of the line, the faster the oil absorption. One can see that the SLS exhibits highest adsorption rate for hexadecane, which can be interpreted by the fact that hexadecane is less volatile than other oils. The oil absorption capacity (g/g) of the SLS for different kinds of oil was influenced by both the density and the volatility of the oils. The greater the density of the oil, the greater the oil absorption capacity of the SLS. Additionally, the evaporation of oil will reduce the actual oil absorption capacity of the SLS. Both the density and the boiling point (boiling range) of gasoline is the lowest [see SM, Table S1]. Therefore, the SLS has the lowest adsorption capacity for gasoline [see Fig. 8(c)]. As a comparison, the density of hexadecane is slightly less than kerosene and diesel, but its boiling point is much higher than that of kerosene and diesel. As a result, the SLS has the largest adsorption capacity for hexadecane [see Fig. 8(c)]. The influence of temperature on the absorption capacities of the SLS for different kinds of oils was measured and the result was shown in Fig. 8(d). One can see that the absorption capacities for all of the four kinds of oils show a slight decrease with increasing temperature. For example, the absorption capacitors for kerosene are 11.4, 10.8 and 10.5 g/g at 10, 25 and 40 °C, respectively. The decrease of absorption capacities with increasing temperature is interpreted by the fact that the density of the oil becomes smaller and the volatility becomes larger with the increase of temperature. Moreover, the SLS could continuously float on the surface of 20 wt% NaCl solution for a week, which demonstrates its stability against the continuous contact of salty water. Fig. S7 (see SM) depicts the separation efficiencies for different oil/water mixtures with different NaCl concentration are above 92%. This can be attributed to the following two reasons. Firstly, both RW and CW are chemical inertness. Secondly, the surface superhydrophobicity of the wax coating could reduce the direct contact area between the wax coating and the salt solution. The above two factors make the SLS have good salt water erosion resistance [13].
while CW particles suspended in EA. As a result, a particles/film-like composite wax coating was formed on the surface of LS fiber [see Figs. 4(f) and (h), S5(c)]. 3.4. Stability investigation The SLS was designed for oil/water separation. Therefore, the SLS must be stable against contacting with both water and oil. One can see that SLS is very stable against water immersing from Fig. 2(f). The high stability of SLS against water contacting can be attributed to the following two aspects. On the one hand, both CW and RW cannot be wet by water. On the other hand, the adhesion force between wax coating layer and LS fiber substrate is strong. From the SEM images [see Fig. 4(f) and (h)], it can be observed that the micro-protrusions of wax are firmly anchored on the fiber surface of LS. In addition, the SLS is also stable against oil immersing, including kerosene, hexadecane, diesel and gasoline. The samples remain superhydrophobicity after being soaked in these oils respectively for 5 days [see Fig. 6(a)]. Furthermore, when corrosive solutions with different pH values (pH = 1–6: HCl solution; pH = 7: 3.5 wt% of NaCl solution and pH = 8–14: NaOH solution) are used as measuring liquid, the surface of SLS still exhibits high CA (> 150°) and low SA (< 10°), which demonstrates that the SLS possesses excellent stability against the contact of corrosive solutions and can be used in a variety of water environments [Fig. 6(b)] [31]. 3.5. Separation of oil from the water surface Fig. 7 illustrates the oil/water separation properties using the superhydrophobic and superoleophilic LS. n-Hexadecane was used to mimicking the oil spill on water surface. The oil was dyed red for good observation. One can see that the oil is adsorbed by the SLS immediately when the SLS is just in contact with oil [Fig. 7(b) and (c)]. There is no oil left on water surface after removing the SLS from water [Fig. 7(e) and (f)], which demonstrates the excellent oil/water separation performance of our SLS. Besides, the SLS can also separate kerosene, diesel and gasoline from water surface with absorption capacitor larger than 10 g/g [Fig. 8(a)]. Additionally, after 10 cycles of separation, the absorption capacitor of SLS reduces < 10%, showing its excellent recyclability. Another important parameter for an oil/water separation material is oil separating efficiency [Fig. 8(b)]. The adsorbed oil in the pores of SLS can be readily collected using a simple centrifugal separation method for reuse. The separation efficiency is above 91% for all of the four kinds of oils even after 10 cycles of separation. Fig. 8(c) shows the relationship between the weight (Wt, g/g) of adsorbed oil and time (t). The SLS sample exhibits fast oil adsorption rate and achieves the saturation oil absorption < 15 s. The oil absorption
3.6. Recovery, recycle and biodegradation of the SLS The SLS becomes a solid waste after long oil/water separation cycles. Based on the 3R principle (namely, reduce, reuse and recycle) [40], we should make full use of the waste and reduce waste discharge. In the present case, the wax coating can be recovered by hot EA [see Fig. 1(d)]. The recycled wax solution can be used again. In addition, the skeleton structure of the LS after dewaxing still exists, so it can be modified again due to the simple preparation process. Thus, both wax and LS can be recycled. Fig. 9 shows the weight loss of the dewaxed LS 89
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Fig. 7. Separation of oil from water surface using the SLS sample. Hexadecane was used to simulate oil on water surface and dyed red to distinguish it from water. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
with increasing biodegradation time. One can see that the weight losses of both the pretreated and untreated LS increase with biodegradation time, which demonstrates their biodegradability. Natural LS is
composed of cellulose, hemicellulose and lignin, so they can be biodegraded without causing any pollution [34–36]. However, the cellulosic polymers of LS are surrounded by complex hemicellulose-lignin
Fig. 8. (a) Absorption capacitor and (b) separation efficiency for different kind of oils as a function of separation cycles; (c) absorption capacitor as a function of time, the inset shows the linear fit result using pseudo-first-order kinetic model; (d) absorption capacitor for different kind of oils as a function of temperature. 90
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Acknowledgements The authors acknowledge with pleasure the financial support of this work by the Natural Science Foundation of China (Grant Nos. 51662032 and 11864024) and the Natural Science Foundation of Jiangxi Province (No. 20161 BAB206112). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.05.002. References [1] J.C. Gu, P. Xiao, P. Chen, L. Zhang, H.L. Wang, L.W. Dai, L.P. Song, Y.J. Huang, J.W. Zhang, T. Chen, Functionalization of biodegradable PLA nonwoven fabric as superoleophilic and superhydrophobic material for efficient oil absorption and oil/ water separation, ACS Appl. Mater. Interfaces 9 (2017) 5968–5973. [2] Q.Y. Cheng, C.S. Guan, M. Wang, Y.D. Li, J.B. Zeng, Cellulose nanocrystal coated cotton fabric with superhydrophobicity for efficient oil/water separation, Carbohydr. Polym. 199 (2018) 390–396. [3] Y. Liu, B. Zhan, K.T. Zhang, C. Kay, T. Stegmaier, Z.W. Han, L.Q. Ren, On-demand oil/water separation of 3D Fe foam by controllable wettability, Chem. Eng. J. 331 (2018) 278–289. [4] B. Wang, W.X. Liang, Z.G. Guo, W.M. Liu, Biomimetic super-lyophobic and superlyophilic materials applied for oil/water separation: a new strategy beyond nature, Chem. Soc. Rev. 44 (2015) 336–361. [5] A. Matin, U. Baig, M.A. Gondal, S. Akhtar, S.M. Zubair, Facile fabrication of superhydrophobic/superoleophilic microporous membranes by spray-coating ytterbium oxide particles for efficient oil-water separation, J. Membrane Sci. 548 (2018) 390–397. [6] Y. Yang, H. Yi, C.Y. Wang, Oil absorbents based on melamine/lignin by a dip adsorbing method, ACS Sustain. Chem. Eng. 3 (2015) 3012–3018. [7] K. Hou, Y. Jin, J.H. Chen, X.F. Wen, S.P. Xu, J. Cheng, P.H. Pi, Fabrication of superhydrophobic melamine sponges by thiol-ene click chemistry for oil removal, Mater. Lett. 202 (2015) 99–102. [8] O. Oribayo, X.S. Feng, G.L. Rempel, Q.M. Pan, Synthesis of lignin-based polyurethane/graphene oxide foam and its application as an absorbent for oil spill clean-ups and recovery, Chem. Eng. J. 323 (2017) 191–202. [9] X. Zhao, L.X. Li, B.C. Li, J.P. Zhang, A.Q. Wang, Durable superhydrophobic/superoleophilic PDMS sponges and their applications in selective oil absorption and in plugging oil leakages, J. Mater. Chem. A 2 (2014) 18281–18287. [10] H.Y. Mi, X. Jing, H.X. Huang, X.F. Peng, L.S. Turng, Superhydrophobic graphene/ cellulose/silica aerogel with hierarchical structure as superabsorbers for high efficiency selective oil absorption and recovery, Ind. Eng. Chem. Res. 57 (2018) 1745–1755. [11] Q.Y. Cheng, M.C. Liu, Y.D. Li, J. Zhu, A.K. Du, J.B. Zeng, Biobased super-hydrophobic coating on cotton fabric fabricated by spraycoating for efficient oil/water separation, Polym. Test. 66 (2018) 41–47. [12] C.P. Su, H. Yang, H.P. Zhao, Y.L. Liu, R. Chen, Recyclable and biodegradable superhydrophobic and superoleophilic chitosan sponge for the effective removal of oily pollutants from water, Chem. Eng. J. 330 (2017) 423–432. [13] Xuejie Yue, Jiaxin Li, Tao Zhang, Fengxian Qiu, Dongya Yang, Mengwei Xue, In situ one-step fabrication of durable superhydrophobic-superoleophilic cellulose/LDH membrane with hierarchical structure for efficiency oil/water separation, Chem. Eng. J. 328 (2017) 117–123. [14] D.L. Zang, M. Zhang, F. Liu, C.Y. Wang, Superhydrophobic/superoleophilic corn straw fibers as effective oil sorbents for the recovery of spilled oil, J. Chem. Technol. Biotechnol. 91 (2016) 2449–2456. [15] Q.Y. Cheng, X.P. An, Y.D. Li, C.L. Huang, J.B. Zeng, Sustainable and biodegradable superhydrophobic coating from epoxidized soybean oil and ZnO nanoparticles on cellulosic substrates for efficient oil/water separation, ACS Sustain. Chem. Eng. 5 (2017) 11440–11450. [16] D. Aslanidoua, I. Karapanagiotis, C. Panayiotoua, Superhydrophobic, superoleophobic coatings for the protection of silk textiles, Prog. Org. Coat. 97 (2016) 44–52. [17] I. Karapanagiotis, D. Grosu, D. Aslanidou, K.E. Aifantis, Facile method to prepare superhydrophobic and water repellent cellulosic paper, J. Nanomater. 2015 (2015) 219013. [18] Y. Huang, D.K. Sarkar, X. Grant Chen, Superhydrophobic aluminum alloy surfaces prepared by chemical etching process and their corrosion resistance properties, Appl. Surf. Sci. 356 (2015) 1012–1024. [19] J. Yang, Z.Z. Zhang, X.H. Xu, X.T. Zhu, X.H. Men, X.Y. Zhou, Superhydrophilic–superoleophobic coatings, J. Mater. Chem. 22 (2012) 2834–2837. [20] L. Ejenstam, L. Ovaskainen, I. Rodriguez-Meizoso, L. Wågberg, J.S. Pan, A. Swerin, P.M. Claesson, The effect of superhydrophobic wetting state on corrosion protection-the AKD example, J. Colloid Interface Sci. 412 (2013) 56–64. [21] C.L. Zhou, Z.D. Chen, H. Yang, K. Hou, X.J. Zeng, Y.F. Zheng, J. Cheng, Natureinspired strategy toward superhydrophobic fabrics for versatile oil/water separation, ACS Appl. Mater. Interfaces 9 (2017) 9184–9194.
Fig. 9. Weight loss of the pretreated and untreated LS as a function of degradation time.
composite structure, which impedes the access of cellulase [41,42]. At present, pretreatment is an essential step for the biodegradation of lignocellulose biomass into biofuels (ethanol) [43]. Dilute acid pretreatment can destroy the structure of lignocellulose, hydrolyze hemicellulose, and make the biodegradable cellulose more accessible to microorganisms and enzymes. The weight loss of the LS caused by pretreatment was as high as 15.6% and increases sharply from 15.6% to 27.1% within a day. This can be explained by the fact that in the early stage of biodegradation, the cellulose content in the LS is relatively high, and it is easy to be degraded by cellulase because of pretreatment. The higher the weight loss of the LS due to cellulase hydrolysis, the lower the cellulose content left in the LS. As a consequence, the weight loss of the LS increases slowly with further increasing time. Hence, the weight loss rate of untreated LS is larger than that of the pretreated one in the late stage of hydrolysis (from 1 to 10 day), which can be interpreted by the higher cellulose content remained in the untreated LS. Cheng et al. reported the hydrolytic degradation of epoxidized soybean oil modified superhydrophobic cellulosic filter paper [15]. However, the degradation process is carried out under chemical conditions without the use of biological enzymes, so it is not a technically biodegradation. In addition, it takes up to 70 days to reach a degradation rate of about 15%. In the present case, the weight loss of untreated LS was > 15%, while that of the pretreated LS was > 30% after only 10 days of biodegradation. Hence, our oil/water separation material is more conducive to the recycling of materials.
4. Conclusions In summary, a renewable and biodegradable material using natural LS as substrate and natural waxes as coating was fabricated via a simple dip-coating method. All raw materials are natural ingredients derived from plants. The waxes modified LS exhibits both superoleophilicity and superhydrophobicity. The coating composing of two types of wax (CW and RW) possesses a particles/film-like composite micro-structure, which can guarantee the reasonable stability of the coating. The obtained SLS selectively separated different oils from water surface with oil/water separation capacity larger than 9.5 (g/g) (taking the lowest value for example) and separation efficiency higher than 91%, as well as good reusability (> 10 cycles). Importantly, the wax coating and LS substrate can be separated easily in hot ethyl acetate, thus achieving the reuse of both wax and LS. Particularly, the LS can be biodegradated. This work provides a new idea and method to develop oil/water separation materials by employing all-biomass as raw materials.
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