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water separation
Journal of Membrane Science 525 (2017) 1–8
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Facile fabrication of superhydrophilic membranes consisted of fibrous tunicate cellulose nanocrystals for highly efficient oil/water separation
MARK
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Qiaoyun Chenga, Dongdong Yea, Chunyu Changa, , Lina Zhanga,b,⁎⁎ a b
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Tunicate cellulose nanocrystals Filtration membrane Oil-in-water nanoemulsion Efficient separation Cholesteric liquid crystal
Faced on the threat of oil-contaminated wastewater to the environment and health of human body, we demonstrated, for the first time, a facile method for fabrication of novel nanoporous cellulose membranes derived from renewable marine resources (shell of tunicate). The fibrous tunicate cellulose nanocrystals (TCNCs) were prepared by acid hydrolysis, and exhibited high degree of crystallinity and distinct cholesteric liquid crystal behavior. Thus, the TCNC membranes were constructed by vacuum-assisted filtration of TCNCs suspensions, showing hierarchical structure, and superhydrophilic/underwater superoleophobic characters. The experimental results confirmed that the TCNC membranes were beneficial for highly efficient separation of oily water, which not only could separate various oil-in-water nanoemulsions, but also were applicable for oil-inwater microemulsions and water-in-oil emulsions. The thickness, pore size, water flux, and oil rejection of the TCNC membranes could be controlled by the dosage of TCNCs. Moreover, they exhibited high mechanical strength, excellent pH- and temperature-stability, and good cycling performance. On the basis of the combining of cholesteric liquid crystal structure and superhydrophilicity of fibrous tunicate cellulose nanocrystals, a new strategy to construct novel filter membranes for the highly effective oil/water separation was provided here.
1. Introduction With the rapid economic growth and social development, large amounts of oily wastewater are produced in many industrial processes and human daily life, which has become a leading global risk factor for environment and human health [1]. The separation of oil from industrial water, polluted oceanic waters, and oil-spill mixtures, especially those stabilized by surfactants, is a worldwide challenge [2,3]. Traditional technologies including oil skimmer, gravity separation, air flotation, flocculation, and coagulation, are useful to treat the phase separated oil/water contaminant, but not effective for emulsified oil/water mixtures because of the small droplet size of microemulsions or nanoemulsions ( < 20 μm) [4,5]. So efficient and broadly applicable strategies for separation of various emulsions with oil droplets of nanometer-scale are highly desired. Membrane technology is considered as the most useful method for oily water treatment because of its high separation efficiency and relatively simple operational process [6]. Recently, a myriad of filtration membranes with superwetting properties have been developed for oil/water separation, which includes ceramic filtration membranes, polymer-based filtration membranes, and nanomaterial dominated membranes [2]. Ceramic mem⁎
branes possess highly chemical, thermal and mechanical stabilities, but exhibit low separation efficiency and are easy to be fouled for oily water treatment [7]. Despite polymer-dominated membranes with superwetting surfaces can offer superior antifouling performance and high liquid flux, many extra approaches involving blending, combination, coating and surface modification have been obliged to overcome the intrinsic shortage of polymers [8–13]. Lately, constructing nanomaterial-based filtration membranes have drawn significant attention for oil/ water separation, because their high surface area and special surface properties, which are conducive to the liquid flux and rejection rate of membrane productions [14]. However, the advanced performance of nanomaterial-based filtration membranes also accompanied by the high cost of raw materials such as graphene oxide and carbon nanotubes, and complicated preparation process [15–18]. Therefore, a simple, low cost method to develop filtration membrane with high liquid flux, high rejection, and high cycling performance is critical need. The nanocellulose as a sustainable replacement for carbon nanotube in water treatment technologies has attracted much attentions [19]. It is worth noting that tunicate cellulose nanocrystals (TCNCs), isolated from the mantles of sessile sea creatures known as tunicates,
Corresponding author. Corresponding author at: College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China. E-mail addresses: [email protected] (C. Chang), [email protected] (L. Zhang).
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http://dx.doi.org/10.1016/j.memsci.2016.11.084 Received 29 September 2016; Received in revised form 28 November 2016; Accepted 29 November 2016 Available online 08 December 2016 0376-7388/ © 2016 Elsevier B.V. All rights reserved.
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freely deposited onto the nylon filter to form chiral nematic structure, which was finally dried to obtain TCNC membranes.
are comprised by more stable cellulose Iβ form [20], and exhibit higher aspect ratio and Young modulus, compared with the cellulose nanocrystals from other bioresources [21,22]. Moreover, different from carbon nanotubes, TCNCs might be superhydrophilic and underwater superoleophobic properties without any further modification, according to the wettability of cellulose based materials which has large amounts of hydroxyl groups on their surface [23,24]. Importantly, the fiber-like TCNCs can self-assemble to form chiral nematic membrane after removal of water from their suspensions [25]. The cholesteric or chiral nematic liquid crystal consist of stacked planes of rod-like unit aligned along a direction, with the orientation of each director rotated about the perpendicular axis from one plane to the next [26]. The unique structure inspired us to establish hierarchical porous material. It is not hard to imagine that the TCNC membranes directly constructed by the physical crosslinking of fibrous TCNCs would possess homogeneous small pore size and no pore wall, which was beneficial to the permeation of water. Herein, we prepared novel filtration membranes by a simple vacuum assisted filtration of TCNCs suspensions. The thickness and pore size of TCNC membranes could be controlled by the amount and concentration of TCNCs suspensions. The hydrophilic and porous TCNC membranes are beneficial for passing through of water, whereas their underwater superoleophobicity can prevent oil-wetting and consequent fouling. The structure, hydrophilicity, and underwater oleophobicity of the TCNC membranes were characterized. Moreover, the mechanical strength, stability, and cycling performance of membranes as well as separation performances, including water flux and oil rejection, were studied and demonstrated via oil/water emulsions separation experiments. We proposed a facile strategy for fabrication of TCNC membranes with highly efficient separation of various oil-inwater nanoemulsions and extended to the separation of water-in-oil emulsions, and a seafood waste (shell of tunicate) was used to fabricate functional materials, which may be extended to the high-value utilization of other bioresource.
2.3. Oil/water emulsion separation All the emulsions used in this work were stabilized by surfactants. Four kinds of oil-in-water nanoemulsions were prepared by mixing 1 mL oil with 99 mL water (containing 0.1 wt% Tween 80 for chloroform, toluene and 0.2 wt% Tween 80 for n-hexane, isooctane), the mixtures were treated by ultrasonic processor (SCIENTZ, 40 kHz) for 2 h. Subsequently, the emulsions were diluted 2, 10, 15, and 20 times for chloroform, toluene, n-hexane, and isooctane to form nanometer size oil droplets. For the preparation of oil-in-water and water-in-oil microemulsions, isooctane containing 0.2 wt% Tween 80 and water containing 0.2 wt% Span 80 were mixed together with oil/water volume ratio of 1/9 (oil-in-water), 3/7 (oil-in-water), 7/3 (water-inoil), and 9/1 (water-in-oil), respectively. Afterwards, the mixtures were stirred at 1000 rpm for 12 h to obtained stable emulsions. The size of droplets in the emulsions were characterized by optical microscope. The oil/water emulsion separation experiments were conducted on a vacuum filter apparatus equipped with the as-prepared TCNC membranes, where the support was a nylon filter with an effective diameter of 42 mm (Fig. 5b). The emulsion was poured onto the membrane and water was permeated under a pressure difference of 0.5 bar. After separation, the filtrate was collected for testing the water flux and oil rejection. The water flux J (L m−2 h−1 bar−1) was calculated from the volume of the filtrate within 5 min according to the equation: (1)
J = V / AtΔP 2
Where V (L) is the volume of the permeated water, A (m ) is the effective filtration membrane area, t (h) is the permeate time, and ΔP (bar) is the suction pressure across the membrane. The oil rejection R (%) was calculated according to the equation:
R = (1 − Cf / C0 ) × 100% −1
(2) −1
Where Cf (mg mL ) and C0 (mg mL ) are the concentration of oil in filtrate and the original emulsion, respectively. Cf was determined by using gas chromatography. The standard curves of oils are shown in Fig. S4.
2. Experimental 2.1. Materials Tunicate (Halocynthia roretzi Drasche) was purchased from Weihai Evergreen Marine science and technology Co. Ltd (Shandong, China), and used as raw material. Nylon filter paper with a cut-off of 0.22 µm and 50 mm in diameter was obtained from Xinya purification Co. Ltd (Shanghai, China) as the support for tunicate cellulose nanocrystals (TCNCs) membranes. All other reagents were analyticalgrade purchased from Shanghai Chemical Agents Co. Ltd (Shanghai, China), and used without further purification.
2.4. Characterization TEM image was measured with transmission electron microscopy (TEM) using a JEM-2010 microscope (JEOL, Japan). The samples were prepared by evaporating a drop of TCNCs dispersion (0.05 wt%) on a carbon-coated copper grid. Scanning electron micrograph (SEM) measurements were carried out on a HITACHI 5-4800 microscope (Tokyo, Japan) at an accelerating voltage of 5 kV. The membranes were frozen in liquid nitrogen, fractured immediately, and then freeze dried. Both the surface and cross-section of samples were sputtered with gold, and then observed and photographed with SEM. Atomic force microscopy (AFM) was performed on an Asylum Research Cypher system (Oxford, UK) to characterize the thickness of membranes. To evaluate the effective pore size of TCNC membranes, standard PEO solutions with different molecular weight were pass through the TCNC membranes. The concentrations and hydrodynamic radius of PEO were determined by the combination system of size exclusion chromatography (SEC), multi-angle static light scattering (DAWN HELEOS-II), refractometer (Optilab T-rEX), and viscometer (ViscoStar II) (Wyatt Technology Co., US). The pore size was thought to be the hydrodynamic diameter of the molecular weight of a molecule which is rejected by 90%. The contact angle was measured by DSA100 (Krüss, Germany). Particle size of nanoemulsions and microemulsions were measured by dynamic light scattering (Malvern, UK), EX20 optical microscopy (Sunny, China), respectively. The concentration of oil was analyzed by using gas chromatography (Agilent 6890, US). Mechanical
2.2. Fabrication of tunicate cellulose nanocrystal membranes To isolate tunicate cellulose from Halocynthia roretzi Drasche, alkaline treatment and bleaching process of tunics were conducted to remove proteins, fats and pigments. From the isolation of tunicate cellulose (Fig. S1), the white fibrous product was obtained with a yield of 20%. Subsequently, 10 g tunicate cellulose was dispersed in 65 wt% sulfuric acid (500 mL) and hydrolyzed for 2 h at 70 °C under stirring to obtain tunicate cellulose nanocrystals (TCNCs). The TCNCs solutions were diluted 10-folds to terminate reactions, and the suspensions were washed with distilled water by repeated centrifugation under 1×104 rpm until turbid. The TCNCs suspensions were dialyzed against distilled water until constant pH value (Fig. S2). The concentration of suspensions was calculated by the weight ratio of TCNCs suspensions and solid after freeze-drying. The TCNC membranes were prepared by filtrating the TCNCs suspensions onto nylon filter membrane with a pore size of 0.22 µm by vacuum filtration (Fig. S3). During the filtration, the TCNCs were 2
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Fig. 1. Appearance and photograph under crossed polarizers of the TCNC membrane at dry state (a), POM image of the fibrous TCNCs suspension (b), SEM image of the TCNC membrane with TCNCs usage of 0.73 g m−2 (c), TEM image of fibrous TCNCs (d), and a scheme to describe the cholesteric structure of TCNCs solution and the porous TCNC membranes fabricated with fibrous TCNCs by vacuum-assisted filtration (e).
Fig. 2. (a) Statistical curve of the thickness and water flux of filter membranes with different usage of TCNCs. (b) The water flux as a function of 1/thickness. (c) Effective pore size of filter membranes for different usage of TCNCs determined by the rejection ( > 90%) of polyethylene oxide (PEO) with different molecular weight (100 kDa, 300 kDa, and 600 kDa). (d) Pore size distribution of TCNC membranes determined by N2 gas adsorption.
filtration of TCNCs suspension is shown in Fig. 1a. The transparency of TCNC membrane proved the nanoscale structure of TCNCs. An anisotropic phase appeared when the concentration of TCNCs reached a critical value (Fig. 1b), TCNCs trended to shape chiral nematic architecture through self-assembling [26]. The chiral nematic structure of TCNC membrane could be confirmed by two crossed polarizers (inset, Fig. 1a). The TEM image of TCNCs in suspension (Fig. 1d) displayed that the TCNCs adopted fiber-like morphologies with 10– 30 nm in width (W), 400–3000 nm in length (L), and 72 in average aspect ratio (L/W). Interestingly, in our findings, the fibrous cellulose
properties of TCNC membranes were tested by using an Electromechanical Universal Testing Machine CMT6503 (Xinsansi, China). The measurements were conducted at 25 °C with an elongation rate of 2 mm min−1. 3. Results and discussion 3.1. The structure and morphology of TCNC membranes The photograph of TCNC membrane fabricated by vacuum-assisted 3
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Fig. 3. Photographs of a water droplet on the TCNC membrane in air (a) and under oil (d), and an oil (isooctane) droplet on the TCNC membrane in air (b) and underwater (c). Dynamic approach-compress-detach oil-adhesion (chloroform) test of TCNC membrane (e). The influence of surface architecture on the wettability of membranes: oil (chloroform) droplet underwater (f) and water droplet under oil (chloroform) (g). The dosage of TCNCs for a-e was 0.73 g m−2, and that for f and g were 0, 0.0011, 0.11, and 0.73 g m−2, respectively (from left to right).
nearly linear increase of water flux with the reciprocal of TCNC membrane thickness appeared. Finally, the TCNC membrane with a thickness of 600 nm, effective pore size of 48–70 nm, and effective filtration area of 14 cm−2 obtained by filtrating 2 mL TCNCs suspension (0.05 wt%), and was selected for the evaluation of oil/water separation. For the separation of oil/water emulsions, especially oil-in-water nanoemulsion, the effective pore size of membrane was required to be small enough to remove oil droplets [17]. To accurately determine the effective pore size of membranes, polyethylene oxide (PEO) with molecular weights of 100 kDa, 300 kDa, and 600 kDa were employed as model compounds, whose hydrodynamic diameter were 24 nm, 48 nm, and 70 nm, respectively, measured by the combination system of size exclusion chromatography, multi-angle static light scattering, refractometer, and viscometer. The effective pore sizes of the membranes were determined by the points located in the domains that the rejection rate of PEO was greater than 90%, and the results are presented in Fig. 2c. The pore size of membrane with TCNCs dosage of 21.7 g m−2 was in the range of 24–48 nm. The pore size distribution of TCNC membranes determined by N2 gas adsorption is shown in Fig. 2d. The average pore size of 4 nm was obtained, which was much lower than that estimated by PEO. This result could be attributed to the shrinking of pore during the drying process and different mechanism of two measurements.
nanocrystals obtained from tunicate had larger diameter and length than those from other bioresources such as wood and cotton linter [21], which were easily to be cut off and propitious to form membrane. SEM images (Fig. 1c) further proved that the TCNC membranes were weaved with fibrous TCNCs. The unique assemble behavior of TCNCs made the rough surface and homogeneous porous morphology of the membrane. Based on these results, a scheme to describe the cholesteric structure of TCNCs solution and the porous TCNC membranes fabricated with fibrous TCNCs is provided in Fig. 1e. The fibrous TCNCs with cholesteric liquid crystal stacked planes aligned along a direction, with the orientation of each director rotated about the perpendicular axis to from one plane to the next. Clearly, the resultant membrane was consisted of the fibrous TCNCs layers and no pore wall existed, leading to the high water permeability. The strategy for fabrication of TCNC membranes in this work was novel, exceedingly simple, low cost, eco-friendly, and easily scale-up. 3.2. Effects of TCNCs usage on water flux and rejection To design and fabricate membranes, the precise control of the dosage of TCNCs was crucial, which were directly related to their thickness, water flux, and effective pore size. Fig. 2a shows the thickness and water flux of the TCNC membranes as a function of the usage of TCNCs. By adjusting the amount of the TCNCs suspension to be filtered, the thickness and water flux of the membrane could be tuned. The thickness of membrane was determined by the SEM and AFM images (Fig. S5), and the water flux of membrane was calculated by Eq. (1). When the usage of TCNCs was 32.6 g m−2, the thickness and water flux of membrane were 27 ± 2 µm, and 66 ± 5 L m−2 h−1 bar−1, respectively, whereas the thickness of membrane decreased to 300 ± 20 nm and water flux of membrane increased to 5400 ± 177 L m−2 h−1 bar−1 with TCNCs usage of 0.36 g m−2. The relationship between water flux and 1/thickness is shown in Fig. 2b. Observably, a
3.3. Wettability of TCNC membranes The wettability of membrane is fatal for its application in oil/water emulsion separation, which greatly depends on the chemical composition and architecture of the surface of membrane. The surface chemical composition determined the intermolecular forces at the interfaces between the solid, liquid/air, and the environmental phase, while the surface roughness enhanced these types of intermolecular forces [27]. 4
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Fig. 4. Emulsions separated by TCNC membranes: (a) photographs and microscope images of isooctane-in-water microemulsion before and after filtration, (b) photographs and microscope images of water-in-isooctane emulsion, and (c) photographs and size distribution of isooctane-in-water nanoemulsion.
gradually removed, it could overcome the adhesion force to detach from the surface of TCNC membrane and no deformation was observed, indicating very low oil-adhesiveness of TCNC membrane. The similar behaviors were also observed for other oil droplets underwater (Fig. S6). Generally, vertical tensile stress was generated by the adhesion force between the oil droplet and membrane surface when oil droplet was detached, which promoted the deformation of oil droplet [15]. Therefore, the dynamic underwater superoleophobicity of the TCNC membranes could be explained by the existence of the nanofibers and nanopore of TCNC membranes containing hydrophilic hydroxyl groups, preventing the contact of oil droplet. The dosage of TCNCs directly affected the surface morphology of membrane, for example, the surface architecture of 0.11 g m−2 TCNC membrane (Fig. S7) was totally different from the TCNC membrane (Fig. 1c) with TCNCs usage of 0.73 g m−2. To estimate the influence of surface architecture on the wettability, membranes with different surface structure were investigated. As shown in Fig. 3f, the contact angles of oil droplet (chloroform) on nylon membrane underwater was 88.7°, which reached 106.8° after coated with TCNCs and gradually increased to 175.9° for TCNC membrane with a thickness of 600 nm (rightmost). These results revealed that hydroxyl groups on the surface of TCNCs made the membrane oleophobic, and the formation of cholesteric architecture in the membrane produced superoleophobicity [23,24]. On the other hand, the contact angle of water on the membrane under oil decreased from 132.7 to 31.6° with the increase of thickness of TCNC membranes, indicating the variation of wettability of membranes (from hydrophobicity to hydrophilicity), which will give them the ability to separate water-in-oil emulsion. We should emphasized that the wettability of TCNCs membrane was independent
To evaluate the wetting behavior of TCNC membranes, both apparent water contact angles in air (or oil) and apparent oil contact angles in air (or water) were obtained, as shown in Fig. 3. In the liquid/air/solid three-phase system, both water (Fig. 3a) and oil (isooctane) (Fig. 3b) contact angle on the TCNC membrane in air was 0°, because plentiful hydroxyl groups on the surface of TCNC membrane, and its high surface energy and porous morphology allowed the liquid droplets to permeate through the membrane quickly, indicating its superamphiphilicity. In contrast, the TCNC membrane could not be wetted by oil (isooctane) underwater with an oil contact angle of 155° (Fig. 3c). Analogously, the contact angles of chloroform, n-hexane, and toluene, were 175.9, 157, and 171°, respectively (Fig. S6), revealing the underwater superoleophobicity of TCNC membrane. The superhydrophilicity in air and superoleophobicity underwater of TCNC membrane were similar with the properties of some organisms in nature, such as fish scales, which was composed of hydrophilic chemical component with hierarchical structure [28]. These results could be explained that the formation of unique hierarchical nanoporous structures as a result of the self-assembly of fibrous TCNCs and the presence of a large number of hydroxyl groups on the membrane. In addition, the contact angle of water under oil (isooctane) was about 40° (similar results for water under other oil are shown in Fig. S6), indicating hydrophilicity of TCNC membrane (Fig. 3d), beneficial for the separation of water-in-oil emulsions. To further characterize the dynamic wetting behavior of oil on the surface of TCNC membrane, the underwater oil-adhesion test was conducted by an approach-compress-detach process (Fig. 3e). After contacting with membrane, oil (isooctane) droplet was compressed to deform from a spherical to ellipsoidal shape. When the oil droplet was 5
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Fig. 5. (a) Water flux and oil rejection of TCNC membranes in separation of various nanoemulsions. (b) Photographs of filtration system and TCNC membranes before and after separating chloroform-in-water nanoemulsion. Chloroform was dyed by oil red. (c) Cycling performance of TCNC membranes. (d) Stress-strain curve for the dried TCNC membranes with thickness of 10 µm.
TCNC membranes not only possessed nanoporous morphology and special wettability, but also showed some advantages such as facile procedure and low cost, which may benefit their large-scale fabrication and practical application in effective separation of surfactant-stabilized nanoemulsions. As shown in Fig. 4c, the isooctane-in-water nanoemulsion was transparent and Tyndall effect could be obviously observed with laser illumination, indicating the presence of oil droplets with nano-size in the emulsion. The size of isooctane droplets were determined by dynamic light scattering (DLS) to be around 100 nm. After separation, no Tyndall effect could be observed in the collected filtrate, indicating that no isooctane droplets were left. Furthermore, the DLS curve also confirmed the disappearance of isooctane droplets, but the sharp peaks of residual surfactants appeared around several nanometers in the filtrate. It was indicated that the TCNC membranes could separate various oil/water emulsion effectively. It was noted that the nylon support membrane could not separate oil/water emulsions, due to both water and oil permeate it under pressure (Fig. S8). Similar separation results were also obtained in the cases of the chloroform-in-water, n-hexane-in-water, and toluene-in-water nanoemulsion (Fig. S9). Fig. 5a presents the separation performance of the TCNC membranes for various oil-in-water nanoemulsions. The TCNC membranes allowed all types of nanoemulsions (including toluene, chloroform, isooctane, and n-hexane) to separate completely. The oil rejection (R) of TCNC membranes for toluene, isooctane, chloroform, and n-hexane were 99.59 ± 0.37%, 99.99%, 93.33 ± 0.01%, and 99.99%, respectively, indicating high capacity for the TCNC membranes to separate oil-in-water nanoemulsions. The lower R for chloroform could be attributed to the smaller size (27 nm, Fig. S10) and higher density of its droplet in the nanoemulsions, leading to trace amount of chloroform was infiltrated into filtrate. Furthermore, the R values of the TCNC membranes for all microemulsions (including water-in-oil and oil-in-water water) were higher than 99.99% (Fig. S11). These results demonstrated that the TCNC membranes can effectively separate oil/water emulsions with different size of oil droplets. On the other hand, the water flux is also a key parameter for
of membrane thickness after the nylon membrane was fully covered, and cholesteric architecture completely formed (dosage ≥0.73 g m−2). 3.4. Separation performances of TCNC membranes for oil/water emulsions Usually, membranes with superhydrophilic and underwater superoleophobic properties are propitious to alleviate their fouling phenomenon and maintain high separation efficiency in practical separation of oil/water emulsions, because water with a higher density than oil trended to form a layer on the surface of membrane which avoided directly contact between oil droplets and membrane surface [16]. Considering the aforementioned wettability of TCNC membranes, three types of emulsions including oil-in-water microemulsions, water-in-oil emulsions, and oil-in-water nanoemulsions were prepared to test the universality of TCNC membranes in oil/water emulsion separation. Fig. 4a gives the photographs of surfactant-stabilized isooctane-inwater emulsion (o/w volume ratio=3:7) and the collected filtrate after separation. The as-prepared isooctane-in-water emulsion was milky white due to the presence of numerous oil droplets with several micrometers which flooded the entire field of image of optical microscope. After separated by the TCNC membranes through a vacuum filtration system, the collected filtrate was clear and transparent, where no droplets could be observed by optical microscope, indicating that oil droplets were effectively cut off and the oily water was effectively separated by the TCNC membranes. For water-in-oil-separation, water-in-isooctane emulsion (o/w volume ratio=7:3) was taken as example (Fig. 4b). The turbid emulsion was turned into transparent solution of water after filtration, indicating the water droplets have been successfully filtrated by the TCNC membranes from emulsion. Actually, the emergence of nanoemulsions made the separation encounter more challenges and called for novel membranes with nanoporous architecture. Impressive works including some nanoporous membranes with special wettability based on polyvinylidene fluoride, and superwetting polymer-decorated ultrathin films based on carbon nanotube have been explored [17,29,30]. Fortunately, our 6
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Table 1 Comparison of various filtration membranes for oil/water nanoemulsion separation. Materials
Wettability
Pore size (nm)
Flux (L m−2 h−1)
Rejection
Sources
Ceramic (αAl2O3-ZrO2) Polysulfone/PEG Dopamine/PEI
— Hydrophilic Superhydrophilic/superoleophobic
40–80 (1–5 bar) 120 LMH (1 bar) 40–100 (gravity)
> 90% > 95% > 98%
[31] [32] [33]
Dopamine/PEI+SiO2
Superhydrophilic/underwater superoleophobic Superhydrophilic/underwater superoleophobic Hydrophilic/underwater oleophobic
50 36–116 < 200 (supporting membrane) < 200 (supporting membrane) 10
1200 (0.5 bar)
> 99%
[34]
1500–3000 (0.5 bar)
> 94%
[17]
< 220 (supporting membrane) < 1000
267 (0.5 bar) 1867 (0.5 bar) 500 (gravity)
99.9%
[15]
99.4%
[35]
< 12 48–70
1200–1300 (0.8 bar) 1036–1734 (0.5 bar)
96.5–98% ≥99.99% (except CHCl3)
[24] This work
SWCNTs/dopamine/PEI Graphene oxide(GO) GO/Palygorskite Cu(OH)2 nanowire -coated mesh Cellulose nanosheet TCNC membranes
Superhydrophilic/underwater superoleophobic Hydrophilic/superoleophobic Superhydrophilic/underwater superoleophobic
evaluate the efficiency of oil/water separation. The water flux (J) of TCNC membranes were 1708.7 ± 92.4, 1734.9 ± 22.5, 1036.1 ± 42.5, and 1549.9 ± 109.2 L m−2 h−1 bar−1 for the emulsion of toluene, isooctane, chloroform, and n-hexane, respectively. These results indicated that the TCNC membranes can rapidly separate these nanoemulsions with very high oil rejection. In the view of above results, the TCNC membranes with small pore size still exhibited high water flux, as a result of their hierarchical architecture and superhydrophilic properties. In our finding, the existence of the fibrous TCNCs stacked planes in the TCNC membranes played an important role in the enhancement of the water flux. However, in the separation of microemulsions, the J values of TCNC membranes were low as shown in Fig. S11, which could be attributed to the high content of oil in the emulsions which were easy to form oil layer on the surface of membrane. A comparison of various filtration membranes prepared from different raw materials and methods for nanoemulsion separation is summarized in Table 1. The wettability of both our TCNC membranes and most of other reported membranes were (super) hydrophilic/ (super) oleophobic or (super) hydrophilic/ underwater (super) oleophobic, where water automatically infiltrated through the pores of the membranes but oil was completely repelled. The pore size of the TCNC membranes was in the range of 48–70 nm, which was small enough to reject oil. As we expected, TCNC membranes exhibited high oil rejection (99.99%) and high water flux (1036– 1734 L−1 m−2 h−1 bar−1), which were comparable with those of the membranes listed in Table 1. These results were closely associated with nanopore architecture without the pore wall of TCNC membranes, in which the former prevented the transition of oil, and the latter facilitated the permeation of water.
placed into aqueous solution with temperature in range of 10-80 °C, indicating that TCNC membranes were stable in the aqueous solution with different temperature. In practical oil/water separation, the stability of membranes under complex environments, such as mechanical scratch, acid and alkali conditions, is also important [37]. Owing to the membranes usually need to tolerate large hydrostatic pressure or even violent mechanical vibrations by pumps in the application situation for oil/water emulsion separation, so the mechanical strength of the TCNC membranes was vital. As shown in Fig. 5d, the tensile strengths of TCNC membranes with thickness of 10 µm was about 140 MPa, indicating excellent mechanical properties. Thus, the further enforcement of membrane was needless in the fabrication process, differing from other reported cellulose porous materials [23]. In addition, the oil contact angles for isooctane were 155, 154, and 154°, in NaCl (1 M), HCl (1 M), and NaOH (1 M), respectively (Fig. S13), revealing that the TCNC membranes could tolerate acidic, salty, and alkaline environments. Therefore, TCNC membranes with excellent durability and stability could applied for oil/water separation under harsh conditions. 4. Conclusion Superhydrophilic and underwater superoleophobic TCNC membranes were fabricated successfully with fibrous tunicate cellulose nanocrystals via a facile vacuum-assisted method. The fibrous TCNCs solution displaying cholesteric architecture could be stacked to form the planes aligned along a direction with the orientation of each director rotated about the perpendicular axis, leading to the formation of the hierarchical porous TCNC membranes without pore wall. The TCNC membranes could separate both oil-in-water and water-in-oil emulsions. For isooctane-in-water nanoemulsions, TCNC membranes exhibited excellent separation performance with high separation efficiency (≈100%) and water flux ( > 1700 L m−2 h−1 bar−1), as a result of their unique wettability (superhydrophilicity and underwater superoleophobicity). In our findings, the TCNC membranes weaved with fibrous TCNCs exhibited the high flux of water, excellent mechanical strength, cycling performance, and temperature- and pH-resistance. This work provide a simple, low cost, and environmentally friendly strategy for construction of highly effective oil/water separation device.
3.5. Durability and stability of TCNC membranes Durability and stability are common and tough issues for filtration membranes during the process of oil-water separation. The durability of TCNC membranes was monitored by detecting the change of water flux and oil rejection during several cycles of oil/water separation. As shown in Fig. 5c, there were no obvious changes in the water flux and oil rejection of TCNC membranes in the ten cycles, indicating their good reusability and antifouling properties. The results were consistent with Jung and Bhushan prediction that oleophobic and most hydrophilic surfaces can be made oleophobic with the properties of selfcleaning and antifouling at the solid-water-oil interface by the proposed model [36]. The as-prepared TCNC membranes also exhibited stable underwater superoleophobicity towards various solutions with different temperature and pH values. As shown in Fig. S12, oil contact angles for isooctane were all larger than 155°, when membranes were
Acknowledgment This work was supported by the Major Program of Natural Science Foundation of China (21334005), the Major International (Regional) Joint Research Project (21620102004), the National Natural Science Foundation of China (21304021), Hubei Province Science Foundation for Youths (2015CFB499), Jiangsu Province Science Foundation for 7
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Youths (BK20150382), and Pearl River S & T Nova Program of Guangzhou (201506010101). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2016.11.084. References [1] T. Birkhead, Stormy outlook for long-term ecology studies, Nature 514 (2014) (405-405). [2] Y. Zhu, D. Wang, L. Jiang, J. Jin, Recent progress in developing advanced membranes for emulsified oil/water separation, NPG Asia Mater. 6 (2014) e101. [3] Q. Ma, H. Cheng, A.G. Fane, R. Wang, H. Zhang, Recent development of advanced materials with special wettability for selective oil/water separation, Small (2016). [4] S. Yang, Y. Si, Q. Fu, F. Hong, J. Yu, S.S. Al-Deyab, M. El-Newehy, B. Ding, Superwetting hierarchical porous silica nanofibrous membranes for oil/water microemulsion separation, Nanoscale 6 (2014) 12445–12449. [5] Q. Chang, J.E. Zhou, Y. Wang, J. Liang, X. Zhang, S. Cerneaux, X. Wang, Z. Zhu, Y. Dong, Application of ceramic microfiltration membrane modified by nano-TiO2 coating in separation of a stable oil-in-water emulsion, J. Membr. Sci. 456 (2014) 128–133. [6] M. Matos, G. Gutiérrez, A. Lobo, J. Coca, C. Pazos, J.M. Benito, Surfactant effect on the ultrafiltration of oil-in-water emulsions using ceramic membranes, J. Membr. Sci. 520 (2016) 749–759. [7] P. Mittal, S. Jana, K. Mohanty, Synthesis of low-cost hydrophilic ceramic– polymeric composite membrane for treatment of oily wastewater, Desalination 282 (2011) 54–62. [8] R. Ou, J. Wei, L. Jiang, G.P. Simon, H. Wang, Robust thermoresponsive polymer composite membrane with switchable superhydrophilicity and superhydrophobicity for efficient oil–water separation, Environ. Sci. Technol. 50 (2016) 906–914. [9] Z. Wang, X. Jiang, X. Cheng, C.H. Lau, L. Shao, Mussel-inspired hybrid coatings that transform membrane hydrophobicity into high hydrophilicity and underwater superoleophobicity for oil-in-water emulsion separation, ACS Appl. Mater. Interfaces 7 (2015) 9534–9545. [10] J.P. Chaudhary, N. Vadodariya, S.K. Nataraj, R. Meena, Chitosan-based aerogel membrane for robust oil-in-water emulsion separation, ACS Appl. Mater. Interfaces 7 (2015) 24957–24962. [11] A.K. Kota, Y. Li, J.M. Mabry, A. Tuteja, Hierarchically structured superoleophobic surfaces with ultralow contact angle hysteresis, Adv. Mater. 24 (2012) 5838–5843. [12] J.-h. Wang, Y.-l. Wu, Y.-h. Zhang, Y.-y. Xu, Fabrication and performance of a low operating pressure nanofiltration poly (vinyl chloride) hollow fiber membrane, Chin. J. Polym. Sci. 32 (2014) 377–384. [13] H.J. Li, Y.M. Cao, J.J. Qin, X.M. Jie, T.H. Wang, J.H. Liu, Q. Yuan, Development and characterization of anti-fouling cellulose hollow fiber UF membranes for oil– water separation, J. Membr. Sci. 279 (2006) 328–335. [14] K. Jayaramulu, K.K.R. Datta, C. Rösler, M. Petr, M. Otyepka, R. Zboril, R.A. Fischer, Biomimetic superhydrophobic/superoleophilic highly fluorinated graphene oxide and ZIF-8 composites for oil-water separation, Angew. Chem. Int. Ed. 55 (2016) 1178–1182. [15] X. Zhao, Y. Su, Y. Liu, Y. Li, Z. Jiang, Free-standing graphene oxide-palygorskite nanohybrid membrane for oil/water separation, ACS Appl. Mater. Interfaces 8 (2016) 8247–8256. [16] T. Huang, L. Zhang, H. Chen, C. Gao, Sol–gel fabrication of a non-laminated graphene oxide membrane for oil/water separation, J. Mater. Chem. A 3 (2015) 19517–19524. [17] S.J. Gao, Y.Z. Zhu, F. Zhang, J. Jin, Superwetting polymer-decorated SWCNT composite ultrathin films for ultrafast separation of oil-in-water nanoemulsions, J.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Facile fabrication of superhydrophilic membranes consisted of fibrous tunicate cellulose nanocrystals for highly efficient oil/water separation
MARK
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Qiaoyun Chenga, Dongdong Yea, Chunyu Changa, , Lina Zhanga,b,⁎⁎ a b
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Tunicate cellulose nanocrystals Filtration membrane Oil-in-water nanoemulsion Efficient separation Cholesteric liquid crystal
Faced on the threat of oil-contaminated wastewater to the environment and health of human body, we demonstrated, for the first time, a facile method for fabrication of novel nanoporous cellulose membranes derived from renewable marine resources (shell of tunicate). The fibrous tunicate cellulose nanocrystals (TCNCs) were prepared by acid hydrolysis, and exhibited high degree of crystallinity and distinct cholesteric liquid crystal behavior. Thus, the TCNC membranes were constructed by vacuum-assisted filtration of TCNCs suspensions, showing hierarchical structure, and superhydrophilic/underwater superoleophobic characters. The experimental results confirmed that the TCNC membranes were beneficial for highly efficient separation of oily water, which not only could separate various oil-in-water nanoemulsions, but also were applicable for oil-inwater microemulsions and water-in-oil emulsions. The thickness, pore size, water flux, and oil rejection of the TCNC membranes could be controlled by the dosage of TCNCs. Moreover, they exhibited high mechanical strength, excellent pH- and temperature-stability, and good cycling performance. On the basis of the combining of cholesteric liquid crystal structure and superhydrophilicity of fibrous tunicate cellulose nanocrystals, a new strategy to construct novel filter membranes for the highly effective oil/water separation was provided here.
1. Introduction With the rapid economic growth and social development, large amounts of oily wastewater are produced in many industrial processes and human daily life, which has become a leading global risk factor for environment and human health [1]. The separation of oil from industrial water, polluted oceanic waters, and oil-spill mixtures, especially those stabilized by surfactants, is a worldwide challenge [2,3]. Traditional technologies including oil skimmer, gravity separation, air flotation, flocculation, and coagulation, are useful to treat the phase separated oil/water contaminant, but not effective for emulsified oil/water mixtures because of the small droplet size of microemulsions or nanoemulsions ( < 20 μm) [4,5]. So efficient and broadly applicable strategies for separation of various emulsions with oil droplets of nanometer-scale are highly desired. Membrane technology is considered as the most useful method for oily water treatment because of its high separation efficiency and relatively simple operational process [6]. Recently, a myriad of filtration membranes with superwetting properties have been developed for oil/water separation, which includes ceramic filtration membranes, polymer-based filtration membranes, and nanomaterial dominated membranes [2]. Ceramic mem⁎
branes possess highly chemical, thermal and mechanical stabilities, but exhibit low separation efficiency and are easy to be fouled for oily water treatment [7]. Despite polymer-dominated membranes with superwetting surfaces can offer superior antifouling performance and high liquid flux, many extra approaches involving blending, combination, coating and surface modification have been obliged to overcome the intrinsic shortage of polymers [8–13]. Lately, constructing nanomaterial-based filtration membranes have drawn significant attention for oil/ water separation, because their high surface area and special surface properties, which are conducive to the liquid flux and rejection rate of membrane productions [14]. However, the advanced performance of nanomaterial-based filtration membranes also accompanied by the high cost of raw materials such as graphene oxide and carbon nanotubes, and complicated preparation process [15–18]. Therefore, a simple, low cost method to develop filtration membrane with high liquid flux, high rejection, and high cycling performance is critical need. The nanocellulose as a sustainable replacement for carbon nanotube in water treatment technologies has attracted much attentions [19]. It is worth noting that tunicate cellulose nanocrystals (TCNCs), isolated from the mantles of sessile sea creatures known as tunicates,
Corresponding author. Corresponding author at: College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China. E-mail addresses: [email protected] (C. Chang), [email protected] (L. Zhang).
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http://dx.doi.org/10.1016/j.memsci.2016.11.084 Received 29 September 2016; Received in revised form 28 November 2016; Accepted 29 November 2016 Available online 08 December 2016 0376-7388/ © 2016 Elsevier B.V. All rights reserved.
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freely deposited onto the nylon filter to form chiral nematic structure, which was finally dried to obtain TCNC membranes.
are comprised by more stable cellulose Iβ form [20], and exhibit higher aspect ratio and Young modulus, compared with the cellulose nanocrystals from other bioresources [21,22]. Moreover, different from carbon nanotubes, TCNCs might be superhydrophilic and underwater superoleophobic properties without any further modification, according to the wettability of cellulose based materials which has large amounts of hydroxyl groups on their surface [23,24]. Importantly, the fiber-like TCNCs can self-assemble to form chiral nematic membrane after removal of water from their suspensions [25]. The cholesteric or chiral nematic liquid crystal consist of stacked planes of rod-like unit aligned along a direction, with the orientation of each director rotated about the perpendicular axis from one plane to the next [26]. The unique structure inspired us to establish hierarchical porous material. It is not hard to imagine that the TCNC membranes directly constructed by the physical crosslinking of fibrous TCNCs would possess homogeneous small pore size and no pore wall, which was beneficial to the permeation of water. Herein, we prepared novel filtration membranes by a simple vacuum assisted filtration of TCNCs suspensions. The thickness and pore size of TCNC membranes could be controlled by the amount and concentration of TCNCs suspensions. The hydrophilic and porous TCNC membranes are beneficial for passing through of water, whereas their underwater superoleophobicity can prevent oil-wetting and consequent fouling. The structure, hydrophilicity, and underwater oleophobicity of the TCNC membranes were characterized. Moreover, the mechanical strength, stability, and cycling performance of membranes as well as separation performances, including water flux and oil rejection, were studied and demonstrated via oil/water emulsions separation experiments. We proposed a facile strategy for fabrication of TCNC membranes with highly efficient separation of various oil-inwater nanoemulsions and extended to the separation of water-in-oil emulsions, and a seafood waste (shell of tunicate) was used to fabricate functional materials, which may be extended to the high-value utilization of other bioresource.
2.3. Oil/water emulsion separation All the emulsions used in this work were stabilized by surfactants. Four kinds of oil-in-water nanoemulsions were prepared by mixing 1 mL oil with 99 mL water (containing 0.1 wt% Tween 80 for chloroform, toluene and 0.2 wt% Tween 80 for n-hexane, isooctane), the mixtures were treated by ultrasonic processor (SCIENTZ, 40 kHz) for 2 h. Subsequently, the emulsions were diluted 2, 10, 15, and 20 times for chloroform, toluene, n-hexane, and isooctane to form nanometer size oil droplets. For the preparation of oil-in-water and water-in-oil microemulsions, isooctane containing 0.2 wt% Tween 80 and water containing 0.2 wt% Span 80 were mixed together with oil/water volume ratio of 1/9 (oil-in-water), 3/7 (oil-in-water), 7/3 (water-inoil), and 9/1 (water-in-oil), respectively. Afterwards, the mixtures were stirred at 1000 rpm for 12 h to obtained stable emulsions. The size of droplets in the emulsions were characterized by optical microscope. The oil/water emulsion separation experiments were conducted on a vacuum filter apparatus equipped with the as-prepared TCNC membranes, where the support was a nylon filter with an effective diameter of 42 mm (Fig. 5b). The emulsion was poured onto the membrane and water was permeated under a pressure difference of 0.5 bar. After separation, the filtrate was collected for testing the water flux and oil rejection. The water flux J (L m−2 h−1 bar−1) was calculated from the volume of the filtrate within 5 min according to the equation: (1)
J = V / AtΔP 2
Where V (L) is the volume of the permeated water, A (m ) is the effective filtration membrane area, t (h) is the permeate time, and ΔP (bar) is the suction pressure across the membrane. The oil rejection R (%) was calculated according to the equation:
R = (1 − Cf / C0 ) × 100% −1
(2) −1
Where Cf (mg mL ) and C0 (mg mL ) are the concentration of oil in filtrate and the original emulsion, respectively. Cf was determined by using gas chromatography. The standard curves of oils are shown in Fig. S4.
2. Experimental 2.1. Materials Tunicate (Halocynthia roretzi Drasche) was purchased from Weihai Evergreen Marine science and technology Co. Ltd (Shandong, China), and used as raw material. Nylon filter paper with a cut-off of 0.22 µm and 50 mm in diameter was obtained from Xinya purification Co. Ltd (Shanghai, China) as the support for tunicate cellulose nanocrystals (TCNCs) membranes. All other reagents were analyticalgrade purchased from Shanghai Chemical Agents Co. Ltd (Shanghai, China), and used without further purification.
2.4. Characterization TEM image was measured with transmission electron microscopy (TEM) using a JEM-2010 microscope (JEOL, Japan). The samples were prepared by evaporating a drop of TCNCs dispersion (0.05 wt%) on a carbon-coated copper grid. Scanning electron micrograph (SEM) measurements were carried out on a HITACHI 5-4800 microscope (Tokyo, Japan) at an accelerating voltage of 5 kV. The membranes were frozen in liquid nitrogen, fractured immediately, and then freeze dried. Both the surface and cross-section of samples were sputtered with gold, and then observed and photographed with SEM. Atomic force microscopy (AFM) was performed on an Asylum Research Cypher system (Oxford, UK) to characterize the thickness of membranes. To evaluate the effective pore size of TCNC membranes, standard PEO solutions with different molecular weight were pass through the TCNC membranes. The concentrations and hydrodynamic radius of PEO were determined by the combination system of size exclusion chromatography (SEC), multi-angle static light scattering (DAWN HELEOS-II), refractometer (Optilab T-rEX), and viscometer (ViscoStar II) (Wyatt Technology Co., US). The pore size was thought to be the hydrodynamic diameter of the molecular weight of a molecule which is rejected by 90%. The contact angle was measured by DSA100 (Krüss, Germany). Particle size of nanoemulsions and microemulsions were measured by dynamic light scattering (Malvern, UK), EX20 optical microscopy (Sunny, China), respectively. The concentration of oil was analyzed by using gas chromatography (Agilent 6890, US). Mechanical
2.2. Fabrication of tunicate cellulose nanocrystal membranes To isolate tunicate cellulose from Halocynthia roretzi Drasche, alkaline treatment and bleaching process of tunics were conducted to remove proteins, fats and pigments. From the isolation of tunicate cellulose (Fig. S1), the white fibrous product was obtained with a yield of 20%. Subsequently, 10 g tunicate cellulose was dispersed in 65 wt% sulfuric acid (500 mL) and hydrolyzed for 2 h at 70 °C under stirring to obtain tunicate cellulose nanocrystals (TCNCs). The TCNCs solutions were diluted 10-folds to terminate reactions, and the suspensions were washed with distilled water by repeated centrifugation under 1×104 rpm until turbid. The TCNCs suspensions were dialyzed against distilled water until constant pH value (Fig. S2). The concentration of suspensions was calculated by the weight ratio of TCNCs suspensions and solid after freeze-drying. The TCNC membranes were prepared by filtrating the TCNCs suspensions onto nylon filter membrane with a pore size of 0.22 µm by vacuum filtration (Fig. S3). During the filtration, the TCNCs were 2
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Fig. 1. Appearance and photograph under crossed polarizers of the TCNC membrane at dry state (a), POM image of the fibrous TCNCs suspension (b), SEM image of the TCNC membrane with TCNCs usage of 0.73 g m−2 (c), TEM image of fibrous TCNCs (d), and a scheme to describe the cholesteric structure of TCNCs solution and the porous TCNC membranes fabricated with fibrous TCNCs by vacuum-assisted filtration (e).
Fig. 2. (a) Statistical curve of the thickness and water flux of filter membranes with different usage of TCNCs. (b) The water flux as a function of 1/thickness. (c) Effective pore size of filter membranes for different usage of TCNCs determined by the rejection ( > 90%) of polyethylene oxide (PEO) with different molecular weight (100 kDa, 300 kDa, and 600 kDa). (d) Pore size distribution of TCNC membranes determined by N2 gas adsorption.
filtration of TCNCs suspension is shown in Fig. 1a. The transparency of TCNC membrane proved the nanoscale structure of TCNCs. An anisotropic phase appeared when the concentration of TCNCs reached a critical value (Fig. 1b), TCNCs trended to shape chiral nematic architecture through self-assembling [26]. The chiral nematic structure of TCNC membrane could be confirmed by two crossed polarizers (inset, Fig. 1a). The TEM image of TCNCs in suspension (Fig. 1d) displayed that the TCNCs adopted fiber-like morphologies with 10– 30 nm in width (W), 400–3000 nm in length (L), and 72 in average aspect ratio (L/W). Interestingly, in our findings, the fibrous cellulose
properties of TCNC membranes were tested by using an Electromechanical Universal Testing Machine CMT6503 (Xinsansi, China). The measurements were conducted at 25 °C with an elongation rate of 2 mm min−1. 3. Results and discussion 3.1. The structure and morphology of TCNC membranes The photograph of TCNC membrane fabricated by vacuum-assisted 3
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Fig. 3. Photographs of a water droplet on the TCNC membrane in air (a) and under oil (d), and an oil (isooctane) droplet on the TCNC membrane in air (b) and underwater (c). Dynamic approach-compress-detach oil-adhesion (chloroform) test of TCNC membrane (e). The influence of surface architecture on the wettability of membranes: oil (chloroform) droplet underwater (f) and water droplet under oil (chloroform) (g). The dosage of TCNCs for a-e was 0.73 g m−2, and that for f and g were 0, 0.0011, 0.11, and 0.73 g m−2, respectively (from left to right).
nearly linear increase of water flux with the reciprocal of TCNC membrane thickness appeared. Finally, the TCNC membrane with a thickness of 600 nm, effective pore size of 48–70 nm, and effective filtration area of 14 cm−2 obtained by filtrating 2 mL TCNCs suspension (0.05 wt%), and was selected for the evaluation of oil/water separation. For the separation of oil/water emulsions, especially oil-in-water nanoemulsion, the effective pore size of membrane was required to be small enough to remove oil droplets [17]. To accurately determine the effective pore size of membranes, polyethylene oxide (PEO) with molecular weights of 100 kDa, 300 kDa, and 600 kDa were employed as model compounds, whose hydrodynamic diameter were 24 nm, 48 nm, and 70 nm, respectively, measured by the combination system of size exclusion chromatography, multi-angle static light scattering, refractometer, and viscometer. The effective pore sizes of the membranes were determined by the points located in the domains that the rejection rate of PEO was greater than 90%, and the results are presented in Fig. 2c. The pore size of membrane with TCNCs dosage of 21.7 g m−2 was in the range of 24–48 nm. The pore size distribution of TCNC membranes determined by N2 gas adsorption is shown in Fig. 2d. The average pore size of 4 nm was obtained, which was much lower than that estimated by PEO. This result could be attributed to the shrinking of pore during the drying process and different mechanism of two measurements.
nanocrystals obtained from tunicate had larger diameter and length than those from other bioresources such as wood and cotton linter [21], which were easily to be cut off and propitious to form membrane. SEM images (Fig. 1c) further proved that the TCNC membranes were weaved with fibrous TCNCs. The unique assemble behavior of TCNCs made the rough surface and homogeneous porous morphology of the membrane. Based on these results, a scheme to describe the cholesteric structure of TCNCs solution and the porous TCNC membranes fabricated with fibrous TCNCs is provided in Fig. 1e. The fibrous TCNCs with cholesteric liquid crystal stacked planes aligned along a direction, with the orientation of each director rotated about the perpendicular axis to from one plane to the next. Clearly, the resultant membrane was consisted of the fibrous TCNCs layers and no pore wall existed, leading to the high water permeability. The strategy for fabrication of TCNC membranes in this work was novel, exceedingly simple, low cost, eco-friendly, and easily scale-up. 3.2. Effects of TCNCs usage on water flux and rejection To design and fabricate membranes, the precise control of the dosage of TCNCs was crucial, which were directly related to their thickness, water flux, and effective pore size. Fig. 2a shows the thickness and water flux of the TCNC membranes as a function of the usage of TCNCs. By adjusting the amount of the TCNCs suspension to be filtered, the thickness and water flux of the membrane could be tuned. The thickness of membrane was determined by the SEM and AFM images (Fig. S5), and the water flux of membrane was calculated by Eq. (1). When the usage of TCNCs was 32.6 g m−2, the thickness and water flux of membrane were 27 ± 2 µm, and 66 ± 5 L m−2 h−1 bar−1, respectively, whereas the thickness of membrane decreased to 300 ± 20 nm and water flux of membrane increased to 5400 ± 177 L m−2 h−1 bar−1 with TCNCs usage of 0.36 g m−2. The relationship between water flux and 1/thickness is shown in Fig. 2b. Observably, a
3.3. Wettability of TCNC membranes The wettability of membrane is fatal for its application in oil/water emulsion separation, which greatly depends on the chemical composition and architecture of the surface of membrane. The surface chemical composition determined the intermolecular forces at the interfaces between the solid, liquid/air, and the environmental phase, while the surface roughness enhanced these types of intermolecular forces [27]. 4
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Fig. 4. Emulsions separated by TCNC membranes: (a) photographs and microscope images of isooctane-in-water microemulsion before and after filtration, (b) photographs and microscope images of water-in-isooctane emulsion, and (c) photographs and size distribution of isooctane-in-water nanoemulsion.
gradually removed, it could overcome the adhesion force to detach from the surface of TCNC membrane and no deformation was observed, indicating very low oil-adhesiveness of TCNC membrane. The similar behaviors were also observed for other oil droplets underwater (Fig. S6). Generally, vertical tensile stress was generated by the adhesion force between the oil droplet and membrane surface when oil droplet was detached, which promoted the deformation of oil droplet [15]. Therefore, the dynamic underwater superoleophobicity of the TCNC membranes could be explained by the existence of the nanofibers and nanopore of TCNC membranes containing hydrophilic hydroxyl groups, preventing the contact of oil droplet. The dosage of TCNCs directly affected the surface morphology of membrane, for example, the surface architecture of 0.11 g m−2 TCNC membrane (Fig. S7) was totally different from the TCNC membrane (Fig. 1c) with TCNCs usage of 0.73 g m−2. To estimate the influence of surface architecture on the wettability, membranes with different surface structure were investigated. As shown in Fig. 3f, the contact angles of oil droplet (chloroform) on nylon membrane underwater was 88.7°, which reached 106.8° after coated with TCNCs and gradually increased to 175.9° for TCNC membrane with a thickness of 600 nm (rightmost). These results revealed that hydroxyl groups on the surface of TCNCs made the membrane oleophobic, and the formation of cholesteric architecture in the membrane produced superoleophobicity [23,24]. On the other hand, the contact angle of water on the membrane under oil decreased from 132.7 to 31.6° with the increase of thickness of TCNC membranes, indicating the variation of wettability of membranes (from hydrophobicity to hydrophilicity), which will give them the ability to separate water-in-oil emulsion. We should emphasized that the wettability of TCNCs membrane was independent
To evaluate the wetting behavior of TCNC membranes, both apparent water contact angles in air (or oil) and apparent oil contact angles in air (or water) were obtained, as shown in Fig. 3. In the liquid/air/solid three-phase system, both water (Fig. 3a) and oil (isooctane) (Fig. 3b) contact angle on the TCNC membrane in air was 0°, because plentiful hydroxyl groups on the surface of TCNC membrane, and its high surface energy and porous morphology allowed the liquid droplets to permeate through the membrane quickly, indicating its superamphiphilicity. In contrast, the TCNC membrane could not be wetted by oil (isooctane) underwater with an oil contact angle of 155° (Fig. 3c). Analogously, the contact angles of chloroform, n-hexane, and toluene, were 175.9, 157, and 171°, respectively (Fig. S6), revealing the underwater superoleophobicity of TCNC membrane. The superhydrophilicity in air and superoleophobicity underwater of TCNC membrane were similar with the properties of some organisms in nature, such as fish scales, which was composed of hydrophilic chemical component with hierarchical structure [28]. These results could be explained that the formation of unique hierarchical nanoporous structures as a result of the self-assembly of fibrous TCNCs and the presence of a large number of hydroxyl groups on the membrane. In addition, the contact angle of water under oil (isooctane) was about 40° (similar results for water under other oil are shown in Fig. S6), indicating hydrophilicity of TCNC membrane (Fig. 3d), beneficial for the separation of water-in-oil emulsions. To further characterize the dynamic wetting behavior of oil on the surface of TCNC membrane, the underwater oil-adhesion test was conducted by an approach-compress-detach process (Fig. 3e). After contacting with membrane, oil (isooctane) droplet was compressed to deform from a spherical to ellipsoidal shape. When the oil droplet was 5
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Fig. 5. (a) Water flux and oil rejection of TCNC membranes in separation of various nanoemulsions. (b) Photographs of filtration system and TCNC membranes before and after separating chloroform-in-water nanoemulsion. Chloroform was dyed by oil red. (c) Cycling performance of TCNC membranes. (d) Stress-strain curve for the dried TCNC membranes with thickness of 10 µm.
TCNC membranes not only possessed nanoporous morphology and special wettability, but also showed some advantages such as facile procedure and low cost, which may benefit their large-scale fabrication and practical application in effective separation of surfactant-stabilized nanoemulsions. As shown in Fig. 4c, the isooctane-in-water nanoemulsion was transparent and Tyndall effect could be obviously observed with laser illumination, indicating the presence of oil droplets with nano-size in the emulsion. The size of isooctane droplets were determined by dynamic light scattering (DLS) to be around 100 nm. After separation, no Tyndall effect could be observed in the collected filtrate, indicating that no isooctane droplets were left. Furthermore, the DLS curve also confirmed the disappearance of isooctane droplets, but the sharp peaks of residual surfactants appeared around several nanometers in the filtrate. It was indicated that the TCNC membranes could separate various oil/water emulsion effectively. It was noted that the nylon support membrane could not separate oil/water emulsions, due to both water and oil permeate it under pressure (Fig. S8). Similar separation results were also obtained in the cases of the chloroform-in-water, n-hexane-in-water, and toluene-in-water nanoemulsion (Fig. S9). Fig. 5a presents the separation performance of the TCNC membranes for various oil-in-water nanoemulsions. The TCNC membranes allowed all types of nanoemulsions (including toluene, chloroform, isooctane, and n-hexane) to separate completely. The oil rejection (R) of TCNC membranes for toluene, isooctane, chloroform, and n-hexane were 99.59 ± 0.37%, 99.99%, 93.33 ± 0.01%, and 99.99%, respectively, indicating high capacity for the TCNC membranes to separate oil-in-water nanoemulsions. The lower R for chloroform could be attributed to the smaller size (27 nm, Fig. S10) and higher density of its droplet in the nanoemulsions, leading to trace amount of chloroform was infiltrated into filtrate. Furthermore, the R values of the TCNC membranes for all microemulsions (including water-in-oil and oil-in-water water) were higher than 99.99% (Fig. S11). These results demonstrated that the TCNC membranes can effectively separate oil/water emulsions with different size of oil droplets. On the other hand, the water flux is also a key parameter for
of membrane thickness after the nylon membrane was fully covered, and cholesteric architecture completely formed (dosage ≥0.73 g m−2). 3.4. Separation performances of TCNC membranes for oil/water emulsions Usually, membranes with superhydrophilic and underwater superoleophobic properties are propitious to alleviate their fouling phenomenon and maintain high separation efficiency in practical separation of oil/water emulsions, because water with a higher density than oil trended to form a layer on the surface of membrane which avoided directly contact between oil droplets and membrane surface [16]. Considering the aforementioned wettability of TCNC membranes, three types of emulsions including oil-in-water microemulsions, water-in-oil emulsions, and oil-in-water nanoemulsions were prepared to test the universality of TCNC membranes in oil/water emulsion separation. Fig. 4a gives the photographs of surfactant-stabilized isooctane-inwater emulsion (o/w volume ratio=3:7) and the collected filtrate after separation. The as-prepared isooctane-in-water emulsion was milky white due to the presence of numerous oil droplets with several micrometers which flooded the entire field of image of optical microscope. After separated by the TCNC membranes through a vacuum filtration system, the collected filtrate was clear and transparent, where no droplets could be observed by optical microscope, indicating that oil droplets were effectively cut off and the oily water was effectively separated by the TCNC membranes. For water-in-oil-separation, water-in-isooctane emulsion (o/w volume ratio=7:3) was taken as example (Fig. 4b). The turbid emulsion was turned into transparent solution of water after filtration, indicating the water droplets have been successfully filtrated by the TCNC membranes from emulsion. Actually, the emergence of nanoemulsions made the separation encounter more challenges and called for novel membranes with nanoporous architecture. Impressive works including some nanoporous membranes with special wettability based on polyvinylidene fluoride, and superwetting polymer-decorated ultrathin films based on carbon nanotube have been explored [17,29,30]. Fortunately, our 6
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Table 1 Comparison of various filtration membranes for oil/water nanoemulsion separation. Materials
Wettability
Pore size (nm)
Flux (L m−2 h−1)
Rejection
Sources
Ceramic (αAl2O3-ZrO2) Polysulfone/PEG Dopamine/PEI
— Hydrophilic Superhydrophilic/superoleophobic
40–80 (1–5 bar) 120 LMH (1 bar) 40–100 (gravity)
> 90% > 95% > 98%
[31] [32] [33]
Dopamine/PEI+SiO2
Superhydrophilic/underwater superoleophobic Superhydrophilic/underwater superoleophobic Hydrophilic/underwater oleophobic
50 36–116 < 200 (supporting membrane) < 200 (supporting membrane) 10
1200 (0.5 bar)
> 99%
[34]
1500–3000 (0.5 bar)
> 94%
[17]
< 220 (supporting membrane) < 1000
267 (0.5 bar) 1867 (0.5 bar) 500 (gravity)
99.9%
[15]
99.4%
[35]
< 12 48–70
1200–1300 (0.8 bar) 1036–1734 (0.5 bar)
96.5–98% ≥99.99% (except CHCl3)
[24] This work
SWCNTs/dopamine/PEI Graphene oxide(GO) GO/Palygorskite Cu(OH)2 nanowire -coated mesh Cellulose nanosheet TCNC membranes
Superhydrophilic/underwater superoleophobic Hydrophilic/superoleophobic Superhydrophilic/underwater superoleophobic
evaluate the efficiency of oil/water separation. The water flux (J) of TCNC membranes were 1708.7 ± 92.4, 1734.9 ± 22.5, 1036.1 ± 42.5, and 1549.9 ± 109.2 L m−2 h−1 bar−1 for the emulsion of toluene, isooctane, chloroform, and n-hexane, respectively. These results indicated that the TCNC membranes can rapidly separate these nanoemulsions with very high oil rejection. In the view of above results, the TCNC membranes with small pore size still exhibited high water flux, as a result of their hierarchical architecture and superhydrophilic properties. In our finding, the existence of the fibrous TCNCs stacked planes in the TCNC membranes played an important role in the enhancement of the water flux. However, in the separation of microemulsions, the J values of TCNC membranes were low as shown in Fig. S11, which could be attributed to the high content of oil in the emulsions which were easy to form oil layer on the surface of membrane. A comparison of various filtration membranes prepared from different raw materials and methods for nanoemulsion separation is summarized in Table 1. The wettability of both our TCNC membranes and most of other reported membranes were (super) hydrophilic/ (super) oleophobic or (super) hydrophilic/ underwater (super) oleophobic, where water automatically infiltrated through the pores of the membranes but oil was completely repelled. The pore size of the TCNC membranes was in the range of 48–70 nm, which was small enough to reject oil. As we expected, TCNC membranes exhibited high oil rejection (99.99%) and high water flux (1036– 1734 L−1 m−2 h−1 bar−1), which were comparable with those of the membranes listed in Table 1. These results were closely associated with nanopore architecture without the pore wall of TCNC membranes, in which the former prevented the transition of oil, and the latter facilitated the permeation of water.
placed into aqueous solution with temperature in range of 10-80 °C, indicating that TCNC membranes were stable in the aqueous solution with different temperature. In practical oil/water separation, the stability of membranes under complex environments, such as mechanical scratch, acid and alkali conditions, is also important [37]. Owing to the membranes usually need to tolerate large hydrostatic pressure or even violent mechanical vibrations by pumps in the application situation for oil/water emulsion separation, so the mechanical strength of the TCNC membranes was vital. As shown in Fig. 5d, the tensile strengths of TCNC membranes with thickness of 10 µm was about 140 MPa, indicating excellent mechanical properties. Thus, the further enforcement of membrane was needless in the fabrication process, differing from other reported cellulose porous materials [23]. In addition, the oil contact angles for isooctane were 155, 154, and 154°, in NaCl (1 M), HCl (1 M), and NaOH (1 M), respectively (Fig. S13), revealing that the TCNC membranes could tolerate acidic, salty, and alkaline environments. Therefore, TCNC membranes with excellent durability and stability could applied for oil/water separation under harsh conditions. 4. Conclusion Superhydrophilic and underwater superoleophobic TCNC membranes were fabricated successfully with fibrous tunicate cellulose nanocrystals via a facile vacuum-assisted method. The fibrous TCNCs solution displaying cholesteric architecture could be stacked to form the planes aligned along a direction with the orientation of each director rotated about the perpendicular axis, leading to the formation of the hierarchical porous TCNC membranes without pore wall. The TCNC membranes could separate both oil-in-water and water-in-oil emulsions. For isooctane-in-water nanoemulsions, TCNC membranes exhibited excellent separation performance with high separation efficiency (≈100%) and water flux ( > 1700 L m−2 h−1 bar−1), as a result of their unique wettability (superhydrophilicity and underwater superoleophobicity). In our findings, the TCNC membranes weaved with fibrous TCNCs exhibited the high flux of water, excellent mechanical strength, cycling performance, and temperature- and pH-resistance. This work provide a simple, low cost, and environmentally friendly strategy for construction of highly effective oil/water separation device.
3.5. Durability and stability of TCNC membranes Durability and stability are common and tough issues for filtration membranes during the process of oil-water separation. The durability of TCNC membranes was monitored by detecting the change of water flux and oil rejection during several cycles of oil/water separation. As shown in Fig. 5c, there were no obvious changes in the water flux and oil rejection of TCNC membranes in the ten cycles, indicating their good reusability and antifouling properties. The results were consistent with Jung and Bhushan prediction that oleophobic and most hydrophilic surfaces can be made oleophobic with the properties of selfcleaning and antifouling at the solid-water-oil interface by the proposed model [36]. The as-prepared TCNC membranes also exhibited stable underwater superoleophobicity towards various solutions with different temperature and pH values. As shown in Fig. S12, oil contact angles for isooctane were all larger than 155°, when membranes were
Acknowledgment This work was supported by the Major Program of Natural Science Foundation of China (21334005), the Major International (Regional) Joint Research Project (21620102004), the National Natural Science Foundation of China (21304021), Hubei Province Science Foundation for Youths (2015CFB499), Jiangsu Province Science Foundation for 7
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