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membrane composite with large pore sizes for efficient separation of surfactant-stabilized water-in-oil emulsions
Journal Pre-proofs Laminated superwetting aerogel/membrane composite with large pore sizes for efficient separation of surfactant-stabilized water-in-oil emulsions Xuejie Yue, Woyuan Li, Zhangdi Li, Fengxian Qiu, Jianming Pan, Tao Zhang PII: DOI: Reference:
S0009-2509(19)30940-6 https://doi.org/10.1016/j.ces.2019.115450 CES 115450
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
10 October 2019 12 December 2019 18 December 2019
Please cite this article as: X. Yue, W. Li, Z. Li, F. Qiu, J. Pan, T. Zhang, Laminated superwetting aerogel/ membrane composite with large pore sizes for efficient separation of surfactant-stabilized water-in-oil emulsions, Chemical Engineering Science (2019), doi: https://doi.org/10.1016/j.ces.2019.115450
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Laminated superwetting aerogel/membrane composite with large pore sizes for efficient separation of surfactant-stabilized water-in-oil emulsions Xuejie Yue a, Woyuan Li a, Zhangdi Li a, Fengxian Qiu a, Jianming Pan a,b, Tao Zhang a,b* a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China. b
Institute of Green Chemistry and Chemical Technology, School of Chemistry
and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu
*Corresponding authors: Tel./fax: +86 511 88791800. E-mail: [email protected] (T. zhang) 1
Abstract: Although superwetting filtration membranes have been widely used, they suffer from the disadvantages of lower flux and membrane fouling caused by the nanoscale pore size. Herein, a laminated cellulose aerogel/membrane composite with large pore size is fabricated via combining facile freeze-drying and hydrophobic modification using cellulose fibers as building blocks. An ultrathin silanization coating layer on cellulose fibers surface is constructed via directly coupling alkoxysilane and endows the composite with excellent superhydrophobicity (161o) and superoleophilicity (0o). Upon contact with a water-in-oil emulsion, the aerogel layer causes the micrometersized water droplets to coalesce. The coalesced water droplets are then repelled by the membrane layer with large pore size, achieving ultrafast gravity-driven separation with high separation efficiency (99.5%) and separation flux (12890 L m−2 h−1). Moreover, the composite exhibits ease to cycle and good usability. The outstanding performance of composite highlights its potential applications in the field of oil related industry.
Keywords: aerogel; membrane; water-in-oil emulsions; superwetting; cellulose
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1. Introduction In recent years, oil/water separation is an important pursuit as a result of the increasing release of oily wastewater produced from industrial processes and daily human activities (Guo et al., 2019; Zhang et al., 2019). Compared to the immiscible oil/water mixtures, stable emulsified oil/water mixtures are more difficult to separate, presenting tremendous threats to the terrestrial ecosystem (Li et al., 2018; Zhang et al., 2017). Emulsified oil/water mixtures, usually containing toxic chemicals, may also harm human health through the biologic chain (Yue et al., 2019b). Moreover, frequent oil spill accidents, such as Fergana Valley oil spill, Gulf of Mexico oil spill and the Chang Qing oil spill, have released millions of tons of oils into river and sea, not only forming numerous refractory emulsions and but also causing a huge waste of precious oil resources (Che et al., 2015; Yue et al., 2018). Various cleanup technologies, such as centrifuges, skimming, and flotation, are very useful for the separation of immiscible oil/water mixtures, but are not applicable for emulsified oil/water mixtures with particle sizes less than 20 μm, which are quite common in real-life scenarios (Cai et al., 2016; Yue et al., 2019a). Besides, they suffer from the limits of tedious processing, high cost, and secondary pollution. Currently, superwetting membrane-based technology draws special interest in the separation of emulsions since it excels itself in acceptable discharge quality, simple operation, energy saving, low cost, and high efficiency (Chakrabarty et al., 2008; Che et al., 2015; Zhang et al., 2013). Effective separation of emulsions with high flux is thought to be the goal in the field of chemical industry (Qiu et al., 2020; Shi et al., 2013). On the basis of “size3
sieving” effect, superwetting membranes which selectively allows materials of certain sizes to pass through the membrane pores, have been applied for the separation of industrial emulsions (Si et al., 2015). Unfortunately, the their indispensable small pore size leads to low permeation flow and high energy consumption, which significantly hamper their upscaling and practical applications (Chen et al., 2019). According to permeation theory, the permeation flow is proportional to the square of the membrane pore radius and inversely proportional to the total distance of the liquid running through the membrane (Si et al., 2015). It indicates the ultrathin separation membrane without sacrificing the pore size can be a choice for effective separation of emulsions with high flux. Nevertheless, the ultrathin membrane structure with low porosity, short permeation channels, and low volume capacity usually causes serious membrane fouling and degradation due to surfactant adsorption and pore plugging (Shi et al., 2013; Zeng et al., 2017). The counter liquid can pass through it when the pressure fluctuation occurs, leading to a quick decline of separation efficiency (Wang et al., 2017b). A breakthrough was recently made by Liu group, who first proposed the concept of using Janus filters with asymmetric wettability to perform both de-emulsion and separation for high-flux separation of oil from oil-in-water emulsions (Wang et al., 2017a; Wang et al., 2016a; Wang et al., 2016b). Nevertheless, the complex asymmetric decoration cannot be ignored and the membrane is merely valuable for water-in-oil emulsion. Three-dimensional (3D) fibrous aerogels are distinguished for the merits of low density, interconnected porous structure, and high internal surface area (Li et al., 2017; Zhang et al., 2017; Zhang et al., 2018). To date, functional aerogels including silica 4
colloid aerogels, polymer sponges, and cellulose aerogels are used as oil sorbents to achieve high oil-absorption capacity, ignoring time-consuming recovery procedures by squeezing. Moreover, these aerogels are failed to treat emulsions, because demulsification is hard to be achieved and the tiny droplets from emulsions can easily pass through them (Si et al., 2015). In spite of this, aerogels, especially functional aerogels with special wettability and tortuously porous structure, still have the potential to facilitate the coalescence of emulsified droplets. The high tortuosity provides a longer permeation path of emulsions than the thickness of aerogel, and the superwetting surface enhances the interception and coalescence of emulsified droplets (Agarwal et al., 2013; Si et al., 2015). Such properties accelerate emulsified droplets coalescence into large droplets, and the coalesced droplets can be easily repelled by filtration membrane having large pore diameters. Thus, the integration of droplet coalescence from aerogels and blocking coalesced droplets from the membrane with large pore sizes can be an effective and promising strategy for effective high-flux separation of emulsions. For the first time, a superhydrophobic and laminated cellulose aerogel/membrane composite (CAMC) was fabricated as an advanced emulsion separation material via the combining of freeze-drying and hydrophobic modification. Comprehensive characterization of the obtained CAMC was carried out to study the structure character, interface compatibility and chemical stabilities. The aerogel layer can coalesce the tiny droplets from emulsions, and the large droplets can be blocked by the membrane layer with large pores, resulting in the effective emulsions separation with high flux. 5
Separation performance of CAMC for surfactant-stabilized water-in-oil emulsions was investigated and compared with that of cellulose membrane and cellulose aerogel. Besides, the internal emulsion separation mechanism for emulsion separation was proposed. Based on its unique structure, the CAMC exhibited both high permeation flux and separation efficiency in water-in-oil emulsions separation.
2. Materials and methods 2.1. Materials The ordinary printing paper was collected directly from local suppliers, as a source of cellulose fibers. The concentrated hydrochloric acid (HCl, 37 wt%), ethanol (C2H6O), Span 80 (C24H44O6), linseed oil, chloroform (CHCl3), n-hexane (C6H14), toluene (C7H8), and kerosene (C15H32) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. Triethoxyvinylsilane (commercially known as A151) with a purity of 98% was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China) and used as received. The cellulose ester filter membrane (pore diameter: 0.45 μm) was purchased from the Tianjin Jinteng Experiment Equipment Co, Ltd. (China). 2.2. Preparation of dispersed cellulose fibers The dispersed cellulose fibers derived from printing paper were obtained by acid soaking and subsequent mechanical dispersion. Typically, 1.0 g of printing paper scraps was dispersed in the 200 mL of water with 5 mL of HCl solution for 48 h to remove impurity ions. The printing paper scraps in the mixture were thoroughly washed with
deionized water to remove the absorbed chloride and then dried at 80 oC in vacuum for 6
5 h. The obtained paper was re-dispersed in the 200 mL of water under strongly stirring for 12 h and dispersed cellulose fibers mixture (cellulose pulp; concentration = 5 mg mL-1) was obtained before use. 2.2. Fabrication of superhydrophobic and laminated CAMC Functional CAMC was fabricated through the combining freeze-drying and subsequent hydrophobic modification. Briefly, 10 mL of the aforementioned cellulose pulp was filtered on the cellulose ester filter membrane with pores of 0.45 μm to form a cellulose membrane layer. The obtained cellulose membrane was transferred to the bottle of Teflon lining and 25 mL of cellulose pulp was then poured into Teflon lining carefully. Subsequently, the obtained mixture was frozen at -20 oC for 36 h, and then freeze-dried for 48 h to obtain the pristine aerogels. The superwetting CAMC was obtained via a simple steaming modification process to ensure a uniform modification. Typically, 50 μL of A151 and pristine CAMC were added into a sealed breaker with aluminum foil. The breaker was then moved to the oven pre-heated to 100 oC for steaming within 10 h. After this streaming treatment, the modified CAMC was dried in a vacuum drying oven at 100 oC for 2 h to remove the residual A151, and the superhydrophobic and laminated CAMC was obtained. 2.3. Tests of separation capacities and regeneration To preparing the water-in-oil emulsions, 1 mL of water and 99 mL of oil were first mixed with violent agitation for 20 min, and 1 mL of Span 80 was added into the aforementioned mixture. Following, the mixture was emulsified via stirring at a speed of 2000 rpm for more than 5 h to form stable milk emulsions. The emulsion separation 7
progress was carried out on a sand core funnel equipped with the CAMC (cellulose aerogel layer up). 100 mL of prepared emulsion was poured onto the sample and the emulsion separation process was driven solely by gravity. The flux (L m-2 h-1) was evaluated with Equation 1: V
(1)
𝐹𝑙𝑢𝑥 = 𝐴 × 𝛥𝑡
where V (L) is the permeation volume, A (m2) is the effective area of the CAMC, and Δt (h) was the separation time. Separation efficiency R (%) was calculated using Equation 2:
(
𝑅= 1―
𝐶𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒 𝐶𝑓𝑒𝑒𝑑
) × 100%
(2)
where Cfiltrate and Cfeed are the concentrations of filtrate and the original oil/water feed after separation, respectively. To verify the reusability, the used CAMC was rinsed by ethanol/water 50:50 (vol/vol) solution and dried at 50 oC for 10 h after each cycle. The flux and separation efficiency of each cycle was calculated by equations 1 and 2, and the emulsion progress was repeated for 10 cycles. 2.4. Sample characterization The structure of the CAMC was performed using scanning electron microscopy (SEM, JSM-6010, Japan). Fourier transform infrared (FT-IR) spectra were collected on a FTIR spectrophotometer (AVATAR 360, Nicolet, USA) in the attenuated total reflection (ATR) mode. X-ray diffraction (XRD) measurements were conducted using a Shimadzu XRD-6100 instrument with Cu Kα radiation (λ = 1.5418 Å) for the crystalline structure of CAMC. X-ray photoelectron spectroscopy (XPS) was 8
performed on a Thermo ESCALAB 250Xi-AER to analyze the chemical composition of CAMC. Water and oil contact angles were captured on a commercial CAM200 optical system and the final contact angles were obtained by three measurements at different positions. Dynamic light scattering (DLS) measurements were used by a Zetasizer Nano ZS 90 to analyze the droplet sizes of emulsions. The water content in the filtrate was measured by a coulometer (KF831, Metrohm, Switzerland).
3. Results and discussion 3.1. The morphology of the aerogel/membrane composite The CAMC allows for an integration of cellulose aerogel and membrane (Figure 1A), and the surface morphologies of it are investigated by SEM. The aerogel layer and membrane layer of CAMC are both derived from cellulose fibers, but they exhibit obvious structural differences. In Figure 1B, the cellulose aerogel shows an interconnected pore structure formed by numerous cellulose microfibers with lengths up to a few centimeters or even longer. The neighboring cellulose fibers are entangled with each other, thus forming pores with diameters ranging from a few tens of μm to 100 μm, which are larger than the droplet sizes (< 20 μm) of microemulsions. Moreover, these randomly entangled microfibers form numerous tortuous microchannels, which are profitable to coalesce the emulsified droplets in emulsions. The detailed information of the aerogel layer is shown in Figure 1C and the inset image in Figure 1C. Most of the cellulose microfibers are belt-like, and they showed a broad diameter distribution varying from 14 to 30 μm. More importantly, after silanization process with the aid of A151, many grooves on the fiber surface are not destroyed compared to that of the 9
pristine cellulose microfibers surface (Figure S1). The plentiful grooves indicate the effective construction of roughness, which is indispensable in fabricating superwetting surfaces. The morphology of the membrane layer of the CAMC is shown in Figure 1D and E. The microfibers in the membrane layer are ultralong and have wide diameters. These microfibers are randomly interconnected, forming a fluctuant and continuous surface with roughness. The pore sizes of the membrane layer are concentrated at 6.6 μm, no type peak is evident from 30 to 100 μm which means cracks existed in the membrane layer (Figure S2). Moreover, these micro-sized pores provide permeation channels for emulsion separation. The significant difference between cellulose membrane layer and cellulose aerogel layer is that the cellulose membrane layer exhibits a tight membrane structure, which tends to repel the micro-sized emulsified droplets. In Figure 1E, the cellulose microfibers are interlaced with each other tightly. The silanization process remained the surface rough structure of cellulose fibers, indicating its limited influence on the construction of roughness. That is, the formed silanized layers on the cellulose fibers surface are ultrathin and cannot be directly observed under SEM.
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Figure 1. Schematic illustration of the laminated CAMC (A). The SEM images of the cellulose aerogel layer (B and C) and cellulose membrane layer (D and E) in the CAMC. The successful silanization process was further demonstrated by FT-IR spectroscopy. Figure 2A shows the FT-IR spectra of pristine CMAC and modified CAMC. In Figure 2A, they both show similar characteristic absorption peaks at 3648 cm−1, 2880 cm−1, 1460 cm−1, and 1120 cm−1, which can be assigned to stretching vibration of -OH groups, stretching vibration of -CH2- bonds, bending vibration of CH2- bonds, and stretching vibrations of C-O-C, respectively. Nevertheless, after silanization, adsorption peak of hydroxyl groups becomes weaker, which could be caused by the consumption of hydroxyl group during hydrophobic modification. Moreover, the new absorption peaks at 1605 cm−1 and 1298 cm−1 are attributed to the C=C bending vibration and Si-C bending vibration arisen from A151, suggesting that CMAC was successfully silanized. More importantly, these new peaks are extremely weak and noticeable difference is not observed, implying the silanization layer on the CAMC surface is thin. 11
Crystallographic structure of CMAC after silanization was determined by XRD analysis as shown in Figure 2B. As expected, the pristine CMAC represent three characteristic peaks located at around 2θ = 14.6°, 16.8°, and 22.5°, corresponding to the typical (110), (110) and (020) planes of cellulose I (JCPDS No. 50-0926). The strong diffraction peaks suggest the high crystal quality of CMAC. No evidence of other impurities is observed, suggesting a high purity of CMAC. After silanization, the crystal structure of CMAC is not destroyed, indicating that the simple modification process is mild. No characteristic peaks driven from the silanization layer are detected, since it is too thin to be detected, in accord with the results of the FT-IR analyses.
Figure 2. FT-IR spectra (A) and XRD patterns (B) of pristine CAMC and treated CAMC, respectively. To further surface analysis the change in chemical compositions after silanization, the samples are analyzed with a sensitive XPS technique. As shown in Figure 3A, the XPS spectrum of CAMC shows the peaks at 530.6, 258.5, 102.5 and 156.5 eV with no indication of any impurities, which are assigned to O, C, Si 2p and Si 2s elements, respectively. The XPS analysis of pristine CAMC confirmed the absence of Si element both in the survey curve and the high resolution spectrum (Figure S3). After the 12
streaming treatment, the Si peak Si 2s and Si 2p peaks are significantly enhanced, indicating the successful silanization of CAMC, and this result was in accordance with the FT-IR analysis. In Figure 3B, the deconvoluted C 1s spectra consisted of three carbon groups: a dominant peak at 286.7 eV C-O due to the presence of C–O bonding, a higher binding energy C=C peak at 285.2 eV due to the presence of C–O bonding, and a peak at 284.7 eV likely due to the photoelectron transition of adventitious carbon. In addition, the high resolution O 1s XPS spectrum was deconvoluted into two contributions of O-C (531.9 eV) and Si-O-Si (533 eV). The presence of the Si-O-Si clearly confirmed A151 was successfully grafted onto the surface. To investigate the silanization mechanism between cellulose and A151, the high resolution Si 2p XPS spectra was also studied. As shown in Figure 3D, two deconvoluted peaks at 102.3 and 101.9 eV represented the Si-O-Si and C-O-Si bonds and explained the chemical combination of the Si on the CAMC surface.
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Figure 3. XPS spectra of (A) CAMC and high resolution (B) C1s, (C) O1s, and (D) Si 2p spectra on CAMC. Based on the direct proofs provided by the abovementioned FT-IR and XPS results, the silanization process was elucidated. Vaporized A151 was physically absorbed on the surface of cellulose fibers. The absorbed A151 can be hydrolyzed with the aid of water in air, transforming methoxyl into hydroxyl. The hydroxyl groups in the hydrolyzed A151 can react with the various hydroxyl groups on cellulose fibers surface, resulting in the coupling A151 on the CAMC via C-O-Si bonds and the reduction of hydroxyl content on the CMAC surface. In addition, the unreacted hydroxyl groups in the hydrolyzed A151 reacted with each other, forming Si-O-Si bonds by selfcondensation reaction. As a result, an ultrathin silanization layer on the CMAC fibers was obtained via a facile streaming treatment. The surface modification mechanism 14
including hydrolysis of A151 and dehydration reactions is provided in Scheme 1.
Scheme 1. (A) The hydrolysis of A151. (B) Coupling of alkoxysilane on pristine CAMC. 3.2. Wettability property of CAMC Comprehensive spreading and wetting tests were carried out to the surface wettability of both sides of CAMC. Owing to the low surface free energy caused by the silanization layer and rough surface structure driven from micro- and nanoscale composite structure, CAMC exhibits superhydrophobic behavior in air with a high water contact angle of above 160o (Figure S4A). In Figure 4A, a water droplet (about 5 μL) was forced to sufficient contact with the aerogel layer surface. Although the water droplet shows an apparent deformation, it can be easily lifted up. Moreover, no water remains when leaving the aerogel layer surface, thus suggesting its extremely low water adhesion. Simultaneously, the membrane layer shows a similar water repelling property. Apart from the superamphiphobic surface, the superhydrophobic surface with low surface free energy always has a high oil wettability (Li et al., 2020). As expected, both sides of CAMC display superior superolephilicity in air with a low oil contact angle of about 0o (Figure S4B). The dynamic wettability tests are also studied and the results are 15
shown in Figure 4B. When the oil droplets are dropped into the two sides of CAMA, the oil droplets can penetrate quickly into the CAMC within 0.5 s due to the superolephilicity and highly porous nature. Interestingly, membrane side displays a faster absorption rate than cellulose membrane side of that, which is attributed to the stronger capillary effect caused by the smaller pores in the cellulose membrane side. In short, two sides of the CAMC show promising superhydrophobic/superoleophilic wettability, which ensures fast oil permeation and reduces the direct contact between water and CAMC during emulsions separation, endowing CAMC with enhanced separation ability and stable separation efficiency with time. Moreover, CAMC was soaked into different organic solvents (toluene, octane, and petroleum ether) for 5 days (Figure S5). After soaking, CAMCs exhibited a satisfactory stability and no detachment of membrane layer was observed. It can be attributed to the strong compatible entanglement between two separated layers.
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Figure 4. Spreading and wetting behaviors of water and oil droplets on both sides of CAMC. 3.3. Separation performance for water-in-oil emulsions To test the separation performance of CAMC, the CAMC was sandwiched by a sand core funnel and a surfactant-stabilized water-in-octane emulsion was separated. The as-prepared water-in-octane emulsion was poured into CAMC to carry out separation progress driven by gravity, which can be seen in Movie S1. During the separation progress, oil can immediately permeate through the CAMC, while the water droplets can be demulsified and were retained above. The separation results are shown in Figure 5. From the digital photos of emulsions in Figure 5A and B, the milky waterin-octane emulsion is successfully converted to transparent oil after filtration separation through CAMC. Moreover, densely packed water droplets were observed from the 17
optical microscope images of the milky emulsion, while no droplets were detected in the whole image of filtrate, suggesting that all the tiny water droplets in the emulsion are removed by CMAC. The water droplet size distribution of emulsion and filtrate are also studied. Most of the water droplets in original emulsion are in a broad size distribution ranging from 300 nm to 6180 nm, which is smaller than the pores size of CAMC. The filtrate exhibits a droplet size distribution ranging from 34 nm to 55 nm, indicating the excellent tiny water droplets removal ability from the surfactantstabilized water-in-oil emulsion. It is worth noting that all the micrometer-sized water droplets in the emulsion can be effectively separated even the size of them are smaller than the pores size of aerogel layer and membrane layer. To understand the separation mechanism of water-in-oil emulsions through the CAMC, the separation capacity of a single cellulose aerogel layer is studied. When the milky emulsion was poured into the cellulose layer, the emulsion can permeate through the aerogel layer. Interestingly, the filtrate is opaque and the emulsion is not successfully separated. The optical microscopic images of collected filtrate showed that the distribution density of the water droplets decreased and the size of them became larger, suggesting the coalescence of water droplets, which can be explained that the emulsified droplets can collide with each other in the numerous tortuous microchannels. Moreover, the water droplet size distribution of filtrate showed various water droplets with the range of 5-10 μm, proving the coalescence ability of the cellulose aerogel layer (Figure S6). Although these coalesced water droplets cannot be intercepted by the aerogel, the large size of them reduced the requirements for the membrane pore size 18
and they might be more likely to be separated by the membrane with micrometer-sized pores.
Figure 5. Digital image and optical image of the surfactant-stability water-in-octane emulsion before (A) and after (B) separation. Dynamic light scattering measured the water size distribution of surfactant-stabilized water-in-octane emulsion before (C) and after (D) separation. For further understanding the separation mechanism of water-in-oil emulsions through the CAMC, a schematic illustration of the separation process is provided in Scheme 2. CAMC consists of two parts including aerogel layer and membrane layer, and the water-in-oil emulsions are poured onto the CAMC, which can be divided into two stages: the coalescence of emulsified water droplets in the cellulose layer and the size-sieving filtration in cellulose layer. The details of the separation mechanism are illustrated. In cellulose aerogel layer, the tiny water droplets can be repelled by the 19
cellulose fibers and the oil phase continuously passes through the aerogel layer due to the superhydrophobic/superoleophilic properties of the aerogel layer. Moreover, in the tortuous microchannels of aerogel layer, the small water droplets can collide and adhere to other droplets, forming larger droplets and then migrating through the depth of aerogel. Thus, the emulsions can be demulsified and then coalesced in the cellulose aerogel layer. However, due to the compressibility, the coalesced water droplets are prone to deformation and redispersion when they try to fit to the pore profile. Under the action of hydrodynamic forces, the coalesced droplets can pass through the aerogel layer. The membrane layer can intercept the coalesced droplets because of the sizesieving surface filtration and grant the continuous oil phase to pass through. In short, the water-in-oil emulsion is efficiently separated via a separation strategy based on a synergistic effect of demulsification, coalescence, and rejection. The exploration of the separation mechanism of water-in-oil emulsions on CMAC could be useful in exploring new strategies for effective separation of emulsions with high flux even though pore size is greater than that of emulsified droplets.
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Scheme 2. The water-in-oil emulsions separation procedure through CAMC. Furthermore, the separation performance of CAMC was evaluated by measuring the water content in filtrates and corresponding separation efficiencies, as shown in Figure 6. In Figure 6A, CAMC shows a high separation efficiency of more than 99.5% for surfactant-stabilized water-in-oil emulsions including water-in-petroleum ether, water-in-diesel, water-in-octane, and water-in-kerosene emulsions, while the water content in the filtrates is less than 45 ppm, which indicated the great separation capacity for various emulsions. Moreover, all the emulsions exhibit high flux directed only by gravity. The flux of 12890, 1820, 7980, 2300 L m−2 h−1 for water-in-petroleum ether, water-in-diesel, water-in-octane, water-in-kerosene, respectively, are obtained. These values are significantly higher than that of a commercial membrane driven by extra applied pressure (less than 300 L m−2 h−1). From the viewpoint of energy conservation, the separation performance of CAMC is especially attractive due to the high separation flux and gravity driven operation, compared to the ultrafiltration membranes where a transmembrane pressure of one to several bars is usually applied. Interestingly, the 21
separation flux of various emulsions shows grant difference, which can be explained by the Hagen–Poiseuille law where the filtrate flux of water-in-oil emulsion is inversely proportional to the oil viscosity. Thus, the water-in-petroleum ether and water-in-octane emulsions had higher separation flux than water-in-diesel and water-in-kerosene emulsions (Figure 6B). Moreover, the cycling performance of CAMC for the treatment of surfactant-stabilized water-in-octane emulsions is evaluated. The CAMC exhibits high separation efficiency of above 95.6% even after 10 cycles (Figure S7), indicating the superior recyclability and excellent antifouling property of CAMC.
Figure 6. Separation efficiency and water content in filtrates (A) and filtrate flux (B) of various surfactant-stabilized water-in-oil emulsions.
4. Conclusions In summary, the preparation of a superhydrophobic and laminated cellulose aerogel/membrane composite via combining freeze-drying and hydrophobic modification is demonstrated. The obtained ultrathin silanized layer and rough surface structure endow CAMC with excellent superhydrophobic/superoleophilic properties. With the aid of superwetting property, high pore tortuosity of the aerogel layer as well as the membrane structure of the cellulose layer, the CAMC can efficiently separate 22
various surfactant-stabilized water-in-oil emulsions with high separation efficiency (99.5%) and separation flux (12890 L m−2 h−1). Based on the analysis of separation mechanism, surfactant-stabilized emulsions were separated via a novel separation strategy based on a synergistic effect of demulsification, coalescence, and rejection, achieving the ultrafast gravity-driven separation using the separation materials with large pore size. More importantly, the CAMC exhibits ease to cycle and good usability, which match well with the requirements for treating the real emulsions on a massive scale. Therefore, utilization of superwetting CAMC with large pore size for the separation of various water-in-oil emulsions may inspire new aspects for solving the problems produced by oil-related activities, including cleanup of oil spills, fuel purification, and the separation of commercially relevant emulsions.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21706100, 21878132 and 21822807), Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRF18003), Natural Science Foundation of Hebei Province (B2019108017).
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Wang, Z., Lehtinen, M., Liu, G., 2017a. Universal Janus filters for the rapid separation of oil from emulsions stabilized by ionic or nonionic surfactants. Angew. Chem. Int. Ed. 129, 13072-13077. Wang, Z., Liu, G., Huang, S., 2016a. In situ generated janus fabrics for the rapid and efficient separation of oil from oil-in-water emulsions. Angew. Chem. Int. Ed. 55, 14610-14613. Wang, Z., Wang, Y., Liu, G., 2016b. Rapid and efficient separation of oil from oil-inwater emulsions using a Janus cotton fabric. Angew. Chem. Int. Ed. 55, 1291-1294. Wang, Z., Xiao, C., Wu, Z., Wang, Y., Du, X., Kong, W., Pan, D., Guan, G., Hao, X., 2017b. A novel 3D porous modified material with cage-like structure: Fabrication and its demulsification effect for efficient oil/water separation. J. Mater. Chem. A 5, 5895-5904. Yue, X., Chen, H., Zhang, T., Qiu, Z., Qiu, F., Yang, D., 2019a. Controllable fabrication of tendril-inspired hierarchical hybrid membrane for efficient recovering tellurium from photovoltaic waste. J. Cleaner Prod. 230, 966-973. Yue, X., Zhang, T., Yang, D., Qiu, F., Li, Z., 2018. Hybrid aerogels derived from banana peel and waste paper for efficient oil absorption and emulsion separation. J. Cleaner Prod. 199, 411-419. Yue, X., Zhang, T., Yang, D., Qiu, F., Li, Z., 2019b. Superwetting rape pollen layer for emulsion switchable separation with high flux. Chem. Eng. Sci. 203, 237-246. Zeng, X., Qian, L., Yuan, X., Zhou, C., Li, Z., Cheng, J., Xu, S., Wang, S., Pi, P., Wen, X., 2017. Inspired by stenocara beetles: from water collection to high-efficiency water-in-oil emulsion separation. ACS Nano 11, 760-769. Zhang, F., Zhang, W.B., Shi, Z., Wang, D., Jin, J., Jiang, L., 2013. Nanowire-haired inorganic membranes with superhydrophilicity and underwater ultralow adhesive superoleophobicity for high-efficiency oil/water Separation. Adv. Mater. 25, 4192-4198.
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Zhang, T., Kong, L., Dai, Y., Yue, X., Rong, J., Qiu, F., Pan, J., 2017. Enhanced oils and organic solvents absorption by polyurethane foams composites modified with MnO2 nanowires. Chem. Eng. J. 309, 7-14. Zhang, T., Yuan, D., Guo, Q., Qiu, F., Yang, D., Ou, Z., 2019. Preparation of a renewable biomass carbon aerogel reinforced with sisal for oil spillage clean-up: Inspired by green leaves to green Tofu. Food Bioprod. Process. 114, 154-162. Zhang, Y.-G., Zhu, Y.-J., Xiong, Z.-C., Wu, J., Chen, F., 2018. Bioinspired ultralight inorganic aerogel for highly efficient air filtration and oil–water separation. ACS Appl. Mater. Interfaces 10, 13019-13027.
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Highlights
Laminated cellulose aerogel/membrane composite was fabricated via successive steps.
Directly
coupling
alkoxysilane
endows
the
composite
with
good
superhydrophobicity.
The composite with large pore size separate water from water-in-octane emulsions.
High separation flux of 12890 L m−2 h−1 and separation efficiency of 99.5% are achieved.
The composite exhibits ease to cycle and good usability.
27
X. Yue, F. Qiu and T. Zhang developed the ideas. X. Yue, W. Li and Z. Li performed the experiment. X. Yue, W. Li and J. Pan conducted the structure and morphology characterization. X. Yue and W. Li wrote the manuscript. T. Zhang supervised the research.
28
Declaration of Interest Statement No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described was original research that has not been submitted previously.
29
S0009-2509(19)30940-6 https://doi.org/10.1016/j.ces.2019.115450 CES 115450
To appear in:
Chemical Engineering Science
Received Date: Revised Date: Accepted Date:
10 October 2019 12 December 2019 18 December 2019
Please cite this article as: X. Yue, W. Li, Z. Li, F. Qiu, J. Pan, T. Zhang, Laminated superwetting aerogel/ membrane composite with large pore sizes for efficient separation of surfactant-stabilized water-in-oil emulsions, Chemical Engineering Science (2019), doi: https://doi.org/10.1016/j.ces.2019.115450
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© 2019 Published by Elsevier Ltd.
Laminated superwetting aerogel/membrane composite with large pore sizes for efficient separation of surfactant-stabilized water-in-oil emulsions Xuejie Yue a, Woyuan Li a, Zhangdi Li a, Fengxian Qiu a, Jianming Pan a,b, Tao Zhang a,b* a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China. b
Institute of Green Chemistry and Chemical Technology, School of Chemistry
and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu
*Corresponding authors: Tel./fax: +86 511 88791800. E-mail: [email protected] (T. zhang) 1
Abstract: Although superwetting filtration membranes have been widely used, they suffer from the disadvantages of lower flux and membrane fouling caused by the nanoscale pore size. Herein, a laminated cellulose aerogel/membrane composite with large pore size is fabricated via combining facile freeze-drying and hydrophobic modification using cellulose fibers as building blocks. An ultrathin silanization coating layer on cellulose fibers surface is constructed via directly coupling alkoxysilane and endows the composite with excellent superhydrophobicity (161o) and superoleophilicity (0o). Upon contact with a water-in-oil emulsion, the aerogel layer causes the micrometersized water droplets to coalesce. The coalesced water droplets are then repelled by the membrane layer with large pore size, achieving ultrafast gravity-driven separation with high separation efficiency (99.5%) and separation flux (12890 L m−2 h−1). Moreover, the composite exhibits ease to cycle and good usability. The outstanding performance of composite highlights its potential applications in the field of oil related industry.
Keywords: aerogel; membrane; water-in-oil emulsions; superwetting; cellulose
2
1. Introduction In recent years, oil/water separation is an important pursuit as a result of the increasing release of oily wastewater produced from industrial processes and daily human activities (Guo et al., 2019; Zhang et al., 2019). Compared to the immiscible oil/water mixtures, stable emulsified oil/water mixtures are more difficult to separate, presenting tremendous threats to the terrestrial ecosystem (Li et al., 2018; Zhang et al., 2017). Emulsified oil/water mixtures, usually containing toxic chemicals, may also harm human health through the biologic chain (Yue et al., 2019b). Moreover, frequent oil spill accidents, such as Fergana Valley oil spill, Gulf of Mexico oil spill and the Chang Qing oil spill, have released millions of tons of oils into river and sea, not only forming numerous refractory emulsions and but also causing a huge waste of precious oil resources (Che et al., 2015; Yue et al., 2018). Various cleanup technologies, such as centrifuges, skimming, and flotation, are very useful for the separation of immiscible oil/water mixtures, but are not applicable for emulsified oil/water mixtures with particle sizes less than 20 μm, which are quite common in real-life scenarios (Cai et al., 2016; Yue et al., 2019a). Besides, they suffer from the limits of tedious processing, high cost, and secondary pollution. Currently, superwetting membrane-based technology draws special interest in the separation of emulsions since it excels itself in acceptable discharge quality, simple operation, energy saving, low cost, and high efficiency (Chakrabarty et al., 2008; Che et al., 2015; Zhang et al., 2013). Effective separation of emulsions with high flux is thought to be the goal in the field of chemical industry (Qiu et al., 2020; Shi et al., 2013). On the basis of “size3
sieving” effect, superwetting membranes which selectively allows materials of certain sizes to pass through the membrane pores, have been applied for the separation of industrial emulsions (Si et al., 2015). Unfortunately, the their indispensable small pore size leads to low permeation flow and high energy consumption, which significantly hamper their upscaling and practical applications (Chen et al., 2019). According to permeation theory, the permeation flow is proportional to the square of the membrane pore radius and inversely proportional to the total distance of the liquid running through the membrane (Si et al., 2015). It indicates the ultrathin separation membrane without sacrificing the pore size can be a choice for effective separation of emulsions with high flux. Nevertheless, the ultrathin membrane structure with low porosity, short permeation channels, and low volume capacity usually causes serious membrane fouling and degradation due to surfactant adsorption and pore plugging (Shi et al., 2013; Zeng et al., 2017). The counter liquid can pass through it when the pressure fluctuation occurs, leading to a quick decline of separation efficiency (Wang et al., 2017b). A breakthrough was recently made by Liu group, who first proposed the concept of using Janus filters with asymmetric wettability to perform both de-emulsion and separation for high-flux separation of oil from oil-in-water emulsions (Wang et al., 2017a; Wang et al., 2016a; Wang et al., 2016b). Nevertheless, the complex asymmetric decoration cannot be ignored and the membrane is merely valuable for water-in-oil emulsion. Three-dimensional (3D) fibrous aerogels are distinguished for the merits of low density, interconnected porous structure, and high internal surface area (Li et al., 2017; Zhang et al., 2017; Zhang et al., 2018). To date, functional aerogels including silica 4
colloid aerogels, polymer sponges, and cellulose aerogels are used as oil sorbents to achieve high oil-absorption capacity, ignoring time-consuming recovery procedures by squeezing. Moreover, these aerogels are failed to treat emulsions, because demulsification is hard to be achieved and the tiny droplets from emulsions can easily pass through them (Si et al., 2015). In spite of this, aerogels, especially functional aerogels with special wettability and tortuously porous structure, still have the potential to facilitate the coalescence of emulsified droplets. The high tortuosity provides a longer permeation path of emulsions than the thickness of aerogel, and the superwetting surface enhances the interception and coalescence of emulsified droplets (Agarwal et al., 2013; Si et al., 2015). Such properties accelerate emulsified droplets coalescence into large droplets, and the coalesced droplets can be easily repelled by filtration membrane having large pore diameters. Thus, the integration of droplet coalescence from aerogels and blocking coalesced droplets from the membrane with large pore sizes can be an effective and promising strategy for effective high-flux separation of emulsions. For the first time, a superhydrophobic and laminated cellulose aerogel/membrane composite (CAMC) was fabricated as an advanced emulsion separation material via the combining of freeze-drying and hydrophobic modification. Comprehensive characterization of the obtained CAMC was carried out to study the structure character, interface compatibility and chemical stabilities. The aerogel layer can coalesce the tiny droplets from emulsions, and the large droplets can be blocked by the membrane layer with large pores, resulting in the effective emulsions separation with high flux. 5
Separation performance of CAMC for surfactant-stabilized water-in-oil emulsions was investigated and compared with that of cellulose membrane and cellulose aerogel. Besides, the internal emulsion separation mechanism for emulsion separation was proposed. Based on its unique structure, the CAMC exhibited both high permeation flux and separation efficiency in water-in-oil emulsions separation.
2. Materials and methods 2.1. Materials The ordinary printing paper was collected directly from local suppliers, as a source of cellulose fibers. The concentrated hydrochloric acid (HCl, 37 wt%), ethanol (C2H6O), Span 80 (C24H44O6), linseed oil, chloroform (CHCl3), n-hexane (C6H14), toluene (C7H8), and kerosene (C15H32) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. Triethoxyvinylsilane (commercially known as A151) with a purity of 98% was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China) and used as received. The cellulose ester filter membrane (pore diameter: 0.45 μm) was purchased from the Tianjin Jinteng Experiment Equipment Co, Ltd. (China). 2.2. Preparation of dispersed cellulose fibers The dispersed cellulose fibers derived from printing paper were obtained by acid soaking and subsequent mechanical dispersion. Typically, 1.0 g of printing paper scraps was dispersed in the 200 mL of water with 5 mL of HCl solution for 48 h to remove impurity ions. The printing paper scraps in the mixture were thoroughly washed with
deionized water to remove the absorbed chloride and then dried at 80 oC in vacuum for 6
5 h. The obtained paper was re-dispersed in the 200 mL of water under strongly stirring for 12 h and dispersed cellulose fibers mixture (cellulose pulp; concentration = 5 mg mL-1) was obtained before use. 2.2. Fabrication of superhydrophobic and laminated CAMC Functional CAMC was fabricated through the combining freeze-drying and subsequent hydrophobic modification. Briefly, 10 mL of the aforementioned cellulose pulp was filtered on the cellulose ester filter membrane with pores of 0.45 μm to form a cellulose membrane layer. The obtained cellulose membrane was transferred to the bottle of Teflon lining and 25 mL of cellulose pulp was then poured into Teflon lining carefully. Subsequently, the obtained mixture was frozen at -20 oC for 36 h, and then freeze-dried for 48 h to obtain the pristine aerogels. The superwetting CAMC was obtained via a simple steaming modification process to ensure a uniform modification. Typically, 50 μL of A151 and pristine CAMC were added into a sealed breaker with aluminum foil. The breaker was then moved to the oven pre-heated to 100 oC for steaming within 10 h. After this streaming treatment, the modified CAMC was dried in a vacuum drying oven at 100 oC for 2 h to remove the residual A151, and the superhydrophobic and laminated CAMC was obtained. 2.3. Tests of separation capacities and regeneration To preparing the water-in-oil emulsions, 1 mL of water and 99 mL of oil were first mixed with violent agitation for 20 min, and 1 mL of Span 80 was added into the aforementioned mixture. Following, the mixture was emulsified via stirring at a speed of 2000 rpm for more than 5 h to form stable milk emulsions. The emulsion separation 7
progress was carried out on a sand core funnel equipped with the CAMC (cellulose aerogel layer up). 100 mL of prepared emulsion was poured onto the sample and the emulsion separation process was driven solely by gravity. The flux (L m-2 h-1) was evaluated with Equation 1: V
(1)
𝐹𝑙𝑢𝑥 = 𝐴 × 𝛥𝑡
where V (L) is the permeation volume, A (m2) is the effective area of the CAMC, and Δt (h) was the separation time. Separation efficiency R (%) was calculated using Equation 2:
(
𝑅= 1―
𝐶𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒 𝐶𝑓𝑒𝑒𝑑
) × 100%
(2)
where Cfiltrate and Cfeed are the concentrations of filtrate and the original oil/water feed after separation, respectively. To verify the reusability, the used CAMC was rinsed by ethanol/water 50:50 (vol/vol) solution and dried at 50 oC for 10 h after each cycle. The flux and separation efficiency of each cycle was calculated by equations 1 and 2, and the emulsion progress was repeated for 10 cycles. 2.4. Sample characterization The structure of the CAMC was performed using scanning electron microscopy (SEM, JSM-6010, Japan). Fourier transform infrared (FT-IR) spectra were collected on a FTIR spectrophotometer (AVATAR 360, Nicolet, USA) in the attenuated total reflection (ATR) mode. X-ray diffraction (XRD) measurements were conducted using a Shimadzu XRD-6100 instrument with Cu Kα radiation (λ = 1.5418 Å) for the crystalline structure of CAMC. X-ray photoelectron spectroscopy (XPS) was 8
performed on a Thermo ESCALAB 250Xi-AER to analyze the chemical composition of CAMC. Water and oil contact angles were captured on a commercial CAM200 optical system and the final contact angles were obtained by three measurements at different positions. Dynamic light scattering (DLS) measurements were used by a Zetasizer Nano ZS 90 to analyze the droplet sizes of emulsions. The water content in the filtrate was measured by a coulometer (KF831, Metrohm, Switzerland).
3. Results and discussion 3.1. The morphology of the aerogel/membrane composite The CAMC allows for an integration of cellulose aerogel and membrane (Figure 1A), and the surface morphologies of it are investigated by SEM. The aerogel layer and membrane layer of CAMC are both derived from cellulose fibers, but they exhibit obvious structural differences. In Figure 1B, the cellulose aerogel shows an interconnected pore structure formed by numerous cellulose microfibers with lengths up to a few centimeters or even longer. The neighboring cellulose fibers are entangled with each other, thus forming pores with diameters ranging from a few tens of μm to 100 μm, which are larger than the droplet sizes (< 20 μm) of microemulsions. Moreover, these randomly entangled microfibers form numerous tortuous microchannels, which are profitable to coalesce the emulsified droplets in emulsions. The detailed information of the aerogel layer is shown in Figure 1C and the inset image in Figure 1C. Most of the cellulose microfibers are belt-like, and they showed a broad diameter distribution varying from 14 to 30 μm. More importantly, after silanization process with the aid of A151, many grooves on the fiber surface are not destroyed compared to that of the 9
pristine cellulose microfibers surface (Figure S1). The plentiful grooves indicate the effective construction of roughness, which is indispensable in fabricating superwetting surfaces. The morphology of the membrane layer of the CAMC is shown in Figure 1D and E. The microfibers in the membrane layer are ultralong and have wide diameters. These microfibers are randomly interconnected, forming a fluctuant and continuous surface with roughness. The pore sizes of the membrane layer are concentrated at 6.6 μm, no type peak is evident from 30 to 100 μm which means cracks existed in the membrane layer (Figure S2). Moreover, these micro-sized pores provide permeation channels for emulsion separation. The significant difference between cellulose membrane layer and cellulose aerogel layer is that the cellulose membrane layer exhibits a tight membrane structure, which tends to repel the micro-sized emulsified droplets. In Figure 1E, the cellulose microfibers are interlaced with each other tightly. The silanization process remained the surface rough structure of cellulose fibers, indicating its limited influence on the construction of roughness. That is, the formed silanized layers on the cellulose fibers surface are ultrathin and cannot be directly observed under SEM.
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Figure 1. Schematic illustration of the laminated CAMC (A). The SEM images of the cellulose aerogel layer (B and C) and cellulose membrane layer (D and E) in the CAMC. The successful silanization process was further demonstrated by FT-IR spectroscopy. Figure 2A shows the FT-IR spectra of pristine CMAC and modified CAMC. In Figure 2A, they both show similar characteristic absorption peaks at 3648 cm−1, 2880 cm−1, 1460 cm−1, and 1120 cm−1, which can be assigned to stretching vibration of -OH groups, stretching vibration of -CH2- bonds, bending vibration of CH2- bonds, and stretching vibrations of C-O-C, respectively. Nevertheless, after silanization, adsorption peak of hydroxyl groups becomes weaker, which could be caused by the consumption of hydroxyl group during hydrophobic modification. Moreover, the new absorption peaks at 1605 cm−1 and 1298 cm−1 are attributed to the C=C bending vibration and Si-C bending vibration arisen from A151, suggesting that CMAC was successfully silanized. More importantly, these new peaks are extremely weak and noticeable difference is not observed, implying the silanization layer on the CAMC surface is thin. 11
Crystallographic structure of CMAC after silanization was determined by XRD analysis as shown in Figure 2B. As expected, the pristine CMAC represent three characteristic peaks located at around 2θ = 14.6°, 16.8°, and 22.5°, corresponding to the typical (110), (110) and (020) planes of cellulose I (JCPDS No. 50-0926). The strong diffraction peaks suggest the high crystal quality of CMAC. No evidence of other impurities is observed, suggesting a high purity of CMAC. After silanization, the crystal structure of CMAC is not destroyed, indicating that the simple modification process is mild. No characteristic peaks driven from the silanization layer are detected, since it is too thin to be detected, in accord with the results of the FT-IR analyses.
Figure 2. FT-IR spectra (A) and XRD patterns (B) of pristine CAMC and treated CAMC, respectively. To further surface analysis the change in chemical compositions after silanization, the samples are analyzed with a sensitive XPS technique. As shown in Figure 3A, the XPS spectrum of CAMC shows the peaks at 530.6, 258.5, 102.5 and 156.5 eV with no indication of any impurities, which are assigned to O, C, Si 2p and Si 2s elements, respectively. The XPS analysis of pristine CAMC confirmed the absence of Si element both in the survey curve and the high resolution spectrum (Figure S3). After the 12
streaming treatment, the Si peak Si 2s and Si 2p peaks are significantly enhanced, indicating the successful silanization of CAMC, and this result was in accordance with the FT-IR analysis. In Figure 3B, the deconvoluted C 1s spectra consisted of three carbon groups: a dominant peak at 286.7 eV C-O due to the presence of C–O bonding, a higher binding energy C=C peak at 285.2 eV due to the presence of C–O bonding, and a peak at 284.7 eV likely due to the photoelectron transition of adventitious carbon. In addition, the high resolution O 1s XPS spectrum was deconvoluted into two contributions of O-C (531.9 eV) and Si-O-Si (533 eV). The presence of the Si-O-Si clearly confirmed A151 was successfully grafted onto the surface. To investigate the silanization mechanism between cellulose and A151, the high resolution Si 2p XPS spectra was also studied. As shown in Figure 3D, two deconvoluted peaks at 102.3 and 101.9 eV represented the Si-O-Si and C-O-Si bonds and explained the chemical combination of the Si on the CAMC surface.
13
Figure 3. XPS spectra of (A) CAMC and high resolution (B) C1s, (C) O1s, and (D) Si 2p spectra on CAMC. Based on the direct proofs provided by the abovementioned FT-IR and XPS results, the silanization process was elucidated. Vaporized A151 was physically absorbed on the surface of cellulose fibers. The absorbed A151 can be hydrolyzed with the aid of water in air, transforming methoxyl into hydroxyl. The hydroxyl groups in the hydrolyzed A151 can react with the various hydroxyl groups on cellulose fibers surface, resulting in the coupling A151 on the CAMC via C-O-Si bonds and the reduction of hydroxyl content on the CMAC surface. In addition, the unreacted hydroxyl groups in the hydrolyzed A151 reacted with each other, forming Si-O-Si bonds by selfcondensation reaction. As a result, an ultrathin silanization layer on the CMAC fibers was obtained via a facile streaming treatment. The surface modification mechanism 14
including hydrolysis of A151 and dehydration reactions is provided in Scheme 1.
Scheme 1. (A) The hydrolysis of A151. (B) Coupling of alkoxysilane on pristine CAMC. 3.2. Wettability property of CAMC Comprehensive spreading and wetting tests were carried out to the surface wettability of both sides of CAMC. Owing to the low surface free energy caused by the silanization layer and rough surface structure driven from micro- and nanoscale composite structure, CAMC exhibits superhydrophobic behavior in air with a high water contact angle of above 160o (Figure S4A). In Figure 4A, a water droplet (about 5 μL) was forced to sufficient contact with the aerogel layer surface. Although the water droplet shows an apparent deformation, it can be easily lifted up. Moreover, no water remains when leaving the aerogel layer surface, thus suggesting its extremely low water adhesion. Simultaneously, the membrane layer shows a similar water repelling property. Apart from the superamphiphobic surface, the superhydrophobic surface with low surface free energy always has a high oil wettability (Li et al., 2020). As expected, both sides of CAMC display superior superolephilicity in air with a low oil contact angle of about 0o (Figure S4B). The dynamic wettability tests are also studied and the results are 15
shown in Figure 4B. When the oil droplets are dropped into the two sides of CAMA, the oil droplets can penetrate quickly into the CAMC within 0.5 s due to the superolephilicity and highly porous nature. Interestingly, membrane side displays a faster absorption rate than cellulose membrane side of that, which is attributed to the stronger capillary effect caused by the smaller pores in the cellulose membrane side. In short, two sides of the CAMC show promising superhydrophobic/superoleophilic wettability, which ensures fast oil permeation and reduces the direct contact between water and CAMC during emulsions separation, endowing CAMC with enhanced separation ability and stable separation efficiency with time. Moreover, CAMC was soaked into different organic solvents (toluene, octane, and petroleum ether) for 5 days (Figure S5). After soaking, CAMCs exhibited a satisfactory stability and no detachment of membrane layer was observed. It can be attributed to the strong compatible entanglement between two separated layers.
16
Figure 4. Spreading and wetting behaviors of water and oil droplets on both sides of CAMC. 3.3. Separation performance for water-in-oil emulsions To test the separation performance of CAMC, the CAMC was sandwiched by a sand core funnel and a surfactant-stabilized water-in-octane emulsion was separated. The as-prepared water-in-octane emulsion was poured into CAMC to carry out separation progress driven by gravity, which can be seen in Movie S1. During the separation progress, oil can immediately permeate through the CAMC, while the water droplets can be demulsified and were retained above. The separation results are shown in Figure 5. From the digital photos of emulsions in Figure 5A and B, the milky waterin-octane emulsion is successfully converted to transparent oil after filtration separation through CAMC. Moreover, densely packed water droplets were observed from the 17
optical microscope images of the milky emulsion, while no droplets were detected in the whole image of filtrate, suggesting that all the tiny water droplets in the emulsion are removed by CMAC. The water droplet size distribution of emulsion and filtrate are also studied. Most of the water droplets in original emulsion are in a broad size distribution ranging from 300 nm to 6180 nm, which is smaller than the pores size of CAMC. The filtrate exhibits a droplet size distribution ranging from 34 nm to 55 nm, indicating the excellent tiny water droplets removal ability from the surfactantstabilized water-in-oil emulsion. It is worth noting that all the micrometer-sized water droplets in the emulsion can be effectively separated even the size of them are smaller than the pores size of aerogel layer and membrane layer. To understand the separation mechanism of water-in-oil emulsions through the CAMC, the separation capacity of a single cellulose aerogel layer is studied. When the milky emulsion was poured into the cellulose layer, the emulsion can permeate through the aerogel layer. Interestingly, the filtrate is opaque and the emulsion is not successfully separated. The optical microscopic images of collected filtrate showed that the distribution density of the water droplets decreased and the size of them became larger, suggesting the coalescence of water droplets, which can be explained that the emulsified droplets can collide with each other in the numerous tortuous microchannels. Moreover, the water droplet size distribution of filtrate showed various water droplets with the range of 5-10 μm, proving the coalescence ability of the cellulose aerogel layer (Figure S6). Although these coalesced water droplets cannot be intercepted by the aerogel, the large size of them reduced the requirements for the membrane pore size 18
and they might be more likely to be separated by the membrane with micrometer-sized pores.
Figure 5. Digital image and optical image of the surfactant-stability water-in-octane emulsion before (A) and after (B) separation. Dynamic light scattering measured the water size distribution of surfactant-stabilized water-in-octane emulsion before (C) and after (D) separation. For further understanding the separation mechanism of water-in-oil emulsions through the CAMC, a schematic illustration of the separation process is provided in Scheme 2. CAMC consists of two parts including aerogel layer and membrane layer, and the water-in-oil emulsions are poured onto the CAMC, which can be divided into two stages: the coalescence of emulsified water droplets in the cellulose layer and the size-sieving filtration in cellulose layer. The details of the separation mechanism are illustrated. In cellulose aerogel layer, the tiny water droplets can be repelled by the 19
cellulose fibers and the oil phase continuously passes through the aerogel layer due to the superhydrophobic/superoleophilic properties of the aerogel layer. Moreover, in the tortuous microchannels of aerogel layer, the small water droplets can collide and adhere to other droplets, forming larger droplets and then migrating through the depth of aerogel. Thus, the emulsions can be demulsified and then coalesced in the cellulose aerogel layer. However, due to the compressibility, the coalesced water droplets are prone to deformation and redispersion when they try to fit to the pore profile. Under the action of hydrodynamic forces, the coalesced droplets can pass through the aerogel layer. The membrane layer can intercept the coalesced droplets because of the sizesieving surface filtration and grant the continuous oil phase to pass through. In short, the water-in-oil emulsion is efficiently separated via a separation strategy based on a synergistic effect of demulsification, coalescence, and rejection. The exploration of the separation mechanism of water-in-oil emulsions on CMAC could be useful in exploring new strategies for effective separation of emulsions with high flux even though pore size is greater than that of emulsified droplets.
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Scheme 2. The water-in-oil emulsions separation procedure through CAMC. Furthermore, the separation performance of CAMC was evaluated by measuring the water content in filtrates and corresponding separation efficiencies, as shown in Figure 6. In Figure 6A, CAMC shows a high separation efficiency of more than 99.5% for surfactant-stabilized water-in-oil emulsions including water-in-petroleum ether, water-in-diesel, water-in-octane, and water-in-kerosene emulsions, while the water content in the filtrates is less than 45 ppm, which indicated the great separation capacity for various emulsions. Moreover, all the emulsions exhibit high flux directed only by gravity. The flux of 12890, 1820, 7980, 2300 L m−2 h−1 for water-in-petroleum ether, water-in-diesel, water-in-octane, water-in-kerosene, respectively, are obtained. These values are significantly higher than that of a commercial membrane driven by extra applied pressure (less than 300 L m−2 h−1). From the viewpoint of energy conservation, the separation performance of CAMC is especially attractive due to the high separation flux and gravity driven operation, compared to the ultrafiltration membranes where a transmembrane pressure of one to several bars is usually applied. Interestingly, the 21
separation flux of various emulsions shows grant difference, which can be explained by the Hagen–Poiseuille law where the filtrate flux of water-in-oil emulsion is inversely proportional to the oil viscosity. Thus, the water-in-petroleum ether and water-in-octane emulsions had higher separation flux than water-in-diesel and water-in-kerosene emulsions (Figure 6B). Moreover, the cycling performance of CAMC for the treatment of surfactant-stabilized water-in-octane emulsions is evaluated. The CAMC exhibits high separation efficiency of above 95.6% even after 10 cycles (Figure S7), indicating the superior recyclability and excellent antifouling property of CAMC.
Figure 6. Separation efficiency and water content in filtrates (A) and filtrate flux (B) of various surfactant-stabilized water-in-oil emulsions.
4. Conclusions In summary, the preparation of a superhydrophobic and laminated cellulose aerogel/membrane composite via combining freeze-drying and hydrophobic modification is demonstrated. The obtained ultrathin silanized layer and rough surface structure endow CAMC with excellent superhydrophobic/superoleophilic properties. With the aid of superwetting property, high pore tortuosity of the aerogel layer as well as the membrane structure of the cellulose layer, the CAMC can efficiently separate 22
various surfactant-stabilized water-in-oil emulsions with high separation efficiency (99.5%) and separation flux (12890 L m−2 h−1). Based on the analysis of separation mechanism, surfactant-stabilized emulsions were separated via a novel separation strategy based on a synergistic effect of demulsification, coalescence, and rejection, achieving the ultrafast gravity-driven separation using the separation materials with large pore size. More importantly, the CAMC exhibits ease to cycle and good usability, which match well with the requirements for treating the real emulsions on a massive scale. Therefore, utilization of superwetting CAMC with large pore size for the separation of various water-in-oil emulsions may inspire new aspects for solving the problems produced by oil-related activities, including cleanup of oil spills, fuel purification, and the separation of commercially relevant emulsions.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21706100, 21878132 and 21822807), Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRF18003), Natural Science Foundation of Hebei Province (B2019108017).
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Highlights
Laminated cellulose aerogel/membrane composite was fabricated via successive steps.
Directly
coupling
alkoxysilane
endows
the
composite
with
good
superhydrophobicity.
The composite with large pore size separate water from water-in-octane emulsions.
High separation flux of 12890 L m−2 h−1 and separation efficiency of 99.5% are achieved.
The composite exhibits ease to cycle and good usability.
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X. Yue, F. Qiu and T. Zhang developed the ideas. X. Yue, W. Li and Z. Li performed the experiment. X. Yue, W. Li and J. Pan conducted the structure and morphology characterization. X. Yue and W. Li wrote the manuscript. T. Zhang supervised the research.
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Declaration of Interest Statement No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described was original research that has not been submitted previously.
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