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water separation
Journal Pre-proofs 3D printed robust superhydrophilic and underwater superoleophobic composite membrane for high efficient oil/water separation Xipeng Li, Huiting Shan, Wei Zhang, Baoan Li PII: DOI: Reference:
S1383-5866(19)34179-6 https://doi.org/10.1016/j.seppur.2019.116324 SEPPUR 116324
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
13 September 2019 5 November 2019 17 November 2019
Please cite this article as: X. Li, H. Shan, W. Zhang, B. Li, 3D printed robust superhydrophilic and underwater superoleophobic composite membrane for high efficient oil/water separation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116324
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© 2019 Published by Elsevier B.V.
3D
printed
robust
superhydrophilic
and
underwater
superoleophobic composite membrane for high efficient oil/water separation
Xipeng Lia,b,c,d, Huiting Shana,b,c,d, Wei Zhanga,b,c,d,Baoan Lia,b,c,d* a Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China b State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300350, PR China c Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300350, PR China d Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300350, PR China
Corresponding author Baoan Li Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China. Tel.: +86 22 2740 7854, Fax: +86 22 2740 4496 E-mail: [email protected] 1
Abstract The preparation of superhydrophilic and underwater superoleophobic membrane for achieving highly efficient oil/water separation through a facile method is still a great challenge. In this work, a superhydrophilic and underwater superoleophobic membrane with an ordered porous structure is fabricated via a facile direct inkjet writing (DIW) 3D printing technology for the application of oil/water separation. To print the membrane, a solid-like solution containing cellulose acetate (CA), poly (viny1 alcohol) (PVA), and silica nano-particles (SiO2 NPs) is used as the printed ink. As a result, the optimal 3D printed membrane exhibits a quite low water contact angles in-air about 18.14 ± 2.61° and a high underwater oil contact angle about 159.14 ± 0.59°, demonstrating its superhydrophilic and underwater superoleophobic characteristics. In oil/water separation process, the optimal printed membrane maintains high oil/water separation efficiency about 99.0% driven by gravity. In contrast to the traditional composite mesh preparing by coating CA/PVA/Si layer on the surface of commercial steel meshes, the printed membrane exhibits superior mechanical stability even after 30min sonication or 100 bending cycles. Most importantly, the printed membrane still maintains good anti-fouling characteristics and high separation efficiency for separating oil/water mixture after 50 cycles. Key words ： 3D printing; membrane; superhydrophilicity; underwater superoleophobicity; oil/water separation; antifouling
2
1. Introduction Currently, oil/water separation technology has attracted extensive attention due to the increasing industrial oily wastewater and the frequent oil spill accidents which causes serious environmental and ecological problems around the world [1-3]. Super-wetting porous materials that can selectively filtrate water (or oil) while completely repelling oil (or water) are widely researched for achieving effective oil/water separation [4]. Compared with traditional oil/water separation techniques, such as gravity separation, skimming, and flotation, super-wetting oil/water separation porous materials are believed to be promising for highly efficient oil/water separation with low cost and easy operation [3, 5, 6]. To date, three types of super-wetting porous materials, superhydrophobic and superoleophilic porous materials[79], superhydrophilic and underwater superoleophobic porous materials[10-12], and smart switched wetting porous materials[13, 14], have been developed for dealing with various oily wastewaters. Among various super-wetting porous materials, superhydrophilic and underwater superoleophobic porous materials possess advantages in high water flux, good anti-oil-fouling, and high separation efficiency, thus they are widely utilized in oil/water separation[10, 11, 15]. Meanwhile, superhydrophilic and underwater superoleophobic porous materials can be designed on the basis of hydrophilic chemical composition and hierarchical rough structure. For designing superhydrophilic and underwater superoleophobic porous materials, the hydrophilic chemical groups on the materials surface that can interact with water molecules result in the formation of a water-barrier layer on the surface of the material for effectively preventing oil from wetting the surface of the material [10, 16-18]. Moreover, the rough micro-nanostructure surface can decrease the contact area between the surface of the materials and oil, thus the adhesion between the oil droplets and the material’s surface is extremely low. Although superhydrophilic and underwater superoleophobic porous materials have great potential in dealing with oily wastewater, fabricating the materials through a low-cost, one-step, and time-saving process is still a great challenge. To fabricate superhydrophilic and underwater superoleophobic porous materials for oil/water separation, various technologies, such as surface coating[19], surface grafting[20, 21], and additive blending[11, 22], are widely employed for providing superhydrophilic and underwater superoleophobic characteristics to porous materials. Among those technologies, surface modification technology is mainly utilized for developing superhydrophilic and underwater superoleophobic porous materials by directly 3
coating a superhydrophilic and underwater superoleophobic layer on porous substrates. For example, in order to prepare a superhydrophilic and underwater superoleophobic membrane for oil/water separation, Ge et.al [12] coated a polyacrylonitrile (PAN) layer with SiO2 nano-particles on the surface of PAN nanofiber membrane by electro-spraying methods to prepare a superhydrophilic and underwater superoleophobic membrane for oil/water separation. Similarly, Ao et al. [23] also coated a cellulose hydrogel on steel mesh for developing a superhydrophilic and underwater superoleophobic cellulose hydrogel modified composite mesh with high oil/water separation efficiency. A novel TiO2 nanowires (NWs) membrane with varied wettability ranging from superhydrophilic to superhydrophobic was easily prepared by Pan et.al, and the membrane exhibited high separation efficiency and outstanding antifouling performances for oil/water separation[24]. In our previous work, we obtained a superhydrophilic and underwater superoleophobic polytetrafluoroethylene (PTFE) composite membrane by depositing polydopamine (PDA) on the PTFE membrane surface in tetrahydrofuran-Tris buffer mixture[25]. According to the results, the as-prepared membranes can be employed for separating varieties of oil-inwater emulsions with good antifouling performance. Nevertheless, surface modification is limited by the disadvantages of coating shedding and multi-step for preparing oil/water separation porous materials on a large scale. Three-dimensional (3D) printing technology, which is developed from additive manufacturing (AM) technology in the late 20th century, can create almost any geometrically complex shape or feature in a range of materials across different scales, enabling the potential to design oil/water separate porous materials [26-30]. Until now, various 3D printing techniques have been utilized for preparing oil/water separation materials, such as direct inkjet writing(DIW)[26], fused deposition modeling (FDM)[31], and selective laser sintering (SLS)[32-34], and digital light processing (DLP)[35]. Lv et.al[26] printed a mechanically durable superhydrophobic and superoleophilic porous membrane with an ordered porous structure via DIW printing technology using hydrophobic nano-silica-filled polydimethylsiloxane (PDMS) solution as printed ink for oil/water separation. Furthermore, a 3D printed polysulfone (PSU) membrane was fabricated by SLS printing, and a self-assembled candle soot loose network was coated on the printed membrane [33]. This novel composite membrane with a switchable wettability could be applied in oil/water separation. Yang et.al[36] fabricated a biomimetic superhydrophobic surface with eggbeater structure on various materials using immersed surface accumulation based on 3D printing technology, and the special surface can provide oil/water separation characteristics to the porous 4
materials. A whiskers-coated ceramic meshes were also prepared by DLP 3D printing technology for achieving highly effective oil/water separation in various situations[35]. Among various 3D printing technologies, direct inkjet writing (DIW) 3D printing technology, as a cost-effective and easy-to-use technique, offer a huge opportunity for printing a wide variety of materials for oil/water separation. Meanwhile, this technique is based on the computer-controlled deposition of continuous extrusion-ink with special rheological behavior, thereby enabling the rapid fabrication of 3D microstructures by layerby-layer building sequence [37]. Among various polymeric materials that can be used for the preparation of superhydrophilic and underwater superoleophobic membranes via DIW 3D printing, cellulose acetate (CA) would be accepted as a potential material due to its low cost, mechanically robust, and chemically versatile, etc. [38, 39]. Due to its advantages, CA has been widely employed as a substrate for preparing reverse osmosis membranes, microfiltration membranes, and gas separation membranes in various applications [40-42]. However, the pure CA membrane is not suitable for preparing superhydrophilic and underwater superoleophobic membrane for oil/water separation due to its limited hydrophilic property. Many researchers found that the water contact angle of the pure CA membrane was about 60-100° [42, 43]. Hence, polyvinyl alcohol (PVA), as a strong hydrophilic material, is employed for improving the hydrophilic property of the CA membrane [43]. Zhou et al. [43] has demonstrated that PVA can enhance the hydrophilic performance of CA membrane when it is introduced into the CA membrane. Moreover, Fan et al [44] found that PVA possessed good anti-oil-fouling characteristics, thus it could provide good anti-oil-fouling characteristics to CA membranes. In addition, silicon dioxide (SiO2) nanoparticles is also introduced to the printed ink for enhancing the hydrophilic characteristics, mechanical strength, and printability of the CA ink. Many researchers found that when SiO2 nanoparticles were added into the printed ink, the printed ink would transform from liquid-like fluid into solid-like fluid, thus can be printed with an ordered structure via DIW printed technology. [26, 45] Herein, we fabricate a superhydrophilic and underwater superoleophobic CA/PVA/Si composite membrane for oil/water separation via DIW 3D printing technology in one-step. Both PVA and SiO2 nanoparticles were introduced into the CA ink for enhancing the printability and hydrophilic of the CA ink. Under the control of a computer program, the CA/PVA/Si composite membranes with an ordered porous structure were prepared by tuning printed parameters during the 3D printed process. After that, the printed membranes were immersed into DI water for curing. Then, the effect of printed parameters 5
on the performance of the printed membrane was also investigated. Meanwhile, the morphology of printed membrane was observed by scanning electron microscopy (SEM) and the chemical structure of the printed membrane was performed by Fourier transform infrared spectroscopy (FTIR). For the application in oil/water separation, the wetting properties of the printed membrane were performed by water contact angle and underwater oil contact angle, respectively. Finally, the oil/water separation performance and anti-fouling property of the printed membranes were also investigated under different operation conditions.
2.Experimental 2.1 Materials Polyvinyl alcohol (Mowiol® PVA-105, average molecular weights of about 47,000 Da, and alcoholysis degree of 98) and cellulose acetate (CA, 39.8 wt% acetyl content, average molecular weights of about 30000 Da) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Dopamine hydrochloride and tris (hydroxymethyl)-aminomethane were purchased from Tianjin Heowns Biochem Technologies Co., Ltd. (Tianjin, China). Silicon oxide nanoparticles (SiO2, 20 nm average diameter) were purchased from Nanjing XFNANO Materials Tech Co.,Ltd. (Nanjing, China). Ethanol (AR), dimethyl sulfoxide (DMSO, AR), n-hexane (AR), hydrochloric acid (36.5 wt%)was purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. Diesel oil and lubricating oil were purchased from Petro China Co., Ltd. (Tianjin, China). Vegetable oil was purchased from Shandong Luhua Group Co., Ltd. 2.2. Preparation of the membranes 3D printing of PVA/CA/Si composite membranes Ink preparation ： Firstly, SiO2 nanoparticles, CA powders and PVA powders were dissolved in dimethyl sulfoxide (DMSO) with vigorous stirring at 90℃ for 6 h to get a blended solution, respectively. Afterward, the as-prepared mixture solution was transferred to a syringe barrel, and the syringe barrel containing the solution was centrifuged at 10000 rpm for 30 min to remove residual bubbles. Then, the PVA/CA/Si composite solution, as ink, was obtained for 3D printing. In the prepared ink, the content of all the polymers (containing CA and PVA) in the ink was 28% by weight, and the content of SiO2 in the ink were 0, 10, 12, 14, and 16 wt% by weight, respectively. In the polymers, the ratio of CA and PVA 6
are controlled at 100 and 0 wt%, 95 and 5 wt%, 90 and 10 wt%, 85 and 15%, 80 and 20 wt%, respectively. 3D printed membranes were printed by a customized 3D printing system which was composed of two parts. One part was an extrusion system which consisted of an air-powered fluid dispenser (Dispenser 983A, YichuanLong Dianzi Co., Ltd, China), and a syringe barrel with an about 200 μm nozzle. Meanwhile, the air-powered fluid dispenser was employed for extruding the prepared ink on a glass slide from the syringe barrel at 9.0 bar with the line velocity of 5.0 mm/s. The other part was a 3D printer (ANYCUBIC i3 MEGA, China) which could control the barrel for printing membranes with the designed structure at room temperature. During the printed process, the barrel was moved on the glass slide along programmed paths by the 3D printer, which was controlled by G-code commands generated through a model conversion software (Ultimaker cura 3.6.0) from STL files designed by a CAD program. To print the designed membrane with different pore size, the center-to-center filament spacing (Fsp) was controlled. After the printed process, the printed membranes were immersed into DI water for 60min for curing at room temperature, and then all membranes were kept in DI water. Finally, the compositions of the printed membranes were shown in Table 1, and the printed process was exhibited in Fig.1. Table 1 Various printed ink with different CA/PVA ratio and SiO2 concentration. CA/PVA
CA/PVA
SiO2 concentration
Inks
SiO2 concentration
Inks Content (%)
Ratio
(%)
Content (%)
Ratio
(%)
CP(1)-14
28
100/0
14
CP(4)-0
28
85/15
0
CP(2)-14
28
95/5
14
CP(4)-10
28
85/15
10
CP(3)-14
28
90/10
14
CP(4)-12
28
85/15
12
CP(4)-14
28
85/15
14
CP(4)-14
28
85/15
14
CP(5)-16
28
80/20
14
CP(4)-16
28
85/15
16
Preparation of PVA/CA/Si mesh Firstly, stainless steel meshes (pore size is about 300 μm) were immersed in NaOH solution (0.1M) for 120 min. Then the meshes were washed by DI water until the water was neutral. Next, the washed meshes were immersed in 2.0 g/L dopamine Tris solution (pH=8.5) for 24h. After the treated process, the meshes were washed in DI water to remove unreacted dopamine. Subsequently, the meshes were immersed in CA/PVA/Si solution ( 85g/L CA, 15g/L PVA, and 50g/L SiO2 nano-powders in DMSO 7
solvent) for 30 min, and then the meshes were dried in air for 10 min. Afterwards, the modified meshes were immersed in DI water for 30min. Finally, the meshes were cured at 130℃ for 30min, and kept in DI water.
Fig. 1 The printed process of the 3D printed composite membrane.
2.3. Characterization 2.3.1 Rheological characterization The rheological behaviors of the prepared inks with different SiO2 content and CA/PVA ratios were performed using a stress-controlled rheometer DHR-2 (TA Instruments, USA) at room temperature. The relationship between the viscosity and shear rate for the ink was obtained as a function of shear rate (0 to 100 s-1) in a logarithmically ascending series. In addition, the shear storage modulus (G’) and viscous loss modulus (G”) of the inks were also characterized in an oscillatory mode by increasing the shear stress gradually from 10 to 1000 Pa at a frequency of 1 Hz with increasing amplitude sweep. 2.3.2 Morphology and Chemical composition The surface morphology of the printed membranes was obtained by a field emission scanning electron microscope (FESEM, Hitachi S-4800, Japan). Additionally, atomic force microscopy (AFM, Cypher,Asylum Research, USA) was also exploited to analyze the surface morphologies of the membranes. For the AFM measurement, the tapping mode with scan rate of 0.5 - 1.0 Hz was utilized at room temperature, then the AFM linearity images of the printed membrane were obtained to analyze the hierarchical rough structure of the membrane surface. Moreover, the chemical composition of the membranes was performed by Fourier-transform infrared spectroscopy technique (FTIR, Nicolet 8700, Thermo Fisher Scientific Co., Ltd, USA) via the attenuated total reflection method, and the absorption spectrum of the membranes was obtained in the wavenumber range from 500 to 4000 cm-1. 8
2.3.3 Wettability The wettability of the printed membrane was performed by a contact-angle measurement apparatus (OCA 15EC, China Dataphysics Co., Ltd, China). In order to analyze the wettability of the printed membrane, both water contact angle (WCA) in air and underwater oil contact angle (OCA) of the printed membranes were characterized. For WCA measurement, a droplet of 5 μL water droplet was syringed and dropped on the membrane, and the WCA value of the membranes was computed by the drop analysis of ImageJ software. In order to characterize the underwater OCA of the membrane, a captive-bubble method was utilized. In detail, the membrane was adhered on a sample holder, and the membrane surface was faced down. Then, the sample holder was immersed in a transparent container filled with DI water, then 5 μL diesel oil droplet was deposited beneath the membrane surface via a syringe equipped with a J-hook needle. Finally, the reported underwater OCA of the membranes was the average value of five obtained data. Furthermore, the oil intrusion pressure of printed membrane was another crucial property for demonstrating the anti-oil wettability of the printed membrane during the oil/water separation, and hexane was used as typical oil compound. The maximum height of oil that the printed membrane could support determines the oil intrusion pressure of printed membrane. Therefore, the experimental intrusion pressure (Pexp) values are calculated by equation (1): (1)
𝑃𝑒𝑥𝑝 = 𝜌𝑔ℎ𝑚𝑎𝑥
where ρ is the density of hexane, g is acceleration of gravity, and hmax is the maximum height of hexane that the printed membrane can support.
2.3.4 Mechanical strength The mechanical strength of the printed membranes was assessed by a universal testing machine (XWW-20A, China) at room temperature with an invariable elongation rate of 10 mm/min. At the same time, the membrane samples were cut to a length of 40 mm before performing mechanical properties. Tensile stress is computed by equation (2) 𝐹
𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑠𝑠 = 𝑆
(2)
where F is the force exerted on an object under tension, and S is cross-sectional area of the sample. In addition, Young's modulus is obtained by equation (3): 9
𝑌𝑜𝑢𝑛𝑔′𝑠 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 =
𝐹∙𝐿 𝑆 ∙ ∆𝐿
(3)
where ΔL is the amount by which the length of the sample changes, and L is the original length of the object.
2.3.5 Oil/water separation tests For characterizing the oil/water separation efficiency of the membranes, various immiscible oil/water mixtures were prepared by mixing oil (including n-hexane, vegetable oil, lubricating oil, and diesel oil) and water in a ratio of 1:1 (v/v). The printed membrane was fixed between two quartz tubes with the diameter of 30 mm. Subsequently, the prepared oil/water mixtures were poured onto the surface of the membranes. Meanwhile, the oil content of the permeate water solution after separation was investigate by an infrared spectrometer oil content analyzer (JKY-2A, I Ke Yi Qi You Xian Gong Si, China). The separation efficiency (R (%)) is calculated according to equation (4):
(
𝐶𝑓
)
(4)
𝑅 = 1 ― 𝐶0 100%
where R (%) is the oil rejection of membrane, cf and c0 are the oil concentrations of the permeate and the original oil/water mixture, respectively. Then, the permeation flux of emulsion is calculated by equation (5) 𝑉
𝐹 = 𝐴∙𝑡
(5)
where F (L/m2.h) is the permeation flux, V (L) is the volume of filtrate, A (m2) is the effective separation area of the membranes, t (h) is the permeation time. All obtained rejection data are the average values of five measured data.
3. Results and discussion 3.1 Rheological behaviors and printability of the inks In order to successfully print the membranes with an ordered pore structure via DIW 3D printing technology, the printed inks should possess a relatively low elastic shear modulus under high shear stress for easily passing through the nozzle, and a large enough static elastic modulus to make the extruded filament “set” immediately[26, 46]. For studying the printing quality of the prepared ink, the rheological 10
behavior of the ink is characterized as a function of shear stress [26, 47-49]. The storage modulus (G’) indicates the elastic property of the ink, and loss modulus (G’’) reflects the viscous property of the inks. Hence, the effect of the SiO2 concentration and the ratio of CA/PVA on the rheological behavior of the ink were investigated, respectively, and the results were shown in Fig.2 and 3. According to Fig.2 (A), the viscosity of all the ink decrease with the increase of the shear ratio, indicating a non-Newtonian fluid property of the inks. Among the inks, the prepared ink (CP(4)-0) without SiO2 NPs displays a quite low viscosity of 76.55 Pa.s at a shear of about 1 s-1, and the storage modulus (G’) of CP (4)-0 ink is much lower than its loss modulus (G’’),as illustrated in Fig.2 (B). Besides, it can be pointed out that CP (4)-0 displays a non-pore and smooth structure. When the storage modulus (G’) of ink is much lower than its loss modulus (G’’), the ink possesses a liquid-like behavior, thus it cannot restrict the flow of ink under the force of gravity due to its too weak shear yield stress[26]. Accordingly, the ink would quickly spread on the substrate and cannot be assembled into the desired porous patterns when extruded from the nozzle, leading to the formation of a non-pore structure. Nevertheless, when SiO2 NPs was introduced into the ink, the viscosity of the ink (from CP (4)-10 to CP (4)-16)) obviously increased from 76.55 to 3227.30 Pa.s at a shear of about 1 s-1. Besides, the G’ of the ink appears to be higher than G’’ of the ink in this region at large when the SiO2 content in the ink is over 12 wt%, indicating that the inks possess a solid-like behavior. This phenomenon is caused by that the strong interaction between the SiO2 NPs and the hydroxyl from both CA and PVA is formed, thus the ink transforms into a viscoelastic fluid that exhibits obvious shear-thinning behavior[50, 51]. The shear thinning and the solid-like behaviors are necessary for the extrusion flow under pressure [52-55]. Thus, the inks with 10 wt% SiO2 NPs and 12 wt% SiO2 NPs can be printed into the porous structure, respectively, and the structure of the printed membranes still slightly collapses and form a relatively smooth porous surface due to the insufficient yield stress of the inks. However, when the SiO2 NPs content is higher than 12wt%, the printed membranes (CP (4)-14 and CP (4)-16) exhibit an order porous structure as a result of their solid-like behavior.
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Fig.2 Rheological behavior and printed structure of the inks with different SiO2 concentration. (Fsp = 500μm, the scale bars in SEM images are 300 μm)
Fig.3 Rheological behavior and printed structure of the inks with different ratio of CA/PVA. (Fsp = 500μm, the scale bars in SEM images are 300 μm) Furthermore, PVA is added into the ink for enhancing the hydrophilic of the printed membrane, which can impact the rheological behavior of the ink. As shown in Fig.3, the viscosity increase dramatically from 663.92 to 9627.30 Pa.s at a shear of about 1 s-1 when increasing the PVA ratio in the polymer of the ink from 0 to 20%. Additionally, when the ratio of PVA is higher than 5%, the G’ of ink is much higher than G’ of the inks, demonstrating that the ink exhibits a liquid-like behavior. Therefore, the printed membranes display obvious pore structure. This is because the chain entanglements could 12
occur between PVA molecular chains and CA molecular chains, thus leading to an increase of the viscosity of the inks [56]. Moreover, the increase of viscosity may be an indication of intermolecular hydrogen bonding between hydroxyl groups of PVA molecules and SiO2 NPs.
3.2 Wettability and mechanical properties of printed membranes
Fig.4 WCA and underwater OCA of printed composite membrane with (A) different SiO2 concentration and (B) different ratio of CA/ PVA. (Fsp = 500μm). The hydrophilic and underwater anti-oil fouling properties are key factors for the application of the printed membrane in oil/water separation, thus performed by WCA and underwater OCA measurements in Fig.4. As depicted in this figure, the WCA of the printed membranes (from CP (4)-0 membrane to CP (4)-16 membrane) is decreased from 42.9 ± 1.95 to 16.14 ± 2.61° with the increase of SiO2 NPs concentration. As SiO2 NPs concentration is higher than 14%, the WCAs of the printed membranes (CP (4)-14 membrane and CP (4)-16 membrane) is changed to zero in 10s. As anticipated, the underwater OCA of the printed membranes is in directly proportional to SiO2 concentration, and the underwater OCA of the printed membrane is higher than 150 ° when the SiO2 concentration is higher than 10%. Therefore, the above results indicate that both the hydrophilicity and underwater oleophobicity of the printed membrane are enhanced significantly when introducing SiO2 NPs in the membrane. This is can be explain by Young model in solid/oil/water three-phase systems (equation (S1)). Firstly, according to Fig.S1, aggregated SiO2 NPs can be observed on the membrane surface when SiO2 NPs is introduced into the printed inks. Furthermore, from the AFM linearity images in Fig.S2, the printed membranes with SiO2 NPs possess obvious hierarchical rough structures. As a result, water molecules can be trapped in the hierarchical rough structures of the membrane surface, forming a composite water-solid interface. 13
Hence, trapped water serves as a support to prevent the penetration of the oil droplets and reduce contact area between the oil and the surface of the membranes, forming an underwater superoleophobic and lowadhesive surfaces on the printed membrane. Besides that, SiO2 NPs can easily interact with water molecules due to its inherent superhydrophilicity [12, 57]. Thus, SiO2 NPs with inherent superhydrophilicity can interact with water molecules due to its water uptake characteristics, leading to produce a mass of the created hydroxyl groups on the membrane surface.[57] These polar groups can interact with water molecules through van der Waals' force and hydrogen bond, forming a water layer on the printed membrane’s surface for preventing the membranes from being wetted by oil underwater condition. Thus, the surface with polar chemical groups and hierarchical rough structure can provide superhydrophilic and underwater superoleophobic characteristics to the printed membranes. The similar results also can be observed on the printed membranes when increasing the PVA ratio in the printed membrane. PVA molecular contains a mass of polar hydroxyl group which can interact with water molecules, thus providing superhydrophilic and underwater superoleophobic characteristics to the printed membrane [43, 44]. Apart from these, depending on Fig.S3, PVA has little effect on the surface structure of the printed membranes. The content of hydrophilic hydroxyl groups in the printed membrane is proportional ton PVA ratio as illustrating in Fig.S4. Fan et.al.[44] have found that PVA with inherent hydrophilicity and underwater oleophobicity can provide superhydrophilicity and underwater oleophobicitity to filter paper for oil/water separation.
Fig. 5 The tensile stress and Young’s modulus of the printed membranes with (A) different ratio of CA/PVA and (B) different SiO2 concentration. (Fsp = 500μm) The mechanical property of the printed membranes is another crucial factor for the practical application in oil/water separation, thus the tensile stress and Young’s modulus of the printed membranes 14
are also measured, as exhibited in Fig.5. When the SiO2 NPs concentration increase in the printed membrane, resulting in the continually enhancement of the mechanical strength of the printed membrane in Fig.5 (A). This is because the strong interaction is formed between the SiO2 NPs and the hydroxyl group from CA and PVA when SiO2 was introduced in the printed membranes. Furthermore, by increasing PVA ratio in the printed inks, both of the tensile strength and Young's modulus of the printed decrease in Fig 5(B). This is because the phase separation occurred in the printed membranes after the treatment of water coagulation bath.
Fig.6 The pore size, WCA, and intrusion pressure of CP (4)-14 membrane with different Fsp. The various pore size of the printed membrane was mainly obtained by setting various FSp during the printing process, the optical images of membranes were exhibited in Fig.S5, and the effect of pore size of the printed membrane on the wettability and intrusion pressure of the membrane was also investigated in Fig.6. According to those figures, for the CP (4)-14 membrane, the pore size of the membranes increases from 103 ± 25 to 631 ± 31μm when the Fsp is changed from 300 to 800 μm. Meanwhile, when the pore size of the printed membrane is changed from 631 ± 31 to 103 ± 25 μm, the WCAs of the printed membrane decrease from 34.35 ± 3.15 to 18.31 ±1.01 °. This is because decreasing Fsp can lead to the reduction of air fraction supporting the droplets. Therefore, the intrusion pressure of CP(4)-14 also decreases from 674.51 ± 5.67 to 186.49 ± 21.62 Pa with the increase of pore size of the CP(4)-14.
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Fig. 7 Optical images, WCAs and underwater anti-oil fouling property of CP(1)-14 membrane, CP(4)14 membrane and CA/PVA/Si mesh (Fsp = 500μm, the scale bars in SEM images are 300 μm). Additionally, CP (4)-14 membrane with about 200 μm pore size is compared with CP (1)-14 membrane with about 200 μm pore size and CA/PVA/Si composite mesh with about 300 μm pore size. The hydrophilic, underwater anti-oil-fouling property, wetting stability of all the membranes were performed by dynamic WCA, dynamic oil absorption measurements, respectively, and the results were exhibited in Fig. 7. According to this figure, it is can be pointed out that the print membranes (CP (1)-14 membrane and CP (4)-14 membrane) display an ordered porous structure. Specially, when the CA/PVA/Si layer is coated on the steel mesh surface, part of the mesh pore is blocked, resulting in forming inhomogeneous porous structures. As indicated in Fig.7, water droplet keep a stable shape on CP (1)-14 membrane surface in 10 s, and oil droplet is attached on the surface of the membrane underwater condition due to its poor hydrophilic characteristics. Nevertheless, water droplet can immerse into the surface of CP (4)-14 membrane and CA/PVA/Si mesh in 10s, and no significant deformation and residual oil were observed on the membrane surface during the process, demonstrating their superhydrophilic and quite low oil adhesion characteristics. Thus, they can be utilized to separate oil/water mixture.
16
3.3 Oil/water separation performance of the membranes
Fig.8 Separation process of CP (4)-14 membrane for (A) diesel oil/water mixture, (B) n-hexane/water mixture (n-hexane was marked with Sudan Ⅲ), (C) Separation efficiency and water flux of CP (4)-14 membrane with different pore size for various oil/water mixtures, (D) Separation efficiency of CP (4)14 for oil/water mixture with different pH.
Fig.9 The wetting stability of CP (1)-14, CP (4)-14, and CA/PVA/Si mesh before and after sonication (A)
and bending cycles (B), and SEM images (Fsp = 500μm, the scale bars in SEM images are 300 17
μm ) of CP (1)-14, CP (4)-14, and CA/PVA/Si mesh before and after sonication (C) and bending cycles (D), separation efficiency (E) and water flux (F) of CP (1)-14, CP (4)-14, and CA/PVA/Si mesh after 30 min sonication or 100 bending cycles. The oil/water separation experiment was performed by utilizing the printed membranes for dealing with various oil/water mixture, and n-hexane, diesel oil, vegetable oil, and lubricating oil were employed as model oil compounds. Before tested, the membranes were pre-wetted by DI water, and the results were exhibited in Fig.8 (A) and (B). As shown in those figures, when pouring n-hexane (dyed by Sundan Ⅲ)/water mixture on CP(4)-14 membrane surface, water can quickly pass through the membrane under the driving force of gravity, while n-hexane is rejected and kept in the upper glass tube because of superhydrophilic and underwater superoleophobic characteristics of CP(4)-14 membrane. The similar phenomenon is also observed when diesel oil/water mixture is poured on the surface of CP (4)-14 membrane. This phenomenon can be explained by the fact that both PVA and SiO2 NPs with inherent hydrophilicity are easy to form hydrogel with CA, thus water can be trapped in the CA/PVA/Si molecules network, resulting in the formation of a water layer on the surface of the membrane [44, 58]. Hence, oil can be repelled from the water layer on the surface of the membrane. Among the oil/water separation experiments, the separation efficiency and water flux of the CP (4)-14 membrane with different pore size was also investigated in Fig.8 (C). According to the results, when the pore size of the printed membrane is higher than about 230 μm, the separation efficiency of the printed membrane is significantly reduced. Additionally, the water flux of the membranes with different pore size were also exhibited in this figure. Depending on the results, the water flux of the printed membrane increased from about 149 000 to about 2160 000 L/(m2.h) when the pore size of membrane increased from about 115 to about 527 μm. Koh, et al [59] also found that 3D printed membrane with a pore size of ∼187 μm possessed extremely high water flux of about 430 000 L/(m2.h). The main reason for such a high flux is because the pore size of printed membrane is much bigger than common membranes and the printed membrane possesses superhydrophilic characteristics. Considering the practical application, CP (4)-14 membrane should possess good stability in acidic or alkaline environments for dealing with various oily wastewater, and their stability in acidic and alkaline environments was evaluated by testing the separation efficiency of n-hexane/water mixture with different pH values. According to Fig.8 (D), CP (4)-14 maintains a good stable separation efficiency about above 99.0% in acid, neutral, alkali, and seawater conditions. For comparing property, CP (1)-14 with pore size of about 200μm, CP (4)-14 with pore size of 18
about 200μm, and CA/PVA/Si mesh with pore size of about 250μm were also employed for dealing with n-hexane/water mixture. Also, the sonication and bending cycle measurements were employed for performing the wetting stability and oil/water separation property of the printed membranes in Fig.9. Firstly, both CP (4)-14 and CA/PVA/Si mesh possess good hydrophilic property and oil//water separation efficiency about 99% due to CA/PVA/Si layer’s superhydrophilic and underwater superoleophobic characteristics. However, CP (1)-14 membrane exhibits poor hydrophilic property and poor separation efficiency about 78% for oil/water mixture due to the poor hydrophilic and underwater oleophobic property of cellulose acetate. Additionally, depending on Fig.9 (E), the pristine water flux of CP (1)-14, CP (4)-14, and CA/PVA/Si mesh are about 318 000, 364 000, 117 000 L/(m2.h), respectively. The water flux of the mesh is lower than the printed membranes, which is because the CA/PVA/Si layer blocks part of the mesh pore. After 30 min sonication or 100 bending cycles, the printed membranes (containing CP (1)-14 and CP (4)-14) still possess stable hydrophilic property and oil/water separation efficiency, and the water flux of CP (1)-14 and CP (4)-14 only has a little change. This is because of that CP (1)-14 and CP (4)-14 still possess ordered structure even after the 30 min sonication or 100 bending cycles according to Fig.9 (C) and (D). The above results demonstrate that both CP (1)-14 and CP (4)-14 possess good stability even after 30 min sonication (200W) or 100 bending cycles. However, after 30 min sonication, the hydrophilic property of the CA/PVA/Si mesh obviously decrease. At the same time, the oil/water separation efficiency of CA/PVA/Si mesh is obviously reduced from 98.15± 0.21 to 46.74 ± 1.75 %, and the water flux of CA/PVA/Si mesh significantly increase. This is because CA/PVA/Si layer on the steel mesh surface is peeled off from the mesh under the sonication condition according to Fig.9(C). Similarly, when the mesh was treated with 100 bending cycles, the hydrophilic property and separation efficiency of CA/PVA/Si mesh was also reduced and the water flux of the mesh increased. As displayed in Fig.9(D), the bending cycles can irreparably destroy the pore structure of the mesh, thus the water and oil can easily pass through the destroy pore of the mesh. Thus, the results mentioned above demonstrate that CP (4)14 membrane with good mechanical strength, superhydrophilicity, and underwater superoleophobicity can be utilized to separate oil-in-water wastewater.
19
Fig.10 The stability of separation efficiency of CP(4)-14 membrane after 50 times cycles, and the WCAs and OCA of CP(4)-14 before and after measurement. To further investigate the anti-fouling and stability of CP (4)-14 membrane, a cyclic filtration experiment was carried out by utilizing a n-hexane/water mixture as an example, and the membrane was washed by DI water every 10 times cycles. Meanwhile, the separation efficiency, WCA and underwater OCA of the membrane before and after 50 times cycles are exhibited in Fig.10. It is noticeable that CP (4)-14 membrane displays a high separation efficiency (above 99.0 %) even after 50 times cycles. Additionally, CP (4)-14 still maintains superhydrophilic and underwater superoleophobic characteristics even after 50 times cycles, demonstrating its good stability and anti-fouling property.
4. Conclusions In summary, a superhydrophilic and underwater superoleophobic membrane with an ordered porous structure is successfully prepared by DIW 3D printing technology, and the solution containing CA, PVA, and SiO2 nano-powders is utilized as the ink. The optimal printed membrane (CP (4)-14 membrane) exhibits good superhydrophilic and underwater superoleophobic characteristics. In addition, the membrane also displays good mechanical strength and mechanical stability even after sonication treatment or bending cycles. For the application of oil/water separation, the membrane can selectively separate various oil/water mixture with high separation efficiency of about 99.0%. Meanwhile, the printed membrane also maintains anti-oil fouling ability, thus it can be easily cleaned and reused during 20
the long time oil/water separation process. Compared with CA/PVA/Si composite steel mesh, the printed membrane possesses superior mechanical stability and oil/water separation efficiency even after sonication treatment or bending cycles. Thus the printed membrane exhibits impressive oil/water separation efficiency, mechanical stability, and reusability, which are desirable for a variety of oil/water separation application.
Acknowledgements This research was supported by National Natural Science Foundation of China (Grant nos. 21878218) and Tianjin Science and Technology Major Project (Grant nos. 18ZXSZSF00030).
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Graphical Abstract 3D
printed
robust
superhydrophilic
and
underwater
superoleophobic composite membrane for high efficient oil-water separation Xipeng Lia,b,c,d, Huiting Shana,b,c,d, Wei Zhanga,b,c,d, Baoan Lia,b,c,d * a
Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300350, PR China b
State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300350, PR China
c
Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University,
Tianjin 300350, PR China d
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300350, PR China
27
Highlight：
1. CA/PVA/Si membrane is fabricated by direct inkjet writing (DIW) 3D printing technology 2. CA/PVA/Si membrane displays superhydrophilic and underwater superoleophobic characteristics. 3. CA/PVA/Si membrane possesses good oil/water separation efficiency. 4. CA/PVA/Si membrane keeps good fouling-resistant and recyclability.
28
S1383-5866(19)34179-6 https://doi.org/10.1016/j.seppur.2019.116324 SEPPUR 116324
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
13 September 2019 5 November 2019 17 November 2019
Please cite this article as: X. Li, H. Shan, W. Zhang, B. Li, 3D printed robust superhydrophilic and underwater superoleophobic composite membrane for high efficient oil/water separation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116324
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© 2019 Published by Elsevier B.V.
3D
printed
robust
superhydrophilic
and
underwater
superoleophobic composite membrane for high efficient oil/water separation
Xipeng Lia,b,c,d, Huiting Shana,b,c,d, Wei Zhanga,b,c,d,Baoan Lia,b,c,d* a Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China b State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300350, PR China c Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300350, PR China d Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300350, PR China
Corresponding author Baoan Li Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China. Tel.: +86 22 2740 7854, Fax: +86 22 2740 4496 E-mail: [email protected] 1
Abstract The preparation of superhydrophilic and underwater superoleophobic membrane for achieving highly efficient oil/water separation through a facile method is still a great challenge. In this work, a superhydrophilic and underwater superoleophobic membrane with an ordered porous structure is fabricated via a facile direct inkjet writing (DIW) 3D printing technology for the application of oil/water separation. To print the membrane, a solid-like solution containing cellulose acetate (CA), poly (viny1 alcohol) (PVA), and silica nano-particles (SiO2 NPs) is used as the printed ink. As a result, the optimal 3D printed membrane exhibits a quite low water contact angles in-air about 18.14 ± 2.61° and a high underwater oil contact angle about 159.14 ± 0.59°, demonstrating its superhydrophilic and underwater superoleophobic characteristics. In oil/water separation process, the optimal printed membrane maintains high oil/water separation efficiency about 99.0% driven by gravity. In contrast to the traditional composite mesh preparing by coating CA/PVA/Si layer on the surface of commercial steel meshes, the printed membrane exhibits superior mechanical stability even after 30min sonication or 100 bending cycles. Most importantly, the printed membrane still maintains good anti-fouling characteristics and high separation efficiency for separating oil/water mixture after 50 cycles. Key words ： 3D printing; membrane; superhydrophilicity; underwater superoleophobicity; oil/water separation; antifouling
2
1. Introduction Currently, oil/water separation technology has attracted extensive attention due to the increasing industrial oily wastewater and the frequent oil spill accidents which causes serious environmental and ecological problems around the world [1-3]. Super-wetting porous materials that can selectively filtrate water (or oil) while completely repelling oil (or water) are widely researched for achieving effective oil/water separation [4]. Compared with traditional oil/water separation techniques, such as gravity separation, skimming, and flotation, super-wetting oil/water separation porous materials are believed to be promising for highly efficient oil/water separation with low cost and easy operation [3, 5, 6]. To date, three types of super-wetting porous materials, superhydrophobic and superoleophilic porous materials[79], superhydrophilic and underwater superoleophobic porous materials[10-12], and smart switched wetting porous materials[13, 14], have been developed for dealing with various oily wastewaters. Among various super-wetting porous materials, superhydrophilic and underwater superoleophobic porous materials possess advantages in high water flux, good anti-oil-fouling, and high separation efficiency, thus they are widely utilized in oil/water separation[10, 11, 15]. Meanwhile, superhydrophilic and underwater superoleophobic porous materials can be designed on the basis of hydrophilic chemical composition and hierarchical rough structure. For designing superhydrophilic and underwater superoleophobic porous materials, the hydrophilic chemical groups on the materials surface that can interact with water molecules result in the formation of a water-barrier layer on the surface of the material for effectively preventing oil from wetting the surface of the material [10, 16-18]. Moreover, the rough micro-nanostructure surface can decrease the contact area between the surface of the materials and oil, thus the adhesion between the oil droplets and the material’s surface is extremely low. Although superhydrophilic and underwater superoleophobic porous materials have great potential in dealing with oily wastewater, fabricating the materials through a low-cost, one-step, and time-saving process is still a great challenge. To fabricate superhydrophilic and underwater superoleophobic porous materials for oil/water separation, various technologies, such as surface coating[19], surface grafting[20, 21], and additive blending[11, 22], are widely employed for providing superhydrophilic and underwater superoleophobic characteristics to porous materials. Among those technologies, surface modification technology is mainly utilized for developing superhydrophilic and underwater superoleophobic porous materials by directly 3
coating a superhydrophilic and underwater superoleophobic layer on porous substrates. For example, in order to prepare a superhydrophilic and underwater superoleophobic membrane for oil/water separation, Ge et.al [12] coated a polyacrylonitrile (PAN) layer with SiO2 nano-particles on the surface of PAN nanofiber membrane by electro-spraying methods to prepare a superhydrophilic and underwater superoleophobic membrane for oil/water separation. Similarly, Ao et al. [23] also coated a cellulose hydrogel on steel mesh for developing a superhydrophilic and underwater superoleophobic cellulose hydrogel modified composite mesh with high oil/water separation efficiency. A novel TiO2 nanowires (NWs) membrane with varied wettability ranging from superhydrophilic to superhydrophobic was easily prepared by Pan et.al, and the membrane exhibited high separation efficiency and outstanding antifouling performances for oil/water separation[24]. In our previous work, we obtained a superhydrophilic and underwater superoleophobic polytetrafluoroethylene (PTFE) composite membrane by depositing polydopamine (PDA) on the PTFE membrane surface in tetrahydrofuran-Tris buffer mixture[25]. According to the results, the as-prepared membranes can be employed for separating varieties of oil-inwater emulsions with good antifouling performance. Nevertheless, surface modification is limited by the disadvantages of coating shedding and multi-step for preparing oil/water separation porous materials on a large scale. Three-dimensional (3D) printing technology, which is developed from additive manufacturing (AM) technology in the late 20th century, can create almost any geometrically complex shape or feature in a range of materials across different scales, enabling the potential to design oil/water separate porous materials [26-30]. Until now, various 3D printing techniques have been utilized for preparing oil/water separation materials, such as direct inkjet writing(DIW)[26], fused deposition modeling (FDM)[31], and selective laser sintering (SLS)[32-34], and digital light processing (DLP)[35]. Lv et.al[26] printed a mechanically durable superhydrophobic and superoleophilic porous membrane with an ordered porous structure via DIW printing technology using hydrophobic nano-silica-filled polydimethylsiloxane (PDMS) solution as printed ink for oil/water separation. Furthermore, a 3D printed polysulfone (PSU) membrane was fabricated by SLS printing, and a self-assembled candle soot loose network was coated on the printed membrane [33]. This novel composite membrane with a switchable wettability could be applied in oil/water separation. Yang et.al[36] fabricated a biomimetic superhydrophobic surface with eggbeater structure on various materials using immersed surface accumulation based on 3D printing technology, and the special surface can provide oil/water separation characteristics to the porous 4
materials. A whiskers-coated ceramic meshes were also prepared by DLP 3D printing technology for achieving highly effective oil/water separation in various situations[35]. Among various 3D printing technologies, direct inkjet writing (DIW) 3D printing technology, as a cost-effective and easy-to-use technique, offer a huge opportunity for printing a wide variety of materials for oil/water separation. Meanwhile, this technique is based on the computer-controlled deposition of continuous extrusion-ink with special rheological behavior, thereby enabling the rapid fabrication of 3D microstructures by layerby-layer building sequence [37]. Among various polymeric materials that can be used for the preparation of superhydrophilic and underwater superoleophobic membranes via DIW 3D printing, cellulose acetate (CA) would be accepted as a potential material due to its low cost, mechanically robust, and chemically versatile, etc. [38, 39]. Due to its advantages, CA has been widely employed as a substrate for preparing reverse osmosis membranes, microfiltration membranes, and gas separation membranes in various applications [40-42]. However, the pure CA membrane is not suitable for preparing superhydrophilic and underwater superoleophobic membrane for oil/water separation due to its limited hydrophilic property. Many researchers found that the water contact angle of the pure CA membrane was about 60-100° [42, 43]. Hence, polyvinyl alcohol (PVA), as a strong hydrophilic material, is employed for improving the hydrophilic property of the CA membrane [43]. Zhou et al. [43] has demonstrated that PVA can enhance the hydrophilic performance of CA membrane when it is introduced into the CA membrane. Moreover, Fan et al [44] found that PVA possessed good anti-oil-fouling characteristics, thus it could provide good anti-oil-fouling characteristics to CA membranes. In addition, silicon dioxide (SiO2) nanoparticles is also introduced to the printed ink for enhancing the hydrophilic characteristics, mechanical strength, and printability of the CA ink. Many researchers found that when SiO2 nanoparticles were added into the printed ink, the printed ink would transform from liquid-like fluid into solid-like fluid, thus can be printed with an ordered structure via DIW printed technology. [26, 45] Herein, we fabricate a superhydrophilic and underwater superoleophobic CA/PVA/Si composite membrane for oil/water separation via DIW 3D printing technology in one-step. Both PVA and SiO2 nanoparticles were introduced into the CA ink for enhancing the printability and hydrophilic of the CA ink. Under the control of a computer program, the CA/PVA/Si composite membranes with an ordered porous structure were prepared by tuning printed parameters during the 3D printed process. After that, the printed membranes were immersed into DI water for curing. Then, the effect of printed parameters 5
on the performance of the printed membrane was also investigated. Meanwhile, the morphology of printed membrane was observed by scanning electron microscopy (SEM) and the chemical structure of the printed membrane was performed by Fourier transform infrared spectroscopy (FTIR). For the application in oil/water separation, the wetting properties of the printed membrane were performed by water contact angle and underwater oil contact angle, respectively. Finally, the oil/water separation performance and anti-fouling property of the printed membranes were also investigated under different operation conditions.
2.Experimental 2.1 Materials Polyvinyl alcohol (Mowiol® PVA-105, average molecular weights of about 47,000 Da, and alcoholysis degree of 98) and cellulose acetate (CA, 39.8 wt% acetyl content, average molecular weights of about 30000 Da) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Dopamine hydrochloride and tris (hydroxymethyl)-aminomethane were purchased from Tianjin Heowns Biochem Technologies Co., Ltd. (Tianjin, China). Silicon oxide nanoparticles (SiO2, 20 nm average diameter) were purchased from Nanjing XFNANO Materials Tech Co.,Ltd. (Nanjing, China). Ethanol (AR), dimethyl sulfoxide (DMSO, AR), n-hexane (AR), hydrochloric acid (36.5 wt%)was purchased from Tianjin Jiangtian Chemical Technology Co., Ltd. Diesel oil and lubricating oil were purchased from Petro China Co., Ltd. (Tianjin, China). Vegetable oil was purchased from Shandong Luhua Group Co., Ltd. 2.2. Preparation of the membranes 3D printing of PVA/CA/Si composite membranes Ink preparation ： Firstly, SiO2 nanoparticles, CA powders and PVA powders were dissolved in dimethyl sulfoxide (DMSO) with vigorous stirring at 90℃ for 6 h to get a blended solution, respectively. Afterward, the as-prepared mixture solution was transferred to a syringe barrel, and the syringe barrel containing the solution was centrifuged at 10000 rpm for 30 min to remove residual bubbles. Then, the PVA/CA/Si composite solution, as ink, was obtained for 3D printing. In the prepared ink, the content of all the polymers (containing CA and PVA) in the ink was 28% by weight, and the content of SiO2 in the ink were 0, 10, 12, 14, and 16 wt% by weight, respectively. In the polymers, the ratio of CA and PVA 6
are controlled at 100 and 0 wt%, 95 and 5 wt%, 90 and 10 wt%, 85 and 15%, 80 and 20 wt%, respectively. 3D printed membranes were printed by a customized 3D printing system which was composed of two parts. One part was an extrusion system which consisted of an air-powered fluid dispenser (Dispenser 983A, YichuanLong Dianzi Co., Ltd, China), and a syringe barrel with an about 200 μm nozzle. Meanwhile, the air-powered fluid dispenser was employed for extruding the prepared ink on a glass slide from the syringe barrel at 9.0 bar with the line velocity of 5.0 mm/s. The other part was a 3D printer (ANYCUBIC i3 MEGA, China) which could control the barrel for printing membranes with the designed structure at room temperature. During the printed process, the barrel was moved on the glass slide along programmed paths by the 3D printer, which was controlled by G-code commands generated through a model conversion software (Ultimaker cura 3.6.0) from STL files designed by a CAD program. To print the designed membrane with different pore size, the center-to-center filament spacing (Fsp) was controlled. After the printed process, the printed membranes were immersed into DI water for 60min for curing at room temperature, and then all membranes were kept in DI water. Finally, the compositions of the printed membranes were shown in Table 1, and the printed process was exhibited in Fig.1. Table 1 Various printed ink with different CA/PVA ratio and SiO2 concentration. CA/PVA
CA/PVA
SiO2 concentration
Inks
SiO2 concentration
Inks Content (%)
Ratio
(%)
Content (%)
Ratio
(%)
CP(1)-14
28
100/0
14
CP(4)-0
28
85/15
0
CP(2)-14
28
95/5
14
CP(4)-10
28
85/15
10
CP(3)-14
28
90/10
14
CP(4)-12
28
85/15
12
CP(4)-14
28
85/15
14
CP(4)-14
28
85/15
14
CP(5)-16
28
80/20
14
CP(4)-16
28
85/15
16
Preparation of PVA/CA/Si mesh Firstly, stainless steel meshes (pore size is about 300 μm) were immersed in NaOH solution (0.1M) for 120 min. Then the meshes were washed by DI water until the water was neutral. Next, the washed meshes were immersed in 2.0 g/L dopamine Tris solution (pH=8.5) for 24h. After the treated process, the meshes were washed in DI water to remove unreacted dopamine. Subsequently, the meshes were immersed in CA/PVA/Si solution ( 85g/L CA, 15g/L PVA, and 50g/L SiO2 nano-powders in DMSO 7
solvent) for 30 min, and then the meshes were dried in air for 10 min. Afterwards, the modified meshes were immersed in DI water for 30min. Finally, the meshes were cured at 130℃ for 30min, and kept in DI water.
Fig. 1 The printed process of the 3D printed composite membrane.
2.3. Characterization 2.3.1 Rheological characterization The rheological behaviors of the prepared inks with different SiO2 content and CA/PVA ratios were performed using a stress-controlled rheometer DHR-2 (TA Instruments, USA) at room temperature. The relationship between the viscosity and shear rate for the ink was obtained as a function of shear rate (0 to 100 s-1) in a logarithmically ascending series. In addition, the shear storage modulus (G’) and viscous loss modulus (G”) of the inks were also characterized in an oscillatory mode by increasing the shear stress gradually from 10 to 1000 Pa at a frequency of 1 Hz with increasing amplitude sweep. 2.3.2 Morphology and Chemical composition The surface morphology of the printed membranes was obtained by a field emission scanning electron microscope (FESEM, Hitachi S-4800, Japan). Additionally, atomic force microscopy (AFM, Cypher,Asylum Research, USA) was also exploited to analyze the surface morphologies of the membranes. For the AFM measurement, the tapping mode with scan rate of 0.5 - 1.0 Hz was utilized at room temperature, then the AFM linearity images of the printed membrane were obtained to analyze the hierarchical rough structure of the membrane surface. Moreover, the chemical composition of the membranes was performed by Fourier-transform infrared spectroscopy technique (FTIR, Nicolet 8700, Thermo Fisher Scientific Co., Ltd, USA) via the attenuated total reflection method, and the absorption spectrum of the membranes was obtained in the wavenumber range from 500 to 4000 cm-1. 8
2.3.3 Wettability The wettability of the printed membrane was performed by a contact-angle measurement apparatus (OCA 15EC, China Dataphysics Co., Ltd, China). In order to analyze the wettability of the printed membrane, both water contact angle (WCA) in air and underwater oil contact angle (OCA) of the printed membranes were characterized. For WCA measurement, a droplet of 5 μL water droplet was syringed and dropped on the membrane, and the WCA value of the membranes was computed by the drop analysis of ImageJ software. In order to characterize the underwater OCA of the membrane, a captive-bubble method was utilized. In detail, the membrane was adhered on a sample holder, and the membrane surface was faced down. Then, the sample holder was immersed in a transparent container filled with DI water, then 5 μL diesel oil droplet was deposited beneath the membrane surface via a syringe equipped with a J-hook needle. Finally, the reported underwater OCA of the membranes was the average value of five obtained data. Furthermore, the oil intrusion pressure of printed membrane was another crucial property for demonstrating the anti-oil wettability of the printed membrane during the oil/water separation, and hexane was used as typical oil compound. The maximum height of oil that the printed membrane could support determines the oil intrusion pressure of printed membrane. Therefore, the experimental intrusion pressure (Pexp) values are calculated by equation (1): (1)
𝑃𝑒𝑥𝑝 = 𝜌𝑔ℎ𝑚𝑎𝑥
where ρ is the density of hexane, g is acceleration of gravity, and hmax is the maximum height of hexane that the printed membrane can support.
2.3.4 Mechanical strength The mechanical strength of the printed membranes was assessed by a universal testing machine (XWW-20A, China) at room temperature with an invariable elongation rate of 10 mm/min. At the same time, the membrane samples were cut to a length of 40 mm before performing mechanical properties. Tensile stress is computed by equation (2) 𝐹
𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑠𝑠 = 𝑆
(2)
where F is the force exerted on an object under tension, and S is cross-sectional area of the sample. In addition, Young's modulus is obtained by equation (3): 9
𝑌𝑜𝑢𝑛𝑔′𝑠 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 =
𝐹∙𝐿 𝑆 ∙ ∆𝐿
(3)
where ΔL is the amount by which the length of the sample changes, and L is the original length of the object.
2.3.5 Oil/water separation tests For characterizing the oil/water separation efficiency of the membranes, various immiscible oil/water mixtures were prepared by mixing oil (including n-hexane, vegetable oil, lubricating oil, and diesel oil) and water in a ratio of 1:1 (v/v). The printed membrane was fixed between two quartz tubes with the diameter of 30 mm. Subsequently, the prepared oil/water mixtures were poured onto the surface of the membranes. Meanwhile, the oil content of the permeate water solution after separation was investigate by an infrared spectrometer oil content analyzer (JKY-2A, I Ke Yi Qi You Xian Gong Si, China). The separation efficiency (R (%)) is calculated according to equation (4):
(
𝐶𝑓
)
(4)
𝑅 = 1 ― 𝐶0 100%
where R (%) is the oil rejection of membrane, cf and c0 are the oil concentrations of the permeate and the original oil/water mixture, respectively. Then, the permeation flux of emulsion is calculated by equation (5) 𝑉
𝐹 = 𝐴∙𝑡
(5)
where F (L/m2.h) is the permeation flux, V (L) is the volume of filtrate, A (m2) is the effective separation area of the membranes, t (h) is the permeation time. All obtained rejection data are the average values of five measured data.
3. Results and discussion 3.1 Rheological behaviors and printability of the inks In order to successfully print the membranes with an ordered pore structure via DIW 3D printing technology, the printed inks should possess a relatively low elastic shear modulus under high shear stress for easily passing through the nozzle, and a large enough static elastic modulus to make the extruded filament “set” immediately[26, 46]. For studying the printing quality of the prepared ink, the rheological 10
behavior of the ink is characterized as a function of shear stress [26, 47-49]. The storage modulus (G’) indicates the elastic property of the ink, and loss modulus (G’’) reflects the viscous property of the inks. Hence, the effect of the SiO2 concentration and the ratio of CA/PVA on the rheological behavior of the ink were investigated, respectively, and the results were shown in Fig.2 and 3. According to Fig.2 (A), the viscosity of all the ink decrease with the increase of the shear ratio, indicating a non-Newtonian fluid property of the inks. Among the inks, the prepared ink (CP(4)-0) without SiO2 NPs displays a quite low viscosity of 76.55 Pa.s at a shear of about 1 s-1, and the storage modulus (G’) of CP (4)-0 ink is much lower than its loss modulus (G’’),as illustrated in Fig.2 (B). Besides, it can be pointed out that CP (4)-0 displays a non-pore and smooth structure. When the storage modulus (G’) of ink is much lower than its loss modulus (G’’), the ink possesses a liquid-like behavior, thus it cannot restrict the flow of ink under the force of gravity due to its too weak shear yield stress[26]. Accordingly, the ink would quickly spread on the substrate and cannot be assembled into the desired porous patterns when extruded from the nozzle, leading to the formation of a non-pore structure. Nevertheless, when SiO2 NPs was introduced into the ink, the viscosity of the ink (from CP (4)-10 to CP (4)-16)) obviously increased from 76.55 to 3227.30 Pa.s at a shear of about 1 s-1. Besides, the G’ of the ink appears to be higher than G’’ of the ink in this region at large when the SiO2 content in the ink is over 12 wt%, indicating that the inks possess a solid-like behavior. This phenomenon is caused by that the strong interaction between the SiO2 NPs and the hydroxyl from both CA and PVA is formed, thus the ink transforms into a viscoelastic fluid that exhibits obvious shear-thinning behavior[50, 51]. The shear thinning and the solid-like behaviors are necessary for the extrusion flow under pressure [52-55]. Thus, the inks with 10 wt% SiO2 NPs and 12 wt% SiO2 NPs can be printed into the porous structure, respectively, and the structure of the printed membranes still slightly collapses and form a relatively smooth porous surface due to the insufficient yield stress of the inks. However, when the SiO2 NPs content is higher than 12wt%, the printed membranes (CP (4)-14 and CP (4)-16) exhibit an order porous structure as a result of their solid-like behavior.
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Fig.2 Rheological behavior and printed structure of the inks with different SiO2 concentration. (Fsp = 500μm, the scale bars in SEM images are 300 μm)
Fig.3 Rheological behavior and printed structure of the inks with different ratio of CA/PVA. (Fsp = 500μm, the scale bars in SEM images are 300 μm) Furthermore, PVA is added into the ink for enhancing the hydrophilic of the printed membrane, which can impact the rheological behavior of the ink. As shown in Fig.3, the viscosity increase dramatically from 663.92 to 9627.30 Pa.s at a shear of about 1 s-1 when increasing the PVA ratio in the polymer of the ink from 0 to 20%. Additionally, when the ratio of PVA is higher than 5%, the G’ of ink is much higher than G’ of the inks, demonstrating that the ink exhibits a liquid-like behavior. Therefore, the printed membranes display obvious pore structure. This is because the chain entanglements could 12
occur between PVA molecular chains and CA molecular chains, thus leading to an increase of the viscosity of the inks [56]. Moreover, the increase of viscosity may be an indication of intermolecular hydrogen bonding between hydroxyl groups of PVA molecules and SiO2 NPs.
3.2 Wettability and mechanical properties of printed membranes
Fig.4 WCA and underwater OCA of printed composite membrane with (A) different SiO2 concentration and (B) different ratio of CA/ PVA. (Fsp = 500μm). The hydrophilic and underwater anti-oil fouling properties are key factors for the application of the printed membrane in oil/water separation, thus performed by WCA and underwater OCA measurements in Fig.4. As depicted in this figure, the WCA of the printed membranes (from CP (4)-0 membrane to CP (4)-16 membrane) is decreased from 42.9 ± 1.95 to 16.14 ± 2.61° with the increase of SiO2 NPs concentration. As SiO2 NPs concentration is higher than 14%, the WCAs of the printed membranes (CP (4)-14 membrane and CP (4)-16 membrane) is changed to zero in 10s. As anticipated, the underwater OCA of the printed membranes is in directly proportional to SiO2 concentration, and the underwater OCA of the printed membrane is higher than 150 ° when the SiO2 concentration is higher than 10%. Therefore, the above results indicate that both the hydrophilicity and underwater oleophobicity of the printed membrane are enhanced significantly when introducing SiO2 NPs in the membrane. This is can be explain by Young model in solid/oil/water three-phase systems (equation (S1)). Firstly, according to Fig.S1, aggregated SiO2 NPs can be observed on the membrane surface when SiO2 NPs is introduced into the printed inks. Furthermore, from the AFM linearity images in Fig.S2, the printed membranes with SiO2 NPs possess obvious hierarchical rough structures. As a result, water molecules can be trapped in the hierarchical rough structures of the membrane surface, forming a composite water-solid interface. 13
Hence, trapped water serves as a support to prevent the penetration of the oil droplets and reduce contact area between the oil and the surface of the membranes, forming an underwater superoleophobic and lowadhesive surfaces on the printed membrane. Besides that, SiO2 NPs can easily interact with water molecules due to its inherent superhydrophilicity [12, 57]. Thus, SiO2 NPs with inherent superhydrophilicity can interact with water molecules due to its water uptake characteristics, leading to produce a mass of the created hydroxyl groups on the membrane surface.[57] These polar groups can interact with water molecules through van der Waals' force and hydrogen bond, forming a water layer on the printed membrane’s surface for preventing the membranes from being wetted by oil underwater condition. Thus, the surface with polar chemical groups and hierarchical rough structure can provide superhydrophilic and underwater superoleophobic characteristics to the printed membranes. The similar results also can be observed on the printed membranes when increasing the PVA ratio in the printed membrane. PVA molecular contains a mass of polar hydroxyl group which can interact with water molecules, thus providing superhydrophilic and underwater superoleophobic characteristics to the printed membrane [43, 44]. Apart from these, depending on Fig.S3, PVA has little effect on the surface structure of the printed membranes. The content of hydrophilic hydroxyl groups in the printed membrane is proportional ton PVA ratio as illustrating in Fig.S4. Fan et.al.[44] have found that PVA with inherent hydrophilicity and underwater oleophobicity can provide superhydrophilicity and underwater oleophobicitity to filter paper for oil/water separation.
Fig. 5 The tensile stress and Young’s modulus of the printed membranes with (A) different ratio of CA/PVA and (B) different SiO2 concentration. (Fsp = 500μm) The mechanical property of the printed membranes is another crucial factor for the practical application in oil/water separation, thus the tensile stress and Young’s modulus of the printed membranes 14
are also measured, as exhibited in Fig.5. When the SiO2 NPs concentration increase in the printed membrane, resulting in the continually enhancement of the mechanical strength of the printed membrane in Fig.5 (A). This is because the strong interaction is formed between the SiO2 NPs and the hydroxyl group from CA and PVA when SiO2 was introduced in the printed membranes. Furthermore, by increasing PVA ratio in the printed inks, both of the tensile strength and Young's modulus of the printed decrease in Fig 5(B). This is because the phase separation occurred in the printed membranes after the treatment of water coagulation bath.
Fig.6 The pore size, WCA, and intrusion pressure of CP (4)-14 membrane with different Fsp. The various pore size of the printed membrane was mainly obtained by setting various FSp during the printing process, the optical images of membranes were exhibited in Fig.S5, and the effect of pore size of the printed membrane on the wettability and intrusion pressure of the membrane was also investigated in Fig.6. According to those figures, for the CP (4)-14 membrane, the pore size of the membranes increases from 103 ± 25 to 631 ± 31μm when the Fsp is changed from 300 to 800 μm. Meanwhile, when the pore size of the printed membrane is changed from 631 ± 31 to 103 ± 25 μm, the WCAs of the printed membrane decrease from 34.35 ± 3.15 to 18.31 ±1.01 °. This is because decreasing Fsp can lead to the reduction of air fraction supporting the droplets. Therefore, the intrusion pressure of CP(4)-14 also decreases from 674.51 ± 5.67 to 186.49 ± 21.62 Pa with the increase of pore size of the CP(4)-14.
15
Fig. 7 Optical images, WCAs and underwater anti-oil fouling property of CP(1)-14 membrane, CP(4)14 membrane and CA/PVA/Si mesh (Fsp = 500μm, the scale bars in SEM images are 300 μm). Additionally, CP (4)-14 membrane with about 200 μm pore size is compared with CP (1)-14 membrane with about 200 μm pore size and CA/PVA/Si composite mesh with about 300 μm pore size. The hydrophilic, underwater anti-oil-fouling property, wetting stability of all the membranes were performed by dynamic WCA, dynamic oil absorption measurements, respectively, and the results were exhibited in Fig. 7. According to this figure, it is can be pointed out that the print membranes (CP (1)-14 membrane and CP (4)-14 membrane) display an ordered porous structure. Specially, when the CA/PVA/Si layer is coated on the steel mesh surface, part of the mesh pore is blocked, resulting in forming inhomogeneous porous structures. As indicated in Fig.7, water droplet keep a stable shape on CP (1)-14 membrane surface in 10 s, and oil droplet is attached on the surface of the membrane underwater condition due to its poor hydrophilic characteristics. Nevertheless, water droplet can immerse into the surface of CP (4)-14 membrane and CA/PVA/Si mesh in 10s, and no significant deformation and residual oil were observed on the membrane surface during the process, demonstrating their superhydrophilic and quite low oil adhesion characteristics. Thus, they can be utilized to separate oil/water mixture.
16
3.3 Oil/water separation performance of the membranes
Fig.8 Separation process of CP (4)-14 membrane for (A) diesel oil/water mixture, (B) n-hexane/water mixture (n-hexane was marked with Sudan Ⅲ), (C) Separation efficiency and water flux of CP (4)-14 membrane with different pore size for various oil/water mixtures, (D) Separation efficiency of CP (4)14 for oil/water mixture with different pH.
Fig.9 The wetting stability of CP (1)-14, CP (4)-14, and CA/PVA/Si mesh before and after sonication (A)
and bending cycles (B), and SEM images (Fsp = 500μm, the scale bars in SEM images are 300 17
μm ) of CP (1)-14, CP (4)-14, and CA/PVA/Si mesh before and after sonication (C) and bending cycles (D), separation efficiency (E) and water flux (F) of CP (1)-14, CP (4)-14, and CA/PVA/Si mesh after 30 min sonication or 100 bending cycles. The oil/water separation experiment was performed by utilizing the printed membranes for dealing with various oil/water mixture, and n-hexane, diesel oil, vegetable oil, and lubricating oil were employed as model oil compounds. Before tested, the membranes were pre-wetted by DI water, and the results were exhibited in Fig.8 (A) and (B). As shown in those figures, when pouring n-hexane (dyed by Sundan Ⅲ)/water mixture on CP(4)-14 membrane surface, water can quickly pass through the membrane under the driving force of gravity, while n-hexane is rejected and kept in the upper glass tube because of superhydrophilic and underwater superoleophobic characteristics of CP(4)-14 membrane. The similar phenomenon is also observed when diesel oil/water mixture is poured on the surface of CP (4)-14 membrane. This phenomenon can be explained by the fact that both PVA and SiO2 NPs with inherent hydrophilicity are easy to form hydrogel with CA, thus water can be trapped in the CA/PVA/Si molecules network, resulting in the formation of a water layer on the surface of the membrane [44, 58]. Hence, oil can be repelled from the water layer on the surface of the membrane. Among the oil/water separation experiments, the separation efficiency and water flux of the CP (4)-14 membrane with different pore size was also investigated in Fig.8 (C). According to the results, when the pore size of the printed membrane is higher than about 230 μm, the separation efficiency of the printed membrane is significantly reduced. Additionally, the water flux of the membranes with different pore size were also exhibited in this figure. Depending on the results, the water flux of the printed membrane increased from about 149 000 to about 2160 000 L/(m2.h) when the pore size of membrane increased from about 115 to about 527 μm. Koh, et al [59] also found that 3D printed membrane with a pore size of ∼187 μm possessed extremely high water flux of about 430 000 L/(m2.h). The main reason for such a high flux is because the pore size of printed membrane is much bigger than common membranes and the printed membrane possesses superhydrophilic characteristics. Considering the practical application, CP (4)-14 membrane should possess good stability in acidic or alkaline environments for dealing with various oily wastewater, and their stability in acidic and alkaline environments was evaluated by testing the separation efficiency of n-hexane/water mixture with different pH values. According to Fig.8 (D), CP (4)-14 maintains a good stable separation efficiency about above 99.0% in acid, neutral, alkali, and seawater conditions. For comparing property, CP (1)-14 with pore size of about 200μm, CP (4)-14 with pore size of 18
about 200μm, and CA/PVA/Si mesh with pore size of about 250μm were also employed for dealing with n-hexane/water mixture. Also, the sonication and bending cycle measurements were employed for performing the wetting stability and oil/water separation property of the printed membranes in Fig.9. Firstly, both CP (4)-14 and CA/PVA/Si mesh possess good hydrophilic property and oil//water separation efficiency about 99% due to CA/PVA/Si layer’s superhydrophilic and underwater superoleophobic characteristics. However, CP (1)-14 membrane exhibits poor hydrophilic property and poor separation efficiency about 78% for oil/water mixture due to the poor hydrophilic and underwater oleophobic property of cellulose acetate. Additionally, depending on Fig.9 (E), the pristine water flux of CP (1)-14, CP (4)-14, and CA/PVA/Si mesh are about 318 000, 364 000, 117 000 L/(m2.h), respectively. The water flux of the mesh is lower than the printed membranes, which is because the CA/PVA/Si layer blocks part of the mesh pore. After 30 min sonication or 100 bending cycles, the printed membranes (containing CP (1)-14 and CP (4)-14) still possess stable hydrophilic property and oil/water separation efficiency, and the water flux of CP (1)-14 and CP (4)-14 only has a little change. This is because of that CP (1)-14 and CP (4)-14 still possess ordered structure even after the 30 min sonication or 100 bending cycles according to Fig.9 (C) and (D). The above results demonstrate that both CP (1)-14 and CP (4)-14 possess good stability even after 30 min sonication (200W) or 100 bending cycles. However, after 30 min sonication, the hydrophilic property of the CA/PVA/Si mesh obviously decrease. At the same time, the oil/water separation efficiency of CA/PVA/Si mesh is obviously reduced from 98.15± 0.21 to 46.74 ± 1.75 %, and the water flux of CA/PVA/Si mesh significantly increase. This is because CA/PVA/Si layer on the steel mesh surface is peeled off from the mesh under the sonication condition according to Fig.9(C). Similarly, when the mesh was treated with 100 bending cycles, the hydrophilic property and separation efficiency of CA/PVA/Si mesh was also reduced and the water flux of the mesh increased. As displayed in Fig.9(D), the bending cycles can irreparably destroy the pore structure of the mesh, thus the water and oil can easily pass through the destroy pore of the mesh. Thus, the results mentioned above demonstrate that CP (4)14 membrane with good mechanical strength, superhydrophilicity, and underwater superoleophobicity can be utilized to separate oil-in-water wastewater.
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Fig.10 The stability of separation efficiency of CP(4)-14 membrane after 50 times cycles, and the WCAs and OCA of CP(4)-14 before and after measurement. To further investigate the anti-fouling and stability of CP (4)-14 membrane, a cyclic filtration experiment was carried out by utilizing a n-hexane/water mixture as an example, and the membrane was washed by DI water every 10 times cycles. Meanwhile, the separation efficiency, WCA and underwater OCA of the membrane before and after 50 times cycles are exhibited in Fig.10. It is noticeable that CP (4)-14 membrane displays a high separation efficiency (above 99.0 %) even after 50 times cycles. Additionally, CP (4)-14 still maintains superhydrophilic and underwater superoleophobic characteristics even after 50 times cycles, demonstrating its good stability and anti-fouling property.
4. Conclusions In summary, a superhydrophilic and underwater superoleophobic membrane with an ordered porous structure is successfully prepared by DIW 3D printing technology, and the solution containing CA, PVA, and SiO2 nano-powders is utilized as the ink. The optimal printed membrane (CP (4)-14 membrane) exhibits good superhydrophilic and underwater superoleophobic characteristics. In addition, the membrane also displays good mechanical strength and mechanical stability even after sonication treatment or bending cycles. For the application of oil/water separation, the membrane can selectively separate various oil/water mixture with high separation efficiency of about 99.0%. Meanwhile, the printed membrane also maintains anti-oil fouling ability, thus it can be easily cleaned and reused during 20
the long time oil/water separation process. Compared with CA/PVA/Si composite steel mesh, the printed membrane possesses superior mechanical stability and oil/water separation efficiency even after sonication treatment or bending cycles. Thus the printed membrane exhibits impressive oil/water separation efficiency, mechanical stability, and reusability, which are desirable for a variety of oil/water separation application.
Acknowledgements This research was supported by National Natural Science Foundation of China (Grant nos. 21878218) and Tianjin Science and Technology Major Project (Grant nos. 18ZXSZSF00030).
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Graphical Abstract 3D
printed
robust
superhydrophilic
and
underwater
superoleophobic composite membrane for high efficient oil-water separation Xipeng Lia,b,c,d, Huiting Shana,b,c,d, Wei Zhanga,b,c,d, Baoan Lia,b,c,d * a
Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300350, PR China b
State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300350, PR China
c
Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University,
Tianjin 300350, PR China d
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300350, PR China
27
Highlight：
1. CA/PVA/Si membrane is fabricated by direct inkjet writing (DIW) 3D printing technology 2. CA/PVA/Si membrane displays superhydrophilic and underwater superoleophobic characteristics. 3. CA/PVA/Si membrane possesses good oil/water separation efficiency. 4. CA/PVA/Si membrane keeps good fouling-resistant and recyclability.
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