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water separation induced by prewetting with a superamphiphilic self-cleaning mesh
Accepted Manuscript Short communication Switchable and Simultaneous Oil/Water Separation Induced by Prewetting with a Superamphiphilic Self-Cleaning Mesh Xin Du, Shijie You, Xiuheng Wang, Qiuru Wang, Jiandong Lu PII: DOI: Reference:
S1385-8947(16)31864-2 http://dx.doi.org/10.1016/j.cej.2016.12.092 CEJ 16257
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
8 November 2016 19 December 2016 20 December 2016
Please cite this article as: X. Du, S. You, X. Wang, Q. Wang, J. Lu, Switchable and Simultaneous Oil/Water Separation Induced by Prewetting with a Superamphiphilic Self-Cleaning Mesh, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.12.092
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Short Communication
Switchable and Simultaneous Oil/Water Separation Induced by Prewetting with a Superamphiphilic Self-Cleaning Mesh
Xin Du,a Shijie You, a Xiuheng Wang,*, a Qiuru Wang, a and Jiandong Lu a
a
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of
Technology, Harbin, 150090, P. R. China * Corresponding author. E-mail: [email protected]; Fax: +86-451-86282110; Tel: +86-45186282008
Abstract: To address the challenges for the common single-mesh oil/water separation, prewetting-induced simultaneous separation methodology with the same material is developed and achieved by a self-fabricated superamphiphilic TiO2-coated stainless steel mesh with switchable transportation manners of oil and water based on its superhydrophobicity under oil and strong oleophobicity under water. Furthermore, the mesh is highly recyclable in its separation efficiency (more than 99.9%), flux and self-cleaning property, and tends to be a promising material for controllable oil/water separation.
Keywords: oil/water separation; superamphiphilic mesh; prewetting; under-water oleophobicity; under-oil superhydrophobicity
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1. Introduction Oil pollution from oil spillages and industrial processes has driven the development of technology for effective separation of oil/water mixture in recent years. Despite significant advances made, conventional methods like gravity separation, skimming, flotation, absorption and electrocoagulation are still hampered by low efficiency, high energy consumption as well as high operational complexity [1-3]. Within this context, gravity-driven filtration using materials with special surface wettability, i. e. superhydrophobicity/superoleophilicity [4-6] or under-water superoleophobicity [3,7-9], draws a growing interest for oil/water separation by virtue of their selective affinity towards oil and water [2]. However, there remain technological challenges using these materials for several reasons [10]. First, the purity of the rejected phase is so low that it is difficult to be reclaimed for use directly, unless further separation and purification are implemented. Second, the rejected liquid will accumulate gradually over the materials as the separation proceeds, which may cause the failure of the separation once the liquid pressure exceeds the maximum value the materials are able to support. To address these problems, it will be highly desirable to develop a strategy that can achieve simultaneous oil/water separation. That is to say, oil and water can pass through the separation materials simultaneously in one system without the retaining of liquid by modifying the separation materials and devices. For example, recent works reported the dual-channel separation devices for continuous and simultaneous oil/water separation on the basis of two kinds of different mesh materials with antagonistic wetting properties for oil and water [10,11]. Inspired by these phenomena, were it feasible to obtain similar results with one kind of material, the separation process would be expected more simplified, more efficient and more cost-effective.
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Recently, smart materials with the ability of responding to pH [12-14], ions [15,16], and prewetting [17-21], etc. are emerging for controllable oil/water separation. Through prewetting strategies, switchable transportation of oil and water could be realized by applying superamphiphobic or superamphiphilic materials with under-water oleophobic and under-oil hydrophobic properties. Taking advantages of the unique properties of prewetting-responsive materials, simultaneous oil/water separation will be expected to be achieved in a specially designed separation device based on just one kind of separation material (Fig. 1). To the best of our knowledge, such method has not been reported previously yet. For the reported superamphiphobic materials [17], the penetration of oil or water necessitates two-step prewetting procedures, which makes the reuse of the harvested oil or water less practical in association with the introduction of the impurities into the permeates. Besides, the application of these materials is also limited by the diminution of separation performance and lifespan as a consequence of surface fouling and pore blocking [22]. Thus, it will be highly desirable to create a self-cleaning material (e. g., TiO2 composite membranes or meshes with photoactivity [8,9,19]) in the specific field of application. Based on these considerations, we herein attempt to fabricate a superamphiphilic TiO2-coated stainless steel mesh (TSSM) to achieve prewetting-induced simultaneous oil/water separation using one kind of mesh material. Fig. 1
2. Materials and methods Sol-gel and dip-coating methods were adopted to fabricate the TSSM. Briefly, the TiO2 sol was adhered onto the hydroxylated SSM through a dip-coating procedure. Then the TiO2 solcoated SSM was dried at 100
for 2 h to obtain the TiO2-xerogel, calcined at 400
for 2 h to
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remove organic groups, and cooled naturally to room temperature. The dip-coating, drying and calcining procedures were conducted twice to repair any possible defects. More fabrication details were presented in Text S2 in Supplementary materials. The switchable and simultaneous oil/water separation performance of the TSSM were tested with single and dual-channel conceptual filtration devices with available mesh area of 2 cm2. For switchable oil/water separation, the TSSM was first prewetted with 2 mL n-hexane or water before the addition of 10 mL n-hexane/water free mixture (volume ratio=1:1) into the singlechannel filtration device. Once one separation process was finished, the TSSM was washed with ethanol and dried naturally before the next separation cycle. The separation-washing procedures were repeated 10 times to test the flux and separation efficiency variations of the TSSM with the used times. In terms of simultaneous oil/water separation, two TSSMs were prewetted by 4 mL n-hexane and water respectively before the addition of 80 mL n-hexane/water free mixture (volume ratio=1:1) into the dual-channel filtration device. In oil/water separation experiments, water was dyed blue with methylene blue. The flux of permeate (J) was calculated according to J=∆V/A, where ∆V is the volume of permeate, A is the available mesh area, and t is the penetrating time. The separation efficiency (R) of the TSSM was calculated according to R(%)=100(1-Cp/C0), where C0 and Cp are the concentrations of the rejected liquid in the pristine oil/water mixture and permeate, respectively. The chemicals and reagents, detailed fabrication procedures of the TSSM, instruments and characterizations, and self-cleaning experiments can be found in Text S1-S4 in Supplementary material.
3. Results and discussion
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First, the morphology and chemical characterizations of the TSSM were investigated. As shown in Fig. 2A, the TSSM was fabricated via sol-gel and dip-coating methods. Initially, the pristine stainless steel mesh (SSM) was hydroxylated by using 10 M HNO3 solution [23], which was confirmed by the characteristic peaks of the COH groups with enhanced intensity (Fig. S1A, B). Then, TiO2 sol was adhered onto the hydroxylated SSM via dehydration and dealcoholisation reactions between the sol and the hydroxyl-covered surface of the SSM during dip-coating process, indicated by the decrease in intensity of the characteristic peaks accounting for both COOH and COH (Fig. S1C, D). After drying and calcining treatment, the asymmetrical TiO2coated SSM could be obtained, as demonstrated by the energy dispersive X-ray (EDX) spectra (Fig. S2) and the mapping images of Ti element (Fig. 2E, G) of the two sides of the fabricated mesh, as well as the presence of Ti2p1/2 and Ti2p3/2 peaks compared with the pristine and pretreated SSM (Fig. 2I). Clearly visible that, the fabricated mesh consisted of a rough front side (Fig. 2C) and a smooth back side (Fig. 2B), with a thickness of ∼225.7 µm (Fig. 2H). The front side was composed of evenly coated mesh fibres and orderly distributed rough rectangle pores with an average size of approximately 167.8 µm × 65.2 µm (Fig. 2D). The back side (Fig. 2F) displayed smooth morphology that was similar as the uncoated pretreated SSM (Fig. S3). The main reasons for the formation of the asymmetrical structure lie in the fluidity of the TiO2 sol and the drying condition difference between the two sides due to the use of a tile substrate (Fig. 2A, more details can be found in Text S5). As illustrated by Fig. 2K, the unique asymmetrical structure of the TSSM is favorable for oil/water separation because of its capability to decrease the resistance to the transportation of liquid across the mesh and thus enhance the liquid flux (see Text S5 for more details). In particular, the TiO2 coating the SSM was anatase (Fig. 2J), the
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crystalline pattern of the highest photocatalytic activity [24,25], making it possible for the TSSM to realize self-cleaning property during separation. Fig. 2 To examine the wettability of the as-prepared TSSM in the air, contact angles were measured and compared with the pretreated SSM. As illustrated in Fig. S4, the uncoated pretreated SSM was hydrophilic (contact angle of 83.7±2.2 ° for water) and superoleophilic (contact angle of 0 ° for n-hexane). As expected, the coating of TiO2 onto the SSM led to a remarkable superhydrophilicity and superoleophilicity (Fig. 3A). This is due to the higher surface energy of TiO2 (490.0 mN m-1) [25] than that of water (72.8 mN m-1) and n-hexane (18.4 mN m-1) together with the increase of the surface roughness originating from the coating of TiO2 according to the Cassie’s mode that delineates the wettability enhancement induced by surface microstructure (Fig. S5A, B) [26,27]. Obviously, the intrinsic superamphiphilic property in the air makes it facile for the TSSM to be prewetted by both water and oil. The under-liquid wettability of the TSSM was also tested. Under water, n-hexane (density of 0.66 g cm-3) droplet was found to suspend below the surface of the TSSM and exist stably with contact angle of 147.2±3.9 ° for the front side (Fig. 3A), which was in good agreement with previous literatures [8,9]. The strong oleophobicity of the front side under water is the result of the significant decrease in the contact area between n-hexane and mesh surface caused by water adsorption [18,28]. Interestingly, a similar phenomenon could also be observed for the water contact angle under n-hexane (150.8±4.5 °), indicating the under-oil superhydrohobicity of the front side of the same material (Fig. 3A). In contrast, for the SSM uncoated with TiO2, both the n-hexane contact angle under water (115.6±11.6 °) and water contact angle under n-hexane (135.5±8.4 °) were much smaller than that of the TiO2-coated SSM (Fig. S4). This clearly
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suggests the essential role of the TiO2 coating for the TSSM in realizing the strong under-water oleophobicity and under-oil superhydrophobicity. Furthermore, the heavy oil (i. e., 1,2dichloroethane with density of 1.257 g cm-3) was also selected to demonstrate such property, giving both the under-water oil contact angle and under-oil water contact angle for the front side of the TSSM of approximately 150 ° (Fig. S6). As to the back side of the TSSM, the n-hexane contact angle under water was 123.6±2.2 ° and the water contact angle under n-hexane was 116.6±3.3 ° (Fig. 3A). The appearance of the higher contact angle of the front side than that of the back side under liquid results from the difference in roughness according to the Cassie’s mode. For better microstructure-enhanced surface wettability under liquid (Fig. S5C, D), the front side of the TSSM is designated as the working side in subsequent oil/water separation tests. Fig. 3 Then, switchable transportation manners of the TSSM induced by prewetting were substantiated by proof-of-concept experiments of free oil/water mixture separation with available mesh area of 2 cm2. Following the addition of the mixture of water and n-hexane, water was able to pass through the water-prewetted TSSM, and then was collected in a vial, whereas n-hexane was completely rejected over the mesh (Fig. 3B, Video S1). On the contrary, after being prewetted by n-hexane, the TSSM rejected water, and at the same time allowed n-hexane to penetrate (Fig. 3C, Video S2). The prewetting-induced switchable oil/water separation of the TSSM can be interpreted by Fig. 3D, showing that n-hexane is prevented from penetrating the water-prewetted mesh by an upward additional pressure (∆P) if the n-hexane contact angle θ > 90 ° under water, and vice versa (see Text S6 for theory details) [29]. The pressure ∆P, known as the intrusion pressure,
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serves as a key role in oil/water separation: a larger θ gives rise to a higher ∆P, and thus a larger maximum height (hmax) of the rejected liquid that the mesh is able to support (Eq. S2, S3). The hmax values of the TSSM were experimentally measured to be 26.5 cm for n-hexane and 10.0 cm for water (Fig. S8), suggesting that once the height of oil or water over the TSSM exceeds the hmax, the rejected liquid will penetrate the mesh, thereby failing the separation. Meanwhile, the rejected liquid was usually with low purity (cf., Fig. 3B, C) and difficult to be reclaimed for use directly [10]. To solve the aforementioned problems, simultaneous oil/water separation was realized with two TSSMs in a specially designed dual-channel separation device (Fig. 3E). As shown in Fig. 3F, the left mesh was prewetted by n-hexane and the right one by water. Subsequently, nhexane/water mixture was added into the upper part of the device. As expected, n-hexane passed through the n-hexane-prewetted mesh and then was collected in the left-hand beaker, followed by water passing through the right-hand water-prewetted mesh and being collected, too (Fig. 3F, Video S3). This is the first report about the simultaneous recovery of oil and water with one kind of mesh material but different prewetting strategies. Different from common single-mesh separation, oil and water are both the penetration moieties in the observed simultaneous oil/water separation, thus avoiding the failure of the separation caused by accumulation of the rejected liquid. Moreover, the purified oil and water can be reclaimed directly if the separation efficiency of the TSSM is high enough. The oil/water separation performance of the TSSM as to the separation efficiency and liquid flux was also explored. As can be seen in Fig. 4A, B, the TSSM mesh kept its high separation efficiency of more than 99.9% under both water-removal and oil-removal modes, and water flux and n-hexane flux undamaged even after 10 cycles of oil/water separation tests. As illustrated by
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Fig. 4B, the water flux (average value of 13554 L m-2 h-1) was higher than the n-hexane flux (average value of 7281 L m-2 h-1), though water has a higher viscosity (1.0 mPa s) than that of nhexane (0.3 mPa s). The reason is that, under water-removal mode, water is forced to pass through the water-prewetted mesh by its own gravity and hydrostatic pressure generated by nhexane due to the density difference between water and n-hexane (Video S1). For n-hexane, gravity is the only driven force (Video S2). Fig. 4 Finally, the self-cleaning property of the TSSM was verified with stearic acid as the model contaminant. After being immersed in ethanol solution of stearic acid for 1 h, the mesh was found to lose its superhydrophilicity with a water contact angle of 129.0±3.5 ° (Fig. 4C). When exposed to UV illumination (365 nm, 175 W) for 30 min, the superhydrophilicity of the TSSM recuperated owing to the degradation of the organic contaminant by highly reactive species like superoxide anions and hydroxyl radicals generated through UV illumination on anatase TiO2 (Fig. 2J) [30]. Furthermore, the TSSM was able to keep its superhydrophilicity unaffected after ten contamination-recovery cycles as shown in Fig. 4D, indicating good recyclability of the selfcleaning property of the TSSM.
4. Conclusions In summary, taking advantages of a self-fabricated TiO2-coated stainess steel mesh, the present communication provides the first demonstration of simultaneous oil/water separation with the same material to address the challenges for the common single-mesh methodology. The superamphiphilic TSSM fabricated in this work is facile to obtain under-oil superhydrophobicity and strong under-water oleophobicity through prewetting strategies, thus showing switchable
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transportation manners of oil and water and making it possible for simultaneous oil/water separation when used in a dual-channel separation device. Compared with previous reported simultaneous oil/water separation on the basis of two kinds of different mesh materials with antagonistic wetting properties for oil and water, the prewetting-induced simultaneous separation with the same material as reported in this study is expected to be more simplified, more efficient and more cost-effective. Furthermore, with highly recyclable separation efficiency (more than 99.9%), liquid flux and good self-cleaning property, the TSSM tends to be a promising candidate for oil/water separation material.
Acknowledgements This work was supported by the National Key R&D Program (No. 2016YFC0401105) and National Natural Science Foundation of China (No. 51378143). We thank the anonymous reviewers for their efforts to improve the paper.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.XXXXX.
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Figure Captions
Fig. 1. Schematic illustration of switchable and simultaneous oil/water separation induced by prewetting based on one kind of mesh material.
Fig. 2. (A) Scheme of the fabrication procedures of the TSSM. Low (B) and high (F) magnification scanning electron microscope (SEM) images of the back side of the TSSM. Low (C) and high (D) magnification SEM images of the front side of the TSSM. EDX mapping images of Ti element of the front side (E) and back side (G) of the TSSM. (H) Cross sectional SEM image of the TSSM. (I) Ti2p X-ray photoelectron spectra of the pristine SSM, pretreated SSM and TSSM. (J) X-ray diffractometer pattern of the TiO2 powder prepared by sol-gel method. (K) Schematic diagram of the variations of the flux (J) and resistance to permeate (R) with the porosity (ε), pore radius (rp ) and the distance travelled by the liquid (L) of the TSSM. The subscripts f and b refer to the front side and back side of the TSSM, respectively.
Fig. 3. (A) Static contact angle measurements for the two sides of the TSSM. W/a, n/a, n/w and w/n correspond to water contact angle in the air, n-hexane contact angle in the air, nhexane contact angle under water and water contact angle under n-hexane, respectively. (B, C) Photos of switchable water/n-hexane free mixture separation with water-prewetted (B) and n-hexane-prewetted (C) TSSMs. (D) Scheme of oil/water separation mechanism. (E) Scheme of the dual-channel oil/water separation device employed in this work. (F) Photos
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of simultaneous water/n-hexane free mixture separation using the dual-channel separation device with two TSSMs prewetted by water and n-hexane, respectively. Water is dyed blue with methylene blue in these photos.
Fig. 4. (A) Separation efficiency variations of the TSSM prewetted by water or n-hexane during multi-cycles of oil/water separation tests. (B) Water and n-hexane flux variations of the TSSM during multi-cycles of oil/water separation tests. (C) Water contact angle changes of the stearic acid-contaminated TSSM with UV exposure time. (D) Water contact angle changes of the TSSM during multi-cycles of stearic acid contamination and UV illumination-based recovery.
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Highlights
A superamphiphilic TiO2-coated stainless steel mesh is fabricated.
The mesh is superhydrophobic under oil and strongly oleophobic under water.
The mesh realizes switchable and simultaneous oil/water separation via prewetting.
The mesh shows good oil/water separation performance and self-cleaning property.
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S1385-8947(16)31864-2 http://dx.doi.org/10.1016/j.cej.2016.12.092 CEJ 16257
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
8 November 2016 19 December 2016 20 December 2016
Please cite this article as: X. Du, S. You, X. Wang, Q. Wang, J. Lu, Switchable and Simultaneous Oil/Water Separation Induced by Prewetting with a Superamphiphilic Self-Cleaning Mesh, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.12.092
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Short Communication
Switchable and Simultaneous Oil/Water Separation Induced by Prewetting with a Superamphiphilic Self-Cleaning Mesh
Xin Du,a Shijie You, a Xiuheng Wang,*, a Qiuru Wang, a and Jiandong Lu a
a
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of
Technology, Harbin, 150090, P. R. China * Corresponding author. E-mail: [email protected]; Fax: +86-451-86282110; Tel: +86-45186282008
Abstract: To address the challenges for the common single-mesh oil/water separation, prewetting-induced simultaneous separation methodology with the same material is developed and achieved by a self-fabricated superamphiphilic TiO2-coated stainless steel mesh with switchable transportation manners of oil and water based on its superhydrophobicity under oil and strong oleophobicity under water. Furthermore, the mesh is highly recyclable in its separation efficiency (more than 99.9%), flux and self-cleaning property, and tends to be a promising material for controllable oil/water separation.
Keywords: oil/water separation; superamphiphilic mesh; prewetting; under-water oleophobicity; under-oil superhydrophobicity
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1. Introduction Oil pollution from oil spillages and industrial processes has driven the development of technology for effective separation of oil/water mixture in recent years. Despite significant advances made, conventional methods like gravity separation, skimming, flotation, absorption and electrocoagulation are still hampered by low efficiency, high energy consumption as well as high operational complexity [1-3]. Within this context, gravity-driven filtration using materials with special surface wettability, i. e. superhydrophobicity/superoleophilicity [4-6] or under-water superoleophobicity [3,7-9], draws a growing interest for oil/water separation by virtue of their selective affinity towards oil and water [2]. However, there remain technological challenges using these materials for several reasons [10]. First, the purity of the rejected phase is so low that it is difficult to be reclaimed for use directly, unless further separation and purification are implemented. Second, the rejected liquid will accumulate gradually over the materials as the separation proceeds, which may cause the failure of the separation once the liquid pressure exceeds the maximum value the materials are able to support. To address these problems, it will be highly desirable to develop a strategy that can achieve simultaneous oil/water separation. That is to say, oil and water can pass through the separation materials simultaneously in one system without the retaining of liquid by modifying the separation materials and devices. For example, recent works reported the dual-channel separation devices for continuous and simultaneous oil/water separation on the basis of two kinds of different mesh materials with antagonistic wetting properties for oil and water [10,11]. Inspired by these phenomena, were it feasible to obtain similar results with one kind of material, the separation process would be expected more simplified, more efficient and more cost-effective.
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Recently, smart materials with the ability of responding to pH [12-14], ions [15,16], and prewetting [17-21], etc. are emerging for controllable oil/water separation. Through prewetting strategies, switchable transportation of oil and water could be realized by applying superamphiphobic or superamphiphilic materials with under-water oleophobic and under-oil hydrophobic properties. Taking advantages of the unique properties of prewetting-responsive materials, simultaneous oil/water separation will be expected to be achieved in a specially designed separation device based on just one kind of separation material (Fig. 1). To the best of our knowledge, such method has not been reported previously yet. For the reported superamphiphobic materials [17], the penetration of oil or water necessitates two-step prewetting procedures, which makes the reuse of the harvested oil or water less practical in association with the introduction of the impurities into the permeates. Besides, the application of these materials is also limited by the diminution of separation performance and lifespan as a consequence of surface fouling and pore blocking [22]. Thus, it will be highly desirable to create a self-cleaning material (e. g., TiO2 composite membranes or meshes with photoactivity [8,9,19]) in the specific field of application. Based on these considerations, we herein attempt to fabricate a superamphiphilic TiO2-coated stainless steel mesh (TSSM) to achieve prewetting-induced simultaneous oil/water separation using one kind of mesh material. Fig. 1
2. Materials and methods Sol-gel and dip-coating methods were adopted to fabricate the TSSM. Briefly, the TiO2 sol was adhered onto the hydroxylated SSM through a dip-coating procedure. Then the TiO2 solcoated SSM was dried at 100
for 2 h to obtain the TiO2-xerogel, calcined at 400
for 2 h to
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remove organic groups, and cooled naturally to room temperature. The dip-coating, drying and calcining procedures were conducted twice to repair any possible defects. More fabrication details were presented in Text S2 in Supplementary materials. The switchable and simultaneous oil/water separation performance of the TSSM were tested with single and dual-channel conceptual filtration devices with available mesh area of 2 cm2. For switchable oil/water separation, the TSSM was first prewetted with 2 mL n-hexane or water before the addition of 10 mL n-hexane/water free mixture (volume ratio=1:1) into the singlechannel filtration device. Once one separation process was finished, the TSSM was washed with ethanol and dried naturally before the next separation cycle. The separation-washing procedures were repeated 10 times to test the flux and separation efficiency variations of the TSSM with the used times. In terms of simultaneous oil/water separation, two TSSMs were prewetted by 4 mL n-hexane and water respectively before the addition of 80 mL n-hexane/water free mixture (volume ratio=1:1) into the dual-channel filtration device. In oil/water separation experiments, water was dyed blue with methylene blue. The flux of permeate (J) was calculated according to J=∆V/A, where ∆V is the volume of permeate, A is the available mesh area, and t is the penetrating time. The separation efficiency (R) of the TSSM was calculated according to R(%)=100(1-Cp/C0), where C0 and Cp are the concentrations of the rejected liquid in the pristine oil/water mixture and permeate, respectively. The chemicals and reagents, detailed fabrication procedures of the TSSM, instruments and characterizations, and self-cleaning experiments can be found in Text S1-S4 in Supplementary material.
3. Results and discussion
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First, the morphology and chemical characterizations of the TSSM were investigated. As shown in Fig. 2A, the TSSM was fabricated via sol-gel and dip-coating methods. Initially, the pristine stainless steel mesh (SSM) was hydroxylated by using 10 M HNO3 solution [23], which was confirmed by the characteristic peaks of the COH groups with enhanced intensity (Fig. S1A, B). Then, TiO2 sol was adhered onto the hydroxylated SSM via dehydration and dealcoholisation reactions between the sol and the hydroxyl-covered surface of the SSM during dip-coating process, indicated by the decrease in intensity of the characteristic peaks accounting for both COOH and COH (Fig. S1C, D). After drying and calcining treatment, the asymmetrical TiO2coated SSM could be obtained, as demonstrated by the energy dispersive X-ray (EDX) spectra (Fig. S2) and the mapping images of Ti element (Fig. 2E, G) of the two sides of the fabricated mesh, as well as the presence of Ti2p1/2 and Ti2p3/2 peaks compared with the pristine and pretreated SSM (Fig. 2I). Clearly visible that, the fabricated mesh consisted of a rough front side (Fig. 2C) and a smooth back side (Fig. 2B), with a thickness of ∼225.7 µm (Fig. 2H). The front side was composed of evenly coated mesh fibres and orderly distributed rough rectangle pores with an average size of approximately 167.8 µm × 65.2 µm (Fig. 2D). The back side (Fig. 2F) displayed smooth morphology that was similar as the uncoated pretreated SSM (Fig. S3). The main reasons for the formation of the asymmetrical structure lie in the fluidity of the TiO2 sol and the drying condition difference between the two sides due to the use of a tile substrate (Fig. 2A, more details can be found in Text S5). As illustrated by Fig. 2K, the unique asymmetrical structure of the TSSM is favorable for oil/water separation because of its capability to decrease the resistance to the transportation of liquid across the mesh and thus enhance the liquid flux (see Text S5 for more details). In particular, the TiO2 coating the SSM was anatase (Fig. 2J), the
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crystalline pattern of the highest photocatalytic activity [24,25], making it possible for the TSSM to realize self-cleaning property during separation. Fig. 2 To examine the wettability of the as-prepared TSSM in the air, contact angles were measured and compared with the pretreated SSM. As illustrated in Fig. S4, the uncoated pretreated SSM was hydrophilic (contact angle of 83.7±2.2 ° for water) and superoleophilic (contact angle of 0 ° for n-hexane). As expected, the coating of TiO2 onto the SSM led to a remarkable superhydrophilicity and superoleophilicity (Fig. 3A). This is due to the higher surface energy of TiO2 (490.0 mN m-1) [25] than that of water (72.8 mN m-1) and n-hexane (18.4 mN m-1) together with the increase of the surface roughness originating from the coating of TiO2 according to the Cassie’s mode that delineates the wettability enhancement induced by surface microstructure (Fig. S5A, B) [26,27]. Obviously, the intrinsic superamphiphilic property in the air makes it facile for the TSSM to be prewetted by both water and oil. The under-liquid wettability of the TSSM was also tested. Under water, n-hexane (density of 0.66 g cm-3) droplet was found to suspend below the surface of the TSSM and exist stably with contact angle of 147.2±3.9 ° for the front side (Fig. 3A), which was in good agreement with previous literatures [8,9]. The strong oleophobicity of the front side under water is the result of the significant decrease in the contact area between n-hexane and mesh surface caused by water adsorption [18,28]. Interestingly, a similar phenomenon could also be observed for the water contact angle under n-hexane (150.8±4.5 °), indicating the under-oil superhydrohobicity of the front side of the same material (Fig. 3A). In contrast, for the SSM uncoated with TiO2, both the n-hexane contact angle under water (115.6±11.6 °) and water contact angle under n-hexane (135.5±8.4 °) were much smaller than that of the TiO2-coated SSM (Fig. S4). This clearly
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suggests the essential role of the TiO2 coating for the TSSM in realizing the strong under-water oleophobicity and under-oil superhydrophobicity. Furthermore, the heavy oil (i. e., 1,2dichloroethane with density of 1.257 g cm-3) was also selected to demonstrate such property, giving both the under-water oil contact angle and under-oil water contact angle for the front side of the TSSM of approximately 150 ° (Fig. S6). As to the back side of the TSSM, the n-hexane contact angle under water was 123.6±2.2 ° and the water contact angle under n-hexane was 116.6±3.3 ° (Fig. 3A). The appearance of the higher contact angle of the front side than that of the back side under liquid results from the difference in roughness according to the Cassie’s mode. For better microstructure-enhanced surface wettability under liquid (Fig. S5C, D), the front side of the TSSM is designated as the working side in subsequent oil/water separation tests. Fig. 3 Then, switchable transportation manners of the TSSM induced by prewetting were substantiated by proof-of-concept experiments of free oil/water mixture separation with available mesh area of 2 cm2. Following the addition of the mixture of water and n-hexane, water was able to pass through the water-prewetted TSSM, and then was collected in a vial, whereas n-hexane was completely rejected over the mesh (Fig. 3B, Video S1). On the contrary, after being prewetted by n-hexane, the TSSM rejected water, and at the same time allowed n-hexane to penetrate (Fig. 3C, Video S2). The prewetting-induced switchable oil/water separation of the TSSM can be interpreted by Fig. 3D, showing that n-hexane is prevented from penetrating the water-prewetted mesh by an upward additional pressure (∆P) if the n-hexane contact angle θ > 90 ° under water, and vice versa (see Text S6 for theory details) [29]. The pressure ∆P, known as the intrusion pressure,
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serves as a key role in oil/water separation: a larger θ gives rise to a higher ∆P, and thus a larger maximum height (hmax) of the rejected liquid that the mesh is able to support (Eq. S2, S3). The hmax values of the TSSM were experimentally measured to be 26.5 cm for n-hexane and 10.0 cm for water (Fig. S8), suggesting that once the height of oil or water over the TSSM exceeds the hmax, the rejected liquid will penetrate the mesh, thereby failing the separation. Meanwhile, the rejected liquid was usually with low purity (cf., Fig. 3B, C) and difficult to be reclaimed for use directly [10]. To solve the aforementioned problems, simultaneous oil/water separation was realized with two TSSMs in a specially designed dual-channel separation device (Fig. 3E). As shown in Fig. 3F, the left mesh was prewetted by n-hexane and the right one by water. Subsequently, nhexane/water mixture was added into the upper part of the device. As expected, n-hexane passed through the n-hexane-prewetted mesh and then was collected in the left-hand beaker, followed by water passing through the right-hand water-prewetted mesh and being collected, too (Fig. 3F, Video S3). This is the first report about the simultaneous recovery of oil and water with one kind of mesh material but different prewetting strategies. Different from common single-mesh separation, oil and water are both the penetration moieties in the observed simultaneous oil/water separation, thus avoiding the failure of the separation caused by accumulation of the rejected liquid. Moreover, the purified oil and water can be reclaimed directly if the separation efficiency of the TSSM is high enough. The oil/water separation performance of the TSSM as to the separation efficiency and liquid flux was also explored. As can be seen in Fig. 4A, B, the TSSM mesh kept its high separation efficiency of more than 99.9% under both water-removal and oil-removal modes, and water flux and n-hexane flux undamaged even after 10 cycles of oil/water separation tests. As illustrated by
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Fig. 4B, the water flux (average value of 13554 L m-2 h-1) was higher than the n-hexane flux (average value of 7281 L m-2 h-1), though water has a higher viscosity (1.0 mPa s) than that of nhexane (0.3 mPa s). The reason is that, under water-removal mode, water is forced to pass through the water-prewetted mesh by its own gravity and hydrostatic pressure generated by nhexane due to the density difference between water and n-hexane (Video S1). For n-hexane, gravity is the only driven force (Video S2). Fig. 4 Finally, the self-cleaning property of the TSSM was verified with stearic acid as the model contaminant. After being immersed in ethanol solution of stearic acid for 1 h, the mesh was found to lose its superhydrophilicity with a water contact angle of 129.0±3.5 ° (Fig. 4C). When exposed to UV illumination (365 nm, 175 W) for 30 min, the superhydrophilicity of the TSSM recuperated owing to the degradation of the organic contaminant by highly reactive species like superoxide anions and hydroxyl radicals generated through UV illumination on anatase TiO2 (Fig. 2J) [30]. Furthermore, the TSSM was able to keep its superhydrophilicity unaffected after ten contamination-recovery cycles as shown in Fig. 4D, indicating good recyclability of the selfcleaning property of the TSSM.
4. Conclusions In summary, taking advantages of a self-fabricated TiO2-coated stainess steel mesh, the present communication provides the first demonstration of simultaneous oil/water separation with the same material to address the challenges for the common single-mesh methodology. The superamphiphilic TSSM fabricated in this work is facile to obtain under-oil superhydrophobicity and strong under-water oleophobicity through prewetting strategies, thus showing switchable
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transportation manners of oil and water and making it possible for simultaneous oil/water separation when used in a dual-channel separation device. Compared with previous reported simultaneous oil/water separation on the basis of two kinds of different mesh materials with antagonistic wetting properties for oil and water, the prewetting-induced simultaneous separation with the same material as reported in this study is expected to be more simplified, more efficient and more cost-effective. Furthermore, with highly recyclable separation efficiency (more than 99.9%), liquid flux and good self-cleaning property, the TSSM tends to be a promising candidate for oil/water separation material.
Acknowledgements This work was supported by the National Key R&D Program (No. 2016YFC0401105) and National Natural Science Foundation of China (No. 51378143). We thank the anonymous reviewers for their efforts to improve the paper.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.XXXXX.
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Figure Captions
Fig. 1. Schematic illustration of switchable and simultaneous oil/water separation induced by prewetting based on one kind of mesh material.
Fig. 2. (A) Scheme of the fabrication procedures of the TSSM. Low (B) and high (F) magnification scanning electron microscope (SEM) images of the back side of the TSSM. Low (C) and high (D) magnification SEM images of the front side of the TSSM. EDX mapping images of Ti element of the front side (E) and back side (G) of the TSSM. (H) Cross sectional SEM image of the TSSM. (I) Ti2p X-ray photoelectron spectra of the pristine SSM, pretreated SSM and TSSM. (J) X-ray diffractometer pattern of the TiO2 powder prepared by sol-gel method. (K) Schematic diagram of the variations of the flux (J) and resistance to permeate (R) with the porosity (ε), pore radius (rp ) and the distance travelled by the liquid (L) of the TSSM. The subscripts f and b refer to the front side and back side of the TSSM, respectively.
Fig. 3. (A) Static contact angle measurements for the two sides of the TSSM. W/a, n/a, n/w and w/n correspond to water contact angle in the air, n-hexane contact angle in the air, nhexane contact angle under water and water contact angle under n-hexane, respectively. (B, C) Photos of switchable water/n-hexane free mixture separation with water-prewetted (B) and n-hexane-prewetted (C) TSSMs. (D) Scheme of oil/water separation mechanism. (E) Scheme of the dual-channel oil/water separation device employed in this work. (F) Photos
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of simultaneous water/n-hexane free mixture separation using the dual-channel separation device with two TSSMs prewetted by water and n-hexane, respectively. Water is dyed blue with methylene blue in these photos.
Fig. 4. (A) Separation efficiency variations of the TSSM prewetted by water or n-hexane during multi-cycles of oil/water separation tests. (B) Water and n-hexane flux variations of the TSSM during multi-cycles of oil/water separation tests. (C) Water contact angle changes of the stearic acid-contaminated TSSM with UV exposure time. (D) Water contact angle changes of the TSSM during multi-cycles of stearic acid contamination and UV illumination-based recovery.
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Highlights
A superamphiphilic TiO2-coated stainless steel mesh is fabricated.
The mesh is superhydrophobic under oil and strongly oleophobic under water.
The mesh realizes switchable and simultaneous oil/water separation via prewetting.
The mesh shows good oil/water separation performance and self-cleaning property.
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