Water Separation with High

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Surface Design of Separators for Oil/Water Separation with High Separation Capacity and Mechanical Stability Yong Taek Lim, Nara Han, Wooree Jang, Wooyoung Jung, Min Oh, Seung Whan Han, Hye Young Koo, and Won San Choi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01800 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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Surface Design of Separators for Oil/Water Separation with High Separation Capacity and Mechanical Stability Yong Taek Lim‡a, Nara Han‡a, Wooree Jangb, Wooyoung Junga, Min Oha, Seung Whan Hanc, Hye Young Koob, and Won San Choia*

a

Department of Chemical and Biological Engineering, Hanbat National University, San 16-1,

Dukmyoung

dong,

Yuseong-gu,

Daejeon,

305-719,

Republic

of

Korea,

E-mail:

[email protected], Fax: +82-42-821-1692, Tel.: +82-42-821-1540 b

Korea Institute of Science and Technology (KIST) Jeonbuk Institute of Advanced Composite

Materials, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeollabuk-do, Republic of Korea c

Department of Biology, Adelphi University, 1 South Ave, 701, Garden City, NY 11530-0701

‡These authors contributed equally.

KEYWORDS: Oil/water separation, Cu mesh, simulation, convection, green method

ACS Paragon Plus Environment

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ABSTRACT

A convection heat treatment that can replace existing chemical oxidation methods was developed for the preparation of hierarchically oxidized Cu meshes with various surface morphologies, representing a very simple and green route that does not involve toxic chemicals. Three types of Cu meshes [bumpy-like (BL) and short and long needle-like (NL) structures] exhibited similar separation efficiencies of 95-99% over 20 separation cycles, as indicated by their similar water contact angles (WCAs) (147-150°). However, these Cu meshes exhibited different flux behaviours. Excessively rough and excessively smooth surfaces of the Cu mesh resulted in increased resistance to flow and to a decrease of the penetration of oil. A surface with intermediate smoothness, such as the BL-Cu mesh, was necessary for high flux over a broad range of oil viscosities. Furthermore, a less rough surface was more suitable for the separation of highly viscous oil. Computational fluid dynamics (CFD) simulations were carried out to support our experimental results. The BL-Cu meshes also showed outstanding mechanical stability because of their low resistance to the flow of fluids.


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1. INTRODUCTION Oily wastewater emitted from industry and due to daily activities results in serious worldwide environmental contamination.1,2 Frequent oil-spill accidents also pose a threat to ecosystems and human health. Thus, the clean-up of oily wastewater, including oils and organic solvents, is essential. Various materials with special wettability, such as metallic meshes, nanoparticles, polymer sponges, and filter papers, have been proposed as materials for oil/water separation.3-18 Among them, metallic meshes are widely used because of their strong mechanical properties and ease of structural modulation. Oxidized forms of metallic meshes have been generally used to obtain a selective affinity for oil over water. Wang et al. fabricated superhydrophobic Cu meshes with needle-like surface structures using a chemical treatment.19 Li et al. developed a simple and inexpensive method to prepare superhydrophobic and oleophobic Cu meshes with rough structures using a chemical etching procedure.20 Double-layer Cu meshes composed of oxidized Cu meshes have been prepared for oil/water separation and pollutant purification.21-23 Applied devices based on oxidized Cu meshes have also been fabricated for active oil/water separation benefiting from easy structural deformation of the metallic meshes.25-26 However, most previous studies have used multiple reactions involving toxic chemicals, including oxidants, strong acids and bases, for preparing oxidized Cu meshes with rough surfaces.20,23,27 Recently, several researchers have reported facile and green methods to fabricate oxidized Cu meshes. Zhang and co-workers fabricated highly dense, ordered Cu2O nanorods on a commercial phosphor-Cu mesh by varying the temperature of the aqueous solution.28 Yanlong and Zhang also reported oxidized Cu meshes prepared by a thermal oxidation method.29,30 The adopted heat treatment methods, however, were unable to synthesize oxidized Cu meshes with various surface morphologies or otherwise did not apply to oil/water separation. Hence, developing a one-step method for the


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preparation of oxidized Cu meshes with various surface morphologies without the use of toxic chemicals remains a challenge. The surface morphology of separators, including metallic meshes, is one of the important factors for effective oil/water separation. Most previous studies have focused on the preparation of rough surfaces by treating the surfaces of separators.3-18 Relatively little attention has been directed to fundamental studies explaining the relationship between the separation performance and the surface morphology of the separators. Hierarchical surfaces of metallic meshes are well known to be favourable for achieving special wettability, although their surface structures are vulnerable to strong fluid flow or mechanical stresses.3-5,23 Thus, the surface design of metallic meshes with sufficient mechanical strength to compensate for the weakness of the oxidized metallic meshes is highly desirable for high performance oil/water separation applications. Herein, we report a facile and green approach for preparing surface-structured Cu meshes and, on the basis of a comparison of our experimental and simulation results, suggest a guideline for the mechanically stable surface design of Cu meshes to withstand high fluid flux.

2. EXPERIMENTAL SECTION 2.1. Materials The Cu mesh and olive oil were purchased from Jinsung Co. and CJ Co., respectively. Diesel fuel was purchased from a local gas station. Stearic acid, Oil red O, methylene blue trihydrate, NaOH, and ammonium persulfate (APS) were purchased from Sigma-Aldrich. Hydrochloric acid (>35%), acetone, hexane (>95%), and ethanol (>99.9%) were purchased from Daejung


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Chemicals. All chemicals were used as received without further purification. Deionized (DI) water with a resistivity of 18.2 MΩ cm was obtained from a Millipore Simplicity 185 system. 2.2. Preparation of BL-, short NL-, and long NL-Cu meshes A pristine Cu mesh was sonicated sequentially in acetone, EtOH, and DI water for 5 min in each solvent to remove organic contaminants. After washing, the cleaned Cu mesh was placed in an oven at 50 °C for 30 min. To prepare bumpy-like (BL)-, short needle-like (NL)-, or long NL-Cu meshes, the Cu meshes (3 × 3 cm2) were placed in a furnace at 300, 400, or 500 °C, respectively, for 12 h under air atmosphere. The resulting Cu meshes were cooled to room temperature over 2 h. 2.3. Hydrophobic coating with stearic acid For the hydrophobic coating with low surface energy, the resultant meshes were immersed into a 0.025 M stearic acid solution (in EtOH) for 30 min. Afterwards, the meshes were washed 3 times with EtOH and dried at 50 °C for 30 min. 2.4. Characterization Scanning electron microscopy (SEM) analyses were carried out using a Hitachi S-4800. XRD patterns were obtained using an X-ray diffractometer equipped with a Cu Ka source (Rigaku). FT-IR spectra were recorded using a Fourier transform infrared spectrometer (Sinco Nicolet IS5). The UV-vis absorption spectra were recorded on a UV-vis spectrophotometer (Sinco Evolution 201). WCA measurements were carried out using a contact angle metre (SEO Phoenix 300Touch) at ambient temperature, and the volume of the probing liquid was 4 µL. CFD simulations were performed on a CFD simulator (Ansys Fluent).

3. RESULTS AND DISCUSSION


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3.1. Morphology observations and structure analyses of the surface-structured Cu meshes

Figure 1 shows a schematic for the preparation of the surface-structured Cu meshes via a simple heat treatment in a convection oven. By choosing reaction temperatures of 300 °C, 400 °C and 500 °C, we controlled the surface structures of the Cu meshes to range from BL structures to short and long NL structures with increasing needle length. After applying a hydrophobic coating to the hierarchically oxidized Cu meshes, we compared the oil/water separation performances and mechanical stabilities of the Cu meshes. Furthermore, the relationship between the surface morphology of each Cu mesh and the flux of oil was investigated. Figure 2 shows the SEM images of the resulting Cu meshes prepared at various temperatures in a convection oven; these images reveal various surface morphologies. In the case of the reaction temperature of 300 °C, relatively uniform BL structures with an average diameter of 320 nm were generated (Figures 2a and 2b). At the reaction temperature of 400 °C, a short NL morphology with flexible and slender structures started to appear, and these wires grew longer and thicker (long NL morphology) when the reaction temperature was increased to 500 °C (Figures 2de and 2gh). Temperatures lower than 300 °C or higher than 500 °C did not produce a notable surface structure on the Cu mesh. An increase in reaction temperature resulted in the growth of the NL structures, whereas a decrease in reaction temperature induced BL structures. When copper is oxidized in air, the major product is Cu2O, and CuO is formed slowly through a second oxidation step.31 In this case, Cu2O served as a precursor to CuO. The oxidation reactions of the Cu mesh upon heat treatment can be summarized as follows: 4Cu + O2 2Cu2O (1) 2Cu2O + O2 4CuO (2) The temperature effect can be understood by considering the temperature


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dependence of the Gibbs free energy of the above reaction 2.32 Heat treatment can induce the formation of additional Cu2O and accelerate the second oxidation step to form CuO,31-33 which is consistent with our XRD results. The additional Cu2O transformed into additional CuO in the long NL-Cu mesh heated at 500 ºC, compared to that in the BL-Cu mesh heated at 300 ºC (Figure S1, red and black lines). However, too high of a temperature prevents the formation of CuO needles, and thus, no needlelike structures were observed. Since the change in entropy for reaction 2 has a negative sign, the change in free energy for this reaction will change sign (from negative to positive) when the temperature is sufficiently high.32 When the temperature dropped below 400 °C, the formation of CuO was too slow to maintain sufficient growth of CuO, and thus, no needlelike growth occurred on the Cu mesh. In the case of the NL structures, similar structures have been prepared using chemical oxidation treatments.20,23,27 However, this process involves toxic chemicals such as strong acids/bases or oxidants as well as multiple steps. The convection heat treatment in our study is a very simple and green route for preparing surface-structured oxidized Cu meshes. Furthermore, the surface morphology of the Cu mesh can be easily tuned by varying the heat-treatment conditions, meaning that only slight adjustments of the experimental conditions are necessary to induce dramatic changes in morphology. To obtain the structured Cu meshes with low surface energies, stearic acid was introduced. The hydrophobic hydrocarbon chains of the stearic acid were coated onto the surface of the Cu mesh because the carboxylic acid groups of the stearic acid reacted with the hydroxyl groups of the Cu mesh.25,34 After the stearic acid coating was applied, the sizes of the bumpy particles observed in the BL structures slightly increased; however, the bumpy surface morphology was maintained (Figure 2c). The WCA of the BL-Cu mesh promptly increased and then reached a steady-state value of 147° (inset of Figure 2c). To


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examine the stearic acid coating, Fourier transform infrared (FT-IR) spectroscopy of the BL-Cu mesh was performed before and after chemical treatment with stearic acid (Figure S2). After the stearic acid coating was applied, new peaks corresponding to stearic acid appeared, demonstrating the successful coating of stearic acid onto the BL-Cu mesh (red line). The absorption bands at 2915-2848 cm−1 are attributed to the aliphatic CH (-CH3 and -CH2-) of stearic acid, and the absorption bands at 1585 and 1538 cm−1 are assigned to the carboxylate (COO) of stearic acid. Similarly, the surface morphologies of the NL-Cu meshes were maintained after the stearic acid coating was applied (Figures 2f and 2i). The WCAs of the short and long NL meshes reached 147° and 150°, respectively (insets of Figures 2f and 2i). Interestingly, although the surface morphologies of the BL and the NL-Cu meshes markedly differed from each other, the WCAs of the two structures were similar. These characteristics are attributed to the dual structures composed of grooves of several tens of nanometres in size formed on the several-hundred-nanometre-sized particles observed in the BL structures (Figure 2c). The water roll off angles of the BL-, short NL-, and long NL-Cu meshes were found to be approximately 14-15º, indicating that each mesh shows relatively low adhesion characteristics based on their analogous WCAs (147-150º) (Figure S3). 3.2. Characterization of BL- and NL-Cu meshes by XRD

The BL-Cu and long NL-Cu meshes were characterized by their XRD patterns (Figure S1). The diffraction peaks detected for the two Cu meshes were indexed to the standard patterns of Cu and oxidized Cu (black and red lines) (JCPDS Nos., CuO: 01-076-7800, Cu2O: 01-0782076). The main peaks of the BL- and long NL-Cu meshes were well matched. To compare the BL- and long NL-Cu meshes with Cu mesh obtained by chemical treatments, NL-Cu meshes were additionally prepared by chemical oxidation.20,23,27 For the chemical treatment, oxidation of


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the Cu was more progressed compared to the other samples (blue line). However, the main peaks of the three different Cu meshes were relatively well matched to each other. These results indicate that the three types of Cu meshes prepared by different oxidation methods possess analogous chemical compositions. 3.3 Oil/water separation efficiency of the BL-, short NL-, and long NL-Cu meshes

To investigate the effects of the surface morphologies of the Cu meshes on oil/water separation performance, we tested the BL-, short NL-, and long NL-Cu meshes with hydrophobic coatings for gravity-driven oil/water separation. Figure 3a-d shows progressive snapshots of the oil/water separation processes using the BL- and NL-Cu meshes. A container equipped with each mesh was used for the rapid separation of oils from an oil/water mixture. Only hexane rapidly penetrated through the mesh as soon as a mixture of a solution of hexane (red-coloured with oil red O dye) and water (blue-coloured with methylene blue (MB)) was poured into the right compartment of the chamber. However, water was not able to pass through the mesh and stayed on the right side of the chamber (Figure 3d). No visible water was observed in the collected oil, clearly exhibiting the effective separation of the oil/water mixture. The separation efficiencies of hexane were 98.2% for the BL-Cu mesh, 96.8% for the short NL-Cu mesh, and 98.2% for the long NL-Cu mesh in the first oil/water separation test (Figure 3e). The three types of Cu meshes all showed similar separation efficiencies. The recycling abilities of the Cu meshes were also investigated. After the first separation of the hexane/water mixture, all Cu meshes were thoroughly washed with ethanol and then reused for the same separation. The meshes retained hexane/water separation efficiencies greater than 96% for 20 separation cycles (Figure 3f). To further investigate the separation performance of the BL-, short NL-, and long NL-Cu meshes,


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their water intrusion pressures were measured. The intrusion pressure (P) was calculated using the following eqn (1): P = ρghmax (1) where r is the density of water, g is the acceleration of gravity, and hmax is the maximum height of water that the Cu mesh can endure. The collected water cannot spontaneously permeate the mesh below this intrusion pressure. Although the intrusion pressure (2.2 kPa) of the long NL-Cu mesh was slightly higher than that (2.1 kPa) of the BL-Cu mesh, the three types of Cu meshes showed relatively analogous intrusion pressures (2.0-2.2 kPa) (Figure 3g). 3.4. Relationship between surface morphology of the Cu mesh and oil flux - I

Various oils with different viscosities, including hexane (viscosity, µ = 0.29 cP), diesel fuel (µ = 2.0 cP), and soybean oil (µ = 51.3 cP), were used to investigate the separation capacities (flux) of the Cu meshes. Among the three types of Cu meshes-BL (WCA = 147°), short NL (5 µm, WCA = 147°), and long NL (13 µm, WCA = 150°) structures, the BL-Cu mesh exhibited the best flux performance for all three types of oils (Figure 4a). The fluxes of the BL-Cu meshes were 13,043 L/m2h for hexane, 4,477 L/m2h for diesel fuel, and 2,000 L/m2h for soybean oil, which were 145%, 125%, and 432% higher than the corresponding fluxes of the long NL-Cu meshes, respectively. We speculate that the differences in surface morphologies of each Cu mesh was responsible for such differences in flux abilities because the WCA of the BL-Cu mesh was almost the same as those of the short and long NL-Cu meshes (insets of Figures 2c, 2f, and 2i). As the surface roughness of the Cu mesh decreased, the flux of the Cu mesh increased. These phenomena were more remarkably observed in the case of the high-viscosity soybean oil than in the case of the low-viscosity hexane (Figure 4a).


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To confirm the aforementioned results indicating that a less rough surface results in a higher flux, a Cu mesh with a surface less rough than the BL-Cu mesh was prepared. Convection heat treatment of the Cu mesh at 200 °C resulted in the appearance of some striped cracks and tiny particles at the surface of the Cu mesh (Figures 4b and 4c). This mesh was used as the lessrough BL-Cu mesh. After hydrophobic coating, the original surface structure was maintained (Figures 4d and 4e); its WCA was 135°. The less-rough BL-Cu mesh exhibited lower fluxes than all types of Cu meshes for hexane and diesel fuel (Figure 4a and Table 1), suggesting that a surface less rough than the BL-Cu mesh was an excessively smooth surface, resulting in a decrease in the flux of the oil. Although the less-rough BL-Cu mesh presented lower fluxes than the BL-Cu mesh, it showed higher fluxes than the short (5 µm) and long (13 µm) NL-Cu meshes for soybean oil, even though the WCA of the less rough BL-Cu mesh was lower than those of the short and long NL-Cu meshes (Table 1). These results indicate that a surface with an intermediate roughness, like the BL-Cu mesh, is good for high flux in a broad range of oil viscosities; in particular, a less-rough surface is more suitable for the separation of highly viscous oil.

3.5. Relationship between surface morphology of the Cu mesh and oil flux - II

To clarify the relationship between the surface morphology of the Cu mesh and the oil flux, pore sections of the BL- and long NL (13 µm)-Cu meshes were thoroughly observed by SEM. The Cu mesh with a 150-µm pore was used. The BL-Cu mesh showed a negligible invasion of the pore section by its bumpy structures (Figures 5a and 5b), while the long NL (13 µm)-Cu mesh exhibited a remarkable invasion of the pore section by its long needle-like structures (Figures 5c and 5d). These results suggest that the hierarchical surface morphology of


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the Cu mesh obstructs the flow of fluids. The flow of fluids can be disturbed by this kind of obstacle within the channel due to the formation of back flow, which can disturb the main stream of fluids and retard the penetration of fluids.35,36 The flow of highly viscous fluids is more affected by obstacles than that of low viscosity fluids because highly viscous fluids can form a strong back flow when they collide with obstacles due to their viscoelastic properties.35,36 This leads to low flow rates and long retention times of the fluids. 3.6. Flow rates and retention amounts of soybean oil

To investigate the flow behaviours of highly viscous oils such as soybean oil, we measured the flow rates and retention amounts of the soybean oils upon passing the soybean oils through the various types of Cu meshes. The flow rates of the fluids were carefully measured by a manual method.35,36 The various types of Cu meshes were equipped in the interlayer of cabinet consisting of the upper and the lower chambers. Time was measured when soybean oil in upper chamber passed through the various types of Cu meshes and reached the bottom of the lower chamber with a certain distance. The flow rates of the soybean oils were calculated using the following equation: S=Vt where S is the migration distance of the fluid (m), V is the velocity of the fluid (m/s), and t is the elapsed time (s) to reach the bottom. Five repeated measurements were conducted, and the average values were recorded. The flow rates of the soybean oils were 0.014 m/s, 0.010 m/s, and 0.004 m/s when the soybean oil passed through the BL-, short NL-, and long NL-Cu meshes, respectively (Figure 6a). The flow rate through the BL-Cu mesh was more than 3 times faster than that through the long NL-Cu mesh. The amount of soybean oil retained within each structure of Cu mesh was also measured after the soybean oil passed through each Cu mesh. The


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retained oils were extracted by dipping each mesh in ethanol after the oil/water separation. The amount of soybean oil retained by the long NL-Cu mesh was 1.63 times greater than the amount retained by the BL-Cu mesh (Figure 6b). The separated volume of soybean oil was also measured as a function of increasing time. The long NL- and BL-Cu meshes showed higher outputs in the early stage of the separation because of the high initial amounts of oil (Figure 6c). For the BL-Cu mesh, 89% of the input amount was separated within 5 min, whereas only 58% of the input amount was separated for the long NL-Cu mesh within 5 min. As the surface roughness of the Cu mesh decreased, the flow rate of soybean oil increased and the retention amounts decreased. These results indicate that the main flow of the soybean oil was remarkably disturbed and delayed by the back flow formed from the rough surface of the long NL-Cu mesh. In summary, although the WCA is an important parameter for oil/water separation, the aforementioned results indicate that a simple surface morphology (herein, the BL-Cu mesh) that does not disturb the flow of fluids is more critical for achieving high flux if the morphology provides a minimum WCA necessary for oil/water separation (cf. excessively smooth surfaces of the Cu mesh result in a decrease of the penetration of oil). 3.7. Comparison of the experimental and CFD simulation results - I

To compare our experimental results with computational fluid dynamics (CFD) simulation results, we carried out dynamic simulations of the BL- and long NL-Cu meshes. The CFD simulation of the oil/water separation was carried out using a multiphase volume of fluid (VOF) model. The VOF model is a Eulerian-Eulerian formulation suitable for the separation of two or more immiscible fluids-specifically, air, water and oil. The operating and simulation conditions used in the simulation are shown in Figure S4. In the VOF formulation, a single set of momentum equations is solved while the volume fraction of each phase is tracked throughout the


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domain. Equations (1) and (2) describe the governing equations of the mass and momentum, respectively. Continuity equation n r 1 ∂ [ (α q ρ q ) + ∇ ⋅ α q ρ q vq = Sα q + ∑ ( m& pq − m& qp )]

ρq

(

∂t

)

(1)

p =1

Momentum balance equation

∂ ∂t

r

rr

r

r

r

r

( ρ v ) + ∇ ⋅ ( ρ vv ) = −∇p + ∇ ⋅ [ µ (∇v + ∇v T )] + ρ g + F

(2)

Prior to the separation of the soybean oil/water mixture, a steady-state simulation was carried out at zero separation speed to achieve numerical stability. The resulting concentration profiles of oil and water from the steady-state simulation are shown in Figure S5. The steady-state results were then used as the initial values (t = 0) for the transient simulations. Iso-surfaces of the oil volume fraction were used to show the behaviour of the oil separated at the initial stage in each case. The CFD simulation showed that the separation capacity (flux) of the BL-Cu mesh is higher than that of the long NL-Cu mesh at the initial step (Figures 7a and 7b). The simulation was further conducted for a period of 5 min. On the basis of the dynamic simulation, a higher oil separation rate occurs in the BL-Cu mesh during the separation process compared with that in the long NLCu mesh (Figures 7a1-4 and 7b1-4). After 5 min, no residual oil was observed in the right compartment for the BL-Cu mesh, whereas a large amount of oil remained for the long NL-Cu mesh (Figures 7a4 and 7b4). These CFD results indicate that the BL-Cu mesh shows a faster flow rate than the long NL-Cu mesh, in agreement with the flow-rate data shown in Figure 6a. 3.8. Comparison of experimental and CFD simulation results - II


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The CFD simulation results for the separated oil volumes were compared with experimental data. Figure 8a shows the oil volumes separated using the BL and the long NL-Cu meshes over a period of 5 min. CFD simulation results indicate that 80% of the input amount was separated for the BL-Cu mesh, whereas only 40% of the input amount was separated for the long NL-Cu mesh, consistent with the experimental results in Figure 6c. To confirm the formation of the back flow of oil, the flow pattern of the soybean oil was investigated as the oil passed through the BLand the long NL-Cu meshes. An enlarged view of the surface structure of each mesh is presented in Figure 8b. The real average dimensions of the BL and the NL structures were used for the simulated surface structures. The flow pattern across each structure is pertinent to the overall performance of each mesh. Figure 8c shows the flow patterns expressed by the velocity vectors of the oil phase near the two different structures. The flow patterns are regular around the BL structures and less regular around the NL structures. Yellow arrows are mainly observed for the BL-Cu mesh, whereas blue arrows are remarkably observed for the long NL-Cu meshes. These observations suggest that the oil smoothly passes through the BL structures, whereas oil is congested around the NL structures. This behaviour also accounts for the fast oil separation of the BL structure. That is, oil passes through the bumpy structures with no resistance, whereas the flow of oil is disturbed and delayed by a back flow formed from collision with the needle structures. The flux can be improved by controlling the pore size of the mesh. Mesh with large pores have been reported to show higher fluxes than mesh with small pores.37,38 These studies report simple relationships between the flux and pore sizes of several hundred of micrometers. However, we studied the effects of several micrometer-sized surface structures within the pores of the mesh on the flux and the stream of fluids resulting from the several micrometer-sized surface structures. Our results show that the pore size and surface structures within the pore


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affect the flux, and the stream of fluids can be controlled by varying the surface design of the pore. Thus, our study is distinguished from previous reports by the analyses of the stream of fluids, as well as the presentation of a guideline for the surface design of oil/water separators to achieve high flux and mechanical stability. 3.9. Comparison of the mechanical stability of the BL- and NL-Cu meshes under various hydraulic pressures

Oil/water separators in real applications are generally exposed to various hydraulic pressure conditions. Thus, strong mechanical strengths of the separators are necessary. We tested the mechanical stabilities of the BL- and long NL-Cu meshes by observing changes in their surface morphologies and WCAs after each mesh was exposed to different hydraulic pressures for a period of 10 min. The hydraulic-pressure tests were performed using a micro-fluid injector. A jet of fluid was focused onto an area (circle, 5 mm diameter) of the Cu mesh. After the hydraulicpressure test, the long NL-Cu mesh exhibited seriously damaged surfaces at the initial pressure of 5 bar (Figure 9a). As the hydraulic pressure increased, more remarkable damages were observed all over the mesh surface (Figure 9b and 9c). On the other hand, the BL-Cu mesh exhibited outstanding mechanical stability under the same conditions of increasing hydraulic pressure (Figure 9d-f). Under various hydraulic pressures (5, 10, 15, 20, and 25 bar), the differences (∆θ) of the WCAs for each Cu mesh before and after the hydraulic pressure tests increased gradually; however, the differences were more significant for the long NL-Cu mesh, suggesting that the hydrophobicity of the BL-Cu mesh is more mechanically robust than that of the long NL-Cu mesh (Figure 9g). These results suggest that separators with rougher surfaces are vulnerable to strong flows of fluids because of their high resistance to flow, whereas separators with less-rough surfaces offer less resistance to the flow of fluids. After the hydraulic pressure


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tests, the separation efficiencies of the BL- and long NL-Cu meshes decreased with increasing hydraulic pressure (Figure 9h). More remarkable decreases in the separation efficiency were observed for the long NL-Cu mesh, while the BL-Cu mesh exhibited a slight decrease of only 3%. These results indicate that the separation efficiency of the BL-Cu mesh was relatively maintained even after the hydraulic pressure test at various pressures. The chemical stability of the BL-Cu mesh was tested by measuring variations in the WCA after exposing the mesh to different conditions for 12 h periods. Under various chemical conditions (i.e., at different pH and in the presence of salt, NaCl), the WCAs of the BL-Cu mesh decreased slightly but remained above 143º, suggesting that the hydrophobicity is relatively robust against corrosive environments (Figures 10a and 10b). After performing the chemical stability tests on the BL-Cu mesh, hexane was used to investigate the separation capacity of the resulting BL-Cu mesh. Although the flux through the BL-Cu mesh slightly decreased to 95% and 96% under low (2) and high (14) pH compared to that under the standard conditions (pH=6, 0 M), respectively, the fluxes were 138% and 140% higher than those the long NL-Cu meshes, respectively (Figure 10c). At various NaCl concentrations, the BL-Cu mesh exhibited almost analogous flux performances without deterioration, compared to the standard conditions (pH=6, 0 M) (Figure 10c). These results indicate that the separation capacity of the BL-Cu mesh was maintained even after exposure to various chemical stimuli.

4. CONCLUSIONS Surface-structured oxidized Cu meshes with various surface morphologies were readily formed by simple convection heat treatments. Because the surface morphology of the Cu mesh can be easily tuned by changing the heat-treatment temperature without using any toxic chemicals, this


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green approach can be utilized as an alternative to the chemical methods used for the oxidation of Cu mesh. Three types of Cu meshes (BL, short NL, and long NL structures) showed similar separation efficiencies of 95-99% over 20 separation cycles, as indicated by their similar WCAs (147-150°). However, the Cu meshes exhibited different flux behaviours. In particular, a surface with intermediate smoothness, such as the BL-Cu mesh, was necessary for high flux over a broad range of oil viscosities. Furthermore, a smoother surface was more suitable for the separation of highly viscous oil. Although the WCA is an important parameter for oil/water separation, our experimental and CFD simulation results indicate that a smoother surface (herein, the BL-Cu mesh) that does not disturb the flow of fluids is more important for high flux if it maintains a certain WCA threshold for oil/water separation. The BL-Cu mesh also exhibited outstanding mechanical stability because of its low resistance to the flow of fluids. We believe that the results described herein provide a guideline for the surface design of oil/water separators for high flux and mechanical stability, which will be useful in the design of ships and facilities for oil/water separation for clean-up work and industrial processes, respectively.

ASSOCIATED CONTENT Supporting Information. XRD patterns, schematic and conditions of CFD simulation, CFD concentration profiles under steady state. The following files are available free of charge.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation

of

Korea

(NRF)

funded

by

the

Ministry

of

Education

(NRF-

2017R1D1A1B03028182).


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(15) Zhu, Q.; Pan, Q. M.; Liu, F. T. Facile Removal and Collection of Oils from Water Surfaces through Superhydrophobic and Superoleophilic Sponges. J. Phys. Chem. C 2011, 115 (35), 17464-17470. (16) Zhu, Q.; Chu, Y.; Wang, Z. K.; Chen, N.; Lin, L.; Liu, F. T.; Pan, Q. M. Robust superhydrophobic polyurethane sponge as a highly reusable oil-absorption material. J. Mater. Chem. A 2013, 1 (17), 5386-5393. (17) Xue, Z.; Cao, Y.; Liu, Na.; Feng, L.; Jiang, L. Special wettable materials for oil/water separation. J. Mater. Chem. A 2014, 2, 2445-2460. (18) Lin, X.; Chen, Y.; Liu, Na.; Cao, Y.; Xu, L.; Zhang, W.; Feng, L. In situ ultrafast separation and purification of oil/water emulsions by superwetting TiO2 nanocluster-based mesh. Nanoscale 2016, 8, 8525-8529. (19) Pan, Q.; Wang, M.; Wang, H. Separating small amount of water and hydrophobic solvents by novel superhydrophobic copper meshes. Appl. Surf. Sci. 2008, 254 (18), 6002. (20) Yin, L.; Yang, J.; Tang, Y.; Chen, L.; Liu, C.; Tang, H.; Li, C. Mechanical durability of superhydrophobic and oleophobic copper meshes. Appl. Surf. Sci. 2014, 316, 259. (21) Gao, C.; Sun, Z.; Li, K.; Chen, Y.; Cao, Y.; S. Zhang, S.; Feng, L. Integrated oil separation and water purification by a double-layer TiO2-based mesh. Energy Environ. Sci. 2013, 6 (4), 1147-1151. (22) Islam, M. S.; Choi, W. S.; Kim, S. H.; Han, O. H.; Lee, H.-J. Inorganic Micelles (Hydrophilic Core@Amphiprotic Shell) for Multiple Applications. Adv. Funct. Mater. 2015, 25 (38), 6061-6070. (23) Boakye-Ansah, S.; Lim, Y. T.; Lee, H.-J.; Choi, W. S. Structure-controllable


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superhydrophobic Cu meshes for effective separation of oils with different viscosities and aqueous pollutant purification. RSC Adv. 2016, 6 (21), 17642-17650. (24) Han, N.; Lim, Y. T.; Jang, W.; Koo, H. Y.; Choi, W. S. Polydopamine-mediated all-in-one device with superhydrophilicity and superhydrophobicity for one-step oil/water separation and pollutant purification. Polymer 2016, 107, 1-11. (25) Khosravi, M.; Azizian, S. Preparation of superhydrophobic and superoleophilic nanostructured layer on steel mesh for oil-water separation. Sep. Purif. Technol. 2017, 172, 366373. (26) Wang, F.; Lei, S.; xue, M.; Qu, J.; Li, C.; Li, W. Superhydrophobic and Superoleophilic Miniature Device for the Collection of Oils from Water Surfaces. J. Phys. Chem. C 2014, 118 (12), 6344-6351. (27) Zhang, W. X.; Wen, X. G.; Yang, S. H.; Berta, Y.; Wang, Z. L. Single-Crystalline ScrollType Nanotube Arrays of Copper Hydroxide Synthesized at Room Temperature. Adv. Mater. 2003, 15 (10), 822-825. (28) Kong, L-H.; Chen, X-H.; Yu, L-G.; Wu, Z-S.; Zhang, P-Y. Superhydrophobic Cuprous Oxide Nanostructures on Phosphor-Copper Meshes and Their Oil–Water Separation and Oil Spill Cleanup. ACS Appl. Mater. Interfaces 2015, 7, 2616-2625. (29) Yanlong, S.; Wu, Y.; Xiaojuan, F.; Yongsheng, W.; Guoren, Y.; Shuping, J. Fabrication of superhydrophobic-superoleophilic copper mesh via thermal oxidation and its application in oil– water separation. Appl. Surf. Sci. 2016, 367, 493-499.


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(30) Wang, S.; Song, Y.; Jiang, L. Microscale and nanoscale hierarchical structured mesh films with superhydrophobic and superoleophilic properties induced by long-chain fatty acids. Nanotechnology 2007, 18, 015103. (31) Adegboyega, G. A. Preparation and characterization of thermally oxidized copper substrates for photothermal and photovoltaic energy conversion. Niger, J. Renew. Energy 1990, 1, 21. (32) Jiang, X.; Herricks, T.; Xia, Y. CuO Nanowires Can Be Synthesized by Heating Copper Substrates in Air. Nano letters 2002, 2, 1333-1338. (33) Zhang, K.; Rossi, C.; Tenailleau, C.; Alphonse, P.; Chane-Ching, J-Y. Synthesis of largearea and aligned copper oxide nanowires from copper thin film on silicon substrate. Nanotechnology 2007, 18, 275607. (34) Zhang, J.; Liu, Z.; Liu, J.; E, L.; Liu, Z. Effects of seed layers on controlling of the morphology of ZnO nanostructures and superhydrophobicity of ZnO nanostructure/stearic acid composite films. Mater. Chem. Phys. 2016, 183, 306-314. (35) Barnes, H. A.; Hutton, J. F.; Walters, K. An introduction to rheology. Elsevier Science Publishers, 1989. (36) Middleman, S. Fundamentals of polymer processing. Houghton Mifflin Company, 1985. (37) Xiao, C.; Si, L.; Liu, Y.; Guan, G.; Wu, D.; Wang, Z.; Hao, X. Ultrastable coaxial cable-like superhydrophobic mesh with self-adaption effect: facile synthesis and oil/water separation application. J. Mater. Chem. A 2016, 4, 8080-8090. (38) Song, B.; Xu, Q. Highly Hydrophobic and Superoleophilic Nanofibrous Mats with Controllable Pore Sizes for Efficient Oil/Water Separation. Langmuir 2016, 32, 9960-9966.


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Figure 1. Schematic of the hierarchically oxidized Cu meshes prepared by convection heat treatments.


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Figure 2. SEM images of the hierarchically oxidized Cu meshes (ab, de, and gh) before and (c, f, and i) after a stearic acid coating was applied. (a, b) BL-, (d, e) short NL-, and (g, h) long NL-Cu meshes formed at reaction temperatures of (a, b) 300 °C, (d, e) 400 °C, and (g, h) 500 °C, respectively. (c) BL-, (f) short NL-, and (i) long NL-Cu meshes after the hydrophobic coating was applied. The insets show the corresponding WCAs. Needle length of short (5 µm) and long (13 µm) NL-Cu meshes was calculated by measuring length of needles shown in the highmagnification SEM images using a scale bar. Average length of needles was determined from measuring over 50 needles.


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Figure 3. (a-d) Photographs showing the oil/water separation process. (e) Oil/water separation efficiencies of the stearic-acid-coated Cu meshes with various types of surface morphologies (BL, short NL, and long NL structures). (f) Comparative oil/water separation efficiencies of the Cu meshes with various surface morphologies over 1-20 cycles of use. (g) Water intrusion pressures of the BL, short NL-, and long NL-Cu meshes.


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Figure 4. (a) Fluxes of oils of various viscosities, such as hexane, diesel fuel, and soybean oil, measured using four types of oxidized Cu meshes. SEM images of the less-rough BL-Cu mesh (b, c) before and (d, e) after the stearic acid coating was applied. The inset shows the corresponding WCAs.


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Length (µ µm) o

WCA ( ) Flux of hexane -2 -1 (Lm h ) Flux of diesel fuel -2 -1 (Lm h ) Flux of soybean oil -2 -1 (Lm h )

Less rough BL-Cu mesh

BL-Cu mesh

NL-Cu mesh (short)

NL-Cu mesh (long)

-

-

5 µm

13 µm

135

o

8,902 (99%) 3,162 (96%) 1020 (220%)

147

o

13,043 (145%) 4,477 (136%) 2,000 (432%)

147

o

11,538 (128%) 4,101 (124%) 1,013 (218%)

150

o

8,955 (100%) 3,296 (100%) 463 (100%)

Table 1. Comparison of the flux data of various types of Cu meshes.


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Figure 5. Low- and high-magnification SEM images of (a, d) BL-, (b, e) short NL-, and (c, f) long NL-Cu meshes prepared at reaction temperatures of (a, d) 300 °C, (b, e) 400 °C, and (c, f) 500 °C, respectively.


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Figure 6. (a) Flow rates of soybean oils through BL-, short NL-, and long NL-Cu meshes. (b) The amounts of soybean oil that remained in each Cu mesh after oil/water separation. (c) The volumes of soybean oil separated using BL- and long NL-Cu meshes as a function of time.


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Figure 7. Iso-surfaces of the oil volume fraction (value = 0.01) for monitoring the behaviour of the separated oil: (a) BL-Cu mesh and (b) long NL-Cu mesh. The concentration profile of the oil phase for (a-1-4) BL- and (b-1-4) long NL-Cu meshes as a function of increasing separation time: (a-1, b-1) 2 min, (a-2, b-2) 3 min, (a-3, b-3) 4 min, and (a-4, b-4) 5 min.


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Figure 8. (a) Simulation results for the oil volumes separated by BL- and long NL-Cu meshes. (b) The design of the mesh surface structures: (1) sketch of mesh, (2) one mesh cell, (3) bumpy, and (4) needle types. (c) The velocity vectors of the oil phase near the (left) bumpy and (right) needle structures.


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Figure 9. SEM images of the (a-c) long NL- and (d-f) BL-Cu meshes after the hydraulic pressure tests. (g) The differences (∆θ) in the WCAs of each Cu mesh before and after the hydraulic pressure tests. (h) Separation efficiencies of the BL- and long NL-Cu meshes after the hydraulic pressure tests.


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Figure 10. WCA data of the stearic acid-coated BL-Cu mesh under various stimuli: (a) pH and (b) salt. (c) Fluxes of the BL-Cu mesh after chemical stability tests under various stimuli.


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SYNOPSIS

Surface Design of Separators for Oil/Water Separation with High Separation Capacity and Mechanical Stability Yong Taek Lim‡a, Nara Han‡a, Wooree Jangb, Wooyoung Junga, Min Oha, Seung Whan Hanc, Hye Young Koob, and Won San Choia*


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Water Separation with High

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