Opposite Superwetting Nickel Meshes for On-Demand and

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Opposite Superwetting Nickel Meshes for OnDemand and Continuous Oil/Water Separation Cailong Zhou, Jinxin Feng, Jiang Cheng, Hui Zhang, Jing Lin, Xinjuan Zeng, and Pihui Pi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04517 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Industrial & Engineering Chemistry Research

Opposite Superwetting Nickel Meshes for OnDemand and Continuous Oil/Water Separation Cailong Zhou,† Jinxin Feng,† Jiang Cheng,*,† Hui Zhang,† Jing Lin,*,‡ Xinjuan Zeng,† and Pihui Pi† †

School of Chemistry and Chemical Engineering, South China University of Technology,

Guangzhou 510640, China ‡

School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006,

China

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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KEYWORDS. Nickel mesh; Underwater superoleophobicity; Superhydrophobicity; Surface modification; Oil/water separation

ABSTRACT. Oil/water separation is widely studied because of the growing discharge of industrial and domestic oily water, as well as the frequent petroleum spills, however, the ondemand and continuous oil/water separation has seldom been reported. For this purpose, we prepared opposite wetting nickel meshes by creating FeNiOx(OH)y micro/nanostructures on the surface and further modification. The surface morphology, chemical composition and the wetting property were investigated by SEM, EDS, XRD, Raman spectrum, XPS, and contact angle measurement.

The

as-prepared

superhydrophilic/underwater

superoleophobic

and

superhydrophobic/superoleophilic meshes could be used for reusable on-demand oil/water separation with high efficiency. Additionally, continuous oil/water separation was realized by integrating the opposite meshes. The current work will be beneficial for the design and development of materials with special wettabilities and the practical application of oil/water separation.

INTRODUCTION The separation of oil/water mixtures has attracted world-wide attention due to the increase of industrial oily wastewater produced by petrochemical, mining, metal/steel, textile, and food industries, as well as the frequent offshore oil spill accidents induced oil pollution.1-4 Over the past two decades, in order to separate oil/water mixtures, a great effort has been dedicated by researchers to fabricate functional materials with special wettabilities inspired from nature.5-8 These separation materials could be generally divided into two types: oil-removal type and water-removal type.9 The former oil/water separation filter based on super-wetting theory was


2

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firstly reported by Feng et al. in 2004.10 In their work, via a facile spray method, a low surfaceenergy material of polytetrafluoroethylene was coated on the stainless steel mesh, obtaining a superhydrophobic/superoleophilic (SHBOI) property that could separate oil from water. Since then, by regulating the surface hierarchical micro/nanostructures and chemical composition, various superhydrophobic/superoleophilic filters were prepared for oil/water separation.11-16 Nevertheless, the superhydrophobic/superoleophilic filter has a poor separation ability for oil/water mixtures if ρoil < ρwater. Water would sink down and stay between oil and membrane because of its higher density than that of oil, causing an appearance of water barrier layer and the discontinuity of separation proceeding.17 To address this problem, the commonly adopted solution was to apply a titled device, which could increase the contact area of oil and separation membrane.18,19 However, this method has the defects of hard to control and inferior handling capability. In contrast, a superhydrophilic/in-air superoleophobic (IASOB) membrane with oil repellence and water affinity could easily separate light oil/water mixture.20,21 But this kind of materials is difficult to design because the surface tension of water (72 mN/m) is always larger than those of oils. So the surfaces that simultaneously possess superhydrophilicity and superoleophobicity in air are energy unfavorable in theory.22,23 The fluorine constituent is requisite in fabricating superhydrophilic/superoleophobic surface for its contribution of ultralow surface energy.24 However, the fluorine compounds are quite expensive and some of them are known to be toxic such as perfluorooctanoic acid25 and perfluorooctanesulphonate26. In 2011, Jiang et al. prepared a polyacrylamide hydrogel-coated mesh with superhydrophilic/underwater superoleophobic (UWSOB) property that could be used for oil/water separation.27 This new generation of separation membrane possesses the same features of oil anti-wetting and water wetting as IASOB surface does.28-33 Even so, such underwater oil-repellent filters still are still


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facing with the difficulty of separating heavy oil/water mixtures (ρoil > ρwater) for the probable separation interruption caused by the accumulated oil layer between water and membrane. Generally, single membrane such as a SHBOI or an UWSOB filter could not satisfy the ondemand oil/water separation. Membranes possess UWSOB and underoil superhydrophobic (UOSHB) properties simultaneously34-37 or Janus membranes38-40 are the popular smart materials that could used for on-demand oil/water separation recently. The design and construction of these two kinds of materials, however, are unconventional and much difficult. Liu et al. previously designed an integrated dual-channel system that combines SHBOI and UWSOB membranes, thus achieving on-demand and continuous oil/water separation.41 Unfortunately, a 1H,1H,2H,2H-perfluorodecyltriethoxysilane modified CuO nanoneedles-coated copper mesh and a polyacrylamide hydrogel-coated copper mesh were chosen respectively in their work for fabricating SHBOI and UWSOB membranes, the two different kinds of materials might increase the complexity during the preparation. Therefore, considerable efforts need be made to develop novel materials through facile and inexpensive methods for selective and continuous separation and removal of oil pollutants from water effectively. Recently, magnetic oil/water separation materials especially for the SHBOI ones, such as macroporous

Fe/C

nanocomposites,42

Fe3O4@SiO2/PNIPAM

composite

particles,43

functionalized iron particles,44 iron oxide functionalized polyurethane foam,45 Fe3O4@PVDF/F fibrous film,46 and Fe3O4-coated cellulose sponge,47 etc. have aroused intensive interest by researchers because they could be placed on the oily water zone for absorbing oil and subsequently be removed just by an external magnetic field. However, particle-based magnetic SHBOI materials possess relatively poor oil adsorption capability, meanwhile, the difficulty in separating the as-absorbed oil from the adsorbent also limits their reusability. Therefore, coating


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porous substrates such as plastic foam with SHBOI magnetic materials seems to be a better strategy. The adhesion between substrate and the magnetic material, however, is a particular issue that needs to be considered, because once the functional magnetic materials exfoliated off the substrate, the oil/water separation ability would be decreased. In this work, we prepared a hierarchical FeNiOx(OH)y structure on nickel mesh via simple and green redox-etching and chemical precipitation reaction. Owing to the in-situ growth reaction, the adhesion between the surface structures and substrate was much desirable. Metallic nickel was selected because of its excellent properties of low cost, wide availability, inherent magnetism as well as chemical and mechanical robustness. The as-prepared mesh exhibited stable underwater superoleophobicity and could separate various oil/water mixtures. The wetting performance of the mesh could easily transform from superhydrophilicity/underwater superoleophobicity

to

superhydrophobicity/superoleophilicity

after

modification

with

polydimethylsiloxane (PDMS). By virtue of the opposite wettabilities, we strategically combined the two kinds of meshes into a dual-channel separation device and realized the continuous oil/water separation no matter how the density of oil is. Furthermore, we made a protocol to collect the oil spill by applying the natural magnetism property of the nickel mesh. Such conceptions may offer exciting opportunities either to treat complex oily wastewater in industry or to solve the problem of oil spill accidents. EXPERIMENTAL SECTION Materials.

Ethanol,

isopropanol,

hydrochloric

acid,

n-hexane,

xylene,

chloroform,

dichloromethane, Methylene Blue (MB), Methyl Orange (MO), Sudan II, ferrous chloride tetrahydrate (FeCl2·4H2O) and trisodium citrate (TSC) were of analytical grade and purchased


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from Damao Chemical Reagent Co., Ltd. (Tianjin, China). PDMS prepolymer (Sylgard 184A) and its curing agent (Sylgard 184B) were purchased from Dow Corning Co. (Shanghai, China). Gasoline was purchased from SINOPEC (Beijing, China). Soybean oil was purchased from a local market. All the above reagents were used as purchased without further purification. Nickel mesh (600 mesh, nickel content of 99.6%) was received from Hongyun Metallic Materials Co., Ltd. (Anping, China). Preparation of UWSOB-FNM and SHBOI-FNM. Nickel mesh (NM) with a size of 30 mm × 30 mm was sequentially cleaned in ethanol, isopropanol, 0.1 M hydrochloric acid solution in an ultrasonic cleaner followed with distilled water washing. In a typical preparation of underwater superoleophobic FeNiOx(OH)y-coated nickel mesh (UWSOB-FNM), the cleaned mesh was immersed into a 50 mL beaker containing of 40 mL H2O, 2 mmol of FeCl2·4H2O and 0.15 mmol of TSC. For comparison, the amount of TSC was changed gradient by 0.05, 0.10 and 0.20 mmol. The beaker was then put in an oven for 5 h at a temperature of 90 oC. After the reaction, the mesh was taken out and rinsed with distilled water. The SHBOI-FNM was obtained by immersing the UWSOB-FNM in chloroform solution containing 1 wt% PDMS prepolymer and 0.1 wt% curing agent for 5 min and subsequent curing at 80 °C for 2 h. Reversible Wettability Conversion of the FNM. The conversion of superhydrophobic PDMSmodified FNM and superhydrophilic mesh was performed by oxygen plasma treatment and heat treatment. When the superhydrophobic mesh was bombarded with oxygen plasma (provided by HARRIC PLASMA CLEANER PDC-32G-2) for 2 min, the wetting property of the surface converted into superhydrophilicity. After the superhydrophilic mesh was heated in an oven at 100 oC for 3 h, it restored the superhydrophobicity. The reversible wettability transition between


6

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superhydrophobicity and superhydrophilicity was investigated by switching between oxygen plasma treatment and heat treatment alternatively for several times. Oil/Water Separation. The oil/water separation process included two types: "filtration" of oil from oily wastewater and "removal" of oil on the water surface. For the former type, an UWSOB-FNM or a SHBOI-FNM was closely coupled into a Teflon joint, a mixture of oil/water was then poured into the upper glass tube of the separation device, the surface-affinitive liquid would pass through the membrane while the surface-repellent one could be blocked. For the latter type, a SHBOI-FNM was folded into an "oil gathering box", which could collect the oil on water surface owing to its superoleophilic feature, the collected oil could be further removed by a peristaltic pump. Instrumentation and Characterization. A field-emission scanning electron microscopy (FESEM, ZEISS Merlin) was used to observe the surface morphology. The surface roughness was analyzed by a scanning probe microscopy instrument (SPM, Bruker Multimode 8). The surface elements and composition were investigated by an energy dispersive X-ray spectroscopy (EDS, Oxford Instruments Inca400) and an X-ray photoelectron spectroscopy (XPS, PHI-5000C ESCA System). The crystal phase of the samples were confirmed by an X-ray diffractometer (XRD, Bruker D8 Advance). Raman spectrum were recorded by a micro-Ramam spectroscopy (HORIBA LabRAM Aramis) with the excitation wavelength at 532 nm. A contact angle analyzer (Shanghai Zhongchen Powereach JC2000C1) was applied to test the water contact angle (WCA) and oil contact angle (OCA) of the sample. Static magnetic property of the FNM was measured by a vibrating sample magnetometer (VSM, Quantum Design) at the temperature of 300 K. RESULTS AND DISCUSSION


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Characterization of Materials. The purchased nickel mesh was mat-like weaved by using two size of wires with the diameters of 45 and 60 µm, forming filter pores of about 37 µm × 68 µm. The surface of the bare mesh is relatively smooth (Figure 1a). After the reaction, plenty of flakelike FeNiOx(OH)y micro/nanostructures in situ grew on the mesh surface densely (Figure 1b and c). Obvious variation could be found that the appearance of nickel mesh color turned from silver to yellow (Figure S1). The surface roughness of the NM and FNM was further analyzed via the scanning probe microscopy as shown in Figure 1d and e. The surface of the NM wire is quite smooth with the Rq and Ra values of 23.9 and 18.1 nm, respectively (Figure 1d), while it turns rough after reaction in FeCl2·4H2O solution containing of TSC, which well agrees with the results from SEM images. The Rq and Ra values reach to 257 and 205 nm, respectively, significantly larger than those of pristine nickel mesh surface (Figure 1e).

Figure 1. SEM images of (a) pristine NM and (b, c) FNM. SPM and 3D profile images of the surfaces of (d) NM and (e) FNM.


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The chemical composition of FNM was characterized by an energy-dispersive X-ray spectroscopy (EDS) at a specific area as shown in Figure 2a. After reacted with FeCl2·4H2O and TSC, sharp peaks corresponding to C, O, Ni and Fe could be found, from which the atom ratio of Ni/Fe was about 4.33:1 (Table 1). The EDS-mapping implies that the chemical composition distributed homogeneously on the surface. Except for a strong peak that belongs to the Ni substrate at 44.6o in XRD pattern, there is no apparent peak assigning to other compounds (Figure 2b and Figure S2), revealing that the as-prepared micro/nanostructured coating on the nickel mesh is amorphous. To better identify the component, further characterization was conducted by XPS and Raman. The XPS results confirm the existence of Fe and Ni as shown in Figure 2c. The atom ratio of Ni/Fe was near 0.61:1 from XPS, which seems much smaller than the value measured by SEM-EDS (see Table 1). The main reason might be the detection depth of EDS is much deeper than that of XPS,48 part of Ni element came from the substrate was detected by EDS. In the Ni 2p region, two core-level peaks located at 855.3 and 872.9 eV are related to Ni2+ 2p3/2 and Ni2+ 2p1/2, respectively, along with two satellite peaks at 861.4 and 880.7 eV (Figure 2d).49 The Fe 2p peaks at 711.2 and 724.5 eV belong to the Fe 2p3/2 and Fe 2p1/2, respectively, revealing an oxidation state of Fe3+ (Figure 2e).50,51 The O 1s region can be divided into three peaks (Figure 2f): metal-hydroxyl binding energy (M-OH, 530.8 eV),52 metal-oxide binding energy (M-O-M, 529.1 eV),53 and the surface adsorbed hydroxyl groups from air (531.8 eV)54. Results of XPS indicate the co-formation of FeNi(OH)x and FeNiOx that could be integrated as FeNiOx(OH)y. However, the proportion of M-O-M is only about 8.7% in the O 1s, suggesting that only a little FeNiOx was formed on the surface. Figure 2g displays the Raman spectrum of the FNM. In detail, the two broad peaks at 455 and 533 cm-1 could belong to the symmetric NiII-OH stretching mode55 and to vibrations of the NiII-O stretching mode56,57,


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respectively. The peaks at 310 and 390 cm-1 could be attributed to a lattice E-type vibration of the NiII-O(H) modes.58,59 Notably, Wang et al. previously found that the amorphous phase of NiFe(OH)x resembles the structure of α-Ni(OH)2 according to their Raman spectra.51 Combining with the XPS and Raman results, the formation of FeNiOx(OH)y could be confirmed.

Figure 2. (a) EDS result of the FNM, the inset is the elements mapping of the single wire. (b) XRD pattern of the FNM. XPS spectra of the FNM: (c) full spectrum, (d) Ni 2p, (e) Fe 2p and (f) O 1s. (g) Raman spectrum of the FNM.


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Table 1. Elemental Concentration of the FNM Measured by EDS and XPS in Atomic% Test technique

C

O

Fe

Ni

Total

SEM-EDS

32.97

43.09

4.49

19.46

100

XPS

47.57

40.75

7.24

4.44

100

The possible formation mechanism of the FeNiOx(OH)y was further discussed. The added Fe2+ was firstly oxidized to Fe3+ with assistance of water and oxygen (eq 1), then the generated Fe3+ could redox-etch the nickel mesh and release Ni2+ (eq 2).60 Simultaneously, Ni2+ together with Fe3+ tend to precipitate on the sacrificial Ni mesh to form FeNi(OH)x nanostructure (eq 3).61 4Fe 2+ + O 2 + 2H 2 O ↔ 4Fe3+ + 4OH −

2Fe3+ + Ni → 2Fe2+ + Ni 2+

(1) (2)

Fe3+ + xNi 2+ + (2x +3)OH − → FeNi x (OH) 2x+3

(3)

On the other hand, the added TSC could combine Fe3+ in the solution to form the metal-ligand [Fe(C6H4O7)2]5- (eq 4).62 The [Fe(C6H4O7)2]5- might also react with Ni2+ to obtain the FeNi(OH)x similar as the formation of Ni2+-Fe3+-CO3 LDH (eq 5).63,64 When changed the amount of TSC, different surface morphologies would be obtained, more amount of TSC seemed to contribute to the generation of rougher micro/nanostructures within the same reaction duration (see Figure S3).

Fe3+ + 2OH − + 2C6 H 5 O 7 3− ↔ [Fe(C6 H 4 O 7 )2 ]5− + 2H 2 O

(4)

[Fe(C6 H 4 O 7 ) 2 ]5− + xNi 2+ + (2x-8)OH − + 4H 2 O → FeNi x (OH) 2x+3 + C6 H 5O 7 3−

(5)

In addition, the small quantity of FeNiOx detected by XPS probably formed through the dehydration of iron nickel hydroxide (eq 6).

FeNi x (OH) 2x+3 → FeNi x O x +1.5 + (x+1.5)H 2O


(6)

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Wetting Performance. As shown in Figure 3a, the pristine nickel mesh displays hydrophobicity with a static WCA of 119 ± 1.5o. After reaction, the uniform nanoflake structures coated surface drastically turned hydrophobic into a superhydrophilic state with a WCA of about 0o (Figure 3b). The unique surface morphology provided considerable roughness and the natural hydrophilicity of hydroxide constituent contributed the special superwetting properties. It is well known that, a superhydrophilic surface may exhibit low underwater oil adhesion. When an oil droplet contacted the as-prepared FNM surface under a certain pressure underwater, it could leave the mesh without any residue, showing an ultralow oil adhesion indeed (Figure 3c). Figure 3d and Video S1 display an experiment of typical underwater oil droplet manipulation. The oil droplets could be easily dragged to move and mix with each other rather than pinning on the mesh by using a syringe needle as the manipulator. Such an underwater superoleophobic surface possesses a unique combination of minimal contact area and an insulating water layer, resulting in an avoidance of oil contamination. For a series of oils, the pristine NM has the underwater oil contact angles (UWOCAs) between 120o and 140o, as a comparison, the corresponding values of FNM are all larger than 150o (Figure 3e), demonstrating that the as-prepared FNM has universal underwater superoleophobic property regardless of oil type. The stability of superoleophobicity is quite important for an oil-prevent filter. In our previous work, we found that a superhydrophilic/underwater superoleophobic membrane generated by fabricating sulfide and oxide micro/nanostructures on the surface may lose its superhydrophilicity and underwater superoleophobicity after storage in air.17 The surface-adsorbed hydrophobic air-born organics on the inorganic materials (especially for metals and oxides) as well as the air pockets caused by storage could cooperatively induce a tendency to more hydrophobic state. Notable that, Yu et al. recently

prepared

a

NiO

hierarchical

micro/nanostructure


coated

Ni

mesh

with

12

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superhydrophilicity and underwater superoleophobicity that can separate oil/water mixtures in harsh environments with high permeate flux.65 The storage stability of the superwetting performance, however, was regretfully not concerned. In this work, after stored in air for two weeks, the UWOCA of the as-prepared FNM maintained above 150o with low oil adhesion, exhibiting stable underwater superoleophobicity (Figure S4).

Figure 3. WCAs of (a) pristine NM and (b) FNM. (c) Underwater oil adhesion test of the FNM. (d) Underwater oil (dichloromethane dyed red) manipulation based on the UWSOB-FNM substrate. (e) UWOCAs of the NM and FNM.

After modification with PDMS, owing to the combination of the low surface energy endowed by PDMS and considerable roughness provided by the hierarchical surface structures, the resultant PDMS-modified FNM showed excellent superhydrophobicity. The EDS result confirms the existence of PDMS on the surface as shown in Figure 4a. Besides O, C, Fe and Ni elements,


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there is 5.93 At% of Si from PDMS, causing a slightly decrease of Fe and Ni compared to the unmodified FNM (Table 1). A series of experiments were taken out to observe the superhydrophobicity of the modified FNM. Water droplets could stay on the PDMS modified FNM (SHBOI-FNM) surface spherically with a CA of 153 ± 2o (Figure 4b). Not only in air, but also underoil the SHBOI-FNM possessed remarkable superhydrophobicity, water droplets exhibited spherical on its surface under n-hexane with ultralow adhesion (Figure 4c). A selfcleaning test of the superhydrophobic FNM was carried out with MO powders as probe contaminant. The mesh was fixed on a transparent glass slide with a tilted angle of ~15°. A layer of MO powder was distributed on the mesh surface and subsequently rinsed by a jet of water flow using an injector, the powder was immediately carried away by the water flow during the sliding process, just like a lotuslike self-cleaning phenomenon (Figure 4d). For the purpose of a comparison between the superhydrophobicity and superoleophilicity of the PDMS modified FNM, we took a series of snapshots to analyze the dynamic behaviors of droplet impact by using a high-speed video camera. The water droplet (10 µL) could bounce on the surface for four times in 180 ms without any obvious adhesion (Figure 4e), while oil droplet (dichloromethane, 5 µL) quickly permeated into the surface in 90 ms, showing the intensive affinity (Figure 4f).


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Figure 4. (a) EDS test of the PDMS modified FNM. Water droplets stay spherically on the SHBOI-FNM surface in air (b) and under n-hexane (c). (d) The self-cleaning process of the SHBOI-FNM, MO power was used as the model pollutant. Time-lapse photographs of (e) a water droplet bouncing on and (f) an oil droplet permeating into the SHBOI-FNM, respectively.

The relationship of superhydrophilicity and underwater superoleophobicity can be explained as below. In a water-oil-solid interface underwater, the OCA can be described as eq 7:66

cos θ OW =

γ OA cos θ OA − γ WA cos θ WA γ OW

(7)

where θOA, θWA and θOW are CAs of oil in air, water in air, and oil in water, respectively. γOA, γWA and γOW are surface tensions of oil-air, water-air, and oil-water interfaces, respectively.

When γOAcosθOA < γWAcosθWA, resulting in cosθOW < 0 (γOW > 0, constantly), hence an oleophobic state can be assured. Moreover, the oil-in-air surface tension is always lower than the value of water (γOA < γWA), so most hydrophilic surfaces can exhibit oleophobic condition at the


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water-oil-solid interface.67 Specially, underwater superoleophobic surface could be easily created when it is superhydrophilic in air. In addition, considering that when adopting a superhydrophobic/superoleophilic membrane for separating oil/water mixtures, the actual working situation is underoil-water-repellency because oil would wet the framework during the separation, so we reasonably believe that underoil superhydrophobicity is the main mechanism for a SHBOI filter to realize oil/water separation. The difference between UWSOB and SHBOI filters for oil/water separation process is that the UWSOB type membrane needs to be pre-wetted by water to form an oil-repelled water layer first, while a SHBOI membrane does not need an oil-pre-wetting process because it is inherently superhydrophobic in air. Once the separation is proceeding, a new oil-water-solid interface would form, the underoil WCA can be expressed in eq 8: cos θ WO =

γ WA cos θ WA − γ OA cos θ OA γ WO

(8)

where θOA, θWA and θWO are CAs of oil in air, water in air, and water in oil, respectively. γOA, γWA and γWO are surface tensions of oil-air, water-air, and water-oil interfaces, respectively.

When γWAcosθWA < γOAcosθOA, it will make cosθWO < 0, so a hydrophobic state can be generated. If a surface is hydrophobic in air with the CA larger than 90o (cosθWA < 0o), the resultant value of γWAcosθWA is facilely lower than that of γOAcosθOA, so that the underoil superhydrophobicity

seems easy to be achieved. The more hydrophobic in air, the more hydrophobic underoil can be obtained. The above discussion confirms the usability of a superhydrophobic/superoleophilic filter for oil/water separation.


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Figure

5.

Switchable

transition

of

the

FNM

between

superhydrophilicity

and

superhydrophobicity realized by plasma treatment and heat treatment.

By alternatively switching oxygen plasma treatment and heat treatment, we found that reversible wetting ability could be formed between a superhydrophilic mesh and a superhydrophobic mesh. Upon oxygen plasma treating for only 2 min, the superhydrophobic mesh transformed into a superhydrophilic one that was similar to the initially prepared FNM. It is because the polar functional groups containing oxygen (such as -COOH and -OH) that can lead to hydrophilicity could be introduced to the surface when treated with the oxygen plasma (Figure S5).68,69 A water droplet spreads out on the surface very fast with a WCA of 0o when dropped onto the treated mesh. Nevertheless, when the plasma treated mesh was heated at 100 oC for 3 h, the superhydrophobicity of the surface restored dramatically. During the heating, the hydrophilic surface is unstable and the non-oxidized chains of PDMS could transfer to the exterior and form a thermodynamically more stable surface, and thereby recovered the wettability, as previous work reported.70 The reversible transition of wettability was repeated three times with good reproducibility (Figure 5). Though there was slight decrease of the restored WCA, it can be fully recovered after a further simple modification using PDMS.


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Oil/Water Separation. As described above, via a facial route, we have prepared two kinds of meshes, on which water and oil displayed opposite wetting properties. Generally speaking, the UWSOB-FNM allows the passage of water, but hinders the permeation of oil, while the SHBOIFNM works oppositely. We hence used the polar opposing meshes to realize on-demand oil/water separation according to the difference of oil densities, as illustrated in Figure 6.

Figure 6. Schematic illustration of preparation of UWSOB-FNM and SHBOI-FNM for ondemand oil/water separation.

The pristine Ni mesh had no oil/water separation ability (Figure S6), while the UWSOB-FNM possessed outstanding water affinity and underwater oil repellency, making it feasible for separating of various oil/water mixtures. As shown in Figure 7a, the oil water separation device in our work consisted of two glass tubes that fixed together by a Teflon flange. Before pouring


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into the oil/water mixture, as an important step, the UWSOB-FNM should be pre-wetted with water. During the separation, oil was repelled by the surface but simultaneously water passed through the mesh rapidly (Video S2). Four kinds of light oils (xylene, n-hexane, gasoline and soybean oil) and two kinds of heavy oils (dichloromethane and chloroform) were used to observe the separation capability of the UWSOB-FNM. The separation efficiency (ES) was calculated by eq 9:71 ES =

MF × 100% MM

(9)

where MF and MM are the mass of water (or oil for the SHBOI type mesh) in the filtrate and original oil/water mixture, respectively. For all the oil/water mixtures, the separation efficiencies were above 95%, showing good separation effect (Figure 7b). Moreover, the 50 times of repeated separation results showed that the UWSOB-FNM had stable separation ability with the achieved efficiencies over 97% for n-hexane/water mixture (Figure 7c). It is notable that, the operation of separating heavy oil/water mixtures was different from that of light oil/water mixtures when adopted the UWSOB-FNM. The device should be tilted to avoid the oil barrier layer and let water permeate the membrane (Figure S7). So SHBOI-FNM is a better choice for separating the oil/water mixture when ρoil > ρwater because its hydrophobicity/oleophilicity (Figure 7d and Video S3). Due to the more suitability, the separation efficiency of SHBOI-FNM for dichloromethane (or chlorofoem)/water mixture reached above 97.5%, which is better than the UWSOB-FNM did (Figure 7e). Results of repeated separation test using the SHBOI-FNM also indicated the excellent stability (Figure 7g).


19


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Figure 7. (a) Light oil (dyed red with Sudan II)/water (dyed blue with MB) separation process using the UWSOB-FNM. (b) Oil/water separation efficiency and oil intrusion pressure of the UWSOB-FNM. (c) Separation efficiency of the UWSOB-FNM versus number of cycles. (d) Heavy oil (dyed red with Sudan II)/water (dyed blue with MB) separation using the SHBOIFNM. (e) Oil/water separation efficiency of the UWSOB-FNM for two heavy oil/water mixtures. (f) Water intrusion pressure of the SHBOI-FNM (water in the inset was dyed orange with MO). (g) Separation efficiency of the SHBOI-FNM versus number of cycles.

In practical separation applications, the filters are usually applied with a high external pressure, which has the risk of exceeding the breakthrough pressure (also called intrusion pressure, ∆p) of non-wetting phase. The breakthrough once occurs, oil (or water) is forced to permeate through the filter, sharply decreasing the separation efficiency. Therefore, the intrusion pressure of non-wetting phase is an extremely significant factor for oil/water separation filters. The experimental intrusion pressure (∆pexp) can be expressed as eq 10:11,28


20

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∆pexp =ρ ghmax

(10)

where ρ, g, and hmax are the density of the non-wetting liquid, the acceleration of gravity, and the maximum height of the non-wetting liquid layer that the mesh can support. The ∆pexp of the six tested oils on the UWSOB-FNM are all above 2.0 kPa (Figure 7b), and the value of water on the SHBOI-FNM is at least 5.88 kPa with the hmax greater than 60 cm (Figure 7f), exhibiting good pressure loading capacity of the meshes. On the other hand, the theoretical intrusion pressure (∆ptheor) can be formulated as eq 11:29,72,73

∆ptheor =

lγ (cos θ a ) 2γ =− R A

(11)

where γ is the surface tension, R is the radius of the meniscus, l and A are circumference and cross-sectional area of the mesh pore, respectively, and θa is the advancing contact angle on the surface. From eq 11, we can see that when θa is greater than 90o, the mesh can withstand some extent of pressure since ∆p > 0; While when θa < 90o, the liquid would spontaneously permeate through the mesh because ∆p < 0. According to this mechanism, water could easily pass through the as-prepared superhydrophilic FNM due to the ∆p < 0 (Figure 8a), however, the PDMS modified FNM displayed superhydrophobicity, so the θa is apparently over 90o, making water no possibility to pass the mesh spontaneously (Figure 8b). The maximum ∆ptheor that the SHBOIFNM can support is about 5.35 kPa from eq 11. In addition, water can permeate the superhydrophilic FNM and be held among the nanostructures, which would cause effective oil repellence because water is polar while oils are nonpolar. The resultant enhanced oil-repellent force leads to a remarkable superoleophobicity with the θa value larger than 90o, which implies ∆p > 0, so oil could not pass through the water-pre-wetted FNM within a certain pressure (Figure 8c). However, either the pristine FNM or the PDMS modified FNM is ultra-oleophilic in air, so


21


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oil could spontaneously permeate through the meshes because ∆p < 0 (Figure 8d). Given this fact, we could make optimal choices for on-demand oil/water separation by taking advantage of the reversibility between superhydrophilic and superhydrophobic states of the meshes.

Figure 8. Schematic diagram of oil and water wetting modes. (a) Water can permeate through the UWSOB-FNM because ∆p < 0; (b) water cannot pass through the SHBOI-FNM because ∆p > 0; (c) oil cannot permeate through the water-pre-wetted UWSOB-FNM because ∆p > 0; (d) oil can pass the UWSOB-FNM or SHBOI-FNM because ∆p < 0.

Although the UWSOB-FNM and SHBOI-FNM could be applied to achieve on-demand oil/water separation according to the density of oil, the single separation process is still discontinuous. In order to realize continuous oil/water separation disregarding the density of oil, the UWSOB-FNM and SHBOI-FNM were installed on the openings on each side of an inverted T-shape three limb tube. When the oil/water mixture is poured into the upright tube, water can only penetrate through the UWSOB-FNM while oil just goes through the SHBOI-FNM. As shown in Fig. 9a and b (see Video S4 and Video S5 in detail), no matter whether the density of oil is lower or higher than that of water, the continuous oil/water separation process could be


22

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accomplished by using a peristaltic pump as the transmission equipment. Moreover, both the UWSOB-FNM and SHBOI-FNM displayed high fluxes of about 1.47 × 104 and 2.39 × 104 L m-2 h-1, respectively (Figure 9c), which are calculated as eq 12:

F=

V S ×t

(12)

where F is the liquid flux (L m-2 h-1), V is the permeate volume (L), S is the effective separation area (m2), and t is the permeation time (h).

Figure 9. An inverted T-shaped device for continuous separation of oil/water mixtures without considering density: (a) ρoil < ρwater and (b) ρoil > ρwater. (c) The liquid flux of the UWSOB-FNM and SHBOI-FNM.

As mentioned above, the oil/water separation process generally includes two types: "filtration" and "removal". When treating the oil spill in seawater, the "filtration" method is inappropriate because the oily seawater should be collected firstly, herein, the direct "removal" of oil from water surface seems much more suitable. Figure 10a (see also Video S6) shows a continuous removal process realized by using a SHBOI-FNM folded "oil gathering box" and a peristaltic pump. Upon placing the box on the oil spill and running the pump at 150 rpm, we can clearly see that the red oil could be quickly absorbed into the box, transported through the pipe and collected


23


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in the beaker, and the whole process takes only a few seconds. There was no obvious water in the gathered oil or oil remained on the water surface, indicating good treating ability (Figure 10b and c). As shown in Figure 10d and e, owing to the special magnetic property of the Ni mesh (saturation magnetization Ms = 49.2 emu·g-1, Figure 10f), the box can be easily taken out by a magnet, which facilitates remote control after the oil retrieving.

Figure 10. (a) Continuous oil collection by using the SHBOI-FNM folded box and a peristaltic pump. Final state of (b) the collected oil and (c) the box when the floating oil was completely eliminated. Inset of (c) is the SHBOI box floating on water surface, showing good superhydrophobicity. (d) The box could easily be removed by a magnet after the oil collection. (e) Photograph showing the FNM folded box tightly adsorbed on a magnet. (f) Roomtemperature magnetic hysteresis loop of the FNM.

Only a few works reported nickel based oil/water separation materials, and we summarized some of them as shown in Table 2. Most of the previous references chose 3D nickel foam as the


24

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substrate, coating with inorganic material (such as NiCo2O4) and organic materials (such as polypyrrole and polyacrylamide hydrogel) to generate roughness or to obtain special wetting films. Nevertheless, 2D nickel mesh based oil/water materials are quite infrequent. Compared with the other literatures, the fabrication route in our work seems to be more facile and meaningful.

Table 2. Summary of Some Nickel Based Oil/Water Separation Materials Substrate

Chemical composition

Method

Wettability

Operability

Ref.

Ni foam

C/Ni

H2/CH4 plasma

SHBOI

complex

74

Ni foam

NiCo2O4

hydrothermal method

UWSOB

moderate

75

Ni foam

polypyrrole/fluoroalkylsilane

electrodepositing

SHBOI

moderate

76

Ni foam

polyacrylamide hydrogel

solution-immersion

UWSOB

simple

77

Ni foam

dopamine/octadecylamine

solution-immersion

SHBOI

simple

78

Ni foam

candle soot/PDMS

flame envelope

SHBOI

simple

79

Ni mesh

zwitterionic coatings

UWSOB

complex

80

Ni mesh

NiO

UWSOB

moderate

65

Ni mesh

FeNiOx(OH)y, FeNiOx(OH)y/PDMS

initiated chemical vapor deposition (iCVD) heat treatment at 1000 oC solution-immersion

UWSOB, SHBOI

simple

This work

CONCLUSIONS In summary, facile and environmentally friendly redox-etching and chemical precipitation reaction were demonstrated for the preparation of superhydrophilic FeNiOx(OH)y nanostructured nickel mesh. The mesh transformed to superhydrophobic after modification with PDMS. By alternatively switching oxygen plasma treatment and heat treatment, the wetting ability could be obtained between the superhydrophilic mesh and the superhydrophobic mesh. We could make optimal choices for on-demand oil/water separation according to the density of oil by taking


25


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advantage of the opposite wettability of the meshes. Furthermore, continuous oil/water separation disregarding the density of oil was realized by integrating the opposite meshes in an inverted T-shape three limb tube. By virtue of superhydrophobicity/superoleophilicity of the PDMS modified mesh, continuous oil removal was also achieved. This w considers of the practical oily water treating conditions and provides reasonable and systemic solutions.

ASSOCIATED CONTENT Supporting Information. Other information of the pristine NM and FNM. (PDF) Manipulation of oil droplet underwater. (AVI) Separation of n-hexane/water mixture. (AVI) Separation of dichloromethane/water mixture. (AVI) Continuous separation of light oil/water mixture. (AVI) Continuous separation of heavy oil/water mixture. (AVI) Continuous removal of oil from water surface. (AVI)

AUTHOR INFORMATION Corresponding Author Jiang Cheng Jing Lin

E-mail: [email protected]; E-mail: [email protected];

Tel: +86-20-87112057. Tel: +86-13798199726.

ORCID Jiang Cheng: 0000-0002-9947-6193 Jing Lin: 0000-0003-0486-8280


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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21776094), Science and Technology Planning Project of Guangdong Province (2014A010105052, 2017A010103039), Natural Science Foundation of Guangdong Province (2015A030313506) and National Undergraduate Innovative and Entrepreneurial Training Program (201710561091).

ABBREVIATIONS NM = nickel mesh FNM = FeNiOx(OH)y-coated nickel mesh IASOB = superhydrophilic/in-air superoleophobic UWSOB = superhydrophilic/underwater superoleophobic SHBOI = superhydrophobic/superoleophilic UOSHB = underoil superhydrophobic CA = contact angle WCA = water contact angle UWOCA = underwater oil contact angle MB = methylene blue MO = methyl orange


27


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TSC = trisodium citrate PDMS = polydimethylsiloxane SEM = scanning electron microscopy XRD = X-ray diffractometer EDS = energy dispersive X-ray spectrometry XPS = X-ray photoelectron spectroscopy SPM = scanning probe microscopy VSM = vibrating sample magnetometer

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Opposite Superwetting Nickel Meshes for On-Demand and

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