Water Separations from Nanosized Superhydrophobic to

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Oil/water separations from nano-sized superhydrophobic to micro-sized under-oil superhydrophilic dust Mengying Long, Shan Peng, Wanshun Deng, Ni Wen, Qiannan Zhou, and Wenli Deng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00615 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Applied Nano Materials

Oil/water separations from nano-sized superhydrophobic to micro-sized under-oil superhydrophilic dust Mengying Long, Shan Peng, Wanshun Deng, Ni Wen, Qiannan Zhou, Wenli Deng* College of Materials Science and Engineering, South China University of Technology, Wushan Road, Tianhe District, Guangzhou 510640, PR China E-mail: [email protected]. Tel.: (+86)020-22236708.

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Abstract Dust which can be seen almost everywhere in real life, often dirties the building, car, cloth, and so on. Humans have come up with many ideas to wash it out for a cleaning world. While in the point of material researchers, dust is composed of particles in different scales. In this work, for the first time, we try to make full use of the dust to separate various types of oil/water mixtures, recycling the waste materials. The tiny dust particles mixed with polydimethylsiloxane could realize superhydrophobicity. The superhydrophobic dust coating could be spray-coated onto glass, sponge, and steel net substrates, and achieved self-cleaning, oil absorption, and immiscible heavy oil/water separation, respectively. The tiny dust particles and a kind of organic binder were adhered on the steel net. The membrane was superamphiphilic in air and superoleophobic under water, and could separate immiscible light oil/water mixtures efficiently. The larger dust particles could be stacked to form a dust layer. This layer was underoil superhydrophilic, and quickly separated the surfactant stabilized water-in-oil emulsion. Dust with different simple treatments could separate various types of oil/water mixtures only under gravity.

Keyword: dust, spray-coating, oil/water separation, emulsion separation, superwettability

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Introduction Learning from the superhydrophobic lotus leaf, rose leaf, and underwater superoleophobic fish skin, artificial superwetting surfaces have been widely fabricated in recent years.1–6 The lotus inspired superhydrophobic surfaces usually have unique properties, such as self-cleaning, anti-fouling, anti-icing, and anti-corrosion properties.7–10 Since then, a series of superlyophobic and superlyophilic materials in air or liquid phase were broadly researched. Owing to the different superwettability toward water and oils, the surfaces are the ideal materials to separate oil/water mixtures. Since the oil-polluted water and frequent oil-spill accidents have become worldwide problems, it is urgent to explore some approaches to solve these problems. Generally, there are three different states of oil/water mixtures, including floating oil, immiscible oil/water mixtures, and emulsion. Different strategies should be chosen to separate various types of oil/water mixtures. By coating of superhydrophobic/superoleophilic particles, sponge was an effective substrate to absorb the floating oil.11–14 Initially, Lin et al. firstly spray-coated a superhydrophobic/superoleophilic coating on steel net to efficiently separate immiscible oil/water mixtures.15 The superhydrophobic/superoleophilic materials were desired to remove the heavy oil from the mixtures. Since then, lots of superhydrophobic/superoleophilic materials were prepared on filter substrates, like filter paper, fabric, metal net, and so forth, to separate immiscible oil/water mixtures.16–19 Various approaches for successfully obtaining superhydrophobic/superoleophilic materials have been published, such as electrospinning, plasma, chemical vapor deposition, hydrothermal methods, and so on.20–23 However, these approaches always need the expensive equipment, complex processes for unique structures, high energy consumption, or poisonous pristine materials. Our superhydrophobic/superoleophilic coating composed by dust and polydimethylsiloxane (PDMS) was accessible, cheap, and environmental friendly. Materials with opposite superwettability toward water and oil often remove heavy oil and remain water. In addition, by mixing dust particles and an inorganic binder, we also prepared the superamphiphilic membrane, which usually remove water from the immiscible light oil/water mixtures. Compared with the oil floating and immiscible oil/water mixtures, it is harder to separate emulsified oil/water mixtures stabilized by surfactant, commonly known as the emulsion, which usually contains lots of steadily-linked micro-size droplets (diameter less than 20 µm). In order to separate stable emulsion, the membrane should usually have smaller pores.24 Superwettable particles with special treatments were often coated on the metal net, filter paper, and fiber to fabricate small pores to separate water-in-oil emulsion.25–27 However, the tight combination among the particles and substrates was a problem. In addition, the membranes were very thin, and surfactant would be absorbed in the pores to block them, leading to the decreasing flux. The extremely small-sized pores also resulted in some challenges to separate surfactant stabilized emulsion, such as complex fabrication, energy consumption, and slow flux. Therefore, a simple and low-cost fabrication of large numbers of pores was 3



required to solve these problems. On a raining day, the rain would quickly infiltrate into the dust, and completely wet them. The life experience reveals that the dust is superhydrophilic. In theory, dust with special superwettability could separate oil/water mixtures. Besides, dust is common to find in daily life, and composed of inorganic particles with varying sizes. Different from the complex fabrications of particles with specific rough topography, the surface of naturally obtained dust particles was rough with irregular microstructure and flake-like nanostructure. Herein, dust was divided into tiny and large particles to separate diverse kinds of oil/water mixtures. Low-surface-energy polydimethylsiloxane (PDMS) and tiny dust particles were mixed to realize superhydrophobicity/superoleophilicity. Floating oils and immiscible heavy oil/water mixtures could be successfully separated by coating these superhydrophobic/superoleophilic particles on sponge and steel net, respectively. A superamphiphilic coating combined with tiny particles and inorganic adhesive was used to separate the immiscible light oil/water mixtures. Furthermore, the large particles formed a layer with large numbers of pores between each particle. The layer with superhydrophilic under oil would capture tiny water droplets in emulsion to purify the oil. Our treatments are quite simple, green, and recycling the waste dust, which may be beneficial for current oil/water separations. Moreover, making full use of the dust to separate various types of oil/water mixtures is never reported as far as we know.

Experimental Materials The dust was collected from the garden (SCUT in Guangzhou, China). The collected dust is a kind of red clay, which widely distributed in low latitudes around the world. After the drying treatment at 120 oC for 6 h, the dust was sifted by the steel net (250#). After the drying and sifting treatments, other dopants, such as the stones and tree branches, were cleared out. The subsequently ball-milling treatment (400 r/min, 4 h) would be conducted to obtain the tiny dust particles in more homogeneous size. The jammed dust was further sifted by another steel net (24#) to obtain the larger dust particles. PDMS prepolymer and its curing agent were purchased from Dow Corning Company in USA, and they were controlled at a weight ratio of 10:1 in the work. Other chemicals were analytical grade and used as received. Preparation of the superhydrophobic dust coating PDMS prepolymer (between 0.05 g and 1.0 g) and its curing agent were dispersed into 5 g of octane by magnetically stirring for 10 min. Next, tiny dust particles (1 g) were respectively poured into the each coating formulation, and the solutions were magnetically stirred for 30 min at room temperature. Immediately, the solutions were spray-coated on a glass slide by using an airbrush with 0.3 MPa of spraying pressure at a distance of about 15 cm, and the spray-coating conditions were used as the same in this work. Next, the samples were cured in an oven at 80 ºC for 30 min. The superhydrophobic coating (0.4 g of PDMS) also can be spray-coated on a clean sponge or steel net (1000#) substrate to realize 4


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superhydrophobicity/superoleophilicity for absorption of a small amount of oil and separation of immiscible heavy oil/water mixtures, respectively. Preparation of the superamphiphilic dust coating The AP binder was prepared according to the article.28 The processes were as follows: 85% of H3PO4 was attenuated to 60%, and then Al(OH)3 (H3PO4:Al(OH)3=3:1, molar ratio) was poured under magnetically stirring at 100 oC for 3 h. The AP binder (2 g) was diluted into 20 mL of deionized water, and tiny dust particles (1 g) were dispersed into the solution and magnetically stirred for 30 min. After that, the prepared coating was spray-coated on a steel net (1000#) substrate. This membrane was used to separate the immiscible light oil/water mixtures. Preparation of the dust layer The larger dust particles were washed by deionized water for several times until the washed water was transparent and clean, and dried at 120 oC for 3 h in preparation. A piece of non-woven fabric is fixed at one end of a glass tube (diameter = 1.34 cm) by a latex ring, and then the larger dust particles were poured into the glass tube with a certain height. The dust layer was applied for separation of the surfactant stabilized water-in-oil emulsion. Preparation of surfactant stabilized water-in-oil emulsion Span 80, one of the surfactants, was used to stabilize the water-in-oil emulsion. 0.02 g of span 80 was first dissolved into 49 mL of isooctane, and then 1 mL of deionized water was dropped into the oil with a magnetic stirring speed at 1000 r/min for 6 h. Finally, the stable milk-like water-in-oil emulsion was obtained. Characterization A field emission scanning electron microscopy (FESEM, Zeiss Company, Ltd, Germany) was used to measure the surface morphology. Wettability of all samples was carried out on OCA35 (DataPhysics, Germany) with 5 µL of liquid at room temperature (∼21 °C). Five places on each surface were chosen to measure the contact angle (CA) and sliding angle (SA), and the final data were their average values. All of the energy dispersive spectroscopy (EDS) spectra were obtained by using the EDS appurtenance (Oxford XMax 20, England). Fourier transform infrared spectroscopy (FT-IR) (Vector 33, Germany) was applied to achieve the FT-IR spectra. Karl Fischer moisturemeter (DK-8A, China) was introduced to character the tiny water content in emulsion or filtrate. Dynamic light scatting (DLS) data were collected from Malvern Zetasizer (Nano ZS 90, England). Photos and videos were vividly recorded by a digital camera (Sony, Japan). The pore diameter of the airbrush used in the spray process was 0.3 mm.

Results and discussion After being sifted by the steel net (250#), dust particles with the diameter lower than 61 µm could be gathered. It can be clearly seen in the low-magnification SEM image (Figure S1a1) that the dust particles are in irregular granular texture with different sizes, and the longest diameter is about 60 µm, which is as long as the mesh size. The surface of each particle is very rough as shown in Figure S1a2. As can be 5



seen in the high-magnification SEM image (Figure S1a3), the morphology of each particle surface is irregularly flake-like nanostructure. After the ball-milling treatment, the dust particles become smaller (Figure S1b1). As shown in Figure S1b2, the diameters most of the particles are lower than 2 µm, and the sizes of particles are more homogeneous. The morphology of each particle is in irregularly flake-like nano size as well (Figure S1b3), demonstrating that the ball-milling treatment cannot break the nanostructure of the dust. With the treatments of sifting and ball-milling, the size of the tiny dust particles is more homogeneous, and which is very suitable for the fabrication of a coating.

Figure 1. Effects of PDMS amount on surface wettability. The relationship between (a) water CAs, (b) water SAs and the mass ratio (g/g) between PDMS and tiny dust particles. (c) Top view and (d) side view digital photos of water droplets on each sample with different mass ratios.

PDMS used as the hydrophobic modification and adhesive, played a significant role in fabricating the superhydrophobic and stable dust coating.29–31As presented in Figure 1a, the tiny particles was hydrophobic by modification of PDMS after the spray-coating process. Low concentration of PDMS is not enough to modify all the dust particles, and also lack of the adhesion between each particle.32 Therefore, the water CAs are first increased as the mass ratio (PDMS:particle, g/g) smaller than 0.4. The coating shows the largest water CA of 156° when the mass ratio is at 0.4. As the amount of PDMS increased, the water CAs decreased with the mass ratio in the range of 0.6–1.0. The dust coating finally loses superhydrophobicity when the mass ratio over 0.8. When the mass ratio is at 0.8, the water droplet cannot roll off the surface even the glass slide inclined to 90°. As displayed in the top view (Figure 1c) and side 6


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view (Figure 1d), water droplets stand spherically and expose shining shadow on the coatings when the mass ratio is lower than 0.8, while water droplets stand semi-spherically on the coating with 1.0 of mass ratio. The results indicated that the PDMS concentrations have huge effects on the wettability of the coating.

Figure 2. SEM images of tiny dust particle coatings with different mass ratio. The mass ratio of PDMS and particles are (a1, a2) 0.05, (b1, b2) 0.2, (c1, c2) 0.4, (d1, d2) 0.6, (e1, e2) 0.8, and (f1, f2) 1.0, respectively.

Morphology structures could explain the variation tendency of wettability of the tiny dust coating. As shown in Figure 2, many tiny dust particles are not well adhered at the mass ratio of 0.05 (Figure 2a1), and it is hard to discern the trace of PDMS in the high-magnification SEM image (Figure 2a2). At 0.2 of the mass ratio, tiny dust particles begin to flock together (Figure 2b1), and they are bonded together by PDMS. It is clear that the flake-like nanostructure is replaced by the pot-like nanostructure. SEM images of mass ratios at 0.4 and 0.6 (Figure 2c1–d2) show the similar structures to these of the mass ratio at 0.2. However, when the mass ratio reaches to 0.8, the gaps between each particle are filled with PDMS as can be clearly seen in Figure 2e1 and e2. It was harder for the surfaces to capture air in this case, and the surface shows the high adhesion state. Certainly, when the concentration of PDMS is further raised, the gaps would be deeply filled (Figure 2f2), and the surface structure turns flatter (Figure 2f1). The gaps between each particle played a significant role in capture of air, which would determine the CA and SA on a superhydrophobic surface. Therefore, compared with the coating with many microscopic gaps, the coating without gaps would have smaller CA and larger SA.

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Figure 3. (a) EDS and (b) FT-IR spectra of the pristine tiny dust particles. (c) FT-IR spectrum of the PDMS modified tiny dust particles with 0.4 of mass ratio.

In consideration of the bonding between the tiny dust particles and substrates (more PDMS would improve the bonding of the tiny dust particles), and the wettability as well, the superhydrophobic coating with the mass ratio at 0.4 was selected for further investigations. As shown in Figure 3a, the tiny dust particle was composed of the C, O, Al, Si, K, Ca, Ti, and Fe elements before PDMS modification. It can be inferred that some of the chemical compounds are silicon oxide, metal oxide, and their coordination compound. The organic groups of the particles before modification were confirmed by FT-IR. The peaks of –OH group clearly exist in the range of 3700–3100 cm-1 in the FI-IR spectrum (Figure 3b), revealing that the particles are covered with hydroxyl groups. All of the new appearance of peaks are marked in green arrows in Figure 3c, and they are at 2963 and 1267 cm-1, corresponding to C-H stretching in -CH3, and -CH3 symmetric stretching vibration in Si-CH3, respectively.34, 35 The variation of wavenumbers in the range of 3200–3650 cm-1 is on account of the introduced Si-OH organic group on PDMS. These emerging peaks belonging to cured PDMS revealed the successful cladding of PDMS for tiny dust particles. PDMS endowed the coating low surface energy and bonding, and the particles provided the coating rough structure. Both of them lead to the formation of superhydrophobic film on the glass slide substrate. For our prepared superhydrophobic dust coating, white silica powder (20 µm) was applied to the self-cleaning tests. It can be observed in Figure S2a–c that water droplets quickly roll away from the superhydrophobic surface, and take away all of the silica powder which cover on the surface, leaving a cleaning brown surface. The superhydrophobic coating performs self-cleaning behavior as the same as the lotus leaf washes dust by rain droplets. In the anti-fouling tests, the immersed part of glass slide should make sure completely spray-coating of treated particles. As shown in Figure S3a–c, after being immersed into the methyl blue dyed water, there is no blue water droplets on the surface, and the surface is completely dry. Moreover, upon further immersion in transparent water, water in the beaker is not stained, and the surface acts like a bright silver mirror. The mirror-like phenomenon of the surface in water revealed that an air layer existed between the superhydrophobic coating and water.33 The air layer was locked due to capability of capturing air of the gaps between each dust particle. In addition, dust was dispersed into water with ultrasonic treatment, and then the mud water was obtained. Interestingly, the PDMS modified dust coating could resist the fouling of mud water (Figure S3d, e). After immersion into the muddy water, the coated glass was further immersed into clean water. 8


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Similarly, water in the beaker is very clean, and the superhydrophobic dust coating forms a mirror-like surface as well (Figure S3f). The results demonstrated the excellent self-cleaning and anti-fouling of the superhydrophobic tiny dust coating.

Figure 4. SEM images of (a) uncoated and (b) superhydrophobic tiny dust coated sponge. (c) An air layer occurs when coated sponge immersed into water. (d–f) Isooctane and (g–i) dichloromethane absorptions of the coated sponge.

Oil/water mixtures are usually required to separate for further usage or environmental protection. In order to separate a few floating oils from water, the sponge which has great absorbability is believed to efficiently absorb a few oil. As can be seen from the SEM image (Figure 4a), numbers of pores are existence in the sponge surface, giving rise to the great absorbability. After the spray-coating treatment, tiny dust particles were adhered into the sponge (Figure 4b). The flake-like nanostructure and the gaps between each particle on the sponge surface would generate the capillary force, leading to the easy absorption of oils. Owing to the adhesion of superhydrophobic dust particles, an air layer generates between the sponge and water (Figure 4c), indicating the superhydrophobicity of the coated sponge. As shown in Figure 4d–f, 2 mL of isooctane dyed with Sudan III floats on water surface. It was quickly absorbed by the coated sponge, leaving clean water in culture dish. Dichloromethane was also applied to the oil absorption tests as the heavy oil. As recorded in Figure 4g–i, when 2 mL of Sudan III dyed dichloromethane sinks under water, the coated sponge would absorb dichloromethane, and the pure water could be obtained. As shown in Table S1, this sponge even could absorb dichloromethane 75 times heavier than its own weight, and the purity of the oil absorbed from the oil/water mixtures was very high, demonstrating a good absorption capability compared with other published works. 12,36,37

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Figure 5. Low- and high-magnification SEM images for the (a1–a3) pristine steel net, and (b1–b3) superhydrophobic dust coated steel net (1000#).

Steel net is a kind of suitable filter membrane for the separation of immiscible oil/water mixtures. Figure 5a1–a3 shows the almost flat structure on each stainless steel wire. The superhydrophobic tiny dust particles exist on mesh after the spray-coating process (Figure 5b1). In the high-magnification SEM images (Figure 5b2 and b3), lots of dust protrusions, gaps between particles, and the flake-like nanostructure on the dust particles composed of the multiple scale rough structures on the particle coated steel net (membrane-A). Water CA of the original steel net was 128° (Figure S4a), while water CA reached to 156° on the membrane-A (Figure S4b). Furthermore, oils would quickly infiltrate into the membrane-A. In other words, water would be resisted, and the heavy oil would pass through the superhydrophobic/superoleophilic membrane-A under gravity.

Figure 6. SEM images with low- and high-magnification of steel net surfaces with (a1–a3) AP binder dip-coating and (b1–b3) the pristine tiny dust particles and AP binder mixtures sprayed-coating.

For the separation of immiscible light oil/water mixtures, superamphiphilic membrane blocked oil after pre-wetted with water was fabricated. Introducing of a binder would enhance the combination between the steel net substrate and coating. The AP binder prepared according to the fabrication processes in the experimental 10


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section was deposited onto the steel net with a dip-coating method. As displayed in the low-magnification SEM image (Figure 6a1), the AP binder is uniformly covered on the steel net, and the flake-like binder particles (Figure 6a3) also overlay on each steel wire (Figure 6a2). The AP binder mixed with the pristine tiny dust particles would spray-coat on the steel nets, fabricating another membrane to separate immiscible light oil/water mixtures, which named of membrane-B. The morphology of the membrane-B could be clearly seen in Figure 6b1–b3. The coating was uniformly spray-coated on each steel wire as well (Figure 6b1, b2), and the coating turned into a coral-like structure under the effect of AP binder (Figure 6b3). The gaps between each particle provided the passageway of liquid, which was great significant for the oil/water separation. Wettability of the membrane-B was shown in Figure S5. When the water and isooctane droplets were dropped on the membrane-B surface, they would infiltrate into the surface, and almost spread out. The water CA (Figure S5a) and isooctane CA (Figure S5b) are of 9° and 0° on membrane-B surface, respectively. The environment of membrane-B surface was changed after immersion into water instead of exposure in air. Water would completely immerse into the membrane-B surface, in formation of a thin water film on the membrane-B surface. Therefore, the oils were impeded from the surface. As discerned in Figure S5c–e, the spherical isooctane, kerosene, and petroleum ether shapes are obtained on the membrane-B surface in water environment. The CAs of isooctane, kerosene, and petroleum ether in water are 150.4°, 152.2°, and 146°, respectively. The results revealed that the membrane-B was superamphiphilic in air and superoleophobic under water.

Figure 7. Immiscible oil/water separations of the superhydrophobic/superoleophilic membrane-A and the underwater superoleophobic membrane-B. (a) Water is dyed with methylene blue before heavy oil/water separation. (b) Schematic illustration of separation of the superhydrophobic membrane-A. (c) Light oil is dyed with Sudan III before light oil/water separation. (d) Schematic illustration of separation of the superhydrophilic membrane-B. (e) Separation efficiency of various 11



oil/water separations. aWater and oil in the schematic illustrations were the blue and red droplets, respectively.

Owing to the different wettabilities toward water and oil of membrane-A and membrane-B, oil or water could be selectively removed from immiscible oil/water mixtures according to the density of the liquid. Dichloromethane and trichloromethane which density larger than that of water were used to fabricate the immiscible heavy oil/water mixtures, while isooctane, kerosene, and petroleum ether which density smaller than that of water were applied to the fabrication of immiscible light oil/water mixtures. The same volumes of oils and water (15 mL/15 mL) were mixed to prepare for separation. As shown in Figure 7a, the membrane-A is tightly fixed between two glass tubes, and water dyed blue is floating on the heavy oil. Since membrane-A is superhydrophobic and superoleophilic, heavy oil would pass through the membrane under the gravity, and water would be resisted as can be seen from the schematic illustration (Figure 7b). For separation of membrane-B, oil dyed red is floating on water (Figure 7c). After being pre-wetted with water, a thin water film was formed on the superhydrophilic membrane-B, and it achieved underwater superoleophobicity. As shown in Figure 7d, when the mixtures are poured into the tube, water would pass through the membrane-B due to superhydrophilicity and gravity, while light oil is impeded by the water pre-wetted membrane-B. Therefore, water is selectively removed from light oil/water mixtures. Separation efficiency, intrusion pressure, and flux are usually used to evaluate the separation ability, and their calculations are according to the equations in the following.38 The separation efficiency of various oils was displayed in Figure 7e. The separation efficiency of immiscible oil/water separation was calculated as the equation: Ԑ=V1/V, where Ԑ, V1, and V are the separation efficiency, the obtained oil volume, and the original oil volume (15 mL), respectively. The separation efficiency of all the oil/water mixtures (oils including dichloromethane, trichloromethane, isooctane, kerosene, and petroleum ether) were above 95%. In addition, as shown in Figure S6, after separation for 10 cycles (the membrane would be washed by ethanol and dried completely after each cycle), the separation efficiency remained about 95% and 97% for membrane-A and membrane-B, respectively. Moreover, the membrane-A could support the maximum water height of 30 cm, which means that if the height of the injected water was over 30 cm, water would pass through the superhydrophobic membrane. Membrane-B pre-wetted by water could sustain the maximum isooctane height of 21 cm. According to the equation: P=ρgh, where P, ρ, g, and h represent the intrusion pressure, density of the introduced liquid, acceleration of gravity, and the height of liquid, respectively, the maximum intrusion pressure of water and isooctane for membrane-A and membrane-B were calculated to be about 3 and 1.6 KPa, respectively. The equation of flux is shown as follows: F=V/(ST), where F, V, S, and T are liquid flux, the volume of liquid, the area of passed liquid on the mesh, time for passed liquid, respectively. Dichloromethane and water with the same volume (1 L) were applied to measure the flux of membrane-A and membrane-B, respectively. The results of dichloromethane flux of membrane-A is about 47000 L m-2 h-1, while water flux of membrane-B is about 34500 L m-2 h-1. As vividly recorded in Video S1, heavy 12


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oil (dichloromethane as the example) quickly passed through the membrane-A for about 15 s, while blue water was remained above the membrane. The immiscible light oil/water mixtures separated by membrane-B were also recorded in Video S2, and it took about 27 s for the separation. Under this condition, the red oil (isooctane as the example) was reserved over the membrane-B, and water quickly flowed through the membrane-B. The results demonstrated that membrane-A and membrane-B could separate immiscible heavy or light oil/water mixtures, respectively.

Figure 8. (a–c) Low- and high-magnification SEM images of pristine large dust particles (sifted by steel net with 24#). (d) EDS spectrum of the pristine particles.

After removing the tiny dust particles, the remains were further sifted by steel net (#24) to collect the large dust particles. The SEM images in Figure 8 clearly illustrate the morphology of the pristine large dust particles. Compared with the tiny dust particles with diameter of 2 µm, the size of the large dust particles are about 200 µm, which is almost 100 times higher than that of the tiny one. In the high-magnification SEM image, the surface structure of a large dust particle is irregularly flake-like nanostructure, which is similar to the nanostructure of the tiny dust particles. The size and shape are different for each large dust particle, but the nanostructure of each particle is the same. As shown in Figure 8d, the chemical composition of the large dust particles was measured by EDS. The EDS spectrum displays that the pristine large dust particles are composed of C, O, Al, Si, and Fe elements. The two strongest peaks in the spectrum belong to the O and Si elements, and their mass percentages are 45.0 and 41.8%, respectively.

Figure 9. Wettability of the layer composed of large dust particles. In air environment, (a) water 13



and (b) oil (isooctane as an example) droplets on the layer. (c1–c5) The movements of a water droplet on the layer in oil environment.

In order to measure the wettability, numbers of large dust particles were spread on a glass slide to form a layer. As shown in Figure 9a and b, when water and oil drop on the layer, they would quickly infiltrate into the layer, showing 0° of water and oil CAs. When the layer is in isooctane, the oil would immerse into each particle. In isooctane environment, the water droplet firstly takes a spherical shape on the layer (Figure 9c1) due to the inhibition of isooctane film, and then infiltrates into the layer (Figure 9c2). After a while, some particles would float on the water droplet (Figure 9c3). It would form a semi-spherical shape fully covered with large dust particles (Figure 9c4), and finally completely infiltrate into the layer (Figure 9c5). After the infiltration of the water droplet, the freely flowed large dust particles bonded together in isooctane. The results revealed that the layer was superamphiphilic in air and superhydrophilic in oil.

Figure 10. The surfactant stabilized water-in-isooctane emulsion separation performances of the layer composed of large dust particles. (a) Optical image of the simple device for the emulsion separation. Optical images of the comparison of the (b) filtrate and (c) emulsion. (d) DLS spectrum of the emulsion. The relationship between the height of layer and the (e) separation efficiency and (f) filtrate flux.

Inspired by the work finished by Li et al. that the sand layer owned the underoil superhydrophilicity could separate the water-in-oil emulsion.39 Our layer composed of large dust particles performed superhydrophilicity in isooctane might have the similar ability. The preparations of the span 80 stabilized water-in-isooctane emulsion were described in experimental section. The emulsion separation results could be clearly seen in Figure 10. As shown in Figure 10a, the dust layer in a glass tube is applied for separating the surfactant stabilized water-in-isooctane emulsion. The emulsion (25 mL) was poured into the layer in glass tube, and separation process would finish automatically only under gravity. When the height of the layer was at 10 cm, the 14


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emulsion passed through the layer was separated into clean and transparent filtrate (Figure 10b). The DLS results of the filtrate were not found, which signified that the size of the filtrate was over the measuring range of the machine (smaller than 2 nm), further demonstrating the extremely clean filtrate. However, the emulsion is milk-like before separation (Figure 10c), the diameter of the water droplets in the emulsion is in the range of 122.4–712.4 nm, and the average diameter is 317.4 nm based on the DLS results (Figure 10d). Video S3 also recorded the efficient separations of the surfactant stabilized water-in-oil emulsion by using a layer composed of large dust particles. With the stabilization of span 80, the tiny water droplets and oil in the emulsion were steadily linked. Large numbers of pores between each dust particle were fabricated, leading to the excellent filtration of the layer. The filtration of the superamphiphilic layer would destroy the linking forces between water and oil. Therefore, the surfactant stabilized emulsion would firstly demulsify after being poured into the layer. Owing to the superhydrophilicity in isooctane of the layer, the suspended water droplets would be captured by the layer filled with water-in-oil emulsion. The capture of water in oil was on account of the capillary cohesion.40, 41After water droplets in emulsion were absorbed by the layer, the purified oil would be obtained. The demulsification and capture of tiny water droplets would require for enough pores of the layer, so that the height of the layer would influence the separation. The water content of the emulsion and filtrate was measured by Karl Fischer moisturemeter, and the separation efficiency was calculated according to the method we had reported.42 The separation efficiency (ŋ) of emulsion separation was calculated according to the equation: ŋ=(m-m1)/m, where m and m1 are the mass of emulsion infiltrated into the Karl Fischer machine, the measured mass of water in emulsion. According to the equation: FF=V/(ST), where FF, V, S and T are filtrate flux, the volume of collected filtrate, the area of the non-woven with passed filtrate, time for passing, respectively, the filtrate flux was calculated by measuring the time collected 10 mL of filtrate after separation. As shown in Figure 10e, the separation efficiency increases with the raise of the height of the layer. When the height is at 2 cm, the emulsion cannot be separated, and while the height is over 4 cm, the separation efficiency is higher than 99.5%. The separation efficiency could reach over 99.99% as the height over 8 cm. The filtrate flux demonstrated the opposite trend as the height of the layer increased (Figure 10f). In other words, the filtrate flux decreased with the increasing of the height of the layer. In summary, at the height of 6 cm, the filtrate flux could reach to 5900 L m-2 h-1, and the separation efficiency is 99.99%. The layer composed of large dust particles was high-efficiently separated surfactant stabilized water-in-oil emulsion under gravity.

Conclusion In summary, we have efficiently used the dust for three types of oil/water separations. The easily collected and no-paid dust was sifted into tiny particles and large particles. The tiny dust particles combinations with PDMS (PDMS:particle=0.4, g/g) spray-coated on the glass was of superhydrophobicity, anti-foiling, and self-cleaning. 15



When the mixture coated onto sponge and steel net, the coating could absorb oils from water, and separate immiscible light oil/water mixtures. Superamphiphilic membrane was obtained by spray-coating the mixtures composed of tiny dust particles and inorganic AP adhesive. After being pre-wetted with water, the membrane achieved underwater superoleophobic, and could quickly separate the immiscible heavy oil/water mixtures. The large dust particles formed a layer in a glass tube with large numbers of pores between each particle, and the layer demonstrated underoil superhydrophilicity. The layer could efficiently separate the surfactant stabilized water-in-oil emulsion. Along with the increasing height of the layer, the separation efficiency would improve, while the filtrate flux would decrease. When the height of the layer was 6 cm, the separation is over 99.99%, and filtrate flux could reach to 5900 L m-2 h-1. Only under gravity, different kinds of oil/water mixtures could be efficiently separated by selectively choosing the dust coatings with simple treatments. These facile methods by using the dust provided an efficient way to purify oils and water. Acknowledgements The National Natural Science Foundation of China (21573077 and 51373055) and the Fundamental Research Funds for the Central Universities (SCUT) are gratefully acknowledged. Supporting Information Available The contents of Supporting Information are as follows: Video S1 records the immiscible heavy oil-water separation by the superhydrophobic/superoleophilic membrane-A (AVI) Video S2 records the immiscible light oil-water separation by the underwater superoleophobic membrane-B (AVI) Video S3 records the highly efficient separation of surfactant stabilized water-in-oil emulsion by the underoil superhydrophilic large dust particle layer (AVI)

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Graphic abstract

Dust with different simple treatments could be selected to separate various types of oil/water mixtures only under gravity.


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Water Separations from Nanosized Superhydrophobic to

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