Water Separation
3.4.1 Mesh- and Textile-Based Superwetting Films for Separation of Oil/Water-Free Mixtures and Emulsions
Metal meshes, such as stainless steel meshes and copper meshes, are very common and commercially available 2d pore-structured materials com- posed of weaved metal strands. these metal meshes usually possess a very high mechanical strength and are widely adopted by researchers as the sub- strates to fabricate superwetting filtration films for oil/water separation.
during the recent decade, a series of mesh-based superwetting filtration films were intensely fabricated by modifying them with special surface com- position and hierarchical roughness.54,55,62,89–97 the superhydrophobic–supe- rhydrophilic mesh-based films are usually modified with low surface energy materials and used for removing water from oil-rich mixtures. the super- hydrophilic–underwater superoleophobic mesh-based films are usually modified with high surface energy materials and used for removing oil from water-rich mixtures. to endow the mesh-based films with superwettabilities, the micro- and nanoscale hierarchical roughness is indispensable.
the superhydrophobic–superoleophilic ptFe coating mesh film which is constructed by micro- and nanostructured rough surfaces of a low sur- face energy fluorine-containing material on a stainless steel mesh is the first reported mesh-based superwetting film for oil/water separation.21 this coat- ing mesh film was prepared by a spray-and-dry method. Ball-like ptFe with a diameter of 2–5 µm is uniformly coated on the skeleton of stainless steel mesh and no coating materials exists in the pores of the mesh. Some balls are glued to and embedded in each other, aggregating to form the blocks.
Craters with a diameter of about 71 ± 8 nm are densely and evenly distrib- uted on the surface of each ball, forming a structure that resembles that of a golf ball. the ptFe coating mesh film exhibits an excellent water-repellent
property with a water Ca of 156.2 ± 2.8° and a water Sa of 4°. Moreover, the ptFe coating mesh film shows a superoleophilicity with a diesel oil Ca of 0 ± 1.38°. a droplet of diesel oil can spread quickly on the film and permeate thoroughly within only 240 ms. this unique superwetting property provides the ptFe coating mesh film with the ability for the effective separation of a diesel oil/water mixture, which allows diesel oil to permeate through the film but the film to reject water.
Feng and co-workers reported a mesh-based film with high hydrophobicity and superoleophilicity for highly efficient separation of oil/water mixtures.94 this superwetting mesh-based film was fabricated by combining mussel- inspired chemistry and Michael addition reaction. to be specific, a polydo- pamine (pda) layer was firstly coated on the strands of the stainless steel mesh and then N-dodecyl mercaptan (ndM) was conjugated with pda. the as-prepared pda–ndM mesh film exhibits a water Ca of 144°, but a diesel oil droplet can quickly spread and permeate the film with an oil Ca of 0°.
the opposite wettability for water and oil endows the mesh-based film with an excellent capacity to remove a series of oils, including hexane, gasoline, diesel, etc., from water driven only by gravity. Separation efficiencies for these oils are all above 98.12% and remain very high after 30 times use. Further- more, the relatively high intrusion pressure (2.2 kpa) guarantees the pda–
ndM mesh film can treat a large amount of oil/water mixtures.
Since the hydrophilic fish scales were reported to exhibit a superoleop- hobic property underwater in 2009, a strategy to fabricate superhydrophilic and underwater oil-repellent materials without the assistance of fluoride compounds has been followed.57–61 Feng and jiang reported a superhydro- philic–underwater superoleophobic hydrogel-coated mesh film made up of a nanostructured hydrogel coating and a stainless steel mesh.62 this super- wetting mesh-based film was fabricated by the photo-initiated in situ radical polymerization of polyacrylamide (paM) with N,N′-methylene bisacrylamide as a chemical cross-linker. after the polymerization process, a dense layer of paM hydrogel with random papillae structures of 80–500 nm is uniformly coated on the stainless steel strands (Figure 3.9a–c). thickness of the paM hydrogel layer is about 1.2 µm and pore size of the paM hydrogel-coated mesh film is approximately 50 µm, which is very similar to that of raw stainless steel mesh (300 mesh size). the microporosity of the paM-coated mesh film can be controlled by selection of different raw mesh sizes. the hydrophilicity of the paM hydrogel and the micro- and nanoscale hierarchical roughness give rise to the superhydrophilicity and underwater superoleophobicity (underwater oil Ca = 155°) of the resulting paM-coated mesh film (Figure 3.9d). When the mesh film is immersed in water, the oil droplet of 1,2-dichloroethane is unstable and can easily roll off the film surface (Figure 3.9e). Mean- while, the mesh film exhibits an ultralow oil adhesion force of 0.8 ± 0.3 µn.
When an oil/water mixture was poured onto the paM-coated mesh film, water quickly permeated through the film while the oil was retained above the film due to the superwetting property of the film (Figure 3.9f and g).
the separation efficiencies for a variety of oils, including vegetable oil,
gasoline, diesel and even crude oil, are all above 99%, indicating its potential for practical separation of oil/water mixtures.
Besides the coating method, jin’s group reported a different strategy to fabricate an all-inorganic superwetting Cu(oh)2 nanowire-haired film via an in situ growing method for separation of free oil/water mixtures and surfac- tant-free oil-in-water emulsions.95 the all-inorganic mesh-based film is fab- ricated by the surface oxidation of copper and in situ growing of Cu(oh)2
nanowires in an alkaline aqueous solution with (nh4)2S2o8. as shown in Figure 3.10a and b, Cu(oh)2 nanowires with length of 10–15 µm and diame- ters of 200–500 nm are thickly and uniformly grown on the copper skeleton.
the micro- and nano-structured hierarchical surface roughness enhances the hydrophilicity of Cu(oh)2 to the extreme and generates the superhydro- philicity (water Ca = 0°) and underwater ultralow adhesive superoleophobic- ity (underwater oil Ca > 150°, underwater oil adhesion force < 1 µn for all the oils investigated, see Figure 3.10c) of the Cu(oh)2 nanowire-haired film. the authors also expressed the underwater superoleophobicity of the mesh-based film according to the Cassie model. pore size of the Cu(oh)2 nanowire-haired mesh film can also be controlled by the selection of different raw meshes (mesh numbers 200, 300, 400 and 500). this superwetting mesh-based film can effectively separate a series of free oil/water mixtures and oil-in-water emulsions solely driven by gravity, with extremely high separation efficiency Figure 3.9 (a) and (b) SeM images of the paM hydrogel-coated stainless steel mesh.62 (c) higher magnification image of the paM hydrogel, in which nanostructured papillae can be clearly observed. (d) photographs of an oil droplet (1,2-dichloroethane, 2 µL) on the hydrogel-coated mesh with an oil Ca of 155.3° ± 1.8° and oil Sa of 2.6° ± 0.5°. (e) photographs of the dynamic underwater oil-adhesion measurements on the hydro- gel-coated mesh. (f) and (g) digital photographs showing the oil/water separation studies of the paM hydrogel-coated mesh. Z. X. Xue, et. al., a novel superhydrophilic and underwater superoleophobic hydrogel- coated mesh for oil/water separation, Adv. Mater., 2011, 23, 37. Copyright © 2011 john Wiley & Sons, inc.
73erwetting Nanomaterials for Advanced Oil/Water Separation Figure 3.10 (a) SeM image of nanowire-haired membrane, scale bar: 50 µm. the inset is a low-magnification SeM image with scale bar of
200 µm.95 (b) SeM image of the Cu(oh)2 nanowires, scale bar: 5 µm. (c) underwater Ca and adhesion force of a series of oils on the nanowire-haired membrane. (d) and (e) oil/water separation results of a nanowire-haired membrane. (f) real-time monitoring of the variation of flux and oil content in the filtrate as a function of permeated volume of the oil/water mixture up to 10 L. F. Zhang, et. al., nanowire-haired inorganic membranes with superhydrophilicity and underwater ultralow adhesive superoleophobicity for high-efficiency oil/water separation, Adv. Mater., 2013, 25, 30. Copyright © 2013 john Wiley & Sons, inc.
and separation fluxes (Figure 3.10d and e). Moreover, this film exhibits an excellent anti-oil fouling property for long-term use, which can continuously separate 10 L of an oil/water mixture without a decrease in flux (Figure 3.10f).
the excellent separation performance, low cost, easily scaled-up fabrication process, solvent resistance and antifouling property of the mesh film indi- cate its promising application in treating large amounts of oil/water mixtures and emulsions from industry and oil spills.
except for the frequently introduced superwetting and superantiwetting properties, some other functions like self-cleaning and photo-catalytic abilities are also extensively explored and integrated on the mesh-based superwetting films.93,96 Wang and co-workers fabricated an all-inorganic underwater superoleophobic mesh film with a self-cleaning property by layer-by-layer assembly of sodium silicate and tio2 nps on a stainless steel mesh.93 the sodium silicate and tio2 nps are densely and uniformly coated on the skeleton of the mesh film, which alters the wettability of the mesh film from hydrophobicity (water Ca = 127.5°) to hydrophilicity (water Ca = 21.9°). the silicate/tio2-coated mesh film shows an underwater superoleophobic property for a series of typical oils with underwater oil Ca larger than 150° and can separate these oils from the oil/water mixtures effectively driven by gravity. the integration of the self-cleaning property into the silicate/tio2-coated mesh film endows the film with removal abil- ity for contaminants by uV illumination, and enables the facile recovery of separation performance of an oil-fouled mesh. Feng’s group reported a double-layer tio2-based mesh film with the multifunctions of oil/water separation and soluble pollutant degradation due to the photo-catalysis of tio2.96 the upper layer is a superhydrophilic tio2-coated mesh film fabricated by a hydrothermal approach with micro- and nanostructures.
the lower layer is a superhydrophobic–superoleophilic (water Ca = 165°) tio2-coated mesh film modified with octadecyl phosphonic acid (odp).
during the separation process, the double-layer tio2-based mesh film was obliquely fixed to be easily contacted by the oil in the oil/water mixture.
hence, the oil can penetrate the mesh film and flow down while water is repelled above the film because of the superhydrophobic–superoleophilic property of the upper layer, consequently achieving the oil/water separa- tion process. after ultraviolet (uV) light illumination, the odp is degraded because of the photo-catalysis of tio2 and the upper layer becomes supe- rhydrophilic. hence, the repelled water permeates the mesh film and flow down. the soluble pollutants in water are simultaneously degraded during the uV illumination and the water purification process is completed. the superhydrophobic–superoleophilic lower layer can be re-obtained by the modification of odp again.
textiles and fabrics are another kind of common and commercially avail- able micro-textured 2d porous materials with similar structures to metal meshes, and are widely chosen as the substrates to fabricate superwetting textile- and fabric-based separation films for oil/water separation. Wang’s group grafted the as-mentioned ph-responsive p2Vp-b-pdMS on a non-woven
textile and used it for controllable separation of free oil/water mixtures.78 the p2Vp-b-pdMS-functionalized textile behaves superhydrophilically and super- oleophobically in acidic water (ph = 2.0) with an underwater oil Ca of 165.3°, but behaves superoleophilically and superhydrophobically in water with a ph of 6.5. Based on the ph-responsive property of the p2Vp-b-pdMS-func- tionalized textile, it was used as a separation film to achieve a controllable separation of gasoline and water from their mixture. When the ph of the mixture was 6.5, the gasoline passed through the textile quickly, but water was kept above the textile because of its superoleophilicity and superhydro- phobicity. however, when the textile was firstly wetted by acidic water with a ph of 2.0 and used under the same the ph condition, the opposite sep- aration result was realized due to its superhydrophilicity and underwater superoleophobicity.
a breakthrough was achieved by tuteja and co-workers who fabricated the novel blend of f-poSS and cross-linked poly(ethylene glycol) diacrylate (x-peGda).97 Mesh- and fabric-based separation films modified with this f-poSS + x-peGda blend via a dip coating method exhibit a superhydrophilic and superoleophobic property both in air and under water (Figure 3.11a and b), which are barely reported. a f-poSS + x-peGda blend-coated mesh film (or fabric film) can achieve the gravity-driven separation of both surfac- tant-stabilized oil-in-water emulsions and surfactant-stabilized water-in-oil emulsions with very high separation efficiency above 99.9% in a single-unit operation (Figure 3.11c–f). the superoleophobicity under water and in air is very essential for the separation of oil-in-water and water-in-oil emulsions, respectively. in addition, the superwetting mesh-based film can continually separate several litres of oil/water mixtures (Figure 3.11g) or continually sep- arate the mixtures for 100 hours without a decrease in flux (Figure 3.11h) by using a scaled-up apparatus.
however, these mesh- and textile-based superwetting films remain with limitations for oil/water separation. the major shortcoming is that they are only capable for free oil/water mixtures and oil/water emulsions with oil or water droplets larger than the pore size of these films usually at microme- tre scale. For example, the f-poSS + x-peGda blend-coated mesh film (mesh 400) can only remove the oils or water with droplet size above 30 µm in the separation process.97 For the oil/water emulsions with oil or water droplets at submicrometre and nanometre scale, these mesh- and fabric-based films don’t work well.62,82–84,94,96