Water Separation
3.4.2 Polymer-Dominated Superwetting Filtration Membranes for Separation of Oil/Water Emulsions
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
3.4.2 Polymer-Dominated Superwetting Filtration
emulsions with oil or water droplets at the submicrometre and nanome- tre scale, the above-mentioned separation technologies are not effective.
instead, filtration membranes are capable and widely used to separate various oil/water emulsions, especially for surfactant-stabilized emul- sions.20,98–103 however, traditional polymer-dominated filtration mem- branes in various industrial fields for treating oily wastewater still have drawbacks. the major problem is that most of these filtration membranes are not superwetting and thus suffer from oil-fouling and quick decline of permeation flux and separation efficiency due to pore plugging by oil Figure 3.11 droplets of water (dyed blue) and rapeseed oil (dyed red) on stainless steel mesh 100 (a) and polyester fabric (b).97 Both surfaces have been dip coated with a 20 wt% f-poSS + x-peGda blend, scale bars, 5 mm.
insets are morphologies of the respective dip coated mesh and fabric surfaces, scale bars, 500 µm. Batch separation of hexadecane-in-water (c and d) and water-in-hexadecane (e and f) emulsions by a stainless steel mesh 400 modified with 20 wt% f-poSS + x-peGda blend. inserts show a hexadecane droplet on the modified surface under water. (g) photographs showing the continuous separation of water-in-oil emul- sions by a scaled-up apparatus. (h) Measured fluxes as a function of time. reprinted by permission from Macmillan publishers Ltd: Nat.
Commun. (ref. 97), copyright 2012.
droplets.104–107 this weakness of traditional filtration membranes bring about high costs for treating large effluent volumes of oily water. devel- oping new advanced polymer-dominated filtration membranes with superwetting and antifouling properties is extremely critical for efficient separation of emulsified oil/water mixtures and other separation applica- tions. to improve the hydrophilicity and antifouling performance of poly- mer-dominated membranes, a variety of methods such as blending, block copolymer grafting and surface coating with hydrophilic components have been adopted by researchers.108–112 however, the hydrophilicity of the mem- branes obtained via these methods are usually nondurable because of the releasing of the water-favouring additives during long-term use. recently, a series of advanced works on developing inherently superwetting polymeric membranes have been carried out by controlling the surface morphology with micro-/nano-scale hierarchical structures.85–88
jin’s group reported a novel salt-induced phase-inversion method to fabricate superhydrophilic and underwater superoleophobic poly(acrylic acid)-grafted polyvinylidene fluoride (paa-g-pVdF) membranes for effective separation of oil-in-water emulsions.88 Completely different from the gen- eral approach of adding salts as a pore-forming additive to change the pore structure of the membrane,113,114 in their report, the salt (naCl) added into the coagulation bath at a nearly saturated concentration functions as nucle- ater during the solvent exchange and induces the assembly of paa-g-pVdF micelles around them (as shown in Figure 3.12a). the micelle assembly on the surface of the paa-g-pVdF membrane gives rise to a micro-/nano-scale hierarchical structure (Figure 3.12b and c) and endows the membrane with superhydrophilic and underwater superoleophobic properties. this super- wetting paa-g-pVdF membrane can separate both surfactant-free and surfac- tant-stabilized oil-in-water emulsions with oil droplet size from micrometre to nanometre, either under a small applied pressure (0.1 bar) or solely driven by gravity, with very high permeation flux (up to 2300 L m−2 h−1, see Figure 3.12d) and very high separation efficiency (>99.99 wt% after one-time sepa- ration, see Figure 3.12e). due to the underwater superoleophobicity of this membrane, it also exhibits an excellent anti-oil fouling property and is easily recycled for long-term use. the outstanding separation performance of this membrane for oil-in-water emulsions and its industrially scalable fabrica- tion process indicate its great potential for treating real oily water. they also reported a facile method to fabricate superhydrophobic and superoleophilic pVdF membrane via an inert solvent-induced phase-inversion process (Figure 3.13a).85 the surface of the as-prepared pVdF membrane was com- posed of spherical micro-particles and rather rough (Figure 3.13b), which results in the superhydrophobicity (Figure 3.13c) and superoleophilicity (Figure 3.13d) of the membrane. this pVdF membrane can effectively separate both surfactant-free and surfactant-stabilized water-in-oil emulsions solely driven by gravity, with very high separation efficiency (oil purity after one- time separation >99.95%) and very high permeation flux up to 3400 L m−2 h−1 (Figure 3.13e and f).
a novel superhydrophilic–superoleophilic pVdF membrane with multi- scale surface structure was reported by Liu and co-workers.87 this pVdF membrane was obtained by two steps including fabrication of the porous pVdF membrane on a non-woven fabric (nWF) via phase inversion and then peeling off the pVdF membrane from the nWF. discrete pores of 50–100 µm in width are disorderly distributed on the membrane surface with numerous randomly oriented grooves of ∼10 µm in width (Figure 3.14a and b). petaloid structures with sizes of hundreds of nanometres form the tightly packed spherulites with size of several micrometres (Figure 3.14c). the authors believe the nWF is essential for the formation of membrane surface topogra- phy. this superhydrophilic and superoleophilic pVdF membrane exhibits a low adhesive superhydrophobicity under oil and an ultralow adhesive super- oleophobicity under water (Figure 3.14d). hence, the superwetting pVdF Figure 3.12 (a) Schematic showing the formation of the superhydrophilic–under-
water superoleophobic paa-g-pVdF membrane by a salt-induced phase-inversion process.88 (b) and (c) are the cross-section and top- view SeM images of the membrane, respectively. permeation flux (d) and oil concentration in the corresponding filtrate (e) for a series of surfactant-free and surfactant-stabilized oil-in-water emulsions per- meating the membrane. W. B. Zhang, et. al., Salt-induced fabrication of superhydrophilic and underwater superoleophobic paa-g-pVdF membranes for effective separation of oil-in-water emulsions, Angew.
Chem., Int. Ed., 2014, 53, 3. Copyright © 2014 john Wiley & Sons, inc.
membrane can be used to separate both oil-in-water and water-in-oil emul- sions (Figure 3.14e) due to the switchable transport property and it displays excellent permeation flux (Figure 3.14f), separation efficiency, recyclability and antifouling performance.
Wang and co-workers developed a dual-scaled perforated nitrocellulose (p-nC) membrane with superhydrophilicity and underwater superoleophobic- ity by a simple and reproducible perforating method using a desktop robot punch system equipped with a home-made needle owing a conical tip.86 a series of p-nC membranes with different pore sizes were obtained via controlling needle punching depth. the dual-scaled hierarchical porosity improves the permeation flux of the nC membrane and ensures the rapid separation of water from a variety of oil/water mixtures including gasoline, diesel, hexane, petroleum ether and even high-viscosity crude oil/water mixture without exter- nal power and with high separation efficiency (>99%). Separation time and intrusion pressure of the superwetting p-nC membrane can be readily tuned by controlling the sizes of the perforated micro-pores. the superwetting p-nC membrane also possesses good environmental stability and recyclability.
Figure 3.13 (a) Schematic illustration of the formation of a superhydrophobic–
superoleophilic pVdF membrane via a modified phase-inversion pro- cess.85 (b) SeM image of the membrane, scale bars: 50 µm. the insert is a higher magnification image of membrane, scale bars: 1 µm. (c) and (d) are optical images of a water droplet and an oil droplet on the mem- brane in air, respectively. oil purity in filtrate (e) and permeation flux (f) for various water-in-oil emulsions separated by the pVdF membranes.
W. B. Zhang, et. al., Superhydrophobic and superoleophilic pVdF mem- branes for effective separation of water-in-oil emulsions with high flux, Adv. Mater., 2013, 25, 14. Copyright © 2013 john Wiley & Sons, inc.
Besides the inherently superwetting membranes discussed above, some other advanced works on fabricating polymer-dominated superwetting membranes are also worth introducing.115–118 a superhydrophilic and under- water superoleophilic nanosilica-decorated polypropylene (pp) microfiltra- tion (MF) membrane with a mussel-inspired intermediate layer was reported by Xu’s group for separation of oil-in-water emulsions.115 the superwetting property of the membrane was obtained by regulating and controlling the surface chemistry and morphology, too. herein, pda is introduced as an adhesive component in the intermediate layer, polyethylenimine (pei) is co-deposited onto the membrane surface for the polycondensation of silicic acid via a biomineralization process. after the surface modification, silica nps are uniformly decorated on the membrane surface and inherent micro- pores, resulting in the superhydrophilicity (water Ca = 0°) and the under- water oil-repelling property (underwater oil Ca > 152° for a series of oils) of the nanosilica-decorated pp membrane. Because of the robust superwetting Figure 3.14 (a)–(c) SeM images for the membrane surface with scale bar of 200 µm, 20 µm and 2 µm, respectively.87 (d) Wettability of the membrane toward water (upper) and oil (lower). (e) Separation results for oil-in-water and water-in-oil emulsions by the pVdF membrane. (f) permeate flux for various oil-in-water and water-in-oil emulsions by the pVdF membrane.
M. tao, et. al., an intelligent superwetting pVdF membrane showing switchable transport performance for oil/water separation, Adv. Mater., 2014, 26, 18. Copyright © 2014 john Wiley & Sons, inc.
property, this membrane exhibits very high permeation fluxes up to 1400 L m−2 h−1 and very high oil rejections (>99%) for four kinds of surfactant- stabilized oil-in-water emulsions under a very low trans-membrane pressure of 0.04 Mpa. the nanosilica-decorated membrane also possesses excellent flux recovery after simply rising with water. Lu and co-workers reported a mus- sel-inspired hybrid coating on pVdF MF membranes via the simultaneous polymerization of dopamine and hydrolysis of silane by one step to fabricate superhydrophilic and underwater superoleophilic pVdF MF membranes.117 the superwetting coating endows the pVdF MF membranes with the abil- ity for separation of oil-in-water emulsions with high water flux and excel- lent antifouling performance. poly(3-(N-2-methacryloxyethyl-N,N-dimethyl) ammonatopropanesultone) (pMapS), a novel zwitterionic polyelectrolyte, was fabricated by jin’s group and grafted on commercially available pVdF MF membranes for separation of oil-in-water emulsions.118 instead of the hydrophobicity of the original pVdF membrane, the pMapS-g-pVdF mem- brane behaves superhydrophilically and underwater superoleophilically with ultralow underwater oil adhesion force. Because of the superwetting property, the pMapS-g-pVdF membrane can thoroughly separate oils from oil-in-water emulsions with ultrahigh separation efficiency (oil content after one-time separation less than 10 ppm).