Water Separation
3.4.3 Nanomaterial-Based Ultrathin Superwetting Films for Separation of Oil/Water Emulsions
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).
3.4.3 Nanomaterial-Based Ultrathin Superwetting Films for
reported for various separation applications.119,122–134 Benefiting from the ultrathin film thickness, these nanomaterial-based films exhibit excellent separation performance, including both very high permeation flux and very high separation efficiency. among these ultrathin films, superwetting films made up of 1d nanomaterials such as Cnts, nanofibres and nanowires have been successfully developed and used for oil/water separation.
jin’s group fabricated a series of hydrophobic and superoleophilic sin- gle-walled carbon nanotube (SWCnt) network films with tuneable thickness and pore size at nanometre scale (Figure 3.15a and b), which are assembled by SWCnts via a simple vacuum-filtering method.131 this hydrophobic–
superoleophilic SWCnt network film can allow oil to permeate through the film, but reject water above the film (Figure 3.15c), eventually achieving the separation of a water-in-oil emulsion. it is worth noting that the thickness of the film can be tuned from 30 to 120 nm and the pore size of the film can be tuned from 200 to 20 nm by adjusting the dosage of SWCnt. the ultrathin SWCnt network films with thickness of tens of nanometres can effectively separate both micrometre- and nanometre-sized surfactant-free
Figure 3.15 (a) and (b) digital photograph and teM image of a 70-nm-thick SWCnt film.131 (c) optical images of a water droplet and an oil (dichloromethane) droplet on the SWCnt film in air. plots of fluxes for surfactant-free (d) and surfactant-stabilized (e) water-in-oil emulsions as a function of the thickness of SWCnt films. Z. Shi, et. al., ultrafast separation of emulsified oil/water mixtures by ultrathin free-standing single-walled carbon nanotube network films, Adv. Mater., 2013, 25, 17.
Copyright © 2013 john Wiley & Sons, inc.
and surfactant-stabilized water-in-oil emulsions with ultrahigh fluxes up to 100 000 L m−2 h−1 bar−1 (Figure 3.15d and e), which are 2–3 orders of magni- tude higher than the fluxes of traditional filtration membranes with simi- lar separation performance, and with very high separation efficiency (above 99.95 wt%) and excellent antifouling ability.
Because of the inherent hydrophobicity and superoleophilicity of SWCnt film, it is incapable of separating oil-in-water emulsions. jin’s group then designed the photo-induced superhydrophilic and underwa- ter superoleophobic SWCnt/tio2 ultrathin film by coating tio2 nps onto SWCnt network film via a sol–gel process and used it for an ultrafast sep- aration of oil-in-water emulsions.132 anatase-type tio2 nps are uniformly and robustly coated on every SWCnt. after uV irradiation, the SWCnt/
tio2 nanocomposite film exhibits a superhydrophilic and underwater superoleophobic property ascribed to photo-responsive wettability of tio2 nps. a SWCnt/tio2 nanocomposite film with thickness of ∼60 nm and pore size of 20–60 nm can effectively separate both surfactant-free and surfactant-stabilized oil-in-water emulsions in an ultrafast manner with fluxes up to 30 000 L m−2 h−1 bar−1, which is 2 orders of magnitude higher than commercial uF membranes with similar separation perfor- mance, and with very high separation efficiency (oil content after one-step separation is lower than 40 ppm, meeting the standards for wastewater discharge). Moreover, this SWCnt/tio2 nanocomposite film exhibits excellent self-cleaning performance duo to the photo-catalytic property of anatase-type tio2 nps. jin and co-workers also designed and fabricated an inherently superhydrophilic–underwater superoleophobic polymer-dec- orated SWCnt ultrathin film with modification of pda and pei.133 the SWCnt/pda/pei nanocomposite film with thickness of ∼160 nm and pore size of ∼10 nm can effectively separate the nano-sized oils and oil-in-water nanoemulsions with ultrahigh permeation fluxes up to 6000 L m−2 h−1 bar−1, which is about 10 times the fluxes of traditional ultrafiltration membranes with a similar rejection property. Meanwhile, the SWCnt/pda/pei nano- composite film exhibits excellent ph stability and antifouling property.
these works point a direction for designing and fabricating new types of ultrathin superwetting films by using 1d nanomaterials for emulsion separation.
Besides the SWCnt-based separation films, Wang and co-workers fabri- cated a Mno2 nanowire membrane via a vacuum-filtering method.135 this Mno2 nanowire membrane behaves hydrophilically because of the hydrogen bond between water molecules and oxygen atoms exposed on the surfaces of Mno2 nanowires. the hydrophilic Mno2 nanowire membrane with pore size of tens of nanometres can be used to achieve the separation of both free and emulsified oil/water mixtures. these SWCnt- and nanowire-based ultrathin superwetting films have a promising potential for treating real emulsified oily wastewater in large quantities, as an alternative to traditional polymer- and ceramic-dominated membranes.