Nanostructured Three-Dimensional Percolative Channels for Separation of Oil-in-Water Emulsions

Abstract

Separation of oil/water mixtures has been one of the leading green technologies for applications such as oil recovery and water purification. Conventional methods to separate oil from water are based on phase separation via physical settlement or distillation. However, challenges still remain in the effective extraction of micron-sized oil droplets dispersed in water, in which case gravity fails to work as separating force. Here, we conformably decorate porous titanium (average pore size 30 μm) with superhydrophilic nanotubes. The resulting three-dimensional superhydrophilic micro channels thus provide a driving force for oil-water separation at the nanotube/emulsion interface, enhancing significantly the water infiltration rate. The high efficiency (>99.95%, with oil droplets of average diameter 10 μm) and strong mechanical durability make the structure a reusable oil/water separator. Our findings pave the way for future applications of oil-in-water emulsion separation, which can be readily scaled up for massive demulsification.

Keywords: Chemical Engineering; Composite Materials; Materials Science; Separation Science.

Graphical abstract

Figure 1

Morphology and Wetting Properties of 3D-TNTAs (A) 3D topological morphology of the pristine Ti foam, obtained by the 3D micro-CT technique. (B–J) Photograph (B, E) and scanning electron micrographs (C, F, and G) of an original Ti foam (B, C) and that conformably coated with TNTs (E–G). Corresponding illustrations are given in (D) and (H), respectively. Time sequence of video frames showing the evolution of water droplet on original (I) and anatase-TNT-coated (J) foams. (K and L) Contact base diameter (K) and contact angle (L) as a function of time for superhydrophilic (amorphous nanotubes, Am-TNTs; anatase nanotubes, An-TNTs) and superhydrophobic (amorphous nanotubes with TPFS treatment, Am-TPFS; anatase nanotubes with TPFS treatment, An-TPFS) foams. Each of the data point in (K) and (L) was based on at least three samples. See also Figures S1−S7, and S10, and Table S1.

Figure 2

Demonstration of Oil-in-Water Emulsion Separation by 3D-TNTs (A–D) (A) Experimental setup of vertical deployment, where local magnifications are given in (B–D). (E) Illustration showing the separation mechanism. (F) The amount of total organic carbon in filtrates and the corresponding separation efficiency using different emulsions. (G) Recycling tests of the superhydrophilic foam (Type III) employing octane-in-water emulsion. Each of the data point in (F) and (G) was based on at least three samples. See also Figures S6, S8, and S9.

Figure 3

Siphon-like Demulsificator and the XANES Analysis (A and B) An oil filter for octane-in-water emulsion in the configuration of a siphon. (C) Filtrate volume per unit medium area (ν) of the prototype siphon filter as a function of time. Each of the data point was based on at least three independent measurements. (D and E) The first derivative of XANES of Ti K-edge for TiO₂ nanotubes (TiO₂-NT, yellow) and that immerged in water (TiO₂-NT + H₂O, orange) and octane-in-water emulsion (TiO₂-NT + H₂O + oil, blue). See also Figures S11–S13.

Figure 4

Statistics on Various Demulsificators The diagram compares the oil-in-water emulsion separation properties of the current work with that of other reports. (Chaudhary et al., 2014, Chen et al., 2017, Fan et al., 2015, Hu et al., 2015, Huang et al., 2015, Joo et al., 2017, Li et al., 2015a, Li et al., 2015b, Li et al., 2016, Liu et al., 2016a, Liu et al., 2016b, Liu et al., 2015, Luo et al., 2017, Ma et al., 2017, Rohrbach et al., 2014, Teng et al., 2016, Zeng and Guo, 2014, Zhang et al., 2013, Zhang et al., 2017, Zhou et al., 2017b).