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

摘要

class="head no_bottom_margin" id="sec1title">IntroductionSeparation of oil from water has been an issue that has developed along with the development of modern technology (, ). For example, petroleum manufacturing process has aroused environmental concerns such as oil waste and water pollution (). Recent advancement of green technologies has also boosted the need of recovery of oil from water for domestic uses such as kitchen waste recycling (, ). It is known that by utilizing gravity (), centrifugal force (), electrochemical means (), or adsorption (), oil/water mixtures can be separated in a macro scale.However, those conventional methods often fail to work when it comes down to the micro scale, which is most often the case in practical applications (). To mitigate this, treatments such as chemical agents are often applied in demulsification technology (). To achieve high-efficiency separation of micron-sized oil droplets in water (i.e., oil-water emulsion), major challenges remain in the high porosity and superhydrophilic surface as concomitants of the material itself. Recently, emerging designs of oil-water emulsion filters, such as organic membranes () and surface-functionalized metal meshes (), have become popular in the field of oil/water separation research. Nano-arrays mimicking cacti surface are also reported to be candidates for the effective collection of micron-sized oil droplets from water (). Nevertheless, they either suffer from poor mechanical durability or from an impotent filtering efficiency. To date, a single reusable demulsificator that combines the merits of mechanical durability, high efficiency, and high throughput has been missing.Among the reported demulsificators, self-organized anodic TiO2 nanotubes (TNTs) vertically grown on non-planar titanium substrates have attracted tremendous interest because of the superb water wettability (, href="#bib41" rid="bib41" class=" bibr popnode">Xiang et al., 2017). For example, TNT-covered titanium meshes, wires, and tubes have been used not only in oil/water separation (href="#bib35" rid="bib35" class=" bibr popnode">Sun et al., 2014, href="#bib24" rid="bib24" class=" bibr popnode">Liao et al., 2012) but also in fields such as organic matter degradation (href="#bib24" rid="bib24" class=" bibr popnode">Liao et al., 2012), flexible solar cells (href="#bib40" rid="bib40" class=" bibr popnode">Wen et al., 2016), and Li-ion battery systems (href="#bib47" rid="bib47" class=" bibr popnode">Zhang et al., 2014). The growth mechanism of vertical TiO2 nanotube arrays (TNTAs) on titanium foams is yet elusive. The reported oil/water separation methods based on TNTAs are mostly limited to the outer surface modification on top of the titanium foams (href="#bib21" rid="bib21" class=" bibr popnode">Li et al., 2015c) and only with low porosity because of the poor oxidation kinetics condition in the micro-pores (href="#bib1" rid="bib1" class=" bibr popnode">Atencia and Beebe, 2005, href="#bib39" rid="bib39" class=" bibr popnode">Wei et al., 2000).In this work, we show that by developing a three-dimensional (3D) percolative anodization technique, high-porosity titanium framework with thickness reaching a few millimeters can be conformably decorated with superhydrophilic TNT vertical arrays. These 3D percolative superhydrophilic micro channels can serve as “highways” for water but prevent the transport of oil droplets (average size 10 μm, see href="#mmc1" rid="mmc1" class=" supplementary-material">Supplemental Information). It thus leads to a low-cost reusable oil/water separator with ultrahigh efficiency (>99.95%) and strong mechanical durability.