The removal of submicrometer-sized oil droplets from water remains a key challenge in engineering the separation of emulsions and has emerged as an urgent imperative given the increasing use of unconventional extractive processes. In this work, the authors demonstrate that a substrate with hierarchical texturation shows pronounced differences in the wettability of water and hexadecane, thereby, facilitating the separation of these two disparate liquids at room temperature and pressure. The multiscale textured substrates are assembled using a facile and readily scalable process, wherein ZnO nanotetrapods are spray-deposited onto a steel mesh with micron-sized features. Separation efficiencies well over 99% are accessible by simply flowing emulsions across these hierarchically textured surfaces.
The steam-assisted gravity-drainage (SAGD) method has emerged as among the leading methods of enhanced oil recovery and is predicated on the injection of steam within the wellbore followed by extraction of emulsions of viscous oil and water. The emulsions are stabilized by endogenous surfactants, necessitating extensive processing such as addition of chemical de-emulsifiers and slow gravity-based separation methods. Here, we show that a hierarchically textured membrane exhibiting orthogonal wettability, specifically, superoleophilic but superhydrophobic behavior, allows for effective separation of the water and viscous oil fractions of SAGD emulsions. The membrane is constructed by integrating ZnO nanotetrapods onto stainless steel meshes using a conformal amorphous SiO 2 layer and is both mechanically resilient and thermally robust. The intrinsic surface energy characteristics of the ZnO tetrapods as well as their three-dimensional texture when arrayed atop the stainless steel mesh substrates contribute to the observed differential wettability between water and oil. Water content in permeated bitumen is reduced to as low as 0.69 vol % through a single-pass filtration step with the further advantage of eliminating silt particles. The permeation temperature and water content are tunable based on modulation of the mesh size and ZnO loading. The membranes allow for operation at SAGD temperatures in excess of 130 °C, thereby enabling the thermal disruption of hierarchical emulsions. The membrane-based separation of SAGD emulsions under realistic process conditions paves the way for entirely new process designs for recovering dry viscous oil.
Achieving the efficacious and rapid separation of mixed water/oil streams has emerged as a fundamental imperative in order to facilitate extraction of fossil fuels by methods such as cyclic steam stimulation as well as to mitigate the potentially calamitous impact of oil spills in natural aquatic environments. Here, we demonstrate functionalized ZnO nanotetrapodal membranes combining the micrometer-scale texturation of underlying stainless steel meshes with the nanoscale texturation of an enmeshed interconnected porous network of ZnO tetrapods, the conformal adhesion afforded by an amorphous silica layer, and the low surface energy of surface-deposited perfluorinated sulfonate layers. The membranes exhibit pronounced differential wettability and selectively permeate water whilst retaining oil. The multivariate design space of the architectures has been evaluated to determine the mesh size and ZnO loading that yield the highest separation efficiencies. Oil content in recovered water is reduced to <300 ppm while maintaining a flux rate of greater than 325 L/(m 2 •h). The functioning of the membranes can be understood in terms of the creation of differential Cassie−Baxter nonwetting and Wenzel wetting regimes for oil and water, respectively.
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