(354j) Selective Wettability Membranes for Continuous Separation of Oil?Water and Purification of Water By in Situ Visible Light?Driven Photocatalysis

Membrane‐based technologies are attractive in oily wastewater treatment because they typically do not require chemical additives, thermal inputs, or regeneration of spent media. Because membranes separate contaminants based on their sizes, they are often limited by their inability to remove the dissolved contaminants from the permeate (e.g., water) phase. Also, the dissolved contaminant can adsorb to the membrane surface and pore walls, resulting in fouling. Membrane fouling not only reduces the permeate flux and purity but also leads to shortened membrane lifespan and thereby an increase of the operating cost. Thus, membranes typically undergo periodic cleaning protocols (e.g., backwashing, forward flushing, chemical treatment) to clean the membrane surface and pores. However, implementing these methods incurs process downtime, and can cause membrane damage and degradation over time, which decrements the membrane's performance.

Modulating the membrane's wettability can enhance its resistance to fouling. Previous reports have demonstrated that hydrophilic (i.e., water contact angle, θ*_water < 90°) or superhydrophilic (i.e., θ*_water ≈ 0°) membranes allow water to permeate through while repelling the oil phase. Further, they prevent adsorption of oils or organic contaminants by forming a thin water film on the surface, which enables oil−water separation without a decline of flux.

These membranes have been incorporated with photocatalytic nanomaterials (e.g., ZnO,TiO₂ ,α‐Fe₂O₃, WO₃) which allows for catalytic degradation of the organic contaminants dissolved in the water‐rich permeate upon light irradiation. Such photocatalytic nanomaterials generate electron hole pairs upon light irradiation with an energy greater than their bandgap energy. The generated electrons and holes react with ambient oxygen or water molecules and produce highly reactive radicals (e.g., hydroxyl, peroxide, and superoxide anion). These radicals can oxidize (or reduce) the organic contaminants, which results in decontamination of the permeate and membrane cleaning.

In this work, we developed an in‐air superhydrophilic and underwater superoleophobic membrane capable of separating surfactant‐stabilized oil‐in‐water emulsions and in situ decontamination of the water‐rich permeate by photocatalytic degradation of dissolved organic contaminants upon visible light irradiation. The membrane was fabricated by utilizing thermally sensitized stainless steel mesh coated with iron (Fe) doped titania (TiO₂) nanoparticles (Fe−TiO₂). Fe−TiO₂ is chosen because it can photocatalytically degrade a variety of organic compounds including phenols, acetaldehyde, oxalic acid, and organic dyes upon visible light irradiation. We showed that post‐annealing increased the adhesion force of Fe−TiO₂ nanoparticles to the membrane by the formation of a fusion‐induced bridge between them. We engineered a cross‐flow apparatus that enables continuous oil−water separation and in situ photocatalytic degradation of the dissolved contaminants in the water‐rich permeate upon visible light irradiation. Finally, we demonstrated complete separation of a surfactant‐stabilized oil‐in‐water emulsion and photocatalytic degradation of toxins such as dioxin and permethrin by utilizing the apparatus. We envision that our separation methodology can offer a wide range of potential applications including petroleum refining, wastewater treatment, and oil spills clean‐up.