(339f) Desalination and Separation of Oil-Saline Water Mixtures Utilizing Wettability and Photocatalysis Decoupled Membranes

Due to increasing concern over the shortage of freshwater, there is a dire need to remediate unconventional water sources such as brackish water and industrial or municipal wastewater beyond what is obtainable from the hydrologic water cycle. The utilization of membrane-based technologies including reverse osmosis (RO), forward osmosis (FO), ultra- and nano-filtration (UF and NF) seems appealing due to their relatively high energy efficiency and resilience to these types of impure water sources. Membranes can prevent dissolved (e.g., salt ions) and/or suspended (e.g., particulates) substances from permeating the feed and, in this way, produce clean water. However, this often leads to irreversible membrane fouling that results in a decline of permeability and separation efficiency. Thus, prevention or mitigation of fouling has become a key research topic in the oil and water industry sectors.

Conventional mitigation of fouling typically involves hydraulic cleaning by backwashing and forward flushing. Chemicals such as acids, bases, surfactants, or chelating agents can be added to facilitate the cleaning process. Although these techniques allow the membranes to recover their inherent permeability and effective separation ability, the high-pressure flushing of the liquids may shorten the membrane’s lifespan.

Manipulating the wettability of the membrane’s surface may be a promising substitute for the current fouling mitigation method. For example, membranes with superhydrophilic (i.e., water contact angle = 0°) and oleophobic (i.e., oil contact angle > 90°) or underwater superoleophobic (i.e., oil contact angle > 150° on a surface submerged in water) wettability showed oil-water separation with good fouling-resistance to oil. These membranes were incorporated by photocatalytic materials (e.g., titania (TiO₂), zinc oxide (ZnO), tungsten oxide (WO₃)) to enhance the fouling resistance. This is possible because it can degrade organic substances adsorbed on the membrane surface upon light irradiation.

For practical applications, these membranes must fulfill the following three conditions. First, the membrane’s wettability must remain unchanged by a photocatalytic reaction, particularly if the membrane is organic. Similarly, its physical and chemical integrity must not be affected by a photocatalytic reaction. Lastly, the photocatalytic materials deposited into the membrane need to remain undetached when a high hydraulic shear force is exerted.

Guided by these principles, we created a visible-light responsive photocatalytic membrane with superhydrophilic and oleophobic wettability both in air and underwater. We coated the membrane (e.g., porous mesh, filter paper, reverse osmosis (RO) membrane) with a UV-curable adhesive, then sprayed it with a mixture of nitrogen-doped titania (N-TiO₂) and perfluoro silane-grafted silica (F-SiO₂) nanoparticles. Subsequently, the membrane was exposed to UV light which resulted in a chemically heterogeneous surface with intercalating high surface energy (N-TiO₂) and low surface energy (F-SiO₂) regions that were securely bound to the membrane. Both the wettability and the integrity of the membrane remained unaffected throughout the photocatalytic degradation process of the organic substances when exposed to visible light irradiation. This can be attributed to the robust chemistry of the cured adhesive that protected the perfluoro silane molecules grafted to the SiO₂ nanoparticles as well as the underlying membrane from the reactive radical species generated when it was exposed to visible light irradiation. Thus, the membrane can be utilized for continuous separation and desalination of an oil-saline water mixture and simultaneous photocatalytic degradation of the organic substances adsorbed on the membrane surface upon visible light irradiation. Finally, we developed a mathematical model to describe the time-dependent evolution of the permeate flux through the membrane when it was exposed to visible light irradiation.