**3. Water treatment**

In recent times due to the ever-increasing urbanization and industrialization, the world is facing freshwater disaster. Heavy metal ions and organic pollutants cause foremost severe threats to aquatic life. Industrial and conventional treatment methods such as oxidation, biodegradation, absorption, coagulation, etc. have been used for water purification. Membrane technology is the most commonly used technique for water treatment due to its green and environment-friendly approach. However, membrane technology has faced a major problem of fouling of membranes due to prolonged use of technique. In this line of research, increasing the hydrophilicity of the membrane is considered an effective way to reduce the fouling behavior of the

membrane which is caused by pore blocking and fouling material adhesion [14, 15]. By using this method of increasing hydrophilicity of membrane, the hydrophilic material also imparts water permeability stiffness and strength to the polymeric matric used for the preparation of these membranes. The introduction of TiO2 in the membranes as hydrophilic photocatalytic materials can add self-cleaning behavior to the membrane, which upon photoexcitation degrades the natural organic volatile compounds adhered at the surface of the membrane or pores. It is also commonly seen that during the process of phase separation, TiO2 particles have been settled down into the bottom of the polymeric bulk due to its high density. However, when TiO2 nanoparticles hybridized with magnetic nanoparticles, the photocatalytic TiO2 nanoparticles can be adhered to the surface of the polymer matrix with the help of an external magnetic field [16]. Oil–water separation is an effective technique to demonstrate the interplay of membranes between antifouling and wettability. The oil–water separation technique is very effective to clean offshores oil spills and discharging oil effluents which are very harmful for aquatic life and biotic environment. Another very helpful technique for oil–water separation of molecular sieving technique which allows only water molecules hence block the larger oil droplets, this technique has been worked under a specific applied pressure, hydrophilicity, and selective wettability. Porous materials such as metal meshes, fibers foams, and textiles can be used in addition to polymers and ceramics for oil–water separation. The major difficulty faced in the oil–water separation technique is rapid decline of water permeation due to the clogging of pores by oil droplets, therefore, lowering the filtration flux [17]. Also, significant fouling will occur over the surface of membrane due to the presence of hydroxide groups. A thin layer of TiO2 has been used for the removal of fouling over the surface of the membrane [18, 19]. In recent times, cellulose acetate nanofiber membrane was fabricated by electro-spinning which is oleophobic in water, superhydrophjilic in oil, and superamphiphilic in air [13]. This membrane can be working for oil/water separation in wastewater treatment; also it has been used for oil/water emulsions and oil/corrosive medium. An interesting Janus membrane is fabricated by in situ growth of TiO2 on poly (phenylene sulfide) membrane with F-TiO2@poly (phenylene sulfide) followed by water–oil interfacial grafting in presence of perfluorodecyltriethoxysilane [20].

In addition to environmental remediation such as offshore spills, such multifunctional membranes find great applications in food industry, textile factories, and other chemical plants.
