Fabrication of a robust inorganic membrane decorated with a photocatalytic Ag₃PO₄/TiO₂ nanocomposite for sustainable separation of emulsified oil from water

A variety of polymeric membranes have been developed over the years for treating wastewater streams. Though organic polymeric membranes have performed remarkably in treating wastewater streams, they have certain drawbacks, such as releasing degradation products into water bodies as pollutants. Hence, the need of the day is to develop alternative options that are environmentally benign and easy to fabricate. In such a scenario, inorganic membranes are highly promising options for treating the complex wastewater streams. Hence, the current study was designed to fabricate a fully inorganic membrane composed of a photocatalytic Ag₃PO₄/TiO₂ (AgP/TiO₂) nanocomposite active layer decorated on the alumina ceramic support using a drop-cast approach onto a ceramic disc, followed by calcination. The synthesized AgP/TiO₂ nanocomposite and the AgP/TiO₂@Aluminum oxide membrane were studied for their physicochemical properties and application in the efficient separation of stabilized and emulsified oil from water. The field-emission transmission electron microscopy (FE-TEM) revealed the detailed internal structure of the nanocomposite, confirming the embedding of AgP in the matrix of TiO₂, whereas the selected area electron diffraction (SAED) confirmed the crystallinity of the nanocomposite. In the case of the AgP/TiO₂@Aluminum oxide membrane, the membrane exhibited unique wettability: superhydrophilic/superoleophilic in air and superoleophobic underwater, as confirmed by water and oil contact angle measurements. The membrane exhibited excellent separation efficiency of approximately 99% across a range of applied feed pressures. The permeate flux was 760.5 L m⁻² h⁻¹ and 549.3 L m⁻² h⁻¹ at 2 bar using distilled water and a 100 ppm crude oil-in-water emulsion, respectively. Despite the special wettability features of the AgP/TiO₂@Aluminum oxide membrane, fouling was inevitable, as evidenced by long-term separation experiments. However, the membrane's photocatalytic self-cleaning properties restored its permeability.