Photo-stimulated self-cleaning oxidized g–C₃N₄-polypyrrole composite membrane for fractionating EBT dyes from salts and separating oil-in-water emulsions

The widespread discharge of industrial wastewater containing dyes and emulsified oils significantly contributes to water pollution. Photoactive self-cleaning membranes can effectively filter out these contaminants; however, developing a stable and efficient photoactive layer remains challenging. This study addresses this by oxidizing graphitic carbon nitride (g–C₃N₄) to enhance its interaction with polypyrrole. The oxidized g–C₃N₄ was then polymerized with pyrrole to form a durable self-cleaning composite membrane capable of separating Eriochrome Black T (EBT) dye from salts and removing surfactant-stabilized oil from water. To prepare the membrane, g–C₃N₄ was synthesized from melamine using a single-step thermal process, followed by oxidation to produce O–g–C₃N₄. The resultant α-Al₂O₃@PPy/O–g–C₃N₄ membrane exhibited superior structural stability, hydrophilicity (contact angle of 13 ± 0.69°), and underwater oleophobicity (153.2 ± 2.14°), attributed to its polar functional groups. The membrane also showed excellent fractionation performance, effectively separating EBT from various salts, with salt permeabilities of NaCl, Na₂SO₄, MgSO₄, and MgCl₂ in the range of 82 to 97 %, and achieving over 96 % rejection of EBT dye. The α-Al₂O₃@PPy/O–g–C₃N₄ membranes also demonstrated outstanding separation efficiency (>99 %) for water's vegetable, diesel, and petroleum ether emulsions. It has been observed that α-Al₂O₃@PPy/g–C₃N₄ membranes have less fouling resistance than α-Al₂O₃@PPy/O–g–C₃N₄. After EBT fouling, membranes were exposed to solar light for 30 min, restoring 96 % of flux with unchanged separation efficiency. The photo-responsive self-cleaning feature of α-Al₂O₃@PPy/O–g–C₃N₄ renders it an advanced anti-fouling membrane suitable for separating various emulsions and dye/salt fractionation.