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

Abstract

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.