<p>A flexible, free-standing SnO₂/g-C₃N₄/Cellulose Acetate (CA) composite film was made using a simple and eco-friendly solution-casting method for effective treatment of dye-contaminated wastewater. The polymer-supported heterojunction design aimed to reduce nanoparticle clumping and eliminate the need for recovery steps often needed with powdered photocatalysts. The resulting film showed strong visible-light-driven degradation of methylene blue and methyl orange. Structural and morphological tests using XRD, FTIR, and SEM confirmed the even distribution of crystalline SnO₂ nanoparticles and two-dimensional g-C₃N₄ nanosheets in the porous CA matrix. This setup promoted good contact between the materials and allowed better access for dye molecules to the active sites. UV-DRS results showed better visible-light absorption with a smaller optical band gap, indicating improved light-harvesting ability. Photoluminescence studies also showed reduced electron–hole recombination in the SnO₂/g-C₃N₄/CA heterojunction system, supporting greater charge separation efficiency. The composite film achieved 91.97% degradation of methylene blue and 80.95% degradation of methyl orange within 90&#xa0;min, outperforming the individual components under the same test conditions. The key novelty lies in the first-time development of a free-standing SnO₂/g-C₃N₄/CA photocatalytic film via a green solution-casting approach. The polymer-immobilized heterojunction design effectively overcomes catalyst separation and nanoparticle agglomeration, while providing a scalable, durable, and reusable platform for practical wastewater treatment.</p> Graphical Abstract <p></p>