Utilising bauxite residue (red mud) to construct Z-type heterojunction for formaldehyde degradation

The photocatalytic treatment of pollutants has garnered significant attention owing to its ability to safely and efficiently treat pollutants without secondary contamination. However, the complexity of synthesis methods and the high costs of photocatalysts remain major challenges for the industrial application of this technology. The high content of α-Fe2O3 metal semiconductors in red mud (RM) presents an opportunity for the effective preparation of RM-based α-Fe2O3 composites for catalytic degradation. Graphitic-phase carbon nitride features a similar energy band structure to α-Fe2O3 and demonstrates excellent photocatalytic properties. Herein, we directly prepared a novel Z-scheme-heterojunction g-C3N4/RM composites via a one-step calcination method by reconstructing the interface between RM and melamine. Comprehensive characterisations revealed the formation of a Z-scheme heterojunction between g-C3N4 and α-Fe2O3. This significantly improved the spatial separation of photogenerated carriers, enabling the efficient degradation of formaldehyde. The degradation efficiency reached 63.04% after 2 h of light exposure, and the composites exhibited excellent recyclability. The optical adsorption and photocatalytic properties of this newly developed composite material were significantly enhanced compared with pristine RM, which degraded 26.96% of formaldehyde under similar conditions. This study marks the first application of Z-type heterojunctions constructed from bauxite residue and nonmetallic semiconductors for degrading formaldehyde, an air pollutant. Our work offers new insights into leveraging solid wastes for high-value-added advanced catalytic applications.