Acid-modified red mud S-scheme heterojunction for enhanced photocatalytic H2 evolution

Citric acid modification reduces red mud alkalinity, enabling effective S-scheme heterojunction with ZnIn₂S₄; • The 4 wt%-ARM/ZIS achieves 2.59 mmol g −1  h −1 H₂ production, 3.28 times higher than pristine ZnIn₂S₄; • Innovative strategy for red mud valorization in sustainable photocatalytic hydrogen production. Hydrogen energy has emerged as a major research focus owing to high combustion heat value, zero carbon emissions, and diverse utilization forms, while photocatalytic hydrogen production, in particular, has attracted considerable interest by virtue of cost-effectiveness and environmental compatibility. Red mud (RM), a large-volume waste from bauxite refining, features a complex composition with harmful and strongly alkaline species that make its disposal costly and environmentally hazardous, thereby demanding urgent and effective treatment solutions. Herein, RM was modified with citric acid to reduce the strong alkalinity of RM, thereby obtaining acid-treated RM (ARM), which is beneficial to be used as reduction photocatalyst and combine with the typical oxidation photocatalyst (ZnIn 2 S 4 , ZIS) to construct the S-scheme heterojunction. The results reveal that the optimal 4 wt%-ARM/ZIS composite achieved the highest photocatalytic hydrogen production rate of 2.59 mmol g −1  h −1 under full-spectrum light irradiation, 3.28 times higher than pure ZIS, demonstrating significant improvement in photocatalytic performance. This work provides innovative approaches and practical solutions for the utilization of RM resources, while also promoting their application in photocatalytic hydrogen production.