Visible-light Cr(VI) reduction on a CdS@MOF@C3N4 heterostructure via cascade S-scheme mechanism: a comprehensive study

A ternary CdS@MOF@C₃N₄ nanocomposite was synthesized and evaluated as a visible-light-driven photocatalyst for the reduction of hexavalent chromium (Cr(VI)). Structural and spectroscopic analyses (XRD, FTIR, TEM, FESEM-EDS, TGA, BET, PL, UV-Vis DRS) confirmed the successful integration of CdS and g-C₃N₄ within the Al-fumarate MOF framework, leading to enhanced light absorption, interfacial contact, and charge-transfer properties. Under optimized conditions (pH 3, catalyst dosage 0.4 g L^-1, initial Cr(VI) 20 mg L^{- 1}), the nanocomposite achieved a Cr(VI) removal efficiency of 86.8%, outperforming the single and binary reference systems. Scavenger experiments and band-structure analysis indicated that the dominant pathway proceeds via a cascade S-scheme mechanism, where electrons migrate from g-C₃N₄ to Al-Fu MOF and then to VB of CdS. The preserved CdS electrons (in CB) serve as the dominant reductive species for Cr(VI) photoreduction. The composite also retained its photocatalytic activity over repeated cycles, demonstrating structural stability and reusability. These findings highlight CdS@MOF@C₃N₄ as a promising photocatalyst for Cr(VI) removal and provide mechanistic insight into charge-transfer processes in ternary heterojunctions.