Design and mechanistic insight into a Na-C3N4/Bi4O5Br2 S-scheme heterojunction for efficient antibiotic degradation via low concentration peroxymonosulfate activation

Advances in medical biotechnology have driven the widespread use of antibiotics, leading to their environmental persistence and associated threats to human health. In this work, a Na-modified S-scheme heterojunction Na-C₃N₄/Bi₄O₅Br₂ was fabricated for photocatalytic activation of PMS to degrade tetracycline (TC). The Na ions are embedded into the heptazine ring of C₃N₄, which alters the symmetrical structure of C₃N₄. Importantly, Na-C₃N₄/Bi₄O₅Br₂ activated a low PMS dosage (5 mg) to generate reactive oxygen species, achieving rapid tetracycline degradation (91 % in 10 min). This superior performance stems from the synergistic effects of efficient charge separation and the strong redox capacity of the heterojunction. In-situ XPS analysis revealed the electron transfer pathway within the Na-C₃N₄/Bi₄O₅Br₂ composite under both dark and illuminated conditions. Combined with DFT calculations, these results support an S-scheme mechanism for PMS activation in TC degradation. Upon light irradiation, electrons in the CB of Na-C₃N₄ preferentially migrate to the VB of Bi₄O₅Br₂, while the remaining electrons in the CB of Bi₄O₅Br₂ and holes in the VB of Na-C₃N₄ react with OH⁻, O₂, and PMS to generate reactive species. This study proposed a reasonable design for an ionic-doped S-scheme heterojunction used for degradation of organic wastewater through photocatalysis and advanced oxidation technology.