In Situ Hydrothermal Synthesis of a g-C3N4–Bi2O3 Heterojunction for Efficient Photocatalytic Degradation of Tetracycline Hydrochloride

Abstract Antibiotics represent persistent chemical pollutants that threaten aquatic ecosystems and public health. Tetracycline hydrochloride (TCH) is one of the most widely used antibiotics, accumulates in water bodies due to incomplete metabolism and resistance to biodegradation. In this study, heterojunction photocatalysts based on graphitic carbon nitride (g-C 3 N 4 ) and bismuth oxide (Bi 2 O 3 ) were synthesized via a hydrothermal method, incorporating different Bi 2 O 3 ratios. The resulting materials were characterized by XRD, TEM, STEM, FTIR, UV–Vis DRS, N 2 physisorption, and XPS to analyze their structural, morphological, and surface properties. Among the tested samples, the optimized gCNBiO-1.5 composite (surface area 9.4 m²/g, band gap 2.73 eV) exhibited the highest activity, achieving 93% TCH degradation within 90 min under UV-A irradiation and 99.8% removal at pH 4. Kinetic analysis confirmed pseudo-first-order behavior with k app = 0.0288 min⁻¹ (t ½ = 24.05 min). Efficient mineralization was evidenced by a decrease in COD from 75.6 to 8.1 mg/L after 90 min. Furthermore, the catalyst maintained 84.6% efficiency after three reuse cycles. These results highlight the potential of g-C 3 N 4 –Bi 2 O 3 composites as stable, efficient photocatalysts for the removal of antibiotic pollutants from aqueous environments.