Magnetic biochar-doped g-C3N4/Fe2O3 S-scheme heterojunction with enhanced photocatalytic degradation of tetracycline by addition of persulfate

Graphitic carbon nitride (g-C 3 N 4 ) exhibits remarkable thermal and chemical stability, enabling effective activation of molecular oxygen and generation of superoxide radicals for photocatalytic pollutant degradation. However, its low surface area and poor photocatalytic activity have limited its development, and the reaction mechanism of pollutant degradation remains unclear. In this study, we synthesized g-C 3 N 4 /BC/Fe 2 O 3 catalysts by combining g-C 3 N 4 , biochar (BC), and Fe 2 O 3 in intimate contact. The highest photocatalytic degradation efficiency of tetracycline (TC) reached 94.9 % in the g-C 3 N 4 /BC/Fe 2 O 3 -2/PDS system, which was 3.01, 1.53, and 2.35 times higher than that of pure g-C 3 N 4 , BC, and BC/Fe 2 O 3 , respectively. The addition of persulfate (PDS) accelerated the formation of reactive oxygen species (ROS), providing more active species and improving photocatalytic performance, thereby enhancing TC degradation. LC-MS analysis and density functional theory (DFT) calculations were used to elucidate possible TC degradation pathways in the g-C 3 N 4 /BC/Fe 2 O 3 -2/PDS system. Electron paramagnetic resonance (EPR) confirmed the generation of multiple ROS in the reaction system, including h + , •OH, SO 4 •− , •O 2 − , and 1 O 2 . This work provides mechanistic insights into TC degradation and offers a theoretical foundation for future studies on advanced oxidation processes for water treatment. • Novel g-C 3 N 4 /BC/Fe 2 O 3 photocatalyst synthesized using coffee grounds as biochar precursor. • g-C 3 N 4 /BC/Fe 2 O 3 /PDS system achieved 94.9 % tetracycline removal under visible light. • 87.4 % TOC removal indicates effective mineralization of tetracycline.