One-Step Calcined Bi-Doped g-C3N4: Surface–Interface Mechanism for Ciprofloxacin Photocatalytic Degradation

The widespread presence of ciprofloxacin (CIP) in aquatic environments threatens ecological and public health, yet conventional treatment processes fail to remove such persistent contaminants. Conventional solvothermal synthesis of Bi-doped g-C3N4 photocatalysts involves complicated procedures and low productivity. Herein, we employ a single-step, template-free and solvent-free green calcination method to construct Bi3+-modified g-C3N4 with strong Bi-N coordination interactions. A series of Bi/g-C3N4 photocatalysts with Bi-doping mass ratios of 0.09–0.34 wt% was prepared, and the structure–performance relationship as well as the surface–interface reaction mechanism for ciprofloxacin (CIP) degradation were systematically elucidated. Experimental results confirm that Bi3+ incorporates into the lattice via Bi-N coordination bonds with nitrogen in the g-C3N4 framework, which narrows the band gap, suppresses photogenerated carrier recombination, and constructs a loose porous morphology beneficial for increasing specific surface area and active sites. Under optimal conditions, 15Bi/g-C3N4 achieves 97.6% degradation of 15 mg L−1 CIP within 90 min, which is 13.7% higher than that of pristine g-C3N4. The effects of catalyst dosage, initial pH, CIP concentration, common coexisting ions, and different real water matrices on the degradation performance were systematically investigated. Radical quenching experiments combined with ESR characterization confirm that h+ is the dominant reactive species responsible for CIP degradation. This green, simple and scalable method yields uniform products, and the resulting materials exhibit high efficiency, economic feasibility and environmental safety, demonstrating promising potential for antibiotic wastewater treatment.