Anion/Cation Modification-Regulated Surface Defects and Interfacial Charge Transfer in Bi2MoO6 for Photocatalytic Degradation of Tetracycline from Water under Visible-Light Irradiation

This work presents a mechanism-oriented comparison of how S and Cu modifications regulate surface defects, band structure, and interfacial charge transfer in Bi 2 MoO 6 for visible-light-driven tetracycline (TC) degradation. Pristine Bi 2 MoO 6 , S-modified Bi 2 MoO 6 , and Cu-modified Bi 2 MoO 6 were synthesized by a one-step hydrothermal method. Structural and spectroscopic analyses show that S modification perturbs the local Mo–O/Bi–O coordination environment, slightly expands the lattice spacing, weakens Mo–O-related vibrations, and narrows the band gap. In contrast, Cu modification induces local lattice distortion, oxygen-vacancy-associated surface defects, and Cu + /Cu 2+ redox-active sites, thereby facilitating interfacial charge transfer and defect-mediated O 2 activation. Radical trapping and EPR analyses reveal that BMS-13 mainly follows an h + -dominated mixed oxidation pathway, whereas BMCu-8 preferentially promotes reactive oxygen species (ROS)-mediated oxidation involving ·O 2 – and ·OH. Under visible-light irradiation, BMS-13 and BMCu-8 achieved TC degradation efficiencies of 75.17% and 78.16%, respectively, with apparent rate constants of 0.01070 and 0.01171 min –1 . LC-MS and TOC analyses indicate that TC degradation proceeds through stepwise molecular transformation, ring opening, fragmentation, and partial mineralization rather than complete mineralization. This work clarifies the modification-dependent regulation mechanisms of Bi 2 MoO 6 and provides useful guidance for designing defect-regulated photocatalysts for antibiotic-contaminated water treatment.