Photocatalytic degradation of tetracycline antibiotic by a novel Bi₂Sn₂O₇/Bi₂MoO₆ S-scheme heterojunction: Performance, mechanism insight and toxicity assessment

The fabrication of step-scheme (S-scheme) heterojunction with superior redox capability has been emerging as a prospective strategy for environmental remediation. Herein, novel Bi₂Sn₂O₇/Bi₂MoO₆ S-scheme heterojunctions have been fabricated via in-situ anchoring Bi₂Sn₂O₇ nanoparticles on Bi₂MoO₆ microspheres. The optimized Bi₂Sn₂O₇/Bi₂MoO₆ (BSO/BMO-6%) attains the highest reaction rate constant (k) in the degradation of tetracycline hydrochloride (TC, k = 0.0397 min⁻¹), which is 3.62 folds higher than that of pristine Bi₂MoO₆. Such an improvement is originated from more exposed active sites, higher photo-excited charge separation efficiency, superior redox ability, and efficient production of reactive h⁺, •OH and •O^2–. Besides, Bi₂Sn₂O₇/Bi₂MoO₆ could efficiently degrade the TC antibiotic in actual water matrix. Significantly, the toxicity evaluation verifies the nontoxicity of Bi₂Sn₂O₇/Bi₂MoO₆. Moreover, the degradation pathways of TC are determined and the toxicity of degradation intermediates is appraised by using HPLC-MS spectra and QSAR prediction. A possible photocatalytic mechanism over S-scheme Bi₂Sn₂O₇/Bi₂MoO₆ has been elucidated based on experimental studies combined with density functional theory (DFT) calculations. This work offers new insights for the design of high-performance S-scheme heterojunctions for environmental remediation.