Photocatalytic degradation of tetracycline by metal-organic frameworks modified with Bi₂WO₆ nanosheet under direct sunlight

Porous metal-organic frameworks (MOFs) with visible-light response have attracted much attention in the field of environmental purification and solar energy conversion. In this study, MIL-100(Fe) was modified with Bi₂WO₆ nanosheets by a facile hydrothermal method to fabricate a photocatalyst with direct Z-scheme heterojunction. When treating the tetracycline (TC) solution under natural sunlight, 12 wt%MIL-100(Fe)/Bi₂WO₆ obtained the highest apparent rate constant of (6.59 ± 0.52)✕10^-3 L mg^-1 min^-1, which was 16.1 and 3.9 times than that of pristine MIL-100(Fe) and Bi₂WO₆, respectively. In addition to explore the feasibility of sunlight-activated MIL-100(Fe)/Bi₂WO₆ to remove TC under various conditions, the degradation intermediates and their possible transformation pathway were provided with the aid of three-dimensional excitation-emission matrix spectra and liquid chromatography-mass spectrometry system. The results of Escherichia coli culture demonstrated that the biotoxicity variation of TC solution would first increase and then decrease with the photodegradation time. Ultimately, based on the results of bandgap calculation, radicals trapping and charge flow tracking experiments, the direct Z-scheme heterojunction between MIL-100(Fe) and Bi₂WO₆ nanosheets was confirmed and the photocatalytic mechanism for TC degradation was rationally proposed. This work enriched MOFs-based heterojunction photocatalysts and provided a promising method to eliminate hazardous TC from aqueous solution.