Specific Adsorption and Efficient Degradation of Cylindrospermopsin on Oxygen-Vacancy Sites of BiOBr

The oxygen-vacancy (OV)-rich BiOBr exhibits higher photocatalytic activity than that of the OV-deficient one. The interaction of the target substrate with the surface OV sites should be essential for the high activity, but it is not well understood so far. Here, we used a cyanotoxin cylindrospermopsin (CYN) as a model substrate to investigate the specific adsorption of pollutants with highly hydrophilic groups on OV sites of BiOBr. We found that the adsorption of CYN on BiOBr with rich OVs is much stronger than the adsorption on BiOBr with poor OVs, and the BiOBr with rich OVs also exhibits a higher degradation rate of CYN. Various experiments indicated that the strong adsorption of CYN on OV-rich BiOBr originates from the strong interaction between the highly hydrophilic sulfate group of CYN and OV sites. Density functional theory calculations suggest that, on OV sites, Brönsted acid sites (OHB) are formed through the dissociative adsorption of water molecules. The combining effect of the hydrogen bonding with these OHB and the coordination with the Lewis acid Bi sites make the adsorption of the sulfate group quite strong. Such a strong interaction can well explain the rapid degradation of CYN, which proceeds through direct oxidation by h+ or by the OHB-derived •OH. Based on ultra-performance liquid chromatography–tandem mass spectrometry analysis, the CYN degradation begins via hydroxylation at the C5–C6 double bond in the uracil ring by •OH, which is further degraded by two pathways. This work provides a deeper understanding of the critical roles of OVs in the processes of adsorption and photocatalysis for organic pollutant removal.