A novel TiO/BiOI heterojunction-activated persulfate photocatalytic removal of tetracycline from pharmaceutical secondary wastewater.

Bismuth oxyiodide (BiOI) can not only generate charge carriers (holes and electrons) under ultraviolet (UV) light irradiation but also activate potassium persulfate (PDS-K) significantly enhancing its catalytic performance. However, the narrow bandgap of single-phase BiOI leads to rapid recombination of hole/electron pairs. Considering the unique property of BiOI and the matching band structure between TiO and BiOI, a novel TiO/BiOI binary heterojunction was constructed by coupling TiO and BiOI. Microstructural characterization confirms that TiO nanoparticles are uniformly loaded on the surface of BiOI nanosheets, forming a heterojunction with an intimate interface. Response surface methodology was employed to optimize the preparation parameters of TiO/BiOI. The as-fabricated TiO/BiOI/PDS-K catalyst demonstrates high and stable photocatalytic activity for tetracycline (TC) degradation under UV irradiation. Its reaction rate constant is 2.76-fold and 8.73-fold higher than those of the TiO/BiOI/UV and PDS-K/UV systems, respectively. Additionally, HCO can react with •OH in water, reducing the concentration of •OH, which also competing with TC for h. This consequently diminishs the degradation efficiency of TC. The potential degradation pathways of TC were explored via liquid chromatography-mass spectrometry. Quenching experiment demonstrated the existence of a free radical pathway in the TiO/BiOI/PDS-K/UV system. In the free radical pathway, the primary reactive oxygen species were •SO, •OH, h and O radicals. This study provides profound insights into the construction of a reusable TiO/BiOI heterojunction integrated with PDS-K for treating industrial water environments.