Visible light-induced N-doped TiO2 nanoparticles for the degradation of microcystin-LR

N-doped nano-crystalline TiO₂ powders have been synthesized by the sol-gel method. The shape and crystal structure of the resulting N-doped TiO₂ were investigated by X-ray Photoelectron Spectroscopy (XPS), X-ray spectroscopy (XRD), Transmission Electron Microscopy (TEM) and UV-vis reflection spectrum. The results showed that doping TiO₂ with nitrogen can lower its band gap and apparently shift its optical response to the visible region. Under the visible light (λ > 420 nm) irradiation, the MC-LR was degraded by the synthesized N-TiO₂ nano-material. The variation of MC-LR amount and its intermediates were detected by high performance liquid chromatography (HPLC) and LC-MS, respectively. The mineralization of MC-LR was determined by total organic carbon (TOC) analysis. Simultaneously, transient oxidative species generated during photocatalysis were tracked by electron spin resonance (ESR) and Peroxidase method. All these results indicated that visible-light excited N-TiO₂ can activate molecular oxygen and thereby achieve degradation of MC-LR completely within 14 h. The removal of 59% of TOC was achieved after 20 h irradiation. The major oxidative species in the system were hydroxyl radical (·OH) and H₂O₂. 13 Kinds of intermediates were primarily identified in the process. Based on these results, a reasonable conclusion was drawn for the degradation of MC-LR wherein its four positions are easy to be attacked by the photo-generated OH radical followed by the hydrolyzation of peptides.