Variable-valence metals catalyzed solid NaBiO3 nanosheets for oxidative degradation of norfloxacin, ofloxacin and ciprofloxacin: Efficiency and mechanism

In this work, we report metal ions catalyzed oxidative degradation of three typical fluoroquinolones norfloxacin (NOR), ofloxacin (OFL) and ciprofloxacin (CIP) by using NaBiO3 nanosheets. It was found that variable-valence metal ions such as Cu2+, Fe2+, Mn2+, Ce3+, Ag+ and Co2+ could obviously enhanced degradation of NOR, OFL and CIP by NaBiO3. The pseudo-first-order kinetic rate for the degradation of 20 mu mol L-1 NOR by NaBiO3 (2 mmol L-1) in the presence of 0.1 mmol L-1 Cu2+, Fe2+, Mn2+, Ce3+, Ag+ and Co-2+ was 0.021, 0.084, 0.019, 0.23, 0.25 and 0.28 min(-1), 2.1, 8.4, 19, 23, 25 and 28 times that by NaBiO3 without any metal ions. In comparison, Ca2+ and Fe3+ exhibited no obviously promotive or depressive effect for the degradation of NOR by NaBiO3. Singlet oxygen (O-1(2)) was suggested as the main reactive species from NaBi03 in the presence of metal ions by electron spin resonance technology and radicals scavenging experiments. The evolution of NaBiO3 was tracked with scanning electron microscope, energy dispersive spectrometer, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. It was found that the metal ions were embedded into the crystal structure of NaBiO3 through ion-exchange between Na in NaBiO3 and metal ions. In the subsequent step, an electron transformation from lattice oxygen to Bi(V) sites was mediated by embedded variable -valence metal species, resulting in an enhanced generation of O-1(2) from the crystal structure of NaBiO3. These results can shed light on the application of NaBiO3 for the organic pollutant decontamination. (C) 2018 Elsevier Ltd. All rights reserved.