Nitric acid treated NaBiO3 (N-NBO) was observed to have the ability of yielding singlet oxygen (O-1(2)) for degrading organic pollutants, and a mechanism study or insight into O-1(2) generation from metastable lattice of treated-NBO was conducted. It was demonstrated that the acid treatment made Bi(V)O-6 octahedron in NBO undergo drastic distortion, leading to the formation of a metastable Bi(III)O-6 octahedron, which could store and release active oxygen as O-1(2) . As confirmed by characterization experiments, the O-1(2)-generating ability of N-NBO could be activated, deactivated and re-activated by different treatments, which corresponded to the formation, consummation and re-formation of the active metastable Bi(III)O-6 octahedron. The formation of the metastable structure was related closely to the conversion of Bi5+ to Bi3+ in N-NBO, and its "collapse" eliminated the activity of the metastable Bi(III)O-6 octahedron "pseudo-interface". This structure was responsible for the release of active oxygen from the lattice in the form of O-1(2), and the deactivated N-NBO could be re-activated by re-constructing this metastable structure through metal-ion doping and iodide reduction. A mechanism of O-1(2) generation was proposed by considering the Bi(V)-Bi(III) conversion and the formation of the metastable Bi(III)O-6 octahedron. (C) 2018 Elsevier B.V. All rights reserved.