The increased oxygen vacancy by morphology regulation of MnO2 for efficient removal of PAHs in aqueous solution.

Manganese dioxide (MnO2) is considered to have a promising future in degrading polycyclic aromatic hydrocarbons (PAHs) in aqueous phase because of its low cost and environmental friendliness. In this study, various MnO2 morphologies were prepared, and their removal performance and mechanism were evaluated using benzo(a)pyrene (B[a]P) as model molecule. Results showed that nanoflower MnO2 with higher concentration of oxygen vacancies exhibited better oxidative and easier oxygen migration properties, and thus enhanced PAHs removal by 14.28%-43.21% compared with other MnO2 samples. Additionally, the transformation rate of PAHs is correlated with their ionization potential (IP) values. Further mechanism studies showed that the degradation of B[a]P by MnO2 process was first to form a combination and then oxidized by non-radical Mn species and superoxide radical (O2-•) to produce degradation product (B[a]P-6-one and B[a]P-6,12-quinone). The specific surface area was not the main factor affecting the removal of B[a]P by MnO2 and oxidation was the main removal mechanism of degrading B[a]P by MnO2. Mn3+ and absorbed oxygen (Oabs) played an important role in the process of removing PAHs by MnO2. Additionally, synergistic effects of oxygen vacancy and Mn3+could be benefit for transforming Oabs to O2-•, leading to the efficient degradation of PAHs.