Shape-controllable synthesis of MnO₂ nanostructures from manganese-contained wastewater for phenol degradation by activating peroxymonosulphate: performance and mechanism

Nanostructured manganese oxide materials were prepared from manganese-contained wastewater (MW) using a facile hydrothermal method and adopted as a catalyst to degrade phenol via activation of peroxymonosulphate (PMS). In the WM environment, δ-MnO₂ (flower-like Mn-2 with nanosheets) was transformed to α-MnO₂ (needle-like Mn-4 with nanowires). Catalytic evaluation experiments demonstrated that the needle-like MnO₂ was highly efficient for phenol removal, with a degradation efficiency of 100% within 15 min at the optimal conditions of catalyst dosage 0.2 g/L, PMS dosage 1.5 g/L, initial phenol concentration 0.025 g/L, initial pH 3 and temperature 25°C. Moreover, the needle-like MnO₂ catalyst could be recycled and the regenerated material after calcination remained excellent catalytic activity. On the surface of catalysts, PMS was activated by Mn^IV to generate ${\mathrm{SO}}_4^{-\bullet }$ which was the major reactive species attacking phenol. Overall, the needle-like MnO₂ prepared from MW was an efficient catalyst with low cost for organic wastewater treatment, realizing both Mn resource recycle and organic wastewater treatment.