Photocatalytic Oxidation of Dissolved Mn2+ by TiO2 and the Formation of Tunnel Structured Manganese Oxides

The redox reaction of manganese (Mn) is of great environmental, geological, and public health significance, as Mn oxides control the distribution and electron flow of numerous nutrients and contaminants in natural and engineered environments. Current understanding on the oxidation pathways of Mn(II) to Mn(III/IV) mainly focuses on biotic processes due to their much higher oxidation rates than those of abiotic processes. This study demonstrates rapid photocatalytic oxidation of Mn2+(aq) under circumneutral conditions catalyzed by naturally abundant semiconducting TiO2 minerals. Notably, the photocatalytic oxidation rates are comparable to or even higher than those of reported biotic/abiotic processes. In addition, the rapid photocatalytic oxidation leads to the formation of large tunnel structured Mn oxides (todorokite and romanechite) on the surface of TiO2. These findings suggest that photocatalytic oxidation of Mn2+(aq) by natural semiconducting minerals is likely an important yet previously overlooked pathway for understanding the occurrence of natural Mn oxide coatings on rock surfaces. In addition, considering the increasing input of photoreactive engineered nanoparticles into environmental systems, this study shows the potential impacts of nanoparticles on influencing natural redox cycles.

Automated summary

The study demonstrates rapid photocatalytic oxidation of Mn2+(aq) under circumneutral conditions catalyzed by naturally abundant semiconducting TiO2 minerals. The photocatalytic oxidation rates are comparable to or even higher than those of reported biotic/abiotic processes. Additionally, the rapid photocatalytic oxidation leads to the formation of large tunnel structured Mn oxides on the surface of TiO2, suggesting a previously overlooked pathway for understanding the occurrence of natural Mn oxide coatings on rock surfaces.