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

Abstract

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 Mn 2+ (aq) under circumneutral conditions catalyzed by naturally abundant semiconducting TiO 2 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 TiO 2 . These findings suggest that photocatalytic oxidation of Mn 2+ (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.

Understanding the role of manganese oxides in retaining harmful metals: Insights into oxidation and adsorption mechanisms at microstructure level

“…Todorokite, a prominent Mn mineral found in marine ferromanganese nodules and crusts [ 37 , [124] , [125] , [126] ], is characterized by its tunnel structure composed of triple chains of edge-sharing Mn(IV)O 6 octahedra that are corner-shared with adjacent triple octahedral chains. This arrangement results in a 3 × 3 tunnel structure, providing a significant tunnel cavity per unit cell measuring 6.9 × 6.9 Å [ 29 , 123 , 127 , 128 ]. The tunnels present in natural samples exhibit a partial occupancy of water molecules and a diverse range of cations, such as Mg 2+ , Ca 2+ , K + , Na + , and Ba 2+ [ 124 , 127 ].…”

A critical review of mineral–microbe interaction and co-evolution: mechanisms and applications

“…Oxidation of Mn(II) by molecular O 2 is slow at neutral pH, and most Mn(II) oxidation is catalyzed by aerobic bacteria and fungi [ 30 , 123 ] via multi-copper oxidases. Mn(II) oxidation catalyzed by iron oxide surface is also possible [ 124 ]. Because biological Mn(II) oxidation depends on availability of Cu for synthesis of multi-copper oxidase and/or formation of Fe oxides for catalysis, Mn deposits generally occur after BIF and GIF (Fig.…”

Manganese Oxidation States in Volcanic Soils across Annual Rainfall Gradients

“…It is primarily soluble and required to support the biochemistry of photosynthesis for plants. , Mn­(IV) exists as Mn­(IV) oxides that are metal scavengers and strong oxidizers, influencing the fate of trace metals, organic compounds, and nutrients in the environment. − Solid Mn­(III) exists as oxyhydroxides, oxides, or substitutes in other mineral structures; dissolved Mn­(III) tends to disproportionate to Mn­(II) and Mn­(IV) unless it binds to organic or inorganic ligands (L) to form stable or metastable Mn­(III)–L complexes. , Mn­(III)–L complexes are potent oxidants and play key roles in depolymerizing lignin and other complex organic compounds during soil organic matter (SOM) and litter decomposition. , Mn cycles among the three oxidation states in response to the redox fluctuations . Mn­(II) can be oxidized by O 2 to Mn­(III) or further to Mn­(IV), and conversely, Mn­(IV) can be reduced to Mn­(II) with Mn­(III) as an intermediate product. , Abiotic processes are important in oxidation of Mn­(II) under oxic conditions and reduction of Mn­(III, IV) under reducing conditions as soils are rich in chemically reactive reductants, such as some labile SOM compounds, and oxidants, such as radicals in addition to O 2 . − However, the redox processes are often mediated by microorganisms, particularly for the formation of Mn­(IV) oxides due to the slow kinetics of homogeneous abiotic oxidation of Mn­(II) by O 2 . , …”