Redox-Driven Formation of Mn(III) in Ice

Area of Science:

• Environmental Chemistry
• Geochemistry
• Water-Ice Interfaces

Background:

• Redox reactions of manganese (Mn) species at Mn(IV) oxide surfaces can release dissolved Mn(III) in natural waters.
• Pyrophosphate (PP) serves as a model ligand for studying Mn(III) complexation and release.

Purpose of the Study:

• To investigate the effect of freezing on Mn(III) formation in the presence of pyrophosphate.
• To elucidate the mechanisms of freeze-promoted Mn(III) production in natural water systems.

Main Methods:

• Time-resolved freezing experiments at -20 °C with varying salt (NaCl) concentrations.
• Consecutive freeze-thaw cycle experiments to assess cumulative Mn(III) yields.
• Temperature-resolved freezing experiments down to -50 °C to determine optimal temperature ranges.

Main Results:

• Freezing significantly accelerates and enhances Mn(III) formation as Mn(III)-PP complexes.
• Concentration of Mn(IV) oxides and solutes in liquid intergrain boundaries (LIB) drives the reaction.
• Low salt content maximized Mn(III) yields, while high salt content initially promoted chloride complexation but was limited by cryosalt formation.
• Consecutive freeze-thaw cycles led to substantial Mn(III) conversion (up to 80%) due to repeated reactant release.
• Reactivity was sustained down to -50 °C, likely due to supercooled water and oxide-bound water films.

Conclusions:

• Freezing is a critical factor promoting Mn(III) production via comproportionation of Mn(II) and Mn(IV) oxides.
• This freeze-driven process can significantly contribute to dissolved Mn(III) pools in cold environments.
• The findings highlight the importance of freeze-thaw cycles in the biogeochemical cycling of manganese in natural waters and sediments.