Enhanced dissolution of manganese oxide in ice compared to aqueous phase under illuminated and dark conditions

Manganese is one of the common elements in the Earth's crust and an essential micronutrient for all living things. The reductive dissolution of particulate manganese oxide is a dominant process to enhance mobility and bioavailability of manganese for the use of living organisms. In this work, we investigated the reductive dissolution of manganese oxides trapped in ice (at -20 °C) under dark and light irradiation (visible: λ > 400 nm and UV: λ > 300 nm) in comparison with their counterparts in aqueous solution (at 25 °C). The reductive dissolution of synthetic MnO₂, which took place slowly in aqueous solution, was significantly accelerated in ice phase both in the presence and absence of light: about 5 times more dissolution in ice phase than in liquid water after 6 h UV irradiation in the presence of formic acid. The enhanced dissolution in ice was observed under both UV and visible irradiation although the rate was much slower in the latter condition. The reductive dissolution rate of Mn(II)(aq) (under both irradiation and dark conditions) gradually increased with decreasing pH below 6 in both aqueous and ice phases, and the dissolution rates were consistently faster in ice under all tested conditions. The enhanced generation of Mn(II)(aq) in ice can be mainly explained in terms of freeze concentration of electron donors, protons, and MnO₂ in liquid-like ice grain boundaries. The outdoor solar experiment conducted in Arctic region (Ny-Ålesund, Svalbard, 78°55'N) also showed that the photoreductive dissolution of manganese oxide is enhanced in ice. The present results imply that the dissolution of natural minerals like manganese oxides can be enhanced in icy environments such as polar region, upper atmosphere, and frozen soil.