Ice at less than 10 degrees Celsius releases more iron from common minerals than liquid water at 4 degrees Celsius, according to a team of researchers from the University of Umeå, the Institute of Chiques of Rennes and CNRS. This discovery could help explain why many arctic rivers are now becoming rusty orange while permafrost in a warming climate.
Schematic representation of the mineral dissolution reactions of iron in the ice. Image credit: Sebaly and al., DOI: 10.1073 / PNAS. 2507588122.
“It may seem counter-intuitive, but ice is not a passive frozen bloc,” said Professor Jean-François at the University of Umeå, Jean-François.
“The gel creates microscopic pockets of liquid water between ice crystals.”
“These act as chemical reactors, where compounds become concentrated and extremely acidic.”
“This means that they can react with iron minerals even at temperatures as low as minus 30 degrees Celsius.”
To understand the process, Professor Boily and his colleagues studied Goethite – a widespread iron oxide mineral – with a natural organic acid.
Using microscopy and advanced experiments, they discovered that repeated freezing cycles make iron to dissolve more effectively.
While the ice freezes and thaw, the organic compounds which were previously trapped in the ice are released, fueling other chemical reactions.
Salinity also plays a crucial role: the dissolution of fresh and brackish water, while seawater can remove it.
The results apply mainly to acidic environments, such as mine drainage sites, frozen dust in the atmosphere, acidic sulfated soils along the Baltic Sea coast or in any acid frozen environment where iron minerals interact with organics.
“While the climate warms up, gel-degel cycles are becoming more frequent,” said Angelo Pio Sebaly, doctoral student at Umeå University.
“Each cycle releases floors and permafrosts in water. This can affect water quality and aquatic ecosystems in large areas. ”
“The results show that ice is not a passive storage support, but an active player.”
“As freezing and defrosting the increase in polar and mountainous regions, for the impact on ecosystems and natural cycling of elements could be significant.”
The team’s document was published on August 26, 2025 in the Proceedings of the National Academy of Sciences.
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Angelo P. Sebaly and al. 2025. Ice as a kinetic and mechanistic driver of the dissolution of iron oxidroxide favored by oxalate. PNA 122 (35): E2507588122; DOI: 10.1073 / PNAS. 2507588122
