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Ice can dissolve iron minerals more effectively than liquid water, according to a new study from Ume氓 University. The discovery could help explain why many Arctic rivers are now turning rusty orange as permafrost thaws in a warming climate.

The study, in PNAS, shows that ice at minus 10 degrees Celsius releases more iron from common minerals than liquid water at 4 degrees Celsius. This challenges the long-held belief that frozen environments slow down chemical reactions.

"It may sound counterintuitive, but ice is not a passive frozen block," says Jean-Fran莽ois Boily, Professor at Ume氓 University and co-author of the study.

"Freezing creates microscopic pockets of liquid water between ice crystals. These act like chemical reactors, where compounds become concentrated and extremely acidic. This means they can react with iron minerals even at temperatures as low as minus 30 degrees Celsius."

To understand the process, the researchers studied goethite鈥攁 widespread iron oxide mineral鈥攖ogether with a naturally occurring organic acid, using advanced microscopy and experiments.

They discovered that repeated freeze-thaw cycles make iron dissolve more efficiently. As the ice freezes and thaws, organic compounds that were previously trapped in the ice are released, fueling further chemical reactions. Salinity also plays a crucial role: fresh and brackish water increase dissolution, while seawater can suppress it.

The findings apply mainly to acidic environments, such as mine drainage sites, frozen dust in the atmosphere, acid sulfate soils along the Baltic Sea coast, or in any acidic frozen environment where iron minerals interact with organics. The next step is to find out if the same is true for all iron-bearing ice. This is what ongoing research in the Boily laboratory will soon reveal.

"As the climate warms, freeze-thaw cycles become more frequent," says Angelo Pio Sebaaly, doctoral student and first author of the study. "Each cycle releases iron from soils and permafrost into the water. This can affect water quality and aquatic ecosystems across vast areas."

The findings show that ice is not a passive storage medium, but an active player. As freezing and thawing increase in polar and mountain regions, the impact on ecosystems and the natural cycling of elements could be significant.