Ice can trigger surprising chemical reactions in frozen regions, such as soils and permafrost. These icy environments play a key role in releasing soluble iron during melting, but scientists still don’t fully understand how these mineral–organic processes work.
A recent study published in PNAS sheds light on how freezing conditions, including temperature, salt levels, organic matter, and cycles of freezing and thawing, affect the breakdown of a key iron oxide nanomineral. These findings help explain how iron is released in icy environments, offering new insights into the chemical activity of frozen landscapes.
This study from Umeå University reveals a surprising twist: ice can break down iron minerals more efficiently than liquid water. Researchers discovered that at –10°C, ice released more iron from common minerals than water did at +4°C. This finding challenges the long-held belief that freezing temperatures slow chemical reactions, showing instead that ice can dissolve iron minerals more effectively than liquid water.
This discovery may shed light on a striking phenomenon in the Arctic: rivers turning rusty orange as the permafrost melts. As frozen soils thaw, iron minerals trapped in the ice begin to dissolve and flow into waterways. The release of soluble iron, accelerated by ice’s unexpected chemical activity, can stain rivers with a reddish hue, much like rust.
Water that simply will not freeze, no matter how cold it gets
Jean-François Boily, Professor at Umeå University and co-author of the study, explained that ice is far from being a passive frozen block. He noted that freezing creates microscopic pockets of liquid water between ice crystals. These act like chemical reactors, where compounds become concentrated and highly acidic. This means they can react with iron minerals even at temperatures as low as minus 30 degrees Celsius.”
To explore the process, researchers utilized advanced tools to study goethite, a common iron oxide mineral, in conjunction with a natural organic acid. They found that freezing and thawing repeatedly helps iron dissolve more easily. As ice melts and refreezes, trapped organic compounds are released, sparking more chemical reactions. Salt levels also matter: fresh and slightly salty water boosts iron release, while seawater tends to slow it down.
The study’s results primarily apply to acidic environments, such as mine drainage areas, atmospheric dust, or coastal soils near the Baltic Sea, where iron minerals mix with organic matter. The next step for researchers is to determine if all types of iron-containing ice behave similarly. That’s what ongoing experiments in the Boily lab aim to uncover.
“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 study shows that ice isn’t just a frozen storage unit; it actively drives chemical changes. As freezing and thawing become more common in polar and mountainous areas, this could have a significant impact on ecosystems and how key elements, such as iron, move through the environment.
Journal Reference:
- Angelo P. Sebaaly, Frank van Rijn et al. Ice as a kinetic and mechanistic driver of oxalate-promoted iron oxyhydroxide dissolution. PNAS. DOI: 10.1073/pnas.2507588122