Carbon doesn’t just vanish into the ground; it gets chemically captured.
Scientists have long known that iron-rich soils quietly lock away vast amounts of carbon, keeping it out of the atmosphere for decades or even centuries.
What remained unclear was how these minerals manage to hold on so tightly to such a wide variety of organic compounds.
Now, engineers at Northwestern University have uncovered the molecular-level tricks behind one of nature’s most effective carbon traps.
Their work shows that ferrihydrite, a common iron oxide mineral found in soils and sediments, uses multiple chemical strategies to grab and stabilize carbon, far beyond simple electrostatic attraction.
The findings help explain why soils, which store roughly 2,500 billion tons of carbon globally, function as one of Earth’s largest and most durable carbon sinks, which is second only to the oceans.
A nanoscale charge mosaic
At first glance, ferrihydrite appears straightforward. Under most environmental conditions, the mineral carries an overall positive electrical charge, which led scientists to assume it mainly binds negatively charged organic molecules.
Using high-resolution molecular modeling and atomic force microscopy, the Northwestern team found a very different reality.
Ferrihydrite’s surface is not uniformly charged. Instead, it resembles a nanoscale patchwork of positive and negative regions.
“It is well documented that the overall charge of ferrihydrite is positive in relevant environmental conditions,” said Northwestern’s Ludmilla Aristilde, who led the study.
“That has led to assumptions that only negatively charged compounds will bind to these minerals, but we know the minerals can bind compounds with both negative and positive charges.”
“Our work illustrates that it is the sum of both negative and positive charges distributed across the surface that gives the mineral its overall positive charge,” Aristilde added.
This surface heterogeneity explains why ferrihydrite can attract a diverse mix of organic matter, including compounds that should not, in theory, stick to a positively charged surface at all.
More than attraction alone
After mapping the mineral’s surface, the researchers tested how real-world organic molecules interact with ferrihydrite. They exposed it to amino acids, plant acids, sugars, and ribonucleotides commonly found in soils.
By measuring how much of each compound adhered to the mineral and analyzing the interactions using infrared spectroscopy, the team identified several distinct binding mechanisms at work.
Positively charged amino acids latched onto negatively charged surface patches, while negatively charged amino acids bound to positive regions. Other molecules followed more complex paths.
Ribonucleotides were first drawn in by electrostatic forces and then formed stronger chemical bonds directly with iron atoms. Sugars, meanwhile, are attached through weaker hydrogen bonds.
“Iron oxide minerals are important for controlling the long-term preservation of organic carbon in soils and marine sediments,” Aristilde said.
“The fate of organic carbon in the environment is tightly linked to the global carbon cycle, including the transformation of organic matter to greenhouse gases.”
“Therefore, it’s important to understand how minerals trap organic matter, but the quantitative evaluation of how iron oxides trap different types of organic matter through different binding mechanisms has been missing,” she added.
The team believes these varied binding strategies help explain why some organic molecules remain protected in soils while others are more easily broken down by microbes.
Next, the researchers plan to study what happens after organic compounds attach to mineral surfaces, including whether they transform into even more stable forms or become vulnerable to further degradation, with the work published in Environmental Science & Technology.
