By closely examining ferrihydrite, a common iron oxide mineral, engineers discovered that it relies on several distinct chemical processes to capture and hold carbon. Rather than using a single method, the mineral employs multiple strategies that allow it to bind many different types of organic material.

Although ferrihydrite carries an overall positive electrical charge, researchers found that its surface is far from uniform. Instead, it is made up of tiny regions with both positive and negative charges. This patchy structure allows ferrihydrite to interact with carbon in more ways than scientists previously understood. In addition to electrical attraction, the mineral forms chemical bonds and hydrogen bonds that create strong links between its surface and organic molecules.

Together, these mechanisms make iron oxide minerals highly adaptable carbon binders. They can capture a wide range of organic compounds and hold them for long periods, sometimes lasting decades or even centuries. This process helps prevent carbon from reentering the atmosphere as greenhouse gases that contribute to climate warming.

The findings were published in the journalEnvironmental Science & Technology and offer the most detailed view yet of ferrihydrite's surface chemistry, a key factor in how soils store carbon.

"Iron oxide minerals are important for controlling the long-term preservation of organic carbon in soils and marine sediments," said Northwestern's Ludmilla Aristilde, who led the study. "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."

Aristilde is a professor of civil and environmental engineering at Northwestern's McCormick School of Engineering and studies how organic materials behave in environmental systems. She is also affiliated with the International Institute for Nanotechnology, the Paula M. Trienens Institute for Sustainability and Energy and Center for Synthetic Biology. Jiaxing Wang served as the study's first author, with Benjamin Barrios Cerda as second author. Both are postdoctoral associates in Aristilde's laboratory.

Soil as One of Earth's Largest Carbon Sinks

Soil stores an estimated 2,500 billion tons of carbon, making it one of the planet's largest carbon reservoirs, second only to the ocean. Despite its importance, scientists are still unraveling the exact processes that allow soil to remove carbon from the active carbon cycle and keep it underground.

Aristilde and her team have spent years studying how minerals and soil microbes influence whether carbon remains trapped or is released back into the atmosphere. Their earlier work examined how clay minerals bind organic matter and how microbes preferentially convert certain organic compounds into carbon dioxide.

In this latest research, the team focused on iron oxide minerals, which are linked to more than one third of the organic carbon found in soils. They concentrated on ferrihydrite, a mineral commonly found near plant roots and in soils or sediments rich in organic material. Even though ferrihydrite often appears positively charged under environmental conditions, it can bind organic compounds with negative, positive, or neutral charges.

How Molecules Attach to Iron Minerals

To understand how ferrihydrite interacts with such a wide range of compounds, the researchers used high-resolution molecular modeling along with atomic force microscopy to closely examine the mineral's surface. While its overall charge is positive, they confirmed that the surface contains a mix of positive and negative regions. This helps explain why ferrihydrite can attract negatively charged substances like phosphate as well as positively charged metal ions.

"It is well documented that the overall charge of ferrihydrite is positive in relevant environmental conditions," Aristilde said. "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."

After mapping the surface charges, the team tested how different organic molecules interact with ferrihydrite. They exposed the mineral to compounds commonly found in soil, including amino acids, plant acids, sugars and ribonucleotides. The researchers measured how much of each compound adhered to the mineral and used infrared spectroscopy to determine how the molecules attached.

More Than Simple Attraction

The experiments revealed that ferrihydrite binds organic molecules through several distinct pathways. Positively charged amino acids attach to negatively charged areas of the mineral, while negatively charged amino acids bind to positively charged regions. Some compounds, such as ribonucleotides, are initially attracted by electrical forces but then form stronger chemical bonds with iron atoms. Sugars, which bind more weakly, attach through hydrogen bonding.

"Collectively, our findings provide a rationale, on a quantitative basis, for building a framework for the mechanisms that drive mineral-organic associations involving iron oxides in the long-term preservation of organic matter," Aristilde said. "These associations may help explain why some organic molecules remain protected in soils while others are more vulnerable to being broken down and respired by microbes."

Next, the researchers plan to study what happens after organic molecules bind to mineral surfaces. Some may be transformed into compounds that microbes can further break down, while others could become even more resistant to decomposition.

The study, "Surface charge heterogeneity and mechanisms of organic binding modes on an iron oxyhydroxide," was supported by the U.S. Department of Energy and the International Institute for Nanotechnology.

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