Research led by Ludmilla Aristilde, professor of civil and environmental engineering at Northwestern University, is providing new insight into one of Earth’s most important natural carbon storage mechanisms. By examining how iron oxide minerals interact with organic matter at the molecular level, the study helps explain why soils can retain carbon for decades or even centuries instead of releasing it back into the atmosphere.
Wang, J., Barrios-Cerda, B., & Aristilde, L. (2025). Surface Charge Heterogeneity and Mechanisms of Organic Binding Modes on an Iron Oxyhydroxide. Environmental Science & Technology. https://doi.org/10.1021/acs.est.5c10850
Soils hold an estimated 2,500 billion tons of carbon, making them one of the largest carbon reservoirs on the planet. A substantial fraction of this carbon is associated with iron oxide minerals, yet the precise mechanisms behind this long-term storage have remained unclear. While scientists have long observed that iron-rich soils tend to preserve organic matter, understanding how this happens has been a major challenge in environmental engineering and soil science.
Ludmilla Aristilde, professor of civil and environmental engineering at Northwestern University stated,
“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. 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.”
The Northwestern team focused on ferrihydrite, a common iron oxide mineral found in soils, particularly near plant roots and in environments rich in organic material. Ferrihydrite has often been described as a positively charged mineral, leading to the assumption that it primarily attracts negatively charged organic compounds. The new study shows that this description is incomplete.
Using a combination of molecular-scale modeling, atomic force microscopy, and spectroscopy, the researchers found that ferrihydrite’s surface is chemically heterogeneous. Rather than carrying a uniform charge, the mineral surface contains intermixed regions of positive and negative charge. This patchwork structure allows ferrihydrite to interact with a much broader range of organic molecules than previously thought.
To test how this surface chemistry influences carbon storage, the researchers examined how different classes of organic molecules bind to ferrihydrite. These included amino acids, sugars, plant-derived acids, and ribonucleotides, all of which are commonly present in soils. The experiments revealed that no single binding mechanism dominates. Instead, multiple processes work together.
Positively charged organic compounds tend to associate with negatively charged regions on the mineral surface, while negatively charged compounds bind to positively charged regions. Some molecules initially attach through electrostatic attraction and then form stronger chemical bonds directly with iron atoms. Others, such as sugars, bind more weakly through hydrogen bonding. Together, these interactions create a network of associations that stabilizes organic carbon and limits its availability to microbes that would otherwise convert it to carbon dioxide.
From an engineering perspective, the findings help clarify why iron oxides are linked to long-term carbon persistence in soils and sediments. The presence of multiple binding modes means that organic matter can remain protected even as environmental conditions change. This helps explain why certain soils act as long-term carbon sinks while others release carbon more readily.
The work also provides a quantitative framework for incorporating mineral-organic interactions into models of the global carbon cycle. Current climate and soil models often treat mineral surfaces in simplified ways, which can underestimate the role of iron oxides in carbon stabilization. More accurate representations could improve predictions of how soils respond to land use changes, warming temperatures, and shifts in microbial activity.
The researchers note that carbon storage does not end once organic molecules bind to mineral surfaces. Future work will examine what happens after attachment, including whether some compounds are transformed into more stable forms or become vulnerable to degradation over time. Understanding these downstream processes will be essential for predicting how long carbon can remain sequestered under different environmental conditions.
By revealing how surface chemistry and molecular interactions govern carbon retention, the study adds an important piece to the broader effort to understand natural carbon sinks. While iron minerals operate quietly beneath our feet, their ability to lock away carbon plays a significant role in regulating Earth’s climate over long timescales.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
