A study from Northwestern University reveals that ferrihydrite, a common iron oxide mineral, captures and stores carbon more effectively than previously understood due to its heterogeneous surface charges. This mineral uses multiple bonding mechanisms to hold diverse organic molecules, contributing to soil's role as a major carbon sink. The findings explain how soils preserve vast amounts of carbon long-term, aiding climate efforts.
Scientists at Northwestern University have uncovered the chemical secrets behind ferrihydrite's ability to lock away carbon in soils. This iron oxide mineral, often resembling rust, features a surface with a nanoscale patchwork of positive and negative charges, allowing it to bind a variety of organic compounds securely.
The research, led by Ludmilla Aristilde, a professor of civil and environmental engineering, examined ferrihydrite's interactions with soil organics using molecular modeling, atomic force microscopy, and infrared spectroscopy. Despite its overall positive charge, the mineral's surface includes regions of both charges, enabling attractions to molecules with negative, positive, or neutral properties. For instance, positively charged amino acids attach to negative areas, while negatively charged ones bind to positive regions. Ribonucleotides form initial electrical bonds that strengthen into chemical links with iron atoms, and sugars connect via hydrogen bonding.
"Iron oxide minerals are important for controlling the long-term preservation of organic carbon in soils and marine sediments," Aristilde said. She emphasized that understanding these mechanisms is crucial for the global carbon cycle, as they prevent organic matter from turning into greenhouse gases.
Soils hold about 2,500 billion tons of carbon, second only to oceans, with iron oxides accounting for over one-third of this storage. Ferrihydrite, common near plant roots and in organic-rich sediments, thus plays a key role in keeping carbon underground for decades or centuries.
The study, published in Environmental Science & Technology in 2025, provides a quantitative framework for mineral-organic associations. Aristilde noted: "It is well documented that the overall charge of ferrihydrite is positive in relevant environmental conditions... Our work illustrates that it is the sum of both negative and positive charges distributed across the surface."
Future research will explore post-binding transformations, determining which compounds resist microbial breakdown. This work, supported by the U.S. Department of Energy, highlights minerals' adaptability in carbon sequestration.