US engineers decode how iron oxides grab carbon at the nanoscale

Researchers at Northwestern University have elucidated the molecular mechanisms by which ferrihydrite, a common iron oxide mineral in soils, captures and stabilizes organic carbon. Contrary to previous assumptions that ferrihydrite’s overall positive charge binds only negatively charged organic molecules, the team discovered that its surface is a nanoscale mosaic of both positive and negative charge patches. This heterogeneous charge distribution enables ferrihydrite to attract a diverse range of organic compounds, including both positively and negatively charged molecules, explaining its role as a highly effective and durable carbon sink in soils. Using high-resolution molecular modeling, atomic force microscopy, and infrared spectroscopy, the researchers identified multiple binding mechanisms between ferrihydrite and soil organic molecules such as amino acids, plant acids, sugars, and ribonucleotides. For example, positively charged amino acids bind to negatively charged surface patches, while negatively charged amino acids attach to positive regions. Ribonucleotides initially adhere via electrostatic attraction and then form stronger chemical bonds with iron