Probing Molecular Level and Subnanoscale Stabilization Mechanisms of Organic Carbon Species during Fe(II)-Induced Ferrihydrite Transformation

Soil dissolved organic matter (DOM) is known to affect iron (Fe) oxide transformation, which may further affect organic carbon (OC) sequestration. However, soil DOM is heterogeneous with varying molecular weights (MW), and how the dynamic interplay between DOM molecules and Fe oxides affects the sequestration of different OC species remains elusive. In this study, we separated DOM into <3, 3-10, and >10 kDa fractions by ultrafiltration to investigate the dynamic distribution of OC species at the molecular level and subnanoscale during ferrihydrite-OC coprecipitate transformation. Compared with the low MW DOM fraction, the higher MW DOM fractions inhibited Fe oxide transformation less, but showed stronger sequestration ability on Fe oxides, due to more aromatic compounds with higher carboxyl number per molecule. Despite the above difference, the released DOM during mineral transformation was similar to mainly nonaromatic compositions. At the nano to subnanoscales, aromatic-rich compounds were sequestered within the defective/porous structure of Fe oxides, whereas high crystalline Fe oxides promoted the sequestration of carboxyl-rich compounds. Our study provides novel insights into the sequestration mechanisms of different OC species during Fe oxide transformation and highlights the significance of incorporating MW-dependent DOM compositions when predicting SOC stability.