Microscopic mechanisms of dissolved organic matter sequestration and transformation at the mineral-water interfaces during the formation and transformation of FeMn (hydr)oxides

Iron (Fe) and manganese (Mn) (hydr)oxides, as important reactive minerals in natural environments, play a critical role in organic carbon (OC) sequestration and turnover, significantly influencing the global carbon cycle. However, Fe and Mn (hydr)oxides usually coexist and undergo progressive phase transformations. There is still lacking a comprehensive understanding of the dynamic reactions of OC with Fe Mn (hydr)oxides during their formation and transformation. In this study, kinetic experiments combined with advanced analytical techniques were employed to investigate, at the nanoscale and molecular level, the microscopic mechanisms of dissolved organic matter (DOM) sequestration and transformation at the mineral-water interfaces during the evolution of Fe Mn (hydr)oxides. Fe Mn (hydr)oxides sequestered significant amounts of OC (57.7 mg C g −1 solid) through coprecipitation during their formation, resulting in a fractionation of DOM on the surface of Fe Mn (hydr)oxides. Compounds with high carboxyl and phenolic content were preferentially sequestered by the Fe Mn (hydr)oxides. The sequestered OC, at the nanoscale, was more associated with Fe (hydr)oxides than Mn (hydr)oxides. With the transformation of Fe Mn (hydr)oxides, Mn(III/IV) (hydr)oxides were formed and further mediated the molecular transformation of DOM, including oxidative degradation and oxidative polymerization. Fe (hydr)oxides primarily facilitated the stabilization of OC, while Mn (hydr)oxides contributed to both stabilization and destabilization of OC. This study, at the nanoscale and molecular level, elucidated the dynamic reactions of DOM with Fe Mn (hydr)oxides during their formation and transformation, contributing to a better understanding of OC sequestration and turnover across aquatic-terrestrial interfaces.