Sorption of paddy soil-derived dissolved organic matter on hydrous iron oxide-vermiculite mineral phases

Sorption of organic matter (OM) onto soil minerals affects OM dynamics, and is strongly controlled by mineral surface properties. Moreover, the degree and mechanism of interaction between different minerals may influence their surface reactivity. We therefore aimed to understand mineral surface modifications brought about by different Fe (hydr)oxide–vermiculite associations, and their influence on OM sorption. This could have important implications on OM dynamics in young hydromorphic soils where seasonal variations in redox conditions may affect mineral surfaces. Paddy soil-derived dissolved OM was used in sorption isotherms on mixed mineral phases obtained by precipitating different amounts of Fe (hydr)oxides on vermiculite. Results evidenced that the surface properties of vermiculite strongly drove Fe (hydr)oxide precipitation, and consequently OM sorption mechanisms. The change in surface charge with increasing Fe loading resulted in a higher retention of dissolved organic carbon. However, dissolved organic nitrogen adsorption isotherms and FTIR carboxyl vibrational shifts revealed the occurrence of two binding mechanisms, one driven by electrostatic attraction of N-containing compounds by the negatively charged vermiculite surface, and another involving ligand exchange of carboxylic compounds with the positive oxides, precipitated in localized nucleation sites. Moreover, the modification of both solution and solid properties during sorption, promoted the selective adsorption of aromatic molecules with increasing Fe coverage.