Sorption Fractionation of Algal Organic Matter by Iron and Manganese Oxides

To elucidate the sorption fractionation behavior of algae-derived organic matter (AOM) on iron-manganese oxides, this study used AOM extracted from Ulva prolifera as a representative to investigate the sorption characteristics of different AOM components on δ-MnO₂, goethite, and hematite under varying organic matter concentrations and pH conditions, as well as the changes in spectral characteristics of organic matter in the medium before and after sorption. The results indicated significant differences in the sorption characteristics of AOM on different iron-manganese oxides at pH=8.0: δ-MnO₂ exhibited the highest sorption capacity and selectively fractionated macromolecular components with high aromaticity and low humification degree. Goethite showed the strongest binding affinity and a higher sorption capacity than hematite. Both iron oxides preferentially fractionated components with high aromaticity and low humification degree; however, they differed in the molecular weight (MW) of the fractionated components. Goethite consistently fractionated low-MW components regardless of concentration, whereas hematite shifted from fractionating low-MW to high-MW components as the concentration increased. By comparing the fractionation characteristics at two pH levels, it was found that the fractionation behavior on iron oxides (goethite and hematite) was significantly influenced by pH, with their primary binding mechanisms being pH-sensitive electrostatic interactions and ligand exchange. In contrast, the fractionation on δ-MnO₂ was largely unaffected by pH. Besides partial redox reactions, its interaction with AOM was governed mainly by pH-insensitive mechanisms such as hydrophobic interactions and van der Waals forces. This study provides a theoretical basis for understanding the sorption fractionation mechanisms between iron-manganese oxides and AOM, offering valuable insights for revealing the transport, transformation, and ecological risk assessment of organic matter in eutrophic waters.