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.

The authors investigated the effects of organic matter (OM) content and composition, as well as medium temperature, salinity and pH, on the sorption of organic phosphorus (OP) using methylphosphonic acid (MPn) and 5′-adenosine monophosphate (AMP) as the model compounds. An 3% increase in the proportion of light fraction organic matter (LFOM) in the HCl treated sediment samples increased the sorption capacity (Q_m) for AMP and MPn by 35% and 32%, respectively, indicating that the LFOM made a significant contribution to OP sorption. The input of algal-derived organic matter (AOM) during algal blooms increased the proportion of LFOM in sediments, enhancing the sorption of OP. However, the released dissolved organic carbon (DOC) inhibited the further enhancement of sorption ability. Fraction analysis after OP sorption revealed that the increase of OM content favored the accumulation bioavailable phosphorus after sorption. The influence of environmental factors on OP sorption was investigated using MPn as an example. Compared to artificial seawater (ASW), the sediment exhibited stronger sorption ability for OP in diluted ASW, with higher sorption observed in 1/2 ASW than in 1/4 ASW. Under pH=7~8 conditions, the electrostatic repulsion on the sediment surface partially inhibited the sorption of MPn. Thermodynamic parameters (ΔG^θ < 0, ΔH^θ>0, ΔS^θ>0) indicated that MPn sorption was a spontaneous and endothermic process.