• • Adsorption capacity of ADOM onto ferrihydrite peaked at 21.59 mg C/g at pH 7.0, with a decline above pH 7.0 due to electrostatic repulsion, indicating optimal pH for carbon sequestration in near-neutral waters.
• • Selective fractionation intensified with pH from 3.0 to 9.0, favoring high-aromaticity, high-molecular-weight CDOM and humified, autochthonous protein-like FDOM, which may reduce bioavailability of these reactive components.
• • Non-linear increase in adsorption with initial DOC concentration (2–100 mg C/L) revealed preferential uptake of low-aromaticity, high-molecular-weight CDOM and low-humification protein-like FDOM, suggesting concentration-dependent molecular sorting.
• • The observed fractionation aligns with an 'onion model' of multilayer adsorption, implying that ferrihydrite can dynamically regulate ADOM composition, impacting carbon turnover and pollutant mobility in eutrophic systems.