Effects of pH and Initial DOC Concentration on Ferrihydrite-Mediated Adsorption of Algal-Derived Dissolved Organic Matter under Eutrophication
The escalating eutrophication of aquatic systems has intensified algal blooms, leading to substantial release and accumulation of algal-derived dissolved organic matter (ADOM), which profoundly influences carbon cycling and pollutant transport. Iron minerals, particularly ferrihydrite, are recognized as critical mediators of DOM sequestration, yet the adsorption fractionation of ADOM under varying environmental conditions remains poorly understood. This study systematically investigated the effects of pH (2.0–10.0) and initial dissolved organic carbon (DOC) concentration (2–100 mg C/L) on the adsorption capacity and selectivity of ADOM onto ferrihydrite, employing UV-Vis spectroscopy and excitation-emission matrix fluorescence with parallel factor analysis (EEM-PARAFAC). Results demonstrated that adsorption capacity increased with pH from 2.0 to 7.0, reaching a maximum of 21.59 mg C/g at pH 7.0, followed by a decline at pH > 7.0 due to enhanced electrostatic repulsion. Within the environmentally relevant pH range of 3.0–9.0, selective fractionation intensified with increasing pH, favoring highly aromatic, high-molecular-weight chromophoric DOM (CDOM) and protein-like/aromatic amino acid fluorescent DOM (FDOM) with high humification and autochthonous characteristics. With increasing initial DOC concentration, adsorption exhibited non-linear growth, with preferential uptake of low-aromaticity, high-molecular-weight CDOM and protein-like FDOM of lower humification and stronger autochthonous features. These findings elucidate that ferrihydrite can effectively sequester reactive ADOM components via pH- and concentration-dependent selective adsorption, potentially altering DOM composition and reactivity in eutrophic waters, thereby providing fundamental data for understanding iron mineral-mediated internal carbon sequestration.