Regulatory mechanisms of initial pH value and DOC concentration on the quantity and quality of ferrihydrite-mediated adsorption of macrophyte-derived DOM

Dissolved organic matter (DOM) plays a pivotal role in carbon cycling and pollutant transport within aquatic ecosystems. Against the backdrop of escalating global eutrophication, macrophyte-derived DOM (MDOM) emerges as an increasingly significant autochthonous input. Its adsorption fractionation behavior at iron oxide interfaces profoundly influences its environmental fate. However, systematic understanding of how environmental factors regulate this fractionation process remains limited. Therefore, this study investigated the effects of initial pH (2.0-10.0) and dissolved organic carbon (DOC) concentration (2-100 mg C/L) on the adsorption amount and substance fractionation of MDOM from the typical submerged macrophyte Myriophyllum verticillatum L. onto ferrihydrite. Combining total organic carbon (TOC) determination, ultraviolet-visible spectroscopy (UV-Vis), and three-dimensional fluorescence spectroscopy-parallel factor analysis (EEM-PARAFAC), the study quantitatively revealed selective fractionation patterns during adsorption. Results indicate that MDOM adsorption exhibits significant pH dependence, with maximum adsorption capacities (14.79-16.27 mg C/g) occurring within the pH range of 4.0-8.0. Ferrihydrite selectively adsorbed chromophoric DOM (CDOM) with high aromaticity and high molecular weight, and fluorescent DOM (FDOM) with high humification, weak autogenic characteristics, and strong microbial characteristics from MDOM primarily via ligand exchange mechanisms. This leads to relative enrichment of aliphatic substances with low aromaticity, low molecular weight, low humification, strong autogenic characteristics such as protein-like compounds in the post-adsorption solution, enhancing their bioavailability characteristics. Within the experimental concentration range, adsorption capacity increased non-linearly with concentration, yet removal efficiency stabilized at 30%. Concurrently, while substance fractionation across the pH range remained broadly consistent, selectivity was markedly more pronounced at low concentrations. As initial concentrations increased, fractionation effects diminished due to heightened molecular competition. Principal Component Analysis (PCA) further confirmed that, across broad pH and concentration conditions, ferrihydrite acts as an efficient “chemical filter”, driving a systematic shift in MDOM from “terrestrial” to “autochthonous” characteristics. This study elucidates those environmental conditions influence MDOM adsorption fractionation by modulating interfacial chemistry rather than altering selectivity direction. This provides crucial scientific evidence for accurately assessing the mineral-mediated cycling pathways and sequestration potential of macrophyte-derived carbon in eutrophic lakes.