Composition regulates organic matter adsorption onto iron (oxy)hydroxides and competition with phosphate: Implications for organic carbon, phosphorus immobilization in lakes

Dissolved organic matter (DOM) is heterogeneous and exhibits diverse adsorption characteristics with or without the competition of phosphorous (P). Uncertainty persists on how these affect the burial and mobilization of organic carbon and P in aquatic ecosystems. In this work, an algae-derived DOM (ADOM) and a commercial humic acid (HA) with distinct composition were assessed for their adsorption behaviors on iron (oxy)hydroxides (FeOx), in the absence and presence of phosphate. ADOM has less aromatics than HA but more protein-like and highly unsaturated structures with oxygen compounds (HUSO). The adsorption capacity of FeOx was significantly greater for ADOM than for HA. Protein-like and HUSO compounds in ADOM and humic-like compounds and macromolecular aromatics in HA were preferentially adsorbed by FeOx. Moreover, ADOM demonstrated a stronger inhibitory effect on phosphate adsorption than that of HA. This observation suggests that substantial autochthonous ADOM released by algae could increase the internal P loading and difficulty to restore eutrophic lakes. In contrast, phosphate competition can also fractionate ADOM, which suppressing the adsorption of protein-like compounds in ADOM onto FeOx, resulting in an increase in their relative abundance in the post-adsorption ADOM, while reducing the abundance of humic-like compounds. However, phosphate had no discernible impact on the compositional fractionation of HA. Collectively, our results show the source-compositions of DOM influence the immobilization of both DOM and P in aquatic ecosystem via adsorption. The preferential adsorption of proteinaceous compounds in ADOM and aromatics in HA highlighted that the attachment with FeOx could remove their source-specific signatures, partly contributing to universal DOM characteristics across aquatic environments.