Coprecipitation of humic acid and phosphate with Fe(III) enhances the sequestration of carbon and phosphorus in sediments

The coprecipitation of dissolved organic matter (DOM) and phosphate with Fe(III) at the sediment-water interface plays an important role in the biogeochemical cycle of organic carbon (OC) and phosphorus (P) in aquatic ecosystems. However, studies remain limited towards understanding the strength of ternary DOM-phosphate-Fe(III) coprecipitation in sequestering OC and P. Here, we investigated the behaviors of humic acid (HA) and phosphate during their coprecipitation with Fe(III) and compared the coprecipitation behaviors with those during the adsorption process. The results showed that the coprecipitation of HA and phosphate with Fe(III) enhanced as pH decreased or initial Fe(III) concentrations increased. The presence of phosphate had a moderate inhibition effect on HA coprecipitation, leading to 3%–37% of reduction in precipitated HA. Changes in optical properties of DOM (i.e., SUVA254, E2/E3, and fluorescent components) showed that macromolecular, aromatic molecules preferentially coprecipitated, and this selectivity pattern was not influenced by the presence of phosphate. Compared to the adsorption of HA and phosphate to goethite, coprecipitation with Fe(III) displayed stronger binding ability, sequestering more than10 times of HA and phosphate, and was more efficient in sequestering protein-like compounds. The inhibitory effect of phosphate on HA sequestration was weaker during coprecipitation than during adsorption. Moreover, coprecipitation led to a lower degree of compositional fractionation of HA than adsorption, which may be explained by that inclusion, an important pathway for coprecipitation, has low selectivity for chemical compositions. Collectively, our results demonstrate that coprecipitation plays a crucial role in sequestering both OC and P in aquatic systems, arguing that the formation of ternary OC-Fe-P coprecipitates may be an important pathway for sequestering OC and P over a long term and reducing the internal loading of P in eutrophic lakes.