Fe(III)-mediated sediment sequestration of phosphate and humic acid: Timing of inputs regulates environmental processes

The co-precipitation of dissolved organic matter (DOM) and phosphate with Fe(III) at the sediment-water interface plays a crucial role in the biogeochemical cycling of organic carbon (OC) and phosphate (P) in aquatic ecosystems. The fluxes of organic matter and phosphate may occur in a sequential manner. This study investigated how phosphate affected pre-equilibrated Fe(III)-humic acid (HA) co-precipitates and how HA influenced pre-equilibrated Fe(III)-phosphate co-precipitates under varying pH and Fe(III) concentrations. The results revealed that the addition of phosphate to the Fe-HA co-precipitation system generally promoted further precipitation of dissolved Fe(III) with phosphate, resulting in about 9.48–33.15 % of the phosphate being precipitated. This addition had minimal impact on the percentage of pre-immobilized HA but facilitated the release of high-aromaticity HA from the Fe-HA co-precipitate. Conversely, adding HA to the Fe P co-precipitation system increased the concentrations of soluble Fe(III) and phosphate, likely due to the strong complexation ability of HA with Fe(III). The percentage of HA remaining dissolved was lower at lower pH levels and higher initial Fe(III) concentrations. These findings highlight how the timing of phosphate or DOM inputs influences their sequestration. This has important implications for carbon and nutrient storage in aquatic sediments, where fluxes of organic carbon and phosphate may occur in a sequential manner.