The influence of iron oxide on the phosphorus speciation transformation mechanisms in the Riparian Zone sediments

Excessive phosphorus content can lead to eutrophication in rivers and lakes. Iron oxide plays a critical role in controlling the concentration, speciation, and transformation of phosphorus in water bodies and their sediments. This research selects the Jiehe Rivers of typical seasonal rivers in northern China as the research object. This study aims to explore the changes in the physicochemical properties of sediments under the influence of exogenous Fe₂O₃, as well as the occurrence forms and release characteristics of phosphorus. It provides important insights into the phosphorus cycle changes in seasonal river sediments. The samples were collected and experiments were carried out in the laboratory. This study investigates the influence of exogenous Fe₂O₃ addition (0 %, 5 %, 10 %, 15 %, 20 %) on phosphorus speciation dynamics in sediments of the Jiehe River under alternating wet-dry conditions. Using the Chang-Jackson (C-J) chemical sequential extraction method, Fourier-transform infrared spectroscopy, three-dimensional fluorescence spectroscopy, and scanning electron microscopy, the structural features and stability of inorganic molecules in sediments with added exogenous Fe₂O₃ were characterized. Experimental results demonstrated that Fe₂O₃ addition significantly altered inorganic phosphorus (IP) speciation. FeP content increased by 0-20 % in sediments with 10-20 % Fe₂O₃, while AlP and CaP decreased by 10-40 % and 2-7 %, respectively, due to competitive adsorption and conversion to FeP. The addition of exogenous Fe₂O₃ transformed the sedimentary dissolved organic matter from tryptophan-like to tyrosine-like. Concurrently, iron ions maintained hydroxyl groups at a stable level, enhancing phosphate adsorption efficiency. SEM revealed that Fe₂O₃-amended sediments exhibited rougher surfaces and larger specific surface areas, further enhancing phosphorus immobilization. This study not only aims to investigate the mechanisms underlying Fe₂O₃-induced transformations of phosphorus speciation under dry-wet cycles, but also to evaluate the potential of γ-Fe₂O₃ as a sediment amendment to immobilize phosphorus and mitigate internal phosphorus pollution in seasonal rivers.