Fe(II)-humus interactions drive phosphorus immobilization: Insights into vivianite formation

The interaction between divalent iron (Fe(II)) and humic substances (HS) is crucial for geochemical cycles in terrestrial and aquatic environments, influencing soil phosphorus (P) cycling and aquatic eutrophication. However, the impact of Fe(II)-HS interactions on P sequestration remains unclear. This research employed controlled lab experiments and advanced characterization techniques, such as X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electron Microscopy (SEM), to study the mechanisms of P fixation by Fe(II)-HS complexes under neutral pH and anaerobic conditions. Results indicate that Fe(II) availability is crucial for P fixation, more so than HS content. Elevated Fe/P ratios boost P removal by encouraging vivianite formation, even in the presence of HS, reducing dissolved P to below 0.3 mg/L at Fe/P of 2.2. HS primarily hinders P adsorption through competitive ligand exchange. Solution pH also impacts P fixation, with higher pH levels (≥7) enhancing P immobilization. While HS binds Fe(II) and prevents hydrolysis, Fe(II) hydrolysis products like Fe(OH)₂/Fe(OH)₃^-can co-precipitate and remove P. XRD analysis indicated vivianite crystals form at pH 7, but their diffraction peak intensity decreases under acidic or alkaline conditions, with formation controlled by Fe species and HS. The potential pathways of P in the Fe(II)-HS system include P combining with Fe(II) to form vivianite, P combining with HS to form HS-P composites, and P combining with Fe(II)-OM species to form carbon-containing vivianite. These insights improve understanding of the Fe-C-P biogeochemical cycle and P mobility in natural environments.