Effects of myo-inositol hexakisphosphate on the Fe(II)-catalyzed transformation of Al-substituted ferrihydrite: A concentration-dependent mechanism and implications for phosphorus geochemical cycling

Poorly crystalline iron hydroxides, such as ferrihydrite (Fh) and its aluminum-substituted counterpart (Al-substituted ferrihydrite, Al-Fh), are ubiquitous in surficial geochemical environments and undergo strong interfacial interactions with organic phosphorus (e.g., myo -inositol hexakisphosphate, phytate, IHP). However, how these interactions regulate the Fe(II)-catalyzed dissolution–recrystallization of Fh and Al-Fh into secondary Fe minerals and consequently influence phosphorus geochemical mobility and sequestration remains unclear. This study systematically investigated the Fe(II)-catalyzed transformation of phytate-preadsorbed Al-Fh via batch experiments and multiple characterizations to elucidate the regulatory role of phytate in governing Fe(II)-catalyzed Al-Fh transformation and phytate repartitioning. Results showed that isomorphous Al substitution inhibited goethite (Gt) nucleation in Fe(II)-catalyzed Al-Fh transformation by decreasing Al-Fh crystallinity and disrupting the periodic Fe–O–Fe crystal lattice arrangement. Fe(II) drove mineral dissolution and secondary phase nucleation via interfacial electron transfer (IET), with catalytic efficiency positively correlated with its concentration. Phytate exerted a concentration-dependent effect: at low concentration (0.5 mM), it impeded the further transformation of lepidocrocite (Lp) to Gt through strong inner-sphere surface complexation of phytate on reactive Fe sites, thereby favoring Lp mineral accumulation. At higher concentrations (≥1.0 mM), it greatly inhibited Al-Fh transformation via a combination of surface complexation, IET inhibition, reactive Fe(II) depletion, and the occupation of mineral dissolution sites, resulting in metastable Lp or amorphous products. Mechanistically, phytate formed stable complexes on Al-Fh surfaces, hindering electron transfer. Concurrently, phytate and Fe(II) generated ternary complexes (≡Fe–IHP–Fe(II)) or surface iron phytate precipitates on minerals, thereby depleting the reactive Fe(II) pool. Throughout transformation, phytate remained strongly sequestered in the solid phase rather than being released into solution due to its strong geochemical interactions with Fe/Al hydroxides. This work elucidates the synergistic regulatory mechanisms of Al and phytate in Al-Fh transformation, offering key theoretical insights into organic phosphorus–iron hydroxide interactions and long-term phosphorus immobilization in Al-rich soils.