Fe(II)-Catalyzed Transformation of Phosphate-Bearing Ferrihydrite: Mineralogical Mechanisms and Phosphate Fate

The transformation of phosphate (P)-bearing ferrihydrite is critical for regulating iron (Fe) speciation and P availability in anaerobic soils and sediments. However, the effects of low P/Fe molar ratios (<0.1) on Fe(II)-catalyzed ferrihydrite transformation and the associated fate of P remain poorly understood. Here, we systematically investigated the influence of varying P/Fe molar ratios (0-0.05) and P incorporation modes (adsorption vs coprecipitation) on the Fe(II)-catalyzed transformation of dispersed and aggregated ferrihydrite and evaluated the resulting redistribution and availability of P. As the P/Fe ratio increased from 0 to 0.01, the predominant transformation products shifted sequentially from goethite to goethite-magnetite-lepidocrocite mixture and then to lepidocrocite, while higher P loading (P/Fe = 0.05) strongly inhibited transformation. Coprecipitated P exerted stronger inhibitory effects than adsorbed P. Mechanistically, P modified transformation pathways by enhancing Fe(II) sorption, suppressing labile Fe(III) generation, and retarding Fe oxides crystallization. Compared with dispersed ferrihydrite, aggregated particles exhibited lower surface reactivity toward Fe(II), favoring magnetite and lepidocrocite formation over goethite. These mineralogical transitions substantially altered P distribution: initially confined within ferrihydrite micropores, P became homogeneously incorporated into goethite and magnetite but preferentially accumulated on lepidocrocite surfaces. Ultimately, Fe(II)-catalyzed ferrihydrite transformation promoted P occlusion within mineral matrices, substantially reducing its availability. These findings elucidate coupled Fe-P transformation pathways and provide new mechanistic insights into P stabilization under reducing conditions.