Pore-Size-Dependent Kinetics and Product Distribution of Fe(II)-Catalyzed Ferrihydrite Transformation

Fe(II)-catalyzed ferrihydrite (Fh) transformation is a widely occurring biogeochemical process in the porous media of anaerobic soils and sediments, but how pore-scale spaces affect the kinetics and product distribution of Fh transformation remains poorly understood. This study investigated the transformation of Fh nanoparticle film across a continuum of sizes of pore-scale spaces created by pressing a glass bead onto the film and immersing in 2 mM and 10 mM FeSO₄ at pH = 7.0. While lepidocrocite (Lp) dominated over goethite (Gt) on the static Fh film, both minerals were observed to preferentially precipitate in micropore regions adjacent to the Fh film-glass bead contact area. The natural logarithm of product formation rates decreased linearly as increasing pore height (h) from 5 to 600 μm, revealing pronounced kinetic inhibition in macropore regions. Product mineral particles in smaller pore regions exhibited reduced primary particle size (146.1 ± 57.6 nm at h = 0.11 μm) compared with those in larger pores (222.5 ± 61.8 nm at h = 561.84 μm). COMSOL-based reaction-diffusion modeling shows that rapid Fe(III)_labile intermediate supply from Fe(II)-catalyzed Fh dissolution drives its localized accumulation and elevated supersaturation within confined micropores, which ultimately dictates the pore-size-dependent distribution, formation kinetics, and primary particle size of the resulting product minerals. These findings provide mechanistic insights into the dominant role of intermediate precursor supply in dictating confinement-regulated phase transformation and highlight the notable effects of micropore size on Fh transformation and element cycling in natural porous media.