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.