Combined Effects of Hematite and Metal Ions on Ferrihydrite Phase Transformation: Unraveling Pathways and Metal Ion Redistribution

Abstract Ferrihydrite (Fhy) is a pivotal precursor of crystalline iron oxides and a key host for environmentally critical metal elements, strongly regulating soil mineral composition and the migration behavior of metal ions through its transformation. Despite the common co-occurrence of Fhy and hematite (Hem) in nature, their dynamic interactions with metal ions during Fhy transformation remain unclear. This study systematically investigated the combined roles of coprecipitated Cd(II)/Zn(II)/Al(III) ions and Hem nanoplates (HNP) in directing Fhy transformation pathways and the consequent redistribution of associated metals. Results show that all three metal cations inhibit Fhy transformation and increase the relative abundance of Hem in the transformation product, following the effectiveness order Al(III) > Zn(II) > Cd(II) that correlates with their binding affinities to iron (oxyhydr)oxides. Conversely, HNP accelerates the transformation of metal-bearing Fhy by facilitating the dissolution-recrystallization pathway and serves as a heterogeneous nucleation template to further enhance the relative abundance of Hem in the final products. Notably, the accelerating effect of HNP is most pronounced for Al-bearing Fhy, due to limited Al(III) release and thus reduced competition between dissolved Al(III) and Fe(III) for nucleation sites. Fhy transformation drives distinct redistribution patterns for the associated metals: Cd(II) and Zn(II) are released to varying degrees, whereas Al(III) is fully retained in the solid phase. Specifically, HNP significantly enhances Zn(II) release, lowering its solid-liquid partition coefficient from 2.74 (without HNP) to 1.90. In comparison, HNP has little effect on the redistribution of Al(III) and Cd(II). Al(III) remains entirely in the solid phase irrespective of HNP presence, while Cd(II) resides predominantly in solution, with partition coefficients of 0.30 (without HNP) and 0.28 (with HNP). The enhanced Zn(II) release is attributed to the HNP-promoted dissolution-recrystallization pathway and the lower structural compatibility of Zn(II) in Hem compared to goethite. These findings elucidate the combined effects of Hem and metal cations in regulating Fhy transformation pathways and metal redistribution, providing mechanistic insights into the evolution of iron oxides and the fate of metals in surficial geochemical systems.