Transformation Cascades in Iron Oxides: Quantitative Resolution of Sequential Precipitation Using the Reaction-Diffusion Framework
Year: 2026
Type: article
Abstract: High Resolution Image Download MS PowerPoint Slide Sequential phase transformations under transport limitation govern mineral formation, corrosion, and diffusion-driven synthesis, yet equilibrium phase diagrams and well-mixed experiments largely obscure transient intermediates, spatial segregation, and kinetic hierarchies. Here, we use precipitation–diffusion in 1.0 wt % agar hydrogels to resolve the transformation cascade of iron oxides as hydroxide (1.0–3.0 M NaOH) diffuses into Fe 2+ /Fe 3+ -loaded gels, producing three sharp, spatially separated fronts: yellow goethite (α-FeOOH), green rust (Fe 2+ –Fe 3+ LDH), and black magnetite (Fe 3 O 4 ). Quantitative tracking shows that front positions follow power-law kinetics, d i ( t ) = α i t β i, with β i spanning 0.39–0.56 and high goodness-of-fit ( R 2 ≥ 0.97; typically >0.99) across all tested conditions. A coupled Stefan moving-boundary analysis links the parabolic front kinetics to phase-specific hydroxide consumption, yielding an alkalinity-demand hierarchy Λ G /Λ GR /Λ M ≈ 1:1.3:1.9, which rationalizes both the transformation sequence and the progressive widening of the goethite region. Increasing outer hydroxide accelerates all fronts, whereas increasing total iron loading slows propagation; after 96 h, fronts typically penetrate 3–12 mm into the gel. Microscopy and spectroscopy support a solution-mediated dissolution–reprecipitation pathway, and Fe 2+ -rich conditions drive a transition from steady fronts to oscillatory Liesegang banding, demonstrating how diffusion–reaction balance controls both cascade formation and periodic precipitation.
Source: ACS Omega
Authors: Nour Abi Aad, Mazen Al-Ghoul
Institution: American University of Beirut
Language: English
Field: Energy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,935
OpenAlex ID: W7128029105