Iron oxyhydroxide colloid formation by gamma-radiolysis

Abstract

Gamma-irradiation of deaerated aqueous solutions containing FeSO₄ leads to the formation of uniform-sized colloidal particles of γ-FeOOH. At short irradiation times, or in solutions with a low initial [Fe²⁺]₀, spherical particles with a size less than 10 nm are formed. These primary particles grow to form a dendritic structure upon longer irradiation, and the final size of the large particles is ∼60 nm with a very narrow size distribution. Further prolonged irradiation does not change the final particle size. The narrow size distribution is attributed to rapid homogeneous radiolytic oxidation of soluble Fe²⁺ to relatively insoluble Fe³⁺ hydroxides [Fe(H₂O)_6−n(OH)_n]³⁻ⁿ leading to particle nucleation by spontaneous condensation. These primary particles then grow into γ-FeOOH particles with a dendritic structure. The final size reached at long times is regulated by the steady-state redox conditions established during long-term irradiation at the aqueous–solid interface.