RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Lepidocrocite (𝛾-FeOOH), an important iron-bearing mineral found to exist with a relatively high abundance in the soils of the Chinese nuclear test sites, has rarely been studied for its sorption of transuranic elements from the nuclear wastes. This work develops a quantitative surface complexation model describing the sorption and speciation of Np(V) on synthetic lepidocrocite (𝛾-FeOOH). Batch sorption experiments on 𝛾-FeOOH were performed under a range of conditions (25 °C, 0.1 M NaClO₄, pH = 4-11, 5 µM Np(V), and atmospheric conditions). The Diffuse Layer Model (DLM) was applied to describe the surface-complexation reaction. The data were best-fitted with a single surface-complexation reaction, (≡XOH + NpO₂⁺ = XONpO₂⁰ + H⁺) that occurred at the lepidocrocite/ water interface to form an inner-sphere Np(V) complex with lepidocrocite. The results are complemented by the Np L_III-edge EXAFS data that show that Np(V) was absorbed on 𝛾-FeOOH as monomeric neptunyl ions, with no observations of multinuclear surface complexes or surface precipitates. A prominent peak at ∼3Å in the EXAFS Fourier Transform spectra can be attributed to aNp-Fe scattering path, consistent with the formation of an inner sphere Np(V)-lepidocrocite surface complex. Formation of aqueous NpO₂(CO₃)_x^1-2x complexes prevents Np(V) sorption at higher pH values but it is unclear if ternary lepidocrocite-Np-carbonate complexes may also form. These data indicate that there are subtle differences in Np(V) interactions with hematite, goethite, and lepidocrocite which is likely a manifestation of the differences in surface reactivity of the three minerals.