Surface complexation modeling of neptunium(V) sorption to lepidocrocite (γ-FeOOH)

Lepidocrocite (gamma-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 (gamma-FeOOH). Batch sorption experiments on gamma-FeOOH were performed under a range of conditions (25 C, 0.1M NaClO4, pH = 4-11, 5 mu M 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+ = XONpO2 + 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 gamma-FeOOH as monomeric neptunyl ions, with no observations of multinuclear surface complexes or surface precipitates. A prominent peak at 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) x 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 amanifestation of the differences in surface reactivity of the three minerals.