Precipitation of crystalline neptunium dioxide from near-neutral aqueous solution

Abstract

We report experimental evidence that crystalline neptunium dioxide, NpO₂(cr), will precipitate from near-neutral aqueous solutions that initially contain NpO₂⁺(aq), even under mildly oxidizing conditions. Observed decreases in aqueous Np concentrations with time are balanced by corresponding decreases in pH and accompanied by formation of high-purity crystals of NpO₂. The crystalline NpO₂ was characterized by X-ray powder diffraction (XRD), X-ray absorption fine structure (XAFS) spectroscopy, and scanning electron microscopy (SEM). Thermodynamic data in the literature suggest that NpO₂(cr) is possibly the stable solid phase in aqueous systems containing dissolved NpO₂⁺(aq); however, the precipitation of NpO₂(cr) from solutions containing initially NpO₂⁺(aq) had not been observed experimentally until now, possibly because of slow reaction kinetics. Our experiments were conducted at 200°C in order to overcome slow reduction reactions and/or precipitation kinetics. Because long time scales may render slow reaction kinetics irrelevant, thermodynamically stable NpO₂(cr) may keep dissolved Np concentrations well below the relatively high Np concentrations (10^-3 M) associated with meta-stable Np(V) solids observed in previous laboratory experiments. Projects investigating the suitability of sites for an underground repository for high-level nuclear waste disposal may not need be so concerned with high dose rates due to migration of Np over long times, because these may not apply to repository-relevant time scales over which NpO₂(cr) is likely to be stable.