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.

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