Neptunium, Radioactive

Environmental Fate

TERRESTRIAL FATE: Neptunium typically occurs as the oxide in the environment(1). In soil, neptunium is generally more mobile than other transuranic elements such as plutonium, americium, and curium, moving with percolating water to lower soil layers(1). Neptunium compounds bind to soil particles, and bind more tightly with clay soils as compared with sandy soils(1). Neptunium is readily taken up by plants, with plant concentrations similar to soil concentrations(1). Neptunium compounds are ionic and would not volatilize from moist or dry soil surfaces(SRC).
AQUATIC FATE: Neptunium ions would be expected to adsorb to suspended particles in water(SRC), since actinide ions with III, IV, and VI oxidation states can be adsorbed to cation-exchange resins(1). Neptunium is a member of the actinide series and would be expected to behave similarly(SRC). Neptunium can exist as the following ions in water: Np3+ (pale purple); Np4+ (yellow green); NpO+ (green blue); and (NpO)2+ (pale pink)(2). The pentavalent state is the most stable ion in aqueous solution(3). Neptunium 3+ ion is stable in water but is readily oxidized by air to the 4+ state(1). Neptunium 4+ ion is stable in water, but is slowly oxidized by air to (NpO2)+(1). (NpO2)+ ion is stable in aqueous solution, and disproportionates only at high activities(1). (NpO2)2+ is stable in aqueous solution, but can be easily reduced(1). (NpO5)3- only exists in alkaline solution(1). Since neptunium compounds are ionic, they will not volatilize from water surfaces(SRC). Bioconcentration is not expected to be an important fate due to the ionic nature of neptunium compounds(SRC).
ATMOSPHERIC FATE: Neptunium compounds are ionic and would not be volatile and would exist solely in the particulate phase in the ambient atmosphere. Particulate-phase neptunium compounds will be removed from the atmosphere by wet or dry deposition. (SRC)
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