EFFECT OF CARBONATE ON THE SOLUBILITY OFNEPTUNIUM IN NATURAL GRANITIC GROUNDWATER

This study investigates the solubility of neptunium (Np) in the deep natural groundwater of the Korea Atomic EnergyResearch Institute Underground Research Tunnel (KURT). According to a Pourbaix diagram (pH-Ehdiagram) that wascalculated using the geochemical modeling program PHREEQC 2.0, the redox potential and the carbonate ion concentrationboth control the solubility of neptunium. The carbonate effect becomes pronounced when the total carbonate concentration ishigher than 1.5×10-2M at Eh= -200 mV and the pH value is 10. Given the assumption that the solubility-limiting stablesolid phase is Np(OH)4(am) under the reducing condition relevant to KURT, the soluble neptunium concentrations were inthe range of 1×10-9M to 3×10-9M under natural groundwater conditions. However, the solubility of neptunium, which wascalculated with the formation constants of neptunium complexes selected in an OECD-NEA TDB review, strongly deviatesfrom the value measured in natural groundwater. Thus, it is highly recommended that a prediction of neptunium solubility isbased on the formation constants of ternary Np(IV) hydroxo-carbonato complexes, even though the presence of thosecomplexes is deficient in terms of the characterization of neptunium species. Based on a comparison of the measurementsand calculations of geochemical modeling, the formation constants for the “upper limit” of the Np(IV) hydroxo-carbonatocomplexes, namely Np(OH)y(CO3)z4-y-2z, were appraised as follows: log Kο122= -3.0±0.5 for Np(OH)2(CO3)22-, log Kο131=-5.0±0.5 for Np(OH)3(CO3)-, and log Ko141= -6.0±0.5 for Np(OH)4(CO3) KCI Citation Count: 4