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The rate and methanism of Np migration were investigated in a flowing water/clayey sand system under oxidizing conditions. A 237-Np-doped borosilicate glass was used as a source of contaminant in the migration experiments. A preliminary study of the chemical speciation of Np in the water leachate allowed identification of soluble cationic and anionic carbonate species of Np(V). The formation constants of Np(V) carbonate complexes were corrected for the ionic strength of the groundwater by making use of the Guggenheim- Scatchard specific interaction theory. The values were then used to model the partitioning of Np between the soil and the water phase. Increasing carbonate concentration in the solution resulted in a decrease of the amount of Np sorbed on the soil surface. Yet such a decrease is lower than that calculated assuming the uptake of the cationic species. This was interpreted as a simultaneous sorption of NpOf5&/2- and NpO-2(CO-3). The individual distribution coefficients of the two species were calculated from the static tests. A kinetic model was developed, including rapid sorption reactions and a slow interconversion between NpOf5&/2- and NpO-2(CO-3). Such a model could account for the chromatographic separation of the two species observed in the flowing system. Analytical solutions of the transport equation were used for best-fit analysis of the Np column profiles. Both the sorption reaction and the dissociation kinetics were considered in the model.

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Bibliographic Reference: APPLIED GEOCHEMISTRY, VOL. 2, PP. 275-284
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