The diffusion of Np(V) in compacted bentonite with a dry density of 1.0 g/cm³ was studied. Nonsteady-state diffusion experiments were performed at temperatures of 40°C and 90°C. The results indicate a negative temperature dependence of the apparent diffusion coefficients: 2×10−12 m²/s and 5×10−13 m²/s, respectively. The comparison between the measured and the calculated apparent diffusion coefficients indicates that Np(V) mi-gration is controlled by the pore diffusion mechanism under the present experimental conditions.
A coupled chemical equilibria/transport code which includes the constant capacitance model to explain the equilibrium sorption of Np(V) onto bentonite was used to analyze the diffusion behavior of Np(V). The profiles were satisfactorily simulated by assuming surface complexation reactions of neptunyl and neptunyl carbonate species which were estimated to be dominant in the pore solution of the bentonite. The calculations show that the complexes NpO2CO3− and NpO2(CO3)23− are dominant in the aqueous and solid phases.
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