Apparent diffusion coefficients and chemical species of neptunium (V) in compacted Na-montmorillonite

Diffusion of neptunium (V) in compacted Na-montmorillonite was studied thro ugh the non-steady state diffusion method. In this study, two experimental attempts were carried out to understand the diffusion mechanism of neptuniu m. One was to establish the diffusion activation energy, which was then use d to determine the diffusion process in the montmorillonite. The other was the measurement of the distribution of neptunium in the montmorillonite by a sequential batch extraction. The apparent diffusion coefficients of neptu nium in the montmorillonite at a dry density of 1.0 Mg m(-3) were from 3.7 x 10(-1)2 m(2) s(-1) at 288 K to 9.2 X 10(-12) m(2) s(-1) at 323 K. At a dr y density of 1.6 Mg m(-3), the apparent diffusion coefficients ranged betwe en 1.5 X 10(-13) m(2) s(-1) at 288 K and 8.7 X 10(-13) m(2) s(-1) at 323 K. The activation energy for the diffusion of neptunium at a dry density of 1 .0 Mg m(-3) was 17.5 +/- 1.9 kJ mol(-1). This value is similar to those rep orted for diffusion of other ions in free water. e.g., 18.4 and 17.4 kJ mol (-1) for Na+ and Cl-, respectively. At a dry density of 1.6 Mg m(-3), the a ctivation energy was 39.8 +/- 1.9 kJ mol(-1). The change in the activation energy suggests that the diffusion process changes depending on the dry den sity of the compacted montmorillonite. A characteristic distribution profil e was obtained by the sequential extraction procedure for neptunium diffuse d in compacted montmorillonite. The estimated fraction of neptunium in the pore water was between 3% and 11% at a dry density of 1.6 Mg m(-3) and at a temperature of 313 K. The major fraction of the neptunium in the montmoril lonite was identified as neptunyl ions sorbed on the outer surface of the m ontmorillonite. These findings suggested that the activation energy for dif fusion and the distribution profile of the involved nuclides could become p owerful parameters in understanding the diffusion mechanism. (C) 2001 Elsev ier Science B.V. All rights reserved.