Diffusion of Ni in compacted Na- and Ca-montmorillonites: results from in-diffusion measurements with characterization of solid phases in the system

Document available in extended abstract form only. In the case of in-diffusion experiment, the diffusion profiles of strongly sorbing nuclides in a bentonite show two parts with different slopes which cannot be fitted by simple one-dimensional diffusion model considering single species (Fijiwara, et al., 2010). There is a lack of the diffusion data of such strongly sorbing nuclides, and the reason for this anomalous diffusion behavior is not understood. Ni diffusion in the bentonite is also belongs to the one. The diffusions of Ni in these compacted Na- and Ca-montmorillonites were studied by in-diffusion experiments. The montmorillonite used in this study was Kunipia-F, a commercial product of Kunimine Industries Co. Ltd., Japan. Ca-montmorillonite was synthesized from Kunipia-F (Na-montmorillonite) by Na/Ca ion exchange. The Na- and Ca-montmorillonites were compacted into an acrylic resin cell (20 mm in diameter, 20 mm in height) to obtain dry density of 0.8 Mg m-3 and fully saturated with 0.1 M NaCl and 0.2 M CaCl2 solutions, respectively. The Ni source concentration in the solution was 660 μM. The diffusion experiments were conducted at room temperature in a glove box under N2 atmosphere (CO2 and O2 < 1 ppm). After the experimental period, the cell was disassembled, and a flat diffraction plane was obtained by sectioning the specimen into 1-mm-thick in steps. The basal spacing of each compacted montmorillonite specimen in the water-saturated state was determined from XRD profiles measured from 1.7 to 8 deg 2θ using a RIGAKU RINT 2100 diffractometer with Cu-Ka radiation. The concentration of Ni ion in each section of the specimen was measured by ICP-AES or ICP-MS after reaching by 1 M HNO3 to obtain the Ni concentration profile. The diffusion of Ni in the compacted Ca-montmorillonite after 83 days showed a normal diffusion profile that could be described using one single effective diffusion coefficient and one sorption value. In the case of Na-montmorillonite, anomalous diffusion profiles were obtained after 84 and 388 days. Ni can uptake into cation exchange site in the interlayer of Na-montmorillonite but, cation exchange sites are blocked in Ca-montmorillonite with the high-ionic strength Ca background electrolyte. These results indicated that these anomalous diffusion profiles were related to uptake into cation exchange sites in the interlayer. Figure 2 shows X-ray diffraction patterns for water-saturated, compacted Na-montmorillonite after 388 days of diffusion with different position of Ni concentration profile. For the sample at 0 to 3.3 mm position of the concentration profile the reflection observed, corresponding to a d-spacing of about 18.8, 15.5 and 12.6 A, indicate the presence of heterogeneous organization of the layers with three, two and one water sheets in the interlayer, respectively. The two and one water sheets in the interlayer decreased and disappeared until the position of 3.3 mm. The one water sheets in the interlayer appeared between 4.1 and 9.1 mm and it disappeared at deeper than 10.1 mm. This observed transition of XRD pattern have corresponded to the transition of concentration profile after diffusion experiment. In this way, it has been shown that different apparent diffusion coefficient (Da) correspond to the structure of different number of interlayer water molecule in the diffusion field. These experiments clearly shows that concentration profile with lower Da have been corresponded to the 1 water sheets in the interlayer. This may be due to the diffusion of Ni into the octahedral layer is one of the uptake mechanisms of Ni onto Na-montmorillonite while it is not onto Ca-montmorillonite. (authors)