Effect of Ionic Charge on Effective Diffusion Coefficient in Compacted Sodium Bentonite
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iffusion species to quantitatively evaluate the degree of surface diffusion and anion exclusion. EXPERIMENTAL Material and Experimental Conditions Sodium bentonite, Kunigel-VI®, which is a crude bentonite and was treated in the reference case of performance assessment in the Table I Experimental Conditions for Diffusin Experiments second progress Bentonite Na-bentonite, Kunigel-Vl® (Kunimine Industries Co. Ltd.) report[1l], was used. Dry density 1.8Mg.m- 3 Major clay mineral of Tracer Sm (SmCb), Se (Na2SeO3), Ni (Ni-63+NiCI2 (carrier)) the bentonite is Namontmorillonite, which Concentration Sm:u0.0M, Se:0.001M, Ni:2kBqpml-l (Ni-63)+0.001M mode is 46-49wt% and Porewater simulated porewater (see Table II) chalcedony, quartz, pH Ni, Sm: 5 - 6, Se: not adjusted (monitoring) plagioclase, calcite, Temperature room temperature dolomite, analcite and Atmosphere Se: Ar-atmosphere (02 concentration < lppm) pyrite are contained as Ni, Sm: aerobic condition impurities. The detailed mineralogy is described Producibility n=2 in the literatures of Ito et al.[12, 13]. A simulated porewater, prepared to obtain certain concentration by dissolving NaCl, Na 2CO 3 and Na 2SO4 in distilled water, was used in all diffusion experiments. The concentration and chemical composition was determined based on the results of bentonite leaching tests for various liquid-solid ratios. Tables I and II show the experimental conditions and the chemical composition of the simulated porewater, respectively.
Table II Chemical Composition of the Simulated Porewater Ion
Concentration(M)
Na+ CGS042C032-
0.83 0.0071 0.12 0.29
Experimental Procedure The diffusion experiments were carried out by through-diffusion method[3]. Figure 1 shows the schematic view of a diffusion cell. Bentonite was dried at 105'C for over night and filled in the sample holder with the size of 20mm in diameter and 3mm in thickness with a dry density of 1.8Mgrn-3 . The bentonite in the holder was saturated with simulated porewater before diffusion experiment. For Ni and Sm, the bentonite was saturated with the simulated porewater adjusted at pH5-6. The saturation was carried out in a vacuum chamber for a week after degassed an hour. The pH of the porewater was monitored and adjusted using HC1 if necessary. All experiments for Se were conducted under Ar atmosphere(0 2 and Se3 3 2- in bentonite for 1.8Mgmrn-3 were obtained. The De values, including De data reported to date, were in the order, Cs+>Sm 3+>HTO>Ni2+>anions for overall density, showing a tendency to be in the order, cations>neutral3 species>anions. 2 2 The FF values for the same density were in the order, Sm +>Cs+>Ni +>HTO>SeO 3 -
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>NpO 2 CO 3 -, showing a tendency to be in the order, cations>neutral species>anions. The degree of surface diffusive effect for a dry density of 1.8Mg-m- 3 was approximately estimated to be 5 times for Sm3÷, 3 times for Cs' and 1.3 times for Ni as much assuming that FF of HTO is a standard. The degree of anion exclusive effect for the same condition was approximately calculated to be 1/7 fo
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