Fe(Ⅲ)-montmorillonite: Basic Properties And Diffusion Of Tracers Relevant To Alteration Of Bentonite In Deep Geological Disposal

摘要

In the deep geological repository of high-level nuclear waste (HLW), the interaction of carbon steel (Fe) overpack container and the back-fill clay mineral, montmorillonite (mont), is an important issue to be clarified in view of the long-term performance of clay mineral as an engineered barrier. To arrive at some understanding of the altered clay at the Fe/clay interface, Fe(Ⅲ)-mont is used here for preliminary investigation to see if there is any considerable difference in the basic properties, mainly diffusion of tracers with respect to Na-mont, although Fe(Ⅲ)-mont is not a realistic analogue of altered clay in the reducing chemistry conditions anticipated in the subsurface of HLW, unlike Fe(Ⅱ)-mont which is difficult to prepare and handle in the ambient atmosphere due to its gradual oxidation. Fe(Ⅲ)-mont was prepared by the conventional cation exchange method with 0.4 M FeCl_3 solution. The total Fe(Ⅲ) ions adsorbed was about 1.42 meq/g, while the CEC of parent clay was 1.2 meq/g. Thus ～ 18% excess Fe (Ⅲ) ions were present presumably as iron oxy hydroxide, i.e., about 2 wt.% with respect to FeOOH or Fe(OH)_3 in the bulk sample. The sample was further characterized by X-ray diffraction (XRD), infrared (IR), thermogravimetry, magnetization, and the methylene blue (MB) adsorption. From these conventional techniques, the minor Fe-oxide phase associated with the Fe(Ⅲ)-mont could not be detected, although we cannot rule out its existence. The paramagnetic behavior of Fe(Ⅲ)-mont was evident from the Curie-Weiss plot (300 K down to 5 K). Some basic properties like osmotic swelling, diffusion of tracers and thermal stability are evaluated here. The osmotic swelling of Fe(Ⅲ)-mont was very low, ～ 5 ml/g, while that of Na-mont was >40 ml/g. Apparent diffusion coefficient (D_a) of tracers viz., ~(22)Na~+, HTO and ~(36)Cl~- was determined using compacted (dry density, ρ_d = 1.0,1.6 Mg m~(-3)), water-saturated Fe(Ⅲ)-mont. There was no considerable change in D_a of these tracers in Fe(Ⅲ)-mont when compared to that in Na-mont, in spite of higher pore water availability in Fe(Ⅲ )-mont due to reduced swelling. For instance at ρ_d = 1.6 and 25 ℃, D_a (×10~(-1)m~2/s) of ~(22)Na~+ (2.6) was same as that in Na-mont whereas D_a of HTO (11.4) and ~(36)Cl~- (1.0) have varied marginally. For comparison, the corresponding D_a (×10~(-11) m~2/s) values in Na-mont were 2.7,7.2 and 3.4, respectively. Also, at ρ_d = 1.6, the diffusion activation energy (E_A, kJ mol~(-1)) of HTO (17.5) in Fe (Ⅲ)-mont was comparable to that in free water, however, E_A of ~(36)Cl~- (13.1) was remarkably low, which may be ascribed to the so-called 'anion exclusion' effect. Furthermore, thermal stability of Fe(Ⅲ)-mont indicated the existence of interlayer Fe(Ⅲ) in the ionic state up to ～190 ℃, based on the XRD and MB adsorption test. On heating to >200 ℃, irreversible dehydration of interlayer cations occurred, and the chemisorption of NH_3 indicated two distinct acid sites at about 300 ℃ and 600 ℃.