Investigation of montmorillonite alteration and form of iron corrosion products in compacted bentonite in contact with carbon steel for ten years

Abstract

In high-level radioactive waste disposal, the alteration of montmorillonite due to the corrosion of carbon steel possibly affects the swelling and self-healing capacity of compacted bentonite used as a buffer material. The nature of the corrosion products in compacted bentonite is also important to evaluate not only the diffusion and sorption behavior of radionuclides but also the chemical composition and redox potential of pore water. In this study, the alteration of montmorillonite in compacted bentonite due to the interaction with carbon steel was analyzed by X-ray diffraction (XRD). The possibility of montmorillonite alteration was also investigated from the cation exchange capacity (CEC) of compacted bentonite and scanning electron microscopy (SEM) observation. The corrosion products distributed in the compacted bentonite were investigated by selective dissolution analysis, which can estimate the crystallinity of Fe-bearing compounds. The valence of Fe in the corrosion products was spectrophotometrically determined. From the XRD analysis, newly formed phyllosilicates resulting from the alteration of montmorillonite could not be identified in compacted bentonite. CEC of compacted bentonite adjacent to the carbon steel, in which high concentration of Fe was extracted, was hardly decreased. No significant differences of clay particles were observed with SEM. Thus, the alteration of montmorillonite was scarcely detected in compacted bentonite in contact with carbon steel for ten years. The selective dissolution and valence analyses suggest that most of the corrosion products of carbon steel existed in Kunipia F, which consists of over 95 wt% montmorillonite, was amorphous, non-crystalline or poorly ordered Fe(OH) 2. This means that Fe(OH) 2 distributed into compacted bentonite was scarcely crystallized within ten years at 80 oC. From the XRD analysis, small amount of green rust one containing Cl − at the interlayers (GR1(Cl −)) and lepidocrocite were also identified in Kunipia F. Therefore, under this experimental condition, Fe(OH) 2 formed in Kunipia F due to the corrosion of carbon steel was oxidized to GR1(Cl −) as intermediates, and then GR1(Cl −) was possibly oxidized to lepidocrocite. On the other hand, GR1(Cl −) was hardly detected in Kunigel V1, which contains 46–49 wt% montmorillonite, from the XRD analysis.