Characterization of homoionic Fe-type montmorillonite: Potential chemical species of iron contaminant

Fe²⁺-montmorillonite with Fe²⁺ ions occupying cation exchange sites is an ideal transformation product in bentonite buffer material. In our previous study on preparation and characterization of Fe²⁺-montmorillonite, the montmorillonite sample that adsorbed Fe²⁺ ions on almost all of the cation exchange sites was prepared using a FeCl₂ solution under an inert gas condition N. Kozai, Y. Adachi, S. Kawamura, K. Inada, T. Kozaki, S. Sato, H. Ohashi, T. Ohnuki, T. Banba, J. Nucl. Sci. Technol. 38 (2001) 1141]. In view of the unstable nature of iron(II) chemical species, this study attempted to determine the potential contaminant iron chemical species in the sample. Nondestructive elemental analysis revealed that a small amount of chloride ions remained dispersed throughout the clay particles. The chloride ion retention may be due to the adsorption of FeCl⁺ ion pairs in the initial FeCl₂ solution and the subsequent containment of the Cl⁻ ions that are dissociated from the FeCl⁺ ion pairs during excess salt removal treatment. Two explanations are advanced for the second process: the slow release of the remaining Cl⁻ ions from the collapsed interlayer of the montmorillonite, and the transformation of a minor fraction of the remaining FeCl⁺ ion pairs to iron(III) hydroxide chloride complexes having low solubility.