Fe(III)- to Fe(II)-montmorillonite preparation and its use in the reduction of aqueous Cr(VI)

Manjanna, J.*; Kozaki, Tamotsu*; Kozai, Naofumi   ; Sato, Seichi*

Fe(II)-montmorillonite with Fe(II) ions occupying cation exchange sites is an ideal alteration product supposed to be formed by adsorption of corrosion products of overpack carbon steel in the buffer material bentonite after closing high-level radioactive waste disposal sites. Because Fe(II) ions are labile with the surrounding chemical conditions including pH and Eh, this study investigated the stability of Fe(II)-montmorillonite in purified water and Cr(VI) solutions. A Fe(II)-montmorillonite sample was prepared by the following two-step process. In the first-step, Fe(III)-montmorillonite was obtained by the conventional ion-exchange method with FeCl solution. Subsequently, it was reduced to Fe(II)-montmorillonite using ascorbic acid in aqueous medium. The stability of exchangeable Fe(II) in Fe(II)-montmorillonite was studied in distilled water (solid /liquid ratio: 0.5 g / 200 ml) with or without Ar bubbling. It took about 40 days to reach a Fe/Fe ratio of about 0.5 even without Ar bubbling, indicating that the exchangeable Fe(II) oxidizes very slowly. When Fe(II)-montmorillonite was suspended in 0.5 mM of KCrO solution (solid /liquid ratio: 0.1g clay/100 ml), more than 80 % of Cr(VI) was reduced within 10 min. No reduction of Cr(VI) was found for the cases of Fe(III)-montmorillonite samples. It can be considered that the exchangeable Fe(II) in Fe(II)-montmorillonite is relatively stable in water even without Ar bubbling but could reduce Cr(VI) with high reaction rate in an aqueous solution. This suggests the potential application of Fe(II)-montmorillonite for the environmental remediation of Cr(VI) contamination.