Dissolution of iron in salicylic acid and cation exchange between Fe(II)-salicylate and Na-montmorillonite to form Fe(II)-montmorillonite

• Fe(II)-Mt is an important redox and altered clay in the repositories of HWL.
• Fe(II)-Mt is prepared by cation exchange between Na-Mt and Fe(II)-salicylate.
• Fe(II)-L is obtained by dissolving Fe0 in salicylic acid at 80 °C in N2-atmosphere.
• Formation of colorless Fe(II)-L and its oxidation to red Fe(III)-L is studied.

Bentonite is used as part of engineered barrier in the geological disposal of high-level nuclear waste (HLW) from nuclear plants. Fe(II)-montmorillonite, Fe(II)-Mt, is a potential altered clay in such a system due to clay mineral interaction with the carbon steel (overpack material) under reducing water chemistry conditions. To understand the long-term integrity of the HLW, the basic studies on Fe(II)-Mt are needed. In this study, we have focused on the synthesis of homoionic Fe(II)-Mt by using weak ferrous complexe for cation-exchange with Na-Mt. We prepared Fe(II)-salicylate by dissolving Fe0 in salicylic acid at 80 °C under N2 atmosphere. Dissolution rate constants are in the range of 0.8–1.6 × 10− 3 min− 1. A colorless Fe(II)-L, was reasonably stable in the inert atmosphere, but was rapidly oxidized in air to a red colored Fe(III)-L (λmax 475 nm). The solution redox potential and UV–Vis absorption spectra of these complexes are recorded here. Na-Mt was subjected for ion-exchange reaction with Fe(II)-L to form Fe(II)-Mt. The formation of Fe(II)-Mt was confirmed from the interlayer Fe2 + ions released by interacting with 0.5 M H2SO4. The cumulative amount of Fe2 + ions are equivalent to CEC of 1.10 meq/g and Fe2 +/Fetotal ≈ 1. The X-ray diffraction patterns (d001) and infrared spectra also support the formation of Fe(II)-Mt. The cation exchange achieved here was possible through dissociation of Fe(II)-L owing to its intermediate stability constant (log K = 6.55), unlike the Fe(III)-L (log K = 16.35). We believe the present method is one of the best approaches for obtaining homoionic Fe(II)-Mt.