Preparation of Fe(II)-montmorillonite by reduction of Fe(III)-montmorillonite with ascorbic acid

A new and convenient method is described here to prepare Fe(II)-montmorillonite (mont) using Fe(III)-mont as the starting material. Fe(III)-mont was treated with ascorbic acid (vitamin C) solution at 70 °C for ~ 12 h in Ar atmosphere. All of the adsorbed iron(III) was reduced to form Fe(II)-mont. The presence of any associated Fe-oxide phase in the starting material, Fe(III)-mont, was rather advantageous to reach stoichiometric Fe(II)-mont. The Fe2+/Fetotal ratio of the iron extracted from freshly prepared Fe(II)-mont was found to be close to unity. The total amount of iron extracted in 1 M NH4Cl was equivalent to the cation exchange capacity of the parent montmorillonite (Kunipia F montmorillonite) in terms of Fe(II), provided there was sufficient iron(III) in the starting material. The basal spacing at a relative humidity (RH) of 40% was 14.7 Å, typical of montmorillonite with divalent interlayer cations. FTIR spectra showed no significant changes in the basic clay mineral structure. The structure was further proved by Mössbauer spectra at room temperature (RT). Thus Fe(II)-mont preparation as described here was found to be highly convenient for routine preparation in the ambient atmospheric condition, also for large quantities. The feasibility of solid-state preparation of Fe(II)-mont was also demonstrated. The stability of Fe(II)-mont was proved in dispersion in water under partially deoxygenated condition. There was about 30% and 70% oxidation of interlayer Fe(II) ions in about 47 days at RT and 65 °C. Thus, the interlayer Fe(II) ions were gradually oxidized (i.e., not rapidly, as anticipated). For long-time preservation of the Fe(II)-mont sample, it is essential to keep deoxygenated, low RH (< 40%) conditions and without exposing to elevated temperatures.