Mössbauer spectra of exchangeable Fe(II)- and Fe(III)-ions on saponite

Intercalation and dynamics of hydrated Fe2+ in the vermiculites from Santa Olalla and Ojén

“…The observed quadrupole splitting for the intercalated Fe2+ site is in good agreement with the values given by Helson and Goodman [7] and Goodman et al [8] and Charlet and Tournassat [12] and assigned to the Fe2+ hexaaquo complex [Fe(H2O)6]2+. Such a Fe2+ site in the interlayer space has also been observed at 4.2 K but not at ambient temperature in the Fe3+ intercalated Ojén vermiculite and the Wyoming smectite after reduction with pyrrole [9].…”

“…This component is attributed to intercalated Fe2+. Its absence in the RT spectra can be explained by a strong decrease of the Lamb-Mössbauer f-factor (f-factor) of this component with increasing temperature, as has been observed previously in smectites [7,8]. It is worth mentioning that after storage of the intercalated Ojén specimen for about 6 months in ambient air, the intercalated Fe2+ component was still observed, although with only about a third of the intensity observed immediately after the preparation of the sample.…”

“…Vermiculites and smectites loaded with Fe3+-especially with Fe3+ isopolyoxo cations-have attracted attention because of their potential application as catalysts [1][2][3][4], although the uptake of Fe3+ into the interlayer space of phyllosilicates is a delicate process because the Fe3+ ions can be maintained in the monomeric hydrated form that is best for ion exchange only at the pH values of the aqueous solution below 1.5, which can lead to a deterioration of the phyllosilicates. At higher pH values, polymerization will set in and eventually result in the formation of iron oxo-hydroxides, which will be deposited preferentially on the external surfaces of the clay minerals because of their positive surface charge [5][6][7][8][9].…”

“…For this reason, the mobility of the hydrated Fe2+ ions in the interlayer space and concomitantly the rate of ion exchange should be higher than for Fe3+. However, except for two early studies [7,8], the uptake of Fe2+ into the interlayer space of phyllosilicates has been investigated only recently [10][11][12][13] and interestingly enough in the context of bentonites used as contamination barriers for nuclear waste. This is surprising since clays are of importance in the iron chemistry of soils, e.g., in connection with the bacterially mediated reduction of iron in phyllosilicates [14][15][16][17] and the concomitant release of Fe2+ [16,18] and the importance of Fe2+ for the formation of chlorites [19][20][21][22].…”