Thermally induced cation migration in Na and Li montmorillonite |
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Authors: | V. Luca C. M. Cardile |
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Affiliation: | 1. Chemistry Department, Victoria University of Wellington, Private Bag, Wellington, New Zealand 2. Chemistry Division, Department of Scientific and Industrial Research, Private Bag, Petone, New Zealand
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Abstract: | In order to determine whether Li+ cations penetrate into the octahedral layers of montmorillonites upon mild heating (Hofmann-Klemen effect) 57Fe Mössbauer spectra of Na+ and Li+ exchanged montmorillonite were obtained before and after treatment at 220 ° C. The 57Fe nucleus was used as a remote probe to detect electronic perturbations which would occur if a Li cation was to move into the octahedral layer from the interlayer after heating. The ambient Mössbauer spectra showed that a high charge density interlayer cation such as Li+ is effective in reducing the phonon energy of VIFe2+. In addition the EFG at octahedral sites can be significantly modified by interlayer cations as evidenced by the larger quadrupole splitting value measured for the Li+-exchanged sample with respect to the Na+-sample. Interlayer collapse and migration of exchange cations into the montmorillonite lattice after heating to 220 ° C resulted in the oxidation of the VIFe2+ and a decrease in site distortion for IVFe3+. Similar spectral parameters for the Fe3+ resonances of both Na+ — and Li+-heated samples suggested the interlayer cations do not penetrate as far as the octahedral layers. In order to utilize the enhanced sensitivity of VIFe2+ Δ values to changes in EFG the Fe3+ in the heated montmorillonites was reduced to Fe2+ with hydrazine. Similar spectral parameters for both the Na+ — and Li+-exchanged montmorillonite were observed giving further evidence that Li cations do not migrate into vacant octahedral sites. |
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