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EPR studies of copper exchanged smectite and the behaviour upon heating

Abstract:

Movement of small cations (e.g. Li+, Cu2+) from the interlayer into the 2:1 layer of montmorillonite upon heating is a well-known phenomenon (Hofmann & Klemen 1950, Heller-Kallai & Moser 1995). However, the position of the cations after migration is not yet clarified. EPR spectroscopy is helpful in monitoring the hydration state and the location of exchangeable d-ions (e.g. McBride & Mortland 1974, Mosser et al. 1997, McBride 1982, Karakassides et al. 1999, Emmerich et al. 2001). The aim of the present work is to improve the knowledge of interaction mechanisms of montmorillonite with heavy metal cations under wet conditions at temperatures up to 100°C, the fixing of heavy metals in the crystal structure of montmorillonite and its effects on the clay characteristics. Homoionic Cu2+-exchanged samples were prepared by washing the <2 µm fraction of bentonite from Linden (Bavaria) with a chloride solution (Emmerich et al. 1999). Air dry samples or suspension with mass content of 10% of these clays were altered in a water steam atmosphere in stainless steel autoclaves at 55 or 90°C up to six month. Samples were characterized by XRD and EPR. Electron Spin Resonance spectra of randomly oriented powder samples of Cu2+-rich montmorillonite were recorded with a X-Band ESR-spectrometer (Bruker ESP 300E) at room temperature after allowing samples to soak humidity in an atmosphere of 75% relative humidity. EPR spectra of Cu2+-rich mont¬morillonite showed two peaks at g = 2.15 and 2.05 which are characteristic for [Cu(H2O)6]2+ and [Cu(H2O)4]2+, respectively, on interlayer positions. The intensity ratio of both peaks moved in favour of the signal at 2.15 after short time alteration (seven days), whereas the peak at 2.05 regained intensity after six months. Dry and suspension samples that were altered in a steam atmosphere at 90°C as well as at 55°C showed a similar behaviour. EPR spectra show that the amount of fully hydratable Cu2+-ions on interlayer positions depends on increasing time of treatment at lower temperatures. The reduction of concentration of [Cu(H2O)6]2+ at interlayer positions is a reversible process at lower temperatures. The Cu2+- cations are fixed by occupying hexagonal cavities of the tetrahedral sheet (Karakassides et al. 1999) and not strongly in previously vacant sites in the octahedral sheet (Mosser et al. 1997).

Authors:

Plötze, Michael and Emmerich, Katja

Index Terms:

Clay; ClayGroup; montmorillonite; EPR

Further Information:

Date published: 2004