Abstract

Li (super +) , Na (super +) , Ca (super 2+) , Sr (super 2+) , Cu (super 2+) , or Zn (super 2+) -saturated samples of a cis-vacant montmorillonite from Linden, Bavaria, were heated to temperatures between 200-700 degrees C. Half of each heated sample was subsequently autoclaved under steam at 200 degrees C ( approximately 1.5 MPa) to promote rehydroxylation. The smectites were characterized by cation-exchange capacity (CEC), determination of exchangeable cations, infrared (IR) spectroscopy, and thermoanalytical investigations of evolved water in a thermobalance linked with a mass spectrometer. Changes in the montmorillonite structure and dehydroxylation behavior are related to three respective mechanisms: type of the interlayer cation, interlayer cation radius, and the movement of the interlayer cation. The migration of the smaller Li (super +) , Cu (super 2+) , and Zn (super 2+) ions after heating produces a strong reduction of CEC due to the Hofmann-Klemen effect before the initiation of dehydroxylation. Thereafter, the CEC of these smectites remains constant over a large temperature interval during dehydroxylation. After rehydroxylation, Cu (super 2+) and Zn (super 2+) -rich samples release 16-23 meq/100 g of Mg (super 2+) from the structure. No Mg (super 2+) release is observed for the Li (super +) -rich montmorillonite. Also the dehydroxylation behavior after rehydroxylation differs between the Cu (super 2+) , Zn (super 2+) , and Li (super +) -rich samples. The mass curves of the evolved water during thermoanalysis of the rehydroxylated Cu (super 2+) and Zn (super 2+) -rich smectites show a peak doublet between 480-700 degrees C. For the Li (super +) , Na (super +) , Ca (super 2+) , and Sr (super 2+) -rich montmorillonites, the second peak disappeared and a third peak at approximately 760 degrees C developed after rehydroxylation. The resulting structure after rehydroxylation of all samples is celadonite-like.

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