Dehydroxylation behavior of heat-treated and steam-treated homoionic cis-vacant montmorillonites

Li+, Na+, Ca2+, Sr2+, Cu2+, or Zn2+-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 (similar to 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+, Cu2+, and Zn2+ 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, Cu2+ and Zn2+-rich samples release 16-23 meq/100 g of Mg2+ from the structure. No Mg2+ release is observed for the Li+-rich montmorillonite. Also the dehydroxylation behavior after rehydroxylation differs between the Cu2+, Zn2+, and Li+-rich samples. The mass curves of the evolved water during thermoanalysis of the rehydroxylated Cu2+ and Zn2+-rich smectites show a peak doubler between 480-700 degrees C. For the Li+, Na+, Ca2+, and Sr2+-rich montmorillonites, the second peak disappeared and a third peak at similar to 760 degrees C developed after rehydroxylation. The resulting structure after rehydroxylation of all samples is celadonite-like.