Reversible collapse and Mg2+ release of de- and rehydroxylated homoionic cis-vacant montmorillonites

Homoionic Na+, Ca2+, Sr2+, Li+, Cu2+ and Zn2+ samples of the < 2 mum fraction of a cis-vacant montmorillonite from Linden (Bavaria) were steam treated at 200 degreesC (approximate to 1.5 MPa), 240 degreesC (approximate to 3.3 MPa) and 300 degreesC (approximate to 8.0 MPa) after dehydroxylation at temperatures up to 630 degreesC. Cation exchange capacity (CEC) measurements, determination of exchangeable cations and X-ray diffraction (XRD), supplemented by thermoanalytical investigations of the evolved water in a thermobalance linked to a mass spectrometer, infrared (IR) and electron spin resonance (ESR) spectroscopy were employed to obtain information about the state of expandability and structural changes of swellable montmorillonite and the sites of interlayer and octahedral cations after heating and rehydroxylation. The XRD pattern of the initial samples showed a well-defined (001) reflection according to the interlayer cation and its hydration state under laboratory atmosphere. After dehydroxylation the pattern exhibited (001) reflections between 9.6 and 9.8 Angstrom, corresponding to a collapsed structure for all samples. The Na+-, Ca2+- and Sr2+-rich montmorillonites regained partial expandability after rehydroxylation at 200 degreesC and full expandability after rehydroxylation at 300 degreesC if the dehydroxylation temperature was less than 630 degreesC. Rehydroxylation at 300 degreesC of the Cu2+- and Zn2+-rich montmorillonites did not cause reexpansion, whereas the Li+-rich samples recovered a partial swellability after rehydroxylation at 240 degreesC and nearly the full swellability after rehydroxylation at 300 degreesC. The Li+-, Cu2+-and Zn2+-rich samples underwent a strong CEC reduction due to migration of the interlayer cations into the 2:1 layer before dehydroxylation started. After rehydroxylation under water steam Cu2+- and Zn2+-rich samples released 16-30 meq/100 g of Mg2+ from the structure, increasing with the steam temperature. Mg2+ release was not observed for the Li+-rich montmorillonite. (C) 2001 Elsevier Science B.V. All rights reserved.