Dehydration of K-exchanged montmorillonite at elevated temperatures and pressures.

Abstract

The dehydration T of K-montmorillonite, obtained by ion exchange of a Na-montmorillonite, was determined to 2 kbar, using high-P DTA. Dehydration reactions were found at approx 50o and 100oC above the liquid-vapour curve of water. At P above the critical point of water, the dehydration T increased only slightly. The T of the first dehydration reaction is 10oC higher than for Na-montmorillonite, indicating a slightly greater stability of the hydration shell around the K interlayer cation. The second dehydration occurs at a slightly lower T. The data were used to determine the enthalpy of the dehydration DELTA H(dh) and the bonding enthalpy of the interlayer water DELTA H(iw) at 1 atm. The first dehydration reaction of the K-exchanged montmorillonite has a DELTA H(dh) = 46.16 + or - 0.06 kJ/mole and a DELTA H(iw) = 7.7 + or - 0.5 kJ/mole, whereas for the second reaction, DELTA H(dh) = 56.7 + or - 2 kJ/mole and DELTA H(iw) = 19.8 + or - 2 kJ/mole. These values compare with a DELTA H(dh) = 46.8 + or - 0.3 kJ/mole and a DELTA H(iw) = 7.8 + or - 0.5 kJ/mole for the first dehydration reaction of the Na-montmorillonite and a DELTA H(dh) = 62.9 + or - 2 kJ/mole and DELTA H(iw) = 27.1 + or - 2 kJ/mole for the second dehydration.-P.Br.