Determining cation exchange capacities of standard montmorillonite clays using solution calorimetry

The enthalpy of hydration (DeltaHhyd) was determined for a series of montmorillonite clays using solution calorimetry. Each clay was modified with four different cations (sodium, potassium, magnesium, and calcium) resulting in 16 unique clay/cation samples. The DeltaHhyd determined depends on two main factors: the identity of the interlayer cation and the number of cations present, which is related to the cation exchange capacity (CEC). Clays used were from the Source Clays Repository and included SWy-2, STx-1, SAz-1, and SCa-3. The results were expected to be linear, as enthalpy is an extensive property, and were, with the exception of the cation/SCa-3 samples. Results indicated that the reported CEC value of 153 meq/100 g for SCa-3 is incorrect. A cation exchange capacity value of 105 meq/100 g was calculated from the results. If this value is used, all the DeltaHhyd for all cation/clay combinations are linear. Further experiments should be conducted to confirm this value. The results of this work indicate solution calorimetry can be utilized to determine the CEC of clay minerals.