A new method for quantifying cation exchange capacity in clay minerals

In this paper, a new method is proposed to theoretically quantify cation exchange capacity (CEC) of pure clay minerals, by use of nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and nitrogen adsorption-desorption isotherm analysis. A theoretical model is introduced to estimate CEC, which is derived based on energy balance between chemical potential and electric potential energy. The change in chemical potential is a function of relative humidity and volume of hydration water. The change in electric potential energy is a function of the CEC, basal spacing, and surface area of clay minerals. XRD measurements were performed on clay minerals before and after exposure to 95% relative humidity. These measurements were used to estimate the basal spacing before and after hydration as well as the number of interlayers in each particle of clay minerals. NMR laboratory measurements were performed on the clay mineral samples before and after hydration to obtain the volume of hydration water. The nitrogen adsorption-desorption isotherms provided the surface area of clay minerals. The estimates of CEC from the new method were cross-validated against the directly-measured CEC, obtained from ammonium acetate method by use of inductively coupled plasma mass spectrometry (ICP-MS) measurements. The introduced method was successfully tested on six types of clay minerals (i.e., illite, kaolinite, chlorite, and three types of montmorillonite). The calculated CEC results from the new method were in agreement with those from the wet chemistry method, with less than 7 meq/100 g difference for all types of clay minerals. The experimental results on the performance of the new method demonstrated reliability of this method for quantification of CEC. The outcomes of this paper can be potentially applied to variety of formations with measurable concentrations of clay minerals such as shaly sands and organic-rich mudrocks for reliable assessment of CEC.