Combining X-ray absorption and Induced Polarization Spectroscopies for in situ monitoring of Cation Exchange in clay materials

The Cation Exchange Capacity (CEC) of clay minerals has been extensively studied in wide applications / purposes, using various imaging techniques to highlight changes of the clay sheet chargeability. Among the clay minerals, swelling clays such as smectite or vermiculite are particularly interesting regarding their adsorption-desorption properties strongly related to their high CEC (80-150 to 100-150 meq/100g respectively). To better monitor and predict cation exchange processes, the CEC has been investigated by different methodological approaches, including X-ray absorption spectroscopy (XAS) and geoelectrical methods. The Spectral Induced Polarization (SIP) is particularly well designed to quantify CEC because its complex conductivity measurements (in phase and quadrature) characterizes the electrical conduction of charge carriers (liquid) and the polarization phenomenon resulting from the local accumulation of electrical charge carriers in the porous medium (mineral interface).

The aim of this work is to investigate in situ how K cations are incorporated within the interlayer of a Ca-montmorillonite by coupling XAS and SIP experimental methods. This novel approach brings multi-scale information at the atomic and clay-sheet levels, providing new insights on enhancing the understanding of CEC mechanisms in terms of time and space and our ability to monitor it with SIP. The experiment was carried out on LUCIA (Soleil synchrotron), at the low energy of potassium K-edge with a microbeam size (2.5 x 2.5 µm²).

We used a 1.7 mm³ cell filled with 0.1 g of Ca-Montmorillonite isolated in a 0.8 µm sieve to avoid flushing of the clay sample during the experiment. The cell was subjected to a solution flux of a few cc per minute with 4 different KCl salinities (0.01, 0.05, 0.1, 1 M of KCl).  In situ SIP spectra are compared with XAS to conjointly monitor the CEC exchange at different scales. Preliminary results are shown to test-proof the methods as a new in situ / in operando cross-scale methods for CEC spatio-temporal characterization. Overall,this work is the first step of a technological development project, merging the approaches of geophysicists, mineralogists and physicists to monitor in real time the cation exchange processes of a Ca-montmorillonite by K in swelling clay minerals.