Spectral induced polarization and Nuclear Magnetic Resonance responses of Ca2+-smectites and Na+-smectites

Montmorillonite clays are the most used material for engineered barriers and geological disposal because of their adsorption properties. However, their microstructural and interfacial properties are not completely elucidated. We designed a laboratory study using two geophysical methods - spectral induced polarization (SIP) and nuclear magnetic resonance (NMR) to investigate these properties. NMR offers the opportunity of directly assessing the water content and pore properties, while SIP provides useful information about the physical mechanisms between the solid/water surface of the clays. We examine several parameters that may influence the adsorption properties of Montmorillonite clays: (1) interlayer cation type (i.e., Ca²⁺-smectites v.s. Na⁺-smectites), (2) dry density (i.e., 1.2 g/cm³ and 1.4 g/cm³), and (3) saturated solutions salinities (i.e. deionized water, 0.1mol/l NaCl, 1 mol/l NaCl and 2 mol/l NaCl solution).The NMR T₂ results show that the adsorbed ability of Ca²⁺-smectites are higher than Na⁺-smectites except in deionized water environment. The water adsorption ability deceases as salinities increases. The SIP results indicated the weak impact of the interlayer cation type of smectite upon the quadrature conductivity of the clays. The real conductivity values of Na⁺-smectites samples are higher than that of Ca²⁺-smectites samples except in deionized water environment. At low salinity (i.e. deionized water) and high salinity (i.e., 2mol/l NaCl) environment, the quadrature conductivity is relatively independent of dry density in the 0.1Hz to ~500 Hz frequency range. For 0.1mol/L NaCl and 1mol/l NaCl salinities environments, the quadrature conductivity of different interlay cation clays increases as frequency increases. Further, it shows that the influence of dry density can be identified within 0.1 mol/l and 1mol/l NaCl solutions. A mechanistic model will be developed to further analyze these results. Our study provides insights into adsorption properties in two smectite sediments and aids broadly in understanding expansive clay surface-water interaction mechanisms.