Complex dielectric permittivity of clay minerals: impact of clay mineralogy, porosity, and ionic concentration

Permittivity can be one of the non-destructive methods to monitor many properties of soils. The objective of this study was to provide a comprehensive understanding of the permittivity of clay minerals through the measurement of permittivity and electrical conductivity as a function of water content, ionic concentration and clay mineralogy. It was found that swelling clay shows a wide variation in real and imaginary permittivity as a function of water content at low ionic concentration. In addition, the variation of permittivity was a strong function of swelling potential where Ca-montmorillonite showed a lower variation of permittivity than Na-montmorillonite. A linear relationship between the slope of real permittivity plotted against porosity and ionic concentration was found, which implies the need to incorporate ionic concentration when estimating porosity from the real permittivity of clay minerals. Overall, the findings and regression models shown in this study suggest that permittivity can be a non-destructive property to estimate the porosity and ionic concentration of clay minerals for monitoring contaminant transport through those clay minerals. The interaction between clay and water molecules at a wide range of water content and ionic concentration is also discussed.