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Soil Physics and Soil Chemistry

Percolation Treatment of Charge Transfer in Humidified Smectite Clays

^a Dep. of Physics and Dep. of Geology, Wright State Univ., Dayton, OH 45435
^b National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011

^* Corresponding author (allen.hunt{at}wright.edu)

Bulk electrical conductivity of soil is generally assumed to be dominated by the electrical conductivity of the soil solution, with perhaps a small contribution from surface charges associated with soil solids. Soils high in smectites often exhibit high electrical conductivity due to water associated with the clays. These charges are either associated with the exchangeable cations or with proton migration in the clay-associated water. The purpose of this study was to use percolation principles to elucidate mechanisms of charge transfer in humidified smectites. The theory predictions were compared with electrical conductivity spectra measured for mono-ionic humidified smectites. The measured spectra were consistent with a percolation model of proton hopping in the maximum interlayer spacing beyond a threshold water content, plus an offset term for some samples. The threshold water content for percolation in the maximum interlayer spacing was 0.07 m³ m^–3, which was assumed the same for all basal spacings. The spectra should converge at the phonon frequency (v_ph), but for some samples at the higher humidity levels, the spectra were offset. The samples with this offset were determined by examining the spectra for the same mono-ionic smectite at different humidity levels. The mean offset level was 0.015 m³ m^–3 and was likely related to charge transfer on external surfaces. The phonon frequency fitted across all samples was 3.54 x 10⁸ Hz, lower than the 10¹² Hz often assumed. These results are generally consistent with the idea that water associated with smectite clays generated the observed electrical conductivity.

Abbreviations: 2D, two-dimensional • 3D, three-dimensional • ac, alter-nating current • dc, direct current