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Electrophoretic Mobility of Illite and Micaceous Soil Clays

U.S. Salinity Lab., USDA-ARS, 4500 Glenwood Dr., Riverside, CA 92501

*Corresponding author.

ABSTRACT

Examination of the colloid chemistry of clays may provide an increased understanding of the structural stability of soil clays under field conditions. Electrophoretic mobility (EM) gives specific information about the net charge of a particle at the shear plane. An evaluation was made of the effect of the important water quality variables, electrolyte concentration, sodium adsorption ratio (SAR), and pH on the EM of Silver Hill illite and three micaceous soil clays. Increases in electrolyte concentrationg enerally increased the mobility of the colloidal particles, contrary to double-layer prediction. The EM behavior is thus not explained by the commonly used double-layer theory of infinite plates; however, results can be explained if this double-layer theory is extended to consider charge spillover into clay particle edges and changes in variable edge charge. The EM increased faster when the SAR increased from 5 to 20 (mmol L^–1)^0.5 than at higher values of this parameter. These results are in agreement with tactoid formation and demixing theory previously developed by others for smectites. The EM for micaceous soil clays and specimen Silver Hill illite was more sensitive to changes in pH as the electrolyte concentration increased. These results are consistent with observations on variable-charge minerals where variable charge increases with increasing ionic strength. Removal of organic matter increased the EM in the pH 4 to 10 range studied.

Received for publication April 17, 1991.