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Electrophoretic Mobility of Illite and Micaceous Soil Clays

“Electrophoretic Mobility of Illite and Micaceous Soil Clays” – Soil Science Society of American Journal 56 (1992): 1106-1115.

I.Lebron and D.L. Suarez*

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 concentration generally 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 2 (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.

RECOMMENDATIONS regarding the dispersion hazard of a soil have typically included salt concentration and sodicity (Quirk and Schofield, 1955; Rhoades, 1982; Shainberg and Letey, 1984; Ayers and Westcot, 1985). Recent studies have indicated that such recommendations may not be sufficient; for example, Suarez et al. (1984) showed that high pH has an adverse effect on the hydraulic conductivity and dispersion of soils. Variable-charge components are considered important parameters in soil stability studies (Goldberg et al., 1988). More recently Pratt and Suarez (1989) showed that, for arid-land soils, a single stability line is not possible due to the high variability among soil types. Likewise it has been reported that, even for samples from the same soil type and mineralogy, large differences in CCC exist (Lebron and Suarez, 1989).