Flocculation of Reference Clays and Arid‐Zone Soil Clays

Abstract The effect of electrolyte concentration, exchangeable sodium percentage (ESP), sodium adsorption ratio (SAR), and pH on the flocculation‐dispersion behavior of reference kaolinite, reference montmorillonite, reference illite, and the clay fraction of three arid‐zone soils was investigated. The clay mineralogy of the soils was dominated by either montmorillonite, kaolinite, or illite. The clays were Na‐ or Ca‐saturated and freeze‐dried before use. Critical coagulation concentrations (CCCs) were investigated in the range of pH 5.5 to 9.5, ESP of (here equated with percent Na clay) 0, 5, 10, 20, 40, 60, 80, and 100, SAR of 0, 5, 10, 20, 40, 60, 80, and ∞, and using electrolyte concentrations of 0 to 500 mmol c L −1 . The CCC values increased with increasing ESP, increasing SAR, and increasing pH. The pH dependence of kaolinite and illite was greater than that of montmorillonite at all ESPs and SARs. The CCC behavior of the soil clays resembled that of illite, especially in its pH dependence at ESP 100 and SAR ∞. The CCC values of the soil clays, kaolinite, and illite showed a sharp increase at high ESP and SAR, whereas montmorillonite showed a sharp increase in CCC at low ESP and SAR. The CCCs for all soil clays were similar in magnitude, despite their differing clay mineralogies. The CCCs for the reference clays were much lower than those for the soil clays, indicating that extrapolation from reference‐clay results is not possible, and that additional factors such as organic‐matter content and Al‐ and Fe‐oxide content may influence the dispersion of soil clays.