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Influence of Electrolyte Concentration and Exchangeable Cations on the Flocculation of Silver Hill Illite

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521

Garrison Sposito^*

Dep. of Plant and Soil Biology, Univ. of California, Berkeley, CA 94720

* Corresponding author.

ABSTRACT

The light transmission and optical density of suspensions of Silver Hill illite were investigated at pH 7 in mixed Na-K-Ca-Mg perchlorate solutions of variable composition, with total charge concentrations varying between 1 and 10 mol_c m^–3. The quantitative relationship between percentage light transmission (T), a measure of flocculation, and bivalent exchangeable cation charge fraction (E_blv) was found to be independent of whether the bivalent cation was Ca²⁺ or Mg²⁺. As the total perchlorate concentration was decreased, the graph of T vs. E_blv changed gradually from an upward-inclined line with nonzero y-intercept, to a sigmoid curve with zero y-intercept, to a "step function" with a definite threshold value of the bivalent charge fraction above which flocculation occurred. For total perchlorate concentrations below 5 mol_c m^–3, a sharp rise in suspension optical density, a measure of dispersion, was observed as the charge fraction of exchangeable Na was increased from 0.05 to 0.20. For perchlorate concentrations above 5 mol_c m^–3, only a gradual increase in turbidity with exchangeable Na charge fraction was found. These results indicate that the relation between flocculation/dispersion and exchangeable cations for illite depends sensitively on the total electrolyte concentration, but not on the type of bivalent exchangeable cation.

Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside.

Received for publication June 20, 1988.