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Chapter 4

Using the similar method, adsorption equilibrium constants and surface sites
density can be calculated from experiment data. Figs. 4.25 and 4.26 show
kaolinite zeta potential as the function of CaCI₂1 MgCI₂ concentration in 0.05 M
NaCI brine at pH 6.5. Dashed line shows zero zeta potential value as reference.

The equilibrium constant of Ca²⁺ ion Kc_a is 0.82 m³∕mol. The equilibrium
constant of Mg²⁺ ion K∣y∣g is 1.71 mɜ/mol. From zeta potential result, cations Ca²⁺
and Mg²⁺ can adsorb on negatively charged surface sites and make zeta potential
of kaolinite less negative.

4.3.5.4. Effect Ofsynthetic brine pH

Based on the site density and dissociation constant of kaolinite, zeta potential
can be calculated at different pH. Figure 4.27 shows zeta potential of kaolinite in
synthetic brine with or without Ca/ Mg ions adding NaOH at different pH.

In the brine with Ca/ Mg, based on the solubility product calculation, Ca²⁺ and
CO₃²^ ions may form CaCO₃ precipitation, which will reduce the concentration of
Ca²⁺. The blue dashed curve shows the calculated zeta potential of kaolinite
without Ca/ Mg. The red solid curve shows the calculated zeta potential of
kaolinite with Ca/ Mg, with equilibrium Ca²⁺ concentration calculated from CaCO₃
solubility product. Increase of pH can make kaolinite zeta potential more negative.
Adding NaOH, zeta potential change of kaolinite is around 15-20 mV∕mM.

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