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Chapter 4

M: ion species (cation or anion).

z,: valency of ion species.

Simplified Gouy-Stern-Grahame model established the relationship between
zeta potential and surface charge. To study the effects of pH and adsorption of
counter ions, site-binding model of oxide/ water interface is widely used ^[18]'^[23].
Surface charge of kaolinite can be explained by proton donor-acceptor reactions
occurring simultaneously on alumina or silica sites of kaolinite ^t2°^l, as expressed in
Eqns. [4.8] and [4.9].

-AlOH₂   -AlOH +H⁺, -AlOH -AlO +H⁺                    [_{4 8}]

-SiOH₂   -SiOH +H⁺, -SiOH ≠= -SiO + H⁺                       [_{4 9}]

To apply the site-binding model the following assumptions are made:

1 ) The surface of kaolinite has amphoteric silica and alumina sites, which are
pH-dependent. H⁺ and 0H^ will react with such surface sites. Permanent
negatively charged sites -B^ are inert sites, which are independent of pH.

2) The indifferent ions, such as Na⁺and CΓ, will not specifically adsorb in the
compact layer.

3) The specifically adsorbed ions, such as Ca²⁺, Mg²⁺ and HSiO3^, will adsorb
in the compact layer.

For counter ion adsorption, we consider kaolinite surface containing

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