Chapter 4
Both -AIO' and -SiO' sites can adsorb divalent cations. In the surface
adsorption of divalent cations, these two sites are not distinguishable. In Eq.
[4.13], -AIO' and -SiO' sites having the same equilibrium constant is assumed.
For positively charged sites, -AIOH2+ and -SiOH2+ sites having the same
equilibrium constant for adsorption of anions is also assumed.
Here the adsorption of HSiOs' ion is considered instead of SiO32' ion,
because the concentration of HSiOs' ion is much higher than SiO32' ion in the
brine. For the similar reason, H3SiO4' ion and CeH5O73' ion have the highest
concentration and are considered in the adsorption. Forweak electrolyte, such as
SiO32' ion, ion dissociation equilibrium needs to be considered.
H2SiO3 H+ + HSiO3 [4.14]
HSiO; f≡ H++ SiO32' [4.15]
a +a a +a 7
h hs,°3 = ∕ς1, h si°3 = κa2 [4.16]
flH2SiO3 flHSiO3
If more than one cation or anion adsorb on the surface, all the possible
reactions need to be considered. Table 4.5 shows equilibrium constants of
different species in bulk solution [24L [25].
The ionic strength of synthetic brine is 0.0478 M. For nonionic solute (e.g.
H2SiO3), the concentration is low enough to Considerthe activity coefficient as 1.0.
But for ions, the activity coefficients need to be calculated using extended
118
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