RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

A surface complexation model is used to describe sorption of ytterbium and cesium on the silica surface. The constant capacitance model gives the description of the solid–solution interface chosen for this work. The first step in the modeling consists of extracting the surface acidity constants. The result is:

SOH ↔ SO⁻ + H⁺     log Ka₂ = −7.60 ± 0.05.

The second step consists of the extraction of surface complexation constants for both ytterbium and cesium. The sorption of the cations is represented as follows:

for the ytterbium sorption,

2H₂O + SOH + Yb³⁺ ↔ SOYb(OH)₂ + 3H⁺     log K = −16.2 ± 0.3

for the cesium sorption,

SOH + Cs⁺ ↔ SOHCs⁺     log K = 2.05 ± 0.05
SOH + Cs⁺ ↔ SOCs + H⁺     log K = −5.5 ± 0.1.

In the case of cesium, the sorption of sodium is competitive and has to be considered:

SOH + Na⁺ ↔ SOHNa⁺     log K = 2.2 ± 0.1.