RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The calculated values of stability constants of surface complexes formed by heavy metal ions depend on the assumed model of the electric double layer and its parameters. Using various parameters of the triple-layer model (TLM) (which fit the titration data almost equally well), one obtains stability constants in a range as wide as two orders of magnitude. On the other hand, the equilibrium constants of the surface reactions ≡Al–O–Na + Cd²⁺ = ≡Al–OCd⁺ + Na⁺ (log K = 7.4 at 15°C and log K = 7.1 at 35°C) and ≡Al–O–Na + Cd²⁺ + Cl⁻ + H⁺ = ≡Al–OHCdCl⁺ + Na⁺ (log K = 15.7 at 15°C and 14.9 at 35°C) are not dependent on the TLM parameters. The surface complexes formed in these two reactions are used in a model which is able to explain cadmium adsorption from NaCl and NaClO₄ solutions up to a concentration of 1 mol dm⁻³. Only one surface complex, ≡Si–OCd⁺, is used in a model which explains the adsorption of cadmium on silica from the same electrolytes, and the equilibrium constant of the surface reaction ≡Si–O–Na + Cd²⁺ = ≡Si–OCd⁺ + Na⁺ (log K = −0.9 at 15°C and −1.0 at 35°C) is not dependent on the TLM parameters used in its calculation. The adsorption of cadmium at constant pH increases with temperature, but adsorption at a constant surface charge density is independent of temperature for both oxides.