ADSORPTION OF CADMIUM ON ALUMINA AND SILICA - ANALYSIS OF THE VALUES OF STABILITY-CONSTANTS OF SURFACE COMPLEXES CALCULATED FOR DIFFERENT PARAMETERS OF TRIPLE-LAYER MODEL

The calculated values of stability constants of surface complexes form ed by heavy metal ions depend on the assumed model of the electric dou ble layer and its parameters. Using various parameters of the triple-l ayer model (TLM) (which fit the titration data almost equally well), o ne obtains stability constants in a range as wide as two orders of mag nitude. On the other hand, the equilibrium constants of the surface re actions =Al-O-Na + Cd2+==Al-OCd+ + Na+ (log K = 7.4 at 15 degrees C an d log K = 7.1 at 35 degrees C) and =Al-O-Na + Cd2+ + Cl- + H+==Al-OHCd Cl+ + Na+ (log K = 15.7 at 15 degrees C and 14.9 at 35 degrees C) are not dependent on the TLM parameters. The surface complexes formed in t hese two reactions are used in a model which is able to explain cadmiu m 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 wh ich explains the adsorption of cadmium on silica from the same electro lytes, and the equilibrium constant of the surface reaction =Si-O-Na Cd2+ ==Si-OCd+ + Na+ (log K = -0.9 at 15 degrees C and -1.0 at 35 deg rees 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.