took on the magnitude of 027 KJ/mol. The chemical interac-The electric double-layer model of physical adsorption originally tion term can be made so overwhelmingly large that adsorpproposed by James and Healy in the early seventies (1-3) has tion of cations over positively charged surfaces can be fitted largely been abandoned in favor of more complex triple-layer with the James and Healy model (4-7). chemical adsorption models. Two refinements have been made to Various formulations of chemical adsorption models, in the original simpler double-layer model and results for the simulaparticular ''triple layer models'' or TLMs (8-10), have tion of metal ion adsorption over silica, iron(III) oxide, chromilargely replaced the physical adsorption model. In these um(III) oxide, and alumina are presented here. With a more accumodels the ''repulsion'' is due to singly valent counterions rate (non-Nernstian) description of surface potential (25, 26), and
such as Cl 0 or Na / which bind to the protonated or deprotothe more accurate solvation free energy term of Levine (8), good fits to the data are obtained with smaller or no adjustable ''chemi-nated hydroxyl groups, so diminishing the potential experical'' interaction terms. The interpretation of the revised model is enced by other adsorbing ionic complexes. In the older directly contrary to the original double-layer model in that TLMs the univalent counterions experience a higher potenmultivalent, unhydrolyzed ions are now predicted to adsorb prefertial than do multivalent ionic complexes, which argues entially to univalent hydrolysis products. This interpretation coinagainst a predominantly electrostatic adsorption mechanism. cides with the results of more recent triple-layer models, indicating Similarly, in James and Healy's model univalent hydrolysis preferential adsorption of multivalent complexes. However, the products such as Co(OH) /1 and Fe(OH) /1 2 were postulated revised physical adsorption model suggests that these adsorption to adsorb preferentially to the multivalent metal cations Co /2 phenomenon are usually physical (electrostatic) in nature and not and Fe /3 (3), due to the domination of the solvation over chemical. แญง 1997 Academic Press
the coulombic free energy term. Newer surface complexation models (11-21), however, predict that multivalent ions can be adsorbed in preference to univalent or lower charged ions. These predictions are based on the best fit of the surface complexation model to the experimental data.