A REVISED PHYSICAL THEORY FOR ADSORPTION OF METAL-COMPLEXES AT OXIDE SURFACES

The electric double-layer model of physical adsorption originally prop osed by James and Healy in the early seventies (1-3) has largely been abandoned in favor of more complex triple-layer chemical adsorption mo dels. Two refinements have been made to the original simpler double-la yer model and results for the simulation of metal ion adsorption over silica, iron(III) oxide, chromium(III) oxide, and alumina are presente d here. With a more accurate (non-Nernstian) description of surface po tential (25, 26), and the more accurate solvation free energy term of Levine (8), good fits to the data are obtained with smaller or no adju stable ''chemical'' interaction terms. The interpretation of the revis ed model is directly contrary to the original double-layer model in th at multivalent, unhydrolyzed ions are now predicted to adsorb preferen tially to univalent hydrolysis products. This interpretation coincides with the results of more recent triple-layer models, indicating prefe rential adsorption of multivalent complexes. However, the revised phys ical adsorption model suggests that these adsorption phenomenon are us ually physical (electrostatic) in nature and not chemical. (C) 1997 Ac ademic Press