A revised physical theory for adsorption of metal complexes at oxide surfaces

The electric double-layer model of physical adsorption originally proposed by James and Healy in the early seventies (1-3) has largely been abandoned in favor of more complex triple-layer chemical adsorption models. Two refinements have been made to the original simpler double-layer model and results for the simulation of metal ion adsorption over silica, iron(III) oxide, chromium(III) oxide, and alumina are presented here. With a more accurate (non-Nernstian) description of surface potential (25, 26), and the more accurate solvation free energy term of Levine (8), good fits to the data are obtained with smaller or no adjustable ''chemical'' interaction terms. The interpretation of the revised model is directly contrary to the original double-layer model in that multivalent, unhydrolyzed ions are now predicted to adsorb preferentially to univalent hydrolysis products. This interpretation coincides with the results of more recent triple-layer models, indicating preferential adsorption of multivalent complexes. However, the revised physical adsorption model suggests that these adsorption phenomenon are usually physical (electrostatic) in nature and not chemical. (C) 1997 Academic Press