Ionic strength effects on cation sorption to oxides: Macroscopic observations and their significance in microscopic interpretation

Sorption of metal ions at oxide mineral-water interfaces is a complex process involving many possible variables, one of them being ionic strength. Depending on the system, adsorption of metal ions at these interfaces has been reported to increase or decrease with increasing ionic strength. However, in most cases it appears to be insensitive to ionic strength. Insensitivity to ionic strength has been taken as an indication for inner sphere surface complexation. Decrease of sorption with increasing ionic strength has been interpreted as outer sphere surface complexation. Promotive effects of ionic strength have not been discussed at all in terms of conventional surface complexation theory. The present paper shows that the classical example of Pa sorption to goethite, where increasing ionic strength decreases Pa adsorption, can be alternatively explained by an inner sphere mechanism if ion pair formation in solution between Pa and nitrate of the swamping electrolyte is taken into account. This suggests that macroscopic observations in cation uptake experiments should not be used to obtain mechanistic interpretations, even for ionic strength effects. Promotive effects of ionic strength on Cd sorption to goethite are also discussed. The present modeling exercises with the help of a conventional one-site surface complexation model suggest that promotive ionic strength effects may be explained in terms of more effective shielding of highly charged surface complexes at high ionic strength. (C) 1997 Academic Press.