Ionic Strength Effects on Cation Sorption to Oxides: Macroscopic Observations and Their Significance in Microscopic Interpretation

✍ Scribed by Johannes Lützenkirchen

• Publisher: Elsevier Science
• Year: 1997
• Tongue: English
• Weight: 137 KB
• Volume: 195
• Category: Article
• ISSN: 0021-9797
• DOI: 10.1006/jcis.1997.5160

✦ Synopsis

Sorption of metal ions at oxide mineral-water interfaces is a surface functional group and the sorbed ion.

complex process involving many possible variables, one of them

The present procedure is to attribute the IS mechanism to being ionic strength. Depending on the system, adsorption of metal systems, where a change in the value of ionic strength (deions at these interfaces has been reported to increase or decrease noted I) does not visibly change the adsorption of a certain with increasing ionic strength. However, in most cases it appears ion on a certain oxide under otherwise equal conditions.

to be insensitive to ionic strength. Insensitivity to ionic strength has However, if an increase in I results in a decrease of ion been taken as an indication for inner sphere surface complexation. uptake, the mechanism is believed to be of the OS type.

Decrease of sorption with increasing ionic strength has been inter-

This latter case will be referred to as classical ionic strength preted as outer sphere surface complexation. Promotive effects of ionic strength have not been discussed at all in terms of conven-effect and is presently explained by competition of an ion tional surface complexation theory. The present paper shows that from the supporting electrolyte for the surface functional the classical example of Ba sorption to goethite, where increasing groups. In the case of metal-ion adsorption, the sorbing cationic strength decreases Ba adsorption, can be alternatively exion would compete with the cation of the background elecplained by an inner sphere mechanism if ion pair formation in trolyte for negatively charged surface groups, both forming solution between Ba and nitrate of the swamping electrolyte is OSSCs. Hence, ion pair (IP) formation is the current phetaken into account. This suggests that macroscopic observations nomenological interpretation attributed to OSSCs, even in in cation uptake experiments should not be used to obtain mechathose cases where the cation sorbs onto a positively charged nistic interpretations, even for ionic strength effects. Promotive surface (such as Ba sorption to positively charged goethite effects of ionic strength on Cd sorption to goethite are also dis-(1)). Depending on the parametrization of surface complexcussed. The present modeling exercises with the help of a convenation models, this is in principle feasible but requires relational one-site surface complexation model suggest that promotive ionic strength effects may be explained in terms of more effective tively high ion-pair formation constants.

shielding of highly charged surface complexes at high ionic

The investigation of ionic strength effects in ion adsorpstrength. ᭧ 1997 Academic Press tion is often combined with kinetic studies (e.g., 3, 4) using Key Words: adsorption; metal ions; ionic strength effects; goethe pressure jump technique (e.g., 5, 6). The observations thite; surface complex. made via conventional batch uptake experiments are used to draw or corroborate conclusions with respect to the mechanism inferred from the reaction kinetics. While these con-