The HSAB concept as a means to interpret the adsorption of metal ions onto activated carbons

While activated carbons (ACs) are extensively used for metal ions trapping in aqueous medium, the physico-chemical factors responsible of this phenomenon are not yet clearly understood, that is an important drawback for improving the adsorption properties of these materials. The main interpretations are related either to a cationic exchange with the acid surface groups or to an interaction with the pi-orbitals of the surface polyaromatic units. However, it remains very unclear why some ions interact mainly with one type of site and rather not the other, and why the interaction might depend whether the ions are coordinated or not. In this paper, we show that the hard and soft acids and bases (HSAB) concept allows quite a large number of published data dealing with ions adsorption on activated carbons to be interpreted. In this concept, the surface of the basal structural units of carbon is soft and can trap soft ions, whereas the oxygen surface groups are the hard sites that fix hard metal ions. On the other hand, using either special treatments for the activated carbon or the coordination of metal ions by various ligands allows the hardness of the two interacting species to be matched for a better control of adsorption. According to the HSAB concept, one is able to predict the potential sites of adsorption on the carbon surface as a function of the hardness of each ion. Changing the experimental conditions, metal ions with a borderline hardness can be adsorbed by the hard and/or the soft sites of carbon. (C) 2004 Elsevier B.V. All rights reserved.