Elucidation of the ion binding mechanism in heterogeneous carbon-compositeadsorbents

A two-stage analysis of the mechanism of metal ion binding by a heterogeneo us adsorbent was developed. In the first stage, a continuous proton affinit y distribution was calculated from potentiometric titration data using the CONTIN method with a Langmuir kernel. Electrostatic effects were accounted for using a diffuse layer model. In the second stage, the parameters obtain ed from the continuous distribution function (i.e. the number of different types of surface sites, site densities and their protonation constants) wer e utilized in a discrete distribution to represent the adsorbent in surface complexation and double layer models using the GRFIT speciation code. This information on the surface groups was applied to metal ion potentiometric titration experiments to calculate the surface complexation equilibrium con stants of the metal ions and hence elucidate the mechanism of ion binding t o these sites. The proposed method was applied successfully to the adsorpti on of Sr and Cu ions on carbon-composite adsorbents, KAU-mod and SCN-mod. T he continuous distribution method (CONTIN) revealed three types of surface sites within these carbon-composite adsorbents with pK values ranging betwe en 3.5-4.1, 5.3-6.3 and 7.7-8.2. The analysis of the metal ion adsorption d ata using the GRFIT speciation code showed that only the first two surface sites were capable of forming surface complexes with the Sr ions, and that only the first site governed the adsorption of the Cu ions. (C) 2001 Elsevi er Science Ltd. All rights reserved.