MODELING SURFACE-ACIDITY OF 2 POWDERED ACTIVATED CARBONS - COMPARISON OF DIPROTIC AND MONOPROTIC SURFACE REPRESENTATIONS

Powdered activated carbon (PAC) can behave as a weak acid in solution. Historically, the amphoteric nature of hydrous solids has been modeled as a single weak diprotic acid. To determine if the single diprotic representation was the most appropriate description of the PAC surface, two commercially available PACs underwent acidimetric-alkalimetric and NaNO3 titrations. The surface complex formation (SCF) model, a variation of a soluble equilibrium chemical model, was used to simulate experimental titration curves using several surface representations. A three-monoprotic site model was chosen to represent the amphoteric nature of both PACs studied, based on the following criteria: the goodness of fit of model results; the coherence of the representations to previously published results; and the agreement of surface site representations at different ionic strengths for the same PAC. Surface acidity parameters (pH(zpc), acidity coefficients and number of surface sites) and surface speciation diagrams are presented.