Modeling surface acidity of two powdered activated carbons: Comparison of diprotic and monoprotic surface representations

Abstract

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 NaNO 3 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.