Activated carbon adsorption of humic substances: II. Size exclusion and electrostatic interactions

Abstract

The activated carbon adsorption isotherms for a wide range of humic substance molecular size (MS) fractions display an inverse dependence on MS, when expressed on an adsorbent mass basis. However, normalizing the amount adsorbed on the basis of available adsorbent surface area accounts for the size exclusion behavior displayed by the MS fractions, resulting in convergence of their isotherms. The available surface area was calculated by relating the hydrodynamic size of the macromolecules to the adsorbent pore size. The effects of adsorbent charge and solution ionic strength conform to adsorption electrostatic principles when the isotherms are expressed on an available surface area basis. Adsorbents with progressively more positively charged surfaces adsorb more of the negatively charged humic substances. Increasing solution ionic strength suppresses adsorption by a positively charged adsorbent and enhances the adsorption by a negatively charged adsorbent. The characteristic, two-segment shape of the MS fraction isotherms is interpreted based upon changes in adsorbed macromolecule orientation.