The role of variable surface charge and surface complexation in the adsorption of humic acid on magnetite

The ionic strength dependence of humic acid (HA) adsorption on magnetite (Fe3O4) was investigated at pH similar to5, similar to8 and similar to9, where variable charged magnetite is positive, neutral and negative, respectively. The adsorption studies revealed that HA has high affinity to magnetite surface especially at lower pH, where interacting partners have opposite charges. However, in spite of electrostatic repulsion at pH similar to9 notable amounts of humate are adsorbed. Increasing ionic strength enhances HA adsorption at each pH due to charge screening. The dominant interaction is probably a ligand-exchange reaction, nevertheless the Coulombic contribution to the organic matter accumulation on oxide surface is also significant under acidic condition. The results from size exclusion chromatography demonstrate that the smaller size HA fractions enriched with functional groups are adsorbed preferentially on the surface of magnetite at pH similar to8 in dilute NaCl solution. (C) 2003 Elsevier B.V. All rights reserved.