Charge adjustments upon adsorption of a weak polyelectrolyte to a mineral oxide: The hematite-humic acid system

The proton adsorption to a mixture of purified Aldrich humic acid (PAHA) an d hematite is investigated. Basic insight into the charge adjustment proces s is obtained by using a self-consistent-field lattice theory for polyelect rolyte adsorption. The calculations indicate that upon adsorption the compo nent with the highest initial charge density tends to induce charges on the other component. The number of induced charges can show a maximum when the surface charge and the charge of the segments in direct contact with the s urface roughly balance each other. Experimentally, the humic acid-hematite system is investigated by proton titrations. The alterations in charge dens ity caused by adsorption of PAHA to hematite are investigated by comparing the proton adsorption on the individual samples with that on their mixtures . Upon adsorption a part of the functional groups of humic acid forms compl exes with some of the surface sites of hematite. This interaction reduces t he proton binding to the humic acid at relatively low pH and it promotes th e proton adsorption on the oxide surface at relatively high pH. (C) 1999 Ac ademic Press.