Adsorption of humic acid to mineral particles. 1. Specific and electrostatic interactions

The adsorption of humic acid to mineral particles can be characterized by specific and electrostatic interactions and by polydispersity effects. In this paper we focus on the adsorbed amount and discuss the importance of specific and electrostatic interactions. The adsorption of purified Aldrich humic acid onto hematite has been measured as a function of pH and salt concentration as well as in the presence of two cadmium concentrations. Besides the adsorbed amount, the thickness of the adsorbed layer has also been studied. The experimental results are discussed in relation to the polyelectrolyte behavior of the humic acid. At high pH and low salt concentration the adsorption is low and the humic acid molecules are adsorbed relatively flat on the surface. At low pH and high salt concentration a substantial fraction of the adsorbed humic acid is not in direct contact with the surface, which results in a relatively high adsorbed amount and a large layer thickness. PAHA adsorption at not too low pH leads to an overcompensation of the hematite charge and to the development of an electrostatic barrier against further adsorption. Substantial PAHA adsorption occurs at the point of zero charge of hematite; this shows that besides electrostatic interactions also specific interactions are important. Calculations based on the SCF theory for polyelectrolyte adsorption show that the trends with respect to the adsorbed amounts and layer thickness as a function of pH and salt concentration correspond well with the measured trends for PAHA. This suggest that at least partly the humic acid adsorption behavior is related to its polyelectrolyte character. Both experiments and theory indicate that a well-developed layer arises upon adsorption and that lateral interactions within this layer determine to a large extent the adsorption process.