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 effe cts. In this paper we focus on the adsorbed amount and discuss the imp ortance of specific and electrostatic interactions. The adsorption of purified Aldrich humic acid onto hematite has been measured as a funct ion of pH and salt concentration as well as in the presence of two cad mium concentrations. Besides the adsorbed amount, the thickness of the adsorbed layer has also been studied. The experimental results are di scussed in relation to the polyelectrolyte behavior of the humic acid. At high pH and low salt concentration the adsorption is low and the h umic acid molecules are adsorbed relatively flat on the surface. At lo w pH and high salt concentration a substantial fraction of the adsorbe d 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 hema tite charge and to the development of an electrostatic barrier against further adsorption. Substantial PAHA adsorption occurs at the point o f zero charge of hematite; this shows that besides electrostatic inter actions also specific interactions are important. Calculations based o n the SCF theory for polyelectrolyte adsorption show that the trends w ith 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 adsorptio n and that lateral interactions within this layer determine to a large extent the adsorption process.