ADSORPTION OF HUMIC ACIDS TO MINERAL PARTICLES - 2 - POLYDISPERSITY EFFECTS WITH POLYELECTROLYTE ADSORPTION

The adsorption of humic acid to mineral particles can be characterized by specific and electrostatic interactions and by adsorption fraction ation. In this paper we focus on adsorption fractionation and the infl uence of the polydispersity (with respect to size) of humic acids on t he adsorption process and the shape of the isotherms at different pH v alues. In the previous paper the effects of electrostatic interactions were described. The adsorption from a mixture of two humic substances , purified Aldrich humic acid (PAHA) and Laurentian fulvic acid, on no nporous hematite is studied. It is shown that, in time, the large humi c acid molecules replace the readily adsorbed, small, fulvic acid mole cules. Preferential adsorption of large humic molecules over small one s also affects the shape of the adsorption isotherm of a single humic acid. After a high affinity start, the adsorbed amount of PAHA gradual ly increases with increasing humic acid concentration. This is mainly due to an increasing adsorption fractionation. For a certain concentra tion in solution the effect of the fractionation increases with increa sing adsorbed amounts. This explains why for PAHA the polydispersity e ffect is much stronger at pH 4 than at pH 9 where the adsorption is lo wer. To obtain a general impression of the effects of polydispersity o n the adsorption of weak polyelectrolytes on a variably charged surfac e, calculations are presented using a self-consistent-field theory. Th e calculated adsorption of a mixture of two acidic polyelectrolytes, t hat (mainly) differ in size, onto an oppositely charged hematite type surface confirm the trends found for humics: adsorption fractionation occurs, and the effect of the fractionation on the isotherm increases with decreasing pH. The calculations also show that the area-to-volume ratio affects the isotherm and results in an adsorption/desorption hy steresis.