Abstract
In the present study we examine the dependence of the interaction of montmorillonite with a brown-coal humic acid on the pH and electrolyte concentration. The effects of these two factors were studied separately. The interaction was always measured by decrease of concentration of humic substances in the solution. The adsorption isotherms of humic acid on H-montmorillonite and of Na-humate on Na-montmorillonite were measured. In the electrolyte-free systems, the isotherms run almost together: a pH difference of about 5 units between the two isotherms did not play an essential role. However, the isotherms measured in the presence of neutral electrolyte were very different at low and at around neutral pH values. At low pH a small amount (10 and 20 mmol dm −3) of NaCl had an extremely large effect on the interaction. At around neutral pH the effect of NaCl was less; a significantly greater NaCl concentration (e.g., 500 mmol dm −3) was needed for the same change in the interaction. The effect of the pH on the interaction can only be evaluated if it is considered along with the effect of the neutral electrolytes. To interpret these effects, the surface charge, potential and the interaction energy of montmorillonite and humic substances were calculated on the basis of a simple model concerning two, mostly different, intrinsic dissociation constants. As a result of these calculations, it is most likely that joint coagulation, i.e., coaggregation, of the components takes place.
Published Version
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