Abstract
Mineral-bound humic substances modify the surfaces of clay minerals, changing the nature and number of adsorption sites for contaminants. Due to their effect on the surface charge of colloidal particles they can also change the particles mobility and thus their transport behavior. In this paper the influence of natural organic matter on the zetapotential of kaolinite and montmorillonite is shown. Adsorption experiments with kaolinite and montmorillonite show that due to adsorption the humic substances are fractionated. The equilibrium pH value does not influence the fractionation due to adsorption. Large hydrophobic molecules showed the strongest affinity towards the clay surfaces. Substances with a small apparent molecular size and a high content of carboxylic functional groups made up the non-adsorbing fraction of the NOM. Ca 2+ had no measurable effect on the adsorption. The low influence of the adsorbed NOM on the zetapotential of the clay minerals suggests physical adsorption as the predominant adsorption mechanism.
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