Investigation of adsorbed humic substances using atomic force microscopy

Atomic force microscopy (AFM) was used to study the effect of adsorption of humic substances onto a goethite-coated mica surface, on the forces between the goethite surface and a silica colloidal probe. The positively charged goethite surface (at pH < 7) resulted in strong adsorption of humic substances. The adsorption process could be controlled by altering the solution concentration, pH and exposure time. Force versus distance curves were measured directly by AFM between a silica sphere colloidal probe and the planar goethite-coated mica surface, with and without the presence of humic substances. The thickness of the adsorbed layer was estimated to be about 5 nm from the force curves. These experiments demonstrate the influence of adsorbed humic substances on the surface charge of goethite, which has direct relevance to the colloid stability of natural aquatic particles.