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Aggregate Structures of Sorbed Humic Substances Observed in Aqueous Solution

Humic substances are high molecular weight, heterogeneous organic materials which dominate the natural organic matter pool in many aquatic environments and often form coatings on mineral surfaces. Humic substances sorbed to mineral surfaces may bind and hence immobilize trace metals, radionuclides, and nonionic organic pollutants, and they may also alter clay-mineral flocculation kinetics. Determining the physical shapes and forms of sorbed humic substances is essential for development of realistic pollutant-binding models. To this end, we are using direct, in-situ (in 0.01 M CaCl 2 , ≤100 mg C L -1 solution, pH ∼5), nanometer-scale atomic force microscopy (AFM) to image the physical shapes and forms of humic substances sorbed to the basal-plane surface of mica. Under our experimental conditions, the sorbed molecules form ring-shaped aggregates with diameters on the scale of several tens of nanometers; smaller nanometer-scale rings present along the circumference could potentially represent hydrophobic domains. The highly porous three-dimensional copolymeric nature of the sorbed humic substances has important implications for reactivity at the soil particle-solution interface. Ongoing research focuses on determining the effects of changing solution conditions and sorbent properties on the shapes and forms of sorbed humic substances.