Surface adsorption of organic substances in aqueous solutions is a ubiquitous mechanism that affects many environmental processes, such as fate and transport of contaminants. In engineered systems, adsorption of organic molecules causes fouling and prompts biofilm formation. Fouling is a major impediment to the successful development of membrane technology to increase the supply of safe water. Control of surface adsorption in aquatic systems requires fundamental knowledge of the physicochemical interactions at water-solid interfaces. The objective of this study is to understand the mechanism of adsorption of humic substances and polysaccharides onto surfaces with well-defined surface composition. In particular, the substrate chemistry will be tailored to represent environmentally relevant surfaces. Surface adsorption/desorption will be investigated either by optical reflectometry or surface plasmon resonance under varying physicochemical conditions. The layer structure, hydration, and conformational properties will be analyzed by surface topography and using the quartz crystal microbalance (QCM). The atomic force microscope (AFM) will be used to image the lateral structure of the adsorbed layers, while the interaction forces involving the dissolved natural compounds and surfaces will be studied by direct force measurements. Using the latter technique, both the adhesion mechanism and the mechanical properties of the organic layer will be investigated using a colloidal probe and tip-surface geometry, respectively. This protocol will allow systematic investigation of the physicochemical determinants of film development and conformation. The experimental data will be analyzed in the light of extended DLVO models. The complex systems are expected to give rise to nonelectrostatic forces originating from acid/base and hydrogen bonding. This work will have implications on manifold environmental processes, including the control of fouling in engineered systems.
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