Thursday, 13 July 2006

Development of Equilibrium-Based Adsorption Modeling: Merging of Data with Theory.

Cristian P. Schulthess, University of Connecticut, 1376 Storrs Road, Storrs, CT 06269-4067

The objective of this presentation is to critically review the advantages and limitations of the current data collected for use in modern adsorption modeling of inorganic compounds on soil constituents. The applicability and rigor of our adsorption modeling theories strongly depend on the quality of the data collected. Our understanding of adsorption processes is a result of the merging of data collection with theory development. There are four basic experimental approaches used for monitoring adsorption phenomena, namely spectroscopy, kinetics, electrokinetics, and equilibrium adsorption studies. The data collected from these experimental approaches often result in the development of new theories, particularly following the development of new research tools. Concurrently, mechanistic adsorption modeling theories also help us focus on which kinds of data (or even on a new kind of data) to be collected. Each of the experimental components contribute to our overall understanding of adsorption processes, and each also has its limitations. Spectroscopy tells us the physical location, orientation, and coordination of the elements adsorbed onto the solid-liquid interface regions, but it lacks information of the processes involved in getting those elements there if it is not coordinated with kinetic experiments. It is easy to misinterpret spectroscopic data unless you also have a working model of the process involved. Development of a working model of the adsorption process involved will also rely strongly on all the other experimental tools (kinetics, electrokinetics, and adsorption studies), which are all blind or “black-box” tools. The development of a working model of the adsorption process is, in turn, also handicapped by the limited data. The stoichiometry of the elements involved in the adsorption reaction is often very poorly understood. The application of mass balance and charge balance over a diffuse double layer is so challenging that an array of different approaches are in use today. Occasionally, the use of equilibrium constants versus equilibrium coefficients is still unresolved. The actual physicochemical characteristics or purity of the interface involved in our adsorption studies are sometimes poorly validated. The propagation-of-error in any one of these aspects of the modeling process can result in very misleading conclusions.

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