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United States Department of Agriculture

Agricultural Research Service

Title: Application of a General Energy Model for Limited Systems to Adsorption

Authors
item Olness, Alan
item Clapp, Charles
item Hayes, Mhb - UNIVERSITY OF LIMERICK

Submitted to: International Humic Substances Society Conference
Publication Type: Proceedings
Publication Acceptance Date: October 23, 2000
Publication Date: N/A

Interpretive Summary: Adsorption of substances to surfaces controls their solubility. Adsorption of pesticides on soil organic matter changes their effectiveness. Most adsorption models were developed for rigid or fixed surfaces. A new model, called a general energy model for limited systems (GEMLS), describes adsorption on flexible surfaces such as soil organic matter. The GEMLS combines two theories to describe limited systems. Use of this model with published data shows that large molecules may change shape during adsorption of small molecules. Rather than a single surface, a change of shape may expose one or more additional surfaces. Thus, a change in shape increases the surface area and results in even greater adsorption. Comparison of several models shows that the GEMLS gives a better description of adsorption. Research scientists and energy modelers will benefit most from this information.

Technical Abstract: Adsorption of a solute or gas on a surface has been described in models by Fruendlich, Langmuir, Brunauer-Emmett-Teller, Frenkel-Halsey-Hill and Harkins-Jura. Many of these models have been developed to describe adsorption of small molecules on a relatively rigid and stable surface. Flexible surfaces such as macromolecules like humic substances may expose additional adsorption sites with increasing concentration of adsorbate. I other cases, molecular rearrangement of the adsorbate molecule may occur. Such adsorption isotherms appear as somewhat irregular or 'stair step' curves when plotted as a function of concentration, often presenting increasing rates of adsorption with increases in the equilibrium adsorbate concentration. Increasing rates of adsorption with increasing equilibrium concentration are poorly described by existing adsorption models. By combining the Arrhenius and Mitscherlich rate model concepts and integrating over time, a general energy model for limited systems (GEMLS) is described that easily accommodates adsorption on flexible surfaces. Unlike other models, GEMLS is capable of describing increases in adsorption which occur at an increasing rate. By combining GEMLS functions, estimates of the change in surface area due to flexible rearrangement are obtained. With this model, adsorption is viewed as an exchange reaction between an adsorbed substance or site occupant and the adsorbate of interest.

Last Modified: 4/20/2014