Distribution coefficient K approach

The distribution coefficient approach - commonly referred to as the K approach - is the most widely applied method in environmental geochemistry for predicting the sorption of contaminant species onto sediments. The distribution coefficient Kd itself is simply the ratio under specific conditions of the sorbed to the dissolved mass of a contaminant. Sorbed and dissolved mass are expressed in units such as 

A K is by nature descriptive, but in the Kt approach the value is taken over the range of chemical conditions considered in a model to be constant and predictive. This assumption, of course, cannot hold in the general sense. A K value varies sharply with pH, contaminant concentration, ionic composition of the fluid, and so on its measurement is specific to the fluid and sediment tested. It is imperative that the modeler keep these points in mind. 

The limitations of the Kt approach stem in part from the fact that it makes no accounting of the number of sorbing sites on the sediment, treating them as if they are in excess supply. The approach allows a solute to sorb without limit, without being affected by the sorption of competing species. As well, the approach treats sorption as a simple process of attachment. It does not consider the possibility of hydrolysis at the interface between sediment and fluid, so it cannot account for the effects of pH. Nor does the approach consider electrostatic interactions between the surface and charged ions. 

For these reasons, the Kt approach works best for describing sorption of trace amounts of nonionized, hydrophobic organic molecules (Stumm and Morgan, 1996). The approach is broadly recognized to describe poorly the behavior of ionic species, especially metal ions, within soils and sediments (e.g., Reardon, 1981 Domenico and Schwartz, 1998). Nonetheless, it is commonly applied for just such purposes, often giving misleading results (Bethke and Brady, 2000). 

The /f( approach, as strictly defined, implies but does not specify a chemical reaction. A variant of the approach known as the reaction Kci model (or activity Kd model) based on a specific chemical reaction is commonly applied in reaction modeling. For example, the sorption and desorption of Cd++ might be taken to occur according to the reaction, 

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These results differ sharply from the behavior predicted by the distribution coefficient (K( ) approach. This approach, despite being broadly acknowledged as too simplistic to describe the behavior of heavy metals, is nonetheless the sorption model most commonly applied in studying aquifer remediation. 

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