Kd concept

From the scientific point of view, however, all approaches in the sense of the Kd concept (Henry, Freundlich or Langmuir isotherm) are unsatisfactory, since the complex processes on surfaces cannot be described by empirical fitting parameters. Boundary conditions like pH value, redox potential, ionic strength, competition reactions for binding sites are not considered. Thus results from laboratory and field experiments are not transferable to real systems. They are only advisable to provide a suitable prognosis model, if no changes concerning boundary conditions are to be expected and if no parameters for deterministic or mechanistic approach can be determined. 

As already explained in chapter 1.1.4.2.3, the Kd concept must be rejected in most cases, because of its oversimplification and its low suitability for application to natural systems. For example, in degradation only the degrading substance is considered. This concept might be applicable for radioactive decay, yet if the decomposition of organic matter is considered, it is crucial to consider decomposition products (metabolites) that form and play an important role in transport themselves. 

Actually, we know that all modeling of natural processes, such as groundwater flow modeling, is inaccurate to some degree. The question really is whether the model results are useful, and more useful than not modeling at all, or modeling with methods that are known to be less accurate than other methods (such as the almost ubiquitous use of the Kd concept in reactive flow modeling). 

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