Isotherm nonlinearity, composite sorption

Distributed Reactivity Model. Isotherm relationships observed for natural systems may well be expected to reflect composite sorption behavior resulting from a series of different local isotherms, including linear and nonlinear adsorption reactions. For example, an observed near-linear isotherm might result from a series of linear and near-linear local sorption isotherms on m different components of soft soil organic matter and p different mineral matter surfaces. The resulting series of sorption reactions, because they are nearly linear, can be approximated in terms of a bulk linear partition coefficient, KDr that is... 

Composite Sorption Magnitude and Isotherm Nonlinearity. Accurate assessment of the extent to which the global isotherm for a system is nonlinear is important for accurate portrayal of sorption processes in that system. From a practical point of view, the extrapolation of linear approximations of weakly nonlinear or near-linear sorption isotherms to concentration ranges beyond which they are valid can result in significant errors in projections of contaminant fate and transport (1). From a conceptual point of view, observations of isotherm nonlinearity over specific concentration ranges may be employed in conjunction with models such as the DRM to probe and evaluate the extent to which multiple sorption mechanisms are operative in a particular system. 

The experimental results given in Figure 7 provide confirmation of the importance of such small but highly reactive nonlinearly sorbing components to the overall sorption behavior of hydrophobic contaminants in subsurface systems. This figure presents a comparison of the carbon-normalized contribution of the nonlinear local TCB isotherm for the shale isolated from the Wagner soil to the composite sorption isotherm for the soil itself. 

When one or more of the component elements of sorption is governed by a nonlinear relationship between the solution and the sorbed phase, the composite isotherm deviates from linearity. In these cases, modifications to the Freundhch isotherm have been developed (e.g., Lambert 1967 Weber et al. 1992) to express these conditions. 

Modified Freundlich sorption models were developed to describe sorption in heterogeneous geosorbents containing both NOM and carbonaceous materials. Composite or distributed reactivity models (DRM) were introduced [28] and applied in a manner which combines the linear partitioning (absorption) part with the nonlinear adsorption part [26, 27, 31, 32]. The overall sorption isotherm can be described in the form of ... 

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