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Modeling Surface Adsorption
programs being a factor of about 0.2 less than the results from the equation (see §3.4.1 for the activity–concentration relationship). The expected results for the inverse plot (Figure 7.3b) are intercept = b−1 = 1/0.01 = 100, and slope = K −1 · b−1 = (1/102.1 ) · 100 = 0.7943. The inverse plot gives the expected results for the equation results. An inverse plot of the computer program results gives a straight line, but with a different slope and intercept.
7.4.2 Why Surface Modeling is Not Perfect Dzombak and Morel (1990) performed a great service for surface modelers in systematizing a huge body of diverse literature on hydrous ferric oxide (HFO) research, and presenting it in a form which readily lent itself to use by programmers. In doing so, they made a number of decisions about HFO, such as the surface area, number of active sites, site densities, and so on (Table 7.7), as well as of the type of model and the data to use. All these choices find their way into the default values of these parameters of the programs written subsequently. There is nothing intrinsically wrong with these choices; they may well be the best possible overall values. However, they cannot be the best values for all possible applications. For one thing, hydrous ferric oxide, although a very important adsorber in natural systems, is an extremely complex substance, having a wide range of physical properties (see Cornell and Schwertmann (1996) for a recent summary). Therefore, it is never certain that the default properties of HFO or the experimental data on HFO chosen by Dzombak and Morel (1990) will be appropriate for given natural systems. Also, the amount of this phase present is often poorly known. It must also be remembered that HFO is only one of the many phases in natural samples, and although an important adsorber, it is not the only one. However, it is the only one in most models. Possibly, other phases, although weak adsorbers, could play important roles by being present in large amounts. Furthermore, most experimental work applies to simple laboratory systems, and a great deal more work will be needed to understand adsorption quantitatively in complex natural systems. Particularly lacking is a mechanistic understanding of the role of the solid phase, and any ability to predict surface complexation constants. A promising start in this direction is provided by Sverjensky (1993, 1994), and Sverjensky and Sahai (1996).
7.5 Retardation of Radionuclides at Oak Ridge Saunders and Toran (1995) used the diffusive double layer model of Dzombak and Morel (1990) to model the role of HFO in retarding radionuclide migration at the Oak Ridge Reservation, Tennessee. Large amounts of low-level radioactive wastes were disposed into shallow, unlined lagoons and trenches, resulting in concerns for their potential migration. Saunders and Toran (1995) used minteqa2 to perform speciation, solubility,