Why Surface Modeling is Not Perfect

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). 

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