An important consideration in constructing certain types of geochemical models, especially those applied to environmental problems, is to account for the sorption of ions from solution onto mineral surfaces. Metal oxides and aluminosilicate minerals, as well as other phases, can sorb electrolytes strongly because of their high reactivities and large surface areas (e.g., Davis and Kent, 1990). When a fluid comes in contact with minerals such as iron or aluminum oxides and zeolites, sorption may significantly diminish the mobility of dissolved components in solution, especially those present in minor amounts. Sorption, for example, may retard the spread of radionuclides near a radioactive waste repository or the migration of contaminants away from a polluting landfill. In acid mine drainages, ferric oxide sorbs heavy metals from surface water, helping limit their downstream movement (see Chapter 23). A geochemical model useful in investigating such cases must provide an accurate assessment of the effects of surface reactions. Many of the sorption theories now in use are too simplistic to be incorporated into a geochemical model intended for general use. To be useful in modeling electrolyte sorption, a theory must account for the electrical charge on the mineral surface and provide for mass balance on the sorbing sites. In addition, an internally consistent and sufficiently broad database of sorption reactions must accompany the theory. The Freundlich and Langmuir theories, which use distribution coefficients Kd to set the ratios of sorbed to dissolved ions, are applied widely in groundwater studies (Domenico and Schwartz, 1990) and used with considerable success to describe sorption of uncharged organic molecules (Adamson, 1976). The models, however, do not account for the electrical state of the surface, which varies sharply with pH, ionic strength, and solution composition. Freundlich theory prescribes no concept of mass balance, so that a surface might be predicted to sorb from solution without limit. Both theories require that distribution coefficients be determined experimentally for individual fluid and rock compositions, and hence both theories lack generality. Ion exchange theory (Stumm and Morgan, 1981; Sposito, 1989) suffers from similar limitations. Surface complexation models, on the other hand, account explicitly for the electrical state of the sorbing surface (e.g., Adamson, 1976; Stumm, 1992).
Keywords: Aluminum, sorption onto, Constant capacitance model, Distribution coefficients, Faraday constant, Gas constant, Ion exchange theory, Newton's method, Protonation reactions, Reduced basis, Surface charge
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