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FIG. 9.12.2 Adsorption of metal ions on amorphous silica as a function of pH. (Reprinted from U.S. Environmental Protection Agency, 1989, Transport and fate of contaminants in the subsurface, Seminar Publication EPA/625/4-89/019, Cincinnati: U.S. EPA.)

ions by exchange processes can aggravate a contamination problem. For example, increases in water hardness resulting from the displacement of calcium and magnesium ions from geological materials by sodium or potassium in landfill leachate have been documented (Hughes, Candon, and Farvolden 1971).

The cation exchange is reversible, and its extent can be described by the adsorption or distribution coefficient (Dragun 1988b) as

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