Sorption and ion exchange

An important consideration in constructing certain types of geochemical models, especially those applied to environmental problems, is to account for the sorption of aqueous species onto sediment surfaces (e.g., Zhu and Anderson, 2002). Because of their large surface areas and high reactivities (e.g., Davis and Kent, 1990), many components of a sediment - especially clay minerals, zeolites, metal oxides and oxyhydroxides, and organic matter - can sorb considerable masses. 

Sorption can significantly diminish the mobility of certain 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 (see Chapters 21 and 32). In acid mine drainages, ferric oxide sorbs heavy metals from surface water, helping limit their downstream movement (see Chapter 31). A geochemical model useful in investigating such cases must provide an accurate assessment of the effects of surface reactions. 

In this chapter, we consider several simple models of ion sorption and exchange that can be applied within the context of a geochemical model. These models include distribution coefficients, Freundlich and Langmuir isotherms, and ion exchange theory. In the following chapter (Chapter 10), we consider surface com-plexation theory, which is more complicated but in some ways more robust than the models presented here. 

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Absorption is the assimilation of a chemical species into the interior of a solid substance. Absorption may include the migration of solutes into the internal pores of a solid material (Fetter, 1993), 117 or the migration or exchange of atoms within the crystalline structure of a mineral (Krauskopf and Bird, 1995), 150. Some researchers use the generic term sorption to refer to a treatment method where both adsorption and absorption may be involved or if adsorption and absorption cannot be distinguished. Sorption and ion exchange have many important roles in immobilizing arsenic in natural environments (Chapter 3). They... 

Since the catalytic, sorption, and ion-exchange properties are strongly dependent on the Al—Si ratio it is important to be able to vary this. The A1 content can be decreased, for example, by treatment with SiCU, which removes Al as A1C13. It can be increased in high silica zeolites by A1C13 vapor ... 

Venkataramani, B. and Gupta, A.R., Water sorption and ion exchange properties of hydrous oxides, Indian J. Chem., 27A, 290, 1988. 

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