Chemisorption and Precipitation Reactions

Authored by: Robert G. Ford

Handbook of Soil Sciences Properties and Processes

Print publication date:  November  2011
Online publication date:  November  2011

Print ISBN: 9781439803059
eBook ISBN: 9781439803066
Adobe ISBN:

10.1201/b11267-19

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Abstract

The partitioning of dissolved components to a solid surface in soils is governed by the competition between sorption and desorption reactions. Chemisorption (or chemical adsorption) of a dissolved component (adsorbate) to a solid surface (adsorbent) has been defined as “adsorption that results from chemical bond formation (strong interaction) between the adsorbent and the adsorbate in a monolayer on the surface” (IUPAC, 1997). These partitioning reactions may be influenced by electrostatic forces between the adsorbate and adsorbent, similar to those controlling ion-exchange reactions (discussed in this volume). It has also been observed that sorption of solution species may result in more than monolayer coverage at the surface of solid surfaces. In these cases, the adsorbate may form polynuclear surface complexes or surface precipitates (e.g., Scheidegger and Sparks, 1996). Surface precipitation can be considered as a special form of homogeneous precipitation reaction where a solid component in the soil participates in the overall reaction process (Chapter 7 in Stumm and Morgan, 1996). In the following discussion, surface precipitation includes all processes that result in incorporation of the sorbate into a solid structure that could not form in the absence of the adsorbent. This includes (1) formation of a dilute solid solution with a structure that is distinct from or shared with the original adsorbent, (2) epitaxial growth of a new solid phase sharing a structural relationship with the adsorbent, and (3) nucleation of a new solid phase at the solid–water interface. Background information and discussion of the literature (dating up to 1998) addressing chemisorption and precipitation reactions in soils were provided by McBride (1999). The following discussion provides an update on the current understanding of these reactions in soil systems.

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