Page 96 - Soil and water contamination, 2nd edition

Solid phase constituents 83

Figure 4.7 Some common functional groups of environmental organic chemicals. If not otherwise indicated, R
represents a carbon-centred substituent (modified from Schwarzenbach et al., 1993).

section discusses the relevance of these groups for the interaction between organic matter and
the liquid phase .

4.3.2 Interaction with the aqueous phase

A number of the characteristic groups of the polymer chains depicted in Figure 4.7 contain
OH, notably the hydroxyl (-OH) and carboxyl (-COOH) groups. These functional groups,
which are weakly to moderately acidic, are the most active. Most of them may dissociate
depending on the pH and the total ion concentration in the bulk solution. Thus, the surface
charge of organic matter constituents is negative and variable, very much as in the case of
the variable surface charge of sesquioxides and clay minerals . Consequently, cation exchange
may also occur and organic matter may contribute considerably to the total CEC of the
soil or sediment. The CEC of colloidal organic matter typically ranges between 100 and
300 meq/100 g (Miller and Gardiner, 2004).

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