A large proportion of trace compounds (typically 40% to 90%) are adsorbed to aquatic colloids via covalent, electrostatic, or hydrophobic interactions.1'2 Based

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upon size and density considerations, colloids will generally remain suspended in the water column and be transported over long distances. On the other hand, coagulation or flocculation can facilitate the elimination of colloids from the water column by producing aggregates that are large enough to sediment.3,4 In complex systems such as natural waters, colloidal aggregation is ubiquitous due to the large number of colloid types and reactive sites. Indeed, size fractionation analysis has often demonstrated that each colloid type may be found in all size fractions.5 This suggests that it is at least as important to understand interactions among the colloids as it is to determine the binding energies of trace compounds to each colloid type.6 While considerable research has focused on determinations of binding constants of trace compounds with colloids (e.g., ref. [1,2,7,8]), relatively few data are available on the precise structural properties of the colloids9 and their interactions in natural systems.

In this context, the exact role of natural organic matter (NOM) in freshwaters is yet to be resolved. Although it is often shown that NOM can stabilize inorganic colloids in natural waters,10,11 the opposite phenomenon, that is, destabilization, has also been shown to occur with specific groups of NOM, in particular, the polysaccharides.12-14 Therefore, in order to understand colloidal aggregation in freshwaters, it is necessary to take into account the specific behavior of each of the major groups of NOM by considering relevant physicochemical parameters such as molar mass, size, electric charge density, and conformational information such as the persistence length, radius of gyration, or fractal dimensions.15,16

This information is more difficult to obtain than the corresponding parameters that are necessary to model the behavior of inorganic colloids (mainly hydrodynamic radius and electric charge density17). It is therefore not surprising that no generalized predictive model of colloidal interactions exists that includes the different types of organic macromolecules that comprise the majority of NOM. The main objective of this chapter will be to improve our understanding of the role(s) of NOM on colloidal aggregation in freshwaters in order to facilitate the development of such a model.

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