Primary Colloidal Interactions In Freshwaters

In most oxic waters, Pareto law size distributions50 and microscopic imaging (e.g., ref. [14,83]) obtained in the nanometer to micrometer size range suggest that colloids are often found associated in aggregate structures84 rather than existing solely as isolated entities. Based upon the physicochemical nature of natural organic matter (Section 7.2) and the known mechanisms leading to stabilization or flocculation (Section 7.3), Section 7.4 summarizes our current understanding of the aggregation properties of the major aquatic colloids.

7.4.1 Homocoagulation

Colloids may aggregate among themselves (homoaggregation) or among other colloid types (heteroaggregation). In homoaggregation, the electric field is always repulsive, whereas it may be attractive or repulsive in heteroaggregation, depending on the nature of particles. An apparent contradiction in environmental colloidal sciences is that models invoking homoaggregation have been reasonably successful in predicting colloidal and aggregate size distributions in natural waters,5,50 despite the obvious large chemical heterogeneity of colloids in freshwaters. Such an observation is probably due in part to the fact that, in natural waters, most inorganic colloids are covered by an adsorbed layer of HS11,62,85 that can modify their surface charge, resulting in an "effective" single class of compounds with similar surface properties. Homoaggregation of HS covered inorganic colloids is usually a slow process due to a low collision efficiency resulting from a significant repulsive charge of the surface bound HS. This results in compact aggregates that do not exceed 1 micron even after a week50 and are unlikely to be responsible for major sedimentation fluxes.

Little is known about the aggregation of HS.86 More traditional opinion proposes that observations of large HS entities can be attributed to that fact that the HS are polymers that can assume a random coil conformation in solution.87 Another view suggests that large HS entities are, in fact, associations of relatively small molecules held together by weak interaction forces including hydrophobic forces and hydrogen bonds.88 Although the conceptual model for HS aggregation is still under discussion, it is clear that HS can form large macromolecular structures or aggregates in aqueous media at low pH26 (Figure 7.5), in the presence of multivalent ions,89 or in the solid state, such as in soils. Indeed, homoaggregates of HS have been observed by light scattering,90,91 turbidmetry,92 fluorescence spectroscopy,93 atomic force microscopy,26 and fluorescence correlation spectroscopy,27 even in the pH range 5 to 8 for which COOH groups of the HS are predominantly dissociated.24 Under these conditions, HS can be considered as charged globules and DLVO theory would appear to be at least partly applicable,94 although for such small, chemically heterogeneous colloids, hydration effects, hydrogen bonding, non polar interactions, polyvalent cation interactions,95 and reversible aggregation must also be considered. Indeed, in an attempt to explain the observed slow disaggregation kinetics of a peat humic acid (Figure 7.5C), Avena and Wilkinson96 postulated that at least two different disaggregation mechanisms were involved simultaneously. Although the disaggreg-ation rate increased significantly above the pH corresponding to the deprotonation of the carboxylic groups, another mechanism, likely involving the breaking of hydrogen bonds holding the HS aggregate together, was also identified. Note finally, that in contrast to what is generally assumed in the DLVO models, aggregation, though slow, was reversible.

In order to understand the formation of biological flocs and biofilms, it is extremely important to determine the mechanisms for the homoaggregation of other biopolymers than the HS, including polysaccharides, peptidoglycans, and proteins. In recent years, there has been a substantial increase of interest in the biomedical literature with respect to protein aggregation and its implications (reviews see ref. [97,98]). As mentioned above, protein aggregation in freshwaters is not thought to be important due to the high lability of the proteins in the water column. In biofilms, polysacchar-ides appear to play a predominant though not singular role.99 Most investigations of the physicochemical behavior of polysaccharides in solution have concentrated on their conformational helix-coil transitions and their interactions with metal ions. These contributions have demonstrated that transitions between a helical and random coil conformation are dependent upon temperature, pH, ionic strength, and calcium content.46,100 Less attention has been paid to the self association of the

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