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FIGURE 7.7 Variation of the height of adsorbed standard Suwannee River humic acid on mica between pH 3 and pH 11 for ionic strengths of 5 mM NaCl (■), 50 mM NaCl (A) or 5 mM CaCl2 (•). (Figure is taken from Balnois, E., et al. Environmental Science and Technology, 1999. 33(21): p. 3911-17 with permission from the American Chemical Society.)

Indeed, several recent studies of model systems have demonstrated that small concentrations of polymer can facilitate the bridging flocculation process (e.g., polystyrene latex and methylcellulose or polyvinylpyrrolidone,112 silica and polyacrylamide,113 alumina and modified polyacrylamide114,115). In these colloidal systems, flocculation is often first observed at lower concentrations followed by stabilization at significantly higher ones. For example, Solberg and Wagberg113 observed bridging flocculation of silica nanoparticles for an addition of 0.4 mgl-1 polyacryl-amide and dispersion due to electrosterical stabilization for concentrations exceeding 1.2 mgg-1 of the polymer. Nonetheless, only a few studies have examined the bridging flocculation process in the presence of microbial polysaccharides and environmental colloids. For example, Labille et al.116 recently showed that the anionic bacterial polysaccharide succinoglycan induced the destabilization of a polydisperse suspension of montmorillonite, presumably by bridging flocculation. Using dynamic light scattering, Ferretti et al.117 demonstrated bridging flocculation of hematite colloids by the neutral triple helical schizophyllan (Figure 7.9). In that case, environmentally realistic (low) polysaccharide concentrations, that is, <1 mgCl-1 were observed to induce bridging flocculation. At higher and lower polysaccharide concentrations, the hematite suspensions were stabilized.

7.4.4 Heterocoagulation: Contributions of Other Types of Organic Matter

To our knowledge, there is very little specific information on the interactions of the proteins or reserve polysaccharides with inorganic surfaces in natural waters. Some recalcitrant proteins have the potential to play a role similar to that of HS in fresh waters, since they are small (hydrodynamic diameters of 1 to 5 nm) polymers with

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