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0 2 4 6 8 10 Schizophyllan / hematite ratio

FIGURE 7.9 Ratio of flocculation rates of a 10 mgl-1 colloidal hematite suspension as a function of the ratio between the total number of schizophyllan entities and the total number of hematite particles (O) pH 4.5, ionic strength = 1 mM and ) pH 3, ionic strength = 1 mM. The flocculation rate ratio is equal to the flocculation rate under consideration divided by the maximum rate of flocculation obtained using 0.1 M NaCl. (Figure is taken from Ferretti, R., et al. Journal of Colloid and Interface Science, 2003. 266: p. 328-38 with permission from Elsevier.)

0 2 4 6 8 10 Schizophyllan / hematite ratio

FIGURE 7.9 Ratio of flocculation rates of a 10 mgl-1 colloidal hematite suspension as a function of the ratio between the total number of schizophyllan entities and the total number of hematite particles (O) pH 4.5, ionic strength = 1 mM and ) pH 3, ionic strength = 1 mM. The flocculation rate ratio is equal to the flocculation rate under consideration divided by the maximum rate of flocculation obtained using 0.1 M NaCl. (Figure is taken from Ferretti, R., et al. Journal of Colloid and Interface Science, 2003. 266: p. 328-38 with permission from Elsevier.)

Although all of the biopolymers stabilized the colloidal suspensions, the HS and the large molar masses of dextran were the most effective in reducing the rate of aggregation of the colloidal particles. On the other hand, none of the biopolymers fragmented already formed aggregates (in contrast to the results observed in Figure 7.6 and ref. [119,120]). In another study, the adsorption of even extremely large molecules of a flexible polymer, polyacrylic acid (molar masses of 1 to 2 x 106 Da), did not result in surface thicknesses exceeding 13 nm121 discounting the role of a potential steric stabilization. In this case, the polyacrylic acid was clearly demonstrated to induce coagulation by charge neutralization, irrespective of its molar mass.75 Indeed, coil-like polymers will tend to collapse at the colloid surface due to a strong electrostatic attraction, especially for interactions between oppositely charged colloids and biopolymers. Furthermore, most flexible biopolymers in aquatic systems have molar masses which are too small to produce significant layer thicknesses. Nonetheless, the role of large extended biopolymers such as alginic acid have been largely unstudied (with the possible exception of their role in biofilms, e.g., ref. [122]).

Finally, when evaluating the role of proteins, polypeptides, reserve polysacchar-ides, etc., it is important to compare the time scales for the decomposition of the macromolecules with that of their adsorption and coagulation. As discussed in the previous section, most measurements of proteins and reserve polysaccharides in natural freshwaters indicate that they constitute a minor component with a rapid turnover.19 As observed for the HS, it is likely that the adsorption of certain recalcitrant, flexible biomacromolecules onto comparatively large inorganic surfaces will result primarily in a modification of the particle surface charge.

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