FIGURE 7.5 Tapping mode atomic force microscopy (TM-AFM) images and disaggregation rates (rate of decrease of an equivalent spherical aggregate) of the U.K. Geological Survey peat humic acid (PHA). (A) TM-AFM of a 10 mg l-1 PHA solution adsorbed on mica at pH 3.2, 5 mM NaCl. Scan size is 2 jm x 2 jm. (B) TM-AFM of a 10 mgl-1 solution adsorbed on mica atpH 6.8,5 mMNaCl. (C) Disaggregation rates of the PHA as a function of pH. (C) shows that the aggregates observed in (A) were not at equilibrium (at this concentration, equilibrium corresponds to complete disaggregation, a process that will take months). (The figures were taken from [A, B] Balnois, E., et al. Environmental Science and Technology, 1999. 33(21): p. 3911-17; [C] Avena, M.J. and K.J. Wilkinson, Environmental Science and Technology, 2002. 36(23): p. 5100-05 with permission from the American Chemical Society.)

polysaccharides, including their gelation properties. Furthermore, most work in the field of gelation has examined polysaccharides used in encapsulation and physico-chemical techniques such as electrophoresis rather than freshwater polysaccharides. In addition to the intermacromolecular coordination bonds of metal ions, hydrogen bonds and hydrophobic interactions are of great importance in the homocoagulation process.

7.4.2 Heterocoagulation: Role of HS

As discussed in Section 7.4.1, HS diameters are typically 1 to 3 nm. Therefore, the interaction of the HS with colloids larger than 10 to 20 nm corresponds to their adsorption101,102 and results primarily in a modification of the surface properties of the colloid (surface potential and dielectric constant). Indeed, inorganic colloids in contact with HS tend to have a similar negative surface charge,11,62,85,103 irrespective of their intrinsic chemical nature. In the normal ranges of pH (5 to 8), ionic strength (1 to 50 mM) and HS concentrations (0.5 to 10 mg l-1) that are found in freshwaters, HS adsorption should stabilize colloidal suspensions,11,12,104,105 due to a significant electrostatic repulsion among the HS covered colloids. The net effect will depend on surface coverage of the mineral surface by the HS and the corresponding degree of charge neutralization. For model compounds, it has been shown that the adsorption of negatively charged polyelectrolytes on positively charged colloids may also result in destabilization but only for a limited range of surface coverages, that is, those very close to charge neutralization.75,106 Laboratory studies of both metal oxides and clays,11,12,105,107 suggest that strong adsorption of the HS occurs and that very low concentrations (<1 mgl-1) are sufficient to stabilize colloidal particles for colloid concentrations that are typical of natural waters (<10 mgl-1) (Figure 7.6). Where coagulation does occur, a low collision efficiency is expected to lead to compact aggregate structures, especially for low ionic strength waters.

Although steric repulsion of inorganic colloids by adsorbed HS might be thought to increase the stabilization effect compared to electrostatic repulsion alone, it is unlikely to be significant, given that the thickness of adsorbed HS molecules is small (few nanometers on mica; ref. [26,108], Figure 7.7). Indeed, these results26 show that it is unlikely that multiple layers of HS form on mica by adsorption from dilute solution (< 10 mg l-1) at circumneutral pH. Note that it might be possible to arrive at a different conclusion for colloid suspensions in marine systems where due to ionic strength effects (compression of the Debye layer), the Debye layer can be of a similar order of magnitude as the adsorbed HS layer.

7.4.3 Heterocoagulation: Roleof Rigid Polysaccharides and Peptidoglycans

In freshwaters, fibrillar material likely corresponding to rigid polysaccharides or pep-tidoglycans can be observed in the water column, especially in zones dominated by photosynthetic plankton (especially phytoplankton and cyanobacteria) (Figure 7.8). These long chain, semi-rigid biopolymers contrast with the organic material remaining in solution at depth which is primarily "processed" organic material. Furthermore, during periods of high productivity, inorganic colloids are depleted in surface waters at depths corresponding to the maximum production of polysaccharides.13 In laboratory experiments, flocculation has been shown to increase with increasing concentrations of added polysaccharide while showing the opposite tendency in the presence of humic substances. Finally, microscopic images of freshwater aggregates (e.g., ref. [84,109]) often show small inorganic colloids embedded into networks of fibrillar material.

For the heterocoagulation of compact colloids and fibrils, chemical reactivity is expected to depend on the electric charge, van der Waals forces, hydrogen and coordination bonds, as well as hydrophobic interactions of the two entities, as would

0 0

Post a comment