Summary And Future Perspectives

In natural freshwaters, the association of all classes of "natural organic matter" into a single, homogeneous reacting entity with a single set of properties is clearly an oversimplification. Thephysicochemical nature of the organic macromolecules, including their chemical nature, inherent chemical heterogeneity and polydispersity, will greatly influence their role in the flocculation process. The aggregation of inorganic freshwater colloids in the presence of natural organic matter can nonetheless be simplified as the result of at least two major but opposing effects: their stabilization by humic substances and their destabilization by large, rigid biopolymers. In the presence of polysaccharides, large, loose flocs are generally formed (Figure 7.10(A)) in marked contrast with the effect of the humic substances. In that case, the significant repulsive charge of the colloids leads either to colloidal stabilization or to the formation of small, compact aggregates (Figure 7.10(B)) due to a low collision efficiency between the particles. Under such a scenario, colloidal aggregates exceeding 1 ^m are likely to form slowly50 and are thus unlikely to be responsible for major sedimentation fluxes. Nonetheless, the simple explanation based upon each of the flocculation mechanisms in isolation is rarely encountered in natural freshwaters. Typically, the bridging role of the organic biopolymers becomes less and less significant and the application of DLVO/Smoluchowski theory becomes more and more valid for larger primary particles (>100 nm). Rigid biopolymers appear to dominate aggregate structures84 when the size of the primary particles making up the aggregates are small (<100 nm), even though, under optimal conditions, aggregates formed by flocculation will be much larger that those obtained by the coagulation of compact colloids. For the larger colloidal aggregates, stability and aggregate structure will also depend on the shear

FIGURE 7.10 (A) Inorganic colloids associated with fibrillar organic macromolecules. Scale bar corresponds to 1 ^m. (B) Compact heteroaggregate from Lake Bret, Switzerland. The picture shows a spherical silica particle (gray at center) aggregated with smaller iron hydroxide particles (black spheroids), a clay particle and some biological debris. Scale bar corresponds to 250 nm. For experimental detail on transmission electron microscopy (TEM) preparation, see ref. [123,124]; for further discussion, see ref. [6]. (B was taken from Buffle, J., et al. Environmental Science and Technology, 1998. 32(19): p. 2887-99 with permission from the American Chemical Society.)

FIGURE 7.10 (A) Inorganic colloids associated with fibrillar organic macromolecules. Scale bar corresponds to 1 ^m. (B) Compact heteroaggregate from Lake Bret, Switzerland. The picture shows a spherical silica particle (gray at center) aggregated with smaller iron hydroxide particles (black spheroids), a clay particle and some biological debris. Scale bar corresponds to 250 nm. For experimental detail on transmission electron microscopy (TEM) preparation, see ref. [123,124]; for further discussion, see ref. [6]. (B was taken from Buffle, J., et al. Environmental Science and Technology, 1998. 32(19): p. 2887-99 with permission from the American Chemical Society.)

forces in solution, the tensile strength of the bridging biopolymers, and the weight of the particles, all factors that may influence fragmentation and collapse. In any case, environmental systems generally contain mixed heteroaggregates that are likely the result of both a charge modification by the small molecules and bridging flocculation by the large biopolymers, the net result being a network of rigid biopolymers that are interconnected by isolated or aggregated compact colloids.

Unfortunately, only limited data are available that examine the effects of organic biopolymer mixtures (especially HS and polysaccharides) on the flocculation and stabilization processes (e.g., ref. [125-127]). For example, in one recent study on a model system, low concentrations of polyvinyl alcohol (PVA) were shown to induce the flocculation of amidine polystyrene latex particles via polymer bridging;126 an effect that was prevented in the presence of alow concentration of HS (2.5 mg l-1).126 On the other hand, colloidal montmorillonite suspensions (3 mgl-1) were shown to aggregate in the presence of 2 and 4 mgCl-1 of algal exopolymers isolated from a small eutrophic lake, even in the (stabilizing) presence of 0.1 mg C l-1 of a soil fulvic acid. Future research into the three-colloidal component system (compact inorganic colloids; large rigid biopolymers; humic substances) is thus clearly merited.6

Much more detailed information is especially needed on the interactions of the natural rigid biopolymers with freshwater colloids and the modifying role of the HS. Since both natural colloids and biopolymers are physically and chemically heterogeneous, neighboring zones of a same particle surface may have different charges or charge densities. Aggregation might then occur due to interactions of surface patches61,64 which do not represent the average characteristics of the surface. Conclusions drawn from studies using model monodisperse, chemically homogeneous polymers and colloids must therefore be applied with great caution to predictions of the role of natural organic matter in "real" environmental systems.

In addition to developing a better understanding of heterocoagulation in fresh-waters, future research should also examine the nature of the chemical interactions and kinetics of biopolymer associations (HS, polysaccharides, proteins). Indeed, the formation mechanisms of biofilms and humic aggregates and the kinetics of the two processes have been largely ignored in the literature. It is hoped that future research might provide a detailed and quantitative insight into the principle mechanisms regulating the role of each of the main classes of organic macromolecules and their mixtures.

Although it is obvious that measurements of total organic carbon are poor indicators of the role(s) of natural organic matter in a given aquatic system, separate determinations of HS, polysaccharides, peptidoglycans, proteins, etc. are analytically very difficult to perform (e.g., ref. [28,128,129]). Physicochemical separation techniques such as chromatography, filtration, field flow fractionation, etc. have been proposed but are largely inadequate for a complete separation of the principal structures. Studies using enriched fractions of the important components are useful, albeit oversimplified surrogates of natural freshwaters. It is likely that increased resolution of the spectroscopic techniques, including nuclear magnetic resonance spectrometry130,131 may one day be able to provide a quantitative distinction of the important components of freshwaters, especially at environmentally relevant (i.e., low) concentrations.

0 0

Post a comment