Growth And Stability Of Freshwater Flocs

This section deals only with those flocs undergoing flocculation within the water column as these are the "true" flocs. Growth of a floc may occur (a) through continued aggregation in the bulk water (i.e., through collision processes), (b) through invasion by biota, and (c) by the intra-floc generation of particles by microbes.

Increasingly, the roles of microorganisms and their secretions are coming under intensive scrutiny, in efforts to understand floc growth and potential manipulation of growth and behavior.3,14-16,18,20,21,50 Mechanisms of floc formation, the interfacial forces involved, and the effects of physiological factors have been reviewed recently by Liss,21 who emphasizes the importance of surface properties in floc interactions. Important interfacial forces considered by Liss21 are: van der Waals; electrostatic double-layer; hydrophobic/hydrophilic; and steric.

There is increasing interest in the role(s) played by transparent exopolymer particles (TEP) in freshwater ecosystems; in essence, they "glue" small flocs together to yield large flocs. TEP, initially described by Alldredge et al.51 in a marine ecosystem, are loosely defined as (abundant) suspended particles formed from the polysaccharides secreted by bacteria and phytoplankton; individual particles are a sticky mixed material which promotes aggregation, and are difficult to detect in water with simple lenses. Grossart et al.,18 working at a site in Lake Constance (Germany) suggest that TEP may be of major importance for the formation of flocs in lakes, as they are known to be in marine waters where TEP has EPS fibrils as a major component.52 Relative abundances of freshwater and marine flocs in different ecosystems, in relation to TEP as an aquatic adhesive, are shown in Simon et al.22

The aggregation of nanoscale particles in rivers (Rhine River, western Europe) and lakes (Lake Bret, Switzerland) has been investigated for almost two decades,29 with the nanoscale formally referring to particle diameters of 1 to 100 nm.53 Regarding nanoscale aggregation in bulk water, a generalized description of aquatic colloidal interactions has been published by Buffle et al.54 for major classes of colloids. They concluded that aggregation is dominated by three classes of colloids: these are (a) compact inorganic colloids, (b) large rigid biopolymers, and (c) fulvic compounds, with fulvic compounds acting to stabilize the inorganic colloids and rigid biopolymers acting to destabilize them. Buffle et al.54 state that the concentration of stable colloids in a given aquatic ecosystem will depend on the relative proportions of the three general classes of colloids. Factors controlling the stability of colloids in natural waters, with a focus on nanoscale particles, are assessed in Filella and

Buffle.55

In aggregation studies, the term "stability" is not used in the thermodynamic sense; a stable aggregate is one which is slow in changing its state of dispersion during an observation period. The common modes of destabilization and their characteristics have been known and documented for decades.26,56 Current studies of floc stability and integrity focus on the effects of various natural organic substances,57 on interparticle interactions,20 on the composition of the extracellular polymeric substances,21,58 on microbial associations,58 and on freshwater physical processes.5,6,41,50 Recent research on engineered systems, applicable to freshwater flocs, indicates that floc/floc interactions may be dominated by the specific nature of the EPS present at the floc/water interface, as opposed to the overall composition (core plus peripheral layer) of the EPS in a floc.20

While particles are diverse, the colloidal particles are believed to dominate the aggregation process. A colloid is defined operationally as any particle with a least dimension in the range of 1.0 to 0.001 ¡m,26 a range which includes macromolecules.29 The significance of this size range of ultradivided matter is that the individual suspended particles can be adhesive in natural waters. Such colloidal suspensions differ fundamentally from true solutions; colloidal particles are unstable because of large interfacial energies, and the particle-particle interactions are stronger than kT.

There is a growing and valuable school of thought59 which seeks to redefine an aquatic colloid, for environmental science purposes, as any particle that provides a molecular milieu into and onto which chemicals can escape from aqueous solution, and whose movement is not significantly affected by gravitational settling. In this context, it has long been known that aquatic colloids can carry significant burdens of contaminants and nutrients; in this burdened condition, they can aggregate to form subsequent settling particles which are capable of burying the associated contaminants and nutrients.60 Thus, a capture of contaminants by suspended colloids, prior to and during the incorporation of those colloids into sedimenting flocs, can be a linked process leading to water column decontamination. Honeyman and Santschi61 take this concept one step further by providing evidence that colloid-associated elements are likely to have a behavior markedly different from the dissolved version and versions associated with suspended "true" particles. Their studies of "colloidal pumping" suggest that sorption of chemicals onto colloids may be coupled directly to the role of flocculation in regulating the fate of trace metals in aquatic ecosystems.

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