The Structure And Properties Of Fibrils

Aggregates in natural waters are composed of a disparate mixture of material: clay particles, fulvics, fecal material, phytoplankton, extracellular polysaccharides, etc.1 The essential ingredient of floc structure is a matrix composed mainly from structural polysaccharides and peptidoglycans derived from cell exudates.31,70,71 These molecules form nanoscale fibrillar structures, which can be identified in a variety of aquatic environments.8'14'31'33'72 These polysaccharide-rich fibrils form 30% of the organic material in freshwaters9'70 and up to 60% in marine systems.14'73 Fibrils are distinct from terrestrially derived humic substances which account for the largest fraction (40% to 80%) of organic material in freshwater systems70 and which typically behave as small nanoscale spherical particles.74-76

Early work on fibrils31 using transmission electron microscopy (TEM) showed that' in the presence of phytoplankton and bacteria' a large fraction of autochthonous organic material is composed of fibrillar particles rich in acid polysaccharides. These fibrillar particles have been shown to stimulate aggregation (see ref. [31] for a review) and to scavenge colloidal particles.10 These fibrils have been found linked with iron particles (Figure 9.3c'd) in both freshwater systems and batch reactors' leading to the suggestion that fibrils can act as nucleation centers during oxidation reactions.6

Properties of an aggregate' such as its settling speed' are dependent on its architecture. Aggregates typically possess a fractal structure.77-79 For example' Alldredge and Gotschalk'80 demonstrated that marine snow aggregates settle with a velocity' V' proportional to d°-26 rather than the Stokes relationship of d2' where d is the diameter (Figure 9.4).

The relationship between mass (M) and size (L) of an aggregate is M = aLD' where a is a constant and D is the fractal dimension of the aggregate. Aggregates which preserve volume upon collision have D = 3; aggregates with D < 3 are more porous and have a density which decreases as aggregate size increases.80 Fractal dimensions have been measured for aggregates in aquatic systems; in marine systems' D ranges between 1.3 and 2.3.19'81-84 In lacustrine systems' fractal dimensions range between 1.19 and 1.69'81'85'86 and in engineered systems from 1.4 to about 2.0 (see ref. [87]' and references therein). In general' for loose flocs' fractal dimensions are in the 1.7 to 1.8 range' and for more compact aggregates' they are of the order of 2.3 to 2.5.88'89 After addition of small amounts (1 wt%) of cationic polymers' fractal dimensions of aggregates in dewatered sludges from a waste water treatment plant decreased from 2.2 to 1.75' amounting to a 2.5-fold decrease in density and a large increase in permeability.90

Both fractal dimension and aggregate composition affect sinking rate. Aggregates with lower fractal dimensions are more porous and settle at slower rates than those with higher values. Engel and Schartau91 have shown that aggregates with a greater proportion of TEP have lower sinking velocities and a less pronounced size-versus velocity relationship indicating that the amount of TEP affects the architecture of the aggregate, possibly decreasing its fractal dimension. It would therefore be important to investigate the role of TEP in determining aggregate architecture, through structural and modeling studies.

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