Aggregation and Flocculation

Creation of the correct shear conditions. High shear rate may be harmful to the organism and disrupt the cell wall low shear may also be undesirable because of unwanted flocculation and aggregation of the cells, or even growth of bacteria on the reactor wall and stirrer. 

Figure 1.7 Modes of platelet associations in clay suspensions, (a) dispersed, (b) FF aggregated, (c) EF flocculated and dispersed, (d) EE flocculated and dispersed, (e) EF flocculated and aggregated, (f) EE flocculated and aggregated, (g) EF and EE flocculated and aggregated. From Schramm [55]. Copyright 1980, Laurier L. Schramm.

Basic concepts of colloid chemistry that are relevant to soils are discussed in Chap. 10 of G. Sposito, The Chemistry of Soils, Oxford University Press, New York, 1989, and in the first three chapters of R. J. Hunter, Foundations of Colloid Science, Vol. I, Clarendon Press, Oxford, 1987. The definitions of flocculation, floccule, and aggregate given in the present chapter are those of its author. 

Non-Aqueous Processes. Dispersions of composite particles in non-aqueous media (12) have been prepared. The particles were sterically stabilised to prevent flocculation and aggregation. This was achieved by physical absorption of amphipathic graft or block copolymer (13,14) or by covalent attachment of diluent-soluble oligomer or polymer chains (15) at the particle surface so that by definition different polymers were situated at the surface and in the bulk of the particles, even for single-polymer particles. Composite particles were prepared by slow addition of the second monomer which was fully miscible with the diluent phase, obviating a monomer droplet phase further monomer-soluble initiation and amphipathic graft stabiliser was included as appropriate so that the process comprised continued dispersion... 

Volume concentration of solid, (j> Influences hydrodynamic interactions between particles, flocculation and aggregate characteristics. 

If the dispersion particles are attracted to each other, they tend to flocculate and form a stmcture. At low concentrations the particles form open aggregates, which give a fractal stmcture (93,94). At higher concentrations a network stmcture results, which can be so pronounced that the mixture has a yield point and behaves like a soHd when at rest. Shearing breaks up this stmcture, and viscosity decreases. 

Before determining the degree of stabiUty of an emulsion and the reason for this stabiUty, the mechanisms of its destabilization should be considered. When an emulsion starts to separate, an oil layer appears on top, and an aqueous layer appears on the bottom. This separation is the final state of the destabilization of the emulsion the initial two processes are called flocculation and coalescence (Fig. 5). In flocculation, two droplets become attached to each other but are stiU separated by a thin film of the Hquid. When more droplets are added, an aggregate is formed, ia which the iadividual droplets cluster but retain the thin Hquid films between them, as ia Figure 5a. The emulsifier molecules remain at the surface of the iadividual droplets duiing this process, as iadicated ia Figure 6. 

These chemicals ensure the aggregation of the suspended solids during the next treatment step-flocculation. Sometimes polyclectrolytes (usually polyacrylamides) are also added after flocculation and sedimentation as an aid to the filtration step. 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

At pH 5.3 and at low salt concentrations polymer addition has practically no influence on the aggregation and on the sedimentation behaviour. At pH 2.3 and at the highest salt concentration (0.1 m) the polymer addition results in infinite absorbances immediately after mixing these reduce to very small absorbances E., and E, - within a short time. This behaviour is attributed to the significant flocculation and the very fast sedimentation of the flocculated particles. 

It is fairly common to employ the words flocculation and coagulation instead of aggregation . Strictly, aggregation means particles of, for example, colloid coming together without external assistance. 

Because polymer adsorption is effectively irreversible, and because adsorption and floe growth occur simultaneously, flocculation is a non-equilibrium process. As a result, performance is largely determined by the kinetics of adsorption and aggregation. Both of these can be regarded as collision processes involving solid particles and polymer molecules. In each case, collisions can arise due to either Brownian motion or agitation of the suspension. The collision frequency v between particles and polymer molecules can be estimated from °... 

J. Gregory and L. Xing, Influence of Rapid Mix Conditions on Flocculation by Polymers in Dispersion and Aggregation Fundamentals and Applications, B.M. Moudgil and P. Somasundaran (eds.). Engineering Foundation, New York, 1994, pp. 427-441. 

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