Coagulation dominant effects

Large numbers of simulations have shown that the dominant effects of coagulation are the limitation that aggregations put on algal populations and the enhanced rates of export of particulate matter that result from the formation of the large aggregates (36, 37). 

While Fe- and Mn-oxides certainly are important in the binding and transport of the trace metals in the estuarine system, organic matter appears to play a dominant role in the coagulation step. For instance, adjustment of the solution pH to values where humics normally precipitate (pH = l to 3) causes all the components (Fe and Mn, organics, and trace metals) to coagulate into filterable particles. By contrast, adjustment of the pH upward, which would normally favor precipitation of Fe- and Mn-oxides, has no effect. In view of this it is not... 

Studies on orthokinetic flocculation (shear flow dominating over Brownian motion) show a more ambiguous picture. Both rate increases (9,10) and decreases (11,12) compared with orthokinetic coagulation have been observed. Gregory (12) treated polymer adsorption as a collision process and used Smoluchowski theory to predict that the adsorption step may become rate limiting in orthokinetic flocculation. Qualitative evidence to this effect was found for flocculation of polystyrene latex, particle diameter 1.68 pm, in laminar tube flow. Furthermore, pretreatment of half of the latex with polymer resulted in collision efficiencies that were more than twice as high as for coagulation. 

These compressed air nebulizers produce polydisperse aerosols. After the aerosol is produced, the size distribution may change due to evaporation of liquid from the droplets. In addition, the particles may be electrically charged due to an ion imbalance in the droplets as they form if such charges become further concentrated due to evaporation, the particle may break up into smaller particles. Thus electrical neutralization of the aerosol, for example, by exposure to a radioactive source, is usually necessary to prevent electrostatic effects from dominating the particle motion, coagulation, and other behavior. 

So far we have focused our attention on dispersions in which interparticle repulsions dominate. Attractive forces (see Chapter 10) have been assumed to be negligible, and as a consequence the dispersions are stable. When the van der Waals attraction begins to play a significant role (i.e., the net effect of repulsion and attraction is such that we have coagulation of the dispersion), more complicated flow behavior occurs. Such dispersions are more common in practice and deserve our attention. 

Electrostatic stability plays a dominant role in many separation processes, such as filtration of industrial wastewaters. Coagulation aids (known as coagulants) are routinely used to improve the effectiveness of separation processes in such cases. Polymer-induced stability is often the method of choice, particularly in the case of concentrated dispersions for example, many pharmaceutical preparations, paints, inks, and liquid toners depend on surfactants or polymer additives for ensuring stable preparations. We see in Section 13.2 that in the case of concentrated dispersions both thermodynamic and kinetic issues often become very important. 

This review indicates that good solvent conditions (in terms of either x or 0) result in a positive value for AGR. This is what would be expected from a model that assumes that the first encounter between particles with adsorbed layers is dominated by the polymers. Conversely, in a poor solvent AGR is negative and amounts to a contribution to the attraction between the core particles as far as flocculation is concerned. Under these conditions the polymer itself is at the threshold of phase separation. Van der Waals attraction between the core particles further promotes aggregation, but it is possible that coagulation could be induced in a poor solvent even if the medium decreases the effective Hamaker constant to zero. 

These data require extension but in a tentative manner the conclusions can be summarized in Figure 8 where the domains of coagulation and flocculation are represented. Moreover, these ideas have only been applied to sodium chloride. With higher valency electrolytes more specific effects may occur which could dominate the phenomena. 

The sensitivity of the stability ratio to chemical or particle interaction factors can be illustrated by an examination of the model expression for Wn in Eq. 6.75. For example, if temperature and the particle interaction parameters are fixed, then Wn will vary with the concentration, c (also included in /c), of Z-Z electrolyte. At low values of c, k is also small, and the first equality in Eq. 6.75 indicates that Wu will take on its largest values. (Decreasing c also provokes an increase in dm because of Eq. 6.73, but this effect is dominated by that of k.40) Conversely, as c increases, the value of Wu will drop until it achieves its minimum, Wn = 1.0, when Z dm = 2 (Eq. 6.75). At this concentration, termed the critical coagulation concentration (ccc), or flocculation value, the flocculation process has become transport-controlled and therefore is rapid. Thus in general... 

Harbauer (1984) first described a venous model of thrombosis induced by mechanical injury and stenosis of the jugular vein. In a modification, both arterial and venous thrombosis is produced in rabbits by stenosis of the carotid artery and the jugular vein with simultaneous mechanical damage of the endothelium. This activates platelets and the coagulation system and leads to changes in the bloodstream pattern. As a consequence, occluding thrombi are formed as detected by blood flow measurement. The dominant role of platelets in this model is shown by the inhibitory effect of an antiplatelet serum in both types of vessels (Just 1986). The test is used to evaluate the antithrombotic capacity of compounds in an in vivo model of arterial and venous thrombosis where thrombus formation is highly dependent on platelet activation. 

The modeling of algal coagulation demonstrated that such coagulation would be consistent with oceanic conditions. Effects of such coagulation would include the facts that aggregation could constrain the maximum phytoplankton concentration, that it would enhance transport of material out of the euphotic zone, and that the transition to an aggregation-dominated state could be very rapid. 

Finally, it is to be noted that in the foregoing discussion of dry removal we considered the total ensemble of particles. If size ranges are taken into account separately, additional sinks have to be mentioned. Thus, thermal coagulation of particles with very small size, as well as the condensation (below a relative humidity of 100 %) of vapours with low saturation vapour pressure provide effective removal for Aitken particles. It is believed (Hidy, 1973) that these processes are dominant in the removal of aerosol particles in the size range below 0.1 /im radius. 

The coupled biogeochemical cycling of iron and arsenic has also recently been studied in a system dominated by the addition of iron as an engineering practice.14 Since 1996, ferric chloride has been added as a coagulant to the Los Angeles Aqueduct in order to control the levels of naturally occurring arsenic that reach the water distribution system for the City of Los Angeles. The iron- and arsenic-rich floe formed by this process is deposited in North Haiwee Reservoir. Sediments and porewaters from this reservoir were examined to determine the extent to which arsenic and iron are remobilized in the sediments and to probe the speciation of arsenic in the solid phase and its possible effects on arsenic remobilization. 

When a surfactant is adsorbed onto a solid surface, the resultant effect on the character of that surface will depend largely upon the dominant mechanism of adsorption. For a highly charged surface, if adsorption is a result of ion exchange, the electrical nature of the surface will not be altered significantly. If, on the other hand, ion pairing becomes important, the potential at the Stern layer will decrease until it is completely neutralized (see Fig. 9.5). In a dispersed system stabilized by electrostatic repulsion, such a reduction in surface potential will result in a loss of stability and eventual coagulation or flocculation of the particles (Chapter 10). 

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