Suspended particles, electrostatic charges

Water and Waste Water Treatment. PAG products are used in water treatment for removal of suspended soHds (turbidity) and other contaminants such as natural organic matter from surface waters. Microorganisms and colloidal particles of silt and clay are stabilized by surface electrostatic charges preventing the particles from coalescing. Historically, alum (aluminum sulfate hydrate) was used to neutralize these charges by surface adsorption of Al cations formed upon hydrolysis of the alum. Since 1983 PAG has been sold as an alum replacement in the treatment of natural water for U.S. municipal and industrial use. 

Electrical Precipitators When particles suspended in a gas are exposed to gas ions in an electrostatic field, they will become charged and migrate under the ac tion of the field. The functional mechanisms of electrical precipitation may be listed as follows ... 

The nature and intensity of the attractive or repulsive forces among particles in a state of suspension in a liquid medium depend primarily on the electrostatic charges of the particle. Other factors contributing to these forces are particle size and surface area of the solid, the physical properties of the suspending medium, the presence of adsorbed gases or liquids, the proximity of the particles, and Brownian movement (5). 

Thus while the presence of electrostatic charge will tend to hasten the evaporation process because of the extended surface provided by the ejected drops, this is probably not a major consideration since the initial stage of evaporation of a droplet is actually the one that limits the over-all rate. The concentration of charge within the drop, however, means that evaporation of a drop can ultimately yield very highly charged small particles of residual nonvolatile soluble or suspended contaminants. 

It is important to note that the concept of osmotic pressure is more general than suggested by the above experiment. In particular, one does not have to invoke the presence of a membrane (or even a concentration difference) to define osmotic pressure. The osmotic pressure, being a property of a solution, always exists and serves to counteract the tendency of the chemical potentials to equalize. It is not important how the differences in the chemical potential come about. The differences may arise due to other factors such as an electric field or gravity. For example, we see in Chapter 11 (Section 11.7a) how osmotic pressure plays a major role in giving rise to repulsion between electrical double layers here, the variation of the concentration in the electrical double layers arises from the electrostatic interaction between a charged surface and the ions in the solution. In Chapter 13 (Section 13.6b.3), we provide another example of the role of differences in osmotic pressures of a polymer solution in giving rise to an effective attractive force between colloidal particles suspended in the solution. 

An electrostatic precipitator is a gas-solid separator in which particles suspended in a gas stream are charged and removed by electrostatic force. Since the separation force is directly applied to the particles without the need of accelerating the gas phase, an electrostatic precipitator usually requires much less hydraulic power than other separation systems. Hence, electrostatic precipitation is widely recognized as an important technique of gas-solid separation. This technique is characterized by low pressure drop, relative high... 

The electrostatic stabilization theory was developed for dilute colloidal systems and involves attractive van dcr Waals interactions and repulsive double layer interactions between two particles. They may lead to a potential barrier, an overall repulsion and/or to a minimum similar to that generated by steric stabilization. Johnson and Morrison [1] suggest that the stability in non-aqueous dispersions when the stabilizers are surfactant molecules, which arc relatively small, is due to scmi-stcric stabilization, hence to a smaller ran dcr Waals attraction between two particles caused by the adsorbed shell of surfactant molecules. The fact that such systems are quite stable suggests, however, that some repulsion is also prescni. In fact, it was demonstrated on the basis of electrophoretic measurements that a surface charge originates on solid particles suspended in aprotic liquids even in the absence of traces of... 

According to Deijaguin-Landau-Verwey-Overbeek (DLVO) theory, a cornerstone of modem colloid science, two types of forces exist between colloidal particles suspended in a dielectric medium electrostatic forces, which result from an unscreened surface charge on the particle, and London-van der Waals attractive forces, which are universal in nature. The colloidal stability and rheology of oxide suspensions, in the absence of steric additives, can be largely understood by combining these two forces (assumption of additivity). 

Figure 1. Mechanism of electrostatic charging of suspended acidic particles (with acidic sites AH) by basic dispersants (with basic sites B) in solvents of low dielectric...

The starting particles are negatively charged or neutral, and can be electrostatically suspended and even ordered in the plasma. 

Suspended particles acquire an electrostatic charge, which, in water treatment, is usually negative. The charges produce a repulsion between particles, which tends to stabilise the suspension. In colloidal suspensions, which have a maximum particle size of less than 2 pm, this repulsion effectively prevents settling. 

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