Colloidal Material

Although we have been discussing concentrations of dissolved substances, a good deal of material may be present in water as suspended particles. If the particles are small enough, they may pass through filters and travel in aqueous systems very much like material in true solution. Such suspensions of small particles are called colloids. Typically, colloids are particles in the size range of 1-10 ^m, although some suspended material in natural water systems may be larger than this. Mineral materials (clays) such as silica, silicates, aluminosili-cates, metal oxides or hydroxides, and polymeric organic material from the excretions or decay of organisms are common sources. Particles will settle out at a rate that depends on the density of the particle and the square of its diameter (Stokes' law). The density depends on the composition. The size, because of the dependence on the square of the diameter, is most important. Small particles will settle very slowly and are easily kept in suspension by agitation, but coagulation or coalescence into larger particles will lead to much more effective settling. Figure 9-20 shows the size relationships of materials in aqueous systems.

Colloidal material has a high surface area per unit mass; clay particles, for example, may have a specific surface area in excess of 50 m2/g. A large surface area results in high activity, particularly for adsorption. All surfaces tend to adsorb other substances from their surroundings. Polar surfaces, which are typical of the materials that make up natural colloids, will adsorb polar molecules or ions. A result of these surface charges is that coagulation into larger particles that can more readily settle out is hindered by the Coulombic repulsions. While small amounts of electrolytes stabilize a colloidal suspension through contribution to this surface charge, addition of large concentrations of

Molecules

Colloidal particles Viruses Suspended particles

Bacteria Algae

FIGURE 9-20 Size spectrum for particles in water. Adapted from W. Stumm, Environ. Sci. Technol., 11, 1066 (1977).

electrolytes may aid in the aggregation of colloidal material into larger particles. Part of the effect is due to adsorption of ions of opposite charge, which reduces the overall charge on the particle. In addition, with more ions available in solution, the solution layer above the surface that must contain counterions to give electrical neutrality can be thinner, allowing closer approach of the particles. The repulsive forces from the charge on the particles depend on their separation distance squared. However, the attractive forces that can hold them together when they coagulate, van der Waals forces, depend on the separation to the fourth power. Consequently, only when close approach is possible will the attractive forces predominate.

The activity of surfaces in adsorbing other substances is of significant importance in modifying concentrations, reactivities, and movement of such substances in the environment. This applies not only to colloidal suspensions in water, but also to the finely divided clays and humic substances in soils, and to the liquid-gas surfaces in foams and even to the surfaces of water systems themselves. Trace amounts of materials can be built up on the surfaces of particles to levels that are much greater than indicated by bulk analytical values. This represents a potential process for the concentration of toxic materials. Also, it is well established that concentrations at the surface of a water body can be different from the concentrations in the bulk; a surface film perhaps 50 ^m thick may have concentrations several hundred times the bulk value. Although the total amount in such a thin film is small, it is this film that becomes spray. The composition of the film can affect properties such as surface tension and these surface effects can be important in phenomena such as energy transfer or gas transfer across water-air interfaces.

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