Zeta potential, coagulation

Streaming Current Detectors These units produce a measurement closely related to the zeta potential of a suspension and are used successfully in optimizing the coagulant dose in clarification applications. 

Ellis and Powis (1912-15) introduced the concept of the critical zeta potential for the coagulation of colloidal solutions. 

Figure 5.9 Illustration of the effect of electrolyte on colloid stability. The photomicrographs A through D show how 1.1 tm size silica particles are progressively coagulated by increasing additions of alum (0, 10, 30, 40 ppm, respectively). The corresponding zeta potentials are -30 mV (A), -14 mV (B), -6 mV (C), and -0 mV (D). From Zeta-Meter [544], Courtesy L.A. Ravina, Zeta-Meter, Inc., Staunton, Va.

Zeta potentials of floe produced in the plant may also be measured as a means of control. The zeta potential value for optimum coagulation must be determined for a given wastewater by actual correlation with jar tests or with plant performance. The control point is generally in the range of 0 to 10 millivolts. If good correlations can be obtained between some zeta potential values and optimum plant performance, then it is possible to make rapid measurements of particle charge to compensate for major variations in wastewater composition due to storm flows or other causes. 

The W values [65] for a dispersion of AI2O3 as a function of pH and KNO3 salt concentration are shown in Figure 10.27. The AI2O3 particles are colloidally stable far away from their isoelectric point (i.e., pH 8.9). As the salt concentration is increased the zeta potential decreases and the colloid stability ratio, W, decreases. Near the isoelectric point there is no electrostatic repulsion, giving a rapid coagulation. 

This chapter applies the techniques of the unit process of coagulation to the treatment of water and wastewater for the removal of colloids that cause turbidity and color. It also discusses prerequisite topics necessary for the understanding of coagulation such as the behavior of colloids, zeta potential, and colloid stability. It then treats the coagulation process, in general, and the unit process of the use of alum and the iron salts, in particular. It also discusses chemical requirements and sludge production. 

The effects of various electrolytes are usually compared in terms of the flocculation values, or minimal concentrations (expressed in millimoles per liter), required to bring about coagulation. Flocculation values for nonspecific electrolytes can be interpreted in terms of electrostatic repulsion and van der Waals attraction. The van der Waals attractive forces vary with size and shape of the particles, but roughly speaking the force is appreciable between colloidal particles at distances of the order of magnitude of their own radii. The significant feature is that flocculation occurs before the zeta potential reaches zero, that is, when it reaches a small critical value. 

FIGURE 11.9 Stability diagram for SijN particles as produced from calculations (lEP 4.4) assuming 90% probability of coagulation for sobd formation. The particle size is assumed to be 10 nm and the system is at room temperature. Also shown superposed is the zeta potential for the same particles. 

An empirical relationship was developed between the zeta potential and the coagulation behavior of a variety of systems. Deryaguin shows that a rapid... 

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