Surrounding this fixed layer of ions is a diffuse layer of ions Measurement of Surface Potential. If a particle is placed in an electrolyte solution, and an electr current is passed through the solution, the particle, depending on its surface charge, will be attracted to one or the other of the electrodes, dragging with it a cloud of ions. The potential at the surface of the cloud (called the surface of shear) is sometimes measured in wastewater-treatment operations. The measured value is often called the zeta potential. Theoretically, however, the zeta potential should correspond to the potential measured at the surface enclosing the fixed layer of ions attached to the particle. The use of the measured zeta potential value is limited because it will vary with the nature of the solution components Particle-Particle Interactions Particle-particle interactions are extremely important in bringing about aggregation by means of Brownian motion. The two principal forces involved are the forces of repulsion, due to the electrical properties of the charged plates, and the van der Waals forces of attraction. It should be noted that the van der Waals forces of attraction do not come into play until the two plates are brought together in close proximity to each The net total energy shown is the difference between the for he for attraction will predominate at short and long distances. The net energy curve contains a repulsive maximum that must be overcome if the particles, represented as the two plates, are to be held together by the van der Waals force of attraction. There is no energy barrier to overcome. Clearlv. if colloidal particles are to be removed by microflocculation. the repulsive force must be reduced. Although floc particles can form at long distances as shown by the net energy curve for condition 1, the net force holding these particles together is weak and the floc particles that are formed can be raptured easily Particle Destabilization with Potential-Determining lons and Electrolytes To bring about particle aggregation through microflocculation, steps must be taken to reduce particle charge or to overcome the effect of this charge. The effect of the charge can be overcome by(1) the addition of potential-determining ions, which will be taken up by or will react with the colloid surface to lessen the surface charge and(2)the addition of electrolytes, which have the effect of reducing the thickness of the diffuse electric laver and, thereby reduce the zeta potential Use of Potential-Determining lons. The addition of potential-determining ions to promote coagulation can be illustrated by the addition of strong acids or bases to reduce the charge of metal oxides hydroxides to near zero so that coagulation can occur. The magnitude of the effect will depend on the concentration of potential-determining ions added. It is interesting to note that depending on the concentration and nature of the counterions added it is possible to reverse the charge of the double laver he use of potential determining ions is not feasible in either water or wastewater treatment because of the massive concentration of ions that must be added to bring about sufficient compression of the electrical double laver to effect perikinetic flocculation. Use of Electrolytes. Electrolytes can also be added to coagulate colloidal suspensions. Increased concentration of an electrolyte that is needed to destabilize a colloidal suspension is nown as the critical coagulation concentration(CCC). Increasing the concentration of an indifferent electrolyte will not result in the restabilization of the colloidal particles Particle Destabilization and Aggregation with Polyelectrolytes Polyelectrolytes may be divided into two categories: natural and synthetic. Important natural polvelectrolvtes include polvmers of biological origin and those derived from starch products such as cellulose derivatives and alginates. Synthetic polyelectrolytes consist of simple monomers that are polymerized into high-molecular-weight substances, Depending on whether their charge. when placed in ater. is negative, positive, or neutral. these polyelectrolytes are classified as anionic, cationic, and Charge Neutralization In the first category polyelectrolytes act as coagulants that neutralize or lower the charge of the wastewater particles. Because wastewater particles normally are charged negatively. cationic polyelectrolytes are used for this purpose. In this application, the cationic polyelectrolytes are considered to be primary coagulants. To effect charge neutralization, the polyelectrolyte must be adsorbed to the particle. Because of the large number of particles found in wastewater. the mixing intensity mus sufficient to bring about the adsorption of the polvmer onto the colloidal particles. With inadequate mixing the polymer will eventually fold back on itself and its effectiveness in reducing the surface charge will be if the number of colloidal particles is limited. it will be difficult to remove them with Polymer Bridge Formation. The second mode of action of polyelectrolytes is interparticle bridging. In6-4 Surrounding this fixed layer of ions is a diffuse layer of ions. Measurement of Surface Potential. If a particle is placed in an electrolyte solution, and an electric current is passed through the solution, the particle, depending on its surface charge, will be attracted to one or the other of the electrodes, dragging with it a cloud of ions. The potential at the surface of the cloud (called the surface of shear) is sometimes measured in wastewater-treatment operations. The measured value is often called the zeta potential. Theoretically, however, the zeta potential should correspond to the potential measured at the surface enclosing the fixed layer of ions attached to the particle. The use of the measured zeta potential value is limited because it will vary with the nature of the solution components. Particle-Particle Interactions Particle-particle interactions are extremely important in bringing about aggregation by means of Brownian motion. The two principal forces involved are the forces of repulsion, due to the electrical properties of the charged plates, and the van der Waals forces of attraction. It should be noted that the van der Waals forces of attraction do not come into play until the two plates are brought together in close proximity to each other. The net total energy shown is the difference between the forces of repulsion and attraction. The forces of attraction will predominate at short and long distances. The net energy curve contains a repulsive maximum that must be overcome if the particles, represented as the two plates, are to be held together by the van der Waals force of attraction. There is no energy barrier to overcome. Clearly, if colloidal particles are to be removed by microflocculation, the repulsive force must be reduced. Although floc particles can form at long distances as shown by the net energy curve for condition 1, the net force holding these particles together is weak and the floc particles that are formed can be raptured easily. Particle Destabilization with Potential-Determining Ions and Electrolytes To bring about particle aggregation through microflocculation, steps must be taken to reduce particle charge or to overcome the effect of this charge. The effect of the charge can be overcome by (1) the addition of potential-determining ions, which will be taken up by or will react with the colloid surface to lessen the surface charge and (2) the addition of electrolytes, which have the effect of reducing the thickness of the diffuse electric layer and, thereby, reduce the zeta potential. Use of Potential-Determining Ions. The addition of potential-determining ions to promote coagulation can be illustrated by the addition of strong acids or bases to reduce the charge of metal oxides or hydroxides to near zero so that coagulation can occur. The magnitude of the effect will depend on the concentration of potential-determining ions added. It is interesting to note that depending on the concentration and nature of the counterions added, it is possible to reverse the charge of the double layer and develop a new stable particle. The use of potential determining ions is not feasible in either water or wastewater treatment because of the massive concentration of ions that must be added to bring about sufficient compression of the electrical double layer to effect perikinetic flocculation. Use of Electrolytes. Electrolytes can also be added to coagulate colloidal suspensions. Increased concentration of a given electrolyte will cause a decrease in zeta potential and a corresponding decrease in repulsive forces. The concentration of an electrolyte that is needed to destabilize a colloidal suspension is known as the critical coagulation concentration (CCC). Increasing the concentration of an indifferent electrolyte will not result in the restabilization of the colloidal particles. Particle Destabilization and Aggregation with Polyelectrolytes Polyelectrolytes may be divided into two categories: natural and synthetic. Important natural polyelectrolytes include polymers of biological origin and those derived from starch products such as cellulose derivatives and alginates. Synthetic polyelectrolytes consist of simple monomers that are polymerized into high-molecular-weight substances. Depending on whether their charge, when placed in water, is negative, positive, or neutral, these polyelectrolytes are classified as anionic, cationic, and nonionic, respectively. Charge Neutralization. In the first category, polyelectrolytes act as coagulants that neutralize or lower the charge of the wastewater particles. Because wastewater particles normally are charged negatively, cationic polyelectrolytes are used for this purpose. In this application, the cationic polyelectrolytes are considered to be primary coagulants. To effect charge neutralization, the polyelectrolyte must be adsorbed to the particle. Because of the large number of particles found in wastewater, the mixing intensity must be sufficient to bring about the adsorption of the polymer onto the colloidal particles. With inadequate mixing, the polymer will eventually fold back on itself and its effectiveness in reducing the surface charge will be diminished. Further, if the number of colloidal particles is limited, it will be difficult to remove them with low polyelectrolyte dosages. Polymer Bridge Formation. The second mode of action of polyelectrolytes is interparticle bridging. In