Adsorption and charge neutralization Adsorption and interparticle bridging dsorption and charge neutralization involves the adsorption of mononuclear and polynuclear metal hydrolysis species on the colloidal particles found in wastewater. It should be noted that it is also possible to get charge reversal with metal salts, as described previously with the addition of counterions Adsorption and interparticle bridging involves the adsorption of polynuclear metal hydrolysis species and olymer species which, in turn, will ultimately form particle-polymer bridges, as described previously. As the coagulant requirement for adsorption and charge neutralization is satisfied, metal hydroxide precipitates and soluble metal hydrolysis products will form. If a sufficient concentration of metal salt is added. large amounts of metal hydroxide floc will form. Following macroflocculation, large floc particles will be formed that will settle readily. In turn. as these floc particles settle, they sweep through the water ed from the wastewater. In most wastewater applications, the sweep floc mode of operation is used most commonly where particles are to be removed by sedimentation The sequence of reactions and events that occur in the one 1\Zon Definition sketch for the 80 coagulation and removal of ffects of the continued particles can be illustrated addition of a coagulant F (e.g, alum) on the 6-3. In zone 1. sufficient destabilization and Flocculation of colloidal added to destabilize the colloidal particles. even though some reduction ir surface charge may occur due to the presence of Fet and some mor ydrolysis species. In zone 2, the colloidal particles ave been destabilized by the adsorption of mono- and polynuclear hydrolysis species, and, if allowed to flocculate and settle, the residual turbidity would be lowered as shown. In zone 3. as more coagulant is added, the surface charge of the particles has reversed due to the continued adsorption of mono- and polynuclear hydrolySIs S ecies. As the colloidal particles are now positively charged. they cannot be d by perikinetic flocculation ched. where large amounts ill be removed sweep action of the settling floc particles. and the residual turbidity will be lowered as shown. The coagulant dosage required to reach any of the zones will depend on the nature of the colloidal particles and the pH and temperature of the wastewater. Specific constituents (e. g, organic matter) will also have an effect on the nt dose It is also very important to note that the example reaction sequence given and the coagulation process illustrated on Fig. 6-3 are time-dependent. For example, if it is desired to destabilize the colloidal particles in wastewater with mono- and polynuclear species, then rapid and intense initial mixing of the metal salt and the wastewater containing the particles to be destabilized is of critical importance. If the reaction is allowed to proceed to the formation of metal hydroxide floc, it will be difficult to contact the chemical and the particles. As discussed below, it has been estimated that the formation of the mono- and polynuclear and polymer hydroxide species occurs in a fraction of a second Solubility of Metal Salts. To further appreciate the action of the hydrolyzed metal ions, it will be useful to consider the solubility of the metal salts. The operating region for alum precipitation is from a ph range for ron precipitation with minimum solubilIty occurring at a ph of 8.0 Operating Regions for Action of Metal Salts. so p Because the chemistry of the various reactions Optimum there is no complete theory explain the action of hydrolyzed metal ions s: To quantify qualitatively the application of um as a tion of alum6-6 1. Adsorption and charge neutralization 2. Adsorption and interparticle bridging 3. Enmeshment in sweep floc Adsorption and charge neutralization involves the adsorption of mononuclear and polynuclear metal hydrolysis species on the colloidal particles found in wastewater. It should be noted that it is also possible to get charge reversal with metal salts, as described previously with the addition of counterions. Adsorption and interparticle bridging involves the adsorption of polynuclear metal hydrolysis species and polymer species which, in turn, will ultimately form particle-polymer bridges, as described previously. As the coagulant requirement for adsorption and charge neutralization is satisfied, metal hydroxide precipitates and soluble metal hydrolysis products will form. If a sufficient concentration of metal salt is added, large amounts of metal hydroxide floc will form. Following macroflocculation, large floc particles will be formed that will settle readily. In turn, as these floc particles settle, they sweep through the water containing colloidal particles. The colloidal particles that become enmeshed in the floc will thus be removed from the wastewater. In most wastewater applications, the sweep floc mode of operation is used most commonly where particles are to be removed by sedimentation. The sequence of reactions and events that occur in the coagulation and removal of particles can be illustrated pictorially as shown on Fig. 6-3. In zone 1, sufficient coagulant has not been added to destabilize the colloidal particles, even though some reduction in surface charge may occur due to the presence of Fe3+ and some mononuclear hydrolysis species. In zone 2, the colloidal particles have been destabilized by the adsorption of mono- and polynuclear hydrolysis species, and, if allowed to flocculate and settle, the residual turbidity would be lowered as shown. In zone 3, as more coagulant is added, the surface charge of the particles has reversed due to the continued adsorption of mono- and polynuclear hydrolysis species. As the colloidal particles are now positively charged, they cannot be removed by perikinetic flocculation. As more coagulant is added, zone 4 is reached, where large amounts of hydroxide floc will form. As the floc particles settle, the colloidal particles will be removed by the sweep action of the settling floc particles, and the residual turbidity will be lowered as shown. The coagulant dosage required to reach any of the zones will depend on the nature of the colloidal particles and the pH and temperature of the wastewater. Specific constituents (e.g., organic matter) will also have an effect on the coagulant dose. It is also very important to note that the example reaction sequence given and the coagulation process illustrated on Fig. 6-3 are time-dependent. For example, if it is desired to destabilize the colloidal particles in wastewater with mono- and polynuclear species, then rapid and intense initial mixing of the metal salt and the wastewater containing the particles to be destabilized is of critical importance. If the reaction is allowed to proceed to the formation of metal hydroxide floc, it will be difficult to contact the chemical and the particles. As discussed below, it has been estimated that the formation of the mono- and polynuclear and polymer hydroxide species occurs in a fraction of a second. Solubility of Metal Salts. To further appreciate the action of the hydrolyzed metal ions, it will be useful to consider the solubility of the metal salts. The operating region for alum precipitation is from a pH range of 5 to about 7, with minimum solubility occurring at a pH of 6.0, and from about 7 to 9 for iron precipitation, with minimum solubility occurring at a pH of 8.0. Operating Regions for Action of Metal Salts. Because the chemistry of the various reactions is so complex, there is no complete theory to explain the action of hydrolyzed metal ions. To quantify qualitatively the application of alum as a function of pH, taking into account the action of alum as described above, Amirtharajah and Mills (1982) developed the Fig. 6-3