The destabilization of colloids through the addition of counterions should be done in conjunction with the application of the complete coagulation process. Four methods are used to bring about this process: double-layer compression, charge neutralization, entrapment in a precipitate, and intraparticle bridging.
When the concentration of counterions in the dispersion medium is smaller, the thickness of the electric double layer is larger. Two approaching colloid particles cannot come close to each other because of the thicker electric double layer, therefore, the colloid is stable. Now, visualize adding more counterions. When the concentration is increased, the attracting force between the primary charges and the added counterions increases causing the double layer to shrink. The layer is then said to be compressed. As the layer is compressed sufficiently by the continued addition of more counterions, a time will come when the van der Waals force exceeds the force of repulsion and coagulation results.
The charge of a colloid can also be directly neutralized by the addition of ions of opposite charges that have the ability to directly adsorb to the colloid surface. For example, the positively charged dodecylammoniun, C12H25NH+, tends to be hydrophobic and, as such, penetrates directly to the colloid surface and neutralize it. This is said to be a direct charge neutralization, since the counterion has penetrated directly into the primary charges. Another direct charge neutralization method would be the use of a colloid of opposite charge. Direct charge neutralization and the compression of the double layer may compliment each other.
A characteristic of some cations of metal salts such as Al(III) and Fe(III) is that of forming a precipitate when added to water. For this precipitation to occur, a colloidal particle may provide as the seed for a nucleation site, thus, entrapping the colloid as the precipitate forms. Moreover, if several of this particles are entrapped and are close to each other, coagulation can result by direct binding because of the proximity.
The last method of coagulation is intraparticle bridging. A bridging molecule may attach a colloid particle to one active site and a second colloid particle to another site. An active site is a point in the molecule where particles may attach either by chemical bonding or by mere physical attachment. If the two sites are close to each other, coagulation of the colloids may occur; or, the kinetic movement may loop the bridge assembly around causing the attached colloids to bind because for now they are hitting each other, thus bringing out coagulation.
Electrolytes and polyelectrolytes are used to coagulate colloids. Electrolytes are materials which when placed in solution cause the solution to be conductive to electricity because of charges they possess. Polyelectrolytes are polymers possessing more than one electrolytic site in the molecule, and polymers are molecules joined together to form larger molecules. Because of the charges, electrolytes and poly-electrolytes coagulate and precipitate colloids. The coagulating power of electrolytes is summed up in the Schulze-Hardy rule that states: the coagulation of a colloid is affected by that ion of an added electrolyte that has a charge opposite in sign to that of the colloidal particle; the effect of such an ion increases markedly with the number of charges carried. Thus, comparing the effect of AlCl3 and Al2(SO4)3 in coagulating positive colloids, the latter is 30 times more effective than the former, since sulfate has two negative charges while the chloride has only one. In coagulating negative colloids, however, the two have about the same power of coagulation.
The most important coagulants used in water and wastewater treatment are alum, copperas (ferrous sulfate), ferric sulfate, and ferric chloride. Later, we will specifically discuss the chemical reactions of these coagulants at greater lengths. Other coagulants have also been used but, owing to high cost, their use is restricted only to small installations. Examples of these are sodium aluminate, NaAlO2; ammonia alum, Al2(SO4)3 • (NH4)2 • 24H2O; and potash alum, Al2(SO4)3 • K2SO4 • 24H2O. The reactions of sodium aluminate with aluminum sulfate and carbon dioxide are:
6NaAlO2 + Al2(SO4)3 • 14.3H2O ^ 8Al(OH)3 + 3Na2SO4 + 2.3H2O (12.1) 2NaAlO2 + CO2 + 3H2O ^ 2Al(OH)31 + Na2CO3 (12.2)
Difficulties with settling often occur because of flocs that are slow-settling and are easily fragmented by the hydraulic shear in the settling basin. For these reasons, coagulant aids are normally used. Acids and alkalis are used to adjust the pH to the optimum range. Typical acids used to lower the pH are sulfuric and phosphoric acids. Typical alkalis used to raise the pH are lime and soda ash. Polyelectrolytes are also used as coagulant aids. The cationic form has been used successfully in some waters not only as a coagulant aid but also as the primary coagulant. In comparison with alum sludges that are gelatinous and voluminous, sludges produced by using cationic polyelectrolytes are dense and easy to dewater for subsequent treatment and disposal. Anionic and nonionic polyelectrolytes are often used with primary metal coagulants to provide the particle bridging for effective coagulation. Generally, the use of poly-electrolyte coagulant aids produces tougher and good settling flocs.
Activated silica and clays have also been used as coagulant aids. Activated silica is sodium silicate that has been treated with sulfuric acid, aluminum sulfate, carbon dioxide, or chlorine. When the activated silica is applied, a stable negative sol is produced. This sol unites with the positively charged primary-metal coagulant to produce tougher, denser, and faster settling flocs.
Bentonite clays have been used as coagulant aids in conjunction with iron and alum primary coagulants in treating waters containing high color, low turbidity, and low mineral content. Low turbidity waters are often hard to coagulate. Bentonite clay serves as a weighting agent that improves the settleability of the resulting flocs.
Coagulation will not be as efficient if the chemicals are not dispersed rapidly throughout the mixing tank. This process of rapidly mixing the coagulant in the volume of the tank is called rapid or flash mix. Rapid mixing distributes the chemicals immediately throughout the volume of the mixing tank. Also, coagulation should be followed by flocculation to agglomerate the tiny particles formed from the coagulation process.
If the coagulant reaction is simply allowed to take place in one portion of the tank because of the absence of the rapid mix rather than being spread throughout the volume, all four mechanisms for a complete coagulation discussed above will not be utilized. For example, charge neutralization will not be utilized in all portions of the tank because, by the time the coagulant arrives at the point in question, the reaction of charge neutralization will already have taken place somewhere.
Interparticle bridging will not be as effective, since the force to loop the bridge around will not be as strong without the force of the rapid mix. Colloid particles will not effectively be utilized as seeds for nucleation sites because, without rapid mix, the coagulant may simply stay in one place. Finally, the compression of the double layer will not be as effective if unaided by the force due to the rapid mix. The force of the rapid mix helps push two colloids toward each other, thus enhancing coagulation. Hence, because of all these stated reasons, coagulation should take place in a rapidly mixed tank.
In practice, irrespective of what coagulant or coagulant aid is used, the optimum dose and pH are determined by a jar test. This consists of four to six beakers (such as 1000 ml in volume) filled with the raw water into which varying amounts of dose are administered. Each beaker is provided with a variable-speed stirrer capable of operating from 0 to 100 rpm.
Upon introduction of the dose, the contents are rapidly mixed at a speed of about 60 to 80 rpm for a period of one minute and then allowed to flocculate at a speed of 30 rpm for a period of 15 minutes. After the stirring is stopped, the nature and settling characteristics of the flocs are observed and recorded qualitatively as poor, fair, good, or excellent. A hazy sample denotes poor coagulation; a properly coagulated sample is manifested by well-formed flocs that settle rapidly with clear water between flocs. The lowest dose of chemicals and pH that produce the desired flocs and clarity represents the optimum. This optimum is then used as the dose in the actual operation of the plant. See Figure 12.5 for a picture of a jar testing apparatus.
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