Some Water-Technological Considerations in Coagulation and Flotation

8 Some Water-Technological Considerations in Coagulation and Flotation 

Such species adsorb specifically and modify the surface charge of the colloids. As shown in Fig. 7.7 these hydrolysis products have a different effect than Al3+ (at low 

Equilibrium composition of solutions in contact with freshly precipitated, Al(OH)3 and Fe(OH)3, calculated, using representative values for the equilibrium constants for solubility and hydrolysis equilibria. Shaded areas are approximate operating regions in water treatment practice coagulation in these systems occurs under conditions of oversaturation with respect to the metal hydroxide. 

Oversaturation is induced by adjusting the pH of incipiently acidic Fe(III) and AI(III) solutions. After initiation of the oversaturation, the hydrolysis of AI(III) and Fe(III) progresses and charged multimeric hydroxo AI(III) and Fe(III) species, the actual coagulants, are formed as intermediates ultimately these intermediates polymerize to solid Al (III) and Fe(III)(hydr)oxides, respectively. 

AI(III) and Fe(III) salts are acids. The addition of these chemicals to water is similar to an acidimetric titration of the water. As a result, the pH of the system after the addition of these coagulants will depend upon the coagulant dosage and the alkalinity of the water or waste water. Coagulation by Fe(III) and AI(III) polymers would be expected to exhibit restabilization (overdosing) and stoichiometry, if adsorption is important. 

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