Interaction of Hydrolyzed Al and Polyelectrolyte Coagulants

A laboratory jar test study was conducted to determine the mode of action of commercial cationic polyelectrolytes used in combination with aluminum salt coagulants. Electrophoretic mobility and residual turbidity data were collected as a function of coagulant dosage, solution chemistry, and the particle size distribution and concentrations of the silica particle suspension. Two general types of results were obtained depending on the tendency of the hydrolyzed aluminum to form a flocculent precipitate. This tendency is a function of the solution chemistry, including pH, concentrations of adsorbing anions such as sulfate and the ionic strength. With dilute solutions and low pH the aluminum hydrolysis products tend to be stable and under these conditions the cationic polyelectrolyte and the adsorbed hydrolyzed Al have similar effects on particle stability. Maximum turbidity removal is observed near an electrophoretic mobility of zero. A simple trade-off relationship exists between the amounts of Al and polyelectrolyte required to react this point. When the aluminum hydrolysis products are flocculent a coating of aluminum hydroxide precipitate is apparently formed on the particles which, as the aluminum concentration is increased, eventually controls particle stability and the efficiency of flocculation.