What is the optimal pH range for coagulation with aluminum sulfate?

The optimal pH range for coagulation with aluminum sulfate, commonly referred to as alum, is indeed around the isoelectric point. This is the pH level at which the particles in water have a neutral charge, which facilitates the coagulation process. At this point, positively charged aluminum ions can better interact with negatively charged colloidal particles, allowing for effective floc formation.

When the pH is too low (acidic), there may be insufficient hydroxyl ions to form effective aluminum hydroxide, which is the floc that helps in the removal of particles from water. Conversely, if the pH is too high (alkaline), the solubility of aluminum hydroxide increases, leading to ineffective coagulation. Additionally, a neutral pH would not optimize the charge interactions necessary for effective coagulation, as it does not reach the desired point where the colloids are neutral.

Therefore, the isoelectric point is the most effective pH for using aluminum sulfate for coagulation, as it enhances the charge balance needed for optimal interaction and flocculation of suspended particles in water.