In which pH range does alum work most effectively?

Alum, or aluminum sulfate, is a coagulant commonly used in water treatment processes to remove impurities and clarify water. Its effectiveness is highly dependent on the pH of the water being treated. The pH range of 5.8 to 7.5 is optimal for alum to function effectively because, within this range, aluminum ions can hydrolyze effectively to form positively charged species that attract and bind with negatively charged particles in water, leading to their aggregation and eventual removal through sedimentation or filtration.

At a pH below 5.8, the increase in hydrogen ion concentration can inhibit the alum's ability to coagulate effectively, as it affects the solubility and hydrolysis of aluminum ions. Similarly, at a pH above 7.5, alum's effectiveness diminishes due to the formation of aluminum hydroxide, which can precipitate out of solution and become less effective at binding with particulates. Therefore, pH levels in the range of 5.8 to 7.5 ensure that alum can operate efficiently, maximizing its coagulation potential and improving water quality.