Explain the fundamental mechanism by which high alkalinity mitigates pH depression during alum coagulation.

Alum, or aluminum sulfate (Al2(SO4)3·nH2O), is a common coagulant used in water treatment. When alum is added to water, it undergoes a chemical process called hydrolysis. During hydrolysis, aluminum ions from the alum react with water molecules to form insoluble aluminum hydroxide precipitates, which are crucial for coagulation—the process of destabilizing suspended particles to facilitate their aggregation. A critical outcome of this hydrolysis reaction is the release of hydrogen ions (H+), which are strong acids. The generation of these H+ ions naturally consumes hydroxide ions already present in the water and increases the concentration of H+, thus causing the water's pH to decrease, a phenomenon referred to as pH depression. A....