Polyaluminum chloride (PAC) is primarily classified as a coagulant, although it also exhibits some flocculant-like properties.
Coagulants neutralize the negative charges on suspended particles (e.g., colloids) in water, allowing them to come closer together. This process is called coagulation.
Flocculants then help these destabilized particles aggregate into larger flocs that can settle or be filtered out—this is flocculation.
PAC works by:
Providing highly charged cationic species (such as Al₁₃ polymers) that effectively neutralize negatively charged colloidal particles → coagulation.
In many cases, the hydrolysis products of PAC can also bridge particles together, aiding in the formation of larger flocs → a flocculation-like effect.
However, in water and wastewater treatment terminology, PAC is predominantly referred to as a coagulant because its main function is charge neutralization, which is the hallmark of coagulation.
In practice, PAC is often used in conjunction with a polymeric flocculant (like polyacrylamide) to enhance floc formation and settling, especially in challenging water conditions.
