Speciation, stability, and coagulation mechanisms of hydroxyl aluminum clusters formed by PACl and alum: A critical review.

The physicochemical property of coagulant species plays a significant role in the coagulation process. Recent progress on speciation, stability, and coagulation mechanisms of the hydroxyl aluminum clusters formed by PACl and alum has been critically reviewed. The complicated nature on species formation, stability, and transformation of various hydrolyzed aluminum clusters formed by PACl and Alum are discussed. Based on the aspects of spontaneous hydrolysis, forced hydrolysis, and dual-hydrolysis models, the special stability of aluminum clusters that results in various coagulation behaviors is compared with the traditional salts. The coagulation behavior of the hydroxyl aluminum clusters in terms of particle aggregation and restabilization, surface adsorption and coverage, microfloc formation and kinetics, modified DLVO simulation, and finally, the coagulation model is then analyzed in detail. It is indicative that the coagulation mechanism of inorganic coagulants can be understood better with the hydroxyl clusters being tailor-made.