Deprotonation and aggregation of Al13 under alkaline titration: A simulating study related to coagulation process

Unraveling the transformation of coagulants and their interaction with contaminants at the micro-level is vital to advancing our understanding of the coagulation mechanism. To the best of our knowledge, the coagulation effectiveness of [AlO 4 Al 12 (OH) 24 (H 2 O) 12 ] 7+ (Al 13 ), regarded as the dominant species in polyaluminum chloride (PACl), is highly related to its aggregation characteristic, but the detailed process of Al 13 aggregation in coagulation time scale was not well studies. Here we systematically studied the deprotonation and aggregation processes of Al 13 by alkaline titration to simulate the reaction in coagulation case. By reacting with OH − , Al 13 can continuously lose protons regardless of pH until its positive charge was well neutralized. The initial Al 13 aggregates (Al 13 agg) appeared at B of 2.70 and large Al 13 agg was generated by coalescence of small initial Al 13 agg. Most Al 13 polycations kept their main structure unchanged during aggregation and part was decomposed into monomers or oligomers. Density functional theory (DFT) results reveal that Al 13 becomes unstable after deprotonation, but the aggregation of Al 13 bridged by Al monomers can stabilize the polycations. Al 13 needs to be hydrolyzed before interacting with colloidal particles, but particles can promote the aggregation of Al 13 by weakening the repulsion force between the polymers. Strong and compact flocs can be generated induced by in-situ aggregation of Al 13 in neutral and alkaline conditions. This study can provide a deep understanding about the role of Al 13 agg in removing particles and instruct the development of new efficient coagulants against the various water qualities.