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 [AlO4Al12(OH)(24)(H2O)(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 insitu 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.