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⁺ (Al₁₃), regarded as the dominant species in polyaluminum chloride (PACl), is highly related to its aggregation characteristic, but the detailed process of Al₁₃ aggregation in coagulation time scale was not well studies. Here we systematically studied the deprotonation and aggregation processes of Al₁₃ by alkaline titration to simulate the reaction in coagulation case. By reacting with OH^-, Al₁₃ can continuously lose protons regardless of pH until its positive charge was well neutralized. The initial Al₁₃ aggregates (Al₁₃agg) appeared at B of 2.70 and large Al₁₃agg was generated by coalescence of small initial Al₁₃agg. Most Al₁₃ polycations kept their main structure unchanged during aggregation and part was decomposed into monomers or oligomers. Density functional theory (DFT) results reveal that Al₁₃ becomes unstable after deprotonation, but the aggregation of Al₁₃ bridged by Al monomers can stabilize the polycations. Al₁₃ needs to be hydrolyzed before interacting with colloidal particles, but particles can promote the aggregation of Al₁₃ by weakening the repulsion force between the polymers. Strong and compact flocs can be generated induced by in-situ aggregation of Al₁₃ in neutral and alkaline conditions. This study can provide a deep understanding about the role of Al₁₃agg in removing particles and instruct the development of new efficient coagulants against the various water qualities.