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.

Aggregation and Dissociation of Aqueous Al13 Induced by Fluoride Substitution

The ε-Keggin ion AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁺ (ε-K Al₁₃⁷⁺) is a double-edged sword, because it commonly acts as a toxic component toward aquatic organisms, but also is considered to be an effective coagulant. Gaining deep insight into the transformation of ε-K Al₁₃⁷⁺ in the presence of coexisting ligands would have significant implications for water environmental science, as well as for practical water purification. The aggregation and dissociation of aqueous Al₁₃⁷⁺ induced by fluoride (F^-) substitution were herein investigated using nuclear magnetic resonance, electrospray ionization-mass spectrometry, and theoretical calculations. The F^- substitution on η-OH₂ sites was extremely fast, reducing the charge of ε-K Al₁₃⁷⁺ so that the repulsive force between fluorinated Al₁₃ species was immediately reduced. Consequently, fluorinated Al₁₃ aggregated, with the formula [Al₁₃F₅]²⁺, which was demonstrated by calculating the Gibbs free energy changes (Δ_rG) of the substitution reactions involved. Moreover, the replacement of η-OH₂ with F^- weakened the strength of Al-OH^a/b bonds and thus prompted the replacement of μ-OH^a/b with F^-. In addition, fluorination prompted [Al₁₃F₅]²⁺ to dissociate to oligomers.