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.