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 (ΔrG) of the substitution reactions involved. Moreover, the replacement of η-OH2 with F- weakened the strength of Al-OHa/b bonds and thus prompted the replacement of μ-OHa/b with F-. In addition, fluorination prompted [Al13F5]2+ to dissociate to oligomers.