Transformation of planar Mögel Al13 to epsilon Keggin Al13 in dissolution process

Epsilon Keggin Al13 (K-Al13, AlO4Al12(OH)24(H2O)127+) is often a dominating hydrolysis aluminum species in polyaluminum chloride coagulants. Another Al13 with planar structure (M-Al13, Al13(OH)24(H2O)24+15) has also been found to be effective coagulant, with higher charge destabilization ability. However, M-Al13 has been only found in crystal and whether the M-Al13 exists in aqueous solution has not been validated yet. This study presents the finding that the transformation of M-Al13 to K-Al13 during the coagulant dissolution process. M-Al13 and K-Al13 were proved to have different structure in crystal with X-ray powder diffraction (XRD) and 27Al MAS NMR. Inferred from the analysis results of electrospray ionization mass spectrometry (ESI-MS), M-Al13 species could spontaneously transform to K-Al13 species, while the latter kept its Keggin structure throughout the dissolution process. This can be further proved by 27Al solution NMR and Al-Ferron kinetic analysis results, i.e., the medium polymer species (Alb) of M-Al13 solution was almost the same with those in K-Al13 solution. The transformation of M-Al13 to K-Al13 is helpful to understand how aluminum species are affected by water in dissolution process.

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► The planar M-Al13 structure is proved to be converted to the Keggin structure in dissolution process.
► M-Al13 is not a stable species and decomposed immediately in aqueous solution.
► A possible mechanism of formation of K-Al13 by M-Al13 dissolution is proposed.
► The mechanism explains how K-Al13 could be formed in non-alkaline environment.