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

Epsilon Keggin Al-13 (K-Al-13 center dot AlO4Al12(OH)(24)(H2O)(12)(7+)) is often a dominating hydrolysis aluminum species in polyaluminum chloride coagulants. Another Al-13 with planar structure (M-Al-13 center dot Al-13(OH)(24)(H2O)(24)(+15)) has also been found to be effective coagulant, with higher charge destabilization ability. However, M-Al-13 has been only found in crystal and whether the M-Al-13 exists in aqueous solution has not been validated yet. This study presents the finding that the transformation of M-Al-13 to K-Al-13 during the coagulant dissolution process. M-Al-13 and K-Al-13 were proved to have different structure in crystal with X-ray powder diffraction (XRD) and Al-27 MAS NMR. Inferred from the analysis results of electrospray ionization mass spectrometry (ESI-MS), M-Al-13 species could spontaneously transform to K-Al-13 species, while the latter kept its Keggin structure throughout the dissolution process. This can be further proved by Al-27 solution NMR and Al-Ferron kinetic analysis results, i.e., the medium polymer species (Al-b) of M-Al-13 solution was almost the same with those in K-Al-13 solution. The transformation of M-Al-13 to K-Al-13 is helpful to understand how aluminum species are affected by water in dissolution process. (c) 2012 Elsevier B.V. All rights reserved.