A study of aluminum speciation in aluminum chloride solutions by small angle x-ray scattering and ²⁷Al NMR

Abstract

Aluminum ions may exist in one of several different types of polynuclear species depending upon the degree to which it is hydrolyzed. The hydrolysis of aluminum can be explained by a metastable equilibrium among A1(H2O)6+, Al(OH)(H2O)s+, Al13O4(OH)24(H2O)i2f, and an oligomer. We have made small angle x-ray scattering and 27Al NMR measurements on 0.3 M aluminum chloride solutions over a range of OH/A1 ratios. 27A1 NMR shows two sharp peaks corresponding to the monomer A1(H2O)6+ and the tetrahedrally coordinated aluminum in Ali3O4(OH)24(H2O)i2f, and a broad peak due to the octahedrally coordinated aluminum in the oligomer. The observed radius of gyration (Rg) of the species in solution increases from 3.0 to 3.6 A, and the intensity at zero angle (which is proportional to the weight average molecular weight of the species) also increases as the 0H/A1 ratio increases from 1.0 to 2.2. The oligomer has an Rg of approximately 3.3 A, a hydrolysis ratio close to 2.0, and may be a six-membered ring with a structure similar to that of a gibbsite layer.

Differentiation of hydroxyl-aluminum species at lower OH/Al ratios by combination of 27Al NMR and Ferron assay improved with kinetic resolution

“…Keggin-Al 13 Singhal and Keefer [7], Pophristic et al [21] assigned Al un formed at lower OH/Al ratios to the six-member-ring Al 6 (Al 6 (OH) 12 6+ ) species. Seichter et al [22], Allouche et al [23] reported the existence of Al 6 -type polycation [Al 13 (OH) 24 include various polymers and species with the structure of octahedrons arrayed similar to the six-member-ring style ( 2 -OH, 3 -OH).…”

“…However, benefiting from the advanced instrumentation the situation is subject to change. Singhal and Keefer [7] by small angle X-ray scattering and Pophristic et al [21] by molecular dynamics demonstrated the existence of the six-member-ring hexamer species in the hydroxyl-Al solution. Moreover, Seichter et al [22] reported that some crystal solid of [Al 13 (OH) 24 [23] reexamined it later with 2D 27 Al 3QMAS NMR and referred to polycation as Mögel-Al 13 15+ .…”

“…However, it can determine only a few Al species like mononuclear (Al m ), oligomers (Al 2 even Al 3 ), and tridecamer (Al 13 ). It can identify currently neither the octahedral coordinated lower polymers as Al(O) 6 series, nor the polymers higher than Al 13 except the recently revealed Al 30 produced at elevated temperature [7][8][9]. All of these species can only be referred to the undetectable Al species (Al un ) in 27 Al NMR spectroscopy.…”

Effect of aging condition on species transformation in polymeric Al salt coagulants

“…Unlike the Al 13 species of Keggin structure, these oligomers often do not have Keggin structure and are undetectable with 27 Al NMR spectroscopy. As the hydroxyl Al speciation results proposed by Singhal and Seichter, these species might be composed of six member ring structure [5,16]. It can also be inferred from above experimental results that these species might be formed without strict synthesis condition and thus could decompose easily into monomers or polymers with lower polymerization degree.…”

“…Thus, many researchers have put emphasis on the transformation process of Al 13 in hydroxyl Al solution under various aging conditions [12][13][14][15]. In fact, besides Al 13 , some other polymers have also been proved to exist in solution by chemical calculation methods and instrumental characterization [16,17]. In our previous studies, Al 13 and some other polymeric Al species have been proved to coexist in hydroxyl Al solutions, especially the solutions with lower basicity [18].…”