Effect of heteroatom substitution on structural transformation and water treatment efficiency of aluminum Keggin polycations

Keggin-type Al3+ polycations belong to a unique class of compound with their large positive charge, nanoscale size, and diverse applications. These species are utilized in wastewater treatment as coagulant, environmentally important due to their potential role in contaminant transport and serve as a precursor for aluminum hydroxide and oxide solid-state materials. While the structural features for the basic Al13 Keggin isomers have been delineated, much less is known regarding heteroatom substitution and oligomerization process. In this thesis work, Ga3+, Ge4+ and Cr3+ have been successfully incorporated into Keggin polycation isomers and this substitution has been study in relation to structural transformations, aggregation behavior, stability, and water treatment performance. Incorporation of these cations impacted the structural transformation of the Keggin isomers by stabilizing different isomers and activating deprotonation at specific H2O sites, which subsequently controlled the mode of condensation. In the case of Ga3+ substitution, the ε-(GaAl12)7+ can transform into [GaO4Al12(OH)24(H2O)12]7+ (δ-GaAl12) and [Ga2O8Al28.5Ga0.5(OH)58(H2O)27(SO4)2]15+ (Ga2.5Al28.5) upon hydrothermal aging. For Ge4+ substitution, the ε-(GeAl12)8+ converts into [Ge4O16Al48(OH)108(H2O)24]20+ (Ge4Al48) and Cr3+ incorporation generated two isomeric structures: [AlO4Al9.6Cr2.4(OH)24(H2O)12]7+ (δ-CrnAl13-n-1) and [AlO4Al9.5Cr2.5(OH)24(H2O)12]7+ (δ-CrnAl13-n-2). Structural analysis of the isolated compounds coupled with density functional theory (DFT) calculation suggested that Ga3+ and Ge4+ prefer to substitute in tetrahedral site, while Cr3+ prefers octahedral positions. The Ga3+ substituted Keggin solution were previously shown to enhanced water treatment performance compared to the other heterometallic systems. Work presented herein on initial and thermally-aged Keggin solution suggest that this is due to greater stability and diversity of Ga3+ substituted larger Keggin oligomers that drive the coagulation process.