Dissertation
Effect of heteroatom substitution on structural transformation and water treatment efficiency of aluminum Keggin polycations
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2021
DOI: 10.17077/etd.006242
Abstract
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.
Details
- Title: Subtitle
- Effect of heteroatom substitution on structural transformation and water treatment efficiency of aluminum Keggin polycations
- Creators
- Mohammad Shohel
- Contributors
- Tori Z Forbes (Advisor)Alexei V Tivanski (Committee Member)Christopher M Cheatum (Committee Member)Edward G Gillan (Committee Member)Johna Leddy (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Chemistry
- Date degree season
- Autumn 2021
- DOI
- 10.17077/etd.006242
- Publisher
- University of Iowa
- Number of pages
- xviii, 187 pages
- Copyright
- Copyright 2021 Mohammad Shohel
- Language
- English
- Description illustrations
- illustrations (chiefly color)
- Description bibliographic
- Includes bibliographical references (pages 172-187)
- Public Abstract (ETD)
Aluminum is one of the most abundant elements on earth surface and important component of materials. In water, aluminum atoms can bond together into a cluster of atoms that is one nanometer in diameter. This type of cluster is called a Keggin cluster and it is made up of aluminum cations (Al3+) surrounded by oxygen atoms, hydroxide groups, or water molecules in tetrahedral and octahedral geometry. These Keggin-clusters are found in coagulants that are used for water purification applications, as precursors for aluminum oxide materials, and may be important pathways for transporting heavy-metal contaminants in the environment. Other metals (heteroatoms or heterometal) can substituted into the Keggin clusters and this changes the overall chemistry into the system. However, the relationship between the heterometallic substitution and the change in the chemistry is not well understood. In the current work, we discovered five novel aluminum Keggin clusters that incorporate gallium (Ga3+), germanium (Ge4+), and chromium (Cr3+). X-ray diffraction techniques were used to determine how the heterometals incorporated into the Keggin structure and these results were coupled with chemical characterization and computational analysis to understand the observed changes. The X-ray diffraction analysis and computational results suggested that gallium and germanium prefer to substitute in tetrahedral position, while chromium prefers octahedral sites. Heteroatom substitution also controlled the way the Keggin clusters interacted with each other or with additional metals in water. Based upon the identity of the heteroatom, the clusters grew in different ways and impacted the stability of the resulting species. Their performance in water treatment was also evaluated and the gallium substituted cluster removed more contaminants that the other systems. Knowledge from this work will help to design functional material and create new methods for water purification.
- Academic Unit
- Chemistry
- Record Identifier
- 9984210748902771
Metrics
8 File views/ downloads
76 Record Views