Chemical Forms of Uranium
Uranium exists in the environment mainly as oxides, organic or inorganic complexes, and rarely as a free metal ion (Mtimunye and Chirwa, 2013). Free elemental uranium primarily exists in higher oxidation states typically bound to oxygen. The oxygen bound uranium exists mainly as triuranium octaoxide also known as pitchblende (U3O8), uraninite (UO2), and uranium trioxide (UO3) (Stefaniak et al., 2009). U3O8 is relatively insoluble in water and relatively stable over a wide range of environmental conditions. UO2 on the other hand is not as stable as U3O8 in the environment as it may undergo alteration under various environmental conditions (Senanayake et al., 2005). Upon exposure to air, UO2 is subjected to oxidation and as a result produces a secondary mineral (UO2 2+) which complexes easily with phosphates, carbonates, silicates, and sulphates (Senanayake et al., 2005; Stefaniak et al., 2009).
The chemistry of uranium and other radionuclides in the environment is totally dependent on their oxidation states. The natural uranium exists in the four oxidation states, i.e., trivalent uranium [U(III)], tetravalent uranium [U(IV]), pentavalent uranium [U(V)], and hexavalent uranium [U(VI)]. U(IV) and U(VI) are the most stable oxidation states in the environment (Francis, 1998; Gavrilescu et al., 2009). Uranium (III) may easily oxidize to U(IV) while U(V) readily disproportionate to U(IV) under most reducing conditions found in nature. The highly soluble U(VI) ion mainly exist as UO2
2+ (uranyl) under oxidising conditions, while U(IV) exist as sparingly soluble UO2 (uraninite) under reducing conditions. In soil of p range 4-7.5, uranium typically exits in the hydrolysed form UO2(OH)- while in water uranium typically exists as hydroxyl carbonate complexes such as (UO2)2CO3(OH)3 -, UO2(CO3)2 2-, UO2CO3 0, and UO2(CO3)4- (Roh et al., 2000).
CHAPTER 1 INTRODUCTION.
1.2 Objectives of the Study
1.3 Outline of Thesis
1.4 Significance of Research and Main Findings
CHAPTER 2 LITERATURE REVIEW
2.1 Occurrence of Uranium in the Environment .
2.2 Radiological Properties
2.3 Chemical Forms of Uranium
2.4 Production of Uranium and Its Use
2.5 Uranium as a Fuel for Nuclear Power
2.6 Radioactive Waste
2.7 Classification of Radioactive Waste
2.8 Waste from High Temperature Fast Reactors
2.9 Chemical and Radiological Toxicity: Risk to Human and Animal Health
2.10 Remediation Strategies
2.11 Enzymatic U(VI) Reduction.
2.12 Cellular Localization
2.13 Emerging Treatment Technologies
CHAPTER 3 EXPERIMENTAL METHODS
3.1 Bacterial Culture .
3.2 Growth Media
3.3 Characterisation of Microbial Community
3.4 Chemical Reagents and Standards
3.5 Experimental Batches
3.6 Continuous Flow Suspended-Cell Bioreactor
3.7 Continuous Flow Biofilm Rector System
3.8 Evaluation of Biomass Yield.
3.9 Analytical Methods
3.10 Statistical Methods
CHAPTER 4 RESULTS FROM BATCH KINETIC STUDIES
4.2 Microbial Analysis .
4.3 Preliminary U(VI) Reduction Studies
4.4 Mixed-Culture Performance
4.5 Modelling Theory .
4.6 Sensitivity Analysis .
CHAPTER 5 KINETIC STUDIES OF CONTINOUS-FLOW SYSTEMS
5.2 Conceptual Basis of Suspended Growth System
5.3 Suspended Growth System Kinetic Studies
5.4 General Principles of Bioremediation Technologi
5.5 Attached Growth System Kinetic Studies
5.6 Microbial Shift Dynamics .
CHAPTER 6 MODELLING OF CONTIONOUS-FLOW SYSTEM
CHAPTER 7 SUMMARY AND CONCLUSIONS
CHAPTER 8 ENGINEERING SIGNIFICANCE AND RECOMMENDATIONS.