Date of Award
Doctor of Philosophy in Environmental Science - (Ph.D.)
Chemistry and Environmental Science
Sanjay V. Malhotra
A. J. Francis
Tamara M. Gund
Room temperature ionic liquids (ILs) are pure ionic compounds with melting points below 100°C, which have drawn great attention due to their many unique properties. They have been widely used in organic synthesis, analytical chemistry, and biochemistry. Although ionic liquids have been used to extract metals and radionuclides, there is no systematic study on the interactions of ILs with uranium and their effects on the biotransformation of uranium.
My research focused on: (1) the interactions between uranium and ionic liquids in aqueous solution; and (2) biotransformation of uranium in the presence of ILs. The interactions between three representative ionic liquids, [EtPy] [BF4] (N-ethylpyridinium tetrafluoroborate), [EtPy][CF3COO] (N-ethyl pyridinium trifluoroacetate), [BMIM][PF6] (l-butyl-3-methylimidazolium hexafluorophosphate), and uranium were determined by various analytical techniques, such as UV-Vis spectroscopy, potentiometric titration, LCMS and X-ray absorption spectroscopy. Extended X-ray absorption fine structure (EXAFS) analysis showed the formation of monodentate complexes between uranium and [BMIM][PF6][EtPy][BF4] and a biodentate complex with [EtPy][CF3COO].
The effects of ionic liquids on the growth of anaerobic bacterium Clostridium sp. were measured by changes in optical density, pH, and gas production. The ionic liquids inhibited the growth of bacterium to varying degrees. TEM and EDS studies showed that the ILs may affect the cell membrane of the bacterium and thereby its growth.
Biosorption of uranium by Clostridium sp. dramatically decreased in the presence of ILs. It decreased in the order of U> U+[BMIM][PF6] > U+[EtPy][BF4] and U+[EtPy][CF3COO].
The bioreduction of U(VI) to U(IV) by Clostridium sp. was affected by the presence of ILs. The rate of reduction of U, in the absence or in the presence of the monodentate complexes U:[EtPy][BF4] and U:[BMIM][PF6] was similar. However, no reduction was observed in the presence of the bidentate U:[EtPy][CF3COO] complex. Also ionic liquids showed different affinities toward reduced U(IV) in solution after bioreduction. In the absence of IL, U(IV) precipitated out rapidly; whereas in the presence of [EtPy][BF4] a substantial amount of U(IV) was present in the solution. This result can be attributed to the complexation of U(IV) with [EtPy][BF4]. Notably, this phenomenon was not observed in the presence of [BMIM][PF6] The formation of U(IV) in solution and in precipitate were confirmed by UV-Vis spectroscope, XANES and EXAFS analysis.
Since ILs are relatively resistant to photodegradation and show toxicity at higher concentrations, the persistence in the environment was determined by the investigation of biodegradation of the three ILs. A soil enrichment culture degraded [EtPy][BF4] and [EtPy][CF3COO] to innocuous products, glyoxylate and acetic acid. In the presence of uranium, the rate and extent of biodegradation was slowed. N-ethyl-(4-(carbosyamino)but-3-enoic acid, semialdehyde, and (4-(carbosyamino)but-3-enoic acid were accumulated in medium as biodegradation products. Imidazolium-based ILs [BMIM][PF6] were resistant to biodegradation. This information is useful for the risk assessment of the "environmentally friendly" compounds.
Zhang, Chengdong, "Interactions of ionic liquids with uranium and its implications on biotransformation" (2006). Dissertations. 782.