Ionic liquids and chitinous biomass: materials, synthesis, and applications for uranium sequestration
The mining of Uranium (U) ore, processing, and applications in weapons manufacturing and nuclear fuel have resulted in a legacy of contamination that require the creation of innovative methods that provide waste to volume minimization. Therefore a major objective of this research was to combine the reactivity of chitosan with the toughness and insolubility of chitin by surface modifications with UO22+ selective moieties. Thus the IL platform consisting of the 1-ethyl-3-methyl-imidazolium cation [C2C1im]+ paired with the acetate anion [OAc]- was selected and demonstrated to overcome solubility limitations of the biomass. This strategy was successful in producing the first report of electrospinning fibers directly from chitinous biomass in ILs. Concurrent to these results, supercritical carbon dioxide was explored as an alternative solvent to high boiling point coagulation baths to reduce economic and engineering challenges of using ILs at scale. With the successful efforts producing high surface area fibers from chitinous biomass, both qualitative and quantitative analysis supported functionalization of fibers with the amidoxime functional group for aqueous uranyl ions. While these research efforts have demonstrated chitin as a versatile polymer back-bone for fiber applications for U recovery, complexity of the waste derived feedstock is challenging and other chemical components that remain in the processed shell waste are key variables. Further characterization of the feedstock led to the discovery that metabolically inactive shrimp shell has the intrinsic ability to mineralize and reduce aqueous metal ions. This represents a new alternative to promote stable secondary U(VI) phosphate U(VI) and insoluble U(IV) phases, providing an effective strategy for immobilizing U. In addition the rediscovery of the simple but understudied salt [NH3OH][CH3COO] not only demonstrates OAc- is an excellent ligand for U(VI) coordination, but its protic ionic liquid properties suggest a much broader application space. Therefore future work is warranted to determine the influence of carboxylic acids in the reduction of U(VI) and the speciation and stability of the U(IV) phase. However these results provide considerable improvements that address constraints of current bioremediation and abiotic precipitation techniques to provide a long term sink for one of the most abundant radionuclides released into the environment.