Toxicity of ionic liquids to Clostridium sp. and effects on uranium biosorption

As green solvents ionic liquids (ILs) show high potential in nuclear industry for extraction and purification of actinides. However, to date relatively little information has been gained on ILs application in microbial processes, for example biosorption of radionuclides. We investigated the effects of three ILs, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆), N-ethylpyridinium trifluoroacetate (EtPyCF₃COO) and N-ethylpyridinium tetrafluoroborate (EtPyBF₄) on the growth and biosorption of uranium by Clostridium sp. The ILs affected the growth of the bacterium as evidenced by decreases in optical density, total gas production, and organic acids production from glucose metabolism. The IC_{50-48 h} of three ILs decreased in the order of BMIMPF₆ (8.26mM)>EtPyBF₄ (7.04mM)>EtPyCF₃COO (4.05mM). Uranium biosorption by the bacterial cells decreased by 75% in the presence of 1% (v/v) BMIMPF₆ and by about 90% with 1% (v/v) EtPyBF₄ or EtPyCF₃COO, in comparison to the control without ILs. The diminished biosorption may be attributed to the membrane damages induced by EtPyBF₄ and EtPyCF₃COO, which can be visualized by Transmission Electron Microscope (TEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) analysis revealed the accumulation of uranium inside peripheral membrane of the cells exposed to uranium alone or with BMIMPF₆, while little or no accumulation was observed in the presence of EtPyBF₄ and EtPyCF₃COO. These results imply that potential toxicity of ILs towards microorganisms is a particularly important issue in limiting its biotechnological applications.