Comparison of Three Bacterial Toxicity Assays for Imidazolium-Derived Ionic Liquids

Ionic liquids have become leading candidates for replacing many common organic solvents used in the chemical process industry. There is, however, a general lack of toxicology data relevant to wastewater treatment facility microbes for these compounds. In this study, we performed three bacterial-based toxicity assays on several imidazolium-derived compounds as well as the precursor compound 1-methylimidazole. Two of the assays, the Shk1 and Microtox assays, are used as surrogate assays for toxicity to bacterial respiration in activated sludge wastewater treatment plants. The third assay was a direct measure of the effect of toxicity on mixed bacterial culture respiration, using a commercially available consortium of naturally occurring bacteria to obtain $I{C}_{50}$ values for direct comparison to the $E{C}_{50}$ values from the surrogate assays. The Shk1 assay is based on a genetically engineered bioluminescent Pseudomonas bacterium and is more highly correlated with the respiration inhibition than the Microtox assay. The Shk1 assay gave $E{C}_{50}$ values more similar to $I{C}_{50}$ values from the bacterial respiration inhibition assay for the compounds tested in this work. The Shk1 $E{C}_{50}$ values were similar to that of 1-butanol, an alcohol with an alkyl chain length similar to that of the cation of the tested compounds, which were 1-butyl-3-methylimidazolium bromide ([bmim][Br]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim] $\left[{\text{PF}}_6\right]$ ), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, ([bmim] $\left[{\text{Tf}}_2\text{N}\right]$ ), and the precursor compound 1-methylimidazole, and were generally smaller than those typical of aromatic organic solvents.