Assessment the ecotoxicity and inhibition of imidazolium ionic liquids by respiration inhibition assays

The ecotoxicity and inhibition of 12 imidazolium ionic liquids (ILs) with alkyl chain from C4 to C10 and chloride (Cl-), tetrafluoroborate (BF4-) and bis(trifluoromethanesulfonyl)imide (NTf2-) anions have been studied by means of respiration inhibition assays using activated sludge collected from a wastewater treatment plant. This test represents an alternative easy, economic and quick way to evaluate the true impact of ILs on activated sludge-based wastewater treatment. For comparison purposes, the EC50 values were also determined by the Microtox test (Vibrio fischeri). It was observed that this widely used microbial test overestimates the effect of the ILs on biological wastewater treatment facilities, especially in the case of ILs with lower ecotoxicity. The results of the biological tests showed that the alkyl chain length plays a crucial role in the ecotoxicity of ILs. A significant increase of the toxicity with the length of the n-alkyl chain was found. Regarding to the impact of the anion, the ecotoxicity measured by respiration inhibition assays follows the order NTf2- > Cl- > BF4- being the anion effect higher as decreasing the length of cation alkyl chain. According to the hazard substances ranking for aquatic organisms (Passino and Smith, 1987), imidazolium ILs with C4 alkyl chain can be classified as practically harmless compounds whereas those with alkyl chains C8 or C10 correspond to highly toxic species.