Toxicity of imidazolium and pyridinium based ionic liquids towards algae. Chlorella vulgaris, Oocystis submarina (green algae) and Cyclotella meneghiniana, Skeletonema marinoi (diatoms)

Abstract

This paper reports on an investigation into the toxicity of 1-alkyl-3-methylimidazolium ionic liquids (ILs) towards two green algae (Chlorella vulgaris and Oocystis submarina) characteristic of freshwater and brackish environments, as well as two brackish and marine diatoms (Cyclotella meneghiniana and Skeletonema marinoi). The test kit of IL compounds consisted of five 1-alkyl-3-methylimidazolium chlorides (from -ethyl to -decyl) for evaluating the expected alkyl chain length effect, together with 1-butyl-3-methylimidazolium tetrafluoroborate, dicyanamide, trifluoromethanesulfonate, methyl sulfate an α-methyl[poly(oxy-1,2-ethanediyl)]sulfate for investigating the influence of the anion on IL toxicity towards various algal species. A pronounced alkyl chain effect was confirmed with all the organisms studied. EC50 values were linearly very well correlated with the number of carbon atoms in the IL alkyl chains. The results indicate that diatoms are far more sensitive than green algae to ILs. Cell size also plays an important part in the intoxication process: a tenfold difference in cell size results in a 100% more sensitive reaction to ionic liquids in both the green algae and diatoms. No significant differences were observed between alkylimidazolium salts and an alkylpyridinium compound of similar lipophilicity. It was also found that the use of tetrafluoroborate and trifluoromethanesulfonate as counteranions in the IL structure gave rise to the most pronounced toxic effects in comparison with the other anions tested. In the case of tetrafluoroborate this is probably caused by the potential hydrolysis of this entity, which leads to the formation of fluoride and a further increase in toxicity. Whereas in the case of trifluoromethanesulfonate it is likely caused by relatively high lipophilicity of this anion, additionally known to be strongly associated with alkylimidazolium cations, that in turn may enhance cell wall penetration.