Ecotoxicity of binary mixtures of ILs and inorganic salts of electrochemical interest

Abstract

The applicability of ionic liquids (ILs) has increased over the last years, and even new opportunities are becoming a reality, i.e. mixtures of pure IL and inorganic salt as electrolytes for smart electrochemical devices, yet the effects on the environment are almost unknown. In this work, the ecotoxicity of two pure protic ILs (Ethylammonium nitrate and Ethylimidazolium nitrate) and two pure aprotic ILs (butylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide and butyldimethylimidazolium bis(trifluoromethylsulfonyl)imide) and that of their binary mixtures with inorganic salts with common cation was tested towards changes in the bioluminescence of the bacteria Aliivibrio fischeri, using the Microtox® standard toxicity test. EC50 of these mixtures was determined over three standard periods of time and compared with the corresponding values to pure ILs. Results indicate that the aprotic ILs are more toxic than protic and that aromatic are more toxic than non-aromatic. The addition of inorganic mono (LiNO3), di (Ca(NO3)2·4H2O, Mg(NO3)2·6H2O) and trivalent (Al(NO3)3·9H2O) salts in binary mixtures with EAN was analysed first. The latter was found to induce an important increase in toxicity. Finally, mixtures of IL-inorganic lithium salt (LiNO3, for the protic ILs and LiTFSI for the aprotic ILs) toxicity was also studied, which showed toxicity levels strongly dependent on the IL of the mixture.

Comprehensive Analysis of the Acute Toxicity of Ionic Liquids Using Microtox® Bioassays

“…The cation families can be also ordered according to the results obtained as follows: imidazolium > pyperidium > piperidinium >pyrrolidinium > phosphonium > ammonium; however, as expected, the alkyl chain length is the key factor influencing toxicity, and the toxicity of the IL increases with longer alkyl chain lengths, as will be detailed below. These results agree with the previous idea that the protic and non-aromatic ILs are less toxic than the aprotic and aromatic ones [12,13]. For instance, the EC 50 values at 15 min for C = 2 compounds are 289.97 and 133 mg/L for APILs C 2 C 1 Im TFSI and C 2 Py TFSI, respectively, while for EAN and C 2 Im NO 3 , as PILs, the values are 10,665.47 and 573 mg/L, respectively, which clearly shows the difference between PILs and APILs and between ammonium cation, which is less toxic, and imidazolium cation, which is more toxic.…”

“…The alkyl chain length plays a fundamental role in the toxicity of the ILs, with six carbons being the critical size to mark the transition from non-toxic to toxic in many cases, although further studies in this line should be performed to verify this conclusion. [12] Relatively harmless Non-toxic PAN [23] Practically harmless Toxic BAN [23] Practically harmless Toxic PEAN [23] Relatively harmless Non-toxic HEAN [23] Toxic Toxic OAN [23] Toxic Very toxic C 2 Im NO 3 [12] Practically harmless Non-toxic…”

“…These concentrations are calculated, along with the corresponding 95% confidence intervals, through a non-linear regression using the least-squares method to fit the data to the logistic equation. The decrease in bioluminescence with increasing sample concentration serves as an integrated measure of physiological impairment of the bacteria, thereby demonstrating the toxic effect of the studied compound [12].…”

“…On the other hand, IL toxicity is more extensively studied with respect to the cation, and therefore, toxicity is primarily attributed to this moiety. The effect of cationic alkyl side chains is one of the most thoroughly investigated aspects of IL toxicity, and numerous studies have established a direct correlation between toxicity and longer alkyl side chain lengths through various trophic levels [11][12][13].…”