Degradability of ionic liquids : Systematic investigations on aerobic and anaerobic biodegradability of ionic liquid cations and anions as well as on the hydrolytical stability of ionic liquid anions - contributions to a sound hazard assessment

Abstract

The presented studies deal with the degradability of ionic liquids under conditions that occur in biological waste water treatment and within the environment. Degradation studies are relevant for the assessment of the technological applicability of chemicals and of their hazard potential, including their environmental fate. In former studies some ionic liquids have shown to remain stable against abiotic and biotic degradation processes. The systematic investigation of chemical structures which are more easier subject to degradation processes shall help to design ionic liquids that, although stable in technical applications, avoid the risk of environmental persistency and is the aim of the presented studies. It is not only the chemical structure that influences the (bio)degradability of a substance; environmental conditions also play a major role. For instance, the availability of oxygen affects the composition of the microbial community and the activity of enzymes that are involved in the degradation process. In previous biodegradation studies the focus was only laid on aerobic conditions, showing a tendency towards low biodegradation potential for most of the investigated ionic liquids. Therefore, the question arose whether the (bio)degradability of ionic liquids can be enhanced, (1) by changing the environmental redox conditions for bacterial growth from aerobic to nitrate-reducing ones, (2) by changing the structural composition of the ionic liquid, namely the anion and the cationic head group, and (3) by specifically selected bacteria for an enhanced biodegradation process. Moreover, the question of whether cyano-based ionic liquid anions that were not biodegradable can be degraded abiotically at different pH values or enzymatically hydrolysed in vitro has been discussed. The investigations showed that the biodegradability of the tested ionic liquid cations with alkyl side chains are even worse under denitrifying conditions than under aerobic ones. However, the one with a hydroxylated side chain could still be biodegraded under the anaerobic conditions. Under aerobic conditions, the change of the cationic head group can enhance the biodegradability of ionic liquids when combined with an appropriate side chain. None of the investigated fluoroorganic and cyano-based ionic liquid anions could be biodegraded neither under aerobic nor denitrifying conditions, nor by the cyanide-degrading bacteria strain Cupriavidus spp. All of these anions were further stable under neutral and slightly basic conditions in the hydrolytical test procedure. However, in strong acidic and basic solutions, N(CN)2- and C(CN)3- showed hydrolytical degradation potential without the formation of hydrogen cyanide. The hydrolytical transformation could further be enhanced enzymatically in vitro at pH 7 by nitrile hydratase leading to the hydrolysis of all of the cyano-based anions. The data concerning the (bio)degradability contribute to the hazard assessment of ionic liquids and support the proactive substance design for inherently safer chemicals.