Ionic liquids (ILs) are expected to be increasingly used by chemical industries as replacements of volatile organic solvents. Consequently, ILs, which are very soluble in water, have the potential to become mobile environmental contaminants. The most common ILs are composed of organic cations containing quaternary amine groups and inorganic or organic anions. While current knowledge of the toxicity of ILs indicates that some IL cations may pose a threat to aquatic ecosystems, little is known about the fate of IL cations in the aquatic environment, particularly regarding abiotic transformation processes. Biotransformation and adsorption studies of ILs suggest that ILs will not be sufficiently removed in waste-water treatment plants and adsorption processes will not lead to a significant removal of ILs from aquatic environments. Consequently, the direct and indirect photochemical transformation of ILs will be an important factor in their persistence in surface waters. In this study, we investigated the kinetics of the photochemical transformation for four common structures of IL cations, namely imidazolium, pyridinium, pyrrolidinium, and piperidinium. Besides direct photochemical transformation, reactions with singlet oxygen and hydroxyl radicals were studied in model systems. Due to the potential association of IL cations with negatively charged moieties of dissolved organic matter, we also determined indirect photochemical transformations of the selected cations in the presence of natural organic matter. In contrast to previous studies, direct photochemical transformation was generally negligible for these IL cations, while the rate, and thus relevance, of different indirect photochemical pathways varied depending on the chemical structure. Overall, these results give a first indication on the persistence of IL cations in aqueous environments. Further studies, however, will be necessary to better understand the kinetics and mechanisms of photochemical transformations of quaternary amine containing contaminants.

Direct and indirect photochemical transformation of imidazolium, pyridinium, pyrrolidinium, and piperidinium ionic liquids

253rd National Meeting (2017)

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