Imidazolium ionic liquids as potential persistent pollutants in aqueous environments: Indirect photochemical degradation kinetics and mechanism

Ionic liquids (ILs) with promising application are likely to become ubiquitous contaminants in water environment for their high hydrophilicity, low biodegradability, and especially its potential toxicity. In this work, we have investigated photochemical transformation of six imidazolium ILs for fate prediction and ecological risk assessment. We found that the reaction rates of the ILs with center dot OH, CO3 center dot- and O-1(2) enhanced with their increasing alkyl chain and varied slightly with the paired anions. Meanwhile, modelled results under different scenarios indicate that the primary contributors to transformation of the ILs are triplet-stated dissolved matter ((CDOM)-C-3*), center dot OH and CO3 center dot-. Besides, the overall half-lives of the ILs can reach 670 days, which indicates persistence of these ILs in the environment. Products for ILs in reaction with center dot OH and triplet-stated sodium anthraquinone-2sulfonate ((3)AQ2S*) were probed by UHPLC-Q-TOF-MS/MS and there is a difference between their products: Products by center dot OH are likely formed by hydrogen abstraction from the side alkyl chain, followed by dehydrogenation, hydroxylation and carbonylation, while one of the products by (3)AQ2S* is formed by dihydroxyl-addition of the imidazolium ring. Furthermore, the ILs and its products were estimated to have toxicity and non-readily biodegradability, suggesting potential eco-risk for the environmental water.

Automated summary

This study investigates the photochemical transformation of ionic liquids (ILs) and assesses their fate and ecological risks. The reaction rates of ILs were found to be influenced by the alkyl chain and showed slight variation with the paired anions. The primary contributors to ILs transformation were triplet-stated dissolved matter, hydroxyl radical, and carbonate radical. The ILs exhibited high persistence with an overall half-life of up to 670 days. The transformation products differed for reactions with hydroxyl radical and triplet-stated sodium anthraquinone-2sulfonate, and both the ILs and their products showed toxicity and low biodegradability, posing potential ecological risks to water environments.