Ionic Liquids: New Emerging Pollutants, Similarities with Perfluorinated Alkyl Substances (PFASs)
By Agneta Oskarsson and Matthew C. Wright
Enviorn. Sci. and Tech.
November 20, 2019
Ionic liquids are organic salts with unique physicochemical properties, including low melting points, negligible vapor pressure, high thermal and chemical stability, and high solubility in both water and lipids (amphiphilic).(1) They are considered to replace volatile organic solvents as environmentally safe “green solvents” in chemical synthesis, biotechnology, and chemical engineering, with numerous applications, such as cellulose processing, as lubricants, corrosion inhibitors and battery electrolytes, for biofuel production, and even for water purification. Due to their biological activity, including antimicrobial and cytotoxic properties, biomedical applications of ionic liquids are under development. Furthermore, ionic liquids are used for chemical separation and extraction and explored for removal of organic pollutants in wastewater treatment.(2) In PubMed 900–1100 publications on “ionic liquids” are registered per year from 2012 to 2018, with the vast majority on technical and chemical aspects. Ionic liquids have been described as a “quiet revolution in material science” and their use and emission to the environment as well as human exposure can be expected to increase rapidly in the near future. The compound annual growth rate is expected to be over 22% and the anticipated market size of ionic liquids predicted to be over 50 kilo tonnes by 2022 (www.gminsights.com).
Due to their water solubility and low biodegradability, ionic liquids are potential persistent aquatic pollutants. Furthermore, due to low sorption to soil particles they may be transported to deeper soil layers and contaminate groundwater, including drinking water.(3)
Despite the increasing use and potential pollution of ionic liquids, the adverse effects on human health have not been considered, and their classification as “green solvents” is not based on any toxicological risk assessment. Data on toxicity of ionic liquids are extremely limited, apart from predominantly academic studies, demonstrating acute toxicity in bacteria and aquatic organisms, for example Vibrio fischeri, green algae, daphnia, mussel, and cytotoxicity in various cell lines.(4)
Among the ionic liquids, imidazolium-based compounds have gained much attention and are among the most studied ionic liquids (Figure 1). These compounds show acute toxicity in bacteria at concentrations significantly lower than those of conventional solvents. The toxicity is dependent on the chain length of the side chain and increasing toxicity with increasing chain length has been demonstrated in various cells and organisms.(3,5)
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