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Ionic Liquids: New Emerging Pollutants, Similarities with Perfluorinated Alkyl Substances (PFASs) Agneta Oskarsson*,† and Matthew C. Wright‡
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Department of Biomedical Sciences and Veterinary Public Health, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden ‡ Institute of Cellular Medicine, Health Protection Research Unit, Newcastle University, Newcastle Upon Tyne NE2 4AA, United Kingdom 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 f ischeri, 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 Recently Probert et al.6 identified the ionic liquid 1-octyl-3onic liquids are organic salts with unique physicochemical methylimidazolium (C8mim) as being present in the environproperties, including low melting points, negligible vapor ment and capable of acting as a potential trigger in the autoimmune liver disease primary biliary cholangitis (PBC). Its pressure, high thermal and chemical stability, and high potential role (along with structurally related ionic liquids) as a solubility in both water and lipids (amphiphilic).1 They are trigger for PBC is postulated on the basis that apoptosis of considered to replace volatile organic solvents as environcholangiocytes is thought to be a critical aspect in autoantigen mentally safe “green solvents” in chemical synthesis, bioexposure. In addition, an hepatic-derived metabolite of C8mim technology, and chemical engineering, with numerous was shown to be capable of replacing lipoic acid in the applications, such as cellulose processing, as lubricants, autoantigen, resulting in generation of antimitochondrial corrosion inhibitors and battery electrolytes, for biofuel antibodies. production, and even for water purification. Due to their In perspective of the high industrial interest and reputation biological activity, including antimicrobial and cytotoxic as environmentally friendly chemicals, surprisingly little data properties, biomedical applications of ionic liquids are under are available regarding the toxicity of imidazolium ionic liquids development. Furthermore, ionic liquids are used for chemical in mammalian systems. With regard to C8mim (as either the separation and extraction and explored for removal of organic Cl- or Br- salt), toxicity has been examined in various cell lines, 2 pollutants in wastewater treatment. In PubMed 900−1100 and toxicity appears to follow an increase in reactive oxygen publications on “ionic liquids” are registered per year from species leading to an apoptotic mode of cell death.7 Ionic 2012 to 2018, with the vast majority on technical and chemical liquids are amphiphilic and can bind to cell membrane aspects. Ionic liquids have been described as a “quiet phospholipids, with the binding increasing with increasing side revolution in material science” and their use and emission to chain length, resulting in decreased membrane stability and ultimately disruption of cell membranes.3,5 Using a rodent liver the environment as well as human exposure can be expected to progenitor cell line and primary human cholangiocytes, C8mim 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 Received: August 7, 2019 by 2022 (www.gminsights.com).
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© XXXX American Chemical Society
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DOI: 10.1021/acs.est.9b04778 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
Figure 1. Ionic liquids, of which C8mim is one example, are considered to be environmentally friendly solvents. However, there are indications that they are a new group of emerging pollutants. High acute toxicity has been demonstrated in cellular systems and aquatic organisms, while long-term toxicity data and studies in mammals are essentially lacking.
ORCID
chloride was shown to directly inhibit mitochondrial oxidative phosphorylation prior to an induction of apoptosis.6 A single limited study in mice suggests the liver is a target organ after acute exposure to C8mim bromide.8 Liver toxicity, associated with oxidative stress and mitochondria-mediated apoptosis, was also demonstrated in silver carps, exposed to C8mim bromide at low water concentrations for 60 days.9 Perfluorinated alkyl substances (PFASs) is a group of emerging aquatic pollutants with chemical−physical properties similar to ionic liquids, that is, high thermal and chemical stability, and with both hydrophobic and hydrophilic functional groups (amphiphilic).10 PFASs have low biodegradability, high mobility in soil and water, and bioaccumulate in the food web. Similar to ionic liquids, PFASs can incorporate into cell membranes, and cellular concentrations and toxicity is dependent on the length of the carbon chain.11 Contamination of PFASs in drinking water is today recognized as a global problem, and PFASs are not removed by conventional techniques in drinking water treatment plants.12 The number of reports on adverse effects of PFASs has increased, and regulatory actions and maximum permitted levels in drinking water are continuously revised. The problems associated with the use of PFASs were not discovered until contamination of soil and water had already occurred and worldwide human exposure demonstrated through their detection in plasma. Even if PFASs pollution ceased today, the problems will remain for decades due to their long half-life in the environment. In order not to repeat the history for ionic liquids, data on in vivo and long-term toxicity are needed for risk assessment as a basis for effective risk management. In conclusion, present data do not justify ionic liquids to be classified as environmentally safe chemicals. In contrast, there are indications that they are both persistent and mobile in the environment, in addition to exerting a high acute toxicity in cellular systems. There is an evident lack of coordination between development and usage of new chemicals, and development of data for safety assessment of chemicals in the environment and human health. From PFASs, we have learnt that once the chemicals have been emitted, they will be long-lasting in the environment, and human exposure will occur with health consequences of which we are not yet fully aware.
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Agneta Oskarsson: 0000-0002-3134-7811 Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.est.9b04778 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology laboratory animals - Relation to current regulations and guidance values. Int. J. Hyg. Environ. Health 2017, 220 (4), 766−775.
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DOI: 10.1021/acs.est.9b04778 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
