Long billed as ‘green’ solvents, some ionic liquids may in fact be persistent ’emerging pollutants’, with similarities to per- and polyfluorinated alkyl substances (PFASs), according to researchers from Sweden and the UK.
Ionic liquids are used in commercial applications from fabric softeners to household paints. The organic salts are liquid at room temperature but do not evaporate under normal conditions when heated and therefore do not escape to the atmosphere, unlike volatile organic compounds. They can also be tailored to fit manufacturing processes, with a vast array of possible structures.
Matthew Wright, chair in toxicology at Newcastle University’s Institute of Cellular Medicine, first came across the liquids after identifying one in a random soil sample. He has since found “surprisingly little” toxicity data on the compounds, except in bacteria, despite the fact that they are long-lived if they enter the environment.
As part of an interdisciplinary team, Professor Wright did a small pilot study to take soil samples from near a UK landfill site and to extract chemicals from them. “We were really surprised that at two sampling sites there was something really toxic,” he said.
He approached the study through an interest in an auto-immune liver disease called primary biliary cholangitis (PBC). “There are lots of studies over 40 years suggesting that there’s some sort of xenobiotic foreign chemical trigger for PBC but no-one had gone really very far into identifying what those chemicals could be,” he told Chemical Watch.
Imidazolium
The team worked out that their toxic chemical was an imidazolium ionic liquid called C8mim (1-octyl-3-methylimidazolium). This was metabolised by human liver cells to a carboxylic acid bearing a structural similarity to a natural substance called lipoic acid, a factor required in part of a protein linked to PBC.
By mimicking lipoic acid, the substance could potentially trigger PBC, says Professor Wright. But there is no direct evidence for this, he cautioned. In addition, C8mim itself “has no widespread commercial application”, commented Leigh Aldous from the department of chemistry at King’s College London, who is not linked to the study.
Professor Wright’s main concern is that imidazolium ionic liquids have low biodegradability and are both water and lipid soluble. “So that means they can get into us quite easily.”
Together with Agneta Oskarsson, professor in food toxicology at the Swedish University of Agricultural Sciences, he has penned a viewpoint article on ionic liquids.
“Present data do not justify ionic liquids to be classified as environmentally safe chemicals,” they write in the journal Environmental Science and Technology. “There are indications that they are both persistent and mobile in the environment, in addition to exerting a high acute toxicity in cellular systems.”
The authors “rightly highlight the commonly used imidazolium-based ionic liquids as being problematic in terms of toxicity, and there are indeed many studies to support this, alongside their poor biodegradability,” said Anna Croft, from the University of Nottingham’s department of chemical and environmental engineering. But there are other ionic liquids that have been demonstrated as safe in biological contexts, she added.
Low spillage risk
“Spillages are low risk where there are good safety structures in place, so I do not see the industrial use being problematic, and certainly nothing has been reported, to the best of my knowledge, to date,” said Dr Croft.
Dr Aldous is concerned that “many of the most popular and commercially available ionic liquids are in fact perfluorinated substances, hence some potential issues are even more significant than raised in this viewpoint.”
But he pointed out that “ionic liquids can be designed to either be extremely toxic or fully biodegradable.” In addition, “most people at the forefront of ionic liquid research are already actively working to avoid the mistakes previously introduced in the past few decades by plastics, PFAS, etc,” he said.