The potential persistence, accumulation in soils and water and biodegradability of ILs was assessed using standardized methods, such as the closed bottle test or the CO2 headspace test [60, 111, 114]. Studies on IL biodegradability included the major types of cations: ammonium, imidazolium, phosphonium, and pyridinium ions [111]. The widely used 1-butyl-3-methylimidazolium IL remains stable after 28 days in an aqueous suspension with waste-water microorganisms [60]. It was found that ILs with halogens, branched alkyl chains, pyridine rings, aliphatic ethers are usually more resistant to biodegradation [111]. With these observations, efforts are underway to develop biodegradable ILs. Current strategies include replacing branched alkyl chains with linear alkyl chains, functionalization to enable enzymatic hydrolysis, and the incorporation of phenyl rings [60].

4.5 Applications