A comparative study of electrochemical degradation of imidazolium and pyridinium ionic liquids: A reaction pathway and ecotoxicity evaluation

The investigation focused on the electrochemical oxidation of imidazolium and pyridinium ionic liquids (ILs), 1-butyl-3-methyl-imidazolium chloride (IM14 Cl), 1-hexyl-3-methyl-imidazolium chloride (IM16 Cl), 1-methyl-3-(2-phenylethyl)-imidazolium chloride (IM1-2Ph Cl), 1-butyl-4-methylpyridinium chloride (Py4-4Me Cl), and 1-butyl-4-(dimethylamino)pyridinium chloride (Py4-4NMe2 Cl) in an aqueous solution at the boron-doped diamond (BDD) electrode. The study demonstrated that the lowest degradation efficiency occurs at alkaline pH, while an increase in temperature has only a slight effect on the electrochemical oxidation process. The intermediate products, identified by LC-(ESI)/MS and GC-MS methods, suggest that the OH and O2− radicals take part in the decomposition of the investigated ILs. The attack of OH at first led to the hydroxylation of ILs followed by the oxidative opening of imidazolium or pyridinium ring. After a 3 h electrolysis, the decomposition of pyridinium salts was more advanced compared to imidazolium salts. Finally, the toxicity of untreated ILs solutions and their effluents after the electrochemical treatment was assessed from the measurements of growth inhibition in Scenedesmus vacuolatus and Lemna minor. The investigated ILs were more toxic to S. vacuolatus than L. minor. In general, the toxicity values decreased after the electrochemical treatment. However, in the case of IM14Cl/NaCl mixture, its post-electrolytic toxicity strongly increased.