Role of the chemical structure of ionic liquids in their ecotoxicity and reactivity towards Fenton oxidation

The production and application of ionic liquids (ILs) is often associated with the formation of aqueous effluents. Due to the well-known toxicity and poor biodegradability of those compounds, effective treatments for their removal from aqueous solutions are required. In this work, the Fenton oxidation methodology under optimum operating conditions (50 mg L-1 Fe3+, stoichiometric dose of H2O2, 70 degrees C and pH(0) = 3) was investigated in eleven different ILs, based in the imidazolium, phosphonium, quaternary ammonium, pyridinium and cholinium families, conjugated with various inorganic and organic anions such as chloride, cyanoborate, cyanamide, thiocyanate, dimethylphosphate and tosylate. The efficacy of the process was evaluated by the analysis of the chemical oxygen demand (COD), total organic carbon (TOC), the acids formed by ILs oxidation and the toxicity of the non-treated and treated aqueous effluents towards the Vibrio fischeri bacteria. It was possible to identify the main compounds produced upon Fenton oxidation in the final aqueous effluents (acetic, oxalic and formic acids) and the results seem to suggest that the Fenton oxidation is a good degradation methodology for most of the ILs tested, achieving TOC conversions in the range of 40-80% and 30-40% for aromatic and aliphatic ILs, respectively. Furthermore, non-toxic effluents were obtained at the end of the treatment for all the ILs investigated with the exception of the cyano-based ones due to the release of highly toxic cyanide species to the aqueous medium. (C) 2015 Elsevier B.V. All rights reserved.