Ionic liquids breakdown by Fenton oxidation

• Fenton oxidation at 70 °C has proved to be efficient for the breakdown of ionic liquids.
• Total conversion of the ionic liquids and a high mineralization (>60%) were reached.
• Reaction pathways have been proposed for the oxidation of the ionic liquids tested.
• The Fenton effluents showed negligible ecotoxicity values.
• The Fenton treatment allows achieving a significant improvement of the biodegradability.

Fenton oxidation has proved to be an efficient treatment for the degradation of ionic liquids (ILs) of different families viz. imidazolium, pyridinium, ammonium and phosphonium, in water. The intensification of the process, defined as the improvement on the efficiency of H2O2 consumption, by increasing the temperature is necessary to avoid high reaction times and the need of large excess of H2O2. In this work, temperatures within the range of 70–90 °C have been used, which allowed an effective breakdown of the ILs tested (1 g L−1 initial concentration) with the stoichiometric amount of H2O2 and a relatively low Fe3+ dose (50 mg L−1). Under these conditions conversion of the ILs was achieved in less than 10 min, with TOC reductions higher than 60% upon 4 h reaction time, except for the phosphonium IL. The remaining TOC corresponded mainly to short-chain organic acids. The treatment reduced substantially the ecotoxicity up to final values below 0.01 TU in most cases and a significant improvement of the biodegradability was achieved. Upon Fenton oxidation of the four ILs tested hydroxylated compounds of higher molecular weight than the starting ILs, fragments of ILs partially oxidized and short-chain organic acids were identified as reaction by-products. Reaction pathways are proposed.

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