Degradation of imidazolium-based ionic liquids by UV photolysis and pulsed corona discharge: The effect of persulfates addition

Abstract

Ionic liquids (ILs) are salts with exceptional properties important for future development, although potentially hazardous for humans and aquatic life due to their high aqueous solubility and toxicity. Biodegradation of ILs depends on the anion nature and cation structure: many ILs are not readily degraded during conventional wastewater treatment, thus requiring alternative cost-effective and energy-efficient solutions. In this research, the degradation of aqueous imidazolium-based ILs by pulsed corona discharge (PCD) and UV photolysis combined with persulfates was studied, using peroxymonosulfate (PMS) and peroxydisulfate (PDS) as extrinsic oxidants. 1-Ethyl-3-methylimidazolium chloride ([Emim][Cl]), 1-methyl-3-octylimidazolium chloride ([Omim][Cl]), and 1-ethyl-3-methylimidazolium bromide ([Emim][Br]) were chosen as imidazolium-based ILs with different alkyl chain lengths and anions in oxidation experiments under variable operation factors - pH, concentrations of persulfates, pulse repetition frequency in PCD. Pulsed corona discharge has previously shown high energy efficiency in the degradation of micropollutants but has not yet been applied to ionic liquids. The experimental results showed the ability of PCD, PCD/oxidant and UV/oxidant combinations to completely degrade the ILs with the degradation rates in descending order [Emim][Cl] > [Omim][Cl] > [Emim][Br], indicating an impact of both the alkyl chain length and the type of anion on the oxidation rate. The strong activation of persulfates was observed in UV/oxidant combinations, whereas PCD demonstrated the effect of persulfates addition between slight acceleration and moderate deceleration of oxidation dependent on the concentration of extrinsic oxidants. The addition of the sulfate radicals’ scavenger, however, showed the presence of those in all combinations indicating a certain replacement of PCD-generated reactive oxygen species with secondary reactants formed from persulfates. The unassisted PCD and UV/PDS combination demonstrated similar energy efficiencies of about 54 and 26 mmol kWh−1 for [Emim][Cl] and [Emim][Br] degradation, respectively, while for [Omim][Cl] the UV/PDS combination showed energy efficiency 1.42 times higher than unassisted PCD.