The long-chain alkylimidazolium ionic liquid 1-butyl-2,3-dimethylimidazolium bromide ([BMMIm]Br) has been viewed as a threat to the ecosphere owning to its toxicity and refractory. Therefore, a classical photocatalyst TiO2 nanosphere was prepared by a gel-sol method for [BMMIm]Br removal. The photocatalytic activity of TiO2 nanospheres for BMMIm degradation (99.01%, 360 min) was superior to commercial TiO2 (34.94%, 360 min), owing to more efficient separation and transfer of photogenerated carriers, lower charge transfer impedance and narrower energy band gap (3.28 eV). The reaction conditions (catalyst dosage and pH) and water components (common inorganic ions and humic acid) had a certain impact on [BMMIm]Br photocatalytic degradation. Besides, [BMMIm]Br could also be degraded efficiently and steadily in four real waters. The main contribution of hydroxyl radicals to the photocatalytic degradation of [BMMIm]Br was verified by electron paramagnetic resonance detection and free radical quenching experiments. A total of 23 products were detected and that hydroxyl substitution, bond breaking, direct oxidation and ring opening were speculated as the main transformation behaviors of [BMMIm]Br photocatalytic degradation. Then the generation of hydroxyl substituted products including structural isomers were further justified by theoretical calculations. The toxicity evaluation of BMMIm and its intermediates showed that hydroxylation was a toxicity-enhancing process, while the bond breaking reaction had great detoxification capacity. We hope that this work can provide theoretical guidance for the development of green and efficient removal methods for the alkylimidazolium ionic liquids in water.

Photocatalytic degradation of alkyl imidazole ionic liquids by TiO2 nanospheres under simulated solar irradiation: Performance, transformation behavior, and DFT calculations

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