Efficient degradation of alkyl imidazole ionic liquids in simulated sunlight irradiated periodate system: Kinetics, reaction mechanisms, and toxicity evolution

As a class of emerging aquatic pollutants, alkylimidazole-based ionic liquids (AM-ILs) have received extensive attention due to the large acute toxicity to aquatic organisms. Therefore, in order to protect both aquatic organisms and human beings, it is necessary to seek an efficient and environmental-friendly technology for removal of AM-ILs from water bodies. In this work, we found that under simulated sunlight (Xe lamp) irradiation, periodate (KIO4, PI) could efficiently degrade 1-hexyl-2,3-dimethylimidazolium bromide ([HMMIm]Br), a representative AM-ILs with six carbon atoms in the side chain. Kinetics experiments on the degradation of [HMMIm]Br were performed, and the results showed that a high degradation efficiency (≥90.00%) of the cation ([HMMIm]+) was still maintained under harsh water conditions of strong acidity/alkaliny or with various non-target inorganic ions. More importantly, the anion of bromide ion (Br-) was not oxidized to the carcinogenic bromate (BrO3-) in current reaction system. The excited stated PI (marked as PI*) was detected by Laser flash photolysis, and it was an important reactive species for [HMMIm]+ degradation. As rationalized by theoretical calculations and scavenging experiments, the main oxidation mechanisms of [HMMIm]+ were hydroxyl radicals induced substitution reaction, PI* initiated electron and double oxygen transfer, and direct photolysis mediated chemical bond cleavage reaction, which contributed to 73%, 21%, and 6% of [HMMIm]+ degradation, respectively. Moreover, toxicity evaluation by ECOSAR software indicated that the oxidation products were generally less toxic to three aquatic organisms (fish, water flea, and green algae) than the target molecule [HMMIm]Br. In conclusion, this work proposed novel oxidation mechanisms of sunlight-activated PI system, and the findings may inspire further researches on the application of photoactivated hypervalent acids in water purification.