Degradation of imidazolium ionic liquids in a thermally activated persulfate system

Abstract

Thermally activated persulfate (PS) oxidation, a clean source of sulfate radicals (SO4•–), has been used as a promising advanced oxidation method for the treatment of organic contaminants. In this study, the degradation of ionic liquids (ILs) by thermally activated PS was investigated with 1-alkyl-3-methylimidazolium bromides (C4mimBr) as a model compound. The effects of various factors including temperature, initial IL and PS concentrations, solution pH, alkyl chain length of imidazolium cation, and common water constituents such as humic acid (HA), HCO3-and Cl- were evaluated. Results showed that C4mimBr can be completely degraded in 2 h at 60 °C, indicating effective removal of C4mimBr by thermally activated PS oxidation. The oxidation followed a pseudo-first-order kinetics model, and the rate constant increased with increasing temperature, PS concentration and initial solution pH. Alkyl chain length had no effect on the degradation. HA and Cl- had negative effects on C4mimBr removal, while 5 mM HCO3- increased C4mimBr degradation. Multiple intermediate products were identified by HPLC-MS/MS and the degradation pathway and mechanism were proposed. The free radicals generated in the activated PS system (e.g., SO4•– and •OH) showed different effects on the degradation. SO4•– radicals select to attack the substituted side chain of ILs cation, producing more chance for •OH to attack the imidazolium core. Moreover, toxicity tests using bioluminescent bacteria Vibrio Qinghaiensis sp.-Q67 and nematode C. elegans demonstrated efficient detoxification of CnmimBr treatment. These findings may offer an environmentally friendly approach for ILs treatment in aqueous solution.