Kinetics and mechanism studies of efficient degradation of 1-hexyl-3-methylimidazolium by zero-valent iron activated persulfate

Abstract

In recent years, ionic liquids (ILs) have captured much attention of environment researchers due to their widespread use and potential biotoxicity. In this work, Fe0 activated persulfate (Fe0/PS) oxidation system was systematically examined for its performance to degrade 1-hexyl-3-methylimidazolium (HMIm), a typical imidazole-based ILs, with the study of degradation kinetics, pathways, mechanisms and ecotoxicity. Under the conditions of pH 3.0, Fe0/PS/HMIm molar ratio of 200:50:1, the removal efficiency of HMIm reached 93.5 % within 10 min. With the increased of the cycle index, the degradation performance became better. The degradation of HMIm was inhibited to some extent in real waters, where the presence of inorganic ions may competitively consume or suppress the production of active species. As evidenced by the results of EPR technology and quenching experiments, •OH, SO4•−, 1O2, and O2•− were generated and involved in the degradation of HMIm, with the corresponding contribution of 9.4 %, 71.4 %, 14.0 %, and 28.7 %, respectively. Twenty-seven oxidation products were identified by HPLC-QTOF-MS analysis, and side chain breakage, hydroxylation, 1O2 initiated oxidation and sulfate addition were the main reactions leading to the four primary products. Based on the frontier molecular orbital calculation, the imidazole ring of HMIm was determined as the main active sites for free radical attack. According to ECOSAR assessment, the products in the four oxidation pathways showed a decreased toxicity than HMIm. This study demonstrates that Fe0/PS is an effective and environmentally friendly method to eliminate HMIm, which also provide some technical reference for the treatment of ILs polluted waters.