Degradation Of Ionic Liquids In An Ultrasound Assisted Zero-valent Iron Activated Carbon Micro-electrolysis System And Their Mecthanistic Elucidations

Ionic liquids (ILs), as novel media and functional materials, are developing rapidly in recent years due to their attracted properties such as negligible vapor pressure, high thermal stability, extended liquid-state temperature range, good dissolving ability and electrochemical properties. Regarded as green solvents and materials, ILs have been successfully used in chemical synthesis and catalysis, extraction and separation, energy materials and electrochemistry. However, it has been reported that the commonly used ILs are toxic and not biodegradable in nature, thus development of efficient chemical methods for the degradation of ILs is imperative. In this work, an ultrasonic irradiation and zero-valent iron activated carbon (US-ZVI/AC) micro-electrolysis system was applied to degradation of ionic liquid residues in water. The experimental conditions were optimized, and the degradation processes of imidazolium ILs with different alkyl side chains, ILs with different head group types and some functionalized ionic liquids were investigated, and some intermediates generated during the degradation were identified. On the basis of these observations, the degradation pathways were suggested. The main contents are as follows.(1)1-Butyl-3-methylimidazolium chloride ([C4mim]Cl), a widely used ionic liquid, was chosen as a model ionic liquid to study the degradation efficiency of ionic liquids in US-ZVI/AC system. The degradation efficiency of [C4mim]Cl was investigated by changing AC/ZVI ratio, dosages of AC and ZVI, solution pH and so on. The degradation processes were analyzed by using UV and HPLC techniques and the degradation efficiency was observed by determination of TOC. It was shown that the optimized experimental conditions for [C4mim]Cl degradation were: AC/ZVI ratio of2:1, the AC dosage of6g·L-1, the ZVI dosage of3g·L-1, and solution pH value of3. The degradation efficiency of [C4mim]Cl could be improved greatly by ultrasonic irradiation frequency of45KHz, and the degradation could be described by a pseudo-first-order kinetics. The presence of excess hydroxyl radical scavenger could greatly reduce the reaction rate but could not prevent degradation of [C4mim]Cl, and the results confirmed that OH· participated in the reaction process and played an important role, but it was not the only reactive species involved in the degradation of [C4mim]Cl. The ultraviolet absorption band of [C4mim]Cl at212nm disappeared almost completely, and more than95%of [C4mim]C1could be degraded in aqueous solution and81.2%of [C4mim]Cl could be mineralized within110min.(2) The relationship between the degradation degree and the chemical structure of ionic liquids was investigated by changing the length of alkyl chain in1-alkyl-3-methylimidazolium bromide ([Cnmim]Br, n=2,4,6,8,10) and the counter ion type of1-butyl-3-methylimidazolium cation ([C4mim]+). The degradation intermediates of [Cnmim]+at different reaction time were analyzed by GC-MS and HPLC-MS, and the degradation pathway was suggested. It was shown that the imidazolium ILs studied here can be degraded rapidly in US-ZVI/AC system, and the degradation of imidazolium ILs was not influenced by the type of counter anion but can be less influenced by their alkyl chain length. The degradation of [Cnmim]Br followed the pseudo-first-order kinetics model with respect to its concentration. In the degradation processes, the2,4,5positioned H atoms of1-alkyl-3-methylimidazolium ring were first oxidized to generate1-alkyl-3-methyl-2,4,5-trioxoimidazolidine, and then the ring was opened to form1-alkyl-3-methylurea which was further decomposed to form N-alkylformamide. More than81%of TOC removal of [Cnmim]Br suggested further mineralization of the N-alkylformamide.(3) The degradation of the ILs with different head group types was investigated in US-ZVI/AC system and their degradation intermediates were identified by GC-MS and HPLC-MS in the same way mentioned above, and the degradation pathways were then suggested. The results showed that1-butyl-1-methylpiperidinium bromide ([C4mpip]Br),1-butyl-1-methylpyrrolidinium bromide ([C4mpyr]Br) and N-butyl-N-methylmorpholinium bromide ([C4mmor]Br) were also effectively degraded in the system, and the level of degradation is dependent on the type of cationic head group of the ILs. Unlike the imidazolium ILs discussed above, oxidation degradation of [C4mpip]Br,[C4mpyr]Br and [C4mmor]Br in the micro-electrolysis system was observed at the N-butyl side chains. The alcohols formed by oxidation were subsequently oxidized via aldehydes to carboxylic acids. N-methyl heterocyclic core was generated via the elimination of the oxidized butyl side chain. The TOC removal of these ILs was in the range of69.6-73.8%and this indicated partial mineralization of N-methyl heterocyclic core.(4) Degradation of the functionalized ionic liquids1-allyl-3-methylimidazolium chloride ([Amim]Cl), N-butyl, methyl benzimidazolium bromide ([Bmbim]Br) and tetrabutyl ammonium chloride ([Tba]Cl) was investigated in US-ZVI/AC micro-electrolysis system. The degradation degree and intermediates were analyzed and degradation mechanisms were presumed. It is shown that the three functionalized ILs could also be effectively degraded and the degradation pathway of [Amim]Cl was similar to that of the imidazolium ILs discussed above: the imidazolium ring of [Amim]C1was first oxidized and then the ring was opened to form urea or formamide. The degradation of [Bmbim]Br was started by the oxidization of2-positioned C atom of the imidazolium ring, and then the alkyl side chain was broken or the imidazolium ring was opened. The difference in degradation pathway is probably caused by the presence of benzene ring. The degradation of [Tba]Cl was carried out through the gradual elimination of butyl groups. The TOC removal of the three ILs was greater than82%, which indicated the further mineralization of the degradation products identified and significant degradation effect of the ILs in the system.The high degradation efficiency and TOC removal suggested that the ZVI/AC micro-electrolysis assisted by ultrasonic irradiation could serve as an efficient technology for the removal of ILs from aqueous solutions, and the degradation products are environmental friendly to a great extent. The results presented here may be useful for the assessment of the factors related to the environmental fate and environmental behavior of these commonly used ILs.