Research On The Degradability Of Imidazolium-Based And Pyridinium-Based Ionic Liquids

Ionic Liquids (ILs) are molten salts at room temperature composed by organic cations and organic/inorganic anions. Because of their non-detectable pressure, stable thermal properties and good solubility, ILs are considered as green substitutes to conventional organic solvents. The study of environmental impact of ILs has mainly centered on their (eco)toxicity and anti-microbial activities, but limited information on biodegradability has been available till now. The biodegradation performance of ILs was studied for five kinds of imidazolium-based ionic liquids and three kinds of pyridinium-based ionic liquids by activated sludge and photocatalysis methods. The main research includes threes parts as follows:(1) The biodegradation of five kinds of imidazolium-based ionic liquids were studied, viz. [BMIM]Cl, [BMIM]BF4, [BMIM]PF6, [HMIM]Cl, [OMIM]Cl. The influence of acclimation time, ILs concentrations, length of side chain and anions were investigated, the biodegradation intermediates wee detected and the possible degradation process proposed. As a matter of fact, [BMIM]Cl, [BMIM]BF4 and [BMIM]PF6 could be particially broken down, the order of their degradation rate is [BMIM]Cl<[BMIM]BF4< [BMIM]PF6. Howerer, [HMIM]Cl and [HMIM]Cl are nearly not biodegradable.(2) In order to investigate the biodegradability of ILs, three kinds of alkyl substituted pyridinium based ionic liquids, viz. N-butylpyridinium, N-hexylpyridinium and N-octylpyridinium chlorides, were studied by aerobic activated sludge using sequencing batch reactor (SBR); and the resulting cationic metabolites were identified by HPLC-MS method. It is found that the rate of biodegreadation of N-butylpyridinium cation is faster than that of N-hexylpyridinium cation, while N-octylpyridinium is nearly not degradable. The biodegradation process follows a zero-order reaction mode. It is assumed that the pyridinium ring cleaves first and then the resulting carbons and the methyl in the farthest end of the alkyl group to the pyridinium ring are oxidized. The metabolized intermediates are further mineralized into NH4+ CO2 and H2O, as proved by the high removal rate of COD, for example, 92.5% of COD is removed for a 300 mg/L N-butylpyridinium chloride solution after 24 h biodegradation.(3)The degradability of three kinds of 1,3-dialkylimidazolium-based ILs was studied using photocatalysis with nano-sized TiO2 p25. The fragments formed in the photocatalysis were identified using HPLC-MS and their possible structures were proposed. The results indicate that the degradation rate of the ILs follows the order of 1-Butyl-3-Methylimidazolium Chlorine ([BMIM]Cl)> 1-Hexyl-3-Methylimidazolium Chlorine ([HMIM]Cl) ≈1-Octyl-3-Methylimidazolium Chlorine ([OMIM]Cl). The degradation process of [BMIM]Cl could be reasonably represented by first-order reaction kinetics, and the optimal loading of TiO2 is 0.5 g/L. It is showed from the HPLC-MS analysis, photocatalysis is associated with cleavage of the imidazolium ring and oxidation, forming different intermediate fragments with various degree of oxidation. As a result, the pretreatment of IL solution via photocatalysis can improve the biodegradability by activated sludge microorgaanisms.