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Title of Journal: Biodegradation

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Abbravation: Biodegradation

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Springer Netherlands

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Biodegradation of imidazolium ionic liquids by activated sludge microorganisms

Authors: Ewa Liwarska-Bizukojc, Cedric Maton, Christian V. Stevens,

Publish Date: 2015/10/13
Volume: 26, Issue:6, Pages: 453-463
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Biological properties of ionic liquids (ILs) have been usually tested with the help of standard biodegradation or ecotoxicity tests. So far, several articles on the identification of intermediate metabolites of microbiological decay of ILs have been published. Simultaneously, the number of novel ILs with unrecognized characteristics regarding biodegradability and effect on organisms and environment is still increasing. In this work, seven imidazolium ionic liquids of different chemical structure were studied. Three of them are 1-alkyl-3-methyl-imidazolium bromides, while the other four are tetra- or completely substituted imidazolium iodides. This study focused on the identification of intermediate metabolites of the aforementioned ionic liquids subjected to biodegradation in a laboratory activated sludge system. Both fully substituted ionic liquids and 1-ethyl-3-methyl-imidazolium bromide were barely biodegradable. In the case of two of them, no biotransformation products were detected. The elongation of the alkyl side chain made the IL more susceptible for microbiological decomposition. 1-Decyl-3-methyl-imidazolium bromide was biotransformed most easily. Its primary biodegradation up to 100 % could be achieved. Nevertheless, the cleavage of the imidazolium ring has not been observed.Ionic liquids, because of their unique physicochemical properties, are a promising group of chemicals that can be widely used in various branches of industry. These compounds have therefore been investigated intensively for the last two decades with regard to their biological properties and their effect on the environment (Jastorff et al. 2003; Gathergood et al. 2006; Stolte et al. 2008; Coleman and Gathergood 2010; Pham et al. 2010; Siedlecka et al. 2011; Markiewicz et al. 2013). This research comprised two main areas: ecotoxicity and biodegradability of ionic liquids. In order to evaluate biodegradability of ILs, standard OECD tests were usually applied. They revealed that many of the ionic liquids were not susceptible to biological decomposition. Particularly imidazolium ionic liquids attracted the most interest in industry and academia. Standard biodegradation tests focused mainly on primary biodegradation, which is an alteration in the chemical structure of a substance, by the biological action, resulting in the loss of the specific property of that substance (OECD 2014).Stolte et al. (2008) tested the primary biodegradation of different N-imidazoles, imidazolium, pyridinium and 4-(dimethylamino)pyridinium compounds substituted with various alkyl side chains and their functionalized analogues. Significant primary biodegradation (up to 100 %) for the ionic liquids carrying long alkyl side chains (C6 and C8) was noticed, whereas in the case of imidazolium ILs with short alkyl (≤C6) and short functionalized side chains, no biological decomposition was found. Docherty et al. (2007) showed that hexyl- and octyl-substituted pyridinium ILs could be totally metabolized; the imidazolium analogues were partially mineralized, whereas butyl-substituted imidazolium ILs were not biodegradable. The results of biodegradation tests indicated that a certain lipophilicity of ionic liquids was required to increase their biodegradability (Docherty et al. 2007; Stolte et al. 2008; Pham et al. 2010). It was also observed that the introduction of various functional groups into the side chain (e.g. terminal hydroxyl, carboxyl, ether and nitrile group) did not lead to the expected improvement of the biodegradation of imidazolium compounds (Stolte et al. 2008). In the case of other cations, the introduction of hydroxyl groups made the ionic liquids more biodegradable (Neumann et al. 2014).Previous studies on the biodegradation of ionic liquids were usually made with mixed cultures of microorganisms (including flocculent and granular activated sludge). Also, the effect of ionic liquids on flocs morphology and metabolic activity of microorganisms was estimated. Anaerobic granular sludge occurred to be less sensitive to pyridinium-based as well as imidazolium-based ionic liquids than the aerobic sludge (Gotvajn et al. 2014). At low concentration (up to 5 mg l−1), imidazolium ionic liquids did not act on the morphology of the flocculent activated sludge, whereas at higher concentrations, they contributed to the decrease of the projected area of sludge flocs (Gendaszewska and Liwarska-Bizukojc 2013). The inhibitory effect on dehydrogenase activity of activated sludge biomass increased with the increase in chain length of the alkyl substituent; however, it was dependent on the origin and properties of activated sludge (Liwarska-Bizukojc 2011). Markiewicz et al. (2009) estimated that at 1-methyl-3-octyl-imidazolium chloride concentration higher than 0.2 mM, the dehydrogenase activity of the cells dropped markedly. Also, Azimova et al. (2009) measured the effect of imidazolium-derived ionic liquids on bacterial respiration rate. It occurred that the values of effect concentration (EC50) were similar to those for 1-butanol, which is the alcohol with the alkyl chain length similar to that of the cation of the tested compound like, for example, 1-butyl-3-methyl-imidazolium bromide (Azimova et al. 2009).Apart from the mixed cultures of activated sludge, the pure cultures of bacteria or the isolated consortia were employed in the biodegradation of ionic liquids, too. Abrusci et al. (2011) found that more than half of 37 studied ionic liquids exhibited biodegradation percentage greater or equal than 60 % after a 28-day incubation with the bacterium Sphingomonas paucimobilis at 45 °C. Megaw et al. (2013) identified the bacterial isolates, out of which two were particularly effective ionic liquid biodegraders and were regarded as the candidates for the bioremediation of 1-ethyl- and 1-butyl-3-methyl-imidazolium chlorides. At the same time, biodegradation of 1-methyl-3-octyl-imidazolium chloride ([OMIM][Cl]) conducted by the isolated consortium of bacteria was lower than that performed with the use of activated sludge organisms (Markiewicz et al. 2014). It might have been a result of lower cell densities in the samples with the isolated consortium (Markiewicz et al. 2014).The next step in the studies on biodegradability of ILs was to check the ability of microorganisms to be adapted to the presence of ionic liquids and finally to use them as a carbon source. Markiewicz et al. (2011) observed a nearly 30-fold increase of the biodegradation rate of 1-methyl-3-octyl-imidazolium chloride ([OMIM][Cl]) during the process of adaptation of activated sludge. At the same time, the supplementation with organic carbon and nitrogen decreased biodegradation rate of this IL (Markiewicz et al. 2011). The results presented by Romero et al. (2008) revealed that 1-alkyl-3-methyl-imidazolium chlorides were poorly biodegradable even if the additional carbon source, in this case glucose, was available. During ten days of aerobic biodegradation, glucose was totally consumed, whereas the concentration of ionic liquids decreased only slightly below the initial value 100 mg l−1. On the contrary, Gotvajn et al. (2014) observed the appearance of co-metabolism in the biodegradation of ionic liquids, when glucose was added.In order to describe biodegradation of any compounds precisely, it is essential to identify the intermediates. The latter was performed using most often liquid chromatographic methods coupled to mass spectrometry including ion trap mass spectrometer (Stolte et al. 2008; Pham et al. 2009; Coleman and Gathergood 2010; Markiewicz et al. 2011; Neumann et al. 2014). In some cases, 1H nuclear magnetic resonance (NMR) analyses can be used (Deng et al. 2011). Nevertheless, the number of publications concerning metabolites of ILs biodegradation and suggestion of their biodegradation pathways is very limited. For 1-methyl-3-octyl-imidazolium cation, different biological transformation products carrying hydroxyl, carboxyl and carbonyl groups were identified (Stolte et al. 2008). This ionic liquid and its hydroxylated and carboxylated analogues were completely degraded if primary biodegradation is considered. At the same time, Pham et al. (2009) observed that biodegradation of 1-butyl-3-methylpyridinium bromide led to the formation of 1-hydroxybutyl-3-methylpyridine, 1-(2-hydroxybutyl)-3-methylpyridine, 1-(2-hydroxyethyl)-3-methylpyridine and methylpyridine. Any product of further degradation, including intermediates of pyridinium ring cleavage, was not found. Markiewicz et al. (2011) proposed that the biodegradation of 1-methyl-3-octyl-imidazolium cation started with the ω-oxidation of the alkyl chain, and this chain was subsequently degraded via β-oxidation. They observed ultimate degradation of the imidazolium ionic liquid [OMIM][Cl]; however, any degradation products of imidazolium ring cleavage were neither shown nor listed (Markiewicz et al. 2011). Neumann et al. (2014) found that imidazolium ionic liquids were the most refractory out of five cation groups tested. No biodegradation was observed for the imidazolium ionic liquid with propyl side chains (Neumann et al. 2014).



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