Biodegradation of imidazolium ionic liquids by act

Authors: Ewa LiwarskaBizukojc Cedric Maton Christian V Stevens

Publish Date: 2015/10/13

Volume: 26, Issue: 6, Pages: 453-463

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Abstract

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 1alkyl3methylimidazolium 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 1ethyl3methylimidazolium 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 1Decyl3methylimidazolium bromide was biotransformed most easily Its primary biodegradation up to 100  could be achieved Nevertheless the cleavage of the imidazolium ring has not been observedIonic 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 2014Stolte et al 2008 tested the primary biodegradation of different Nimidazoles imidazolium pyridinium and 4dimethylaminopyridinium 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 octylsubstituted pyridinium ILs could be totally metabolized the imidazolium analogues were partially mineralized whereas butylsubstituted 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 eg 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 2014Previous 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 pyridiniumbased as well as imidazoliumbased 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 LiwarskaBizukojc 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 LiwarskaBizukojc 2011 Markiewicz et al 2009 estimated that at 1methyl3octylimidazolium chloride concentration higher than 02 mM the dehydrogenase activity of the cells dropped markedly Also Azimova et al 2009 measured the effect of imidazoliumderived ionic liquids on bacterial respiration rate It occurred that the values of effect concentration EC50 were similar to those for 1butanol which is the alcohol with the alkyl chain length similar to that of the cation of the tested compound like for example 1butyl3methylimidazolium bromide Azimova et al 2009Apart 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 28day 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 1ethyl and 1butyl3methylimidazolium chlorides At the same time biodegradation of 1methyl3octylimidazolium chloride OMIMCl 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 2014The 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 30fold increase of the biodegradation rate of 1methyl3octylimidazolium chloride OMIMCl 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 1alkyl3methylimidazolium 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 cometabolism in the biodegradation of ionic liquids when glucose was addedIn 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 1methyl3octylimidazolium 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 1butyl3methylpyridinium bromide led to the formation of 1hydroxybutyl3methylpyridine 12hydroxybutyl3methylpyridine 12hydroxyethyl3methylpyridine 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 1methyl3octylimidazolium cation started with the ωoxidation of the alkyl chain and this chain was subsequently degraded via βoxidation They observed ultimate degradation of the imidazolium ionic liquid OMIMCl 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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